Arynes from Other Cyclic Systems

Fragmentation of cyclic systems to arynes without nitrogen expulsion is invariably accompanied by the loss of other low-energy molecules such as CO2, 

TABLE 4, Generation of Arynes from Cyclic Systems without Nj Loss 

or SOj (Table 4). Phthaloyl peroxide (72) loses COj to give benzyne (1) upon thermolysis in the gas phase,photolysis in solution, or treatment with lithium in hexamethylphosphoramide. The photolysis has been examined in an argon matrix at 8°K and the transient lactone 37 and ketene 38 detected. The solution thermolysis and reduction of 72 also appear to proceed in a stepwise manner. 

Although the major fragmentation pathway of o-sulfobenzoic anhydride (76) in its 70-eV mass spectrum involves loss of SO3 and CO, upon thermolysis SO2 and CO2 are lost to give benzyne (i), - Some evidence was found for the intermediacy of the zwitterion 77.  

Loss of two molecules of CO leads to arynes upon photolysis or severe thermolysis of benzocyclobutenedione (78) and plasmolysis or pyrolysis-mass spectroscopy of acenaphthenequinone (79). With both precursors the process appears to be stepwise as shown except, once again, under pyrolysis-mass spectroscopy conditions, which is consistent with a concerted loss of CO.  

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TABLE 3, Generation of Arynes by Loss of Nitrogen from and other Cyclic Systems... 

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