Reaction of 2,2-Bis(trifluoromethyl)-1,1-dicyanoethylene with 6,6-Dimethylfulvene: Cycloadditions and a Rearrangement

Reaction of 2,2-bis(trifluoromethyl)-1,1-dicyanoethylene (BTF; 1) with 6,6-dimethylfulvene (2) affords
the expected Diels−Alder cycloadduct, 7-(1-methylethylidene)-3,3-bis(trifluoromethyl)bicyclo[2.2.1]hept-5-ene-2,2-dicarbonitrile (3), in good yield. The cycloadduct 3 is unstable and exists in
equilibrium with the starting materials in less polar solvents. In more polar environment, the
[4 + 2] adduct 3 either reverts to starting materials, or, in a competing process, is converted to the
formal [2 + 2] adduct, 2-(1-methylethylidene)-7,7-bis(trifluoromethyl)bicyclo[3.2.0]hept-3-ene-6,6-dicarbonitrile (6). In the presence of acid, 3 is converted to a third isomeric form, 1,3a,5,6-tetrahydro-7-methyl-5,5-bis(trifluoromethyl)-4H-indene-4,4-dicarbonitrile (8). Both 6 and 8 are formed with
complete regiospecificity. Quantum mechanical calculations and X-ray crystallographic studies of
this ensemble of reactions by BTF, and the analogous set of reactions by its progenitor,
tetracyanoethylene (TCNE), reveal several interesting facets. The conversion of 3 to 6 occurs in
certain polar solvents, in the presence of silica gel and alumina, as well as in the solid state. Single
crystals of 3 were observed by X-ray crystallography to undergo crystal-to-crystal rearrangement
to 6. The conversion of 3 to 8 proceeds by the initial retro-Diels−Alder reaction followed by
isomerization of the fulvene to 1-isopropenyl-1,3-cyclopentadiene that then reacts with BTF to give
the alternative Diels−Alder product as a single regioisomer. A hybrid density functional theory
(DFT) method at the B3LYP/6-31G(d) level of theory gave calculated relative energies of 0.0, −9.0,
and −18.8 kcal/mol for 3, 6, and 8, respectively. The same method was also used to correctly predict
the regiochemical outcome of the cycloaddition of BTF with 1-isopropenyl-1,3-cyclopentadiene.
Finally, an explanation is offered for the preference of the persubstituted cyanoolefins BTF and
TCNE to add to the exocyclic diene portion of 1-isopropenyl-1,3-cyclopentadiene and the contrasting
preference of 2-acetyloxy-2-propenenitrile to add to the endocyclic diene.