Energetics and Mechanism of Organolanthanide-Mediated Aminoalkene Hydroamination/Cyclization. A Density Functional Theory Analysis

This contribution focuses on organolanthanide-mediated hydroamination processes and analyzes the hydroamination/cyclization of a prototypical aminoalkene, NH2(CH2)3CHdCH2, catalyzed by Cp2LaCH(TMS)2, using density functional theory. The reaction is found to occur in two steps, namely, cyclization to form La-C and C-N bonds, and subsequent La-C protonolysis. Calculations have been carried out for (i) insertion of the olefinic moiety into the La-N bond via a four-center transition state and (ii) protonolysis by a second substrate molecule. The cyclized amine then dissociates, thus restoring the active catalyst. DFT energy profiles have been determined for the turnover-limiting insertion of the 1-amidopent-4-ene CdC double bond into the La-NH- bond. DFT calculations of geometries and the stabilities of reactants, intermediates, and products have been analyzed. The picture that emerges involves concerted, rate-limiting, slightly endothermic insertion of the alkene fragment into the La-N(amido) bond via a highly organized, seven-membered chairlike cyclic transition state (¢Hq calcd ) 11.3 kcal/mol, ¢Sq calcd ) -14.6 cal/mol K). The resulting cyclopentylmethyl complex then undergoes exothermic protonolysis to yield an amine-amido complex, the resting state of the catalyst. Thermodynamic and kinetic estimates are in excellent agreement with experimental data.