Tetra-tert-butylethylene: An Elusive Molecule with a Highly Twisted Double Bond. Can It Be Made by Carbene Dimerization?

A theoretical evaluation of tetra- tert -butylethylene ( 1 ) at the BLYP/DZd level confirms that it should be a stable molecule with a singlet ground state. The synthesis of 1 from two molecules of di- tert -butylcarbene ( 6 ) is unlikely. Although the formation of singlet 1 from the triplet 3 B ground state of 6 (singlet 6 is only 1−3 kcal/mol higher in energy) is highly exothermic (Δ H = −73.7 kcal/mol), the barrier Δ G ⧧ = 25 kcal/mol (298 K, 1 atm, BLYP/DZd) for the dimerization is too large to compete with the barrier for intramolecular carbene insertion. The barrier for singlet 6 to yield 1,1-dimethyl-2- tert -butylcyclopropane ( 12 ) is only 5 kcal/mol. The CC double bond in singlet 1 is twisted by 45°, and the strain energy is ∼93 kcal/mol in agreement with molecular mechanics results. Triplet 1 has a nearly perfectly perpendicular conformation at the central CC bond (87° torsional angle), but it is still strained by 42 kcal/mol and is 12 kcal/mol higher in energy than singlet 1 . Alkyl substitution decreases the S−T separation of carbenes due to the greater hyperconjugative stabilization of the singlet than the triplet.