bluuebunny/crossref_metadata_2025_split huggingface dataset:
Kinetic Study on Stability of Schiff Base of Pyridoxal 5′‐Phosphate and Leucine in Water Media with Cationic Surfactants
Authors: Vázquez Miguel A. , Muñoz Francisco , Donoso Josefa , García‐Blanco Francisco 
Published: December 2004

AbstractWe studied the stability of the Schiff bases formed between pyridoxal 5′‐phosphate (PLP) and leucine in the presence of (hexadecyl)trimethylammonium bromide (CTAB) over a wide pH range by determining the kinetic constants of formation and hydrolysis of these compounds. The results show that the stability of the Schiff bases is increased by the presence of CTAB as a result of increased rates of formation and decreased hydrolysis rate constants. The ionic head groups of CTAB favour the formation of the bases, while its hydrophobic rests protect the imine double bond from hydrolysis. This model system permits one to obtain partially hydrophobic media with no need for any non‐aqueous solvents.
Synthesis of Coumarins and Derivatives. Part 1. Biomimetic synthesis of esculetin and halogenated derivatives
Authors: Borges Fernanda , Pinto Madalena 
Published: December 2004

AbstractEsculetin (1) and the novel compounds 5‐chloroesculetin (5) and 5‐bromoesculetin (6) were obtained from a light‐induced cyclization of trans‐caffeic acid (3) catalyzed by [FeNa(edta)] and/or H2SO4, HCI, or HBr (Scheme 1). The experimental conditions for trans‐cis‐isomerization of the cinnamic‐acid derivative 3 and subsequent non‐enzymatic cyclization were described. The photoperiod and the presence of air and iron‐chelate catalyst are shown to be important parameters that markedly affect yields. The reactions probably occur by a free‐radical mechanism involving a photo‐initiated one‐electron redox process (Scheme 2).
Ab‐initio‐Studien anN‐ Acyliminium‐Ionen
Authors: Lamatsch Bernd , Seebach Dieter , Ha Tae‐Kyu 
Published: December 2004

Ab initioStudies onN‐Acyliminium IonsThe geometric and electronic structure and reactivity of theN‐formylmethyleneiminium ion are calculated in different conformations byab initiomethods and compared to those ofN‐methylformamide and the methyleneiminium ion. The known preferred attack of the C(5) of 3‐acyl‐2,3‐dihydro‐1,3‐oxazoles by electrophiles is explained by the lower (calculated) energy of the corresponding acyliminium ion relative to that of the oxenium ion. The stereoselectivity of additions to these acyliminium ions can not be explained by our calculations.Ab initiocalculations of the subsystems acyliminium and oxenium ion show that the reactivity can be predicted from investigation of these much simpler systems and make us suppose that a nitro instead of an acyl group will reverse the regioselectivity of the addition to 2,3‐dihydro‐1,3‐oxazoles.
Base Hydrolysis of Pentaaminecobalt (III) Complexes: The [CoX(dien) (dapo)]n+ system. Part 3. The internal conjugate base mechanism
Authors: Comba Peter , Jackson W. Gregory , Marty Werner , Zipper Luc 
Published: December 2004

AbstractAzide anation and racemization of optically pure mer‐exo(H)‐ and mer‐endo(H)‐[Co(OH)(dien)(dapo)]2+ (A and B (X = OH), resp.; dien = N‐(2‐aminoethyl)ethane‐1,2‐diamine; dapo = 1,3‐diaminopropan‐2‐ol) involve the same symmetrical pentacoordinate intermediate as the base hydrolyses of the corresponding mer‐exo(H)‐ and mer‐endo(H)‐[CoX(dien)(dapo)]2+ species A and B, respectively, where X = Cl, Br, or N3. The kinetic parameters of the anation process are fully compatible with the independently measured competition ratio. The rate data reveal that substitution of OH− is unexpectedly fast, viz. it is not consistent with the usual sequence Br− > Cl− > H2O > N > OH−. This behavior is interpreted on the basis of an internal conjugate base mechanism which involves an amino‐hydroxo/aminato‐aqua tautomerism, viz. the reaction is actually an OH− ‐catalyzed substitution of [CoH2O(dien)(dapo)]3+ where deprotonation occurs effectively at the secondary‐amine site NH of dien.
Struktur zweier trans‐Bis (N,N‐dimethylethylendiamin)kobalt(III)‐Komplexe. Simulation der sterischen Ligand‐Ligand‐Wechselwirkungen durch ein ‘Molecular‐Mechanics’‐Modell
Authors: Eminger Roman , Fallab Silvio , Zehnder Margareta , Dobler Max 
Published: December 2004

The Crystal and Molecular Structures of Two trans ‐Bis(N,N‐dimethylethylenediamine)cobalt(III) Complexes. Estimate of Steric Ligand‐Ligand Interactions by a Molecular‐Mechanics ModelTwo trans‐bis (N,N‐dimethylethylenediamine)cobalt(III)m complexes with different axial ligands (SNC−, NO) have been synthesized starting from the corresponding CoII‐amine species. An X‐ray crystal‐structure analysis revealed widely differing CoN bond distances. While for the primary amino groups, as in complexes of unsubstituted bidentate amines, bond distances are close to 196 pm, those for the tertiary amino groups are stretched by ca. 10 pm. The good agreement between the molecular geometry as calculated by a molecular‐mechanics model and experimental data suggests that the differences in bond distances must be primarily due to simple steric effects.
Cycloaddition of Carbodiimides with a Heteroaromatic Substituent to Allenic Acids
Authors: Trifonov Latchezar S. , Christova Nadja I. , Dimitrov Valentin S. , Orahovats Alexander S. 
Published: December 2004

AbstractCycloaddition of the allenic acid 3 with the N ‐cyclohexyl‐N′ ‐heteroaromatic carbodiimides 2a and 2b gave the isomeric pyrido[1,2‐a]pyrimidinones 4 and 5 and thiazolo[3,2‐a]pyrimidinones 6 and 7, respectively, instead of the expected Diels‐Alder adducts analogous to 1. The compounds of the latter type, i.e. 8 and 9, were formed from 3 and carbodiimides 2c and 2d, respectively, containing an N′‐(pyrazin‐2‐yl) or N′‐(pyrimidin‐2‐yl) substituent.
The Vinylidene/Acyl Coupling at Rhenium Centers
Authors: Feracin Sibylle , Hund Hans‐Ulrich , Bosch H. William , Lippmann Elisabeth , Beck Wolfgang , Berke Heinz 
Published: December 2004

AbstractThe reaction of [Re(CO3)L2Cl] (L = (MeI)3P, 1a; L = (i‐PrO)3P, 1b; L = (PhO)3P, 1c; L = Me3P, 1d; L = Et3P, 1e) with t‐BuLi and phenylacetylene in THF affords bis (phosphorus donor) (dicarbonyl) {η2(C,O)‐[(E)‐4,4‐dimethyl‐3‐oxo‐1‐phenylpent‐1‐3n‐2‐yl]}rhenium (L = (MeO)3P, 2b; L = (PhO)3P, 2c; L = Me3P, 2d; L = Et3P, 2c). Compounds 2b and 2d were characterized by X‐ray structure analysis.
Synthesis and Photochemistry of 5,5‐Dimethyl‐1H‐pyrrol‐2(5H)‐one and of Some N‐Substituted Derivatives
Authors: Ihlefeld Andreas , Margaretha Paul 
Published: December 2004

AbstractIn two steps, 5,5‐dimethyl‐1H‐pyrrol‐2(5H)‐one (3a) was prepared from 5,5‐dimethylpyrrolidine‐2,4‐dione ( = dimethyltetramic acid; 4) in 71% overall yield (Scheme 1) and further converted to N‐substituted derivatives 3b–f via acylation, alkylation, or methoxycarbonylation of its anion (Scheme 2). The substituents on the N‐atom exert a strong influence on the photochemical reactivity ([2 + 2] photocycloaddition to 2,3‐dimethylbut‐2‐ene, photocyclodimerisation, photoreduction) of these aza‐enones 3 (Scheme 3). In general, N‐alkyl compounds react much slower and with less efficiency than either the (N‐unsubstituted) title compound 3a or its N‐acetyl and N‐(methoxycarbonyl) derivatives 3e and 3f, respectively. These compounds behave similarly to the corresponding lactone, 5,5‐dimethyl‐2(5H)‐furanone, studied previously.
1,3,5‐Trideoxy‐1,3,5‐tris(dimethylamino)‐cis‐inositol, an Efficient Ligand for Hard Trivalent Metal Ions. Determination of the stability constants of the FeIII, AlIII, and CuII complexes in aqueous solution
Authors: Kradolfer Thomas , Hegetschweiler Kaspar 
Published: December 2004

AbstractThe complexes [Fe(tdci)2]Cl3 and [Al(tdci)2]Cl3 (tdci = 1,3,5‐trideoxy‐1,3,5‐tris(dimethylamino)‐cis‐inositol) were prepared and characterized by mass spectrometry, NMR spectroscopy, and magnetic‐susceptibility measurements. The formation constants were determined in aqueous solution (25°, 0.1M KCl) by potentiometric titration. pK values of H3(tdci)3+: 5.89, 7.62, 9.68; FeIII complexes: log βML = 18.8, log β = 32.6; AlIII complexes: log βML = 14.3, logβ = 26.4. The protonated complex [FeH(tdci)2]4+ has also been identified. In contrast to the high stability of the FeIII and AlIII complexes, only weak interactions of tdci with CuII have been observed in aqueous solution (25°, 0.1M KNO3).
Pteridines. Part XCVII. Synthesis and properties of 6‐thioxanthopterin and 7‐thioisoxanthopterin
Authors: Mohr Detlev , Kazimierczuk Zygmunt , Pfleiderer Wolfgang 
Published: December 2004

Abstract6‐Thioxanthopterin (13) was synthesized in four steps starting from 2‐amino‐4‐(penthyloxy)pteridine (3) via the 8‐oxide 4, its subsequent interconversion to the 6‐chloro (7) and 6‐thio derivative (12) and final hydrolysis of the pentyloxy group. 7‐Thioisoxanthopterin (15) was derived analogously from 2‐amino‐4‐(pentyloxy)pteridine‐7(8H)‐thione (14) by alkaline hydrolysis. The various 6‐ and 7‐thiopteridines were methylated to give the corresponding 6‐ (10, 11) and 7‐(methylthio) derivatives (16, 17). The newly synthesized compounds have been characterized by elemental analyses, their UV spectra, and the determination of the acidic and basic pKa values. The spectral relationships are discussed in detail.
Synthesis of Glycosylphosphine Oxides and Related Compounds
Authors: Łopusiński Andrej , Bernet Bruno , Linden Anthony , Vasella Andrea 
Published: December 2004

AbstractThe benzyl‐protected glycosyl acetates 1, 6, 11, and 15 react with MeOPPh2 under catalysis by TMSOTf to yield diastereoselectively the glycosylphosphine oxides 2, 3, 8, 12, 13, and 16, with a strong preference for the 1,2‐cis‐configurated anomers. Hydrogenolysis of the major products gave the crystalline, unprotected phosphine oxides 4, 9, 14, and 17, of which 4 was transformed in to the acetate 5, and 9 into the benzoate 10. The benzylated phosphine oxides 4, 8, 12, and 16 were reduced with Cl3SiH in the presence of a tertiary amine to form the phosphines 18, 21, 24, and 26, which were transformed into the phosphine sulfides 19, 22, 25, and 27. Moreover, 18 and 21, were characterized as the borane adducts 20, and 23. The structure of the (arabinofuranosyl)phosphine oxide 12, the corresponding sulfide 25, and of the borane complex 20 were established by X‐ray analysis. According to NMR spectroscopy, the equatorial pyranosylphosphine oxide 8, the sulfide 22, and the borane complex 23 adopt a 4C1 conformation. The axial phosphine oxide 2 is a flattened 4C1, the sulfide 19 exists as a B2,5, and the borane complex 20 is a flattened 4C1 in the solid sate and a B2,5 in solution. Thus, the conformational behavior of these α‐D‐glucopyranose derivatives reflects the steric requirement of the P‐substituents.
Regio‐and Stereoselectivity of Transition‐Metal‐Ion‐Mediated Single and Double Dehydrogenation of Tetraline
Authors: Seemeyer Katrin , Prüsse Tilman , Schwarz Helmut 
Published: December 2004

AbstractLabeling experiments provide evidence that the Fe1‐ and CO1‐mediated losses of H2 and 2 H2 from tetraline are extremely specific in that both reactions follow a clear syn‐1,2‐elimination involving C(1)/C(2) and C(3)/C(4), respectively. In the course of the multi‐step reaction, the metal ions do not move from one side of the π‐surface to the other. Independent experiments confirm that the kinetic isotope effect (KIE) associated with the loss of the first H2 molecule is indeed larger than the KIE for the elimination of the second H2 molecule.
Structure of Rapamycin: An NMR and Molecular‐Dynamics Investigation
Authors: Kessler Horst , Haessner Rainer , Schüler Wolfgang 
Published: December 2004

AbstractThe highly active immunosuppressive antibiotic rapamycin is a specific inhibitor of a signal transduction pathway that results in exocytosis and transcription. We report the results of the homo‐ and heteronuclear NMR experiments of rapamycin in DMSO leading to a complete assignment of the 1H‐ and 13C‐NMR signals. With exception of one CH2 group, all diastereotopic assignments could be achieved using heteronuclear long‐range coupling constants. Restrained molecular‐dynamics simulation in the same solvent as the NMR experiments led to a well defined conformation of the rapamycin molecule in solution. Differences between the solution and crystal structures are discussed.
Intramolecular Addition of Nucleophilic Carbenes to Acceptor‐Substituted Alkenyl Groups: Synthesis and transformation of homobenzofurans and synthesis of a homoindole
Authors: Li Chunbao , Vasella Andrea 
Published: December 2004

AbstractThe intramolecular addition of unsaturated alkoxycarbenes leads in high yields and diastereoselectively to fused cyclopropanes (Scheme 1). Reaction of the halodiazirines 2, 10, 11, and 20 with the unsaturated phenolates 1, 8, and 9 yielded intermediate alkoxydiazirines, and hence the homobenzofurans 5, 12–16, 22, and 26 (Scheme 2). The intermediate alkoxydiazirine 25 was isolated at low temperature (Scheme 3). An equilibrium between the cyclopropane derivatives 12 and 27, and 14 and 28 was established at 120°. At 200°, 12 rearranged to the chromene 29, by disrotarory opening of the cyclopropane ring, followed by electrocyclization. Hydrogenation of 29 gave the (all‐cis)‐chroman 32 (Scheme 4). The homoindole 35 was obtained in good yields, presumably by an SRN1 reaction from 34 and 10 (Scheme 5).
A New Synthesis of Benzylidene Acetals
Authors: Li Chunbao , Vasella Andrea 
Published: December 2004

AbstractAryl‐halo‐diazirines react under basic conditions with 1,3‐cis‐, 1,2‐cisand 1,2‐trans‐diols to give acetals. Yields are high. Diastereoselectivities depend upon the diol and upon the reaction conditions. Thus, reaction of the 1,3‐cis‐diol 1 (Scheme 1) with 2 gave 3 as a single diastereoisomer. The 1,2‐cis‐diols 4 and 7 led to the endo‐ and exo‐acetals 5/6 (93:7) and 8/9 (ca.10:1), respectively, The 1,2‐trans‐diols 10, 16, and 19 reacted with 2 to afford 11/12 (90:10), 17/18 (1:1), and 20/21 (6:1), respectively. Reaction of the (4‐nitrophenyl)diazirine 13 with 10 at higher temperatures yielded 14/15 (6:4). The uracil moiety, the acetamido group, and the enol‐ether moiety are compatible with the reaction conditions. The diastereoselectivity is rationalized on the basis of a reaction sequence involving alkoxy‐halogen exchange, which is regioselective or not, thermolysis of the ensuing alkoxydiazirine(s), protonation of the alkoxycarbene to form an (E)‐configurated oxycarbenium ion, and attack of the neighboring oxy or hydroxy group, which is only possible for a limited range of conformers.
Enantioselektive Synthese von Allyl‐ und Propargylaminen durch nucleophile 1,2‐Addition an chirale Aldimine. Preliminary communication
Authors: Enders Dieter , Schankat Jürgen 
Published: December 2004

Enantioselective Synthesis of Allyl‐ and Propargylamines via Nucleophilic 1,2‐ Addition to Chiral AldininesThe asymmetric Synthesis of allylamines and propargylamines 5 in high enantiomeric purity (e.e. ⩽ 97%) is described. Key step is the 1,2‐addition of organocerium reagents to chiral α,β‐unsaturated aldimines 3 to produce secondary animes 4. The Chiral auxiliary (S,S)‐2 is removed in three steps, affording the title compounds 5, useful bifunctional building blocks and compounds of pharmaceutical interest.
On Intrahelical Hydrogen Bonding and Stability of β‐Helices: The behavior of some D,L‐alternating oligoleucines with an N‐methylated residue
Authors: Römer Daniel Urs , Fenude‐Schoch Emma , Lorenzi Gian Paolo , Rüegger Heinz 
Published: December 2004

AbstractAn N methylated residue at the n – 3 position of the chain was used to reduce the maximum number of H‐bonds realizable y some D,L‐alternating oligopeptides in β4.4‐, ↑↑5.6‐ and ↑↓5.6‐helices and thus increase for the oligopeptides, the relative stability of larger β‐helices. With D, L‐alternating oligoleucines of the series Boc‐Leun‐OMe, however, this approach did not produce the helices expected. Although ↑↓7.2‐helices with only one free NH per strand would theoretically be possible, the N‐methylated oligoleucines formed instead flawed β4.4‐helices having three free NH's in CHCl3 as well as in other solvents of low polarity. These observations confirm that the stability of β‐helices does not depend only on the number of intra‐or interstrand H‐bonds, and corroborate the idea that β‐helices with large cavities are inherently unstable.
A Simple Procedure for the Preparation of Chiral ‘Tris(hydroxymethyl)methane’ Derivatives
Authors: Seebach Dieter , Lapierre Jean‐Marc , Jaworek Wilfried , Seiler Paul 
Published: December 2004

AbstractThe aldol adducts 1a–13a of R,R‐2(tertbutyl)‐6‐methyl‐1,3‐dioxan‐4‐one (from 3‐hydroxybutanoic acid) to aldehydes, single diastereoisomers obtained as described previously, are acetylated or benzoylated to the corresponding esters 1b–5b and 6c–13c, respectively, which in turn are reduced with LiAlH4 to the title compounds 14–24. The enantiomerically pure triols thus available may be useful as chiral building blocks, as auxiliaries for enantioselective reactions, and as center pieces for chiral dendrimers.
Structural Effects on the RhII‐Catalyzed Rearrangement of Cyclopropenes
Authors: Müller Paul , Gränicher Christian 
Published: December 2004

AbstractThe thermocatalytic rearrangement of 2‐alkylcycloprop‐2‐ene‐1‐carboxylates (1) in the presence of RhII perfluorobutyrate is regio‐ and stereospecific and leads to the substituted metallocarbenes 3. The latter undergo intramolecular CH bond insertion to form cyclopentylidenes (4). In contrast, the metallocarbenes 19, derived from 2,3‐dialkylcycloprop‐2‐ene‐1‐carboxylates 6c, d, react to dienes (Z)‐20, via 1,2‐H migration. The cyclopropenedicarboxylates 10, in turn, rearrange exclusively to the more substituted metallocarbenes 26, which cyclize to furans 28, With 6e, and 12, products derived from both modes of ring‐opening are observed.
Preparation of optically active cyclohexenones: Chirons for the lipophilic moiety of flowery‐ and woody‐like odorant ketones
Authors: Chapuis Christian , Brauchli Robert , Thommen Walter 
Published: December 2004

AbstractOptically active 2,5,6,6‐ and 2,4,4,5‐tetraalkylcyclohex‐2‐en‐1‐ones ((+)‐2a–d and (−)‐5a–d), important building blocks for flowery‐ and woody‐like odorants, have been prepared. Compounds (+)‐2a–d and (−)‐5a–d were obtained by ozonolysis of the corresponding cyclopentenic precursors, followed by intramolecular aldol condensation. Alternatively, enones (+)‐2a–d were reduced to the corresponding allylic alcohols ad converted to enones (‐)‐5a–d via acidic isomerization and oxidation. 13C‐NMR assignments are presented.
Heteronuclear Alkoxide Compounds Containing Copper and Alkaline Earth Metals
Authors: Bidell Woflgang , Bosch H. William , Veghini Dario , Hund Hans‐Ulrich , Döring Joachim , Berke Heinz 
Published: December 2004

AbstractThe tetrameric Cu(β‐diketonate) alkoxide complex [Cu(thd)(OCH2CH2OCH3)]4 (thd = 2,2,6,6‐tetramethyl‐3,5‐heptanedionate; 1a) reacts with the alkaline earth metal alkoxides [M(OCH2CH2OCH3)2] (M = Ca, 2a; M = Sr, 2b; M = Ba, 2c) to yield the heteronuclear compounds [Cu2M(thd)3(OCH2CH2OCH3)3] (M = Ca, 6a; M = Sr, 6b). These heterometallic complexes were also obtained in the reaction of 1a and the mixed Ca and Sr complexes of β‐diketonate‐alkoxide [Mx(thd)y(OCH2CH2OCH3)2x−y] (M = Ca, x = 7, y = 6, 3; M = Sr, x = 5, y = 3, 4), respectively. In comparison, 1a reacts with the analogous [Ba(thd)(OCH2CH2OCH3)] (5a) to yield a[Ba2Cu2(thd)4(OCH3)4(HOCH2CH2OCH3)2] species (8a.) The in situ prepared mixed‐ligand Ba Compounds [Ba(thd)OR)] (R = CH2CH2OCH2CH2OCH3, (5b); R = CH2CH2CH2OCH3 (5c) react with the corresponding Cu complexes [Cu(thd)(OR)]n (R = CH2CH2OCH2CH2OCH3), n = 4 (1b); R = CH2CH2OCH2CH2OCH3 (8b); R = CH2CH2CH2OCH3 (8c). However, [Cu(hfd)(OCH2CH2OCH3)]4 (hfd = 1,1,1,5,5,5,‐hexafluoroacetylacetonate; 1e) is converted in the presence of 2a–c to the simple metathesis products [M(hfd)2] (M = Ca, Sr, Ba) and [Cu(OCH2CH2OCH3)2]. Crystalline [Ba2Cu2(hfd)2(thd)2(OCH2CH2CH2OCH3)4(HOCH2CH2CH2OCH3)2] (9) was isolated from the reaction of 1a with in situ prepared [Ba((hfd)OCH2CH2CH2OCH3)] (5d) in 2‐, methoxyethanol. X‐Ray crystallographic structure determinations are reported for 6a, 6b, 8b, and 8c.
A Facile Conversion of Tertiary Amines into [2‐(dialkylamino)vinyl]triphenylphosphonium salts
Authors: Vorbrüggen Helmut , Krolikiewicz Konrad 
Published: December 2004

AbstractHalogenation of Et3N, (i‐Pr)2EtN, and N‐ethylmorpholine or of enamines with dichlorotriphenylphosphorane gives in up to 75% yield the corresponding [2‐(dialkylamino)vinyl]triphenylphosphonium chlorides, which can be readily converted into the corresponding stable crystalline tetraphenylborates (Schemes 2 and 3).
A General Method for the Synthesis of 2′‐O‐Modified Ribonucleosides
Authors: Keller Thomas H. , Häner Robert 
Published: December 2004

AbstractA general way for the functionalization of ribonucleosides is described. The method involves the synthesis of the methyl‐ribofuranoside derivative 6 equipped with a linker at the 2‐hydroxy group (Scheme 2). After introduction of the nucleic‐acid bases under standard conditions (Scheme 3), the resulting β‐D‐ribonucleosides 8 and 10 are further transformed to derivatives with lipophilic, intercalating, and aminoalkyl residues at the linker moiety. In this way, 2′‐modified 5‐methyluridines 12, adenosines 13, and 5‐methylcytidines 15 and 16 were prepared (Scheme 4).
Influence of Chelating Groups on the Luminescence Properties of Europium(III) and Terbium(III) chelates in the 2,2′‐bipyridine series
Authors: Mukkala Veli‐Matti , Kwiatkowski Marek , Kankare Jouko , Takalo Harri 
Published: December 2004

AbstractEight different 2,2′‐bipyridine derivatives, i.e. 2, 5, 8, 10, 12, 13, 15, and 19 (Schemes 1 and 2), were prepared to study the influence of the chelating groups on the luminescence properties of their EuIII and TbIII chelates. According to our luminescence results, 2,2′‐(methylenenitrilo)bis(acetic acid) as well as (methylenenitrilo)bis‐(methylphosphonic acid) in 6‐ and 6′‐position of 2,2′‐bipyridine is a suitable group when developing luminescent markers for bioaffinity assays based on time‐resolved luminescence measurement.
Sterically Congested Phosphite Ligands: Synthesis, crystallographic characterization, and observation of unprecedented eight‐Bond 31P,31P coupling in the 31P‐NMR spectra
Authors: Pastor Stephen D. , Shum Sai P. , Rodebaugh Ronald K. , Debellis Anthony D. , Clarke Frank H. 
Published: December 2004

AbstractThe synthesis and characterization of 2‐{1‐{3,5‐bis(1,1‐dimethylethyl)‐2‐{[2,4,8,10‐tetrakis(1,1‐dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin‐6‐yl]oxy}phenyl}ethyl}‐4,6‐bis(1,1‐dimethylethyl)phenyl diphenyl phosphite (6) is described. In the 31P‐NMR spectrum (1H‐decoupled) of 6, an unprecedented eight‐bond P,P coupling of J = 72.8 Hz is observed. In the X‐ray crystal structure of 6, an intramolecular P–P distance of 3.67 Å is found, which is within the sum of the van‐der‐Waals radii of the P‐atoms. The observed intramolecular P–P distance suggests that a through‐space coupling mechanism is operative. The solid‐state conformation of 6 is compared to the conformation obtained by semi‐empirical MO geometry optimizations (PM3 method). The calculated geometry suggests that the solid‐state structure is near a true energy minimum, but that crystal‐packing forces decrease the intramolecular P–P distance in the solid state. In the absence of crystal‐packing forces, however, the collisional and vibrational energy available in solution may lead to the population of states with a shortened intramolecular P–P distance in 6. The proximity of the P‐atoms in 6 is due to restricted conformational freedom resulting from steric congestion within the molecule. The free energy of activation (ΔG* = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo[d,f][1,3,2]dioxaphosphepin ring in 6 is determined by variable‐temperature 31P‐NMR spectroscopy. Semi‐empirical MO calculation on model compounds suggest that the structure of the transition state for ring inversion has the two aryl rings and O‐atoms in a common plane, with the P‐atom lying above this plane.