Cyclobutadiene iron tricarbonyl synthesis

In a series of classic studies, Pettit et al. reported the synthesis of (cyclobutadiene)iron tricarbonyl together with a variety of electrophilic substitution reactions of this aromatic system (Scheme S).25-27... 

Cyclobutadiene iron tricarbonyl complexes can be isolated and have been utilized in organic synthesis. Both intra- and intermolecular [2 + 2] cycloadditions of alkenes with cyclobutadiene complexes are observed upon decomplexation using CAN or TMANO (Schemes 164-165). The stereochemistry of the aUcene is retained in the product. Iron tricarbonyl diene complexes are compatible with metathesis reactions... 

Rosenblum et al. 386> reported a very elegant synthesis of cyclobutadiene iron tricarbonyl starting from oc-pyrone. The corresponding reaction with cyclopentadienylcobalt dicarbonyl produces cyclobutadiene-(cyclopentadienyl)cobalt 88S>. 

Sergheraert s group also attempted the synthesis of an enkephalin derivative with the organometaUic amino acid cyclobutadiene iron tricarbonyl alanine 26 [73]. This compoimd, however, did not withstand the conditions of solid phase peptide synthesis and could not be isolated in pure form. 

A convenient synthesis of a cubane system, in which the cyclobutadiene transfer reaction plays a key role, was reported by Pettit et al. (Scheme 11). By the oxidative decomposition of cyclobutadiene-iron tricarbonyl 56 with Ce + ion in the presence of a dienophile, a molecule of cyclobutadiene can be transferred from the iron to the dienophile. Decomposition of 56 in the presence of 2,5-dibromobenzoquinone 57 yielded the Diels-Alder adduct 58 with e do-configuration. Irradiation of 58 in benzene with a high-pressure Hg lamp afforded the bishomocubane derivative 59, which gave cubane-1,3-dicarboxylic acid 60 (80%) by treatment of 59 with aqueous KOH at 100°C. 

A new route to pleiadienes involves photoaddition of maleic anhydride derivatives to acenaphthene, oxidative removal of the anhydride groups, and ring-opening of the resulting cyclobutenes (Scheme 18). Schmidt has reported further details of the synthesis of optically active tricarbonyl(cyclobutadiene)iron complexes, the first step involving [2 + 2] addition of 1,2-dichloroethylene to dichloromaleic anhydride. 

Phenylbenzocyclobutadiene is an intermediate in the photolysis of diphenyl-acetylene, but is too unstable to be isolated and undergoes dimerization. A cyclobutadiene has also been proposed as an intermediate in the complex reaction of perfluoro-l,l-dimethylallene with chloroform and caesium fluoride. Cyclobutadiene is generally trapped and stored as its iron tricarbonyl complex. A normal co-ordinate analysis of the latter has been reported, as has a convenient synthesis of the carboxylic acid complex (823), ... 

Similar to the first syntheses of cyclobutadiene complexes [2, 9], the first synthesis of a trimethylenemethane complex started from dichloride 2, which was treated with diironenneacarbonyl to give tricarbonyl(trimethylenemethane) iron(O) (3) in 30% yield In addition to iron(II) chloride (Scheme 10.1) [10]. The r -coordination has been confirmed by crystal structure analyses [11, 12]. Very recently, Frenking et al. published a detailed theoretical bonding analysis of some late transition metal sandwich trimethylenemethane complexes [13]. 

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