Synthesis and characterization of a free phenylene bis(N-heterocyclic carbene) and its di-Rh complex: Catalytic activity of the di-Rh and CCC-NHC Rh pincer complexes in intermolecular hydrosilylation of alkynes

1,3-Bis(3-butylimidazolium-1-yl)benzene diiodide (1) was reacted with Li(2,2,6,6-tetramethylpiperidine) yielding the free biscarbene, 1,3-bis(3-butylimidazol-2-ylidene-1-yl)benzene (3), which has been spectroscopically characterized. Combining the free bis-carbene with [Rh(COD)Cl](2) yielded the corresponding di-Rh bis(N-heterocyclic carbene) complex (4) that was structurally characterized. The di-Rh bis-carbene complex was found to exhibit complex solution C-13 and H-1 NMR spectra that have been assigned as a mixture of diastereomers. The crystal structure of the di-Rh bis-carbene compound 4 was composed of a pair of enantiomeric atropisomers. The diastereomeric atropisomers were assigned as the source of the spectral complexities. The di-Rh di-carbene complex 4 and the CCC-NHC Rh pincer complex 2 were applied as catalysts in hydrosilylation reactions of terminal and internal alkynes. Both catalysts are highly active, regioselective, stereo selective, and chemoselective: terminal alkynes give predominantly the beta-(Z) isomer and internal alkynos afford the beta-(E) isomer in chloroform or benzene. One of the strongest attributes of the catalyst systems is that the results were achieved without exclusion of air and without purification of commercially available reagents. (c) 2005 Elsevier B.V. All rights preserved.