bluuebunny/crossref_metadata_2025 huggingface dataset:
Nucleotides. Part XLI. The 2‐dansylethoxycarbonyl (  2‐{[5‐(dimethylamino)naphthalen‐1‐yl]sulfonyl}ethoxycarbonyl; dnseoc) group for protection of the 5′‐hydroxy function in oligodeoxyribonucleotide synthesis
Authors: Bergmann Frank , Pfleiderer Wolfgang
Published: December 2004

Use of the 2‐dansylethoxycarbonyl (  2‐{[5‐(dimethylamino)naphthalen‐1‐yl]sulfonyl}ethoxycarbonyl; Dnseoc) group as an intermediate 5′‐OH protecting group in oligodeoxyribonucleotide synthesis using the automated phosphoramidite approach is described in a model study to an alternative strategy in RNA synthesis.
Calorimetric Measurements of the Complexation of Cyclosporin A, Ascomycin, Fujimycin, and Rapamycin with Lithium Chloride and with an Immunophilin
Authors: Seebach Dieter , Bossler Hans G. , Flowers Robert , Arnett Edward M.
Published: December 2004

Solutions (2 ml) of small linear and cyclic peptides (4–11), of a peptolide containing nine amino acids and a lactate moiety (12), of the cyclic undecapeptide cyclosporin A (CS, 1), and of the macrolides ascomycin, fujimycin, and rapamycin (13–15) in THF were added to excess LiCl, LiBr, or LiClO4 (up to 3000 equiv. in 40 ml THF) in a calorimeter (calorimetric titration). The enthalpies of interaction measured are in the range of ΔH = −8 to −37 kcal/mol. A similar experiment was carried out with one of the binding proteins of cyclosporin, the human cyclophilin A, to give the thermodynamic parameters for the complexation ΔH = −16, ΔG° = −10 kcal/mol, and ΔS° = −20 cal/mol·deg. at 25° which corresponds to an equilibrium constant K = 2·107 l/mol, in good agreement with the result of independent measurements using different methods. NMR Measurements of the macrolides in (D8)THF containing LiCl show strong down‐field shifts of signals of the H‐atoms next to CO and C–OH groups in these molecules.
Radical Reactions in an X‐Irradiated Phosphaalkene: A single‐crystal ESR study
Authors: Bhat Shrinivasa N. , Berclaz Théo , Jouaiti Abdelaziz , Geoffroy Michel
Published: December 2004

Single crystals of 1‐[2,4,6‐tri(tert‐butyl)phenyl]‐2‐phenylphosphaethene (PPPE) and of 2D‐ and 13C‐enriched PPPE were studied by ESR after X‐ray irradiation. Two phosphorus‐centered radicals were trapped in the crystals. The first one was characterized by its 31P, 1H‐, and 13C‐hyperfine tensors, the second one exhibited coupling with 31P only. Comparison of these parameters with those predicted by ab initio calculations on some phosphinyl species indicates that these two radicals probably result, on the one hand, from an addition of a H‐atom to the C‐atom of the PC bond and, on the other, from a cyclization of the parent molecule. The proposed mechanisms are consistent with the mutual orientations of the hyperfine eigenvectors and bond directions in the undamaged molecule. A C‐centered radical which results from an addition of a H‐atom to the P‐atom of the phosphaethylene bond is also detected.
Total Syntheses of Two Self‐polymerizable Carbocyclic Analogues of 2′‐deoxyribonucleosides
Authors: Wang Jianji , Gossauer Albert
Published: December 2004

A short regio‐ and stereoselective synthesis of two carbocyclic 3′‐deoxynucleoside analogues is described, the key step of which consists in the photosensitized addition of MeOH to a cyclopent‐2‐enone derivative. As in both cases functional groups capable to react with each other are present in the same molecule, the synthetic compounds can form polymers similar to oligonucleotides.
3‐Phenyl‐D3‐Trishomocuban‐4‐ol: Structure and Configuration
Authors: Carpy Alain J. M. , Oliver Douglas W.
Published: December 2004

The crystal structure of 3‐phenyl‐D3‐trishomocuban‐4‐ol (4) was determined by X‐ray crystallography (Fig. 1). Compound 4 was prepared from 8‐phenylpentacyclo[5.4.0.02,6.03,10.05,9]undecan‐8‐ol in acidic medium. This is a facile method to prepare substituted trishomocubanes with a specific configuration at C(4).
Carbamate Series of Juvenoids: Variation of the O‐alkyl substituent
Authors: Wimmer Zdenêk , S̆aman David , Nêmec Václav , Francke Wittko
Published: December 2004

By changing the O‐alkyl substituents of the carbamate moiety of alkyl N‐{2‐{4‐[(2‐oxocyclohexyl)methyl]phenoxy}ethyl}carbamates and subsequent transformation of the oxo group in the cyclohexyl substituent, the juvenoids 1–20 were synthesized (Scheme). The methyl (1–4), propyl (9–12), isopropyl (13–16), and prop‐2‐ynylcarbamates (17–20) were subjected to biological screening on several non‐related insect species (Tenebrio molitor, Galleria mellonella, Dysdercus cingulatus, and Pyrrhocoris apterus). Some of the juvenoids showed high biological activity and excellent selectivity with respect to target insect species (Table 2).
3′‐Amino‐Modified Nucleotides Useful as Potent Chain Terminators for current DNA sequencing methods
Authors: Herrlein Mathias K. , Konrad Renate E. , Engels Joachim W. , Holietz Torsten , Cech Dieter
Published: December 2004

We describe the synthesis of modified nucleoside triphosphates of the four DNA bases containing a 3′‐amino group which were prepared from the corresponding 3′‐azido derivatives. Introduction of the triphosphate and subsequent reduction of the N3 to the NH2 group led directly to the target molecules 6a–d. Furthermore, 3′‐amino‐2′,3′‐dideoxynucleoside 5′‐triphosphates proved to be potent inhibitors of the enzymatic synthesis of DNA catalyzed by the standard sequencing enzymes T7 DNA polymerase, sequenase version 2.0, Thermus aquaticus DNA polymerase, and Thermus thermophilus DNA polymerase. Both radioactive and fluorescent sequencing methods were applied successfully to the 3′‐amino‐modified terminators. Investigations in view of using these chain terminators according to Sanger's sequencing method for fluorescence labeling were done.
A Novel Approach to 2,2‐Disubstituted 1,2‐Dihydro‐4‐phenylquinolines
Authors: Walter Harald
Published: December 2004

The reaction of 2‐(1‐phenylvinyl)aniline and 4‐chloro‐2‐(1‐phenylvinyl)aniline with acetophenone derivatives, 1‐(naphthalen‐1‐yl)ethanone and 1‐(furan‐2‐yl)ethanone in toluene at 110–115° with toluene‐4‐sulfonic acid as a catalyst leads in good‐to‐excellent yields to the 2,2‐disubstituted 1,2‐dihydro‐4‐phenyl‐quinolines 1–18 (Scheme 1, Table). The structure of the new racemic 1,2‐dihydroquinolines 1–18 is determined by NMR spectroscopy. A reaction mechanism proceeding via a 6π‐electrocyclic rearrangement of 2‐(1‐phenylvinyl)anils 19 as the key step is proposed for the formation of these compounds (Scheme 1). The scope and limitations of the novel methods are discussed (Scheme 2).
Derivatives of (2R)‐N‐Glyoxylbornane‐10,2‐sultam and Their Use as auxiliaries in the diastereoselective spirocyclization of 2‐substituted tryptamines
Authors: Freund Ralf , Allagiannis Christos , Schönholzer Peter , Bernauer Karl
Published: December 2004

A crude hydrate 6 and a crystalline hemiacetal 7 of glyoxylamide 4 were prepared from crotonamide 5 (Scheme 2). Particularly hemiacetal 7, but also 6 and the ‘dimer’ 8 (obtained from 7) may serve as homochiral auxiliaries. The structure of 8 was determined by X‐ray analysis. By arenesulfonyl halides, tryptimines 12–14 of 4 were diastereoselectively transformed into spirotricycles 15–17 and 19.
A Concise Synthesis of (E)‐ and (Z)‐Neomanoalides
Authors: Jefford Charles W. , Rossier Jean‐Claude , Boukouvalas John , Huang Pingzhong
Published: December 2004

The first synthesis of (Z)‐neomanoalide (4) and an improved synthesis of its (E)‐isomer 3 was accomplished in a concise, regiocontrolled manner by exploiting 2‐[(tert‐butyl)dimethylsiloxy]‐4{[(tert‐butyl)dimethylsiloxy]‐methyl}furan (6) as the key reagent. Lithiation of 6 and subsequent reaction with the (2Z)‐ or (2E)‐isomer of (6E)‐3‐{[(tert‐butyl)dimethylsiloxy]methyl}‐7‐methyl‐9‐(2′,6′,6′‐trimethylcyclohex‐1′‐enyl)nona‐2,6‐dienyl bromide (5), followed by hydrolysis, afforded the corresponding neomanoalide.
Face selectivity of the Diels‐Alder additions and cheletropic additions of sulfur dioxide to 2‐ vinyl‐7‐oxabicyclo[2.2.1]hept‐2‐ene derivatives
Authors: Meerpoel Lieven , Vrahami Maria‐Miranda , Deguin Brigitte , Vogel Pierre
Published: December 2004

Racemic 6‐ethenyl‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐one (23), 5‐ethenyl‐7‐oxabicyclo[2.2.1]hept‐5‐en‐2‐one (25) and their ethylene acetals 24 and 26, respectively, were derived from the Diels‐Alder adduct of furan to 1‐cyanovinyl acetate (27). The Diels‐Alder additions of 26 to dimethyl acetylenedicarboxylate, to methyl propynoate, to N‐phenylmaleimide, and to methyl acrylate were highly exo‐face selective, as were the cycloadditions of methyl propynoate to dienones 23 and 25 and of dimethyl acetylenedicarboxylate to ethylenedioxy‐diene 24. The cheletropic additions of SO2 to 23–26 gave exclusively the corresponding sulfolenes 57–60 resulting from the exo‐face attack of the semicyclic dienes under conditions of kinetic and thermodynamic control.
A Short Synthesis of (−)‐Carbovir
Authors: Hildbrand Stefan , Troxler Thomas , Scheffold Rolf
Published: December 2004

(–)‐Carbovir ((–)‐1) was synthesized via the cyclic carbonate 2 in four steps starting from enantiomerically enriched (–)‐(S)‐(cyclopent‐2‐enyl)methanol ((–)‐3).
Synthesis and Structure‐Activity Relationships of Juvenoids Derived from 2‐(4‐hydroxybenzyl)cycloalkan‐1‐ones
Authors: Rejzek Martin , Wimmer ZdenêK , Šaman David , Říčánková Michaela , Nêmec Václav
Published: December 2004

Juvenoids 16–30, 32, 36, 38–41, 44–46, and 49–56 containing carbamate, carbonate, and urea moieties in the molecule were synthesized and subjected to a biological screening (Schemes 2–7, Table 2). Carbamate juvenoids 1–4 were used as reference compounds for a detailed structure‐activity study of their analogues. A clear relationship between the nature of the side chain functional group and the biological activity was found. Surprisingly, not only the juvenoids 1–4 but also 38–41, the compounds with a reversed carbamate N,O‐substitution pattern, showed very promising biological activity. In contrast, the carbonate and urea derivatives displayed a remarkably low activity. The relationship between the size and substitution at atoms C(2) and C(3) of the saturated ring and the biological activity is very complex and is still not completely understood.
Umsetzung von Diazoessigsäure‐ethylester mit 1,3‐Thiazol‐5(4H)‐thionen
Authors: Kägi Martin , Mlostoń Grzegorz , Linden Anthony , Heimgartner Heinz
Published: December 2004

Reaction of Ethyl Diazoacetate with 1,3‐Thiazole‐5(4H)‐thiones

Reaction of ethyl diazoacetate (2a) and 1,3‐thiazole‐5(4H)‐thiones 1a,b in Et2O at room temperature leads to a complex mixture of the products 5–9 (Scheme 2). Without solvent, 1a and 2a react to give 10a in addition to 5a–9a. In Et2O in the presence of aniline, reaction of 1a,b with 2a affords the ethyl 1,3,4‐thiadiazole‐2‐carboxylate 10a and 10b, respectively, as major products. The structures of the unexpected products 6a, 7a, and 10a have been established by X‐ray crystallography. Ethyl 4H‐1,3‐thiazine‐carboxylate 8b was transformed into ethyl 7H‐thieno[2,3‐e][1,3]thiazine‐carboxylate 11 (Scheme 3) by treatment with aqueous NaOH or during chromatography. The structure of the latter has also been established by X‐ray crystallography. In the presence of thiols and alcohols, the reaction of 1a and 2a yields mainly adducts of type 12 (Scheme 4), compounds 5a,7a, and 9a being by‐products (Table 1). Reaction mechanisms for the formation of the isolated products are delineated in Schemes 4–7: the primary cycloadduct 3 of the diazo compound and the CS bond of 1 undergoes a base‐catalyzed ring opening of the 1,3‐thiazole‐ring to give 10. In the absence of a base, elimination of N2 yields the thiocarbonyl ylide A′, which is trapped by nucleophiles to give 12. Trapping of A′, by H2O yields 1,3‐thiazole‐5(4H)‐one 9 and ethyl mercaptoacetate, which is also a trapping agent for A′, yielding the diester 7. The formation of products 6 and 8 can be explained again via trapping of thiocarbonyl ylide A′, either by thiirane C (Scheme 6) or by 2a (Scheme 7). The latter adduct F yields 8 via a Demjanoff‐Tiffeneau‐type ring expansion of a 1,3‐thiazole to give the 1,3‐thiazine.
Synthese einer Calicen‐Vorstufe für Retro‐Diels‐Alder‐Reaktionen
Authors: Mühlebach Michel , Neuenschwander Markus
Published: December 2004

Synthesis of a Calicene Precursor for Retro‐Diels‐Alder Reactions

In view of retro‐Diels‐Alder reactions (RDA reactions), the calicene precursor 9 has been synthesized in a comparably simple four‐step synthesis by dibromocarbene addition at dibenzobarrelene (10 → 11, 44%), halogenlithium exchange followed by reaction with cyclopentenone (11→12, 91%) and H2O as well as HBr elimination (12→14→9, 43%) (Scheme 5). First experiments with respect to the thermal behavior of 9 show that, although RDA reaction seems to be relatively easily occurring according to the results of ‘Curie‐Point’ pyrolysis, only anthracene and no calicene 2 has been detected so far.
Total Synthesis of Racemic and Optically Active Compounds Related to Physostigmine and Ring‐C Heteroanalogues from 3‐[2′‐(dimethylamino)ethyl]2,3‐dihydro‐5‐methoxy‐1,3‐dimethyl‐1H‐indo l‐2‐ol
Authors: Pei Xue‐Feng , Greig Nigel H. , Flippen‐Anderson Judith L. , Bi Sheng , Brossi Arnold
Published: December 2004

Oxindole 11, obtained on 3‐[2′‐(dimethylamino)ethyl]alkylation of oxindole 12, yielded, on stereoselective reduction with sodium dihydridobis(2‐methoxyethoxy)aluminate, aminoalcohol 8 (Scheme 2). The quaternary methiodide 10, obtained from 8 with MeI, gave, in nucleophilic displacements concurring with a Hofmann elimination, (±)‐esermethole 6, (±)‐5‐O‐methylphysovenol (14), (±)‐5‐O‐methyl‐1‐thiaphysovenol (15), and (±)‐1‐benzyl‐1‐demethylesermethole (16). Syntheses of (±)‐1‐benzyl‐1‐demethylphenserine (18), (±)‐1‐demethylphenserine (19), and (±)‐phenserine (4) from 6 and 16 are described. Optically active 8a and 8b, obtained by chemical resolution, similarly gave the enantiomers 6a and 14a–16a of the (3aS)‐series (prepared earlier from physostigmine (1a)) and their (3R)‐enantiomers. The anticholinesterase activity of (±)‐4, (±)‐18, and (±)‐19 was compared with that of their optically active enantiomers.
Triplet Energy of 2, 2‐Dimethylisoindene from Electron‐Energy‐Loss Spectroscopy and Photoinduced Triplet Energy Transfer
Authors: Allan Michael , Asmis Knut R. , Houar Sarah El , Haselbach Edwin , Capponi Marco , Urwyler Bernhard , Wirz Jakob
Published: December 2004

The excited electronic states of 2, 2‐dimethylisoindene (1) have been studied by electron‐energy‐loss spectroscopy. Its vertical gas‐phase triplet (13B2), and singlet (11B2) excitation energies are 1.61 and 3.19 eV, respectively. The excited states are thus lowered by 0.49 eV and 1.21 eV, respectively, when compared to the corresponding states of (all‐E)‐octatetraene, which serves as a reference compound. These shifts are partially reproduced by ZINDO calculations. The spectra give no evidence for a 21Ag state below the 11B2 state, but this lack of observation does not exclude its existence. The lowest triplet state T1(1) was further characterized by flash photolysis. T1(1) was observed as a transient intermediate, λ ≤ 350 nm, with a lifetime of 8 m̈s in degassed hexane. The adiabatic excitation energy of T1(1) was bracketed to the range of 1.1 ± 0.1 eV by energy‐transfer experiments. Relationships between the energies of the lowest excited singlet and triplet states of 1 and the lowest excited doublet state of its radical cation \documentclass{article}\pagestyle{empty}\begin{document}${1}^{+\kern0pt {.}}$\end{document} – essentially a non‐Koopmans' state – are discussed.
Synthese von Phosphinico‐Analog des Pantetheins
Authors: Neidlein Richard , Li Sheng
Published: December 2004

Synthesis of Phosphinico Analogues of Pantetheine

The synthesis of phosphinico analogues 9a–c of pantetheine is described where the carbonyl group of one of the amide linkage is replaced by a methyl‐, ethyl‐, or phenylphosphinoyl group.
Synthesis of Chiral Starburst Dendrimers from PHB‐Derived Triols as Central Cores
Authors: Seebach Dieter , Lapierre Jean‐Marc , Greiveldinger Guy , Skobridis Konstantinos
Published: December 2004

Chiral triols 1–3 (‘tris(hydroxymethyl)methane’ derivatives), prepared from (R)‐3‐hydroxybutanoic acid and aldehydes, are used as center pieces of dendrimers. The triols may be employed as such or after attachment of spacers containing alkyl or aryl moieties (see 5 and 7). The branches combined with the original or elongated triols are those first reported by Fréchet (9–12, benzyl ethers of 3,5‐dihydroxybenzyl alcohol and bromide). In this way, 1st‐, 2nd‐, and 3rd‐generation chiral dendrimers without (13–15), or with aliphatic (16–18) or aromatic (19–21) spacers are prepared. The molecular weights range from 447 to 2716 Dalton. Two of the chiral triols, i.e., 2 and 3, are used as center pieces for chiral dendrimers containing 6 NH2, or 6 and 12 NO2 groups on the periphery (22–27), with 3,5‐dinitrobenzoyl chloride as the branching unit. All compounds thus synthesized are of course monodisperse and are fully characterized. In some cases, the optical activity of the dendrimers indicates that conformationally chiral substructures might be present. The NH2‐ and NO2‐substituted compounds avidly clathrate smaller molecules; they are sorbents exchanging host molecules through the gas phase.
Stability of Charge‐Transfer Complexes of CS2 with PH3 and its derivatives: Ab initio MRSDCI/CASSCF Study
Authors: Zahradník Rudolf , Jungwirth Pavel , Urban Jiří , Polášek Martin
Published: December 2004

While trialkylamines and dialkyl(phenyl)amines do not react with CS2 in the sense of an addition reaction, the analogous phosphines react smoothly. Attempts to interpret the reaction course on the basis of semiempirical, HF, MP2, and MP4 calculations of energy changes failed completely. To understand why Me3P or Me2PhP react so vigorously (liquid phase, 300 K) with CS2, CASSCF and MRSDCI calculations must be carried out.
Thermal and Ru‐catalyzed Reactions of Styryl‐Substituted Azulenes with Dimethyl Acetylenedicarboxylate
Authors: Briquet Anne Andrée Sophie , Hansen Hans‐Jürgen
Published: December 2004

The thermal reaction of 1‐[(E)‐styrl]azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin at 190–200° does not lead to the formation fo the corresponding heptalene‐1,2‐dicarboxylates (Scheme 2). Main products are the corresponding azulene‐1,2‐dicarboxylates (see 4 and 9), accompanied by the benzanellated azulenes trans‐10a and trans‐11, respectively. The latter compounds are formed by a Diels‐Alder reaction of the starting azulenes and ADM, followed by an ene reaction with ADM (cf. Scheme 3). The [RuH2(PPh3)4]‐catalyzed reaction of 4,6,8‐trimethyl‐1‐[(E)‐4‐R‐styryl]azulenes (R=H, MeO, Cl; Scheme 4) with ADM in MeCN at 110° yields again the azulene‐1,2‐dicarboxylates as main products. However, in this case, the corresponding heptalene‐1,2‐dicarboxylates are also formed in small amounts (3–5%; Scheme 4). The benzanellated azulenes trans‐10a and trans‐10b are also found in small amounts (2–3%) in the reaction mixture. ADM Addition products at C(3) of the azulene ring as well as at C(2) of the styryl moiety are also observed in minor amounts (1–3%). Similar results are obtained in the [RuH2(PPh3)4]‐catalyzed reaction of 3‐[(E)‐styryl]guaiazulene ((E)‐8; Scheme 5) with ADM in MeCN. However, in this case, no heptalene formation is observed, and the amount of the ADM‐addition products at C(2) of the styryl group is remarkably increased (29%). That the substitutent pattern at the seven‐membered ring of (E)‐8 is not responsible for the failure of heptalene formation is demonstrated by the Ru‐catalyzed reaction of 7‐isopropyl‐4‐methyl‐1‐[(E)‐styryl]azulene ((E)‐23; Scheme 11) with ADM in MeCN, yielding the corresponding heptalene‐1,2‐dicarboxylate (E)‐26 (10%). Again, the main product is the corresponding azulene‐1,2‐dicarboxylate 25 (20%). Reaction of 4,6,8‐trimethyl‐2‐[(E)‐styryl]azulene ((E)‐27; Scheme 12) and ADM yields the heptalene‐dicarboxylates (E)‐30A/B, purely thermally in decalin (28%) as well as Ru‐catalyzed in MeCN (40%). Whereas only small amounts of the azulene‐1,2‐dicarboxylate 8 (1 and 5%, respectively) are formed, the corresponding benzanellated azulene trans‐29 ist found to be the second main product (21 and 10%, respectively) under both reaction conditions. The thermal reaction yields also the benzanellated azulene 28 which is not found in the catalyzed variant of the reaction. Heptalene‐1,2‐dicarboxylates are also formed from 4‐[(E)‐styryl]azulenes (e.g. (E)‐33 and (E)‐34; Scheme 14) and ADM at 180–190° in decalin and at 110° in MeCN by [RuH2(PPh3)4] catalysis. The yields (30%) are much better in the catalyzed reaction. The formation of by‐products (e.g. 39–41; Scheme 14) in small amounts (0.5–5%) in the Ru‐catalyzed reactions allows to understand better the reactivity of zwitterions (e.g. 42) and their triyclic follow‐up products (e.g. 43) built from azulenes and ADM (cf. Scheme 15).
Synthesis of Functionalized Cycloprop[f]indenes via the Carbene Addition Route
Authors: Müller Paul , Miao Zhongshan
Published: December 2004

The synthesis of 4,5‐dihydro‐1H,3H‐cycloprop[f]inden‐4‐ol (1) and diethyl 4,5‐dihydro‐1H,3H‐cycloprop[f]‐indene‐4,4‐dicarboxylate (26) starting from diene 4 is described. The cyclopentene ring is constructed by condensation of diethyl malonate to the dibromide 21. The key‐step in the synthesis of 1 consists in a twofold Curtius degradation of 22, with subsequent reduction of the carbonyl group and aromatization. The skeleton of the isomer 31 is synthesized via cycloaddition of butadiene to cyclopent‐4‐ene‐1,3‐dione (7) and addition of dichlorocarbene to the adduct 27 after ketalisation. The attempted synthesis of dihydrocycloprop[f]indene (2) by base‐induced elimination of several appropriately substituted precursors failed.
C2‐Symmetric Bicyclic Diols as Chiral Ligands in the titanate‐catalyzed enantioselective addition of alkylzinc reagents to aldehydes
Authors: Waldmann Herbert , Weigerding Michael , Dreisbach Claus , Wandrey Christian
Published: December 2004

Enantiomerically pure C2‐symmetric 1,4‐diols embodying bicyclic C‐frameworks were synthesized by means of asymmetric carbo‐Diels‐Alder reactions as key steps (Scheme 1). They were investigated as chiral ligands in the enantioselective addition of ZnEt2 to aromatic aldehydes. In the presence of 20–40 mol‐% of the titanates formed from these diols and [Ti(i‐PrO)4] at −78°, the respective 1‐arylpropanols were obtained with enantiomer ratios up to 93:7 (Scheme 2, Table).
Einkristall‐Strukturen eines Donator‐Akzeptor‐substituierten Cyclopentadiens, seines Kations und seines dimeren Anions: Verschiedenartige Störungen eines Kohlenstoff‐Fünfringes
Authors: Bock Hans , Nick Sabine , Näther Christian , Bats Jan W.
Published: December 2004

Single‐Crystal Structures of a Donor‐Acceptor‐Substituted Cyclopentadiene, Its Cation and Its Anion Dimer: Different Distortion of a Five‐Membered Carbon Ring

1,4‐Dimethyl‐2,3,4,6‐tetrahydro‐1 H‐cyclopentapyrazine‐5,7‐dicarbonitrile, its molecular cation, and its dimer dianion, generated by hydride abstraction or deprotonation, and crystallized, differ remarkably in their structures: the five‐membered ring in the neutral compound is of (Z)‐butadiene‐type, whereas, in the tetrafluorborate salt, a peripheric NCCCCCN⊕ cyanine distortion occurs and in the sodium derivatives, solvated by either one or two 1,2‐dimethoxyethane molecules, a cyclopentadienyl anion is formed. The structural perturbations induced by changing molecular charges are rationalized by extensive MNDO calculations, which on geometry optimization, reproduce the experimental structures and, in addition, provide charge distributions, further confirming the nN/π and π/π interactions already recognizable from the structural parameters.
2,3‐Dihydrospiro[1H‐4‐ and 5‐azabenzimidazole‐2,1′ ‐cyclohexane] ( = Spiro[cyclohexane‐1,2′(3′H)‐1′H‐imidazo[4,5‐b]pyridine] and Spiro[cyclohexane‐1,2′(3′H)‐1′H‐imidazo[4, 5‐c]pyridine]): Reactions with Nucleophiles
Authors: Schwoch Stefan , Kramer Walter , Neidlein Richard , Suschitzky Hans
Published: December 2004

The readily available title compounds 4a and 24 react with N‐, O‐, S‐, and C‐nucleophiles in presence of MnO2 to give the corresponding mono‐ or disubstituted 2H‐azabenzimidazoles ( = azaisobenzimidazoles), e.g., 11–18 and 26a–h, respectively, or 2,3‐dihydro‐1H‐azabenzimidazoles ( = dihydro‐azabenzimidazoles) such as 9 and 10 and 27 and 28, respectively, by a 1, 4‐ or 1,6‐Michael addition (Schemes 2 and 4). The bromo‐dihydro‐1H‐azabenzimidazole 4b lost the Br‐atom when treated with piperidine or morpholine yielding the corresponding disubstituted 2H‐azabenzimidazole 21 (Scheme 3). Reductive ring opening of the substituted spiro compounds leads to mono‐ and disubstituted diaminopyridines which are intermediates for fused pyridine ring systems with substituents often not available by conventional routes and of potential pharmaceutical interest (see 32–37). E.g., starting from 4a, a three‐step synthesis of the analgesic flupirtine maleate (= ethyl {2‐amino‐6‐[(4‐fluorobenzyl)amino]pyridin‐3‐yl}carbamate maleate = Katadolon®; 39) and of its non‐fluorinated derivative D‐7195 is described. Its analogue 40 was similarly made from the spiro compound 24.
Gas‐Phase Generation and Characterization of Nitrileimine, HCNNH: A new, stable isomer of diazomethane
Authors: Goldberg Norman , Fiedler Andreas , Schwarz Helmut
Published: December 2004

A new isomer of diazomethane 1, the nitrile imine, HCNNH (2) is reported to be a stable molecule in the gas phase. Upon neutralizing the α‐distonic HCNNH+ cation in a beam experiment, this long‐time predicted ylide can be generated. The experiments are supported by theoretical calculations (DFT/HF hybride method) on the neutral and cationic diazomethane 1, nitrile imine 2, and N‐isocyano amine 3 as well as the transition states for their interconversion.
Tetraethynylethenes: Fully cross‐conjugated π‐electron chromophores and molecular scaffolds for all‐carbon networks and carbon‐rich nanomaterials
Authors: Anthony John , Boldi Armen M. , Rubin Yves , Hobi Markus , Gramlich Volker , Knobler Carolyn B. , Seiler Paul , Diederich François
Published: December 2004

The preparation of tetraethynylethene (3,4‐diethynylhex‐3‐ene‐1,5‐diyne) 1 as well as of a great diversity of differentially mono‐, di‐, and triprotected derivatives by newly developed synthetic routes is described. These fully cross‐conjugated molecules are versatile building blocks and precursors to two‐dimensional all‐C networks and novel C‐rich nanoarchitecture with unusual structural and electronic properties, such as perethynylated expanded radialenes, or molecular wires and polymers with the novel polytriacetylene backbone. A key step in all of these routes was the Corey‐Fuchs dibromoolefination of aldehydes and ketones. Dibromoolefination of silyl‐protected penta‐1,4‐diyn‐3‐ones yielded the corresponding dibromomethylidene derivatives which, by twofold Pd‐catalyzed alkyne coupling, were transformed into tetraethynylethene derivatives. In routes to tetraethynylethenes with free cis‐or trans‐enediyne moieties, dibromoolefination of aldehyde groups produced geminal dibromoethenes which, upon elimination/metallation with LDA followed by quenching with H+ or other electrophiles, yielded free or substituted ethynyl groups in high yields. Tetra‐ and triprotected tetraethynylethenes are rather stable compounds that could be isolated in pure form, whereas derivatives with two or more free CH termini were only stable in dilute solution and polymerized rapidly in pure form. A trans‐bis(triisopropylsilyl)‐protected derivative represented an exception and could be isolated as stable crystals. X‐Ray analysis revealed that the two bulky (i‐Pr)3Si groups isolate the reactive chromophores from one another in the crystal and prevent intermolecular reactions. The structures of several tetraethynylethenes were revealed in high‐quality X‐ray crystal structures.
Salmycin A–D, Antibiotika aus Streptomyces violaceus, DSM 8286, mit Siderophor‐Aminoglycosid‐Struktur
Authors: Vértesy László , Aretz Werner , Fehlhaber Hans‐Wolfram , Kogler Herbert
Published: December 2004

Salmycin A–D, Antibiotics from Strep tomy ces violaceus, DSM 8286, Having a Siderophor‐Aminoglycoside Structure

Salmycin B (2) and C (3) were isolated under acid conditions, under which they are stable, from the culture broth of Streptomyces violaceus, DSM 8286. The acid‐ and alkaline‐labile, native main component salmycin A (1), as well as salmycin D (4), were obtained under strictly neutral pH conditions. The compounds 1 (C41H70FeN7O21), 2 (C41H69FeN6O21), 3 (C40H67FeN6O21), and 4 (C40H68FeN7O21) are classified as sideromycins and are stable when dry. Mild alkaline hydrolysis of 1 and 2 yielded the known siderophor danoxamin (5; C27H46FeN5O11), and amino‐disaccharides. The amino‐glycoside 6 (C14H25NO11) of salmycin B was stabilized by hydrogenation and the structure of the corresponding peracetate 10 determined by 1H,1H‐ and 1H,13C‐correlation NMR spectroscopy (Table 1). Compound 6 consists of a glucos‐2‐ulose unit which is linked to the 2‐position of a 6‐deoxy‐6‐(methylamino)heptopyranose. The danoxamin is bonded via the carboxy group by ester linkage to the primary alcohol of the glucos‐2‐ulose. Salmycin A (1) is a natural oxime of 2, it was synthesized from 2 with hydroxylamine. The salmycins and those derivatives which contain hexapyranos‐2‐ulose form stable ketone hydrates which can be identified by mass spectrometry. Although several recently identified features of the danomycins do not correspond with those of the salmycins, 13C‐NMR spectra show that both groups of antibiotics are closely related. All salmycins, especially component 1, are highly active against Staphylococci and Streptococci, even against resistant strains of these pathogens.
Microscopic Protonation/Deprotonation Equilibria of the Anti‐Inflammatory Agent Piroxicam
Authors: Takács‐Novák Krisztina , Kökösi József , Podányi Benjámin , Noszál Béla , Tsai Ruey‐Shiuan , Lisa Giuseppe , Carrupt Pierre‐Alain , Testa Bernard
Published: December 2004

The microscopic ionization behavior of piroxicam was investigated using two different approaches, i.e., direct UV spectroscopy and an indirect analogue approach (deductive method). The best microscopic pKa values (pKa12 = 4.60, pKa21 = 5.40, pKa22 = 2.72, and pKa11 = 1.92) were obtained by the deductive method using as pKa22 the pKa of the enolic O‐methylated piroxicam 2. The results show remarkable electrostatic effects in the protonation/deprotonation equilibria, a marked increase in the acidity of the enolic function (2.68 pKa units) being caused by the pyridinium group. The electronic structure of piroxicam was studied based on 1H‐NMR chemical shifts at various ionization states, indicating an extended electron conjugation through the molecule. The partition measurements in octan‐1‐ol/H2O of zwitterionic compound 3 (the pyridyl N‐methyl derivative of piroxicam (1)) suggest that the two opposite charges in zwitterionic piroxicam are indeed in a close intramolecular proximity.
L‐Phenylalanine Cyclohexylamide: A simple and convenient auxiliary for the synthesis of optically pure α,α‐disubstituted (R)‐ and (S)‐amino acids
Authors: Obrecht Daniel , Bohdal Udo , Broger Clemens , Bur Daniel , Lehmann Christian , Ruffieux Ruth , Schönholzer Peter , Spiegler Clive , Müller Klaus
Published: December 2004

This work describes L‐phenylalanine cyclohexylamide (5c) as a simple, cheap, and powerful chiral auxiliary for the synthesis of a series of optically pure α,α‐disubstituted (R)‐ and (S)‐amino acids of type 1, such as (R)‐ and (S)‐2‐methyl‐phenylalanine (1a), (R)‐ and (S)‐2‐methyl‐2‐phenylglycine (1b), and (R)‐ and (S)‐2‐methylvaline (1c; Scheme 3). These amino acids were efficiently transformed into the suitably protected and activated amino acid building blocks (R)‐ and (S)‐12b and (R)‐ and (S)‐12c (Scheme 4) which are ready for incorporation into peptides by solution or solid‐phase techniques. Based on the crystal structures of 6b, 6c, and 7a belonging to the diastereoisomeric peptides series 6 and 7, the absolute configurations of each member of the series were determined. β‐Turn geometries of type II′ and I were observed for 6b and 7a, respectively, whereas 6c crystallized in an extended conformation. The impacts of side‐chain variation on conformation and crystal packing of these triamides are discussed.
Synthesis of the Dolabellane Diterpene Hydrocarbon (±)‐δ‐araneosene
Authors: Jenny Luzi , Borschberg Hans‐Jürg
Published: December 2004

The racemic form of the bicyclic diterpene hydrocarbon δ‐araneosene (4), endowed with the dolabellane skeleton, was prepared from geraniol in two different ways. The more efficient route involved 13 steps and proceeded with an overall yield of 3.6% (average: 77% per step). With this reference sample at hand, the hitherto elusive metabolite (−)‐4, a likely biogenetic precursor of cycloaraneosene ((−)‐3) and sordaricin ((−)‐1), could finally be isolated in ≥ 99.5% purity from the neutral fractions of the mold Sordaria araneosa CAIN (Ascomycetes).
Oligodeoxyribonucleotides Containing 4‐Aminobenzimidazole in Place of Adenine: Solid‐phase synthesis and base pairing
Authors: Seela Frank , Wenzel Thomas
Published: December 2004

Oligonucleotides containing 4‐aminobenzimidazole 2′‐deoxyribofuranoside (1,3‐dideaza‐2′‐deoxyadenosine; c1c3Ad, 1) were synthesized. For this purpose, various NH2‐protecting groups were investigated, and the [(9H‐fluoren‐9‐yl)methoxy]carbonyl group was selected for phosphoramidite protection (→ 4c). Apart from the phosphoramidite 3, the phosphonate 2 was prepared. Compound 1 was incorporated in a homooligonuclectide as well as in oligomers containing naturally occurring nucleosides. The Tm values and the thermodynamic data of various duplexes (11 · 10, 17 · 10, 18 · 10) containing 4‐aminobenzimidazole were determined. Although d[(c1c3A)20] (11) does not form a Hoogsteen duplex with d(T20) (10) as observed with d[(c1A)20], it destabilizes the Watson‐Crick duplexes to a much smaller extent than it was expected from a bulged loop structure. Apparently, 4‐aminobenz‐imidazole interacts with regular nucleoside residues within a Watson‐Crick duplex structure, most likely by vertical stacking. According to the low basicity of the amino group, only weak H‐bonding is expected.
Unexpected Hydroxyspirolactone Formation upon Baeyer‐Villiger Oxidation of a trans‐α‐decalone (trans‐octahydronaphthalen‐1(2H)‐one) and X‐ray structure of the product
Authors: Demnitz F. W. Joachim , Freiberger Sabine , Weber Hans‐Peter
Published: December 2004

trans‐4,4,10‐Trimethyl‐9‐decalone ( = trans‐5,5,8a‐trimethyl‐octahydronaphthalen‐1(2H)‐one; 1), when treated with trifluoroperacetic acid, gave the unexpected hydroxyspirolactone 7‐hydroxy‐7,11,11‐trimethyl‐1‐oxaspiro[5.5]undecan‐2‐one (6), in which the two new O‐atoms were introduced in a 1,2‐trans relationship. The structure of this compound was conclusively proven by X‐ray crystallography. The process involves the intermediacy of 7‐membered lactone 2, the expected Baeyer‐Villiger product, which could also be successfully prepared under controlled conditions at 0° in a buffered medium containing Na2HPO4.
Absolute Rate Constants for the Addition of Benzyl (PhĊH2) and Cumyl (PhĊMe2) Radicals to Alkenes in Solution
Authors: Walbiner Manfred , Wu Jie Qiang , Fischer Hanns
Published: December 2004

Absolute rate constants and their temperature dependence were determined by time‐resolved electron spin resonance for the addition of the radicals PhĊH2 and PhĊMe2 to a variety of alkenes in toluene solution. To vinyl monomers CH2=CXY, PhĊH2 adds at the unsubstituted C‐atom with rate constants ranging from 14 M−1S−1 (ethoxyethene) to 6.7 · 103 M−1S−1 (4‐vinylpyridine) at 296 K, and the frequency factors are in the narrow range of log (A/M−1S−1) = 8.6 ± 0.3, whereas the activation energy varies with the substituents from ca. 51 kJ/mol to ca. 26 kJ/mol. The rate constants and the activation energies increase both with increasing exothermicity of the reaction and with increasing electron affinity of the alkenes and are mainly controlled by the reaction enthalpy, but are markedly influenced also by nucleophilic polar effects for electron‐deficient substrates. For 1,2‐disubstituted and trisubstituted alkenes, the rate constants are affected by additional steric substituent effects. To acrylate and styrenes, PhĊMe2 adds with rate constants similar to those of PhĊH2, and the reactivity is controlled by the same factors. A comparison with relative‐rate data shows that reaction enthalpy and polar effects also dominate the copolymerization behavior of the styrene propagation radical.
7‐Substituted 7‐Deaza‐2′‐deoxyguanosines: Regioselective Halogenation of Pyrrolo[2,3‐d]pyrimidine Nucleosides
Authors: Ramzaeva Natalya , Seela Frank
Published: December 2004

The synthesis of 7‐chloro‐, 7‐bromo‐, and 7‐iodo‐substituted 7‐deaza‐2′‐deoxyguanosine derivatives 2b–d is described. The regioselective 7‐halogenation with N‐halogenosuccinimides was accomplished using 7‐[2‐deoxy‐3,5‐O‐di(2‐methylpropanoyl)‐β‐D‐erythro‐ pentofuranosyl]‐2‐(formylamino)‐4‐methoxy‐7H‐pyrrolo[2,3‐d]‐ pyrimidine (4) as the common precursor. A one‐pot reaction (2N aq. NaOH) of the halogenated intermediates 5a–c furnished the desired compounds. Also the 7‐hexynyl derivative 2e of 7‐deaza‐2′‐deoxyguanosine is described.
Synthetic Attempts towards Polymers with Pentafulvene Structural Units
Authors: Zhang Gengyi , Neuenschwander Markus
Published: December 2004

Synthetic attempts towards fully conjugated polymers 9 with pentafulvene‐diyl structural units are described. Cationic polymerization of pentafulvenes 1a (R = X = Me) and 1b (R = X = MeS) nearly quantitatively gives polymers 8a and 8b with typical Mn and Mw values of 38800 and 53750, respectively, for 8a, and 12000 and 35900, respectively, for 8b. Key step of the conversion 8a → 9a (Scheme 6) is a quantitative bromination 8a → 32a, the structure of 32a being confirmed by analytical data as well as by spectroscopic comparison with model compound 23. Best results in view of two‐fold the HBr elimination 32a → 9a are obtained with Et3N, but so far elimination has not been complete. Synthetic sequences are optimized with model compound 21 (Scheme 4). Here again, bromination 21 → 23 is quantitative, while two‐fold HBr elimination 23 → 22 with Et3N proceeds in 51% yield. Dibromide 23 easily undergoes HBr elimination followed by a Br shift to give bromide 29. Contrary to cationic polymerization, anionic polymerization of simple pentafulvenes 1 to 2 (which would be attractive in view of the conjugated polymers 3) is not successful: For pentafulvene 1b (R = X = MeS), the main reaction is Diels‐Alder‐type dimerization 1b → 15b (Scheme 2), even under anionic conditions.
Oligosaccharide Analogues of Polysaccharides. Part 4. Synthesis of a monosaccharide‐derived octamer
Authors: Alzeer Jawad , Vasella Andrea
Published: December 2004

NaSMe in toluene leads to regioselective de‐C‐silylation of the bis[(trimethylsilyl)ethynyl]saccharide 2, but to decomposition of butadiynes such as 1 or 12. We have, therefore, combined the known reagent‐controlled, regioselective desilylation of 2 and of 12 (AgNO2/KCN) with a substrate‐controlled regioselective de‐C‐silylation, based on C‐silyl groups of different size. This combination was studied with the fully protected 3 which was mono‐desilylated to 4 or to 5 (Scheme 1). Triethylsilylation of 5 (→ 6) was followed by removal of the Me3Si group (→ 7), introduction of a (t‐Bu)Me2Si group (→ 8) and removal of the Et3Si group yielded 9; these high‐yielding transformations proceed with a high degree of selectivity. Iodination of 4 gave 10. The latter was coupled with 5 to the homodimer 11 and the heterodimer 12, which was desilylated to 13. The second building block for the tetramer was obtained by coupling 14 (from 7) with 5, leading to 15 and 16. Removal of the Me3Si group (→ 17) and iodination led to 18 which was coupled with 13 to the homotetramer 20 and the heterotetramer 19 (Scheme 2). Deprotection of 19 gave 21, which was, on the one hand, iodinated to 22, and, on the other hand, protected by the (t‐Bu)Me2Si group (→ 23). Removal of the Et3Si group (→ 24) and coupling afforded the homooctamer 26 and the heterooctamer 25. Yields of iodination, silylation, and desilylation were consistently high, while heterocoupling proceeded in only 50–55%. Cleavage of the (i‐Pr)3SiC and MeOCH2O groups of 11 (→ 27), 15 (→ 28), 20 (→ 29) and 26 (→ 30) proceeded in high yields (Scheme 3). Complete deprotection in two steps of the heterocoupling products 16 (→ 31 → 32), 19 (→ 33 → 34), and 25 (→ 35 → 36) gave the unprotected dimer 32, tetramer 34, and octamer 36 in high yields (Scheme 4). Only the dimer 32 is soluble in H2O; the 1H‐NMR spectra of 32, 34, and 36 in (D6)DMSO (relatively low concentration) show no signs of association.
From Colchicine and Some of Its Derivatives to 1,2,3,9,10‐Pentamethoxybenzo[a]heptalenes
Authors: Kouroupis Pavlos , Hansen Hans‐Jürgen
Published: December 2004

A two‐step synthesis of 4‐methylcolchicine (13), starting from colchicine (2), has been developed (Scheme 5). In three steps, 4‐ethylcolchicine (28) is also accessible from 2 (Scheme 8). Colchicine (2) and its derivatives 13 and 28 have been transformed into the benzo[a]heptalene derivatives 9, 18, and 34, respectively, by Hofmann degradation of the corresponding deacetylcolchiceine 3, 19, and 29, respectively, followed by methylation of the two O‐functions first with diazomethane and then with trimethoxonium tetrafluoroborate (Scheme 2 and 6). The thus formed tropylium salts gave, on deprotonation with Me3N in CHCl3, the expected pentamethoxybenzo[a]heptalenes 9, 18, and 34, respectively. X‐Ray crystal‐structure analysis of 9 (Fig.3) and 18 (Fig. 7), determination of the vicinal coupling constants of the H‐atoms at the heptalene skeleton as well as the measurement of the racemization rate of the new benzo[a]heptalenes revealed a marked influence of the substituent at C(4) on the degree of twisting of the heptalene skeleton. The absolute configuration of the resolved heptalenes was deduced from their long‐wavelength CD maxima around 350 nm. The heptalenes with a negative maximum in this range possess (7aP)‐configuration.
1,3‐Dipole mit zentralem S‐Atom aus der Umsetzung von Aziden mit Thiocarbonyl‐Verbindungen: Eine unerwartete MeS‐Wanderung im Abfangprodukt eines ‘Thiocarbonyl‐aminids’ mit Dithiobenzoesäure‐methylester
Authors: Mlostoń Grzegorz , Romański Jaroslaw , Linden Anthony , Heimgartner Heinz
Published: December 2004

1,3‐Dipoles with a Central S‐Atom from the Reaction of Azides and Thiocarbonyl Compounds: An Unexpected MeS Migration in the Trapping Product of a ‘Thiocarbonyl‐aminide’ with Methyl Dithiobenzoate

Reaction of PhN3 with O‐methyl thiobenzoate (11a) and thioacetate (11c) as well as with the dithio esters 11b,d at 80° yields the corresponding imidates and thioimidates 12 (Scheme 3). The formation of 12 is rationalized by a 1,3‐dipolar cycloaddition of the azide and the CS group followed by successive elimination of N2 and S. In the three‐component reaction of 11b, PhN3, and the sterically crowded thioketone 1a, 1,2,4‐trithiolane 13a and 1,4,2‐dithiazolidine 3a are formed in addition to 12b (Scheme 4). The heterocycles 13a and 3a are trapping products of 1a and ‘thiocarbonyl‐thiolate’ 5a and ‘thiocarbonyl‐aminide’ 2a (ArPh), respectively (Scheme 6). These 1,3‐dipoles are formed as reactive intermediates. Surprisingly, in the presence of catalytic amounts of acids, the major product is the (methyldithio)cyclobutyl thioimidate of type 14 (Scheme 5), formed by an acid‐catalyzed MeS migration in dithiazolidine 17. A reaction mechanism is proposed in Scheme 7.
Synthesis of a Type‐VIβ‐Turn Peptide Mimetic and Its Incorporation into a High‐Affinity Somatostatin Receptor Ligand
Authors: Gramberg Dieter , Weber Christoph , Beeli Reto , Inglis Janice , Bruns Christian , Robinson John A.
Published: December 2004

The synthesis of a cis‐Phe‐Pro dipeptide mimetic is described, which adopts a type‐VIβ‐turn conformation. In this mimetic, the α‐positions of Phe and Pro are joined by a CH2CH2 bridge, thereby forming a fused bicyclic system, and fixing a geometry similar to that seen in cis‐Phe‐Pro units in protein crystal structures. The dipeptide mimetic 20 was synthesized in optically pure form starting from (R)‐α‐allylproline (6; Schemes 1, 3, and 4), with a free carboxylic acid and an Fmoc‐protected N‐terminus, thereby allowing its incorporation into linear and cyclic peptides using standard solid‐phase methods. The mimetic 20 was incorporated into the cyclic somatostatin analogue cyclo(‐Phe = Pro‐Phe‐D‐Trp‐Lys‐Thr‐), where Phe = Pro represents the mimetic. This analogue shows a high affinity (pIC50 8.6) for somatostatin receptors on rat‐brain cortex membranes. Based on NMR studies in aqueous solution, likely low‐energy conformations for this analogue were deduced using restrained dynamic simulated annealing. The conformations found, which include a distorted type‐II′ turn at D‐Trp‐Lys, are similar to low‐energy conformations deduced elsewhere for cyclo(‐Phe‐Pro‐Phe‐D‐Trp‐Lys‐Thr‐), as well as to those seen in crystal structures of the somatostatin analogue octreotide.
Molecular Magnetism and Iron(II) Spin‐State Equilibrium as Structural Probes in Heterodinuclear d–f Complexes
Authors: Piguet Claude , Rivara‐Minten Elisabeth , Hopfgartner Gérard , Bünzli Jean‐Claude G.
Published: December 2004

Fe(ClO4)2 reacts with the segmental ligand 2‐{6‐[1‐(3,5‐dimethoxybenzyl)‐1H‐benzimidazol‐2‐yl]pyridin‐2‐yl}‐1,1′‐dimethyl‐5,5′‐methylene‐2′‐(5‐methylpyridin‐2‐yl)bis[1H‐benzimidazole] (L2) in MeCN to give the diamagnetic deep violet complex [Fe(L2)2]2+ where the metal is pseudo‐octahedrally coordinated by two perpendicular tridentate binding units. When L2 reacts with an equimolar mixture of Ln(ClO4)3 (Ln = La, Ce, Pr, Nd, Sm, Eu) and Fe(ClO4)2, electrospray‐mass spectrometric, spectrophotometric, and 1H‐NMR data in MeCN show the selective formation of the deep red heterodinuclear C3‐cylindrical complexes [LnFe(L2)3]5+ where the three ligands L2 are wrapped about the metal‐metal axis. FeII occupies the pseudo‐octahedral capping site produced by the three bidentate units and LnIII lies in the resulting ‘facial’ pseudo‐tricapped trigonal prismatic site defined by the three remaining tridentate coordinating units. The heterodinuclear complexes [LnFe(L2)3]5+ display spin‐state equilibrium (1A ⇄ 5T) and thermochromism in MeCN between 243 and 333 K. Detailed 1H‐NMR, UV/VIS, and magnetic measurements in solution show that the partial spin‐crossover behavior of [LnFe(L2)3]5+ occurs for Ln = LaEu with similar thermodynamic parameters (ΔHsc = 20–23 kJ·mol−1 and ΔSsc = 55–66 J·mol−1·K−1) indicating that the size of LnIII has a negligible influence on the spin‐state equilibrium. However, the smaller LnIII ions have less affinity for the pseudo‐tricapped trigonal prismatic coordination site in the heterodinuclear complexes as demonstrated by the partial decomplexation of [YFe(L2)3]5+ to give [Fe(L2)2]2+ and the absence of the heterodinuclear complex [LuFe(L2)3]5+ under the same conditions. The crucial role played by the sterically demanding FeII in the assembly processes is discussed together with the use of the efficient combination of lanthanide probes with magnetic d‐block probes for the design and investigation of luminescent and magnetic materials with controlled structural and physical properties. Photophysical measurements reveal that efficient ligand → metal and Eu → Fe energy transfer occur in [EuFe(L2)3]5+ which strongly quench both the ligand and the Eu‐centered luminescence.