90
J. Am. Chem. SOC.1983, 105, 90-96
Kinetics and Mechanism of the C-Alkylation of Nitroalkane Anions by 1-Alkyl-2,4,6-triphenylpyridiniums:A Nonchain Reaction with Radicaloid Characteristics' Alan R. Katritzky,* M. Akram Kashmiri, George Z. de Ville, and Ranjan C. Patel Contribution from the Department of Chemistry, University of Florida, Gainesuille, Florida 32611, and the School of Chemical Sciences, University of East Anglia, Norwich, England. Received September 14, 1981
Abstract: The C-alkylation of 2-nitropropanide by N-substituted pyridiniums is first order in each reactant in Me2S0. The effect on the rate of temperature, N substituent, leaving group, and nucleophile was studied quantitatively. Overall, the evidence excludes an S N displacement ~ and suggests that the reactions proceed by a free radical nonchain mechanism.+ We have shown2 that the reaction of N-(primary alkyl)-, N(secondary alkyl)-, and N-benzyl-2,4,6-triphenylpyridiniums(1) with nitronate anions 2a-c derived from nitromethane, nitroethane, and 2-nitropropane yields the corresponding C-alkylated nitro compounds 6 in preparatively useful reactions (Scheme I). We now report kinetic studies with sodium 2-nitropropanide (2c) and nitromethanide (2a) as nucleophiles in M e 2 S 0 solution designed to clarify the mechanisms of these reactions. Preparative Work. The pyridiniums used were prepared by standard methods3 Some preparative reactions between the pyridiniums (1) and nitroalkane anions (2) have not previously been reported. N-@-Nitrobenzyl)-2,4,6-triphenylpyridinium(le) with 2c gives 2-@-nitrobenzyl)-2-nitropropane (80%), with melting point in agreement with the l i t e r a t ~ r e . ~ Similarly, the Nmethylpyridinium If gave 2-methyl-2-nitropropane (57%). N(Neopentyl)-2,4,6-triphenylpyridinium(lj) with 2-nitropropanide (2c) gave 2,4,4-trimethyl-2-nitropentanein 50% yield. Reaction of N-(4-methoxybenzyl)-2,4,6-triphenylpyridinium tetrafluoroborate with 2c at 25 "C gave 4-anisaldehyde (69%), the significance of which is mentioned later. Sodium nitromethanide reacts at 25 OC under N 2 in Me2S0 with the N-benzyl compound l a to give 2-phenylnitroethane in 58% yield. In air, only a very low yield (ca. 6%) of 2-phenylnitroethane was isolated: a separate experiment showed that in alkaline Me2S0, where air was passed, 2-phenylnitroethane had a half-life of a few hours only. Sodium nitromethanide decomposes quickly in the presence of air in an alkaline medium. Kinetic Studies of the Reaction of l-Benzyl-2,4,6-triphenylpyridinium with Sodium 2-Nitropropanide. The kinetic runs reported were carried out in Me,SO containing 2% n-pentanol under pseudo-first-order conditions (excess of 2-nitropropanide) and followed by UV. Good reproducibility WRS obtained with the pyridinium and 2-nitropropane under these conditions and also with pentanol as solvent. The results of a typical experiment are given in the supplementary material (Table A); the pseudofirst-order dependence is illustrated in Figure 1. Figure 2 demonstrates that a t various temperatures the rate is proportional to the nucleophile concentration. Observed rates and derived second-order rate constants are collected in Table I. The Arrhenius plot is shown in Figure 3. A good straight line is obtained from 25-50 OC, but a t 70 OC the rate goes very fast indeed (for discussion, see later). From Figure 3 we calculate E, = 4.3 & 0.6 kcal mol-' and AS* = -58 & 2 cal mol-' K-'. Reaction of 1-benzyl-2,4,6-triphenylpyridinium with sodium 1-nitrocyclohexanide gave very similar kinetic results (Table 11) with k2 a little larger. Rate Variation with N Substituent Transferred. Table I11 gives the rates for a series of N-para-substituted benzylpyridiniums. 'To whom correspondence should be addressed at the University of Florida. In a preliminary communication' we referred to this mechanism as SR.,2. However, this symbol was used in a paper that appeared after submission but before publication of our manuscript by Galli and Bunnett ( J . Am. Chem. Soc. 1979, ZOI, 6139) in a different way, and we therefore discontinue its use for our reaction to avoid confusion.
Scheme I Ph
I
2c, R' = R" = Me
R
l a , R = CH,Ph If, R = CH, Ib, R = CH,Ph-4-C1 lg, R = CH,(CH,),CH, I C , R = CH2Ph-4-Me l h , R = CH(CH,), Id, R = CH2Ph4-OMe l i , R = CH,CH,Ph le, R = CH,Ph-4-NO2 lj, R = CH,C(CH,),
r
Ph
Ph
4
-
RCR'R"NO2
6
Table I. Temperature Dependence of Second-Order Rate Constants" for the Reaction of l-Benzyl-2,4,6-triphenylpyridinium Tetrafluoroborateb with Sodium 2-Nitropropanide in Me,SO (Measured at 31 2 nm)
25ci 1
0.50 1 1.86 (0.992) 3.12 0.83 1 2.99 (0.995) 3.60 0.99 1 3.52 (0.998) 3.55 1.06 4 3.44" (0.999)" 3.25" 4.09 (0.998) 3.55 1 1.15 4.01 1.32 1 5.29 (0.995) 5.52 (0.998) 3.72 1.48 1 3.15 1.65 1 6.19 (0.995) 3.53" 1.06 3 30 1 3.72e (0.999)e 40i 1 1.06 3 4.60e 4.86" (0.999)" 0.33 1 50d+ 1 5.81 1.92 (0.992) 5.78 0.50 1 2.89 (0.993) 5.60 0.66 1 3.70 (0.994) 5.76e 1.06 3 6.11" (0.999)" 5.46 7.22 (0.998) 1.32 1 'Average k , (25 "C) = 3.30 L mol-' s-' ;k ,
