Intermolecular Silacarbonyl Ylide Cycloadditions: A Direct Pathway to Oxasilacyclopentenes

Abstract

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.

Silver‐Catalyzed Silylene Transfer

“…109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides 115 to afford synthetically useful products. 82,116,117 While exploring the scope of silver-catalyzed silylene transfer to olefins, Woerpel and Calad observed that the electrophilic silver silylenoid species reacted preferentially with the enolizable ester group to produce vinylsilane 129 instead of a an allyl substituent (e.g., 130h) to reaction conditions facilitated an Ireland-Claisen rearrangement after the initial electrocyclization. This process was highly diastereoselective; the reaction of ester 130h produced silalactone 135 with three contiguous stereocenters as a single isomer.…”

“…In the absence of an internal olefin, silver-catalyzed di-tert-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.…”

“…117 While inclusion of terminal acetylenes resulted in preferential silacyclopropene 99c or dioxosilacyclopentane 161 formation, the desired oxasilacylopentene (163) was obtained when diethylacetylene dicarboxylatewas added to the reaction mixture. Substitution of the carboethoxy groups with methyl and phenyl groups, unfortunately, resulted in dioxosilacyclopentane 161 formation.…”

Reactions of an Isolable Dialkylsilylene with Ketones

“…Although 2 , 7 , and 10 are all 1:2 cycloadducts, the ring structure of 7 and 10 is isomeric to that of 2 . Recently, a number of interesting and useful synthetic reactions via the addition of dimesitylsilylene and di- tert -butylsilylene to various carbonyl compounds have been explored.…”

Nucleophilic Silylenoid Character of Stable Phosphonium Sila–ylides

“…[7] In contrast, the reaction with carbonyl compounds, leading to silaoxiranes, does not proceed through a [2 + 1] cycloaddition, but rather involves the transient formation of a silacarbonyl ylide, [8] which can be trapped by dipolarophiles, such as olefins, to afford the corresponding oxasilacyclopentanes. [9] Although several reactions of silylene-base adducts (base: imine or amine) with diphenylacetylene, affording 1,2-disilacyclobutene derivatives (2:1 silylene/acetylene adduct) have been described, in the case of donor-stabilised silylenes direct observation of [2 + 1] cycloaddition reactions has never been achieved. [10] Herein we report clear experimental and theoretical evidence for [2 + 1] cycloadditions involving a phosphine-coordinated silylene, the stable phosphonium sila-ylide 1.…”