Cyclisation reactions involving the oxidation of carboxylic acids with lead tetra-acetate. Part I. The conversion of 2′-substituted biphenyl-2-carboxylic acids into 3,4-benzocoumarin

Abstract

The reaction of 2'-substituted biphenyl-2-carboxylic acids (where the 2'-substituent is H, CO,H, NO,, CI, OMe. or C0,Me) with lead tetra-acetate in refluxing benzene solution, under a nitrogen atmosphere, affords 3.4-benzocoumarin as a major organic product. Only a trace is formed when the 2'-substituent is Me. o-Terphenyl-2carboxylic acid gives no 3.4-benzocoumarin, but triphenylene is formed in good yield. It is suggested that these cyclizations involve the intermediacy of free radicals, particularly as the products of the thermal decomposition, in benzene solution, of the diacyl peroxides derived from 2'-substituted biphenyl-2-carboxylic acids, are similar to the products formed on reaction of the acids with lead tetra-acetate.THE most recent studies on the reaction of lead tetraacetate with carboxylic acids 1-5 (acids belonging to the norbornane6 and the bornane' series may be exceptions) indicate that the organic products are derived from free-radical intermediates. A study of the reaction of the 2'-substituted biphenyl-2-carboxylic acids (1a)-(Ig) with lead tetra-acetate in boiling benzene, under a nitrogen atmosphere, has shown that with the exception of the methyl-acid (Ig), 3,4-benzocoumarin (V) is a major product in each case (see Table ). The most likely mechanism for the formation of 3,4-benzocoumarin (V) is therefore via the intermediates ( 11), (111), and (IV), and involves a free-radical displacement of the substituent group in the final steps (111) -w (IV) -+ (V). I n the oxidation of 2'-chlorobiphenyl-2-carboxylic acid (Id), 2-chloro-o-terphenyl and o-chlorobiphenyl are additional reaction products. They provide evidence for the intermediacy of free radicals in that their formation is most satisfactorily explained via 2'-chlorobiphenyl-2-y1 radicals, which either substitute in the solvent benzene or abstract hydrogen from a suitable donor. Competition between the intramolecular substitution reaction (IIId) _t (IVd) __t (V) and decarboxylation to 2'-chlorobiphenyl-2-y1 radicals is readily suggested. 2-Methoxycarbonyl-o-terphenyl is likewise an additional product produced from the methoxycarbonyl-acid (If), but similar products are not formed from the other acids, which indicates that the ease of the displacement of the group X relative to decarboxylation is dependent on the nature of the group X. The thermal decomposition, in benzene solution, of diacyl peroxides,8 even if highly a~ymmetrical,~ invariably proceeds by a free-radical pathway. Further support for the importance of free-radical intermediates in product formation from the lead tetra-acetate oxidt We are grateful to the referees for this suggestion.

Ipso substitution in free-radical aromatic substitution reactions

“…35 Nitro (and to some extent, bromo) rather than carboxy is replaced by hydroxyl radical reacting with 5-nitro-(or 5bromo-) 2-furancarboxylate.42 Some evidence for replacement of nitro and sulfo groups by hydroxyl during electrolytic oxidation of acidic aqueous solutions of substituted benzenes was reported, but the presumed products were not isolated.43 Several 2'-substituents are lost by attack of 2-carbonyloxyl radicals generated by lead tetraacetate oxidations of 2'-substituted 2-biphenylcarboxylic acids in benzene solution (eq 20). 44 C^-p^C^O ,20)…”

Free-Radical Rearrangements

“…Examples include silyl (90, 90a, 101, 362), e.g., PhCH 2 CH 2 CH 2 SiMe 2 , alkoxy (363,364), carboxylate (93,365,366), acyl (356), iminyl, e.g., 167 -* 168 (367), amidyl (334,(368)(369)(370), aminium (371, 371a), and thiyl (372) radicals. Examples include silyl (90, 90a, 101, 362), e.g., PhCH 2 CH 2 CH 2 SiMe 2 , alkoxy (363,364), carboxylate (93,365,366), acyl (356), iminyl, e.g., 167 -* 168 (367), amidyl (334,(368)(369)(370), aminium (371, 371a), and thiyl (372) radicals.…”

Aromatic substitution reactions: when you've said ortho, meta, and para you haven't said it all

“…Lead tetraacetate oxidations of 2-biphenylcarboxylic acids XI generate 2-biphenylcarbonyloxyl radicals XII which then attack the 2'-position intramolecularly to form a lactone (18) (eqn. ( 29…”

“…The distribution along the four pathways to isomeric cationic intermediates shown in eqn. (18) was revealed by data associated with the isolation and acid-catalyzed reaction of the cyclohexadiene adduct V, the product of trapping of one of the ipso intermediates by nucleophilic solvent (eqn. ( 19)).…”