bluuebunny/crossref_metadata_2025_split huggingface dataset:
Synthese und Reaktionen Chinuclidin‐stabilisierter Sulfene, R1R2CSO2←N(CH2CH2)3CH
Authors: Sundermeyer Wolfgang , Waldi Joachim 
Published: July 2007

Synthesis and Reactions of Quinuclidine‐Stabilized Sulfenes, R1R2CSO2←N(CH2CH2)3CHQuinuclidine adducts of the sulfenes 3 and 10 have been isolated and have proven to be good sources for investigating the reactivity of the respective sulfenes. Water and some alcohols react with 3 as well as with “disulfene” (5) in the presence of quinuclidine (2) under formation of the corresponding (alkyl)chinuclidinium methanesulfonyl methanesulfonates 6a–f. The expected intermediate mesyl methanesulfonic acid esters could only be isolated in the case of 4. Similar results were obtained on attempted synthesis of methylsulfene. Only the noncyclic dimer 10 could be prepared, which reacts with alcohols or water forming 11a–c. The formation of “monomeric” sulfene or methylsulfene under these conditions could be ruled out by comparison with their expected hydrolysis products 8 and 13, which were synthesized separately.
N‐Aryl‐O‐(α‐aminoacyl)hydroxylamine: Modellreaktionen mit Desoxyguanosin, Guanosin und 5′‐Guanosinmonophosphat zur Aktivierung monocyclischer aromatischer Amine (z. B. Phenacetin) zu ultimaten Carcinogenen
Authors: Meier Chris , Boche Gernot 
Published: July 2007

N‐Aryl‐O‐(α‐aminoacyl)hydroxylamine: Model Reactions with Deoxyguanosine, Guanosine and 5′‐Guanosinemonophosphate for the Activation of Monocyclic Aromatic Amines (e.g. Phenacetin) into Ultimate CarcinogensIn in vitro model reactions of the activation of monocyclic aromatic amines by α‐amino acids it is shown that α‐amino‐hydroxamic acids 8 and 9 rearrange base‐catalyzed to N‐(α‐aminoacyloxy)arylamines 10 and 11 which react with bionucleophiles such as deoxyguanosine (dG) (12), guanosine (G) (13) and 5′‐guanosinemonophosphate (5′‐GMP) (14) to form adducts. We describe the regioselective formation of the C‐8 adducts of 4‐chloroaniline (15), aniline (16), 4‐methylaniline (17), and 4‐methoxyaniline (18), respectively, [“N‐(deoxyguanosine‐8‐yl)anilines”], and also of N‐(guanosine‐8‐yl)‐4‐methylaniline (21) and 8‐(4‐methylanilino)‐5′‐guanosinemonophosphate (22). Similar reactions of the N‐(acetoxy)arylamines 20, which are very likely to be “ultimate” carcinogens of aromatic amines, lead to the same C‐8 adducts 15–18, 21, and 22 in comparable yields. These in vitro reactions thus show that the N‐(α‐aminoacyloxy)arylamines 10 and 11 react like the N‐(acetoxy)anilines 20 as “ultimate” carcinogens. Therefore, the activation of aromatic hydroxylamines by O‐α‐aminoacylation is of similar quality as by O‐acetylation.
Transition metal catalyzed oxidations, 3. Reaction of 4‐hydroxy[2.2]paracyclophane with the mimoun molybdenum oxodiperoxo complex [Mo(O2)2O]· Py · HMPT
Authors: Krohn Karsten , Rieger Hagen , Hopf Henning , Barrett David 
Published: July 2007

AbstractA four‐step preparation of dimethyl 7‐oxobicyclo[2.2.1]‐heptane‐exo‐2, exo‐3‐dicarboxylate (4) in satisfactory yield from the readily available 6,6‐dimethylfulvene and maleic anhydride is described. Compound 4 represents a well‐functionalized intermediate for the synthesis of norbornane derivatives which is used for an improved synthesis of the tricyclic diester 7, precursor of the cage compound 8. An alternative approach to diester 7 from dimethyl 7‐isopropylidenebicyclo[2.2.1]heptane‐exo‐2, exo‐3‐dicarboxylate (2) is also discussed.
Ein neues PH‐funktionelles Diphosphadistannetan: (tBu2SnPH)2
Authors: Hänssgen Dieter , Aldenhoven Heinz , Nieger Martin 
Published: July 2007

A Novel PH‐Functional Diphosphadistannetane: (tBu2SnPH)2The reaction of tBu2SnCl2 with NaPH2 in liquid ammonia yields the four‐membered ring compound (tBu2SnPH)2 (1). The corresponding silyl compound, (tBu2SnPSiMe3)2 (2), is obtained by metallation of 1 with tBuLi and metathesis with Me3SiCL Both compounds are characterized by NMR analysis (1H, 31P, 119Sn) and 1 by an X‐ray structure analysis; it reveals a planar heterocycle.
Organische Synthesen mit Übergangsmetallkomplexen, 46. Alkenylcarben‐Chrom‐ und ‐Wolframkomplexe; Protonierung und Aktivierung durch Elektronen‐ziehende Substituenten
Authors: Aumann Rudolf , Hinterding Peter , Krüger Carl , Betz Peter 
Published: July 2007

Organic Syntheses via Transition Metal Complexes, 46. — Alkenylcarbene Chromium and Tungsten Complexes; Proton Addition and Activation by Electron‐Withdrawing SubstituentsAlkenylcarbene complexes LnMC(OEt)‐CHC(R)NMe2[1, LnM  Cr(CO)5, W(CO)5; R  H, CH3, C6H5] add HBF4 in ether to give novel cationic carbene complexes [Ln MC(OEt)‐CH2‐C(R)NMe2]BF4 (3) by protonation at C‐2 of the C‐CC ligand. The addition of HBF4 to 7 leads to the formation of the cyclic carbene complexes 8. Structures of 3 and 8 are assigned spectroscopically. X‐ray data are reported for 8a. Protonation of 1 and 7 results in a strong deshielding of the carbene carbon atoms, which parallels an increase of reactivity at the MC bonds. An activation of the MC bond has also been achieved by electron‐withdrawing substituents like N‐acyl groups. We report on the first synthesis of [β‐(acylamino)‐alkenyl]carbene complexes 13, which are more reactive than β‐(aminoalkenyl)carbene complexes 7.
Intra‐ und extraannular funktionalisierte helical‐chirale [2.2]Metacyclophane: Synthese, Circulardichroismus und Struktur‐Chiroptik‐Beziehungen
Authors: Vögtle Fritz , Knops Peter , Ostrowicki Andreas 
Published: July 2007

Intra‐ und Extraannularly Functionalized Chiral [2.2]Metacyclophanes: Synthesis, Circular Dichroism, and Structure ‐ Chiroptic RelationshipsA simple one‐step cyclization reaction for the synthesis of the title compounds 1‐5 is presented; separation (enrichment) of enantiomers is achieved by HPLC mainly on (+)‐PTrMA. The kinetics of interconversion (racemization) of the new dihetera[2.2]metacyclophanes are determined: No racemization of the internally substituted phanes is found on heating until decomposition, whereas the extraannularly substituted compounds exhibit barriers of interconversion of about 130 kJ/mol, similar to that of the unfunctionalized skeleton 1a. For the pyridinophane 2 the barrier (110 kJ/mol) is lower corresponding to the decreased spatial requirement of the nitrogen lone pair compared to a C‐H bond. The circular dichroism curves of the intraannularly functionalized phanes are compared with those of the extraannularly functionalized reference compounds: A bathochromic shift of the Cotton effect at short wavelengths is found, which is increasing with the bulkiness of the internal functional group.
Struktur und Reaktivität des α,p‐Dimers aus 2,2‐Dimethyl‐1‐(1‐naphthyl)propyl‐Radikalen
Authors: Peyman Anuschirwan , Peters Karl , von Schnering Hans Georg , Rüchardt Christoph 
Published: July 2007

Structure and Reactivity of the α,p‐Dimer from 2,2‐Dimethyl‐1‐(1‐naphthyl)propyl RadicalsThe title compound 2 formed by recombination of two 2,2‐dimethyl‐1‐(1‐naphthyl)propyl radicals 1, is the first isolated α,p‐dimer from secondary benzyl radicals. 2 is obtained in crystalline form, its structure is determined by X‐ray analysis and compared with the results obtained by MMPI calculations. 2 exhibits unusual reactivity in the presence of acid. Its thermal stability is much higher than that experienced with comparable α,p‐dimers of tertiary benzyl radicals.
Darstellung und Struktur von Alkali‐penta‐tert‐butoxystannaten(IV)
Authors: Veith Michael , Reimers Michael 
Published: July 2007

Preparation and Structure of Alkali Penta‐tert‐butoxystannates(IV)Potassium, rubidium, and caesium tert‐butoxide react with tin tetra‐tert‐butoxide in toluene to form compounds MSn(OtBu)5 [MK(2), Rb (3), Cs (4)] in contrast to lithium and sodium tert‐butoxide. The compounds 2–4 are soluble in non‐coordinating solvents and seem to be almost monomeric. 2 crystallizes as 2 · 1/2 toluene in the orthorhombic space group Fddd. The X‐ray structure determination shows 2 to be coordinated as a helical polymer with the tin atom in the center of a trigonal bipyramid of tert‐butoxide groups interlinked by two structurally different potassium atoms.
9‐Fluor‐9‐borabicyclo[3.3.1]nonan ‐ Charakterisierung in Lösung und im Kristall
Authors: Köster Roland , Schüßler Wilhelm , Boese Roland 
Published: July 2007

9‐Fluoro‐9‐borabicyclo[3.3.l]nonane — Characterization in Solution and in the Solid State9‐Fluoro‐9‐borabicyclo[3.3.1]nonane (1), prepared from bis(9‐borabicyclo[3.3.1]nonane) with SbF3 or SbF5 in high yields, is characterized by IR, mass, and multi‐NMR spectroscopy. Pure 1 in solution has a small 11B‐NMR singlet signal in contrast to the freshly prepared 1 with the normal JPB‐split 11B resonance. The X‐ray structure analysis of 1 with three differently bonded molecules I–III (Abb. 2) in the cell shows a monomer and a dimer of 1 with DFB coordination bond. – Some Lewis base adducts of 1 are prepared, the others were characterized by 11B‐NMR spectra.
Wasserlösliche Metallkomplexe und Katalysatoren, IV. 2,2′‐Bipyridin‐5‐sulfonsäure: Synthese, Reinigung, Derivate und Metallkomplexe
Authors: Herrmann Wolfgang A. , Thiel Werner R. , Kuchler Josef G. 
Published: July 2007

Water‐Soluble Metal Complexes and Catalysts, IV. — 2,2′‐Bipyridine‐5‐sulfonic Acid Synthesis, Purification, Derivatives and Metal ComplexesThe synthesis of 2,2′‐bipyridine‐5‐sulfonic acid (1a) is achieved by mercury(II)‐catalyzed sulfonation of 2,2′‐bipyridine in oleum (30% SO3) in 10‐50‐g amounts. The crude product is purified by extraction of 1a as tetra(n‐butyl)ammonium salt into dichloromethane and re‐extraction of the free acid with conc, hydrobromic acid. Melting of 1a together with potassium hydroxide gives 5‐hydroxy‐2,2′‐bipyridine (2). The salts 1b–d (Na+, [N(n‐C4H9)4]+, [P(C6H5)4]+) and sulfonamides 1f–h (tert‐butyl, benzyl, 2‐pyridylmethyl) of the acid 1a are ligands of different solubility. The coordination chemistry of these N,N‐chelating ligands is studied. The compounds 4–14 of chromium, molybdenum, tungsten, manganese, rhenium, and osmium containing carbonyl or oxo ligands are prepared as examples of chelate complexes with metals in low and high oxidation states. The solubility of the complexes is mostly determined by the cations: sodium salts are usually soluble in water and/or in short‐chained alcohols. Tetra‐n‐butylammonium and tetraphenylphosphonium salts and the sulfonamides are soluble in polar organic solvents. On the other hand, the distribution of charge has an influence on solubility. Increased polarity of the anion, caused by high oxidation states of the metals, decreases the solubility of the complex in organic solvents.
Mehrfachbindungen zwischen Hauptgruppenelementen und Übergangsmetallen, LXXXII. Zweikernige μ‐Benzylidin‐Komplexe mit Doppelbindung zwischen Rheniumatomen
Authors: Herrmann Wolfgang A. , Kiprof Paul , Felixberger Josef K. 
Published: July 2007

Multiple Bonds Between Main‐Group Elements and Transition Metals, LXXXII. — Dinuclear p‐Benzylidyne Complexes Having a Double Bond between Rhenium AtomsThe dialkylrhenium(V) complex (η5‐C5Me5)Re(O)(CH2Ph)2 (3) reacts with the organotitanium reagents (η5‐C5H5)TiX3 (X  Cl, Br) to yield the dinuclear rhenium(IV) complexes (η5C5Me5)2 Re2X3(CC6H5) (4a, b). According to a single‐crystal X‐ray diffraction analysis, 4a′ (C5Me4Et in place of C5Me5) represents one of the few known examples of μ‐alkylidyne complexes that have metal‐to‐metal multiple bonds. The molecule of C2 symmetry has a planar four‐membered Re,Cl,Re′,C‐ring geometry and two trans‐oriented sets of ligands. Very small angles at the bridging chloro ligand and alkylidyne carbon atom result from the short metal‐metal distance (double bond).
Gold complexes with selenium ligands, I. Preparation and crystal structures of phenylselenolato‐gold(I) complexes
Authors: Jones Peter G. , Thöne Carsten 
Published: July 2007

Abstract(Triphenylphosphine)phenylselenolatogold(I), Ph3PAuSePh (1), and the corresponding dppm derivative, [(dppm)(AuSePh)2] (3), are prepared from Me3SiSePh and the appropriate (phosphine)chlorogold(I) complex. The reaction between Ph3PAuCl, Ph2Se2 and AgSbF6 leads to the salt [(Ph3PAu)2SePh]+SbF−6 (2), with a bridging PhSe ligand. X‐ray structure analyses of 1 and 2 confirm the expected linear geometry at gold and reveal short Au‐Au contacts; both compounds may thus be regarded as loose dimers.
Aza‐Enamine, VIII. Elektrophile Substitutionsreaktionen am Azomethin‐C‐Atom von Aldehyddialkylhydrazonen: Vilsmeier‐Formylierung und Folgereaktionen
Authors: Brehme Rainer 
Published: July 2007

Aza‐enamines, VIII. — Electrophilic Substitution Reactions at the Azomethine C‐Atom of Aldehyde Dialkylhydrazones: Vilsmeier Formylation and Consecutive ReactionsThe reaction of hydrazones 1 with the Vilsmeier reagent yields 3‐phenyl‐1,4,5‐triaza‐1,3‐pentadienium salts according to their aza‐enamine character. Hydrolysis of 2 gives 1‐phenylglyoxal 1‐dialkylhydrazones 3, which rearrange in acidic media to 1‐phenylglyoxal 2‐dialkylhydrazones 4. Compound 3h forms the dihydropyrroloimidazole 5 in boiling ethanol. Pyrrolotriazinium salt 6 is obtained by the reaction of 1b with the isolated Vilsmeier reagent from dimethylformamide/oxalyl chloride.
Organische Synthesen mit Übergangsmetallkomplexen, 47. 1‐Chroma‐1,3‐diene durch Aminoalkenylierung von Methylcarben‐Chromkomplexen mit sekundären Amiden
Authors: Aumann Rudolf , Hinterding Peter 
Published: July 2007

Organic Syntheses via Transition Metal Complexes, 47. — 1‐Chroma‐1,3‐dienes by Aminoalkenylation of Methylcarbene Chromium Complexes with Secondary Amides4‐(NH‐Amino)‐1‐chroma‐1,3‐dienes [= β‐(NH‐Amino)alkenylcarbene chromium complexes] (CO)5CrC(OEt)CHC‐(NHMe)R (Z)‐6 (R  Me, Et, iPr, tBu, Ph) have been obtained with high stereoselectivity in 53–69% yields by the aminoalkenylation of (CO)5CrC(OEt)Me (1a) with secondary amides OC(NHMe)R (5, R  Me, Et, iPr, tBu, Ph) in the presence of POCl3/Et3N.
The laboratory generation of potentially interstellar cumulenes S(Cn)S (n = 2–6) and their radical cations
Authors: Sülzle Detlev , Beye Norbert , Fanghänel Egon , Schwarz Helmut 
Published: July 2007

AbstractThe title compounds are generated and characterized in the gas phase by tandem mass spectrometry (neutralization‐reionization mass spectrometry); it is demonstrated that both even and odd‐numbered polycarbon disulfides S(Cn)S(n = 2–6) are stable species. The elusiveness of the even‐numbered analogues (n = 2, 4, 6) in the condensed phase must be due to facile intermolecular reactions.
Facile catalytic conversion of carboxylic acids into thiocarboxylic S‐acids by the Ph3SbO/P4S10 system
Authors: Nomura RyǒKi , Miyazaki Shin‐Ichiro , Nakano Takahiro , Matsuda Haruo 
Published: July 2007

AbstractThiocarboxylic S‐acids 2 are readily prepared by direct sulfuration of the corresponding carboxylic acids 1 catalyzed by Ph3SbO/P4S10 under mild conditions.
Intramolecularly stabilized organoaluminium, ‐gallium and ‐indium derivatives crystal structures of {o‐[(dimethylamino)methyl]phenyl}dimethylgallium and {o‐[(diethylamino)methyl]phenyl}dimethylindium
Authors: Schumann Herbert , Hartmann Uwe , Wassermann Wilfried , Dietrich Andreas , Görlitz Frank H. , Pohl Ludwig , Hostalek Martin 
Published: July 2007

AbstractIntramolecularly stabilized four‐coordinated organoaluminium, ‐gallium, and‐indium compounds R2MC6H4CH2NR'2‐(2) (1–7), Me2MCH2C6H4NMe2‐(2) (8, 9), Me2MCH2C6H4CH2‐NMe2‐(2) (10, 11), and Me2GaC6H4NMe2 (12) have been synthesized from O ‐[(dialkylamino)methyl]phenyllithium, [O ‐(dimethylamino)phenyl]methyllithium, {O ‐[(dimethylamino)methyl]phenyl}methyllithium and O ‐(dimethylamino)phenyllithium, respectively. The 1H‐, 13CNMR, and mass spectra of the new compounds and the X‐ray crystal structures of 2 and 7 are reported and discussed.
Synthesis of allenes by 1,6‐addition of organocuprates to polarized enynes
Authors: Krause Norbert 
Published: July 2007

AbstractTen β‐allenic esters, ketones, and thioesters 15 bearing alkyl, alkenyl, aryl, and trimethylsilyl groups are synthesized in 57‐ 85% yield by 1,6‐addition of organocuprates to polarized enynes 10. The dependence of the regioselectivity of the protonation of the allenyl enolate intermediate 7 on the protonating agent is studied: pure allenes are obtained by quenching the intermediate with pivalic acid at −80°C. The method is applied in a short synthesis of pseudoionone (18).
Synthese von Imidazolidin‐Derivaten aus Dicyanessigsäureestern
Authors: Neidlein Richard , Sui Zhihua 
Published: July 2007

Synthesis of Imidazolidine Derivatives from DicyanoacetatesBy nucleophilic additions of amino acids to one cyano group of salts of alkyl dicyanoacetates 1 followed by ring‐cyclisation reactions the 4‐imidazolidinone derivatives 3, 5 are obtained.
Phenylazo‐substituierte Triphenylmethylium‐Systeme
Authors: Hellwinkel Dieter , Fritsch Helmut 
Published: July 2007

Phenylazo‐Substituted Triphenylmethylium SystemsTriphenylmethanols (or their ethers) bearing phenylazo groups react with trifluoroacetic acid to give deeply colored solutions of the tritylium ions 7+ ‐ 9+, (RC6H4N  N‐C6H4)n C(C6H5)3‐n (R  H, Me, C1, NOz, MeO, H2N, Me2N), whose long wavelengths VIS absorptions and lowfield 1H‐ NMR signals are consistent with the existence of through‐ conjugated n‐electron systems. After addition of trifluorometh‐ anesulfonic acid significant hypsochromic shifts of the VIS bands as well as additional proton deshieldings in the 1HNMR spectrum are observed, which are in accordance with the formation of proximally azo‐protonated phenylazotrity‐ lium ions 7+·H+, 8+·2H+, and 9+·3H+, (RC6H4NNH‐ C6H4)nC(C6H53 ‐ n. Fully analogous acid‐strength‐dependent ionisation/protonation reactions result for the overall p‐sub‐ stituted ions 17+, 18+, (RC6H4N NC6H4)C(C6H4R)2 (R, R'  Me, MeO, Me2N), as well as for the azobistritylium system,10 (C6H5)2CC6H4NNC6H4(C6H52, and its azoxy deriv‐ ative 11. The linearly extended bis‐ and trisazo systems 19+, 22+, MeC6H4N  N(C6H4N  N)1,2C6H4C(C6H5)2 and the (ph+e‐ny1azo)styryl system 20+, MeC6 H4N  NC6H4CH  CHC6H4C‐ (C6H5), in trifluoroacetic acid do not show any significant color changes relativ to the corresponding monofunctional parent types. In trifluoromethanesulfonic acid, however, compounds 19+, 22+ exhibit the usual bathochromic shifts of the longest wavelength VIS absorptions accompanying the increasing protonation of the polyazo systems. In the case of the tritylium ions containing azomethine bridges, 24+, only for the methoxy derivative 24b+ indications for the existence of the conjuga‐ tively extended tritylium system (MeOC6H4N  CHC6H,)C‐ (C6H5)2 are found. The amide derivatives 26+ ‐ 29+, on the other hand, present no clues for the existence of through‐con‐ jugated systems of the kind ArC(OH)NC6H4Ckz and ArN  C(OH)C6H4CAr2, respectively.
Reactions with aziridines, 53. — Arene hydrides, 9. intermediate substitution in the formation of a benzylic anion by an aromatic radical anion as observed with 1‐benzoyl‐2‐phenylaziridine
Authors: Stamm Helmut , Falkenstein Reinhart 
Published: July 2007

AbstractThe reaction of the title compound 1 with anthracene hydride AH‐ or anthracenide A‐' leads to the formation of the ben‐ zylic anion 9 by fragmentation of the first generated substi‐ tution intermediate 7 or 8. In the reactions with AH‐ the carb‐ anion 9 is completely trapped by protonation with dihydroan‐ thracene AH2 yielding the reduction product 3 (N‐benzoyl‐phenethylamine).phenethylamine). In reactions with A‐' as well as with naph‐ thalenide N‐' the carbanion 9 either abstracts a proton from the solvent THF (yielding 3) or adds to the benzoyl group of unreacted 1 which finally results in P‐benzamido‐ct‐phenyl‐ propiophenone (6).
Acetylierung von N‐(Trimethylsilyl)keteniminen: N‐Acylketenimine und α‐Cyanketone
Authors: Meier Stefan , Würthwein Ernst‐Ulrich 
Published: July 2007

Acylation of N‐(Trimethylsfly1)ketenimines: N‐Acylketenimines and α‐Cyano KetonesThe acylation reactions of N‐(trimethylsilyl)ketenimines 1 with different acyl halides 4 yields N‐acylketenimines 2 and/or α‐cyano ketones 3, depending on the substitution pattern of compounds 1 and the halogen atom in 4. In some cases, novel adducts of enamide type 5, 6 are isolated. The new N‐acylketenimines 2a, b are stable at room temperature and have been fully characterized; compounds 2c, d, however, were found to dimerize yielding the 1,3,5‐oxadiazines 7. For comparison, the analogous acylation reactions of nitrile anions were also studied. MNDO calculations are used to rationalize the regiochemistry in terms of kinetic and thermodynamic control.
Inversionsbarrieren Ortho,ortho′‐verbrückter Biphenyle
Authors: Müllen Klaus , Heinz Willi , Klärner Frank‐Gerrit , Roth Wolfgang R. , Kindermann Islinde , Adamczak Oliver , Wette Michael , Lex Johann 
Published: July 2007

Inversion Barriers of ortho,ortho′‐Bridged BiphenylsThe syntheses and spectroscopic properties of the ortho,ortho′‐bridged biphenyls 2–19 are described. In the case of 5, 15, and 16 the inversion barrier is determined from the temperature dependence of the NMR spectra (lineshape analysis), in the case of 12, 13, and 14 from the temperature of coalescence, and in the case of 2a, 3b, 7, 17, 18, and 19 from the rate of racemization of the optically active compounds. The experimental inversion barriers, including data from the literature, are compared with those calculated using the extended force field (MM2erw). The conformational behavior of the title systems is significantly affected upon transformation into the corresponding dianion salts. The relevant energy terms are discussed.
On the mechanism of diels‐alder reactions of nitroso alkenes: exo/endo selectivity, stereospecificity, E/Z selectivity, and relative reactivity of various olefins
Authors: Reißig Hans‐Ulrich , Hippeli Claudia , Arnold Thomas 
Published: July 2007

AbstractThe mechanism of the hetero Diels‐Alder reaction of nitroso alkenes 2 with silyl enol ethers and other olefins has been investigated. Using the bicyclic nitroso compound 2a a study of the exo/endo selectivity has demonstrated that the exo approach is preferred with the siloxyethene l a as dienophile. On the other hand, the siloxycyclopentene 1 c gives a mixture of cycloadducts 3 c with an excess of endo product (endo: exo = 82: 18). The stereospecificity of the nitroso alkene cycloaddition could be demonstrated with the stereochemically homogeneous silyl enol ethers 1 b and 1 d. Experiments with enol ethers 1 f and 1 g also occur stereospecifically. α‐Nitrosostyrene 2b reveals surprisingly high kE/Z values when E/Z‐isomeric olefins are compared in competition experiments. Also, a detailed reactivity scale of 2b including various structurally different silyl enol ethers and other typical dienophiles shows that a complex interplay of electronic and steric effects is operating. The large influence of steric effects is taken as evidence for a highly ordered transition state in the cycloaddition. All mechanistic details for the Diels‐Alder reactions of nitroso alkenes 2 with (silyl) enol ethers are in strong accord with a concerted mechanism and exclude the involvement of zwitterions or diradicals as intermediates.