Elimination reactions of .alpha.,.beta.-dihydroxysilanes: stereospecific synthesis of silyl enol ethers from vinylsilanes

Abstract

a,P-Dihydroxysilanes were prepared from vinylsilanes by osmium tetroxide catalyzed hydroxylation, and in a few cases by acid-catalyzed hydrolysis of the corresponding a,@-epoxysilanes. When a,@-dihydroxysilanes are treated with base, elimination reactions occur via both a-and 8-oxidosilanes. With sodium hydride in ether, the a-oxidosilane pathway leading to silyl enol ethers is nearly the exclusive process. This reaction is shown to be highly stereospecific and preferentially anti. The overall process (osmium tetroxide catalyzed hydroxylation followed by treatment with sodium hydride in ether) provides a synthetically useful method for conversion of vinylsilanes to silyl enol ethers with overall retention of double-bond configuration.Silyl enol ethers have become one of the more useful and versatile functional groups available to the synthetic organic chemist.' In their initial applications to synthesis, their stereochemistry ( E / Z ) was of little importance. However, in recent years a number of applications have been developed in which this stereochemistry is crucial. One topic of considerable current interest is the use of silyl enol ethers2 (or other enol derivatives such as metal enolates) of defined stereochemistry in aldol condensations and related processes for the synthesis of acyclic molecules of defined stereochemistry? Silyl enol ethers can also be used in stereospecific vinyl substitution reactions with organometallic reagent^,^ and in various cycloaddition reactions.' Many methods exist for the preparation of silyl enol ethers (and generation of metal enolates), but few provide stereochemical control. Although some types of silyl enol ethers can be obtained in high isomeric purity, there is a need for general methods for preparing these intermediates with known, predictable stereochemistry.For some time we have been developing a new method for preparing heteroatom-substituted olefins of known stereochemistry using organosilicon compound^.^ This method is based on the finding that a,@-epoxysilanes undergo regio-and stereospecific (Y opening by a variety of nucleophiles to produce diastereomerically pure /3-hydroxy~ilanes,~,~ and that these P-hydroxysilanes undergo stereospecific syn or anti 8-elimination reactions under basic or acidic conditions, respe~tively.~,' (See Scheme I.)With a view toward extending this method to the generation of stereoisomerically pure enolate anions and silyl enol ethers, we (1) For reviews, see: (a) Rasmussen, J. K.

Preparation of Functionalized Acylsilanes by Diol Cleavage of Cyclic 1,2‐Dihydroxysilanes

“…The spectra matched previously reported data. [17,26] (15). The spectra matched previously reported data.…”

“…To find optimal reaction conditions for the diol cleavage of 1,2‐dihydroxysilanes, we chose the 6‐membered cyclic cis diol 3a , as a model substrate. As depicted in Scheme , 3a was synthesized from the commercially available tosylhydrazone 1a by formation of vinylsilane 2a by using Shapiro reaction and subsequent cis dihydroxylation using catalytic amounts of osmium tetroxide and N ‐methyl‐morpholine N ‐oxide (NMO) .…”

An Expedient and Stereoselective Synthesis of Alkenyl Nonaflates from Silyl Enol Ethers: Optimization, Scope and Limitations

“…The starting silyl enol ethers were obtained by the standard procedures described in the literature. 1‐Trimethylsiloxynon‐1‐ene ( 1k , E / Z = 93:7),36 and 1‐trimethylsiloxyoct‐1‐ene ( 1j , E / Z = 25:75)37 were synthesized stereoselectively as described in the literature. The latter compound was further enriched up to 4:96 E / Z ratio by a kinetic resolution exploiting the considerably higher reactivity of the E isomer in a hetero‐Diels−Alder reaction 38.…”

Silicon‐Directed Rhenium‐Catalyzed Allylic Carbaminations and Oxidative Fragmentations of γ‐Silyl Allylic Alcohols

“…The very small kinetic isotope effect suggests there is no formation of an allylic cation prior to the rate‐limiting step and instead CSi oxidation takes place in a Flemming–Tamao‐type reaction. This could occur either through oxidation of the Si atom followed by C‐migration, as in the Flemming–Tamao, or by an epoxydation of the vinyl silane followed by rearrangement 24. The former is much more likely as no intermediates were ever detected and a very clean reaction occured with the remainder of the mass balance containing starting allylic alcohol.…”

“…This could occur either through oxidation of the Si atom followed by C-migration, as in the Flemming-Tamao, or by an epoxydation of the vinyl silane followed by rearrangement. [24] The former is much more likely as no intermediates were ever detected and a very clean reaction occured with the remainder of the mass balance containing starting allylic alcohol. Modulating the silane used to less oxidizable groups led to poor conversions, which indicates a catalytic oxidative Flemming-Tamao reaction with air as the terminal oxidant.…”