John Whelan

Homogeneous catalysis. transition metal based lewis acid catalysts.

Abstract Transition metal based Lewis acids provide catalysts for the Diels-Alder and Mukaiyama reactions. These catalysts must possess an electron deficient axophilic metal center and a labile coordination position. Unlike traditional Lewis acids, those derived from transition metals can function in the presence of water and have well defined structures. It is shown how a normally electron rich ruthenium atom can be converted to a Lewis acid by incorporation of electron withdrawing ligands and ligands with hard donor atoms. This ruthenium complex, [Ru(salen)(NO)(H2O)]+, is an efficient catalyst for the Diels-Alder reaction, but in the Mukaiyama reaction, it tends to be reduced and thereby deactivated by the silyl enol ether. It is shown that the complex [TiCp*2(H2O)2]2+ (Cp* is pentamethylcyclopentadienyl) is an efficient catalyst for the Diels-Alder reaction even when water is present. Similarly, the triflato complexes [TiCp2(CF3SO3)2] and [ZrCp2(CF3SO3)2] (Cp is cyclopentadienyl) are efficient catalysts for both the Diels-Alder and Mukaiyama reactions. All of these catalysts are effective at very low loadings of ≈ 1 mol%. Catalysis has been shown to occur via substrate-catalyst adducts and moreover these adducts are formed rapidly and reversibly as required for efficient catalysis.

Homogeneous catalysis. Use of the [TiCp2(CF3SO3)2] catalyst for the sakurai reaction of allylic silanes with orthoesters, acetals, ketals and carbonyl compounds.

Abstract The effectiveness of the [TiCp 2 (CF 3 SO 3 ) 2 ] catalyst for the Sakurai reaction has been explored for a variety of allylic silanes reacting with orthoesters, acetals, ketals, aldehydes and ketones.

Homogeneous catalysis. A transition metal based catalyst for the Mukaiyama crossed-aldol reaction and catalyst deactivation by electron transfer.

Abstract The cationic complex [Ru(salen)(NO)H 2 O]SbF 6 is intrinsically a powerful catalyst for the Mukaiyama crossed-aldol reaction at 25°C in nitromethane solutions and at very low catalyst loadings but, for some reactions, electron transfer from the silyl enol ether to the ruthenium catalyst can occur which leads to catalyst deactivation.

Asymmetric catalysis. Asymmetric catalytic intramolocular hydrosilation and hydroacylation

Abstract Catalyst of the type [Rh(chiral Diphosphine)]+ efficiently catalyse the intramolecular hydrosilation of silyl ethers derived from allylic alcohol. The products can be converted to chiral 1,3-dols. High enantiomeric excesses (ees) are observed for substrates bearing an aryl group at the olefin terminus. These same catalyst produce chiral cyclopentanones from 4-substitued 4-pentenals. Tertiary acyl and ester substituents give nearly quantitative ees. The mechanism of hydrosilation is inferred to involve silyl olefing insertion, whereas the key step in hydroacylation probably involves reductive elimination of the metallacyclohexanone intermediate.