Reaction with silane carbanions

Alternatively, some conclusions can be derived from the relative reactivities of car-banions. For example, DePuy and colleagues13 made use of a clever method involving reactions of silanes with hydroxide ion to deduce acidities of such weak acids as alkanes and ethylene. The silane reacts with hydroxide ion to form a pentacoordinate anion that ejects a carbanion held as a complex with the hydroxysilane rapid proton transfer gives the stable silanoxide ion and the carbon acid (equation 5). 

The intramolecular addition of carbon nucleophiles to alkenes has received comparatively little attention relative to heterocyclization reactions. The first examples of Pd-catalyzed oxidative carbocyclization reactions were described by Backvall and coworkers [164-166]. Conjugaled dienes with appended al-lyl silane and stabilized carbanion nucleophiles undergo 1,4-carbochlorination (Eq. 36) and carboacetoxylation (Eq. 37), respectively. The former reaction employs BQ as the stoichiometric oxidant, whereas the latter uses O2. The authors do not describe efforts to use molecular oxygen in the reaction with allyl silanes however, BQ was cited as being imsuccessful in the reaction with stabihzed car-banions. Benzoquinone is known to activate Ti-allyl-Pd intermediates toward nucleophilic attack (see below. Sect. 4.4). In the absence of BQ, -hydride eUm-ination occurs to form diene 43 in competition with attack of acetate on the intermediate jr-allyl-Pd" species to form the 1,4-addition product 44. 

Silylation of a carbanion obtained by enantiotopically differentiating deprotonation of the precursor carbamate in the presence of sparteine leads to the chiral silane 10 with >95 % ee by analogy with the reaction with other electrophiles29 (for analogous derivatives see Section D.8.). 

It is noted that Vedejs used the fluoride/silane carbanion reaction to generate a novel imidate ylid that was used in a [3+2]-cycloaddition reaction with ethyl acrylate (sec. 11.11.F). 23... 

As with the silanes, some of the most useful synthetic procedures involve electrophilic attack on alkenyl and allylic stannanes. The stannanes are considerably more reactive than the corresponding silanes because there is more anionic character on carbon in the C—Sn bond and it is a weaker bond.103 104 There are also useful synthetic procedures in which organotin compounds act as carbanion donors in palladium-catalyzed reactions, as discussed in Section 8.2.3 Organotin compounds are also very important in free-radical reactions, which will be discussed in Chapter 10. 

The [4 + 2] cycloaddition of a-phenylselenopropenoyl trimethylsilane (75) with 2,3-dimethylbuta-1,3-diene is unusual in that a significant portion of product mixture consists of the hetero-Diels-Alder dihydropyran adduct 76. The phenylselenenyl substituent appears to be responsible for this unusual pattern of reactivity, since propenoyl trimethylsilane gives only the expected regioisomer (77, X = H) (Scheme 116)14. a-Selenenyl substituted a,/l-unsaturated acyl silanes such as 75 were used to prepare a series of substituted dienes in excellent yields through the addition of a-sulphinyl carbanions, Brook rearrangement and expulsion of sulphinate, in a reaction pathway recognisably more typical of acyl silanes (Scheme 117). 

The reactive species in fluoride-mediated carbon-carbon bond-forming reactions has been investigated.146 The regio- and diastereo-selectivities of silanes reacting with cyclohexenone in the presence of a catalytic amount of fluoride have been compared with the reactivity of analogous solvent-separated lithium ion pairs. Closely analogous behaviour has been observed, showing that carbanions and not siliconate complexes are the reactive species in the fluoride-catalysed reactions. 

In 1968 Peterson published his results about the reaction of a-silylated carbanions with carbonyl compounds to achieve /3-hydroxyalkyl-silanes. He found that instantaneous elimination affords unsaturated hydrocarbons ( Peterson olefmation )180 Thus, the lithiated trimethyl-benzyl-silane (274) plus benzophenone (135) give via lithium l,l,2,-triphenyl-2-TMS-ethanolate (275) 1,1,2-triphenyl-ethene (276)180 ... 

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