The triplet ground state of 1,3-di been directly observed by matrix-isolation EP l! henyl-1,3-cyclopentanediyl (3) has spectroscopy at both 4 K and 77 K.
Until recently, the direct observation of localized, hydrocarbon biradicals was quite uncommon.
Cyclopentanediyl (1) and some simple methyl derivatives were first observed in their triplet states by
Closs in 19752 and for some time these remained the only examples.3 The cyclopentanediyls observed were quite unstable, decaying via quantum-mechanical tunneling even at 5 K. In addition, several simple methyl-substituted structures could not be observed at all, suggesting that this system was very much on the edge with regards to lifetime under cryogenic, matrix-isolation conditions. In contrast, we recently reported the observation of a ,broad series of cyclobutanediyls (21.4
Cyclobutanediyls with only simple alkyl substituents (R = methyl, ethyl, propyl) also decayed via tunneling at all temperatures.5 However, those with delocalizing substituents (vinyl or phenyll are stable from 4 to roughly 20 K. Above 20 K, conventional Arrhenius decay commences, with some structures being marginally observable at temperatures as high as 60 K.5 Based on these observations we concluded that the four-ring system was inherently more stable than the flve.4b We now report the surprising observation that 1,3-diphenyl-1,3-cyclopentanediyl (31 is much more stables than any of these previously reported systems. Intense signals of the triplet can be generated readily even at 77 K! . . 0
.a, yyPh
The precursor to 3, diasene 6, is easily prepared. Urasole 4s is obtained by hydrogenation of the Diels-Alder adduct of N-methyltriaxolinedione and diphenylcyclopentadiene.9 As in other cases,4Jo
we have found that the two-step procedure involving partial hydrolysis to the semicarbazide (51 0 4 KOH iPrOH, reflux NHCHs NiO,