Letters in Organic Chemistry [A Simple Facile Synthesis of Bifluorenylidenes]

Bifluorenylidenes were synthesized in two steps, by conversion of fluorenones to fluorenethiones followed by reductive dimerization of the latter in boiling benzene. Treatment of 12H-dibenzo[b,h]fluoren-12- one (5) with Lawesson's reagent (LR) in boiling toluene gave12H-dibenzo[b,h]fluorene-12-thione (6). Heating a benzene solution of 6 under reflux gave 12-(12'H-dibenzo[b,h]fluorene-12'-ylidene)-12H-dibenzo[b,h] fluorene (4) in 65% yield. (E)-and (Z)- 11-(11'H-benzo[b]fluorene-11'-ylidene)-11H-benzo[b]fluorene were prepared analogously. The method was applied also to the synthesis of the cross coupling product 9-(11'Hbenzo[ b]fluorene-11'-ylidene)- 9H- fluorene (12). A mechanism of the reductive dimerization of thione 6 via its dimer 17 and carbene 16 leading to 4 was outlined.