Vinyl sulfoxides electrophilic additions

Vinylic sulfoxides such as 1 react readily with electrophiles to give highly reactive species, and the overall reactions have been likened to "generation" of the synthon 2. Treatment of 1 with TFAA, for example, results in what is referred to as an "additive Pummerer reaction", and gives the diester 3. Reaction of 1 with triflic anhydride and sodium acetate in acetic anhydride, by contrast, gives an 85% yield of the protected aldehyde 4. 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

Electrophilic additions to vinyl sulfoxides and sulfones allow for further functionalization of these compounds at the a-position. Deprotonation of the a-hydrogen, under appropriate conditions, gives rise to a-vinyl anions which can be trapped by electrophiles. This protocol has been employed in the synthesis of a diverse range of compounds, such as 34 and 37, 40 and 43, and 46 (see Schemes 9, 10, and 11, respectively). Both enantiomers of optically pure propargylic alcohols (115) were conveniently prepared by the reaction of the a-vinyl anion of 2-(trimethylsilyl) vinyl... 

The reaction of phenyl vinyl sulfoxide 234 with isobutene, in the presence of trifluoroacetic anhydride, yielded the to-alkylated product 238 (Scheme 59).128 It was suggested that this reaction proceeded by a different mechanism than the usual additive Pummerer mechanism. The alkene reacts with the electrophilic sulfur atom of intermediate 235, giving, after loss of a trifluoroacetate ion and a proton, the sulfonium ion 236. Thio-Claisen rearrangement of the ion then gives the thonium ion 237 which reacts with a further molecule of isobutene to give the product 238. 

The first report of asymmetric induction in the electrophilic addition to a,P-unsaturated sulfoxides was made by Abbott and Stirling [12,41] who found that treatment of nonracemic (5)-(-i-)-p-tolyl vinyl sulfoxide (26b) with bromine in acetic acid gave the dibromide (30) with ee (a-induction) of 32% and 92% overall chemical yield (Scheme 5.10). (+)-a,P-Methylvinyl p-tolyl sulfoxide gave a higher... 

There are numerous reports of the P-addition of organometallic reagents to racemic a,p-unsaturated sulfoxides [62,63]. For example, the Michael-type reaction of dialkylcuprates with p-chlorophenyl vinyl sulfoxide (65), followed by reaction of the initially formed a-sulfinyl carbanion (66) with electrophiles such as benzophenone, gave the products (67) (Scheme 5.21) [66]. 

The addition of electrophilic reagents to chiral a,/3-unsaturated sulfoxides is also accompanied by asymmetric induction. Stirling and Abbott (318,322) found that the addition of bromine to the optically active (.R)-vinyl-p-tolyl sulfoxide 319 yields a mixture of diastereo-meric a,/3-dibromosulfoxides 320. Oxidation of this mixture gives the optically active sulfone 321, with a center of chirality at the a-carbon atom only. The optical purity (32%) of this sulfone was estimated by comparing its specific rotation with that obtained as a result of oxidation of diastereomerically pure sulfoxide (/ )-320. The assignment of configuration at the a-carbon atom was based on the analysis of the polarizabilities of substituents. 

Many common reactions of aliphatic amines, ethers, and sulfides 1 involve initial addition of an electrophilic reagent utilizing the lone pair of electrons on the heteroatom salts, quaternary salts, coordination compounds, amine oxides, sulfoxides, and sulfones are formed in this way. Corresponding reactions are very rare (cf. Section 3.3.1.3) with pyrroles, furans, and thiophenes. These heterocycles react with electrophilic reagents at the carbon atoms 2, 3 rather than at the heteroatom. Vinyl ethers and enamines 4 show intermediate behavior, reacting frequently at the -carbon but sometimes at the heteroatom. 

The finding that thionium ions may serve as electrophiles in electrophilic substitution chemistry has greatly extended the synthetic range of the Pummerer reaction. Padwa and Kuethe used intramolecular versions of this process in the preparation of nitrogen-containing heterocycles. Vinyl amido sulfoxide 217 underwent an additive Pummerer reaction, on treatment with triflic anhydride, to yield product 220 (Scheme 56).123 The critical step in this transformation involves a... 

Chiral a,P-unsaturated sulfoxides 1.136 (Y = Tol, R = R CH=CH) also have been used in asymmetric synthesis. These compounds are prepared either by treatment of 1.137 with vinylic organometallic reagents, or from saturated precursors by classical chemical transformations [102, 173, 476, 484-487], Michael additions to these electrophiles are interesting only if R = CF3 [161], Organometallic additions or [4+2] cycloadditions require the introduction of a second electron-withdrawing substituent [73, 102], and acyclic 1.138 and cyclic 1.139 gem-di substituted sulfoxides have seen many interesting applications [101, 102,... 

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