Nitroalkanes proton-transfer reactions

Sodium hydroxide has been the most commonly used base in experimental nitroalkane proton transfer reaction studies.However, the computational studies of these reactions have generaUy been with hydroxide ion without the sodium counter ion. Recently a computational study of the proton transfer reactions of the three simple nitroalkanes in the presence of NaOH in water has been carried out and it was found that the presence of Na had an enormous effect on the energetics of the reactions. Double potential energy well diagrams, much like those found for the Sn2 reactions, were recorded for the proton transfer reactions of NM, NE and 2-NP with hydroxide ion in water. The computations included two explicit water molecules in the water cavity. The Gibbs free energies and enthalpies observed for the reactant complex (CPI), the TS and the product complex (CP2) both in the presence and absence of sodium ion and two explicit water molecules are summarized in Table 1.24. 

We will devote the remaining part of this section to a discussion of the proton transfer reaction of nitroalkanes. 

Without a doubt, the proton transfer reaction which has attracted most attention over the past 15 years is the reaction of a series of nitroalkanes with a base, as indicated in (104) (Bordwell et al., 1969, 1970, 1972). The reason... 

Another acid/base anomaly, the anomalous relationship between rates and equilibria for the proton-transfer reactions of nitroalkanes such as H3C—NO2/H3C— CH2—N02/(H3C)2CH—NO2 found in water, does not exist in the gas phase [244]. Only in aqueous solution is the rate of proton abstraction by HO unexpectedly slower for the more acidic nitro compound. 

An extensive kinetic study for the proton-transfer reactions of phenylnitromethanes under various reaction conditions revealed that, although ac/-nitro species may form, they are not on the main reaction pathways. " The Hammett plot for protonation of nitronate and a Brpnsted plot including P-NO2 showed that the nitroalkane anomaly exists for all substimted phenylnitromethanes except the P-NO2 derivative. This result is consistent with the notion that transition-state imbalance is the source of the anomaly. 

Example 3. Proton Transfer Reactions of Phenylnitroalkanes with Hydroxide ion in Water. The proton transfer reactions of phenyinitromethane (PNM) in water and in water—dimethylsulfoxide (DMSO) mixture have been studied extensively by Bemasconi and the results have played a prominent role in discussions of the principle of nonperfect synchronization. A recent paper on proton transfer reactions of PNM s was concerned with the implications on the nitroalkane anomaly. 

PROTON TRANSFER REACTIONS OF SIMPLE AND ARYL NITROALKANES IN SOLUTION AND IN THE GAS PHASE... 

Several papers dealing with computations of proton transfer reactions of nitroalkanes and aryl nitroalkanes have dealt with the energetics of the processes without finding any reaction intermediates and all resulted in support for the single-step mechanism. ... 

Table 1.24 Gibbs free energies and enthalpies of intermediates and transition states formed in the proton transfer reactions of simple nitroalkanes obtained by computations (SMD/M05-2X/6-31+G(d)) in water with two explicit water molecules and Na in the solvent cavity and in the free state... 

A combined experimental and theoretical study of the proton-transfer reactions of nitroalkanes (CH3NO2 and CH3CH2NO2 with HO and CN ) in the gas phase has established that the asynchronous proton transfer and charge delocalization found is... 

Superoxide anion formed in situ in a solution exposed to air (i.e. with only a small concentration of O2) has been used as an EGB to generate nitroalkane anions that may add to activated alkenes or to carbonyl compounds [130, 131]. An example is shown in Scheme 33. The reaction is catalytic since the product anion can act as a base toward the nitroalkane. Using the nitroalkane as the solvent favors the proton transfer pathway over the competing addition of the product anion to a second molecule of activated alkene, a pathway that may lead to polymerization [130]. In some cases, better yields of the Michael addition product were obtained if a stoichiometric amount of the anion was formed ex situ (with O2 as the PB), and the activated alkene added subsequently ]130, 132]. 

Fig. 1 More O Ferrall-Jencks diagram for the deprotonation of a nitroalkane. The curved line shows the reaction coordinate with charge delocalization lagging behind proton transfer.

The clearest example of the danger in using a as a measure of transition state structure is illustrated in the work of Bordwell et al. (1969, 1970, 1975). In the rate-equilibrium relationship for the deprotonation of a series of nitroalkanes the unprecedented Br nsted slopes of 1 61 for l-aryl-2-nitropropanes and 1-37 for 1-arylnitro-ethanes were obtained. The simple exposition of the mechanistic significance of a disallows values greater than 1. This, coupled with the fact that the transition state for the proton transfer is not product-like (as established by alternative criteria) indicates at best that, in at least some cases, a does not reflect the selectivity of a particular reaction. Several attempts to rationalize these anomalous results have been made. 

The analysis as stated does not allow for values of a and j3 outside the limits of zero and unity. Hence when experimental values for a and (3 were recently obtained which were outside this range [76, 77] considerable doubt was thrown upon the general usefulness of a and (3 as an index of the degree of proton transfer. Detailed discussion of these unusual experimental results will be given in Sect. 4.2, but meanwhile an explanation suggested by Kresge [78] will be described here as it illustrates some of the limitations of the foregoing analysis. Results for the reaction of hydroxide ion with thirteen substituted nitroalkanes in 50 % (v/v) methanol—water (63)... 

The numerical values of the forward and reverse Bronsted coefficients in a simple proton transfer should sum to unity. Neither a nor p should exceed unity or be less than zero. Bordwell and his co-workers [29] discovered that in the nitroalkane acid-base system (Eqn. 36) the introduction of electron-donating substituents into R lowered the rate constant for reaction but increased the overall equilibrium constant. 

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