Theoretical studies of cycloaddition reactions of cationic aluminum β-diketiminate alkyl complexes with alkenes and alkynes

Cycloaddition reactions of cationic {HC(CHNX)(2)}Al(R)(+) aluminum beta-diketiminate complexes (X = variable substituent) with alkenes and alkynes to form bicyclic diimine species, {kappa(3)-N,N,C-HC(CH=NX)(2)(CR2CR2)}Al(R)(+) and {kappa(3)-N,N,C-HC(CH=NX)(2)(CR=CR)}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 sigma-donation. Cycloaddition of {HC(CHNX)(2)}Al(Me)(alkene)(+) species proceeds by a concerted asynchronous process through an unsymmetrical transition state in which the new Al-C bond is almost fully formed, the C=C bond is lengthened ca. halfway between the reactant and product distances, the Al-N and C-C distances within the diketiminate ring are lengthened but the C-N bonds are shortened, and the new C-C distance is long. Alkyl substituents on the alkene and electron-withdrawing substituents on the diketiminate nitrogens disfavor cycloaddition of {HC(CHNX)(2)}Al(Me)(alkene)(+). These substituents enhance the interaction between the alkene HOMO and the {HC(CHNX)(2)}Al(Me)(alkene)(+) LUMO+1 (Al 3p(z)), which stabilizes the {HC(CHNX)(2)}Al(Me)(alkene)(+) species, and decrease the interaction between the {HC(CHNX)(2)}Al(Me)(+) HOMO and the alkene LUMO, which destabilizes the cycloadduct. Cycloaddition of {HC(CHNX)(2)}Al(Me)(alkyne)(+) species is more exothermic than cycloaddition of analogous alkene adducts because the newly formed C-C and Al-C bonds in the {kappa(3)-N,N,C-HC(CH=NX)(2)(CR=CR)}Al(R)(+) alkyne cycloadducts involve sp(2)-hybridized carbons and are stronger than those in the alkene cycloadducts, which involve sp(3)-hybridized carbons.