Nitroalkenes conjugated

TABLE 9.3 Selective Hydrogenation of 1-Nitrocyclooctene (I) and l-Nitro-l-octadecene(II) over Pd-Ca i 

The hydrogenation of P-nitrostyrenes to P-phenethylamines can be accomplished over palladium catalyst in strongly acidic conditions.49-51 Kindler et al. hydrogenated P-nitrostyrene to 2-phenethylamine in 84% yield over a palladium black in acetic acid-sulfuric acid (eq. 9.26).49 In the absence of sulfuric acid the uptake of the same amount of hydrogen required a much longer time and the phenethylamine was obtained in only slight amounts. 

Substituted a-nitrostilbenes were hydrogenated to 1,2-diphenylethylamines in 52-87% yields, conveniently in methanol over in situ-prepared Pd-C catalyst (eq. 9.28).53 In these cases the hydrogenation was carried out in presence of the hydrogen chloride formed from palladium chloride, the amount of which, however, was much smaller (-0.0022 mol) for the nitrostilbenes hydrogenated (0.037-0.042 mol). In one instance, a-nitro-4 -methoxystilbene gave rise to a small amount of the corresponding oxime when the hydrogen uptake was less than theoretical. 

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The reaction of conjugated nitroalkenes v/ith a,fi-unsaturated esters, ketones, nitnles, and sulfones is catalyzed by TMG to give the Michael adduct of allyiic nitro compounds fEq. 4.108. -... 

A convenient preparative method for conjugated nitroalkenes has been developed based on the reaction of nitrogen oxides. Nitric oxide (NO) is commercially available and used in the industry for the mass production of nitric acid. Nitric oxide is currently one of the most studied molecules in the fields of biochemistry, medicine, and environmental science.47 Thus, the reaction of NO with alkenes under aerobic conditions is of a renewed importance.48... 

Replacement of silver nitrite by inexpensive sodium or potassium nitrite enhances the utility of this process. Treatment of alkenes with sodium nitrite and iodine in ethyl acetate and water in the presence of ethylene glycol gives conjugated nitroalkenes in 49-82% yield.63 The method for generation of nitryl iodide is improved by the treatment of iodine with potassium nitrite complexed with 18-crown-6 in THF under sonication, as shown in Eq. 2.32s4... 

In 1978, Corey reported a general synthetic route for the conversion of alkenes to conjugated nitroalkenes via nitro-mercuration and demercuration.74 Since then, many chemists have used this method for the preparation of cyclic nitroalkenes such as 1-nitrocyclohexene. However, the use of mercury salts is not recommended even for the small-scale preparation of nitroalkenes. This reaction is not as clean as expected, and formidable efforts are required to remove the mercury in the waste. 

Conjugated nitroalkenes are isomerized to allylic nitro compounds under basic conditions. Reactions of a,P-unsaturated nitro compounds with aldehydes under basic conditions lead directly to y,8-unsaturated P-nitro alcohols (Eq. 3.24). 3 This reaction is very useful for preparing allylic nitro compounds. 

Nitronates (5) can be synthesized by the Michael addition of in situ generated sulfur or selenium ylides (12) (58-60, 62, 63) to conjugated nitroalkenes... 

Synthesis of Six-membered Cyclic Nitronates by the [4 + 2]-cycloaddi-tion Reaction The [4 + 2]-cycloaddition reaction of conjugated nitroalkenes (42) with olefins (43) is the most powerful and widely used method for the synthesis of six-membered cyclic nitronates (35) (Scheme 3.38). 

The behavior of complexes of nitroalkenes (42) with LA toward conjugated dienes is yet another factor underlying the role of these complexes. Conjugated nitroalkenes (42) are considered as active dienophiles in classical Diels-Alder reactions (104, 105). On the contrary, in the presence of SnCl4, nitroalkenes (42) react with cyclopentadiene and 1,3-cyclohexadiene exclusively at one double bond (103). Therefore, it is highly probable that the 42 + 43 cycloaddition proceeds by a nonconcerted mechanism in the presence of LA (see Scheme 3.40). 

However, the introduction of sterically hindered substituents at the p-car bon atom of nitroalkene (42) completely changes the ring-chain tautomerism of conjugated nitroalkenes. Apparently, steric hindrance caused by two bulky Bu groups in product (42a) (Scheme 3.47) prevents effective conjugation of the jt systems of the C,C double bond and the nitro group, thus causing its deviation from the plane of the C=C bond as a result of which isomer (47a) becomes thermodynamically more favorable. 

Cunico and Motta reported (202) on the very interesting one-pot synthesis of SENAs, such as 3-nitrocarboxylic acid /V./V-dimethylamides derivatives, from silylated N./V-dimethylformamides 57a,b and conjugated nitroalkenes (Scheme 3.57). 

Reactions of Tandem [4 + 2][3 + 2] Cycloaddition of Conjugated Nitroalkenes and their Use in Organic Synthesis... 

This approach has been comprehensively described in Reference 99 and two monographs 427 and 428. Hence, we will not consider this approach in detail, the more so that selected aspects of [4+ 2]-cycloaddition reactions of conjugated nitroalkenes with olefins were discussed in Section 3.2.1.2.2.2. Many concerned with the synthesis of six-membered cyclic nitronates, many problems of [3+ 2]-cycloaddition of six-membered cyclic nitronates were also considered above (see Sections 3.4.3.1.4 and 3.4.3.3). 

This process can be used with advantage for the preparation of conjugated nitroalkenes from nitroalkanes. It is of interest because the majority of known synthetic methods for generating nitroalkenes, the skeletons of target molecules, are generally assembled from simpler molecules (104). 

Viologen salts act as one-electron phase-transfer agents and, in conjunction with sodium dithionite which regenerates the bipyridinium radical cation, they have been used for the debromination of 1,2-dibromoalkanes to yield alkenes in variable yields [13-15]. Nitroarenes are reduced to anilines in high yield (>90%) under similar conditions [16], whereas conjugated nitroalkenes are converted into the oximes of the saturated ketones [17] saturated aliphatic nitro compounds are not reduced by this process. 

The reduction of conjugated nitroalkenes such as S-nitrostyrenes to oximes provides easy access to a large number of versatile organic intermediates. However, despite their potential utility, many of these methods suffer from the use of strongly acidic or basic conditions, requirement of anhydrous conditions, and incompatibility with halogenated arenes. Eurther, some of the methods are inefficient for the preparation of aldoximes due... 

Scheme 6.26 Proposed model of the biomimetic reduction of conjugated nitroalkenes in the presence of thiourea catalyst 9 and 19.

A method that avoids the conventional chlorination of aldoximes corresponds to the reaction of nitroalkanes or conjugate nitroalkenes with titanium tetrachloride (48) (Scheme 6.3). a-Chloro (a), alkyl (a, b, c), aryl (c), a-cyano (d), and a-azido (d) hydroximoyl chlorides have been prepared in good yield in this way (48). Concerning overall efficiency, however, this would necessitate that the nitro... 

As a consequence of the strong electron-withdrawing properties of the nitro group, conjugated nitroalkenes behave as powerful Michael acceptors. An example of conjugate addition of Reformatsky reagents to (2-nitrovinyl)arenes 107 is given in equation 63150. 

The axially chiral guanidine catalyst (155) (0.4-5 mol%) has been developed to facilitate the highly enantioselective Michael addition of 1,3-dicarbonyl compounds (g to a broad range of conjugated nitroalkenes (<98% ee).211... 

On the other hand, conjugated nitroalkenes are very useful electron-poor alkenes, prone to act as nucleophilic acceptor, mainly in the Michael reaction (Berestovitaskaya et al., 1994) or in the Diels-Alder cycloaddition (Denmark and Thorarensen, 1996). Moreover, the nitro group can be easily turned into a respectable array of functional groups such as its reduction to a primary amine, replacement with hydrogen (Ballini et al., 1983 Ono, 2001), conversion into a carbonyl (Nef reaction) (Ballini and Petrini, 2004), and transformation into other important functionalities such as nitrile, nitrile oxide, oximes, hydroxylamines, and thiols (Colvin et al., 1979). 

Nitro olefins are useful intermediates in organic synthesis. Conjugated nitroalkenes are excellent acceptors (Michael reaction and Diels-Alder cycloaddition) and the nitro group can be further transformed, in situ, into several reactive intermediates. 

Trani-fused bicyclic y-lactones (7) have been conveniently prepared, in a one-pot process, from conjugated nitroalkenes, by way of the expected transesterihcation-intramolecular hetero Diels-Alder (HAD) reaction, followed by the transformation of a functional group using ( )-l-ethoxy-2-nitroethylene (5) and prim-, sec-, and tert-y,e-unsaturated alcohols (6) having two methyl substituents at the terminal position as illustrated in Figure 2.2 (Wada and Yoshinaga, 2003). 

The overall yields are favored by the one-pot procedure, which drives the process to the formation of the target compounds (60) and (61). The nitroalkanols (60) are obtained as a diastereomeric mixture. Nevertheless, this seems not to be a problem since the main uses of nitroalkanols are the conversion into a-nitro ketones (Ballini et al., 2005d) or conjugated nitroalkenes (Berestovitaskaya et al 1994 Barrett and Graboski, 1986 Kabalka and Varma, 1987). In addition, the process is mild and avoids complicated workup and complex purification steps with evident waste and energy consumption. 

Kabalka, G.W. and Varma, R.S. 1987. Syntheses and selected reductions of conjugated nitroalkene. Organic Preparations and Procedures International, 19(4—5) 283-328. 

Wada, E. and Yoshinaga, M. 2003. A new methodology of intramolecular hetero-Diels-Alder reaction with (3-alkoxy-snbstituted conjugated nitroalkenes as heterodienes Stereoselective one-pot synthesis of tra -fused bicyclic y-lactones. Tetrahedron letters, 44(43) 7953-6. 

Methoxyl, cyano, and nitro groups may also be present in the molecule during pyrolysis. Examples are y-methoxycrotononitrile (83%), acrylonitrile (64%), 1-cyano-l, 3-butadiene (70%), and a series of conjugated nitroalkenes. The esters of /3-nitro alcohols are pyrolyzed at 195° or merely refluxed with sodium bicarbonate in dilute aqueous methanol. By the latter procedure the yields of conjugated nitro alkenes are. 90-95%. Direct dehydration by heating the nitro alcohols to 180° with phthalic anhydride has also been used. ... 

Trialkylboron hydrides react with conjugated nitroalkenes to yield nitronate intermediates via a 1,4-addition of the hydride. The corresponding nitroalkanes are easily obtained by using silica gel to proto-nate the nitronate salt. These reactions presumably occur through a common intermediate (36), which is then hydrolyzed to the nitroalkane or reduced by a borane complex to yield hydroxylamine, after hydrolysis. 

Even sodium borohydride catalyzes the reaction of borane complexes with conjugated nitroalkenes (equation 24). This straightforward approach affords pure hydroxylamines in high yields. As an... 

In the course of investigations involving the reaction of lithium triethylhydroborate with conjugated nitroalkenes (61),A -ethylamines (62) were consistently observed as by-products (equation 34). The intermediacy of nitroso compounds has been confirmed in this reaction which provides a useful method for the preparation of A -ethylated amine derivatives. These A -alkylated products are not produced when sterically demanding reagents, such as potassium tri-s-butylborohydride, are used. Apparently, the competition between reduction and alkylation of the nitroso group is sensitive to steric effects. 

Corey and Bstreicher I06] developed new methods for the preparation of conjugated 1-nitrocycloalkenes (28). They used nitromercurial chlorides (28a) obtained by Bachman and Whiichouse 1107) or vinyl stannanes. The latter were transformed into conjugated nitroalkenes by the action of letranitromeihane (28b) ... 

Kantam ML, Sreekanth P (1999) One-pot synthesis of conjugated nitroalkenes by diamino-functionalised mesoporous material. Catal Lett 57 227... 

There are reviews dealing with conjugated nitroalkenes in organic synthesis, " nitroenamines, the oxyalkylation of carbonyl compounds with conjugated nitroalkenes and the use of conjugated nitroalkenes in the synthesis of heterocyclic compounds. In this section mainly the preparation of nitroalkenes starting from alkenes is described. 

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