Rhodium-Catalyzed Hetero-(5 + 2) Cycloaddition of Vinylaziridines and Alkynes: A Theoretical View of the Mechanism and Chirality Transfer

A newly
reported density functional theory method, M11-L, was performed
to study the mechanism and chirality transfer for the intramolecular
formal hetero-(5 + 2) cycloaddition of vinylaziridines with alkynes.
Both (E)- and (Z)-olefinic substrates
were considered in the density functional theory calculations. The
computational results suggested a metallahydropyridine pathway for
the generation of azepines, which involves aziridine cleavage, 2π
insertion of the alkyne group into the Rh–C bond, and reductive
elimination from a rhodium­(III) cation. The chirality transfer process
for the (E)-alkene substrate is shown to occur on
the re face of the alkene, whereas the (Z)-alkene cycloaddition chirality transfer occurs on the si face. The high enantioselectivity in this type of reaction is attributed
to the greater ring strain in the trans allylic rhodium
complex.

本研究采用新近报道的密度泛函理论（density functional theory）方法M11-L，用以探究乙烯基氮丙啶与炔烃的分子内形式杂(5+2)环加成反应的机理及手性转移过程。计算过程中同时考量了(E)型与(Z)型烯烃底物。计算结果表明，氮杂卓类化合物的生成遵循金属氢吡啶反应路径，该路径包含氮丙啶开环、炔烃基团对铑-碳键的2π插入，以及三价铑阳离子的还原消除步骤。研究显示，(E)型烯烃底物的手性转移过程发生于烯烃的Re面（re face），而(Z)型烯烃的环加成手性转移则发生于Si面（si face）。该类反应所具备的高对映选择性，可归因于反式烯丙基铑配合物中更大的环张力。