Push-pull allenes-conjugation, (anti)aromaticity and quantification of the push-pull character

Structures, 1H/13C chemical shifts, and π electron distribution/conjugation of an experimentally available and theoretically completed set of push-pull allenes Acc2CCCDon2 (Acc=F, CHO, CF3, CN; Don=t-Bu, OMe, OEt, SMe, SEt, NCH2R) have been computed at the DFT level of theory. Both orthogonal linear and orthogonal bent structures have been obtained. In the latter case the push-pull character could be quantified by the quotient method. The 13C chemical shift of the central allene carbon atom C-2 and chemical shift differences Δδ(C-1, C-2) and Δδ(C-2, C-3) of allene carbon atoms proved to be a quantitative alternative. TSNMRS of ring-closed push-pull allenes have been computed in addition and were employed to identify polar, carbene-like and carbone-like canonical structures of these molecules.

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