Reaction of 2,2-bis(trifluoromethyl)-1,1-dicyanoethylene with 6,6-dimethylfulvene: cycloadditions and a rearrangement.

Abstract

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.