3-Methyl-5-phenylisoxazole undergoes nitration as the conjugate acid at the para-position of the phenyl group. 5-Methyl-3-phenylisoxazole undergoes nitration as the conjugate acid at the rneta-position and also as the free base at the para-position of the phenyl group. Standard ratesfor nitration at 25 "C and H,, -6.6 are calculated and the influence of the charged and neutra I isoxazolyl substituents is discussed.WHEREAS alkylisoxazoles undergo nitration at the 4position of the heterocyclic ring, phenylisoxazoles are frequently substituted in the benzene ring. The precise orientation of such nitrations has been a matter of some controversy : Kochetkov and Khomutova reported that 5-phenylisoxazole (1) with H2S04-HNO, yields both 5-fi-nitrophenyl-(2) and 4-nitro-5-phenyl-isoxazole (3), but Lynch and Shiu claim3 that (2) is the sole mononitro-product and that is further nitrated to (4). 3,5-Diphenylisoxazole is reported to yield the para,para' dinitro-derivative. 3-Phenylisoxazole was first stated to yield the para-nitro-derivative, but reinvestigation 5a indicated that a mixture of 3-metn-and S-$ara-nitrophenyl derivatives was formed with H2S0,-HNO, but the 4-nitro-compound with HNO,-Ac,O.Recently the nitration of 3,5-diphenylisoxazole has been reinvesti-gated5b and found to give the 5-fi-nitrophenyl and the 3-m-nitrophenyl-5-fi-nitrophenyl analogues in mixed acid but the 4-nitro-derivative in acetic anhydride.Following our work 011 the nitration of 3,5-dimethylisoxazole,G we have now investigated preparatively and kinetically the nitration of 3-niethyl-5-phenylisoxazole ( 5 ) , 5-methyl-3-phenylisoxazole (6), and the corresyonding N-methyl cations (8) and (10) which serve as models for the protonated species (7) and (9), respectively. The methylphenylisoxazoles (5) and (6) were prepared by standard methods ; the methoperchlorates (8), (la), (lo),
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