Ab initio computational studies of heterocycloalkynes: Structures, naturalbond orders, ring strain energies, and isomerizations of cyclic iminoboranes and iminoalanes

Ab initio calculations of series of cyclic iminoboranes (CH2)(n)B=N and imi noalanes (CH2)(n)- Al=N show that they adopt angularly distorted structures compared to the isoelectronic cycloalkynes. Natural bond order analyses in dicate that the larger ring iminoboranes contain BN triple bonds, but the s maller ring iminoboranes and all the iminoalanes contain MN double bonds an d a lone pair on the nitrogen atom. Group equivalent calculations confirm t hat cyclic iminoboranes and iminoalanes contain less ring strain energy tha n the corresponding cycloalkynes, but the data do not distinguish between d oubly and triply bonded systems. Examination of the rearrangements of isome ric azacycloalkane borines to cyclic iminoboranes shows that they are exoth ermic, with moderate activation barriers. Thus, a way by which the heterocy cloalkynes might be synthesized is suggested.