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Nef Reaction) Tetrahedron 60 (2004) 1017–1047 Tetrahedron report number 667 Recent synthetic developments in the nitro to carbonyl conversion (Nef reaction) Roberto Ballini and Marino Petrini* Dipartimento di Scienze Chimiche, Universita` di Camerino, via S. Agostino, 1, I-62032 Camerino, Italy Received 21 October 2003 Contents 1. Introduction 1017 2. General aspects of the Nef reaction 1018 3. Recent modifications of the Nef reaction 1018 3.1. Oxidative methods 1018 3.2. Reductive methods 1020 3.3. Other methods 1021 4. Applications of the Nef reaction 1022 4.1. Oxidative methods 1023 4.2. Reductive methods 1031 4.3. Other methods 1036 5. Conclusions 1044 1. Introduction of structurally-defined compounds. Indeed, many activating groups that promote the formation of carbon– Interconversion of functional groups represents an carbon bonds often need to be replaced by other important aspect in every process leading to the synthesis functional entities once they have assisted the main synthetic process. The availability of a consistent number Keywords: Nitro compounds; Carbonyl group; Nef reaction. of such transformations for a particular functional group largely contributes to the success and development of the Abbreviations: BINAP, 2,20-bis(diphenyphoshino)-1,10-binaphthyl; related chemistry. The importance of nitroalkanes 1 in BINOL, 1,10-binaphthalene-2,20-diol; Bn, benzyl; Boc, t-butoxycarbonyl; synthesis is mainly due to their easy conversion into the BY, Baker’s yeast; CAN, ceric ammonium nitrate; Cbz, corresponding nitronate anions 2 because of the high benzyloxycarbonyl; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; DCC, dicyclohexylcarbodiimide; DHP, dihydropyran; DIBALH, electron-withdrawing power of the nitro group that provides diisobutylaluminium hydride; DMD, dimethyldioxirane; DMAP, 4-N,N- an outstanding enhancement of the hydrogen acidity at the 1–5 dimethylaminopyridine; DMF, dimethylformamide; DMI, 1,3-dimethyl-2- a-position (cf. pka MeNO2¼10). Nitronate salts can imidazolidinone; DMSO, dimethylsulphoxide; DPPA, diphenylphosphoryl therefore act as carbon nucleophiles with a range of azide; IBX, 1-hydroxy-1,2-benzodioxol-3(1H)-one-1-oxide; LiHMDS, 6 7,8 m electrophiles including haloalkanes, aldehydes and lithium 1,1,1,3,3,3-hexamethyldisilazide; MCPBA, - 9 chloroperoxybenzoic acid; MOM, methoxymethyl; Ms, Michael acceptors, leading to carbon–carbon bond for- methanesulphonyl; Ni(acac)2, nickel bis(acetylacetonate); NMO, N- mation (Scheme 1). methylmorpholine-N-oxide; Pd2(dba)3, tris(dibenzylideneacetone)- dipalladium; Phth, phthalimidoyl; PMB, p-methoxybenzyl; PPTS, pyridinium p-toluenesulphonate; Py, pyridyl; SET, single electron transfer; TBDPS, t-butyldiphenylsilyl; TBS, t-butyldimethylsilyl; Tf, trifluoromethanesulphonyl; TFAA, trifluoroacetic anhydride; THF, tetrahydrofuran; TMEDA, N,N,N0,N0-tetramethylethylenediamine; THP, tetrahydropyranyl; TMS, trimethylsilyl; TPAP, tetrapropylammonium perruthenate; Ts, p-toluenesulphonyl; pTSA, p-toluenesulphonic acid; TMG, tetramethylguanidine; Tol, p-methylphenyl; Tx, 2,3-dimethyl-2- butyl (thexyl). * Corresponding author. Tel.: þ39-0737-402253; fax: þ39-0737-402297; e-mail address: [email protected] Scheme 1. Nitroalkanes as nucleophiles. 0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2003.11.016 1018 R. Ballini, M. Petrini / Tetrahedron 60 (2004) 1017–1047 Hydrolysis occurs on a protonated form 11 of the corresponding nitronic acid 10, giving as intermediate 12 that, by loss of water and hyponitrous acid 14, gives the carbonyl derivative 13. The product distribution in this reaction is strongly affected by the acidity level of the system. At pH.1, oximes as well as other hydroxynitroso compounds can be formed in appreciable amounts. For this reason, a rapid acidification of the nitronate salt is required and it is very often operationally desirable to add the nitronate salt to the acid solution. The harsh conditions in which this conversion is usually carried out (pH,1) have spurred the development of alternative methods that can be Scheme 2. General transformations of the nitro group. performed in oxidative, reductive, as well as almost neutral, conditions. It is interesting to note that a common factor in Once these adducts have been formed, the nitro group can all of the oxidative procedures is the formation of the be retained in the molecular framework if this is useful for a corresponding nitronate anion as the reactive species; the further nucleophilic addition or it can be transformed into subsequent cleavage occurs on the carbon–nitrogen double other functionalities following a defined synthetic strategy bond to give the carbonyl derivative. Conversely, reductive (Scheme 2). Reduction of the nitro group in compound 4 methods can be carried out both on the nitronate anion or allows the preparation of a primary amine 5, in which a directly on the nitroalkane, even in acidic conditions. An simple modification of the oxidation state of the nitrogen important aspect of this transformation concerns the nature atom is carried out. Alternatively, the nitro group can be of the nitroalkane used as the substrate. Indeed, secondary removed from the molecule by replacing it with hydrogen nitro compounds are conveniently transformed into ketones, giving the corresponding denitrated product 610,11 or by but primary nitro derivatives can be converted into elimination as nitrous acid, introducing a double bond in the aldehydes or carboxylic acids, depending on the reaction molecular structure 7.12 A further option consists of the conditions. Particular care must therefore be taken in this conversion of the nitro group into a carbonyl group 8.13 This reaction, especially when oxidative procedures are chosen process is probably the most exploited transformation of the to transform primary nitroalkanes into aldehydes. Nitro- nitro group, since it definitively reverses the polarity of the alkenes that are powerful Michael acceptors can also be neighbouring carbon atom from nucleophilic to used as substrates for the Nef reaction.17 – 22 In addition, the electrophilic. conjugate addition of nucleophilic reagents to nitroolefins provides the formation of a nitronate anion as an The synthetic opportunity offered by this conversion has intermediate that can usually be transformed by a tandem been focusing the attention on the chemistry of aliphatic process into the corresponding carbonyl derivative. nitro compounds since its discovery by Nef in 1894.14 As a matter of fact, the transformation of sugar nitromethyl groups into aldehydes represents an alternative procedure for the chain elongation of carbohydrates, known for at least 3. Recent modifications of the Nef reaction half a century as the Sowden protocol.15 The aim of this review is to collect the new procedures which have appeared 3.1. Oxidative methods in the last decade after the comprehensive report by Pinnick in 1990.16 In addition, several significant applications of this As previously stated, cleavage of the carbon–nitrogen transformation to the synthesis of pivotal building blocks double bond is the key step in almost all of the oxidative and important biologically active compounds will be methods that are currently used for the Nef reaction. discussed. KMnO4 is certainly the most widely used oxidant for this purpose and, in controlled conditions, it is able to convert primary nitro compounds into aldehydes.23 Buffered permanganate solutions (pH¼11) can oxidise primary 2. General aspects of the Nef reaction nitroalkanes such as 15 into alkanoic acids 16 without affecting other functions such as esters, amides, primary The original procedure for the nitro to carbonyl transform- alcohols and acetals (Scheme 4).24 ation, as described by Nef, was essentially the hydrolysis in strongly acidic conditions of a nitronate salt 9 produced by Dimethyldioxirane (DMD) is a strong oxidising agent basic treatment of a nitroalkane (Scheme 3). readily prepared by the reaction of Oxonew with acetone Scheme 3. Mechanism for the original Nef reaction. R. Ballini, M. Petrini / Tetrahedron 60 (2004) 1017–1047 1019 Scheme 7. Reaction of 1-nitrohexane with sodium percarbonate. Secondary a-alkoxy nitro derivatives such as 25 can be efficiently converted into a-alkoxy ketones 26 using catalytic amounts of tetrapropylammonium perruthenate (TPAP) with N-methylmorpholine-N-oxide (NMO). The presence of 4 A˚ molecular sieves and silver salts is Scheme 4. Permanganate oxidation of primary nitroalkanes to carboxylic mandatory for the efficiency of the procedure (Scheme 8).29 acids. and, among various applications it has been used for the regeneration of the carbonyl group from acetals, hydrazones and other derivatives. DMD attacks nitronate anions obtained from nitro compounds such as 17, giving the corresponding carbonyl derivative 18 in good yields (Scheme 5).25 Scheme 8. Nef reaction on nitrocycloalkanones with TPAP. Molecular oxygen is an appealing system to perform oxidations, since it is a cheap, readily available and environmental friendly reagent. Some nitro compounds such as 27 can be transformed into the corresponding carbonyl derivatives 28 when exposed to air in the presence of copper salts (Scheme 9).30 Scheme 5. Nef reaction using dimethyldioxirane. Oxonew, from which DMD derives, is also able to convert nitroalkanes into carbonyl derivatives, but shows less selectivity since primary nitro compounds such as 19 are converted into carboxylic acids 20. Common protecting groups such as acetals, TBS and acetates are,
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