Synthesis and study of some cyclopentane-1,3-diyl derivatives as potentially long-lived singlet diradicals

The syntheses of several derivatives of cyclopentane-1,3-diyl were undertaken in the hope of generating a long-lived singlet diradical. Azo precursors to derivatives of both 2,2-difluoro-1,3-dimethylcyclopentane-1,3-diyl and 2,2-difluoro-1,3-diphenylcyclopentane-1,3-diyl were produced. As expected, after photolysis generated the 1,3-diradical, the 1,3-dimethyl-substituted derivative underwent ring-closure. Unfortunately, the ring-closed product proved too unstable to purify, characterize, or study. Photolysis of the azo precursor of the 1,3-diphenyl-substituted derivative, on the other hand, not only generated the expected 1,3-diradical, but this diradical proved persistent enough at very low temperatures in non-polar solvents to be characterized. In fact, this derivative of 2,2-difluoro-1,3-diphenylcyclopentane-1,3-diyl proved to be an unusually long-lived singlet diradical, with a lifetime on the order of a day at 77K. Several attempts were made to synthesize an azo precursor to a derivative of either 2,2-bistrimethylsilylcyclopentane-1,3-diyl or 2,2-bis(dimethylsilylethano)-cyclopentane-1,3-diyl. Multiple synthetic attempts failed, mostly due to the unreactivity of 5,5-disilyl-substituted cyclopentadienes towards Diels-Alder dienophiles. More promising results were obtained in an attempted synthesis of a precursor to cyclopentan-2-one-1,3-diyl. After failed attempts to synthesize an azo precursor to 1,3-diphenylcyclopentan-2-one-1,3 diyl, a synthesis of bicyclo[2.1.1]hexane-5,6-dione was devised. Photochemical decarboxylation of this molecule should generate the desired diradical. After much work, it would appear that a workable synthesis towards this diketone is in hand; and the only major question remaining is whether photolysis of bicyclo[2.1.1]hexane-5,6-dione will really furnish cyclopentan-2-one-1,3-diyl under conditions in which this diradical may be studied.