Silylene insertion into allylic ethers occurs suprafacially to provide enantioenriched allylic silanes and disilanes. Silylene insertion to a variety of protected allylic alcohols utilizing silver- and copper-mediated conditions proved to be a general method for allylic silane formation. Allylic disilanes reacted similarly to allylic silanes, undergoing both annulation and allylation reactions.

Allylic ethers undergo insertions of silylenes into C-O bonds to form allylic silanes. Silylene insertion into C-O acetal bonds was also observed. Formation of silylene ylide intermediates led to [1,2]-Stevens rearrangement products as well as [2,3]-sigmatropic products depending upon the steric environment of the starting allylic ether.

Silylene transfer to allylic sulfides results in a formal 1,2-sulfide migration. The rearrangement yields substituted silacyclobutanes, not the expected silacyclopropanes. The silacyclobutanes were elaborated by insertions of carbonyl compounds selectively into one carbon–silicon bond. A mechanism for the 1,2-sulfide migration is proposed involving an episulfonium ion intermediate.

Silacarbonyl ylides, generated by metal-catalyzed silylene transfer to carbonyls, participate in formal intermolecular 1,3-dipolar cycloaddition reactions with carbonyl compounds and alkynes to form dioxasilacyclopentane acetals and oxasilacyclopentenes in an efficient, one-step process.The reactions of silylenes with carbonyl compounds have produced a number of mechanistically and synthetically useful reactions. Silylenes (1) react with aldehydes and ketones (2) to give either oxasilacyclopropanes 3 or silacarbonyl ylides 4 (Scheme 1), 1 and these isomeric products can be interconverted. 2 The silacarbonyl ylide can be intercepted by π-bonds to form products formally arising from 1,3-dipolar cycloadditions. 3 This reaction, which forms new carbon-carbon and carbon-silicon bonds, has not been found to be preparatively useful because yields are often low, even in intramolecular examples. 1a In addition, the generation of silylenes requires forcing conditions such as extended photolysis with excess reagents. 4In this Letter, we report that metal-catalyzed silylene transfer to carbonyl compounds in the presence of an electron-deficient alkyne produced formal 1,3-dipolar cycloaddition products efficiently under mild conditions. The products of these reactions possess functional groups that can be functionalized with control of regioselectivity.We discovered that silylene transfer to carbonyl compounds led to three-component cyclization reactions in our attempts to form oxasilacyclopropanes. Treatment of cyclohexene silacyclopropane 6 5 with two equivalents of benzaldehyde 5 and a catalytic amount of silver triflate did not yield the expected three-membered ring. Instead, it gave the product of silylene transfer to the C-O double bond and subsequent insertion of the second equivalent of aldehyde to form dioxasilacyclopentane 7 (Scheme 2). 6 A screen of metal salts known to catalyze silylene transfer to alkenes 7 did not lead to formation of oxasilacyclopropanes. When nbutyraldehyde (8) was subjected to the same catalyst screen, copper (II) bromide-catalyzed silylene transfer afforded the intermolecular 1,3-dipolar cycloaddition product, dioxasilacyclopentane acetal 9, in 89% yield (Scheme 2). A competition experiment between aldehydes 5 and 8 yielded dioxasilacyclopentane 7 as the sole product, suggesting that the difference in regioselectivity of the corresponding products 7 and 9 is caused by a divergence in the mechanistic pathways.

Abstract(-)-Isopulegol derivatives undergo a ring contraction under silylene-mediated conditions to provide cyclopentane products. Silylene transfer to other homoallylic ethers did not provide the ring contraction products. Allylic silane products were elaborated to determine the stereochemical course of the ring contraction reaction. A mechanism for the transformation is proposed.Cyclopentane units are found in many natural and non-natural products, including alkaloids, steroids, prostaglandins, triquinanes, and guaianes. [1][2][3][4][5][6] The ring contraction of a six-membered carbocyclic compound is an efficient way to assemble a cyclopentane, because the reorganization of the bonds can occur with high selectivity. [7][8][9][10][11][12] This reorganization leads to compounds not easily accessed by other syntheses. 13 In this Note, we report the ring contraction of six-membered carbocyclic homoallylic ethers to form five-membered carbocyclic allylic silanes by treatment with silylene intermediates.As part of our investigations into silylene transfer reactions to homoallylic ethers, 14 we examined the reaction of (-)-isopulegol derivative 1 under the optimized reaction conditions. Subjection of benzyl ether 1 to cyclohexene silacyclopropane 2 and AgO 2 CCF 3 did not provide the expected silylmethyl allylic silane product. Instead, the five-membered ring allylic silane product 3 was isolated as a single stereoisomer (eq 1).