Benzynes from phthaloyl peroxide

The first direct infrared (IR) spectroscopic detection of o-benzyne was accomplished by Chapman et al., " using matrix isolation spectroscopy at very low temperatures to generate 4 starting from phthaloyl peroxide (5) and benzocyclo-... 

Benzyne is produced in the vapor phase by the fragmentation of phthalic anhydride and of o-sulfobenzoic anhydride (11) at 690°C,17 also of 4-phenyl-1,2,3-benzotriazine (12),18 phthaloyl peroxide (13),19 and a variety of other cyclic systems at appropriately high temperatures. From the point of view of studying the reactions of benzyne with heterocycles, these methods are necessarily restricted in their application. 

Such experimental differences and not the nature of the precursor may be responsible for the fact that certain intermediates sometimes apparently go on to arynes and sometimes do not. For example, the valence tautomers 37 and 38 are formed during the photolysis of phthaloyl peroxide (72) but only go on to benzyne upon prolonged irradiation. Similarly, the sulfur analogs 53 and 103 appear to lead to benzyne (1) when their precursor is the thiadiazinone 49c but not from the benzoxathianone 104, " the thianaphthaquinone 105,... 

With selected aryne precursors it has been possible to demonstrate the intermediacy of free arynes in cyclojaddition reactions by other methods. The most convincing of these is the previously mentioned spectroscopic detection of benzyne (1) from the photolysis of phthaloyl peroxide (72) in an argon matrix at 8 K and its subsequent reaction with furan (146a) upon warming to give the adduct 148a. A conceptually similar technique is pseudodilution, in which... 

---