The mechanism of the quinamine rearrangements

Abstract

Acid-catalyzed rearrangement of 6-bromo-2,4-dimethyl-4-(phenylamino)cyclohexa- 1 ,4-dienone (1, a quinamine) in aqueous methanol gives, from a so-called quinamine rearrangement, 4'-amino-6-bromo-2,4-dimethyldiphenyl ether (2) and a number of byproducts. The ratio of yield of 2 to that of byproducts is 76:24. The byproducts are, mostly, 1,3-dimethylcarbazole (7) and some of its derivatives, the relative yields of which depend on the concentration of the catalyzing acid, HCI. The major byproduct in low HC1 concentrations is 1,3-dimethyl-4-methoxycarbazole (9). Kinetic isotope effects (KIE) were measured for the formation of 2 from 1, which was labeled at the carbonyl oxygen atom (('*0)-1), the nitrogen atom ((I5N)-1), and the para position of the aniline ring ((4-I4C)-1). The KIE (averages) were as follows: k('60)/k(180), 1.0399; k(I4N)/k(l5N), 1.0089; /~('*c)/k(~~C), 1.0501. The results suggest that the formation of 2 is a concerted process, a (5,5)-sigmatropic rearrangement, and not a two-step one, going through the rate-determining formation of a r-complex. KIE were measured for the formation of both 2 and 9 from 1, which was labeled in the ortho position of the anilino ring ((2-14C)-1). The KIE (k(12C)/k(14C)) were respectively 0.9895 and 1.0697. These results suggest that the byproduct (9) is formed by a concerted process, too, a (3,3)-sigmatropic rearrangement to an intermediate (14), which continues on to 9 and the other byproducts. The results show also that 2 cannot be formed from 1 by a succession of two (3,3)-sigmatropic rearrangements, the first of which is to 14. Thus, the quinamine rearrangements. on the basis of our results with 1, appear to be concerted, rather than a-complex intermediate, processes. Quinamine rearrangements are the acid-catalyzed conversions of (ary1amino)cyclohexadienones. They are part of the rich collection of rearrangements of N-substituted arylamino com- pounds, which were brought to light in the early 1900s and which are classified as aromatic rearrangementsS2 Most of what is known about the scope of the quinamine rearrangements is to be found in the early publications of Fries and co-~orkers.~-~ All of the quinamines whose rearrangements have been reported carry a number of substituents in their rings, and the substituents affect which of the two major pathways rearrangement may take. Thus, the quinamine 1 rearranges into the 4-aminodiphenyl ether Z3 When a quinamine has a substituent, e.g., methyl, in the para position of the arylamino ring, rearrangement may take a second pathway, leading to a biphenyl, with the expulsion of a labile group from the cyclohexadienone ring. For example, the quinamine 3 rearranged into the biphenyl 4 in 80% yield.5 There is also a third, less common pathway available to some acid-catalyzed quinamine rearrangements, that is, the formation of a diphenylamine. An example given by Fries is the rearrangement of 5 into 6, also in 80% yieldas