Nitroalkanes. Roberto Ballini

secondary nitroalkanes were obtained.

1.4 Reduction of Conjugate Nitroalkenes

      Conjugated nitroalkenes are a very important class of molecules easily prepared by nitroaldol condensation or direct nitration of the corresponding olefins. Their reduction provides a convenient route to a variety of different functionalities including oximes, carbonyl compounds, hydoxylamines, alkylamines, and nitroalkanes [27,28].

      1.4.1 Reduction of Nitroalkenes into Nitroalkanes

The reduction of conjugate nitroalkenes into nitroalkanes (Scheme 1.16) has been achieved under several distinct protocols; however, often there is a loss of product due to dimerization caused by the competitive Michael addition of the formed nitronate to another molecule of nitroalkene.

Scheme 1.16 Reduction of nitroalkenes.

However, because this by-product may be suppressed at reduced pH, the main theme of several reports is the selective reduction of the double bond avoiding the formation of the Michael by-product. Thus, a plethora of reductive methods have been reported. A modified reduction with sodium borohydride in the presence of both silica gel and a mixture of chloroform-propanol as solvent system achieves good results of nitroalkanes (Table 1.1) [29].

Successively, Kabalka et al. [28] proposed three different reductive agents (Table 1.2): (i) trialkylborohydride, (ii) NaBH₄ in a mixed MeOH-THF solvents, and (iii) NaBH₄ supported on an ion exchange resin, for an effective conversion of nitroalkenes to nitroalkanes.

Table 1.1 NaBH₄/SiO₂ reduction of nitroalkenes (selected examples).

Nitroalkene	Nitroalkane	Yield (%)
		93
		92
		94
		94
		98

Table 1.2 Comparative reduction of nitroalkenes with methods (i)–(iii) (selected examples).

Nitroalkene	Nitroalkane	Yield (%)
		(i)	(ii)	(iii)
		69	62	80
		78	64	—
		81	82	80
		69	75	83

Later, Vankar and coworkers [30] developed an efficient, chemoselective procedure for the reduction of conjugated nitroalkenes with the use of NaCNBH₃ (1 equiv) in methanol (Table 1.3), by the help of Zeolite (H-ZSM-5, [Si:Al = 35 : 1], 0.5 equiv), and a careful control of the pH.

      1.4.2 Stereoselective Reduction of Conjugated Nitroalkenes

The stereoselective