Bromides, acyl, from aldehydes halides, alkyl

As noted earlier, [HCr(CO)5] also converts alkyl halides to alkanes, but the reactivity of the acyl chloride is much higher, such that it was possible to selectively convert the acyl chloride to an aldehyde in one step, without interference from the alkyl bromide functionality. A second equivalent of [HCr(CO)5] further reduced the alkyl bromide (Eq. (27)). 

Acyl carbonyl ferrates are involved as intermediates in two preparations of aldehydes. Alkyl bromides react with sodium tetracarbonyl ferrate(—ii) (from iron pentacarbonyl and sodium) by a process of oxidative addition to furnish the ferrate(—i) (124) this species rearranges on treatment with triphenylphosphine to a phosphonium-substituted acyl ferrate(—i) (125), which is subsequently cleaved by acetic acid to the homologous aldehyde, as outlined in Scheme 46. A related procedure employs reaction of an acid halide with sodium tetracarbonyl ferrate(—ii) to afford the acyl ferrate(—i) (126) directly this species is also cleaved by acetic acid, and affords yet another method for the reduction of acid halides to aldehydes. 

---