Balancing protection and activation: Silyl ethers and dicobalt complexes as controlling elements in organic synthesis

Abstract

<p>The tris(trimethylsilyl)silyl (sisyl) ether is photolabile, and possesses the potential to be used as a hydroxyl protecting group which can be photolytically cleaved selectively, in the presence of other silyl ethers. In order to compare its hydrolytic stability with that of a t-butyldimethylsilyl (TBS) ether, the rates of acidic hydrolysis of a series of sisyl and TBS ethers were determined under pseudo-first-order conditions. The sisyl ethers were found to be more hydrolytically stable in CDCl3 containing p -TsOH·H 2 O, but less hydrolytically stable in a AcOH/THF/H2 O solvent mixture. The hydrolysis studies were extended to the corresponding series of alkoxyallyl- t -butylmethylsilanes, which were determined to be more hydrolytically stable than either the sisyl or the TBS ethers. The ability of the alkoxyallyl-t-butylmethylsilanes to function as both allylsilanes (undergoing silicon-carbon bond cleavage), and alkoxysilanes (undergoing silicon-oxygen bond cleavage), was examined under conditions that could lead to both processes. In the presence of Brønsted acids, silicon-oxygen bond cleavage occurred exclusively. Electrophilic substitution of the allyl group was only observed in the presence of the very sterically hindered triphenylcarbenium ion. Allylsilanes were also originally envisioned as ideal nucleophilic partners for the development of a method of forming carbon-carbon bonds at the propargylic carbon atom of dicobalt-complexed acetylenic aldehydes or ketones, diastereoselectively. Allyl transfer onto racemic mixtures of chiral dicobalt pentacarbonyl(triarylphosphine)-complexed acetylenic aldehydes, leading to the formation of the hexa-1,5-enyne system, was accomplished with modest diastereoselectivity.</p>