Density Functional Theory Studies of Selected Transition Metals Catalyzed C-C and C-N Bond Formation Reactions

This dissertation, "Density Functional Theory Studies of Selected Transition Metals Catalyzed C-C and C-N Bond Formation Reactions" by Xufeng, Lin, 林旭鋒, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled DENSITY FUNCTIONAL THEORY STUDIES OF SELECTED TRANSITION METALS CATALYZED C-C AND C-N BOND FORMATION REACTIONS submitted by Xufeng LIN for the degree of Doctor of Philosophy at The University of Hong Kong in September 2007 This thesis presents theoretical investigations on selected transition metals catalyzed C-C and C-N bond formation reactions. Density functional theory (DFT) calculations were performed to explore the potential energy surfaces of (1) alkyl-alkyl cross-coupling reactions catalyzed by nickel-trinitrogen ligand complexes, and (2) amidation of C-H bonds and alkene aziridinations catalyzed by rhodium and ruthenium complexes. In the first part three catalytic cycles were examined for Ni-catalyzed Negishi alkyl-alkyl cross-coupling with the ligand of 2,2',6',2''-terpyridine (tpy). The catalytic cycle that involves a sequence of oxidative addition-reductive elimination- transmetalation with the actual catalyst of Ni(tpy)-CH or Ni(tpy)-I [Ni(I)] was found to be feasible, and this provide some new insight into the chemistry of metal-catalyzed cross-coupling reactions. The oxidative addition undergoes in a two-step radical pathway to produce a Ni(III) species, and the Ni(III) speices undergo reductive elimination to afford the alkyl-alkyl coupled product. Bulk solvation effect was considered in examination of this catalytic cycle. Cross-coupling reactions of both primary and secondary alkyl iodides in reactions with organozinc reagent were compared. Based on these results a Ni-catalyzed Negishi cross-coupling reaction of a secondary alkyl bromide was investigated with the ligands of bis(oxazolinyl)pyridine (Pybox). Reductive elimination step was found to be irreversible and rate-determining in generating the coupled product. When Pybox is replaced (S)-(i-Pr)-Pybox with C symmetry, it was found that the racemic 1-bromoindane can be converted into the coupled product in S enantiomer with high enantioselectivity. This is accounted for higher free energy of activation needed for the reductive elimination step in generating the R enantiomer compared to the S enatiomer in the case of (S)-(i-Pr)-Pybox ligand. In the second part the intramolecular amidation of C-H bonds in carbamates and sulfonamate esters were examined with dirhodium tetracarboxylate [Rh(II)] as the catalyst. "Rh(II)-nitrene" complexes were found to be largely favorable in free energy compared to the staring materials or the "Rh(II)-phenyliodinane" complexes. The singlet and triplet states of a Rh(II)-nitrene complex are very close in energy, which was attributed to the π-π orbital interaction of nitrene and Rh. However, a singlet Rh(II)- nitrene undergoes C-H bond cleavage accompanied by N-H and C-N bond formation in a concerted pathway, whereas a triplet Rh(II)-nitrene undergoes C-H bond cleavage accompanied by only N-H formation to produce a biradical. A singlet pathway is favorable over its corresponding triplet pathway in free energy of activation. Rh(II)- catalyzed nitrene insertion into a C=C double bond in competition with insertion into a C-H bond was also examined with the unsaturated substrate of sulfonamate esters. Similar cases were found for the reaction systems in which intramolecular amidation of C-H bonds in the cyclohexane ring in a sulfamate ester is catalyzed by Ru-porphyrins. In all these catalytic systems, stereoselectiviti