Allyl vinylic thioethers

A route to allyl vinyl thioethers with defined E- or Z-geometry at the vinyl double bond has been reported (Scheme 46) the key step is reaction of a vinyl alanate (89) or (90), produced stereoseleCtively, with an allyl thiosulphonate and proceeds with retention of geometry at the alane carbon. The products are potential substrates for thio-Claisen rearrangements. 

In Entry 5, the carbanion-stabilizing ability of the sulfonyl group enables lithiation and is then reductively removed after alkylation. The reagent in Entry 6 is prepared by dilithiation of allyl hydrosulfide using n-bulyl lithium. After nucleophilic addition and S-alkylation, a masked aldehyde is present in the form of a vinyl thioether. Entry 7 uses the epoxidation of a vinyl silane to form a 7-hydroxy aldehyde masked as a cyclic acetal. Entries 8 and 9 use nucleophilic cuprate reagents to introduce alkyl groups containing aldehydes masked as acetals. 

For the addition of ethylene, EtOAc as solvent was particularly advantageous and gave 418 in 60% yield (Scheme 6.86). The monosubstituted ethylenes 1-hexene, vinylcyclohexane, allyltrimethylsilane, allyl alcohol, ethyl vinyl ether, vinyl acetate and N-vinyl-2-pyrrolidone furnished [2 + 2]-cycloadducts of the type 419 in yields of 54—100%. Mixtures of [2 + 2]-cycloadducts of the types 419 and 420 were formed with vinylcyclopropane, styrene and derivatives substituted at the phenyl group, acrylonitrile, methyl acrylate and phenyl vinyl thioether (yields of 56-76%), in which the diastereomers 419 predominated up to a ratio of 2.5 1 except in the case of the styrenes, where this ratio was 1 1. The Hammett p value for the addition of the styrenes to 417 turned out to be -0.54, suggesting that there is little charge separation in the transition state [155]. In the case of 6, the p value was determined as +0.79 (see Section 6.3.1) and indicates a slight polarization in the opposite direction. This astounding variety of substrates for 417 is contrasted by only a few monosubstituted ethylenes whose addition products with 417 could not be observed or were formed in only small amounts phenyl vinyl ether, vinyl bromide, (perfluorobutyl)-ethylene, phenyl vinyl sulfoxide and sulfone, methyl vinyl ketone and the vinylpyri-dines. 

Bromination of the enol ether product with two equivalents of bromine followed by dehydrobromination afforded the Z-bromoenol ether (Eq. 79) which could be converted to the zinc reagent and cross-coupled with aryl halides [242]. Dehydrobromination in the presence of thiophenol followed by bromination/dehydrobromination affords an enol thioether [243]. Oxidation to the sulfone, followed by exposure to triethylamine in ether, resulted in dehydrobromination to the unstable alkynyl sulfone which could be trapped with dienes in situ. Alternatively, dehydrobromination of the sulfide in the presence of allylic alcohols results in the formation of allyl vinyl ethers which undergo Claisen rearrangements [244]. Further oxidation followed by sulfoxide elimination results in highly unsaturated trifluoromethyl ketonic products (Eq. 80). 

Thioethers (sulfides)319 can be converted to their a-carbanion derivative (296, M = Li, Na, K) by treatment with strong bases such as -butyllithium, sodium amide (NaNH2) or potassium amide (KNH2). The hydrogen atom a-to the sulfur atom of sulfides is a weaker acid than that a-to a carbonyl, nitrile or nitro group and stronger bases are required for deprotonation. Sulfides are stronger acids, than the allylic, vinyl, and aromatic hydrocarbons discussed above, however. The various oxides of sulfur can also be converted to a-carbanions. The most important derivatives are the a-carbanions derived from sulfoxides (297). sulfones (298), and... 

Gold(I)-catalysed ring-opening of 3,3-disubstituted cyclopropenes (249) with thiols, thiophenols, and thioacids has been shown to occur regio- and chemo-selectively, producing either vinyl thioethers (250) or primary allylic thioesters (251), depending on the ligands coordinated to Au and the nature of the nucleophile. ... 

Ether groups, including vinyl ethers, allylic ethers, and thioethers are also compatible. 

Catalytic hydrogenation with platinum liberates the hydrocarbon from methylcobalamin (57) and from alkyl-Co-DMG complexes (161), but not from pentacyanides with primary alkyl, vinyl, or benzyl ligands, though the cr-allyl complex yields propylene (109). Sodium sand gives mixtures of hydrocarbons with the alkyl-Co-salen complexes (64). Dithioerythritol will liberate methane from a variety of methyl complexes [cobalamin, DMG, DMG-BF2, G, DPG, CHD, salen, and (DO)(DOH)pn] (156), as will 1,4-butanedithiol from the DMG complex (157), and certain unspecified thiols will reduce DMG complexes with substituted alkyl ligands (e.g., C0-CH2COOH ->CH3C00H) (163, 164). Reaction with thiols can also lead to the formation of thioethers (see Section C,3). 

This hypothesis has been verified starting from (Z)-y-iodoallyl ethers [109], amines [107], and thioethers [107] (Scheme 7-95) that are easily accessible from a propargyl ether [110] or ethyl propiolate [111]. In each case, the heteroatom chelates the vinylmetal to form a rigid five-membered ring and the allyl moiety approaches the vinyl one with a diastereofacial selectivity anti to the alkyl group [112]. 

The [2+2] cycloaddition reaction of ketenes with vinyl ethers and thioethers also occurs very readily. Even allyl ethers undergo this reaction. The reaction of diphenylketene with vinyl ethers is stereospecific, indicating a concerted one-step process . Also, dimethylketene and -MeOCH=CHMe affords a cycloadduct in which the alkene stereochemistry is maintained In contrast, the [2+2] cycloadduct obtained fl om t-butylcyanoketene and CH2=CHOEt or CH2=CHOAc did not give a 100% stereoselectivity and linear products are often also obtained, indicating the formation of a switter ionic intermediate. The latter are detected in the reaction of bis(trifluoromethyl)ketene with ethyl vinyl ether (see the General Introduction ). The initial reaction occurs across the C=0 bond of the ketene, which rearranges via switter ionic intermediates to form the cyclobutanone reaction product . 

By analogy with the catalytic allylic alkylation performed with C-nucleo-philes described in Section 7.2.2.3, Plietker et al. used benzyl mercaptan as an S-nucleophile toward an isobutyl carbonate in the presence of an NHC-Fe complex 9. Allylic thioethers were obtained in 82% yield as a mixture of two regioisomers [eqn (7.7)]. The same group also investigated the sulfonylation of allylic carbonates with a-sulfonyl succinimides as S-nucleop-hiles. In this case, a base-free catalytic system generated in situ by thermal release of the SIMes carbene from its chloroform adduct in the presence of [Bu4N][Fe(CO)3(NO)] was adopted. Addition of DBU to the reaction mixtures allowed the one-pot synthesis of vinyl sulfones through a tandem iron-catalysed allylic sulfonylation/amine-catalysed isomerisation process. Remarkably, base-induced decomposition of the iron catalyst was not observed. 

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