Michael Karras

Alkylaluminum halides : Lewis acid catalysts which are brønsted bases

Abstract Alkylaluminum halides react with Bronsted acids to liberate an alkane and generate a new Lewis acid. Using these reagents, Lewis acid catalyzed reactions can be run under aprotic conditions, even when acidic protons are produced in the reaction. The use of these reagents for Lewis acid catalyzed ene, Diels-Alder and cycloaddition reactions and Claisen rearrangements is described. These reagents are also useful initiators for cation-olefin addition reactions. In some cases the alkyl groups react as nucleophiles. While this is often undesirable, addition of an alkyl group to carbenium ion intermediates provides novel classes of compounds.

Nickel-catalyzed carbometallation of functionalized silylalkynes by grignard reagents

Abstract Syntheses of vinylsilanes by the nickel catalyzed addition of methyl-magnesium bromide to functionalized silylalkynes are described. Simple syntheses of geraniol and farnesol are reported.

Stereocontrolled carbotitanation of alkynylsilanes

Abstract Carbometalation of alkynylsilanes with 1 1 titanocene dichloride/dialkylaluminum chloride is stereospecifically cis. The alkenyltitanium intermediate can be stereospecifically protonated by aqueous sodium hydroxide or iodinated. Addition of any Lewis base to the alkenyltitanium intermediate induces decomposition of this intermediate to give an alkene by hydrogen atom abstraction from the medium, with loss of stereochemistry.