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1,3-Dipolar cycloadditions of C-carboxymethyl-N-arylnitrile imines with substituted acetylenes bearing thiol or sulfone groups were studied. The sulfur controls the regiochemistry of the reaction, and this protocol was applied to the synthesis of ring-fused thieno[2,3-c]pyrazoles.

1,3-Dipolar cycloaddition of C-aryl-N-aryl- and C-carboxymethyl-N-aryl-nitrile imines with functionalized acetylenes have been studied. Regioisomeric mixtures have been obtained with the 5-substituted pyrazole as the major cycloadduct. Under scandium triflate catalysis a reversal in the regiochemistry was observed, especially in the case of C-carboxymethyl-N-aryl-nitrile imines.

A synthesis of ferrocenyl-oxazolines is described using an iodide-mediated ring expansion of N-ferrocenoyl-aziridine-2-carboxylic esters. The ring enlargements take place with full stereocontrol, namely net retention of configuration. Modification of the ester function by a Grignard reaction leads to three new types of ferrocenyl-oxazoline carbinol ligands, which were used as chiral ligands in the asymmetric addition of diethylzinc to benzaldehyde (e.e.s ranging from 46 to 62%). The planar chiral ferrocenyl-oxazoline carbinol ligand gave a very good result (e.e. 90%) in the palladium-catalyzed allylic substitution of 1,3-diphenyl prop-2-enylacetate with dimethyl malonate.

A detailed investigation of the desilylation of (Z)-α-dimethylphenylsilyl vinyl sulfides with fluoride ion has shown that in substrates containing an electron-withdrawing group β to the sulfur, phenyl group migration occurs as a result of a fluoride ion catalyzed retro-Michael reaction.

1,3-Dipolar cycloaddition of nitrile imines with α,β-unsaturated five- and six-membered lactones, thiolactones and lactams gave ring-fused pyrazoles. Regioisomeric mixtures have been obtained with the 5-substituted pyrazole as the major cycloadduct. Only with the five-membered lactone the major product was the 4-acyl derivative. Computational studies, the use of the topological analysis of the Fukui functions and the potential energy surfaces (PES) theory allowed a theoretical description of the local reactivity in agreement with the observed high regiochemistry and with the role of the heteroatom adjacent to the carbonyl group.

1,3-Dipolar cycloadditions of C-carboxymethyl-N-arylnitrile imines with substituted acetylenes bearing thiol or sulfone groups were studied. The sulfur controls the regiochemistry of the reaction, and this protocol was applied to the synthesis of ring-fused thieno[2,3-c]pyrazoles.

The reaction of acylsilanes 1 with Grignard reagents gives the diastereomeric α-hydroxysilanes 3 . The level of asymmetric induction depends on the nature of R 1 and R 2 . Good results were obtained for R 1 =t-Bu, R 2 =Me and R 1 =α-Napht, R 2 =Ph. Subsequent desilylation occurs with predominant inversion of configuration at carbon.

Photo-induced cycloadditions of phenyl triphenylsilyl thioketone 1 with electron-poor olefins (acrylonitrile, methyl acrylate and cis- and trans-1,2-dichloroethene) gave silyl thietanes in a regio- and highly stereo-selective manner. In contrast, reaction with vinyl ethers gave thietanes without any regio- or stereo-control. Reaction with α-methylstyrene and 2,3-dimethylbut-2-ene gave open chain products. Desilylation of silyl thietanes occurs with prevalent inversion of configuration at the carbon bearing the silicon group.

The palladium-catalyzed desilylation−arylation of substituted vinylsilanes by p-iodoanisole in the presence of bidentate phosphine ligands is described. Apart from enhancing the rate of the reaction considerably, heteroatom-based functional groups in the vinylsilane moiety have a profound influence on the regiochemistry. A catalytic cycle for the chelation-controlled desilylation−arylation reaction involving five- and six-membered chelate rings is proposed.

Treatment of methyl 1-methylsulfanylvinyl sulfoxides with sodium thiophenolate in MeOH gives 1-methylsulfanylalk-1-en-3-ols.

Uncatalyzed and LiClO4 catalyzed cycloadditions of (RS)-1-{1-[(1S)-isoborneol-10-sulfinyl]vinyl}cyclohexene 1 and (RS,E)-3-[(1S)-isoborneol-10-sulfinyl]-1-methoxybuta-1,3-diene 2 with di(p-tolyl)- and di(p-anisyl)thioketones 3 and 4 occur with complete regioselectivity. The lack of facial diastereoselectivity, observed in the cycloadditions of 1 with 3 or 4, appears to be a consequence of the sterical features of both diene and dienophile which, in the transition states, make the topological differentiation induced by the presence of the alkylsulfinyl group as chiral auxiliary uninfluential. No significant improvement in diastereoselectivity is observed in the LiClO4 catalyzed reactions, but the obtained enantiopure cycloadducts are easily separated by column chromatography and isolated in high yields. © 1999 Elsevier Science Ltd. All rights reserved.

Enantiomerically pure β-hydroxyalkyl and β-aminoalkyl ferrocenyl sulfides have been synthesized in good yields from mercaptoferrocene and amino alcohol derivatives. Primary β-aminoalkyl sulfides allowed the synthesis of tetrahydro-1,4-thiazepines containing the ferrocene moiety with good diastereoselectivity and β-iminoalkyl sulfides. Some of these derivatives have successfully been employed as ligands for palladium-catalyzed allylic substitution, with asymmetric induction of up to 99% ee. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

1,3-Dipolar cycloaddition of C-aryl-N-aryl- and C-carboxymethyl-N-aryl-nitrile imines with functionalized acetylenes have been studied. Regioisomeric mixtures have been obtained with the 5-substituted pyrazole as the major cycloadduct. Under scandium triflate catalysis a reversal in the regiochemistry was observed, especially in the case of C-carboxymethyl-N-aryl-nitrile imines.

Intramolecular cyclization of enethiolizable (allylthio)sulfines afford 2-alkylidene-1,3-dithiolane-1-oxides in good yields. The formation of these compounds can be explained by an initial tautomerization of the sulfine to vinyl sulfenic acid, followed by an intramolecular addition of the sulfenic acid to the allylic double bond. The stractures of the products were elucidated by means of 1H-and 13C-NMR spectroscopy as well as LIS effects, deuteriation experiments and an X-ray analysis.

The reaction of acylsilanes 1 with Grignard reagents gives the diastereomeric α-hydroxysilanes 3 . The level of asymmetric induction depends on the nature of R 1 and R 2 . Good results were obtained for R 1 =t-Bu, R 2 =Me and R 1 =α-Napht, R 2 =Ph. Subsequent desilylation occurs with predominant inversion of configuration at carbon.

Six ferrocenyl sulfides, exhibiting planar and central chiralities, have been screened as a catalytic source of asymmetric sulfonium ylides. A one-pot reaction has been achieved, involving the addition of an aldehyde, benzyl bromide, 20% molar equivalent of the ferrocenyl sulfide, sodium iodide in a mixture of tert -butanol and water. The best results were observed with enantiopure sulfide 3a , bearing a tert -butyl group. Good yields of stilbene oxides were obtained, with enantiomeric excesses ranging from 74% to 94%. trans/cis -Diastereomeric ratios ranged from 60:40 to 86:14. The chiral sulfide was recovered. An unexpected case of stereoconvergence was observed with diastereoisomers 3a and 3b . A model is proposed to account for the asymmetric induction, based on a conformation locked by the tert -butyl group and the interplay of planar and remote central chiralities.

The palladium-catalyzed desilylation−arylation of substituted vinylsilanes by p-iodoanisole in the presence of bidentate phosphine ligands is described. Apart from enhancing the rate of the reaction considerably, heteroatom-based functional groups in the vinylsilane moiety have a profound influence on the regiochemistry. A catalytic cycle for the chelation-controlled desilylation−arylation reaction involving five- and six-membered chelate rings is proposed.

t-Butyl and methyl trimethylsilyl thioketones have been synthesized, and the reactivity towards organometallic compounds, dienes, and oxidation, and the desilylation of the resulting adducts, have been investigated. Where possible, a comparison with aryl trimethylsilyl thioketones has been made.

Photo-induced cycloadditions of phenyl triphenylsilyl thioketone 1 with electron-poor olefins (acrylonitrile, methyl acrylate and cis- and trans-1,2-dichloroethene) gave silyl thietanes in a regio- and highly stereo-selective manner. In contrast, reaction with vinyl ethers gave thietanes without any regio- or stereo-control. Reaction with α-methylstyrene and 2,3-dimethylbut-2-ene gave open chain products. Desilylation of silyl thietanes occurs with prevalent inversion of configuration at the carbon bearing the silicon group.

The radical adducts resulting from the reaction between group IVB organometallic radicals MR 3 (M = Si, Ge, Sn; R = alkyl or aryl) and a number of thioketones of the chromone and flavone series have been investigated by ESR spectroscopy. The spectral parameters obtained for these species have been compared with those of similar adducts of the corresponding ketones. Both classes of radicals adopt an orthogonal conformation, and the differences between the former and the latter paramagnetic species can be accounted for by the different electronegativity of oxygen and sulphur. the effects of substitution of the heterocyclic oxygen with a sulphur or selenium atom on the spin density distribution are likewise explained. INDO calculations have been carried out on the model systems CH 2 XSiH 3 (X = O, S).

The reaction of organolithium compounds with stilbene episulphoxides and episulphides leads to stereospecific desulphurization and to ring opening. The stereochemistry of the latter reaction depends on the structural features.

The title compound was prepared by acid catalysed reaction of the corresponding ketone with hydrogen sulphide, and its ability to transfer chirality to the carbon α to silicon in reactions with buta-1,3-diene and methyl-lithium was tested; good levels of asymmetric induction were observed.

(E)-Phenyl and (E)-(p-tolyl) trimethylsilyl thioketone S-oxides can be easily and stereospecifically protodesilylated into the corresponding (Z)-aryl thioaldehyde S-oxides; these stable liquid lachrimatory compounds show good reactivity as heterodienophiles and the reaction with 1,3-dienes is surprisingly non-stereospecific.

Diarylthioketone S-oxides react with the 2H-azirine (1) in the presence of boron trifluoride–diethyl ether to give the oxathiazines (3)via a 1,3-cyclization across the sulphine C–S–O bond; the oxathiazines (3) rearrange thermally to the sulphoxides (5).

Trimethylsilylphenyl- and triphenylsilylphenyl thioketones easily undergo [4 + 2]-cycloadditions with cyclic and open chain 1,3-dienes at room temperature to give silylated thiabicyclo[2.2.1 ]heptenes and dihydrothiopyrans in very good yields. With cyclopentadiene the reaction produces exclusively the endo-silylated adducts. Sulphur oxidation and desilylation of the adducts were also investigated.

Diphenylnitrilimin (II), hergestellt aus de Sulfinen (III) zu den Addukten (IVa) oxidiert werden konnen.

Thermal Intramolecular cyclisation of enethiolisable (allylsulfanyl)sulfines affords 2-alkylidene-1, 3-dithiolane 1-oxides in good yields. The formation of these compounds is explained by an initial tautomerisation of the sulfine to vinylsulfenic acid, followed by an Intramolecular addition of the sulfenic acid to the allylic double bond. Also, sulfines having an S-(prop-2-ynyl) substituent have been investigated and they similarly give 2-alkylidene-5-methylene-1, 3-dithiolane 1-oxides. The results obtained by using thionyl chloride to promote the reaction turned out to be highly dependent upon the structure of the starting sulfine.

A practical route is described for the synthesis of enantiopure β- and γ-amino alcohols with two stereocenters, starting from homochiral α- (1 and 5) and β- (13 and 16) -aminoacylsilanes, and involving stereoselective addition of allylmetal compounds and subsequent stereospecific protiodesilylation of the adducts. The degree of diastereoselectivity achieved in the nucleophilic addition step depends on both the nitrogen-protecting group and the reagents used. Diastereomeric excess (de) values equal to or higher than 98% were obtained in the TiCl4-promoted allylation of the N-Pht-aminoacylsilanes 1 and 13 and of the N-Ts-aminoacylsilane 5 with allyltrimethylsilane. Lower de values were obtained in the Sc(OTf)3-catalyzed allylation of 5 with tetraallyltin and in the additions of both allyltrimethylsilane and tetraallyltin to the N-Ts-β-aminoacylsilane 16. Protiodesilylation of the adducts, leading to the β- and γ-amino alcohols, was accomplished with TBAF, except in the case of the adducts obtained from 5. For these, a preliminary removal of the tosyl group was necessary, which was accomplished with simultaneous desilylation by treatment with Na in liquid ammonia.

(E)-Phenyl and (E)-(p-tolyl) trimethylsilyl thioketone S-oxides can be easily and stereospecifically protodesilylated into the corresponding (Z)-aryl thioaldehyde S-oxides; these stable liquid lachrimatory compounds show good reactivity as heterodienophiles and the reaction with 1,3-dienes is surprisingly non-stereospecific.

Diarylsulphines (diaryl thioketone S-oxides) undergo cycloadditions with benzonitrile oxide to yield 1,4,2-oxathiazole 4-oxides, with the exception of thiofluorenone S-oxide which gives a 1,5,2-oxathiazole 5-oxide as the dominant product. From the nature of the products obtained from geometrically isomeric sulphines, the cycloaddition was shown to be stereospecific.

Diarylthioketone S-oxides react with the 2H-azirine (1) in the presence of boron trifluoride–diethyl ether to give the oxathiazines (3)via a 1,3-cyclization across the sulphine C–S–O bond; the oxathiazines (3) rearrange thermally to the sulphoxides (5).

The cycloaddition reactions of aromatic sulphines, such as thiobenzophenone S-oxide and thiofluorenone S-oxide, with 2-diazopropane lead to 1,3,4-thiadiazoline 1-oxides in high yields. Diazomethane reacts more sluggishly with sulphines: only with thiofluorenone S-oxide was a cycloadduct obtained. The structure of the cycloadducts has been proven by photochemical extrustion of sulphur monoxide from 2,2-dimethyl-5,5-di-(p-tolyl)-1,3,4-thiadiazoline 1-oxide. In solution the cycloadducts readily undergo a retro-cycloaddition. The adduct from thioxanthen-9-thione SSS′-trioxide and 2-diazopropane not only reverts to the starting sulphine, but also shows a reversed retro-cycloaddition reaction to give diazothioxanthen SS-dioxide and thioacetone S-oxide.