New approaches to the synthesis of carbenes and benzynes.

The elimination of stable molecules from o-disubstituted benzenes to give benzyne is reviewed, and the synthesis and reactions of new potential benzyne precursors are described. Benzenetrimethylammonium-2-carboxylate, benzenedimethylsulphonium-2-carboxylate, benzenediazonium-2- sulphonate, the tosylate of salicylic acid, o-sulpho-benzoic anhydride and 2-phenyl-1, 3-benzoxazin-4-one were prepared. The pyrolysis and photolysis of these o-benzobetaines and cyclic o-disubstituted benzenes were studied under a variety of conditions in the presence and absence of benzyne traps (particularly tetracyclone). In no case was benzyne detected, the main reactions being rearrangement of the betaine or the loss of one stable molecule followed by reaction of the resultant zwitterion. The desulphurisation of catechol thiono-carbonate, another potential route to benzyne, gave the trimer 3-benzodioxolidene benzo-1, 4-dioxan-2-one catechol ketal. However, deoxygenation of the analogous 4, 5-diphenyldioxolen-2-one gave stilbene through the intermediacy of diphenylacetylene. Arising from the above use of tetracyclone, its deoxygenation by phosphines and phosphites was studied. With triphenylphosphine the novel octaphenyl-fulvalene was formed but tributylphosphine gave tetra-phenylcyclopentadiene, thus providing a useful route to this compound. The reaction of tetracyclone and phosphites resulted in the formation of various phosphates, phosphonates and reduction products of tetracyclone; these reactions are discussed and rationalised. Independent syntheses of octaphenylfulvalene through the intermediacy of tetraphenylcarbenacyclopenta-diene from 1-diazo- and 1, 1-dibromo- 2,3,4,5-tetraphenyl-cyclopentadiene were also studied. However, hydrogen abstraction or bond insertion by the carbene, rather than its dimerisation, were observed. Unsuccessful attempts to generate tetraphenyl-benzyne are also reported. Mechanisms are proposed for all the new reactions.