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Pericyclic Reactions between Iminoboranes RB⋮NR‘ and Alkynes:  [4 + 2] vs [2 + 2] Transition States

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journal contribution
posted on 11.10.2004, 00:00 by Kirk M. Bissett, Thomas M. Gilbert
Computational comparisons of the [2 + 2] cyclization and [4 + 2] ene-type reactions between iminoboranes RB⋮NR‘ and alkynes HC⋮C(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)B⋮NMe and higher fluorinated analogues, as acting like polar alkynes. The hypothetical 1-aza-2-boracyclohexyne shows remarkable potential reactivity, in that its [4 + 2] ene-type 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.

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