Computational comparison of the [2+2] and [4+2] reactions between aminoboranes R2BNR‘2 and alkenes CH2CH(R‘ ‘) shows that the latter are strongly preferred. The preference arises from the [4+2] reactions exhibiting lower reaction barriers than do the [2+2] reactions; the products formed are of nearly equal stability. This behavior mimics that of reactions between alkenes and supports viewing aminoboranes, particularly (F3C)2BNMe2, as acting like polar alkenes.

Computational comparisons of the [2 + 2] cyclization and [4 + 2] ene-type reactions between iminoboranes RBtNR′ and alkynes HCtC(R′′) show that the latter are strongly preferred. The preference arises from the [4 + 2] reactions exhibiting lower barriers and greater exothermicities than do the [2 + 2] reactions. This behavior mimics that of reactions between alkynes and supports viewing iminoboranes, particularly (F 3 C)BtNMe and higher fluorinated analogues, as acting like polar alkynes. The hypothetical 1-aza-2-boracyclohexyne BC(CF 3 ) 2 CF 2 CH 2 C(Me) 2 N shows remarkable potential reactivity, in that its [4 + 2] enetype reaction with propyne exhibits no activation barrier. This arises from the loss of ring strain energy in the azaboracyclohexyne as the propyne approaches and the B-C bond forms.