Probing the mechanism of rhodium(I) catalyzed dehydrocoupling of di-n-hexylsilane
Date
2008-05-22T20:15:48Z
Authors
Jackson, Sarah Marie
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Abstract
The mechanism of the rhodium(I) catalyzed dehydrocoupling of di-n-hexylsilane was investigated by isolating and characterizing products of stoichiometric reactions, by preparing a series of rhodium(I) phosphine precatalysts and silane substrates and measuring catalytic activity, and by observing catalytic reaction mixtures directly using 31P{ 1H} NMR spectroscopy. Catalyst initiation was found to occur via oxidative addition of silane to a rhodium centre, followed by reductive elimination of a chlorosilane generating an unsaturated rhodium hydride complex, the putative active catalyst fragment. The series of precatalysts screened for catalytic activity include [Rh(PPh3)3Cl] (1), [Rh(PPh3)2(µ-Cl)]2 (2). [Rh(dppe)(µ-Cl)]2 (3) [Rh(dppb)(µ-Cl)]2, (4) [Rh(COD)(µ- Cl)]2 (5), [Rh(PPh3)3H] (6), [Rh(PPh3)4H] (7), [Rh(xantphos)(COD)(Cl)]. (8). Of these eight precatalysts the two hydride complexes (6 and 7) displayed the highest catalytic activity. The flexibility of chelating bis(phosphine) ligands was found to be important for catalyst activity. Of the chelating bis(phosphine) rhodium complexes (3, 4, and 8), 4 displayed the highest catalytic activity. The catalyst resting state for all precatalysts containing the monodentate PPh3 ligand was found to be trans-[Rh(PPh3)2LX] when catalytic reactions were observed in situ by 31P{1H} NMR spectroscopy. The substitution and steric bulk of the silane substrate was found to influence reactivity as well. Of the substrates studied, the least bulky primary n-hexylsilane was most reactive. By monitoring the reaction over by 31P{1H} NMR spectroscopy, decomposition of the catalyst was found to involve phosphine dissociation. Decomposition was found to occur more slowly for precatalysts containing chelating bis(phosphine) ligands than for precatalysts containing monodentate phosphine ligands.
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Keywords
Silicon polymers, Rhodium catalysts