Can tetra-tert-butylethylene be formed by ion-ion recombination or ion-molecule reaction of two di-tert-butylcarbene units? - A DFT study

The reaction channels of di-tert-butylcarbene (2), its radical anion, (3) and its radical cation (4) were investigated theoretically by using DFT/B3LYP with 6-31+G(d) basis set and 6-311+G(2d,p) for single point energy calculations. Conversion of the neutral carbene 2 to the charged species 3 and 4 results in significant geometric changes. In cation 4 two different types of C -(CH ₃)₃ bonds are observed: one elongated sigma bond called "axial" with 1.61 and two normal sigma bonds with a bond length of 1.55 Species 2 and 4 have an electron deficient carbon center; therefore, migration of CH₃ and H is observed from adjacent tert-butyl groups with low activation energies in the range of 6-9kcal/mol like similar Wagner-Meerwein rearrangements in the neopentyl-cation system. Neutral carbene 2 shows C -H insertion to give a cyclopropane derivative with an activation energy of 6.1kcal/mol in agreement with former calculations. Contrary to species 2 and 4, the radical anion 3 has an electron rich carbon center which results in much higher calculated activation energies of 26.3 and 42.1kcal/mol for H and CH₃ migrations, respectively. NBO charge distribution indicates that the hydrogen migrates as a proton. The central issue of this work is the question: how can tetra-tert-butylethylene (1) be prepared from reaction of either species 2, 3, or 4 as precursors? The ion-ion reaction between 3 and 4 to give alkene 1 with a calculated reaction enthalpy of 203.5kcal/mol is extremely exothermic. This high energy decomposes alkene 1 after its formation into two molecules of carbene 2 spontaneously. Ion-molecule reaction of radical anion 3 with the neutral carbene 2 is a much better choice: via a proper oriented charge-transfer complex the radical anion of tetra-tert-butylethylene (11) is formed. The electron affinity of 1 was calculated to be negligible. The ion-neutral reaction between di-tert-butylcarbene and its radical anion is energetically barrierless and leads to the product, the anion of tetra-tert-butylethylene, which can lose an electron to produce tetra-tert-butylethylene.