Cyclopentadienylideneethenone: Pyrolytic Generation and Argon Matrix Infrared Spectroscopic Study

Abstract

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 600-700º; 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 º 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.