Cyclobutene complexes with iron

Cycloarsa(V)azanes, 28 216-223 Cyclobutadienes, 31 144, 166 Cyclobutene complexes with iron, 12 254,... 

Phenyl selenides, in radical reactions, 9, 476 Phenylstannyl complexes, with iron, 6, 104 1-(Phenylthio)cyclobutene, with trinuclear Os clusters, 6, 743... 

The reaction of iron-carbonyl complexes with alkynes led to cyclobutenediones, which is formally a [2 + 1 + 1]-cycloaddition process for the formation of a cyclobutene derivative (Scheme 9.22) [49]. Nevertheless, in this reaction the liberation of the ligand is initiated by addition of stoichiometric amounts of copper] 11) salts and the use of various alkynes leads to interesting products such as 30 in good yields. 

A rare example of a ferracycloheptane 108 was obtained as the product of the photochemical reaction of a Petitt s cyclobutadiene iron complex with dimethyl-maleate [Eq. (43)].118 The ferracycloheptane arises from the insertion of a maleate into each of two Fe-C bonds and might therefore be considered a special case of alkene trimerisation (vide infra). The cyclobutene fragment in the final metallacycle remains coordinated to iron, as established crystallographically (Fig. 34). 

As an application of this procedure, Snapper succeeded in the formal total synthesis of (+)-astericanolide. Treatment of iron complex 62, which is prepared from cyclopentenol derivative 60 and iron complex 61, with MeaNO gives fused cyclobutene derivative 63. ROM of cyclobutene of 63 using Ig under ethylene gas smoothly proceeds to produce an eight-membered ring of 64 via Cope rearrangement. The resulting product 64 is converted into Wender s intermediate for the synthesis of (+)-astericanolide ... 

Complexation has been utilized to control site selectivity. The iron tricarbonyl complex 216 reacts with nitrile imines only at the cyclobutene double bond, whereas the uncomplexed analogue reacts at all three sites (117). 

Substituted cyclobutenes (225) are formed by the reaction of tricarbonylcyclo-butadiene iron complexes (226) with electrophiles. With a chiral complex (226a) the cyclobutenes formed are also chiral, and the inference is that the reaction must involve attack of electrophile on complexed cyclobutadiene rather than on free cyclobutadiene. In contrast, the cyclobutadiene Diels-Alder adducts (227) produced by oxidation of the optically active complexes in the presence of dieneophiles are racemic, indicating the intermediacy of the free cyclobutadienes. ... 

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