Benzyne-platinum intermediates

• Main Author: Graveling, Frederick James
• Published: University of Leicester 1969
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.673962

The evidence for considering benzyme as a symmetric singlet with a partial ? bond between the two one-electron centres is given. Benzyne is compared with strained cyclic acetylenes and a method of stabilizing both systems as 6? complexes on a transition metal is illustrated. Other reactions possibly involving benzyne-metal complexes are reviewed. Two new benzyne precursors are described; the sodium and lithium salts of 1-(toluene-p-sulphonamido) benzotriazole (Ia, Ib), and (1-benzotriazolyl) iminotriphenylphosphorane (II) give benzyne on photolysis. For comparison the new cyclohexyne and cycloheptyne precursors, (4,5-tetramethylene-l-triazolyl) iminotriphenylphosphorane (III) and (4,5-pentamethylene-l-triazolyl) iminotriphenylphosphorane (IV) were prepared. It was also found that photolysis at room temperature of cycloheptenocyclopropenone (V) gave cycloheptyne. 1,2,3-Benzothiadiazole-1,1-dioxides(VI) and benzene- diazonium-2-csrboxylates (VII) were used as benzyne precursors with tetrakis (triphenylphosphine) platinum(O) (VIII) to prepare benzyne-platinum complexes. Benzyne-platinum intermediates are postulated with 1,2,3-benzothiadiazole-1,1-dioxides in refluxing ether - benzene where cyclotrimerisation of benzyne to triphenylenes (not observed in the absence of platinum) occurs. In cold benzene, 1,2,3-benzothiadiazole-1,1-dioxide is trapped by tetrakis (triphenylphosphine)platinum(O) before it decomposes to benzyne to give the adduct (IX), which when irradiated, is also a new benzyne precursor. The reaction with benzenediazonium -2-carboxylates gives carboxylato-platinum complexes (X) which are not benzyne precursors and no cyclotrimerisation of benzyne was detected. The oxidation of l-aminobenzotriazole (XI) in the presence of tetrakis(triphenylphosphine)platinum(O) did not lead to a benzyne-platinum complex and benzyne-platinum intermediates do not appear to be involved. Furthermore the above, new, non-oxidative routes to benzyne with tetrakis (triphenylphosphine) platinum(O) gave no evidence for benzyne-platinum complexes. Analogous cycloalkyne precursors were also explored as routes to cycloalkyne- platinum complexes and cyclooctynebis (triphenylphosphine)- platinum (XII) was isolated. The lead tetraacetate oxidation of 1-aminobenzotriazole in the presence of ethylenebis(triphenylphosphine)platinum (XIII) does, however, appear to give a benzyne-platinum complex which is stable for 10 minutes in solution. Benzyne was again cyclotrimerised to triphenylene in this reaction. Other attempts to synthesise a benzyne-platinum complex and to stabilize an antiaromatic compound with zerovalent platinum are briefly described.