Alkyne hydrosilylation with cationic cyclopentadienylruthenium complexes: Scope, mechanism, and synthetic applications of a metal-catalyzed trans addition process

The discovery, investigation, and synthetic utility of an alkyne hydrosilylation reaction catalyzed by cationic cyclopentadienylruthenium complexes is described. The catalyst provides the first general Markovnikov hydrometallation of terminal alkynes that do not contain directing groups. The same complexes catalyze hydrosilylation of internal alkynes, where (Z)-vinylsilanes from a trans addition process are formed exclusively---the first observation of trans hydrosilylation of internal alkynes with a transition-metal catalyst. Tolerance of variously substituted silanes, including siloxanes, arylsilanes, and chlorosilanes is demonstrated. Regioselectivity is achieved for a variety of internal alkynes.; The unique selectivities demonstrated by cationic cyclopentadienylruthenium complexes are not supported by previously accepted hydrosilylation mechanisms. We present experimental evidence that a new mechanistic pathway is active, and present a proposal based on a computational study of concerted Si-H activation and alkyne insertion. The direct formation of an intermediate ruthenacyclopropene---rather than a traditional vinylruthenium---is also proposed. The mechanistic proposal explains the reactivity and selectivity observed in both intra- and intermolecular hydrosilylation reactions, and is the pathway most consistent with the observed results of all mechanistic postulates in the literature.; The utility of regio- and stereodefined vinylsilanes for target-oriented synthesis is studied through a variety of oxidative and non-oxidative pathways. A 1,2 Si → C migration of carbon atoms in electron deficient vinylsilanes is exploited in a new synthesis of tertiary alcohols bearing beta-carbonyl substituents. Selective oxidation pathways of vinylsilanes from both inter- and intramolecular hydrosilylation reactions are pursued to demonstrate the use of alkynes as surrogates for ketone and hydroxy ketone functionalities.; The hydrosilylation and vinylsilane-based methods described are brought to bear on the synthesis of the trisubstituted piperidine, (+)-spectaline, and on an approach to the natural product soraphen A. In both cases, silicon-based methodologies provide unique retrosynthetic insight to allow for highly efficient approaches to the targets. The use of the same cationic cyclopentadienylruthenium complexes to catalyze stereospecific C-C and C-X bond-forming processes with chiral allylic alcohols and their derivatives is also described, and these studies are applied to the synthesis of aryl ether pharmaceutical agents.