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Double Bonds? Studies on the Barrier to Rotation about the Cumulenic C=C Bonds of Tetraaryl[ n ]cumulenes ( n= 3, 5, 7, 9)
Author(s) -
Bühringer Martina U.,
Padberg Kevin,
Phleps Martin D.,
Maid Harald,
Placht Christian,
Neiss Christian,
Ferguson Michael J.,
Görling Andreas,
Tykwinski Rik R.
Publication year - 2018
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201802137
Subject(s) - isomerization , double bond , chemistry , steric effects , photochemistry , computational chemistry , stereochemistry , polymer chemistry , organic chemistry , catalysis
Bonding is a fundamental aspect of organic chemistry, yet the magnitude of C=C bonding in [ n ]cumulenes as a function of increasing chain length has yet to be experimentally verified for derivatives longer than n =5. The synthesis of a series of apolar and unsymmetrically substituted tetraaryl[ n ]cumulenes ( n =3, 5, 7, 9) was developed and rotational barriers for Z / E isomerization were measured using dynamic VTNMR spectroscopy. Both experiment and theory confirm a dramatic reduction in the rotational barrier (through estimation of Δ G ≠ rot for the isomerization) across the series, from >24 to 19 to 15 to 11 kcal −1 in [ n ]cumulenes with n= 3, 5, 7, 9, respectively. The reduction in cumulenic bonding in longer cumulenes thus affords bond rotational barriers that are more characteristic of a sterically hindered single bond than that of a double bond.