Reduction of acyl halides

The reduction of acyl halides with hydrogen to form aldehydes using Pd catalyst is well known as the Rosenmund reduction[756]. Some acyl chlorides give decarbonyiation products rather than aldehydes under Rosenmund conditions. The diene 890 was obtained by decarbonyiation in an attempted Rosenmund reduction of acetyloleanolic acid chloride (889)[757], Rosenmund reduction of sterically hindered acyl chlorides such as diphenyl- and tnpheny-lacetyl chloride (891) gives the decarbonylated products 892[758],... 

Table 9. organosilane reduction of acyl Halides (continued)... 

For a review of the reduction of acyl halides, see Wheeler, in Patai The Chemistry of Acyl Halides Wiley New York, 1972, pp. 231-251. For a list of reagents, with references, see Ref. 21, p. 549. 

Reduction of carboxylic acids 9-42 Reduction of carboxylic esters 9-43 Reduction of carboxylic esters with titanocene dichloride 9-44 Reduction of anhydrides 9-45 Reduction of acyl halides 9-53 Reduction of nitriles 9-57 Reduction of hydroperoxides 9-60 Reduction of peroxides 9-69 Reaction between aldehydes and base (Cannizzaro)... 

Hydrolysis of enol esters 0-83 Reduction of acyl halides 0-84 Reduction of carboxylic acids, esters, or anhydrides 0-85 Reduction of amides 0-95 Alkylation and hydrolysis of imines, alkylation of aldehydes 0-97 Alkylation and hydrolysis of dithi-anes... 

REDUCTIONS OF ACYL HALIDES, ANHYDRIDES AND RELATED SPECIES... 

In general, the dissolving metal or electrochemical reductions of acyl halides or acyclic anhydrides are not useful for the preparation of primary alcohols. Such reductions invariably provide acyloin esters, ene-diolate diesters or related species. Cyclic anhydrides may be reduced to give lactones. For example, the reduction of phthalic anhydride at a mercury cathode has been used in the synthesis of phthalide (90%). In general, however, such reduction are not widely employed in synthesis. 

W. P. Neumann, Synthesis, 1987, 665 discusses the reduction of acyl halides to aldehydes with tri-n-butyltin hydride. 

One of the standard methods for the preparation of aldehydes involves the reduction of acid halides. A variety of stoichiometric reducing systems are available for this transfomiation, which include NaAlH(OBu-r)3, LiAlHfOBu-O.i, NaBHfOMe). Catalytic hydrogenation with H2 and Pd on carbon is also a popular method. In contrast, methods based on the radical reduction of acyl halides are synthetically less important. Radical reduction methods involve generation and subsequent hydrogen abstraction as key steps, which is complicated by decarbonylation of the intermediate acyl radicals. The first example in Scheme 4-1 shows that this competitive reaction is temperature dependent, where an acyl radical is generated from an acyl phenyl selenide via the abstraction of a phenylseleno group by tributyltin radical [5]. 

The radical reduction of acyl halides and related compounds, such as acyl chal-cogenides, to aldehydes may find significance for primary, vinyl, and aromatic acyl radicals whose decarbonylation rates are significantly slower than those of the corresponding secondary and tertiary radicals [6], In practice, however, this method is restricted to substrates which have serious incompatibilities with more traditional methods. 

Figure 4.52 Reductions of acyl halides with sodium borotritide...

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