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Author: Subject: Pd/C H2-gas reduction of ß-nitrostyrenes
International Hazard

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[*] posted on 22-2-2016 at 10:16

Aluminum and its variants are well-known as reductants for this substrate; using NaBH4/Pd/C is really the same thing as two step NaBH4 >> Pd/C/formate except you waste a lot more borohydride for not very much more yield. Unfortunately real advances here are quite rare.

Copper hydride complexes are known to reduce activated alkenes with good selectivity and might be able to replace borohydride in the nitroalkene >> nitroalkane transformation. Usually copper (I) t-butoxide is used as the precursor with a hydrogen donor to generate CuH in situ. Here's an example converting terminal nitroalkenes to nitroalkanes:

In this case big chiral ligands are used to enforce chirality; there are simpler complexes of CuH including (CuH(PPh)3)6 also known as Stryker's reagent although we might prefer, for simple reasons, a complex without triphenylphosphine. N-heterocyclic carbenes have also been used as ligands although they can be somewhat difficult to prepare. 2-methoxyethanol, in the form of Red-Al, is one of the simplest complexing reagents I've seen:

For at-home purposes it would be nice to find a ligand that doesn't use phosphorus and a reductant that doesn't use silicon or aluminium. Thiamine and H2 might work if you're lucky. It appears that a copper hydride can be generated from CuBr and LiHAl(OMe)3 and used to reduce unsaturated ketones to saturated ketones:

[Edited on 22-2-2016 by clearly_not_atara]
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[*] posted on 22-2-2016 at 10:49

Reference Information

Catalytic Enantioselective Conjugate Reduction of b,b-Disubstituted Nitroalkenes
Prof. Dr. E. M. Carreira, C. Czekelius
Angew. Chem.
2003, 115, 4941 –4943
DOI: 10.1002/ange.200352175

Optically active nitroalkanes are versatile precursors for a wide range of useful building blocks for fine-chemical syn- thesis. However, only a few effective methods for their preparation are available.[1–3] Despite recent advances in the addition of dialkyl zinc reagents to a,b-unsaturated nitro- olefins, the complementary method involving metal-catalyzed enantioselective reduction of b,b-disubstituted nitroalkenes has not been reported.[2] Herein we document such an approach in which bisphosphane–Cu complexes (with tol- binap or josiphos[4]) catalyze the enantioselective reduction of b,b-disubstituted nitroalkenes, giving optically active b,b- disubstituted nitroalkanes in useful yields and selectivities


Reduction of a,@-UnsaturatedNitriles with a Copper Hydride Complex
Morey E. Osborn, James F. Pegues
J. Org. Chem.
1980,45, 168-169

The reduction of conjugated nitriles to their saturated counterparts has long been a vexatious synthetic problem because of oft-encountered overreduction to the amine,* hydrodimeri~ationd,~ecyanation; and/or polymeri~ation.~ Recently, Profitt, Watt, and Corey published results achieved with magnesium metal in methanol.6 Not only did reductions performed with this reagent proceed readily and in high yield but compatibility with a variety of other functional groups was also demonstrated.............


Reductions of conjugated carbonyl compounds with copper hydride - preparative and mechanistic aspects
M. F. Semmelhack, R. D. Stauffer, A. Yamashita
J. Org. Chem.
1977, 42 (19), pp 3180–3188
DOI: 10.1021/jo00439a015

The reaction of lithium trimethoxyaluminum hydride with 0.5 molar equiv of cuprous bromide produces a heter- ogeneous mixture referred to as the Licomplex, while sodiumbis(2-methoxyethoxy)aluminumhydride with 1.0 molar equiv of cuprous bromide gives a similar mixture, the N a complex. B o t h reagents are effective in selective reduction of the olefin unit in conjugated ketones and esters, including two examples of acetylenic esters. The Li c o m p l e x i s m o r e e f f i c i e n t w i t h c y c l o h e x e n o n e s , w h i l e t h e N a c o m p l e x g i v e s b e t t e r y i e l d s i n r e d u c t i o n of a c y c l i c e n - onesandenoates, especially inthepresenceof2-butanol. Deuteriumlabelingexperiments showthat thehydrogen which is transferred to the Pposition of the conjugated carbonyl compound originates from the hydridocuprate re- agent; the 2-butanol appears to serve as a weak acid, inhibiting polymerization. Inthe absence of 2-butanol, reduc- tion of methyl cinnamate produces dimethyl meso-3,4-diphenyladipateas a major product, apparently the result of radical anion intermediates. Aldehyde, ketone, and halide functionality are reduced at rates comparable to the rate of enone reduction, but nitrile and ester units are inert.

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