Kinetics and mechanisms of nucleophilic displacements with heterocycles as leaving groups. Part 9. N-substituted 2,4,6-triphenylpyridiniums, 5,6-dihydro-2,4-diphenylbenzo[h]quinoliniums, and 5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]acridiniums: kinetic rate variation with structure of the N-substituent

Abstract

N-n-Alkyl, N-s-alkyl, N-allyl, and N-benzyl derivatives of the title compounds react with piperidine and other nucleophiles in clearly separable S_N1 and/or S_N2 reactions. For N-s-alkyl compounds, S_N2 rates decrease with nucleophilicity of the nucleophile, whereas the S_N1 rates are unaffected by the nature of the nucleophile. The S_N1 reactions show higher activation entropies than found for the S_N2 reactions. Although for the same N-substituent, the S_N2 rate always increases in the order (1) < (2) < (3), the rate enhancement varies considerably for different N-substituents: the variations can be partially rationalized by the steric shape of the N-substituent. In general, as the N-substituent increases in size, the enhancement grows. For the S_N2 reactions, rates decrease in the order benzyl > methyl ≃ s-alkyl > continuous chain primary alkyl ≃ neopentyl. Comparisons with literature data involving other leaving groups, standardized by reference to the ethyl compound in each series, again show that groups larger than ethyl tend to react faster in series (1), (2), and especially (3) than expected, and methyl tends to react more slowly. Similar but smaller trends are found in literature data. Based on comparison of reactions of the corresponding benzyl compounds with thiourea and piperidine, the benzoquinoline leaving group is about as nucleofugic as chlorine, whereas the dibenzoacridine is considerably better than chlorine, though not quite as good as bromine.