Double Bonds? Studies on the Barrier to Rotation about the Cumulenic C=C Bonds of Tetraaryl[ n ]cumulenes ( n= 3, 5, 7, 9)

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.