Pericyclic reactions between iminoboranes RBNR′ and alkynes:: [4+2] vs [2+2] transition states

Computational comparisons of the [2 + 2] cyclization and [4 + 2] ene-type reactions between iminoboranes RBdropNR' and alkynes HCdropC(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 (F3C)BdropNMe and higher fluorinated analogues, as acting like polar alkynes. The hypothetical 1-aza-2-boracyclohexyne BC(CF3)(2)CF2CH2C(Me)(2)N shows remarkable potential reactivity, in that its [4 + 21 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.