Argon Matrix Infrared Spectroscopic Evidence for the Generation of Pentatetraenone by Flash Pyrolysis of Suitable Precursors

Abstract

Five compounds were investigated as precursors for the pyrolytic generation of pentatetraenone, H₂C=C=C=C=C=O. These were (1)-(4): 3- ethenylidenebicyclo [2.2.1]hept-5-ene with the following 2,2 substituents : H, COOCOCF₃ (1); H, 13COOCOCF₃ (1′); (COOCOCF₃)₂ (2); (COO)₂C(CH₃)(OCH₃) (3); (COO)₂Si(CH₃)₂ (4) and 5-(3′- methylenebicyclo [2.2.1]hept-5′-en-2′-ylidene)-2,2-dimethyl-1,3-dioxan-4,6-dione (5).

The five precursors were pyrolysed in a stream of argon at temperatures in the range 350-725°C and the pyrolysate -argon mixture condensed on a CsI plate at c. 10 K. Infrared spectra were obtained between 4000 and 250 cm^-1. All five precursors gave two strong bands in the spectral region 2070-2250 cm^-1, possibly attributable to pentatetraenone. At lower pyrolytic temperatures the more intense of the two bands was a broad band centred at c. 2128 cm^-1 [precursors (1)- (4)] or at c. 2094 cm^-1 [precursor (5)]. At higher pyrolytic temperatures these bands were diminished in intensity and replaced by a narrow band at 2207 cm^-1 for all five precursors. Bands due to the expected other products for each pyrolysis reaction to form pentatetraenone were observed. H₂C₄¹³CO ( pentatetraenone substituted by ¹³C at the carbonyl carbon atom) was prepared by pyrolysis of precursor (1′).

We assign the broad bands at c. 2128 cm^-1 [precursors (1)-(4)] and at c. 2094 [precursor (5)] to incompletely pyrolysed precursor in which cyclopentadiene has been retained but decomposition in the rest of the molecule has resulted in formation of a =C=C=O group. Bands at 2207, 2068 and 1726 cm^-1 we assign to v₂-v₄ of pentatetraenone. Corresponding bands at 2168, 2056 and 1720 cm^-1 are observed in the spectrum of H₂C₄¹³CO.