A Synthetic and Mechanistic Investigation of the Chromium Tricarbonyl-Mediated Masamune鈥揃ergman Cyclization. Direct Observation of a Ground-State Triplet p-Benzyne Biradical

详细信息    查看全文

• 作者：Kai E. O. Ylijoki ; S茅verine Lavy ; Angelika Fretzen ; E. Peter K眉ndig ; Th茅o Berclaz ; G茅rald Bernardinelli ; C茅line Besnard
• 刊名：Organometallics
• 出版年：2012
• 出版时间：August 13, 2012
• 年：2012
• 卷：31
• 期：15
• 页码：5396-5404
• 全文大小：527K
• 年卷期：v.31,no.15(August 13, 2012)
• ISSN：1520-6041

文摘

A new room-temperature chromium tricarbonyl-mediated cycloaromatization of enediynes is reported. The reaction occurs with both cyclic and acyclic enediynes in the presence of [Cr(CO)₃(畏⁶-naphthalene)] and both a coordinating solvent and a hydrogen atom source, providing chromium鈥揳rene complexes in reasonable yield and good diastereocontrol. The mechanism of the reaction has been probed through DFT computational and spectroscopic methods. These studies suggest that direct C1鈥揅6 bond formation from an 畏⁶-enediyne complex is the lowest-energy path, forming a metal-bound p-benzyne biradical. NMR spectroscopy suggests that enediyne alkene coordination occurs in preference to alkyne coordination, forming a THF-stabilized olefin intermediate; subsequent alkyne coordination leads to cyclization. While biradical quenching occurs rapidly and primarily via the singlet biradical, the triplet state biradical is detectable by EPR spectroscopy, suggesting intersystem crossing to a triplet ground state.