Push-Pull Buta-1,2,3-trienes: exceptionally low rotational barriers of cumulenic C=C bonds and proacetylenic reactivity.

A variety of asymmetrically donor-acceptor-substituted [3]cumulenes (buta-1,2,3-trienes) were synthesized by developed procedures. The activation barriers to rotation ΔG(≠) were measured by variable temperature NMR spectroscopy and found to be as low as 11.8 kcal mol(-1) , in the range of the barriers for rotation around sterically hindered single bonds. The central C=C bond of the push-pull-substituted [3]cumulene moiety is shortened down to 1.22 Å as measured by X-ray crystallography, leading to a substantial bond length alternation (BLA) of up to 0.17 Å. All the experimental results are supported by DFT calculations. Zwitterionic transition states (TS) of bond rotation confirm the postulated proacetylenic character of donor-acceptor [3]cumulenes. Additional support for the proacetylenic character of these chromophores is provided by their reaction with tetracyanoethene (TCNE) in a cycloaddition-retroelectrocyclization (CA-RE) cascade characteristic of donor-polarized acetylenes.