Journal of Organic Chemistry p. 143 - 149 (1980)
Update date:2022-08-10
Topics:
Pomerantz, Martin
Combs, Gerald L.
Fink, Rina
The benzotropyl dimer 7,7'-bis(3,4-benzotropyl) (6) has been prepared and thermolyzed (400 deg C) both without and with added benzene.In addition, 1,2:5,6-dibenzotropilidene (3) and 7,7'-bis(1,2:5,6-dibenzotropyl) (7) have been pyrolyzed (430 deg C) both without and with added naphthalene.The similarity of the products obtained in the former pyrolysis, namely, naphthalene (12), α-methylnaphthalene (13), β-methylnaphthalene (14), 1,2-benzocyclohepta-1,3-diene (15), benzocycloheptene (16) and toluene (whwn benzene is used), with those obtained from pyrolysis of 1,2-benzotropilidene (1) confirms that both thermolyses involve the benzotropyl radical.The similarity of the products from the latter two pyrolyses (of 3 and 7), namely, anthracene (4), 9-methylanthracene (5), 9,10-dihydroanthracene (17), 1,2:4,5-dibenzo-1,4-cycloheptadiene (20), 1- and 2-methylanthracene (18 and 19), and, when naphthalene (12) was used, benzotropilidene (1) and 1- and 2-methylnaphthalene (13 and 14), shows that both of these go through a common intermediate, the 1,2:4,5-dibenzotropyl radical (8). 9-Methylanthracene (5) was shown to thermally produce anthracene (4) and 1- and 2-methylanthracene (18 and 19), but, even with this complication, anthracene (4) was shown to be a primary product from the pyrolysis of dibenzotropilidene (3).Attempts to use di-tert-butyl nitroxide (21) as an initiator (400 deg C) gave primarily Diels-Alder addition of the isobutylene formed by decomposition of the nitroxide with both 1,2:4,5-dibenzotropilidene (the product of a 1,5-hydrogen shift in 3) and 9-methylanthracene (5).A lower temperature (345 deg C) reaction of 3 with naphthalene in the presence of nitroxide 21 did produce anthracene (4) and benzotropilidene (1) in addition to the Diels-Alder adduct, confirming the free-radical nature of the reaction.Low-temperature control experiments employing the benzotropyl dimer (6) (280 deg C) and the dibenzotropyl dimer (7) (345 deg C) showed that they were undergoing cleavage to their respective radicals (2 and 8) and that these were giving product directly, rather than the benzo- and dibenzotropilidenes (1 and 3), which were being formed in the reactions by hydrogen abstraction, giving the products.It has thus been demonstrated that the benzo- and dibenzotropyl radicals (2 and 8) can transfer a CH group to an aromatic acceptor and that the pyrolytic reactions of benzo- and dibenzotropilidenes (1 and 3) to give naphthalene and anthracene are CH-transfer reactions of the tropyl radicals formed by hydrogen abstraction.Yields of up to 65percent of anthracene (4) have been realized in the pyrolyses of dibenzotropilidene (3).Mechanisms for these CH-transfer reactions are presented.A contrast is made between the low-temperature reaction of the dibenzotropyl dimer (7) which (345 deg C) gives only anthracene and dibenzotropilidene (3), by CH loss and hydrogen abstraction, respectively, and the low-temperature (280 deg C) reaction of benzotropyl dimer (6) which not only gives naphthalene (12) and benzotropilidene (1) by CH loss and hydrogen ...
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