Wai Han Lam

Theoretical study of reaction pathways for the rhodium phosphine-catalysed borylation of C-H bonds with pinacolborane

The reaction mechanism of the rhodium-phosphine catalysed borylation of methyl-substituted arenes using pinacolborane (HBpin) has been investigated theoretically using DFT calculations at the B3PW91 level. Factors affecting selectivity for benzylic vs. aromatic C-H bond activation have been examined. It was found that [Rh(PR3)2(H)] is the active species which oxidatively adds the C-H bond leading to an eta3-benzyl complex which is the key to determining the unusual benzylic regioselectivity observed experimentally for this catalyst system. Subsequent reaction with HBpin leads to a [Rh(PR3)2(eta3-benzyl)(H)(Bpin)] complex from which B-C reductive elimination provides product and regenerates the catalyst. The electrophilic nature of the boryl ligand assists in the reductive elimination process. In contrast to Ir(L)2(boryl)3-based catalysts, for which Ir(III)-Ir(V) cycles have been proposed, the Rh(I)-Rh(III) cycle is operating with the system addressed herein.

Understanding the electron counts of aluminum metalloid clusters

The structures and bonding of several Al metalloid clusters have been analyzed with the aid of density functional theory calculations. These clusters include [Al7{N(SiMe3)2}6]− (1), [Al12{N(SiMe3)2}8]− (2), [Al22Br20·12THF] (3) [Al14{N(SiMe3)2}6I6]2− (4), Al14(μ8-Si)Cp*6 (5), [(AlEt)8(μ4-CCH2Ph)5(μ4-H)] (6), [(AlMe)8(μ4-CCH2Ph)5(μ4-CCPh)] (7) and [(AlMe)7(μ4-CCH2Me)4(μ2-H)2] (8). In this paper, an attempt has been made to illustrate the relationship between the observed geometry and electron counts for these metalloid clusters.

DFT Studies of Dimerization Reactions of Boroles

Boroles undergo dimerization reactions to give Diels-Alder (DA) dimers, bridged-bicyclic (BB) dimers or spiro dimers (SD) depending on the substituents on the borole. We performed DFT calculations to investigate how different substituents at the carbon atoms of the butadiene backbone as well as at the boron atom influence the dimerization reaction pathways. The DFT results show that, in general, both the DA and BB dimers are easily accessible kinetically, and the DA dimers are thermodynamically more stable than the BB dimers. When the substituent-substituent repulsive steric interactions are alleviated to a certain extent, the BB dimers are more stable than the DA dimer, and become accessible. The SD dimers are generally kinetically difficult to obtain. However, we found that aryl substituents promote the formation of the SD dimers.