Osmium models of intermediates involved in catalytic reactions of alkylidenecyclopropanes

Abstract

The complex [OsCp{κ3-P,C,C-PiPr 2[C(CH3)î=CH2]}(CH3CN)]PF 6 (1) reacts with (2-pyridyl)methylenecyclopropane, at room temperature, to give initially the cyclobutylidene derivative [Os(η5-C5H5)(î=CCH2CH 2CH-o-C5H4N){PiPr 2[C(Me)î=CH2]}]PF6 (2), as a result of the ring expansion of the alkylidenecyclopropane unit. Over time complex 2 rearranges into the cyclobutene derivative [Os(η5-C 5H5){η2-C(î=CHCH2CH 2)-o-C5H4N}{PiPr2[C(Me) î=CH2]}]PF6 (3). The reaction of 1 with (2-pyridyl)methylenecyclopropane at room temperature also affords the phosphinomethanide metallacycle [Os(η5-C5H 5){κ4-P,Ca,Cb,N-P iPr2[Ca(Me)CH2CH)(C bCH2CH2-o-C5H4N]}]PF 6 (4) as a minor product, which becomes the major product of the reaction at 45 C. This osmacyclopentane results from the C-C coupling of the isopropenyl substituent of the phosphine ligand and the organic substrate. In acetone at 75 C, the reaction of 1 with (2-pyridyl)methylenecyclopropane leads to the 2-alkylidene-1-osmacyclobutane [Os(η5-C5H 5){κ3-N,Ca,Cb-C a(CH2CbH2)(î=CH-o-C 5H4N)}{PiPr2[C(Me)î=CH 2]}]PF6 (5), as a consequence of the oxidative addition of one of the C(sp2)-C(sp3) bonds of the cyclopropane unit of the substrate to the osmium atom, along with 6, a diastereomer of 4. Complexes 3-5 have been characterized by X-ray diffraction analysis. DFT calculations suggest that all of the reaction products are derived from a common key 1-osma-2-azacyclopent-3-ene intermediate (D).