The Rearrangement of 2,2‐Diphenyl‐1‐[(E)‐2‐phenylethenyl]cyclopropane to 3,4,4‐Triphenylcyclopent‐1‐ene: a DFT Analysis

Abstract

Dedicated to Paul von Ragué Schleyer, friend and mentor, on the occasion of his 80th birthday A computational study on the rearrangement of 2,2-diphenyl-1-[(E)-2-phenylethenyl]cyclopropane (1) is presented, using density functional theory (DFT), (U)B3LYP with the 6-31G* basis set (DFT1) and (U)M05-2X with the 6-311 þ G** basis set (DFT2). In agreement with a biradical character of the transition structure (TS) or intermediate, the potential-energy hypersurface is lowered by the influence of three conjugated Ph groups. Surprisingly, two conformations of the geminal diphenyl group (different twist angles) induce two different minimum-energy pathways for the rearrangement. Independent of the functional used, the first hypersurface harbors true biradical intermediates, whereas the second energy surface is a flat, slightly ascending slope from the starting material to the TS. The functional (U)M05-2X with the basis set 6-311 þ G** provides realistic energies which seem to be close to experiment. The activation energy for racemization of enantiomers of 1 is lower than that of rearrangement by 2.5 kcal mol À1, in agreement with experiment.1. Introduction. -The rearrangement of vinylcyclopropane and its derivatives to substituted cyclopentenes has played an important role in the development of mechanistic and theoretical organic chemistry. Ever since the discovery of this reaction, independently by Vogel [1], and by Overberger and Borchert [2], it has been studied experimentally and theoretically. It played a role in the presentation of the WoodwardÀHoffmann rules [3], was recognized early as involving biradical intermediates [3 -5], and was analyzed by quantum-chemical calculations [6] [7]. A critical review on mechanistic aspects of this rearrangement was provided by Baldwin [8].The reaction exhibits both aspects, that of a concerted and a stepwise mechanism. However, calculations on the potential-energy surface of the unsubstituted system display only one transition structure (TS) for rearrangement and no intermediate [6]. The potential-energy surface is rather flat and diradical in nature. The TS is a biradical which undergoes ring closure to cyclopentene. The results of Houks group closely resemble those of Davidson and Gajewski [7], who took a slightly different approach. Both studies agree on the biradical nature of the rearrangement and on the preference for the si-stereochemistry of rearrangement suggested by the WoodwardÀHoffmann rules [3]. For the unsubstituted system, two more TSs, slightly (0.3 -1.1 kcal mol À1 )

1‐[(E)‐2‐Arylethenyl]‐2,2‐diphenylcyclopropanes: Kinetics and Mechanism of Rearrangement to Cyclopentenes

“…With the 1,1'-bond as a vertical axis, the Newman projection in Scheme 6, with R ¼ H, shows the three conformations on the first line; there was no evidence for a s-cis-conformation. Our quantum-chemical calculations confirmed the relative stabilities [19].…”

“…The substantial stabilization, which the allylic diradicals 26 and 27 experience by conjugation with three Ph groups, may well promote the formation of a trough in the energy profile. Concerning intermediates or paraintermediates, our calculations promote a noteworthy dual answer [19].…”

“…It seemed rewarding to study the kinetics of racemization and rearrangement of oligophenylated vinylcyclopropanes and to examine the influence of substitutents and solvents on the rate. The pertinent computational study by Sustmann et al reported in the following paper [19] is not free of surprises.…”

“…[19]). The X-ray diffraction discloses for the 3,4,4-triphenylcyclopent-1-ene (23a) a rigidly frozen envelope conformation in two crystallographically independent molecules with very close geometrical parameters; Fig.…”

Rolf Huisgen, Eminent Chemist and Polymath (1920–2020): In His Own Words and In His Publication Metrics

“…In 2011, Huisgen, Sustmann, and colleagues published back‐to‐back papers on the vinylcyclopropane to cyclopentene rearrangement . The first paper is almost entirely experimental, drawn in part from Johann Mulzer's Ph.D. thesis with a number of likely recently performed X‐ray crystallographic analyses.…”