[2+2] versus [3+2] addition of metal oxides across C=C double bonds: Toward an understanding of the surprising chemo- and periselectivity of transition-metal-oxide additions to ketene

The peri-, chemo-, stereo-, and regioselectivity of the addition of the transition-metal oxides OsO₄ and LReO₃ (L=O^-, H₃PN, Me, Cp) to ketene were systematically investigated using density-functional methods. While metal-oxide additions to ethylene have recently been reported to follow a [3+2] mechanism only, the calculations reveal a strong influence of the metal on the periselectivity of the ketene addition: OsO₄ again prefers a [3+2] pathway across the C=C moiety whereas, for the rhenium oxides LReO₃, the [2+2] barriers are lowest. Furthermore, a divergent chemoselectivity arising from the ligand L was found: ReO₄^- and (H₃PN)ReO₃ add across the C=O bond while MeReO₃ and CpReO₃ favor the addition across the C=C moiety. The calculated energy profile for the MeReO₃ additions differs from the CpReO₃ energy profile by up to 45 kcal/mol due to the stereoelectronic flexibility of the Cp ligand adopting η⁵, η³, and η¹ bonding modes. The selectivity of the cycloadditions was rationalized by the analysis of donor - acceptor interactions in the transition states. In contrast, metal-oxide additions to diphenylketene probably follow a different mechanism: We give theoretical evidence for a zwitterionic intermediate that is formed by nucleophilic attack at the carbonyl moiety and undergoes a subsequent cyclization yielding the thermodynamically favored product. This two-step pathway is in agreement with the results of recent experimental work.