Influence of Bystander Substituents on the Rates of 1,2-H and 1,2-Ph Shifts in Singlet and Triplet Carbenes

Abstract

Substituent effects at the migration origin on the rate of rearrangement of several alkylchlorocarbenes have been studied at the B3LYP/6-311G**//B3LYP/6-31G* level. Methyl, halo, and phenyl groups are found to accelerate 1,2-H shift rearrangements in the order Ph > Me > F > Cl > H. Methyl groups are also found to accelerate both singlet and triplet 1,2-Ph shifts in benzylchloro- and benzylcarbenes. A direct comparison of the 1,2-H shift in methylchlorocarbene and the 1,2-Ph shift in benzylchlorocarbene shows that the inherent migratory preference of a phenyl group is greater than that of a hydrogen atom. Significant bystander substituent effects are found, as originally proposed by Nickon.

Density Functional Study on the Reactivity of Carbenes Toward 1,2‐H Shifts

“…From the ear lier com pu ta tional stud ies, the pre dicted Ea's of CH2Y-C-Cl seem to be larger than those of the cor re spond ing CH 2 Y-C-CH 3 spe cies. 16,17 In our early anal y sis, we found that the dif fer ences in E a for these two sets of carbenes (X=Cl and Me) are rel a tively large. The view point of by stander ef fect may not be the ma jor fac tor ac count ing for the re ac tiv ity of carbenes to ward 1,2-H shift.…”

“…21 Stereochemistry of carbenes has been stressed in pre vious com pu ta tional stud ies. 16,22 In this re search, PESs of both cis and trans con form ers were stud ied. The ex cep tions are CH 3 -C-X and CH 2 Y-C-H, be cause only cis con form ers ex ist in these carbenes.…”

“…[9][10][11][12][13][14][15] The ef fects of by stander substituent groups on the ac ti va tion bar ri ers (E a 's) of chlorocarbenes (CH 2 Y-C-Cl) and sub sti tuted dimethylcarbenes (CH 2 Y-C-CH3) have been stud ied us ing com pu ta tional meth ods. 16,17 Keating, Gar cia-Garibay and Houk found that methyl, halo, and phenyl groups lower the ac ti va tion bar ri ers of 1,2-H shifts in the or der of Ph > Me > F > Cl > H (for CH2Y-C-Cl). 16 Hu re ported that the substituent groups lower the ac ti va tion bar ri ers in the or der of Ph > Me, F > Cl, Br > H (for CH 2 Y-C-CH 3 ).…”

“…16,17 Keating, Gar cia-Garibay and Houk found that methyl, halo, and phenyl groups lower the ac ti va tion bar ri ers of 1,2-H shifts in the or der of Ph > Me > F > Cl > H (for CH2Y-C-Cl). 16 Hu re ported that the substituent groups lower the ac ti va tion bar ri ers in the or der of Ph > Me, F > Cl, Br > H (for CH 2 Y-C-CH 3 ). 17 Sim i lar by stander ef fects were ob served in these two the o ret i cal stud ies.…”

Rearrangement Reactions of Tritylcarbenes: Surprising Ring Expansion and Computational Investigation

“…This leads to the most frequently found migratory aptitudes of H>Ph>Me 3. However, it has been shown that the tendency of the rearrangement is strongly influenced not only by the nature of the migrating group X, but also by the bystander substituents R 1 and R 2 at the migration origin and by the spectator substituent R 3 2b. 4 Furthermore, 1,2‐rearrangement reactions of carbenes may be affected by the type of carbene precursor that is used and the reaction conditions, such as solvent polarity and temperature.…”