Simple Nitroalkanes

Thus, various kmds of bases are effective in inducing the Henry reaction The choice of base and solvent is not crucial to carry out the Henry reaction of simple nitroalkanes v/ith aldehydes, as summarized in Table 3 1 In general, sterically hindered carbonyl or nitro compounds are less reactive not to give the desired ni tro-aldol products in good yield In such cases, self-condensation of the carbonyl compound is a serious side-reaction Several mochfied procedures for the Henry reaction have been developed... 

Simple nitroalkanes such as nitroethane, 1-nitropropane, or 2-nitropropane are generally bad electrophiles for the SN2 reactions.14 In contrast, nitro groups at allylic positions are readily displaced by thiolate ions (Eq. 7.13)15 or lithium dialkylcuprates (Eq. 7.14).16... 

The Hel and Hell spectra of some simple nitroalkanes are depicted in Figure 17. IP values are collected in Table 16. The assignment of the characteristic IPs is analogous to those of nitromethane (Table 15). Comparison of the data reveals that the orbitals are destabilized in a manner parallel to the electron-donating power of the alkyl groups143. The gap between the first two ionizations remains essentially constant, between 0.41 and 0.44 eV, in the series. 

IV. PHOTOCHEMISTRY OF ALIPHATIC NITRO COMPOUNDS A. Simple Nitroalkanes... 

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 ... 

Protiodenitration. Simple nitroalkanes are not reduced by 1, but a-nitro nitriles, esters, and ketones are selectively reduced by 1 under irradiation, with selective replacement of NO by H. The reduction occurs via an electron-transfer chain mechanism. Example ... 

Simple nitroalkanes absorb below 350 nm and are excited to the lowest singlet n,n state, which efficiently intersystem crosses to Homolytic photocleavage is the principal primary process in both the gas phase and solution to produce alkyl radicals and NO2 (Scheme 6.186).1187 Apart from subsequent recombination of the radical intermediates to form nitrites (Section 6.4.2), competing hydrogen abstraction (photoreduction) involving an excited nitro compound and a hydrogen-atom donor may take place. 

Although the Henry reaction has been known for over 90 years and is considered as a classic carbon-carbon bond-forming process, its utilization has so far been limited to simple nitroalkanes and the more... 

Excellent nucleophiles in palladium(0)-eatalyzed allylic alkylations are soft carbanions, i.e.. metal salts of C — H acids with a pKa in the range of 10-20. These are activated methylene compounds which are substituted with at least two geminal electron-withdrawing groups [CO.R, S02R, — CN, —NC. COR, N02, (C6H5)2C = N]. As an exception, deprotonated simple nitroalkanes are sometimes also effective as nucleophiles. 

The observation of the unusual inverse order of kinetic and thermodynamic acidities of the simple nitroalkanes (nitromethane = NM, nitroethane = NE, and 2-nitropropane = 2-NP) during their reactions... 

Li and coworkeis also used the more detailed kinetic procedures developed by the Parker group to study the reactions of the simple nitroalkanes in water. The data in Table 1.23 show the range in the apparent rate constants observed the three nitroalkanes over several HL using the procedures described in Section 2. As expected from a consideration of the plots in Figure 1.15, the degree of the deviations (about a factor of two) in fe pp for the NM proton transfer are small in comparison with the values (about a factor of 10 for NE and 6 for 2-NP) for other two nitroalkanes. The... 

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. 

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... 

The nitro-Mannich reaction has been reviewed (266 references), covering a variety of its manifestations simple nitroalkane versus more functionalized nitro compounds, non-catalytic, metal ion- and organo-catalytic, conjugate and cycloadditions and so on. 0 New chiral modular bifimctional iminophosphorane superbase organocatalysts allow 0 metal-free enantioselective addition of nitromethane to otherwise unreactive ketone-derived imines. The readily scalable reaction yields -nitroamines (20) with a fully 0 substituted carbon atom, in up to 95% ee. 

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