No abstract

Flash vacuum pyrolysis at 1000-1100°C of the allyl esters of the three isomeric biphenylcarboxylic acids, of the allyl esters of the 12 biphenyldicarboxylic acids and of the three biphenyldicarboxylic anhydrides gave pyrolysates which were examined by 1H n.m.r. spectroscopy at temperatures below -50°C. In all cases the spectra showed the presence of cyclopent[a]indene and acenaphthylene together with other products. Possible mechanisms for these ring contraction and cyclization processes are discussed and the results of pyrolyses of [2,3-13C2] biphenyl-2,3-dicarboxylic anhydride, and [3,4-13C2]- and (2-2 H1)-biphenyl-3,4-dicarboxylic anhydrides are reported.

Flash vacuum pyrolysis of 9-methyl-1,3-dihydronaphtho[1,2-c]furan-1,3-dione (8) at 750-880� gave exclusively 1H-cyclopent[ cd ]indene (9) and of 8-methyl-1,2-dihydrocyclobuta[a]naphthalene-1.2-dione (10) at 600-840� gave a mixture of (9) and acenaphthylene. Acenaphthylen-4-ol (12) was synthesized and found to be stable to flash vacuum pyrolysis over thetemperature range 600-900�. These findings are discussed in relation to the rearrangement described in the title.

Two previously reported pyrolytic precursors for cyclopentadienylideneethenone (4) and benzyne produced two strong infrared bands attributable to ketene type compounds when their pyrolysates were examined by argon matrix isolation spectroscopy. To determine which band should be assigned to (4), four new precursors for (4) and benzyne and two analogues labelled with 13C at the carbonyl group have been synthesized. Precursors (8) and (14) are respectively a caged system and a bridged ,system bearing a mixed anhydride with trifluoroacetic acid. Flash vacuum pyrolysis of (8) and (14) at 600-700 with trapping at 77 K gave pyrolysates which contained biphenylene. Pyrolysis of (8) and (14) at 600-700�; in a stream of argon followed by deposition as an argon matrix at about 10 K showed that both produced a pyrolysate absorbing at 2089 cm-1 assigned to (4). Precursors (24) and (25) and the previously reported (18) and (27) are Meldrum's acid derivatives designed to yield the hypothetical cyclopentadienylidene Meldrum's acid (19) by cage fission or retro-Diels-Alder reaction. They all gave their biphenylene on flash vacuum pyrolysis at 600-700 � and their pyrolysates in argon matrices showed absorption both at 2089 cm-1 due to (4) and at 2225 cm-1 due to an unidentified ketene. The frequency shift resulting from substitution of 13C in the carbonyl group (52 cm-l) is in accordance with the assignment of the 2089 cm-1 band to (4). The pyrolysates from precursors 8a), (14), (18a), (25)and (27) were allowed to react with methanol and the resulting mixtures were hydrogenated. In all cases methyl cyclopentylacetate was obtained.

Naphthalene-1,2,-dicarboxylic anhydrides with neighbouring phenyl substituents give on flash vacuum pyrolysis (850-90O0/0. 02-0.04 mm) ring-contracted carbenes which insert into the phenyl groups. The 8-phenyl anhydride (7) gives acephenanthrylene (10) as the major product, and the 3-phenyl anhydride (1 5) gives 1,2 : 4,s-dibenzopentalene (indeno[2,1-alindene) (18). The anhydrides (7) and (15) were synthesized by pyrolysis of the corresponding 1-naphthylmethyl propynoates (2) and (13) through a new one-step process of intramolecular Diels-Alder addition/retro-Diels-Alder elimination of acetylene.1-Phenylnaphthalene-2,3-dicarboxylic anhydride (19) on pyrolysis at 960°/0.02 mm gives fluoranthene (11) as the major product. The behaviour of the l-CsD5 compound (24) suggests involvement of a radical cyclization process. * Dedicated to Professor Wolfgang Kirmse on the occasion of his 60th birthday. t The yield of cyclopent[cd]indene in ref. 2 (50-56%) should be corrected to 85%,