Search

The frontier orbital theory is applied to investigate an organolanthanide-catalyzed reaction at the electronic level, using hydroamination/cyclization reactions of primary aminoalkynes. The insertion of the alkyne functionality into the Ln–N bond in Cp 2 LnNH(CH 2 ) 3 C CH involved in the turnover-limiting cyclization step is found to be favored by the phases of HOMO of HC C(CH 2 ) 3 NH − and LUMO of closed-shell Cp 2 Ln + (Ln = La, Lu) or the LUMO+2 of excessive spins of open-shell Cp 2 Sm + . The LUMO and LUMO+1 of Cp 2 Sm + are 4f orbitals contracted too much to effectively overlap with HOMO of the counterpart. The HOMO–LUMO/LUMO+2 overlaps are visually shown and numerically confirmed to increase in the order of Ln = La

The frontier orbital theory is applied to ruthenium olefin metathesis. The formal [2+2] cycloaddition step, that is, the key step involved in the catalytic cycle of the reaction, is found to be favored by the phases of the HOMO and LUMO, in sharp contrast to [2+2] cycloaddition reactions between olefins. In the LUMO of transition metal part, a d-orbital overlaps out of phase with the vacant p-orbital of the carbene in the inner space of the metal–carbon π bond as is expected, but the remote lobe of the d orbital overlaps in phase in the outer space of the bond. This is a characteristic feature of the antibonding orbitals of transition metal bonds. The outer orbital phase plays more important role in the interaction. The significantly enhanced activity of the ‘second-generation’ catalysts or N -heterocyclic carbenes (NHC’s) comes from the high HOMO energy.

We investigated the electronic structures of the transition states of the oxidative addition, transmetalation, and reductive elimination steps in the catalytic cycle of the title reaction. The frontier orbital theory was surprisingly found to be applicable whereas any d orbitals of transition metals can be a main component of frontier orbitals because of their close energies. Visualizing the actually calculated HOMO and LUMO of the two parts of the transition structure of each step clearly demonstrated their orbital phase matching in favor of overlapping. The HOMO for the transmetalation step suggests that electron-donating ability of the carbon–metal bond of organometallic compounds (RMX) could control the reactivities of related cross-coupling reactions. The energies of the molecular orbitals having large amplitudes of the C–M bonding orbitals of RMX explain why the Suzuki–Miyaura cross-coupling reaction needs a base while the Kumada–Tamao and Negishi reactions take place without any bases.

We have defined diagonalization reactions as a concerted bond formation between the diagonal saturated atoms in cyclic molecules and designed the reactions of 1,2-heteraazetidine N -oxides to produce heteracyclopropanes with the extrusion of HNO. Analysis of the bond interactions at the transition states suggested the reactivities of the diagonalization reactions are controlled by the strains of the incipient rings of the transition structures.