Theoretical Studies of Cycloaddition Reactions of Cationic Aluminum β-Diketiminate Alkyl Complexes with Alkenes and Alkynes

Abstract

Cycloaddition reactions of cationic {HC(CHNX)2}Al(R)+ aluminum β-diketiminate complexes (X = variable substituent) with alkenes and alkynes to form bicyclic diimine species, {κ3-N,N,C-HC(CHNX)2(CR2CR2)}Al(R)+ and {κ3-N,N,C-HC(CHNX)2(CRCR)}Al(R)+, respectively, were studied by density functional theory. Alkenes and alkynes form {HC(CHNMe)2}Al(Me)(substrate)+ adducts with binding energies ranging from 11.4 (ethylene) to 19.4 (2-butyne) kcal/mol. Alkene and alkyne coordination is stronger than CH2Cl2 coordination to {HC(CHNMe)2}Al(Me)+ by 0.4 (ethylene) to 8.4 (2-butyne) kcal/mol. Alkynes bind more strongly than sterically similar alkenes, and alkyl substituents on the alkenes and alkynes enhance binding. Electron-withdrawing groups on the diketiminate nitrogens (X) enhance alkene and alkyne coordination. These trends reflect the fact that the Al−substrate binding is dominated by substrate-to-Al σ-donation. Cycloaddition of {HC(CHNX)2}Al(Me)(alkene)+ species proceeds by a concerted asynchronous process throu...