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Push–Pull Buta‐1,2,3‐trienes: Exceptionally Low Rotational Barriers of Cumulenic CC Bonds and Proacetylenic Reactivity
Author(s) -
Gawel Przemyslaw,
Wu YiLin,
Finke Aaron D.,
Trapp Nils,
Zalibera Michal,
Boudon Corinne,
Gisselbrecht JeanPaul,
Schweizer W. Bernd,
Gescheidt Georg,
Diederich François
Publication year - 2015
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201406583
Subject(s) - cumulene , steric effects , reactivity (psychology) , moiety , chromophore , chemistry , acceptor , cycloaddition , double bond , triple bond , stereochemistry , photochemistry , crystallography , polymer chemistry , molecule , catalysis , organic chemistry , medicine , condensed matter physics , physics , alternative medicine , pathology
A variety of asymmetrically donor–acceptor‐substituted [3]cumulenes (buta‐1,2,3‐trienes) were synthesized by developed procedures. The activation barriers to rotation Δ G ≠ 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 CC 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.