Lithium aryls, reaction with sulfoxides

This procedure was extended to a method for asymmetric synthesis of optically active epoxides starting from optically active sulfoxides. As the oxiranyUithium 131 reacts with the acidic hydrogen of the n-butyl aryl sulfoxide, the introduction of electrophiles to the reaction mixture was problematic. Therefore, the reaction was performed by addition of 1 equivalent of f-C4H9Li at — 100°C to 130 and the sulfoxide-lithium exchange reaction was found to be extremely rapid (within a few seconds at this temperature). Moreover, as f-butyl aryl sulfoxide 138 has now no more acidic hydrogen, the addition of several electrophiles leads to functionalized epoxides 139 (equation 48). ... 

It is known that on treatment of alkyl aryl sulfoxide 1 with alkylmetal (alkyl-lithium or Grignard reagent) sulfur-aryl (path a) or sulfur-alkyl (path b) bond cleavage occurs to give arylmetal 2 or alkylmetal 3 (Scheme 3.9). This reaction is commonly called the hgand exchange reaction of sulfoxides. Although the predominant path of this reaction depends on the structure of the sulfoxide 1, this dependence remains somewhat obscure at present. 

The few examples indicate that sulfone-stabilized carbanions should react normally with electrophilic animating reagents (Eqs. 145158 and 146465) with the caveat that free a-amino sulfones are unstable.158,465 The (3,y-unsaturated sulfone 74 is animated at the y-position (Eq. 147),250 presumably by an ene reaction. The preparation of a-tosyl azides from nitronates was shown above in Eq. 144. The scope of this reaction does not seem to have been determined. Reaction of the anions of nitrobenzyl aryl sulfones with l-oxa-2-azaspiro[2.5]octane (13a) gives nitrobenzaldehydes by cleavage of the initially formed amination products.466 Similarly, reaction of the lithium salt of benzyl phenyl sulfone with phenyl azide gives benzilydeneaniline and phenyl sulfinate.467 No reports on animations of sulfoxide-stabilized carbanions were found. 

Good yields of pyranoid glycals were obtained from acetal or ether protected D-gluco-, T)-galacto- or p-mamo- configured glycosyl sulfoxides by reaction with aryl- or alkyl-lithiums. It was determined that the nature of the lithium reagents... 

Lithium and zinc tert-butyl phenylmethyl sulfoxide (1) and A-phenyl imines 2, in which the substituent R is alkenyl or aryl, react at —78 °C over 2 hours with high anti diastereoselection (d.r. >98.5 1.5)6. Shorter reaction times result in poorer yields, due to incomplete reaction. In contrast, the reaction of the sulfoxide anion with benzaldehyde is complete after 5 seconds, but shows poor diastereoselection. When the substituent R1 or R2 of the imine 2 is aliphatic, the substrates exhibit poor chemical reactivity and diastereoselection. The high anti diastereoselection suggests that if a chelated cyclic transition state is involved (E configuration of the imine), then the boat transition state 4 is favored over its chair counterpart 5. 

The aryl sulfoxide moiety may serve as a good leaving group in the exchange reaction. Thus, 1-haloalkenyl sulfoxide 55 undergo the exchange at —78°C to give carbenoid compounds 56 which can be trapped by electrophiles or converted to acetylenes 57 (equation 39) °. Reaction of carbenoid 58 with lithium acetylides leads to the formation of enynes 59 (equation 40). ... 

For example, as shown in equation 43, Taguchi, Nozaki and coworkers reported in 1974 a one-carbon ring enlargement of cyclododecanone (187) to cyclotridecanone (190) with dibromomethyllithium through / -oxido carbenoid (188) . This reaction was expected to proceed via a one-carbon expanded enolate (189). Cohen and coworkers used the bis(phenylthio)methyllithium whereas Satoh and coworkers used a-sulfinyl lithium car-banion of 1-chloroalkyl aryl sulfoxides as the source of S-oxido carbenoids (equation 44) °. 

Previously, Andersen et al. had attempted to prepare sulfuranes from the reactions of diaryl sulfoxides with aryl Grignard reagents and aryl lithium agents. They reported that the reactions give initially the corresponding triaryl sulfoni-um salts, from which two pathways, namely the formation of tetraaryl sulfuranes and the formation of benzyne, take place simultaneously, on the basis that they obtained diaryl sulfides and biaryls derived from benzyne, as shown in Scheme 7 [37]. 

In order to confirm the formation of these hypervalent compounds directly, H and 13C NMR spectra of the reaction mixtures of sulfoxide or selenoxide with PhLi were measured at -80 °C. However, neither sulfurane nor selenurane was detected. The spectra were consistent with those of a mixture of disproportion-ated sulfoxides (selenoxides) and aryl lithiums [46]. 

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