Silacarbonyl ylides

One novel and interesting method of generating a silacarbonyl ylide occurred through the addition of a carbonyl species with a silylene formed under photolytic conditions. Komatsu and co-workers (177) found that photolysis of trisilane (315) in solution with a bulky carbonyl species led initially to the formation of a silacarbonyl ylide followed by a dipolar cycloaddition of an olefinic or carbonyl substrate. Reaction of simple, nonbulky aldehydes led to only moderate yields of cycloadduct, the siladioxolane. One lone ketone example was given, but the cycloadduct from the reaction was prepared in very low yield (Scheme 4.89). 

The only exception to these uniform spectroscopic properties of silylene-Lewis base complexes, i.e., the hypsochromic shift on complexation, is silacarbonyl ylide 27, the absorption maximum of which is shifted to wavelengths longer than that of free dimesitylsilylene (20a) 31 an explanation of this observation is yet to come. 

Carbonyl and thiocarbonyl complexes of Mes2Si have been observed in a remarkable series of experiments by Ando and coworkers144. Photolysis of Mes2Si(SiMe3)2 was carried out at 77 K in a soft (3-MP/isopentane) matrix in the presence of tetramethyl-2-indanone or its sulfur analog. The free silylene was formed initially but it slowly reacted with the carbonyl (or thiocarbonyl) compound to give the complex, which can be formulated as a silacarbonyl ylide (Scheme 8). Further photolysis converted these ylide complexes to the silaoxirane or silathiirane, reversibly. 

Woerpel and Calad tested for the formation of the silacarbonyl ylide by interrogating the behavior of the electrophilic silver silylenoid intermediate 115 toward a,(3-unsaturated carbonyl compounds (Scheme 7.37).82 They hypothesized that formation of silacarbonyl ylide 131 might trigger a 6jt-electrocyclization to form oxasilacyclopentene 132. As anticipated, exposure of cyclohexene silacyclopropane 58 to substoichiometric amounts of silver trifluoroacetate in the presence of a,(3-unsaturated carbonyl compounds 130 produced oxasilacyclopentenes 132. The reaction tolerated a substitution at the a and/or (3 position and was general for both esters and ketones. 

In the absence of an internal olefin, silver-catalyzed di-tcrt-butylsilylene transfer to aldehydes did not produce an oxasilacyclopropane (Scheme 7.44).117 Instead, the reaction produced a dioxosilacyclopentane 161. Woerpel and Bourque attributed the formation of 161 to the intermediacy of silacarbonyl ylide 159. Subsequent intermo-lecular 1,3-dipolar cycloadditionwith a second molecule of aldehyde would generate the observed product. On the basis of this mechanistic hypothesis, Woerpel and Bourque sought to trap the silacarbonyl ylide intermediate with an acetylenic dipolarophile to generate an oxasilacyclopentene. To achieve the formation of 162, the dipolarophile would have to react with 159 faster than the aldehyde did. 

Scheme 7.46. Scope of silver-catalyzed silacarbonyl ylide cycloaddition.

The a-ketoester 134 participates in a novel metal-catalyzed silylene transfer in the presence of silacyclopropane to generate silacarbonyl ylide 137 which undergoes 6 r-electrocyclization to provide the silylketene acetal 138. Subsequent [3,3]-sigmatropic rearrangement proceeds via the chair-like... 

Photolysis of silicon-silicon bonds generates silylenes, which add to carbonyl compounds to give silacarbonyl ylides. These undergo 1,3-dipolar cycloadditions in low to excellent yields. ... 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

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