Iron carbonyls dehalogenation

Iron carbonyls also mediate the cycloaddition reaction of allyl equivalents and dienes. In the presence of nonacarbonyldiiron a,a -dihaloketones and 1,3-dienes provide cycloheptenes (Scheme 1.5) [14,15]. Two initial dehalogenation steps afford a reactive oxoallyliron complex which undergoes a thermally allowed concerted [4 + 31-cycloaddition with 1,3-dienes. The 1,3-diene system can be incorporated in cyclic or heterocyclic systems (furans, cyclopentadienes and, less frequently, pyrroles). Noyori and coworkers applied this strategy to natural product synthesis, e.g. a-thujaplicin and P-thujaplicin [14, 16]. 

In addition to dihalides such as 2, other organohalides can serve as precursors for the synthesis of trimethylenemethane complexes by dehalogenation. This dehalogenation can be afforded not only by iron carbonyls but also by disodium tetracarbonylferrate(-II). For example, the reaction of 2-(bromomethyl)allyliron halide 6 with Fe2(CO)g afforded trimethylenemethane complex 3 in 91% yield (Scheme 10.3) [16]. 

Dehalogenation of oi,a dihaloalkenes and Ketones. The preparation (eq 24) of the iron carbonyl complex of trimethylen-emethane (42) and some Fc2(CO)9-mediated rearrangements of dibromo ketones constitute the pioneering work in this area. Noyori has extended the dehalogenation reaction of a-halo ketones to a,a -dibromo ketones and o,a,a, a -tetrabromo ketones which can be dehalogenated with Fe(CO)5 or Fe2(CO)9. ... 

The reductive dehalogenation of haloalkanes has also been achieved in high yield using polymer supported hydridoiron tetracarbonyl anion (Table 11.15). In reactions where the structure of the alkyl group is such that anionic cleavage is not favoured, carbonylation of the intermediate alkyl(hydrido)iron complex produces an aldehyde (see Chapter 8) [3]. 

Although nickel-catalyzed carbonylative Pauson-Khand cycloadditions have not been broadly developed, related doubly carbonylative cycloadditions involving allyl halides have been demonstrated as an entry to functionalized cyclopentenones. In recent catalytic versions, iron powder was used as the terminal reductant and dehalogenating agent (Scheme 3-38). [Caution Ni(CO)4, which could potentially be liberated in this reaction, is a highly toxic gas.]... 

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