Computational Studies of Pericyclic Reactions of Iminoboranes. [4+2] Diels−Alder vs [2+2] Dimerization of RB⋮NR‘ Compounds
Thomas M. Gilbert · Organometallics · 2003
Computational studies comparing the energetics of [2+2] dimerization of iminoboranes RB⋮NR‘ (R = H, Me, CF 3, C 6 F 5, t-C 4 F 9; R‘ = H, Me, t-C 4 H 9 ) with their [4+2] Diels−Alder-like cyclizations with cis -butadiene and cyclopentadiene show that generally the product distributions depend on the transition state barrier heights rather than the reaction exothermicities. Dimerization is favored when the iminoborane carries small substituents; a slight preference exists for the [4+2] reaction for (F 5 C 6 )B⋮N(t-C 4 H 9 ), while the very bulky (t-F 9 C 4 )B⋮N(t-C 4 H 9 ) strongly prefers the [4+2] cyclization reaction. These results concur with the limited experimental data available. The data suggest general characteristics for iminoboranes likely to prefer [4+2] cyclization to dimerization. Also, the use of cyclopentadiene as the eneophile is contraindicated because of the strain incorporated into the azaborabicycloheptadiene product. Iminoborane [4+2] reactions employing acyclic dienes such as cis -butadiene or larger cyclic dienes such as 1,3-cyclohexadiene appear more likely to succeed.