Computational studies on the formation of aza‐oxypentadienyl intermediates from alkylidene oxaziridines and keteneimine oxides and their conversion to 1,5‐dihydropyrrolones

Abstract

The rearrangements of alkylidene oxaziridines 1a,b (a, R = Me; b, R = OMe) and keteneimine oxides 1’a,b to 1,5‐dihydropyrrolones 3a,b have been computationally studied at the UM062X/Def2TZVPP(PCM,TFE)//UM062X/Def2SVP(PCM,TFE) level. The mechanism for conversion of 1a‐3a is predicted to be stepwise, with intermediacy of the acyclic aza‐oxypentadienyl species 4a. The stabilization provided by the nitrogen of the oxaziridine makes the ring opening of 1a to diradical/zwitterionic intermediates 4a more feasible than in the case of the vinyl allene oxide counterparts. The formation of aziridinone 2a from 1a showed competing activation energies but 2a was found to be a thermodynamically less stable species than 3a. When the Me group at N is exchanged by the electron‐donating OMe substituent in 1b, the diradical species and the corresponding transition structures are more stabilized and therefore the aziridinones are not required as intermediates in the conversion of 1b to 3b. Keteneimine oxide 1′ is predicted to directly generate diradical 4 by C‐O ring opening although through a less favored transition state.

A Computational Study on Cycloaddition Reactions between Isatin Azomethine Imine and in situ Generated Azaoxyallyl Cation

“…Structure 2 b discussed above is a zwitterionic compound on the close‐shell singlet potential energy surfaces. As previous theoretical works indicated that azaoxyallyl cations may also be described as open‐shell singlet diradicals, [28] we attempted to locate such a spin state by using “guess=mix” in unrestricted DFT calculations. We have successfully optimized such a state of 2 b , which has the <S 2 > value of 0.398 and lies at 0.8 kcal/mol lower‐lying than the corresponding close‐shell singlet state.…”

“…Although several computational studied on the structures and reactivity of azaoxyallyl cations have been available in the literature, [27,28] detailed mechanistic analyses on the [3+2] cycloaddition with a carbonyl group and comparison of different cycloaddition pathways have been lacking. This work will present a series of density functional theory (DFT) calculations on the mechanism of the cycloaddition reactions given in Scheme 2 to address the above issues.…”