C−C bond activation of a cyclopropyl phosphine : isolation and reactivity of a tetrameric rhodacyclobutane

Reaction of the functionalized phosphine P(t)Bu(2)CH(2)(C(3)H(5)) with [Rh(C(8)H(14))(2)Cl](2) (C(8)H(4) = ciscyclooctene) resulted in selective C C bond activation of the cyclopropyl group and formation of a tetrameric rhodacyclobutane, [RhCl(kappa(3)-P(t)Bu(2)CH(2)CH(CH(2))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)-P(t)Bu(2)CH(2)CH(CH(2))(2))](+) fragment, forming a range of new complexes by salt metathesis with NaCp, [RhCp(kappa(3)-P(t)Bu(2)CH(2)CH(CH-chloride abstraction in the presence of arenes, [Rh(eta(6)-arene)(kappa(3)-P(t)Bu(2)CH(2)CH(CH(2))2)][BAr(4)(F)] [arene = C(6)H(5)F, C(6)H(3)Me(3); Ar(F) = 3,5-C(6)H(3)(CF(3)),], or fragmentation by addition aphosphine ligands, trans-[RhCl(PR(3))(kappa(3)-P(t)Bu(2)CH(2)CH(CH(2))(2))] (R = (t)Bu, 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)(P(t)BuCH(2)(C(3)H(5)))]. These complexes were characterized in solution by NMR spectroscopy and in the solid state by X-ray diffraction. Chloride abstraction from [RhCl(B(t)Bu(3))(kappa(3)P(t)Bu(2)CH(2)CH(CH(2))(2))] resulted in the regioselective isomerization of the tethered cyclopropane to a terminal alkene, viz., the formally 14-electron complex [Rh(P(t)Bu(3))(kappa(3)-P(t)Bu(2)CH(2)CH(2)CH=CH(2))][BAr(4)(F)], notable for the presence of a strong agostic interaction with a phosphine (t)Bu substituent, apparent both from the solid-state structure and in solution by (1)H NM R spectroscopy. Addition of hydrogen to this complex resulted in hydrogenation of the alkene and formation of [Rh(H)(2)(P(t)Bu(3))(P(t)Bu(2)(II)Bu)][BAr(4)(F)]. 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