Protodestannylation of carbomethoxy-substituted vinylstannanes: Kinetics, stereochemistry and mechanisms

Abstract

Five carbomethoxy-and four carboethoxy-substituted vinylstannanes have been prepared. Methyl 2-(trimethylstannyl)acrylate and ethyl 2-(trimethylstannyl)acrylate were prepared by hydrostannation of methyl and ethyl propiolate under polar conditions. The former compound was also prepared by Pd(0) catalyzed hydrostannation of methyl propiolate. The E and Z isomers of methyl and ethyl 3-(trimethylstannyl)acrylate were prepared by free radical hydrostannation of methyl and ethyl propiolate. Methyl and ethyl 2-(trimethylstannyl)fumarate were synthesized by hydrostannation of dimethyl and diethyl acetylenedicarboxylate under polar conditions. Methyl 2-(trimethylstannyl)maleate was prepared by Pd(0)-catalyzed hydrostannation of dimethyl acetylenedicarboxylate. Structures were confirmed by and 13C NMR. The stereochemistry of stannyl cleavage was determined by deuteriodestannylation and XH NMR of the products. The methyl 3-(trimethylstannyl)acrylate isomers gave retention of configuration while the methyl 2-(trimethylstannyl)fumarate and -maleate resulted in approximately equal ratios of isomeric deuteriodestannylation products. In this latter case an allenol intermediate is proposed. Second-order rate constants for protodestannylation, in methanol-5 % water, were determined at three temperatures for the carbomethoxy compounds. Activation parameters were calculated from the rate data. The carbomethoxy group was found to be deactivating for all compounds except methyl 2-(trimethylstannyl)maleate. In this case, interaction of the syn carbomethoxy groups may serve to provide a more reactive route to the allenol intermediate.

Hydrostannylation of Propargylic Alcohols Using Mixed Tin Hydrides

“…Typical of the β anti isomer are the following NMR data: a) Tin chemical shift between Ϫ8 and Ϫ20 ppm b) 3 There is a complete change in the regiochemistry (in comparison with the terminal propargylic alcohols), the β-anti isomer almost always being the minor one. Under the free radical conditions used, attack is possible at either acetylenic carbon centre, and is no longer directed by the gain in radical stability due to intramolecular coordination between oxygen and tin.…”

“…For metal-catalyzed hydrostannylations control of stereoselectivity is not a problem, but control of regioselectivity can pose huge challenges. [6] Using tributyltin hydride, arguably the most important organotin reagent, regiocontrol is never a problem, while stereocontrol is however far from perfect [3] , due to isomerization of the reaction product.…”

Total Synthesis of (-)-Reveromycin A via a Hetero-Diels-Alder Approach

“…Under these optimized conditions, the E-vinyl stannane 31 was obtained in 56% yield. A small amount of C19 alkene was also isolated, which was formed by protodestannylation 29 The total synthesis of (-)-reveromycin A (1) has been achieved which utilizes a Lewis acid catalyzed inverse electron demand hetero-Diels-Alder reaction to synthesize the [6,6]-spiroketal core of this complex natural product. In addition, a modified high pressure succinylation was utilized to introduce the C18 succinate ester.…”

Trimethylstannane

“…7 Radical hydrostannation typically affords a mixture of the cis-β-and trans-β-products, with the latter predominating. 4 (7) Reaction of trimethyltin hydride with a cuprate in THF at −78 • C forms the stannylcuprate, which will undergo conjugate additions. The stereochemistry of the product can vary with the substituents on the alkyne.…”