Silacycles strained

Leighton has combined this concept of strained silacycles [62-66] with the asymmetric allylation chemistry in a series of pubhcations [60, 67-70], Leighton s aUyhc silacyclopentane 29 [67] (Scheme 20) works equally for allylation of aromatic and ahphatic aldehydes in the absence of additional Lewis bases (promoter activator) or Lewis acids with high yield and enantioselectivity. The mechanism of... 

VII. STRAINED SILACYCLES A. Silacyclopropyne and Other C2H2Si Isomers... 

Tandem intramolecular silylformylation-allylation reaction of diallylhydrosilyl ethers derived from homoallyl alcohols is convenient for rapid, stereoselective synthesis of 1,3,5-triols convertible to more oxygen-functionalized compounds (Scheme 12).142,142a,142b 143 [ he second uncatalyzed allylation step would be facilitated by the formation of a strained silacycle intermediate, which has enough Lewis acidity to activate the formyl group. A similar tandem reaction via alkyne silylformylation has been reported.144... 

Weyenberg and Nelson (7) reported that the strained silacycles, 1,1-di-methyl-1-silacyclobutane (1) and l,l,3,3-tetramethyl-l,3-disilacyclobu-tane (2) undergo ring opening polymerization in the presence of platinum on carbon or chloroplatinic acid [Eqs. (11) and (12)]. 

Enantioselective Friedel-Crafts alkylation of A -benzylpyrrole and 5-nitroindole with benzoylhydrazones 504 promoted by a simple strained silacycle reagent 505 gives 2- and 3-derivatives 506 and 507, respectively (Scheme 102) <2005JA2858>. Indole itself provides the corresponding product with significantly reduced enantioselectivity (ee 24%). 

Strained silacycles can be used in a ring opening polymerization (ROP) to yield linear polysilanes. 

Extending the application of his strained silacycle reagents, Leighton et al. published a procedure for the enantioselective Friedel-Crafts alkylation with benzoyl hydrazones, catalyzed by a simple chiral silane Lewis acid. The enantiopure silanes were prepared in bulk in a single step from (S,S)- or (k,k)-pseudoephedrine and PhSiCls. After use in the catalytic reaction the pseudoephedrine was recovered in nearly quantitative yield during the workup. Scheme 16.16 shows the best example, in which by employing enantiopure silane 23 a 92% yield and 90% ee of the product was obtained in 48 h [62]. 

Silyl enol ethers containing a strained silacycle and an allylic group are designed to undergo tandem aldol-allylation reactions. The net transformation is understood by... 

Kinnaird JWA, Ng PY, Kubota K, Wang X, Leighton JS. Strained silacycles in organic synthesis a new reagent for... 

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