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 14 = cis -cyclooctene) resulted in selective C−C bond activation of the cyclopropyl group and formation of a tetrameric rhodacyclobutane, [RhCl(κ 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(κ 3 -P t Bu 2 CH 2 CH(CH 2 ) 2 )] + fragment, forming a range of new complexes by salt metathesis with NaCp, [RhCp(κ 3 -P t Bu 2 CH 2 CH(CH 2 ) 2 )], chloride abstraction in the presence of arenes, [Rh(η 6 -arene)(κ 3 -P t Bu 2 CH 2 CH(CH 2 ) 2 )][BAr F 4 ] [arene = C 6 H 5 F, C 6 H 3 Me 3; Ar F = 3,5-C 6 H 3 (CF 3 ) 2 ], or fragmentation by addition of phosphine ligands, trans -[RhCl(PR 3 )(κ 3 -P t Bu 2 CH 2 CH(CH 2 ) 2 )] (R = i 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(P i Bu 3 )(κ 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 i Bu 3 )(κ 3 -P t Bu 2 CH 2 CH 2 CH═CH 2 )][BAr F 4 ], notable for the presence of a strong agostic interaction with a phosphine i Bu substituent, apparent both from the solid-state structure and in solution by 1 H NMR spectroscopy. Addition of hydrogen to this complex resulted in hydrogenation of the alkene and formation of [Rh(H) 2 (P i Bu 3 )(P t Bu 2 n Bu)][BAr F 4 ]. These steps overall correspond to selective C−C bond functionalization (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 alkanes.