C-C Bond Activation of a Cyclopropyl Phosphine: Isolation and Reactivity of a Tetrameric Rhodacyclobutane

Reaction of the functionalized phosphine (PBu2CH2)-Bu-t(C3H5) with [Rh(C8H14)(2)Cl](2) (C8H4 = ciscyclooctene) resulted in selective C C bond activation of the cyclopropyl group and formation of a tetrameric rhodacyclobutane, [RhCl(kappa(3)-(PBu2CH2CH)-Bu-t(CH2)2)]4, which was characterized in the solid state by X-ray crystallography. This complex acts as a latent source of the [Rh(kappa(3)-(PBu2CH2CH)-Bu-t(CH2)(2))](+) fragment, forming a range of new complexes by salt metathesis with NaCp, [RhCp(kappa(3)-(PBu2CH2CH)-Bu-t(CH-chloride abstraction in the presence of arenes, [Rh(eta(6)-arene)(kappa(3)-(PBu2CH2CH)-Bu-t(CH2)2)][BAr4F] [arene = C6H5F, C6H3Me3; Ar-F = 3,5-C6H3(CF3),], or fragmentation by addition aphosphine ligands, trans-[RhCl(PR3)(kappa(3)-(PBu2CH2CH)-Bu-t(CH2)(2))] (R = Bu-t, Cy). In contrast, reaction with carbon monoxide results in the reductive elimination of the tethered cyclopropane, demonstrating reversible C C bond activation, and formation of cis-[RhCl(CO)(2)((PBuCH2)-Bu-t(C3H5))]. These complexes were characterized in solution by NMR spectroscopy and in the solid state by X-ray diffraction. Chloride abstraction from [RhCl((BBu3)-Bu-t)(kappa(PBu2CH2CH)-P-3-Bu-t(CH2)(2))] resulted in the regioselective isomerization of the tethered cyclopropane to a terminal alkene, viz., the formally 14-electron complex [Rh((PBu3)-Bu-t)(kappa(3)-(PBu2CH2CH2CH)-Bu-t=CH2)][BAr4F], notable for the presence of a strong agostic interaction with a phosphine Bu-t substituent, apparent both from the solid-state structure and in solution by H-1 NM R spectroscopy. Addition of hydrogen to this complex resulted in hydrogenation of the alkene and formation of [Rh(H)(2)((PBu3)-Bu-t)((PBu2Bu)-Bu-t-Bu-II)][BAr4F]. These steps overall correspond to selective C C bond frinctionalization (hydrogenation) of the tethered cyclopropane and are of direct significance to a related catalytic process [J Am. Chem. Soc. 2003, 125, 886] and, more generally, to the rhodium-mediated transformation of alkalies.