[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
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- 作者：Dirk V. Deubel ; Sabine Schlecht ; and Gernot Frenking
- 刊名：Journal of the American Chemical Society
- 出版年：2001
- 出版时间：October 17, 2001
- 年：2001
- 卷：123
- 期：41
- 页码：10085 - 10094
- 全文大小：192K
- 年卷期：v.123,no.41(October 17, 2001)
- ISSN：1520-5126
文摘
The peri-, chemo-, stereo-, and regioselectivity of the addition of the transition-metal oxides OsO4and LReO3 (L = O-, H3PN, Me, Cp) to ketene were systematically investigated using density-functionalmethods. While metal-oxide additions to ethylene have recently been reported to follow a [3+2] mechanismonly, the calculations reveal a strong influence of the metal on the periselectivity of the ketene addition: OsO4again 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=Cmoiety. The calculated energy profile for the MeReO3 additions differs from the CpReO3 energy profile by upto 45 kcal/mol due to the stereoelectronic flexibility of the Cp ligand adopting 
5, 3, and 1 bonding modes.The selectivity of the cycloadditions was rationalized by the analysis of donor-acceptor interactions in thetransition 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 carbonylmoiety and undergoes a subsequent cyclization yielding the thermodynamically favored product. This two-step pathway is in agreement with the results of recent experimental work.
5, 3, and 1 bonding modes.The selectivity of the cycloadditions was rationalized by the analysis of donor-acceptor interactions in thetransition 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 carbonylmoiety and undergoes a subsequent cyclization yielding the thermodynamically favored product. This two-step pathway is in agreement with the results of recent experimental work.