Stannyl halides reactions

Compound 211 and several related compounds are readily accessible by stereospecific deprotonation of the appropriate optically active carbamic esters with 5-BuLi/TMEDA ° . Much of the knowledge about the stereochemical course of substitution in benzyUithium derivatives was obtained from experiments with these compounds. Only the reaction with proton acids, aliphatic aldehydes, ketones or esters as electrophiles proceed with retention for alkyl, silyl and stannyl halides, acid chlorides. 

Thiophene undergoes metalation in the 2-position by reaction with -butylhthium. The resulting 2-lithiothiophene is susceptible to various electrophiUc reactions, for example, with alkyl, silyl, or stannyl halides ... 

The stannyl halides can be generated in situ by reactions of allylic halides with tin metal or with stannous halides. 

Reactions of tetrallylstannanes with aldehydes catalyzed by SnCU also appear to involve a stannyl halide intermediate. It can be demonstrated by NMR spectroscopy that there is a rapid-redistribution of the allyl group. ... 

Stannyl cuprates couple widi vinyl halides or trlGales [16c-d, 85], and a vinyl stannane produced diis way has been used in die syntliesis of 7-[f )-alkylidetie]-cepbalosporlns [117]. Vinyl substitution reactions slariing Grom ddiydrofciraiis are... 

Use of di-( -butyl)stannyl dichloride along with an acyl or silyl halide leads to addition of allylstannanes to the aldehydes.1728,172 Reaction is also promoted by butylstannyl trichloride.173 Both SnCl4 and SnCl2 also catalyze this kind of addition. 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

This generalized reaction sequence consumes the halide, the stannane, and the reactant X=Y, and effects addition to the organic radical and a hydrogen atom to the X=Y bond. The order of reactivity of organic halides toward stannyl radicals is iodides > bromides > chlorides. 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

Allylic stannanes are an important class of compounds that undergo substitution reactions with alkyl radicals. The chain is propagated by elimination of the trialkyl -stannyl radical.315 The radical source must have some functional group that can be abstracted by trialkylstannyl radicals. In addition to halides, both thiono esters316 and selenides317 are reactive. 

In the first systematic study on nucleophilic substitutions of chiral halides by Group IV metal anions, Jensen and Davis showed that (S )-2-bromobutane is converted to the (R)-2-triphenylmetal product with predominant inversion at the carbon center (Table 5)37. Replacement of the phenyl substituents by alkyl groups was possible through sequential brominolysis and reaction of the derived stannyl bromides with a Grignard reagent (equation 16). Subsequently, Pereyre and coworkers employed the foregoing Grignard sequence to prepare several trialkyl(s-butyl)stannanes (equation 17)38. They also developed an alternative synthesis of more hindered trialkyl derivatives (equation 18). 

Recent contributions from Vogel s group have shown that, under CO atmosphere and in the presence of Pd2(dba)3 and Ph3As, 1-stannyl glycals can be carbonylated and coupled to organic halides (Scheme 6a),38 or vinyl triflates (Scheme 6b),39 in carbonylative Stifle cross-coupling processes.40 Also of interest is the palladium catalyzed cross-coupling reaction of... 

Scheme 5 Palladium-catalyzed coupling reactions of C-l stannylated glycals with organic halides.

Stannyllithium compounds are important as sources of nucleophilic stannyl anions, and the dialkyltin lithium hydrides, R2SnLiH, have recently come to prominence as their reaction with electrophilic alkyl halides gives hydrides, R1R2SnH, with mixed alkyl groups (see Section 3.14.18.1).397... 

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