Diene-iron carbonyl complexes acyclic dienes

Since 1958 a considerable amount of research activity has centered around these systems, both in the acetylene-iron carbonyl reactions and the direct reactions of olefins with iron carbonyls. The types of unsaturated ligands which are now known to occur in stable iron carbonyl complexes include substituted and nonsubstituted cyclic, acyclic, and nonconjugated dienes as well as some aromatic systems. Furthermore, what may be formally regarded as dienyl cations as well as allyl cations and radicals are found to... 

Acyclic Series. The first complex in the acyclic series was prepared from butadiene by the thermal method. Heating isoprene and pentacarbonyliron at high temperature, however, is inefficient due to competitive Diels-Alder dimerization. Despite the formation of some bis(diene) iron carbonyl complexes on prolonged irradiation, the photochemical method is superior in this case. Complexation of acyclic dienes by Fe(CO)3 is limited to those that can adopt a cisoid conformation, with the syn substitution pattern normally preferred. 2,4-Hexadienolc acid, for example, ean be conveniently complexed by a photolytic procedure. Trialkylsilyl-substituted dienes have also been complexed. ... 

Dienes form very stable complexes with a variety of metal caibonyls, particularly Fe(CO)s, and the neutral V-diene metal carbonyl complexes are quite resistant to normal reactions of dienes (e.g. hydrogenation, Diels-Alder). However, they are subject to nucleophilic attack by a variety of nonstabilized carbanions. Treatment of -cyclohexadiene iron tricarbonyl with nonstabilized carbanions, followed by protonolysis of the resulting complex, produced isomeric mixtures of alkylated cyclohexenes (Scheme 15).24 With acyclic dienes, this alkylation was shown to be reversible, with kinetic alkylation occurring at an internal position of the complexed dienes but rearranging to the terminal position under thermodynamic conditions (Scheme 16).2S By trapping the kinetic product with an electrophile, overall carbo-... 

The analogous cycloheptadienyl complex (5 equation 1) was similarly prepared by Dauben and Ber-telli,4 but the acyclic pentadienyl systems were a little more difficult to obtain. The triphenylmethyl cation does not remove hydride from tricarbonyl(frans-pentadiene)iron (6 equation 2). The corresponding c/s-pentadiene complex (7 equation 3) cannot be prepared directly from the diene and an iron carbonyl,... 

The classical protocol for synthesis of iron-diene complexes starts from the homoleptic pentacarbonyliron complex. In a stepwise fashion, via a dissociative mechanism, two carbonyl ligands are displaced by the diene system. However, thermal dissociation of the first CO ligand requires rather harsh conditions (ca. 140 °C). For acyclic 1,3-dienes, the diene ligand adopts an s-cis conformation to form stable q4-complexes (Scheme 1.18). 

Fot detailed discussion of the use of metal carbonyl 1,3-diene ctnnplexes in organic synthesis, see L.R. Cox and S.V. Ley (1998) Chem. Soc. Rev., vol. 27, p. 301 - Tricarbonyl complexes an approach to acyclic stereoctHitrol W.A. Donaldson and S. Chaudhury (2009) Etdr. J. Org. Chem., p. 3831 - Recent applications of acyclic (diene)irrai complexes and (dienyl)iron cations in organic synthesis . 

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