ALDEHYDES FROM ACID CHLORIDES BY REDUCTION

ALDEHYDES BY OXIDATION OF TERMINAL OLEFINS WITH CHROMYL CHLORIDE 2,4,4-TRIMETHYL-PENTANAL, 51, 4 ALDEHYDES FROM ACID CHLORIDES BY MODIFIED ROSENMUND REDUCTION 3,4,5—TRIMETHOXYBENZ-ALDEHYDE, 51, 8 ALDEHYDES FROM ACID CHLORIDES BY REDUCTION OF ESTER MESYLATES WITH SODIUM BOROHY-DRIDE CYCLOBUTANECARBOXAL-DEHYDE, 51, 11... 

ALDEHYDES FROM ACID CHLORIDES BY REDUCTION OF ESTER-MESYLATES WITH SODIUM BOROHYDRIDE CYCLOBUTANECARBOXALDEHYDE... 

ALDEHYDES FROM ACID CHLORIDES BY MODIFIED ROSENMUND REDUCTION 3,4,5-TRIMETHOXYBENZALDEHYDE... 

A viable iron carbonyl-mediated reduction process converts acid chlorides and bromoalkanes into aldehydes [3, 6]. Yields are high, with the exception of nitro-benzoyl chloride, and the procedure is generally applicable for the synthesis of alkyl, aryl and a,(i-unsaturated aldehydes from the acid chlorides. The reduction proceeds via the initial formation of the acyl iron complex, followed by hydride transfer and extrusion of the aldehyde (cf. Chapter 8). 

The reduction of acid chlorides is particularly easy however, by carefully adjusting the reaction conditions, the reduction process can be controlled. Starting from acid chlorides, aldehydes can be obtained by four different methods [C5, M3] ... 

Reductive Methods.—Aldehydes from Acid Derivatives. Acid chlorides are reduced to aldehydes by sodium borohydride in the presence of cadmium(il)... 

Acid chlorides are reduced by organotin monohydrides as easily as aliphatic bromides. Benzaldehyde is reduced by triphenyltin hydride (4S) and tri-n-butyltin hydride (J2). Inasmuch as aldehydes are reduced quite slowly by the monohydrides this reaction would appear to constitute a convenient method for their synthesis from acid chlorides. However, the reaction product mixture invariably contains ester as well as aldehyde. In the case of benzoyl chloride, for example, reduction with tri- -butyltin hydride in ether provided benzaldehyde (54%) as the major reduction product in the absence of solvent benzyl benzoate (58%) predominated. The reaction sequence (57-60) has been postulated to account for the products formed (32). 

Kinetic studies established that tetra-n-butylammonium borohydride in dichloromethane was a very effective reducing agent and that, by using stoichiometric amounts of the ammonium salt under homogeneous conditions, the relative case of reduction of various classes of carbonyl compounds was the same as that recorded for the sodium salt in a hydroxylic solvent, i.e. acid chlorides aldehydes > ketones esters. However, the reactivities, ranging from rapid reduction of acid chlorides at -780 C to incomplete reduction of esters at four days at 250 C, indicated the greater selectivity of the ammonium salts, compared with sodium borohydride [9], particularly as, under these conditions, conjugated C=C double bonds are not reduced. 

Tetra-n-butylammonium triborono-octahydride, which can be obtained by metathesis from the sodium salt and tetra-n-butylammonium iodide, is a mild reducing agent with a selectivity similar to that of the tetrahydride, but with a much lower overall activity [l]. For example, acid chlorides are reduced five-times more readily than are aldehydes, which are ten-times more susceptible to reduction than acyclic... 

Alkenes are directly oxidized to aldehydes and/or ketones by ozone (O3) at low temperatures (—78 °C) in methylene chloride, followed by the reductive work-up. For example, 2-methyl-2-butene reacts with O3, followed by a reductive work-up to yield acetone and acetaldehyde. This reducing agent prevents aldehyde from oxidation to carboxylic acid. 

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