Sciencemadness Discussion Board

reduction of allylic acid to corres aldehyde

Gargi - 19-3-2008 at 22:14

Are there any reagents for one step reduction of carboxylic acids to aldehydes. Does sodium borohydride works??

guy - 19-3-2008 at 22:16

no

Klute - 19-3-2008 at 22:28

You can turn the carboxylic acid into a acyl chloride and then hydrogenate over Pd/BaSO4 IIRC to obtain the aldehyde., lokk for the Rosemund reduction. NaBH4 alone doesn't reduce -COOH groups. In any case it will reduce any aldehyde as soon as it would be formed (if it could be).

Gargi - 19-3-2008 at 23:09

yes thats true, there's another one pot procedure which vogel describes using Boron dimethyl sulfide complexation followed by PCC oxidation, but are there any other simpler alternative.

wangbinnk - 19-3-2008 at 23:33

I think u can convert carboxylic acids to methyl ester first, then convert ester to aldehydes with DIBAH.
or convert carboxylic acids to alcohol first, then convert alcohol to aldehydes with Swern oxidation or MnO2.

Klute - 20-3-2008 at 03:41

I think (m)ethyl esters can also be reduced by NaBH4 under certain conditions (THF/MeOH), or perhaps with Ni/Co II salts.

Nicodem - 20-3-2008 at 10:20

They, and even carboxylic acids, can be reduced even with plain NaBH4 in refluxing diglyme (Synt. Commun., 33, (2003) 1733–1750), but the products are alcohols not aldehydes. DIBAL is more or less the only general reagent for the reduction of esters to aldehydes.

wangbinnk - 20-3-2008 at 17:56

Actually when esters were reduced by NaBH4, not only esters were turned into alcohols, but also the double bond of allylic acid was also reduced.

Gargi - 20-3-2008 at 20:16

The references were useful, thanks for the help.

Reference Information

solo - 21-3-2008 at 02:38

Reductions of Carboxylic Acids and Esters with NaBH4 in Diglyme at 162°C[
Hua-Jie Zhu a; Charles U. Pittman Jr.
journal Synthetic Communications Volume 33, Issue 10 January 2003 , pages 1733 - 1750


Abstract
Aromatic esters, including the extremely sterically hindered ester: tamyl
2-chlorobenzoate, are readily reduced to the corresponding
benzyl alcohols in high yield with NaBH4 in refluxing diglyme
(162 C). In sharp contrast, aliphatic esters usually gave only low yields
of alcohols. Instead, diglyme fragmentation products are formed
which undergo transesterification reactions, producing complex product
mixtures including products such as RCOOCH2CH2OCH3. The
mechanism of this process involves sodium borohydride-induced SN2
cleavage of diglyme (hydride attack) at high temperatures. However,
when the extremely electron rich, 3,4,5-trimethoxybenzoic acid is
treated with NaBH4/diglyme at 162 C (with or without an equivalent
of LiCl), no 3,4,5-trimethyoxybenzyl alcohol is formed. The electron
rich and hindered ester, t-amyl-3,4,5-trimethoxybenzoate, also does
not reduce under these conditions (with or without LiCl). However,
both methyl and isopropyl 3,4,5-trimethoxybenzoate esters were converted
into 3,4,5-trimethyoxybenzyl alcohol in good yields in
NaBH4/diglyme/LiCl at 162 C. These reductions did not occur
unless LiCl was present, illustrating the electron releasing e ect of
the three methoxy functions which reduce the carbonyl group’s
reactivity.

Key Words: Sodium borohydride; Reductions; Diglyme;
Transesterification; Carboxylic acids; Esters.

Attachment: Reductions of Carboxylic Acids and Esters with NaBH4 in Diglyme at 1628C.pdf (799kB)
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Nicodem - 21-3-2008 at 09:21

Quote:
Originally posted by wangbinnk
Actually when esters were reduced by NaBH4, not only esters were turned into alcohols, but also the double bond of allylic acid was also reduced.

What do you and Gargi mean by allylic acid? You can either have allylic esters or acids with an allyl group elsewhere. But perhaps you are talking about 3,4-unsaturated carboxylic acids or 2,3-unsaturated carboxylic acids (aka acrylic acids)?
Threads like this always make me wonder why people ask for advices when it is obvious no good advice can be expected unless the problem is presented.

Klute - 21-3-2008 at 10:31

According to Syn. Comm. 12(11), 839-846 (1982), NaBH4 and pyridine reduces acyl chlorides to aldehydes in good yields:

Abstract:
Quote:
By use of sodium borohydride in N,N-dimethylformamide solution containing a molar excess of pyridine as a borane scavenger, direct conversion of both aliphatic and aromatic acid chlorides to the corresponding aldehydes can be achieved in >70% yield with minimal (5-10%) alcohol formation.

grind - 21-3-2008 at 13:30

Carboxylic acid ---> acid chloride (SOCl2) ---> acylphosphonate ester (triethyl phosphite) ---> Aldehyde (NaBH4)

It´s a very simple method and a one-pot reduction (when starting with the acid chloride).