Observation of Two Characteristic Methylenecyclopropane Stereomutations in a System That also Forms Trimethylenemethane Dimers. An Experimental Connection between Putative and Directly Observed Biradicals

Article abstract of DOI:10.1021/ja00372a021

A stereospecific synthesis of 7-syn-deuteriomethylene-5-exo-methoxy-2,3-diazabicyclo<2.2.1>hept-2-ene (20-8-syn-d) from 6-bromofulvene is effected in ten steps.Irradiation of the undeuterated analogue (20) at 77 K in a glassy medium gives rise to a long-lived signal characteristic of the triplet state of 2-methylene-4-methoxycycopentane-1,3-diyl.This appears to be the radical's ground state, and a typical dilution effect on the cycloadduct composition in the reaction of the species with cyclopentadiene supports this conclusion.In the reactions of the deuterated diazene 20-8-syn-d with cyclopentadiene, the deuterium label is completely scrambled.Irradiation of 20-8-syn-d at -78 deg C in a fluid medium gives a 3:1 mixture of endo- and exo-2-methoxy-5-deuteriomethylenebicyclo<2.1.0>pentanes (46 and 47).Partial but incomplete scrambling of the deuterium label is observed in both products so formed.Stereomutation by double epimerization (47 -> 46) occurs at -60 deg C; exocyclic torsion (46-syn-d -> 46-syn-anti-d) occurs at -40 deg C; ring opening and dimer formation occur at +5 deg C.Evidence for the rearrangement of the 5-methylenebicyclo<2.1.0>pentane to the bicyclo<3.1.0>hex-1-ene system above -40 deg C is presented.This work shows that the characteristic structural and stereochemical rearrangements of methylenecyclopropanes can be observed in the same systems that permit direct spectroiscopic observation of the biradical's triplet ground state.