Deprotonation of nitroalkanes

Another method for improving the reactivity of nitro compounds is provided by the double deprotonation of nitroalkanes. In this case, the reaction with ketones affords P-nitro alcohols in 40-60% yield (Eq. 3.19).30... 

Silylation of Products of Conjugated Addition of Nucleophiles to a-Nitroolefins Nitroalkane anions can be generated not only by deprotonation of nitroalkanes (various modifications of these process were considered above) but also by the conjugated addition of nucleophiles 56 to a-nitroalkenes (42) (Scheme 3.56, Table 3.2). 

The nitroaldol condensation reaction involving aldehydes and nitronates, derived from deprotonation of nitroalkanes by bases. 

In cases where there is strong solvation of the carbanion, as for example hydrogen bonding solvation of enolate or nitronate ions in hydroxylic solvents, the intrinsic barrier is increased further because the transition state cannot benefit significantly from this solvation. This is the reason why AG for the deprotonation of nitroalkanes in water is particularly high, i.e., much higher than in dipolar aprotic solvents, see, e.g., entry 11 versus 15 and entry 13 versus 16 in Table 1. These solvation effects will be discussed in more detail below. 

The reason why delocalization is not more advanced is that there are constraints imposed on the transition state that prevent extensive delocalization. This was first pointed out by Kresge51 in the context of the deprotonation of nitroalkanes, but it applies to any proton transfer from carbon. The situation is represented in Equation (9) which is a more nuanced version of Equation (3) and allows for a certain degree of charge delocalization into the jt-acceptor (8Y) at the transition... 

For the quantitative deprotonation of nitroalkanes and active-methylene compounds, there is no need to employ the heavy artillery of lithium amides. Rather, it suffices to employ alkaline earth metal alkoxides or alkaline earth metal hydroxides. In addition, equilibrium reactions between these C,H acids and amines form enough enolate to initiate enolate reactions. 

Surprisingly few simple nitroalkanes are commercially available but more complex examples can be prepared readily by alkylation of the anions derived from nitromethane, nitroethane, and 2-nitro-propane. Deprotonation of nitroalkanes with butyllithium followed by the addition of alkyl halides gives the alkylated nitroalkanes in good yield. Some examples of this general method are shown below. These reactions really do have to be done in two steps BuLi is not compatible with alkyl halides ... 

We can complement this type of selectivity with the opposite type. Are there any compounds that can enolize but that cannot function as electrophiles No carbonyl compound can fill this role, but in Chapter 21 we met some enolizable compounds that lacked carbonyl groups altogether. Most notable among these were the nitroalkanes. Deprotonation of nitroalkanes is not enolization nor is the product an enoiate ion, but the whole thing is so similar to enolization that it makes sense to consider them together. The anions, sometimes called nitronates, react well with aldehydes and ketones. 

You will see both enamines and aza-enolates In action In Chapters 25 and 26. Deprotonation of nitroalkanes is discussed in Chapters (p. 177). 

Properties and Preparation of Nitronates Nitroalkanes exist in equilibrium with a tautomeric form known as a nitronic acid (Scheme 16.19) [103]. The aci-form is usually present in minor concentration with an equilibrium constant of 10 [104]. Salts of nitronic acids, metal nitronates, are formed upon deprotonation of nitroalkanes and are potent nucleophiles [105]. Alkylation of the nitronate salts leads to both a-substituted nitro compounds and the isomeric nitronate esters. 

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