Nitromethane proton transfer reactions

Parrinello ab initio/MD study of Decker et al. [39] that showed a proton transfer reaction of liquid nitromethane when compressed by a factor of 3.0 at 150 K. 

The observed increases in k0 upon addition of (CH3)2SO are consistent with similar increases in proton-transfers (13, 15, 16, 25-27) and are easily understood in terms of a reduced 8 log K C-) in the (CH3)2SO-containing medium (equation 6, (5nucn instead of (5). The magnitude of the effects is smaller than in the proton-transfer reactions, though. For example, 8 log k0 = 0.45 for nucleophilic addition to (5-nitrostyrene contrasts with 8 log k0 = 1.35 for the same solvent change in the deprotonation of nitromethane anion by the same amines (C. F. Bernasconi, A. Mullin, and D. Kliner, unpublished results). This attenuation of the solvent effect is another manifestation of the smaller imbalances in the olefin reactions. 

In our study of the proton transfer reactions of three simple nitroal-kanes (nitromethane, nitroethane and 2-nitropropane), absorbance—time profiles due to the formation of reactive intermediates were observed at the isosbestic points where changes in reactant and product absorbance... 

All Pi values obtained for oxygen bases A- are between 0.94 and 1.09 (Table 7), with the exception of two reactions involving nitromethane, with lower p2 values. This indicates that proton transfer between these oxygen bases must be fast as expected. (The best value is probably a little larger than 1.) It follows that the assumption of the constancy of the p, values is essentially correct for these examples. Furthermore, the predicted relationship between p value (of a CH bond) and resonance effect in the carbanion appears to be fulfilled (Table 7). For some reactions of the type RH + A-, the values calculated for n2max = 1 — nmax are of approximately the same magnitude as the experimental Bronsted (5 values [132],... 

The second event in simulation C is a second proton transfer, observed at 60 fs, and is due to an intramolecular 0-H attraction. The fast vibrations of the methyl group induce an umbrella inversion type motion that, along with a stretch in a N-O bond, leads to the hydrogen transfer from the methyl to the nitro group. This reaction yields the aci acid form of nitromethane. The process of bond rearrangement is illustrated as ... 

For the reaction with a stabilized phosphoniutn ylide, the betaine intermediate undergoes proton transfer and even extrudes a sulfonamide group to give a vinyl phosphonium salt that can be trapped with water, a stabilized phosphonium yUde [223], or nitromethane (solvent) [225]. These findings suggest that the conversion of the betaine to the azaphosphetane is much slower than the interconversion between the two betaine diastereomers (Scheme 49). Thus, the Z/E ratio for the alkene product does not correspond to the diastereoselectivity for the initial imine/ ylide addition. Instead, the Z/E selectivity is decided by the different rates for the transformation of the two betaine diastereomers into their corresponding azaphosphetanes. 

Since Corey s group first reported 0(9)-allyl-N-(9-anthracenylmethyl) cinchonidi-nium bromide as a new phase-transfer catalyst [13], its application to various asymmetric reactions has been investigated. In particular, this catalyst represents a powerful tool in various conjugated additions using chalcone derivatives (Scheme 3.2). For example, nitromethane [14], acetophenone [15], and silyl eno-lates [16] produce the corresponding adducts in high enantioselectivity. When p-alkyl substrates are used under PTC conditions, asymmetric dimerization triggered by the abstraction of a y-proton proceeds smoothly, with up to 98% ee [17]. 

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