Carbon nucleophiles silanes

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. 

Acyl silanes can display disparate behaviour when treated with carbon nucleophiles, even of related types5,61149. For example, when aroyl silanes were treated with a Wittig reagent, none of the expected alkenes was obtained, and the only reaction products isolated were silyl enol ether and triphenylphosphine (Scheme 73)182,183. When alkanoyl silanes were treated with Wittig reagents, however, only the normal olefinated vinyl silane products were isolated (Scheme 74)182-184 Under soluble lithium salt conditions, Z-vinyl silanes were produced with very high selectivities the reaction was used to prepare a pheromone component (50) of the sweet potato leaf folder moth (Scheme 75)183. 

Allyl silanes are a species of carbon nucleophile that has performed well in combination with oxidative dearomatization induced by hypervalent iodine reagents [65]. Inter- and intramolecular additions of allyl silanes to putative phenoxonium cations have been reported, and the latter reaction type provided a key transformation in the synthesis of the potent antibiotic (-)-platensimycin (Scheme 15.23) [66]. 

The trapping of these cations with carbon nucleophiles has made them useful for synthesis. The carbon nucleophiles must be stable to the mildly acidic conditions used to generate the cations. Examples are trisubstituted alkenes, in an intramolecular fashion, allyl silanes (Scheme 6.130) and pyrrole. The ti -alkylcobalt complex 6.354 produced may be converted to an alcohol 6.355 by free radical methods as the carbon-cobalt bond undergoes photochemical homolysis. 

Reactions at acid centers to create 5 -2-(trimethylsilyl) ethanethiolesters are also common. Carboxylic 5-thiolesters are formed in high yield by the DCC/DMAP mediated coupling of 2-(trimethylsilyl)ethanethiol and carboxylic acids or by thiol substitution on a carboxylic acid chloride. 5 -2-(Tiimethylsilyl) ethyl p-toluenethiolsulfonate is formed by treating 2-(trimethyl-silyl)ethanethiol with tosyl bromide (eq 4). The product is a useful electrophile for carbon nucleophiles, allowing the introduction of the 2-(trimethylsilyl)ethylthio unit by an alternative mechanism. Independent of the thiol, aryl and alkyl 2-trimethylsilylethyl thioethers may be prepared by the radical addition of the appropriate arene- or aikanethiol to vinyl trimethyl-silane in a reaction comparable to that of eq l7 ... 

Silicon centered radicals can be generated by transfer to silanes and by photolysis of polysilanes. Rate constants for addition to monomer are several orders of magnitude higher than similar carbon centered radicals.453,43 The radicals have nucleophilic character. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

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