Arynes from Aromatic Anhydrides

Arynes from Aromatic Anhydrides A. Reactions of Benzyne with Benzene 

The partial mass spectrum (McLafferty and Gohlke, 1959 Meyerson, 1965) of phthalic anhydride and tentative structures of the ions in the main reaction sequence are  

The large amount of C6Ht formed, most simply formulated as benzyne, although the structure is not established, prompted us to try to duplicate this reaction thermally. 

Accordingly, a 0-1 molar solution of phthalic anhydride in benzene (100 ml.) was pyrolyzed at 690° under nitrogen at a steady rate of 30 ml/hr (Fields and Meyerson, 1965). The pyrolysis tube was Vycor, filled with Vycor beads contact time was 11 5 sec, which gave a 58% conversion of the phthalic anhydride. Acetylene was steadily evolved, along with carbon dioxide and carbon monoxide in a 1 1 ratio these were identified in the gas stream by mass-spectral analysis of samples taken at regular intervals. The benzene was distilled off and the products boiling over 180° (2 g) were analyzed by mass spectrometry on a Consolidated Model 

The major portion of the pyrolysate, apart from 0-62 g of unconverted phthalic anhydride, was 1-05 g of a mixture of naphthalene (15%) and biphenyl (85%). Benzene (100 ml), pyrolyzed under identical conditions, gave no acetylene, and only 0-18 g of high-boiling product, which consisted of about 96% (0-17 g) biphenyl and 4% (0-072 g) terphenyl. Only a trace of naphthalene (about 0-2%, 0-0004 g) was found. 

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C. Arynes from Aromatic Anhydrides Other Than Phthalic. . . 15... 

The reactions of a series of arynes from aromatic anhydrides with thiophene and benzothiophene at 690° revealed some processes not as clearly evident with other reagents. 

With the discovery of benzyne formation by pyrolysis of phthalic anhydride, a new field was opened for the investigation of aryne reactions at high temperatures. A first concern was to determine the generality of aryne formation from aromatic acid anhydrides. Such syntheses could be of considerable significance because of the enormous quantities of aromatic mono- and polyanhydrides available from petroleum aromatics by oxidation. 

Aromatic fused anhydrides are well known to act as a source of arynes by pyrolytic loss of CO2 and CO. In some cases the cyclopropenone resulting from loss of only CO2 can be trapped and this is the case for 294, where FVP at 500 °C allows trapping of both the thienocyclopropenone 295 stabilized as the dipolar form shown, and the thiophyne 296147. 

Diels-Alder cycloaddition provides one of the most useful methods for the construction of polycyclic aromatic ring systems. With the use of maleic anhydride as dienophile, two additional carbon atoms may be fused to the bay region sites of some polycyclic arylenes to generate additional rings. For example, the first step in the synthesis of benzo[ghi]perylene 6 involves a Diels-Alder cycloaddition between perylene and maleic anhydride (Scheme 3.5) [19]. Aryne intermediates generated in situ also serve as dienophiles for Diels-Alder cycloadditions for example, reaction of 1,5-naphthadiyne (generated in situ from 2,6-dibromo-l,5-bis[(p-tolylsulfonyl)oxy] naphthalene with 2-methylisoindole forms an adduct which upon oxidation with m-chloroperbenzoic acid affords dibenzo[fe,k]chrysene 7 (Scheme 3.5) [20]. 

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