[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 regio selectivity of the addition of the tr ansition-metal oxides OSO4 and LReO3 (L = O-, H3PN, Me, Cp) to ketene were systematically investigated using density-functional methods. While metal-o xide additions to ethylene have recently been reported to follow a [3+2] me chanism only, the calculations reveal a strong influence of the metal on th e periselectivity of the ketene addition: OSO4 again prefers a [3+2] pathwa y across the C=C moiety whereas, for the rhenium oxides LReO3, the [2+2] ba rriers are lowest. Furthermore, a divergent chemoselectivity arising from t he ligand L was found: ReO4- and (H3PN)ReO3 add across the C=O bond while M eReO3 and CpReO3 favor the addition across the C=C moiety. The calculated e nergy profile for the MeReO3 additions differs from the CpReO3 energy profi le by up to 45 kcal/mol due to the stereoelectronic flexibility of the Cp l igand adopting eta (5), eta (3), eta (1) bonding modes. The selectivity of the cycloadditions was rationalized by the analysis of donor-acceptor inter actions in the transition states. In contrast, metal-oxide additions to dip henylketene probably follow a different mechanism: We give theoretical evid ence 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 wi th the results of recent experimental work.