Electronic and Steric Control of Regioselectivities in Rh(I)-Catalyzed (5 + 2) Cycloadditions: Experiment and Theory

Ligand steric contours to understand the effects of N-heterocyclic carbene ligands on the reversal of regioselectivity in Ni-catalyzed reductive couplings of alkynes and aldehydes

The regioselectivities of N-heterocyclic carbene (NHC) ligands in Ni-catalyzed alkyne-aldehyde reductive coupling reactions with silane reducing agents are investigated using density functional theory. Reversal of regioselectivity can be achieved by varying the steric bulkiness of the ligand. The steric influences of NHC ligands are highly anisotropic. Regioselectivity is primarily controlled by the steric hindrance at the region of the ligand close to the alkyne. Analysis of 2D contour maps of the NHC ligands indicates that the regioselectivities are directly affected by the shape and orientation of the N-substituents on the ligand.

Mechanism and origins of regio- and enantioselectivities in RhI-catalyzed hydrogenative couplings of 1,3-diynes and activated carbonyl partners: intervention of a cumulene intermediate

The mechanism of the rhodium-catalyzed reductive coupling of 1,3-diynes and vicinal dicarbonyl compounds employing H(2) as reductant was investigated by density functional theory. Oxidative coupling through 1,4-addition of the Rh(I)-bound dicarbonyl to the conjugated diyne via a seven-membered cyclic cumulene transition state leads to exclusive formation of linear adducts. Diyne 1,4-addition is much faster than the 1,2-addition to simple alkynes. The 1,2-dicarbonyl compound is bound to rhodium in a bidentate fashion during the oxidative coupling event. The chemo-, regio-, and enantioselectivities of this reaction were investigated and are attributed to this unique 1,4-addition pathway. The close proximity of the ligand and the alkyne substituent distal to the forming C-C bond controls the regio- and enantioselectivity: coupling occurs at the sterically more demanding alkyne terminus, which minimizes nonbonded interaction with the ligand. A stereochemical model is proposed that accounts for preferential formation of the (R)-configurated coupling product when (R)-biaryl phosphine ligands are used.

Density functional theory study of the mechanisms and stereochemistry of the Rh(I)-catalyzed intramolecular [3+2] cycloadditions of 1-ene- and 1-yne-vinylcyclopropanes

The mechanisms, structures of all stationary points involved, and kinetic and thermodynamic parameters of the Rh(I)-catalyzed intramolecular [3+2] cycloaddition reactions of 1-ene- and 1-yne-vinylcyclopropanes (1-ene-VCPs and 1-yne-VCPs) have been investigated using density functional theory (DFT) calculations. The computational results showed that the [3+2] reactions of 1-ene/yne-VCPs studied here occur through a catalytic cycle of substrate-catalyst complex formation, cyclopropane cleavage, alkene/alkyne insertion, and reductive elimination. Alkene/alkyne insertion is the rate- and stereoselectivity-determining step of these multistep [3+2] cycloadditions. The experimentally observed high reactivity of 1-yne-VCPs compared to 1-ene-VCPs is well rationalized by the differences of steric effects in the alkyne/alkene insertion transition states. DFT calculations unveiled that the relative orientation of the tethers in the 1-ene/yne-VCPs plays a key role in controlling the stereochemistry of the [3+2] cycloadducts. In addition, DFT calculation results are used to explain why, in some cases, the formation of the β-hydride elimination byproduct can compete with the [3+2] pathway.

Synthesis of a Tricyclic Core of Rameswaralide

A tricyclic core containing a 5,7-fused bicyclic unit of rameswaralide was prepared starting from a 1,6-enyne. The synthetic sequence involved (i) ruthenium-catalyzed [5+2]-cycloaddition of 1,6-enyne, (ii) an acyl radical based approach to construct the lactone, and (iii) a regioselective installation of the conjugated double bond by a concomitant sulfenylation-dehydrosulfenylation sequence.