Opening of a Cyclopropyl Ring in (Diphenylcyclopropyl)alkenes Promoted by Electron Transfer from Potassium 4,4'-Di-tert-butylbiphenyl Radical Anion and X-ray and Theoretical Calculations of the Structure of (Z)-1,2-Bis(trans-2,trans-3-diphenylcyclopropyl)ethene

Article abstract of DOI:10.1021/jo00054a029

We have prepared the (diphenylcyclopropyl)alkenes 1a-c and studied their reductions, using potassium 4,4'-di-tert-butylbiphenyl radical anion (DBB^.-) as an electron source.Spectra analyses of the reaction products reveal that the double bond of the alkene stayed intact, whereas the cyclopropyl moiety was cleaved to provide alkenes 2a-c.When the reaction was carried out with a simple cyclopropylalkene without phenyl substituents (1e), the starting material was fully recovered.This indicates that the opening of the cyclopropyl ring in the alkenes is promoted by electron transfer from potassium DDB^.- onto the phenyl rings attached to the cyclopropyl moiety.Two mechanisms were considered: (1) opening of the cyclopropyl via a cyclopropylcarbinyl to homoallylcarbinyl radical rearrangement (CPCRR) with potassium DBB^.- as the le^- reductant, with electron transfer to the phenyl rings attached to the cyclopropyl moiety, and (2) 2e^- transfer to the phenyl rings and opening of the cyclopropyl moiety via an anionic rearrangement(CPCAR). (Z)-1-(trans-2,trans-3-Diphenylcyclopropyl)butene (1c) was used as a model for AM1 calculations which establish that the isomeric form of the radical anion product with opened cyclopropyl ring (1c^.- form 1) is about 7 kcal/mol lower in energy than an isomeric form in which the cyclopropyl ring is closed (1c^.- form 2).This suggests that the opening of the cyclopropyl ring is likely to happen through CPCRR rather than CPCAR.The X-ray crystal structure of (Z)-1,2-bis(trans-2,trans-3-diphenylcyclopropyl)ethene (1a) shows the two phenyl rings to be completely out of the cyclopropyl ring plane.The average C-C bond distance for the cyclopropyl moiety in the trans-2,trans-3-diphenylcyclopropyl rings is 1.562 Angstroem, which is longer than the average bond distance in unsubstituted cyclopropyl rings.The gas-phase structures as calculated by AM1, PM3, and MNDO molecular orbital methods are all quite similar and agree closely with the X-ray structure.However, MNDO gave more satisfactory results than AM1 and PM3 for bond distances and bond angles and deviated more for torsion angles.The latter is reflected by the relatively large difference in the heat of formations (7 kcal/mol) of the gas-phase fully optimized structure and the X-ray structure.