Polyfunctional nitro-compounds

The reduction of polyfunctional nitro compounds, nitroaryl as well as nitroalkyl compounds, to the corresponding amines in basic aqueous alcoholic solutions on Raney copper (RCu) is more selective if carried out by generating chemisorbed hydrogen by electroreduction of water (ECff method) than by generating it by leaching of the alloy in situ (CCff method) except in the case of o-iodonitrobenzene (2) for which the CCH method is more selective, ffowever, the most selective method in all cases studied is ECH in neutral medium (pff 3-7). 

Thus many heterocycles of various sizes can be formed and the reductive cyclization of polyfunctional nitro compounds offers a useful synthetic tool for the synthesis of complex molecules. When the interacting functions are not located at positions conducive to ring formation, bimolecular condensation or even polymerization may occur. 

A simultaneous reduction-oxidation sequence of hydroxy carbonyl substrates in the Meerwein-Ponndorf-Verley reduction can be accomplished by use of a catalytic amount of (2,7-dimethyl-l,8-biphenylenedioxy)bis(dimethylaluminum) (8) [33], This is an efficient hydride transfer from the sec-alcohol moiety to the remote carbonyl group and, because of its insensitivity to other functionalities, should find vast potential in the synthesis of complex polyfunctional molecules, including natural and unnatural products. Thus, treatment of hydroxy aldehyde 18 with 8 (5 mol%) in CH2CI2 at 21 °C for 12 h resulted in formation of hydroxy ketone 19 in 78 % yield. As expected, the use of 25 mol% 8 enhanced the rate and the chemical yield was increased to 92 %. A similar tendency was observed with the cyclohexanone derivative. It should be noted that the present reduction-oxidation sequence is highly chemoselective, and can be utilized in the presence of other functionalities such as esters, amides, rert-alco-hols, nitriles and nitro compounds, as depicted in Sch. 10. 

Other oxidations achieved with this reagent include sulfides to sulfones - and amines to nitro compounds." Actually, polyfunctional compounds show useful chemoselectivities. a-Nitroacetic esters are quantitatively prepared from glycine esters. 

Polyfunctional isocyanates can be formed in many ways. Commercially, the most important way is through reactions of phosgene with amines or amine salts. Other reactions, however, like that of carbon monoxide with nitro compounds, are now also becoming important. Addition of isocyanic acid to olefins is also gaining prominence. 

Scheme 4.19 Preparation of polyfunctional arylmagnesium compounds of type 69 bearing a nitro function.

These reaction conditions also permit the chemoselective quantitative reduction of benzaldehyde to benzyl alcohol without any concomitant reduction of either acetophenone or 3,3-dimethylbutan-2-one present in the same reaction mixture.83 Additionally, this useful method permits the reduction of aldehyde functions in polyfunctional compounds without affecting amide, anhydride, eth-ylenic, bromo, chloro, or nitro groups.79,80,319... 

The reactions of the resulting stabilized ions 153a,b with silicon dioxide produces isolable polyfunctional compounds 155a,b. It should be emphasized that the configuration of the stereocenters in nitronate (153) remains unchanged in the course of the transformation and the reaction is stereoselective with respect to the new stereocenter at the atom bearing the nitro group. 

Types of compounds are arranged according to the following system hydrocarbons and basic heterocycles hydroxy compounds and their ethers mercapto compounds, sulfides, disulfides, sulfoxides and sulfones, sulfenic, sulfinic and sulfonic acids and their derivatives amines, hydroxylamines, hydrazines, hydrazo and azo compounds carbonyl compounds and their functional derivatives carboxylic acids and their functional derivatives and organometallics. In each chapter, halogen, nitroso, nitro, diazo and azido compounds follow the parent compounds as their substitution derivatives. More detail is indicated in the table of contents. In polyfunctional derivatives reduction of a particular function is mentioned in the place of the highest functionality. Reduction of acrylic acid, for example, is described in the chapter on acids rather than functionalized ethylene, and reduction of ethyl acetoacetate is discussed in the chapter on esters rather than in the chapter on ketones. 

The full AM1-SM2 model is discussed in more detail in the final section. Within the context of Tables 2-5, however, it is worth noting that the model enjoys the largest correlation to experimental data for the neutral solutes. All but a few of the molecules listed comprise a subset of a larger data set of 147 neutral solutes spanning many functionalities (e.g., nitro groups, phosphorus compounds, sulfides, bromides, fluoroalkanes, iodides, polyfunctional compounds), not shown because they have not been studied by other methods, for which the AM1-SM2 model has a mean absolute error of 0.6 kcal/mol.203 One apparent flaw illustrated here is that AM1-SM2 consistently underestimates the free energies of solvation for aliphatic and alicyclic ethers. 

---