Mechanism and regioselectivity of the cycloaddition between nitrone and dirhodium vinylcarbene catalyzed by Rh2(O2CH)4: a computational study

Mechanism and enantioselectivity of the asymmetric [3+2]-annulation between N-methylindole and enoldiazoacetamide catalyzed by prolinate-coordinated dirhodium: A theoretical study

The mechanistic study of Rh₂(S-MSP)₄-catalyzed asymmetric [3 + 2]-annulation of N-methylindole and enoldiazoacetamide carbenoid to produce C2,C3-fused indole skeleton has been carried out by combining the density functional theory and semi-empirical method with ONIOM methodology. The mechanism and origin of enantioselectivity were disclosed by our calculations. The cycloaddition process was initiated by vinylogous attack of indole on the enol terminus of carbenoid, which was considered to be the enantio-controlling step. Followed by ring closure step where C-Rh bond adds to the iminium, adduct was afforded enantioselectively. The rate-controlling step, depending on the pathways the reaction proceeds along, could be either one of the two steps. Moreover, we elucidated the effect of weak interaction on the enantioselectivity using the independent gradient model analysis and provided possible insight into reaction design. The catalytic cycle was also kinetically interpreted in a non-conventional way, which is the energetic span model, realized by the AUTOF program.

Catalyst-Dependent Chemoselectivity in the Dirhodium-Catalyzed Cyclization Reactions Between Enodiazoacetamide and Nitrosoarene: A Theoretical Study

N-O heterocycle compounds play an important role in various fields. Doyle et al. (Cheng et al., 2017) have recently reported an efficient catalyst-controlled selective cyclization reactions of tert-butyldimethylsilyl (TBS)-protected enoldiazoacetamides with nitrosoarenes in which the multifunctionalized products 5-isoxazolones and 1,3-oxazin-4-ones were formed through [3+2]- and [5+1]-cyclizations using Rh2(oct)4 and Rh2(cap)4 as catalysts, respectively. The present work studied the mechanism of the reactions in question and the origins of the catalyst-dependent chemoselectivity by the density functional theory (DFT) calculations at the M06-D3/SMD/6-311+G(d,p)//B3LYP-D3/6-31G(d,p) level of theory. The computed results illustrate the importance of the different dirhodium catalysts to gain diversified N-O heterocycle compounds and suggest the specific interaction between the reactants by the real catalyst model. Meanwhile, it is the steric hindrance and electronic effect of the ligands of dirhodium catalysts that control the reaction mechanism. Furthermore, the other auxiliary theoretical analysis, natural bond orbital calculation and distortion/interaction analysis, make the electronic effects, and steric hindrance more distinct.

[1,4]-sigmatropic rearrangement of chiral nitrones and their utilization in the synthesis of new iminosugars

A new mechanism of nitrone epimerization via [1,4]-sigmatropic rearrangement was proposed and a set of epimeric iminosugars was synthesized.