Nitroalkenes reaction with amines

The addition of alkoxides to 2-nitro-l-phenylthio-l-alkenes affords P-nitro-aldehyde acetals.276 The reaction of the same nitroalkenes with amines gives nitroenamines.270 They are important intermediates for organic synthesis and are generally prepared by the reaction of nitroalkanes with triethylorthoformate in the presence of alcohols or secondary amines.2"1 0 The methods of Eqs. 4.20 and 4.21 have some merits over the conventional methods, for variously substituted (3-nitro-aldehydes acetals or nitroenamines are readily prepared by these methods. 

By employing the primary amine catalyst 160, Zhong and coworkers developed the tandem Michael-Henry reaction of ketones with nitroalkenes to provide highly functionalized chiral hexanes and pentanes with high diastereo- and enantioselec-tivity [49]. The selected examples depicted in Scheme 9.56 show that, in the presence of 160 (10-15 mol%), various Michael donors and nitroalkenes smoothly underwent the tandem reaction with almost quantitative yield and extremely high enantios-electivity with the complete diastereoselectivity of the products. Further details of this reaction can be seen in Section 10.4. 

The reaction involves the amine-catalyzed conversion of an aldehyde into a nitroalkene by reaction with nitromethane followed by a transition-metal-catalyzed Michael addition of p-dicarbonyl compounds in the same reaction vessel. Typically, amine catalysts and nickel complexes are incompatible due to their tendency to chelate and to render each other inactive. However, microencapsulation of poly(ethyleneimine) (PEI) forms catalyst 140, which can successfully be used in tandem with the nickel-based catalyst 141 (Figure 3.6). 

Nevertheless, as was pointed out before, a straightforward solution to the rather limited substrate scope of the reaction with regard to the ketone reagent and also a good way to overcome the lack of reactivity of ketones toward enamine activation has been the use of primary amines as organocatalysts. In fact, literature examples indicate that primary amines are much more active catalysts for the Michael addition of both cyclic and acyclic ketones to nitroalkenes compared to the same reaction using a secondary amine catalyst like most of the proline-based derivatives already presented before. 

In this context, a wide variety of different chiral primary amines has been developed and tested in this reaction with different results (Scheme 2.8). The first example of a primary amine-catalyzed Michael reaction of ketones with nitroalkenes was reported by Cordova,who found that alanine-derived amide 20a was an excellent catalyst for this transformation. However, in this report, the reaction scope was also mainly focused on the use of cyclohexanone, with a single example of an acyclic enone (2-butanone) providing the Michael... 

Michael/Henry/dehydration/aromatisation reaction of 2-(2-oxoethyl) benzal-dehydes and nitroalkenes mediated by pyrrolidine to obtain polysubstituted naphthalene derivatives. DBU catalysed the conjugate additions of alcohols to ot,p-unsaturated nitriles, esters and ketones. Perhaps more important are the aza-Michael addition reactions of amines to a,p-unsaturated ketones, nitriles and esters. Recently, Costa, Vilarrasa and coworkers described the addition of lactams, imides, 2-pyridone, pyrimidine-2,4-diones and inosines to methyl propiolate and other similar compounds, DABCO and DMAP being the best catalysts. As mentioned before, tertiary amines give zwitterionic species with activated allynes. It was as early as 1932 when Diels and Alder used the reaction of pyridine with dimethyl acetylenedicarbojylate (DMAD) for the synthesis of heterocycles. The interception of the corresponding intermediate with Al-tosylimines and activated olefins provided access to l-azadienes ° and highly substituted butadienes (Scheme 2.6). When the quenching species of the zwitterionic intermediate is a 1,2-diketone, dibenzoyl maleates or cyclopentenedione derivatives could be obtained (Scheme 2.6). The interception of the zwitterionic species of N-methyl imidazole (NMI) and DMAD with ketenes to obtain unsaturated esters has also been shown. ... 

Amorphous SA treated with 3-aminopropyltriethoxysilane (tertiary amine) and 3-(diethylamino) propyltrimethoxysilane (primary amine) acts as a heterogeneous catalyst for the one-pot synthesis of 1,3-dinitroalkanes from aldehydes and nitromethane (Scheme 6.21) [133]. Trifunctionality of the catalyst surface has been proposed. Aldehydes are activated by surface acid sites and they react with secondary amino group to form imine intermediates. The proton of nitromethane is abstracted by a tertiary amine group accompanied by nucleophilic attack of the deprotonated nitromethane on the imine, which is also assisted by a surface acid site, resulting in nitroalkenes. Another nitromethane is activated by the tertiary amine group, and the Michael reaction with nitroalkenes occurs to give the 1,3-dinitroalkanes. 

Despite the fact that extensive studies have been conducted on secondary-amine-catalyzed Michael addition of unmodified a-monosubstituted aldehydes, there are few reports on the use of a,a-disubstimted aldehydes as nucleophiles, probably due to the high hinderance during the formation of enamine intermediates. By using (5)-l-(2-pyrrolidinylmethyl)pyrrolidme/TFA salt 19 as catalyst, Barbas and co-workers [13] reported the first application of a,a-disubstituted aldehydes in the enantiose-lective Michael reactions with nitroalkenes (Scheme 5.6). The corresponding Michael adducts bearing an all-carbon quaternary stereocenter were obtained in moderate diastereoselectivities and with moderate to good enantioselectivities (up to 91%). 

The same approach was used by Benagha s group for the enantioselective organocatalytic reduction of P-trifluoromethyl nitroaUcenes 73, with the aim of achieving chiral yS-trifluoromethyl amines 75 (Scheme 28) [161]. The authors also performed the organocatalyzed reduction of a-substituted-p-trifluoromethyl nitroalkenes, although with poorer results. The stereochemical result of the reaction and the behavior of thiourea catalyst 74 were discussed based on computational studies and DFT transition-state analysis. 

In 2010, Wang s group reported a copper-catalyzed three-component reaction of a-diazoketones 122 with amines 123 and nitroalkenes 124 under aerobic conditions (Scheme 5.86) [87], The cascade reaction follows a sequence of an N—H insertion, a copper-catalyzed oxidative dehydrogenation of amine, a [3+2] cycloaddition, and finally, an elimination of HNOj/dehydrogenation, affording a variety of polysubsti-tuted pyrroles 125. 

In general, base-catalyzed reactions of aromatic aldehydes with nitroalkanes give nitroalkenes directly (Knoevenagel reaction).54 The reaction is very simple heating a mixture of aromatic aldehydes, nitroalkanes, and amines in benzene or toluene for several hours using a Dean-Stark water separator gives the desired nitroalkenes in good yield, as shown in Eqs. 3.31-3.34.54-58... 

Reaction of the salts of primary and secondary alky Initio compounds with diborane in THF solution at 25 °C yields the corresponding hydroxylamines.117 Kabalka has reported the reduction of nitroalkenes to hydroxylamines or amines with a variety of borane and borohydride reagents (Eq. 6.61).118... 

Dienes with Chiral Auxiliaries The use of dienes with the chiral auxiliary attached to the C-l position of the dienes is the most popular in asymmetric Diels-Alder reactions.59 In 1980, Trost reported high asymmetric induction in the Diels-Alder reaction using l-(S)-0-methylmandeloxy-l,4-butadiene59a However, the result obtained by Trost et al. has remained unique for more than a decade, at least in terms of enantioselectivity. The asymmetric Diels-Alder reaction of chiral diene-amines with nitroalkenes gives aminocyclohexenes with good diastereoselectivity (Eq. 8.37).60 The development in the area of chiral dienes is slow it may be due to the difficulty of preparing these compounds. 

In conjunction with the Knoevenegal reaction, a Michael addition (Figure 22) reaction was also described by Jaekson et al. using the same siliea supported tertiary amines for the catalysis of conjugate addition of nitroalkenes to a,y9-unsaturated carbonyl compounds. At a flow rate of 6.6 pL/min the eonversion rate was constant with a high yield for 7 hours reaction time. 

Nitroalkanols are intermediate compounds that are used extensively in many important syntheses 142). They can be converted by hydrogenation into / -aminoalcohols, which are intermediates for pharmacologically important chemicals such as chloroamphenicol and ephedrine. They are obtained by Henry s reaction by the condensation of nitroalkanes with aldehydes. The classical method for this transformation involves the use of bases such as alkali metal hydroxides, alkoxides, Ba(OH)2, amines, etc. 142-144). However, these catalysts give predominantly dehydrated products—nitroalkenes— which are susceptible to polymerization (Scheme 16). The reaction proceeds by the nucleophilic addition of the carbanion formed by the abstraction of a proton from the nitro compound to the carbon atom of the carbonyl group, finally forming the nitroaldol by abstraction of a proton from the catalyst. 

Various approaches have been used to prepare pyrroles on insoluble supports (Figure 15.1). These include the condensation of a-halo ketones or nitroalkenes with enamines (Hantzsch pyrrole synthesis) and the decarboxylative condensation of N-acyl a-amino acids with alkynes (Table 15.3). The enamines required for the Hanztsch pyrrole synthesis are obtained by treating support-bound acetoacetamides with primary aliphatic amines. Unfortunately, 3-keto amides other than acetoacetamides are not readily accessible this imposes some limitations on the range of substituents that may be incorporated into the products. Pyrroles have also been prepared by the treatment of polystyrene-bound vinylsulfones with isonitriles such as Tosmic [28] and by the reaction of resin-bound sulfonic esters of a-hydroxy ketones with enamines [29]. 

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