Nitromethanes deprotonation

The reactivity of carbon is much enhanced by the double deprotonated intermediates of nitro compounds. Except for nitromethane, other nitroalkanes are alkylated to give the C-alkylated products in 50-60% yield by this procedure (see Eq. 5.4).9 10... 

Recent studies of kinetic acidity involving nitrocarbon acids have included the following benzoate ion-promoted deprotonation of (3- or 4-nitrophenyl)nitromethane and (3,5-dinitrophenyl)nitromethane187 the deprotonation by various bases of a series of... 

Scheme 6.147 visualizes two proposals for the mechanism of the 131-catalyzed Henry addition of nitromethane to benzaldehyde. In (A), benzaldehyde is achvated by the thiourea moiety through double hydrogen bonding to the carbonyl funchon, while the nitromethane is deprotonated and activated by the basic quinuclidine nitrogen [298] proposal (B), however, based on detailed DFT computations... 

N-Methylation of 139 with iodomethane in nitromethane afforded 2,3,4,4,6-pentamethyl-5,6-dihydro-4/7-oxazi-nium iodide 140. Deprotonation of 140 with sodium hydride resulted in formation of the cyclic ketene-A, 0-acetal derivative tetrahydro-l,3-oxazine 141 (Scheme 21) <2006JC0262>. 

The solvent affects the chemical equilibria of reactions. Second-order rate constants and equilibrium constants have been determined for the benzoate ion promoted deprotonation reactions of (m-nitrophenyl)nitromethane, (p-nitrophenyl)nitromethane, and (3,5-dinitrophenyl)nitromethane in methanol solution. The pKa values for the arylnitromethanes in methanol are the following pKa = 10.9, 10.5, and 9.86 for m-nitrophenyl)nitromethane, (p-nitrophenyl)nitromethane, and (3,5-dinitrophenyl)nitro-methane, respectively, relative to benzoic acid (pKa = 9.4). A Bronsted B value of... 

Nitromethane was proposed as a probe molecule of basic sites on catalyst surfaces. The nitro form is in equilibrium with a small amount of the aci tautomer. Kheir and Haw (165) demonstrated that the aci tautomer was deprotonated on MgO and CaO to form chemisorbed aci anion (Scheme... 

A kinetic study of the deprotonation-reprotonation behaviour of (4-nitrophenyl)-nitromcthane in 50% H2O-50% Me2SO mixtures promoted by bases (phenoxide and carboxylate ions, primary amines) has revealed a one-step equilibration at pH > 4.2 the equilibration in acidic media is complicated by protonation of the exocyclic nitro group.142 The results suggest that the substrate acts essentially as a nitroalkane rather than a /j-nitrotolucnc. A further study of kinetics of deprotonation of (4-nitrophenyl)nitromethane has provided evidence of a steric effect on proton tunnelling on reaction with /V -propyl-A./V-dipropylbcnziinidamidc.143... 

A Bronsted fl]t value of 0.5 and occh value of 1.31 have been calculated for deprotonation reactions of (3,5-dinitrophenyl)nitromethane promoted by substituted benzoate ions and of substituted (3-nitro-, 4-nitro- and 3,5-dinitro-)phenyhiitro-methanes promoted by benzoate ion, respectively, in methanol.146 The intrinsic rate constants are (2.0-6.3)x 104 tunes lower than for the same reactions in acetonitrile solution, and this has been attributed to commensurate reduction of strength of the hydrogen bond between the carbon acid and benzoate ion in the imbalanced transition state. The transfer activity coefficient (logMyAN) from methanol to acetonitrile solution have been calculated for (// -nitrophenyl)nitromcthyl anion (3.6) and (m-nitrophenyl)-nitromethane (—1.0). 

There is, unfortunately, no obvious correlation between the impact sensitivity of a compound, its thermal behavior, and its structure. Thus, most polynitro arenes can explode on mechanical shock or rapid heating [96], but the shock sensitivity and thermal stability varies widely. TNT, for instance, is rather insensitive toward mechanical shocks or heating, and even pentanitrotoluene and hexanitrobenzene have been prepared, and their melting points (240 °C and 260 °C, respectively) could be determined [97]. The potassium salt of the radical anion of nitrobenzene, on the other hand, is highly shock sensitive, and detonates on simple agitation [92], Similarly, whereas nitromethane can be handled safely the dry sodium salt of deprotonated nitromethane is potentially explosive [98]. 

Note that the deprotonation of the strong acid nitromethane (pKa = 10) is accomplished using the relatively weak base sodium methoxide (NaOMe). 

Fig. 13.53. Mechanism of a Knoevenagel reaction with nitromethane. Alkaline aluminum oxide powder is sufficiently basic to deprotonate nitromethane. The small amount of the anion generated from nitromethane suffices for the addition to aldehydes to proceed. The elimination of water via an Elib mechanism follows quickly if a conjugated C=C double bond is formed, as in the present case.

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 ... 

Although cyclohexanone forms an enoiate in the absence of nitromethane, when both ketone and nitroalkane are present the base prefers to remove a proton from nitromethane. This is simply a question of pkTa values. The pof a typical ketone is about 20 but that of nitromethane is 10. It is not even necessary to use as strong a base as NaOH (pJfaH — 15.7) to deprotonate nitromethane an amine will do (p fan about 10) and secondary amines are often used. 

When nitromethane is used as the capping agent instead of ammonia, either the interaction of the nitromethane anion with coordinated ethyleneimine or the addition of nitroethylene to deprotonated coordinated amine yields an intermediate nitroethyl complex. Nitroethylene is formed by a base-catalyzed condensation of nitromethane and formaldehyde. The experimental data suggest that the mechanism of formation of the nitroethyl complex via coordinated imine by Scheme 106 is favoured [101]. 

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 enolate 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, anion of nitromethane... 

The secondary amine of the AEP group is responsible for the supported enamine formation with acetone (aldol reaction), the deprotonation of nitromethane (Henry reaction) and the generation of a potential nucleophile from trimethylsilyl cyanide through hypervalent silicate formation (cyanosilylation reaction). Therefore, the presence of both AEP and UDP groups in close proximity can cooperatively activate the electrophile (through hydrogen bond) and the nucleophile by enamine formation, thus enhancing the reaction rate. 

An electrochemical-induced Henry reaction has been achieved by Elinson et al. via electrolysis (undivided cell) of solutions of carbonyl compounds and nitromethane in methanol or in methanol-DMF mixture containing alkali metal iodide as supporting electrolyte. p-Nitroalcohols have been isolated in 60-75% yields. The authors suggest that the Henry reaction could be induced by the deprotonation of methanol at the cathode and by the oxidation of the iodide anion at the anode (Scheme 16.27) [165]. 

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