Enantioselective Arylation of N‐Tosylimines by Phenylboronic Acid Catalysed by a Rhodium/Diene Complex: Reaction Mechanism from Density Functional Theory

Abstract

A DFT study of the reaction mechanism of the rhodium-catalysed enantioselective arylation of (E)-N-propylidene-4-methyl-benzenesulfonamide by phenylboronic acid [Lin et al J. Am. Chem. Soc. 2011, 133, 12394] is reported. The catalyst ([{Rh(OH)(diene)}2]) includes a chiral diene ligand and the reaction is conducted in 1,4-dioxane in the presence of drying agents (4 Å molecular sieves). Because phenylboronic acid is in equilibrium with phenylboroxin and water under the reaction conditions, three catalytic cycles are proposed that differ in the way the transmetallation and the release of the product are brought about, depending on the availability of phenylboronic acid, water and boroxin in the reaction medium. Based on computations, a new mechanism for the title reaction is proposed, in which phenylboronic acid plays the double role of "aryl source" and proton donor. This path does not require the presence of adventitious water molecules, in keeping with a reaction conducted in a dry medium. Comparisons with the generally accepted mechanism for arylation of enones proposed by Hayashi and co-workers (J. Am. Chem. Soc. 2002, 124, 5052) show that the latter mechanism is less favourable and is not expected to operate in the case of the N-tosylimine substrate considered herein. Finally, the possibility that phenylboroxin is the aryl source has also been investigated, but is not found to be competitive.

On the Catalytic Activity of [RuH2(PPh3)3(CO)] (PPh3=triphenylphosphine) in Ruthenium‐Catalysed Generation of Hydrogen from Alcohols: a Combined Experimental and DFT study

“…The refined energies are computed with the B97‐D2 functional, that follows the DFT−D2 general approach of Grimme, in which the functional energies are corrected by an atomic pair‐wise additive term accounting for the long‐range non‐covalent interactions. B97‐D2 has been successfully employed to study Ru and Rh catalysed reactions, and has been recently shown to perform well at describing several “bulky” transition metal‐complexes …”

Hierarchical Silica Inverse Opals as a Catalyst Support for Asymmetric Molecular Heterogeneous Catalysis with Chiral Rh‐diene Complexes

“…A potential reason for deactivation might be pore blocking by Rh boronate precipitates in agreement with recent calculations of the catalytic cycle. [60]…”

“…Then, hierarchical macro-mesoporous SiO 2 À IOs were used for the first-time as a catalyst support for chiral Rh-diene complexes, and their catalytic activity and selectivity with respect to the porous material was studied. As a catalytic model reaction, the Rh-diene-catalyzed 1,2-addition of phenylboroxine to Ntosylimines was chosen, because the corresponding homogeneous catalysis has been previously studied both experimentally and theoretically [41][42][43][57][58][59][60] and thus could serve as a benchmark for our novel solid catalyst.…”

Adventures and Detours in the Synthesis of Hydropentalenes

“…Despite the much larger distance of the substituents R from the Rh center (compared with the diene ligand 140a), a high ee value was expected for the 1,4-addition products derived from aliphatic enones (Scheme 38). 99 When we used a strategy similar to that shown in Scheme 36 for the synthesis of dienes 144 from Weiss diketone 103, we noticed that, depending on the base, considerable amounts of the undesired C s -symmetrical (achiral) diene 145 were formed (Scheme 38). For example, treatment of Weiss diketone 103 with a chiral Simpkins base, trapping with triflimide, and subsequent Suzuki cross-coupling with phenylboronic acid delivered the dienes 144a and 145a in 71% yield as an 84:16 regioisomeric mixture.…”