Cyclobutadiene iron tricarbonyl oxidation

The area of cyclobutadiene-transition metal chemistry has expanded rapidly since these initial findings, largely through the work of Maitlis 163), Nakamura 183), Freedman 104), and others, but details will not be presented here. Several recent important discoveries by Pettit and co-workers 22, 79,102, 24I), however, relate to the formation and chemistry of cyclobutadiene-iron tricarbonyl (XVII). This product is formed from the reaction of cis-3,4-dichlorocyclobutene and diiron nonacarbonyl and can be isolated in the form of yellow crystals of excellent stability. Cyclobutadiene can be liberated by treating the complex with oxidizing agents such as ferric or ceric ion. The free ligand has been trapped and demonstrated to possess a finite lifetime. It has also been shown to... 

Diels-Alder reactions. The diazene (1) undergoes a Diels-Alder reaction with cyclopentadiene to give the expected adduct in >90% yield. The reaction with cyclobutadiene (liberated by oxidation of cyclobutadiene iron tricarbonyl) has been studied in detail. The adduct (2) is formed in 45% yield. Deblocking of (2) by treatment with base and then acid leads to 2,3-diazabicyclo[2.2.0]-hexene-5 (4). Oxidation of a hydrazo group is known to lead to an azo group. 

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. 

The oxidation of cyclobutadiene iron tricarbonyl with ceric ion releases the cyclobutadiene which may be characterized by its in situ reactions ... 

The reaction of o-diphenylcyclobutadiene (generated in situ by oxidation of its iron tricarbonyl complex) with p-benzoquinone yields A as the exclusive product. With tetracyanoethylene, however, B and C are formed in a 1 7 ratio. Discuss these results, and explain how they relate to the question of the square versus rectangular shape of cyclobutadiene. 

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. 

Liberation of cyclobutadiene from its tetrahapto-iron tricarbonyl complex is believed to proceed via a dihapto iron complex, and the first such complex to be prepared by an independent method is [824 Fp — T -CpFe(CO)2]. On oxidation with iodide ion in the presence of diphenylisobenzofuran the adduct (825) of benzo-cyclobutadiene is obtained. A dinuclear complex with cyclobutadiene acting as a bridging ligand between two iron atoms has also been prepared. ... 

Little meaningful physical data on these complexes has appeared yet. The study of their reactivity is complicated by the effects of the substituents on the cyclobutadiene ring and the other ligands present. Until more information is available on the properties of the recently prepared unsubstituted cyclobutadieneiron tricarbonyl (XVIII) 38) and similar molecules, it is hard to be certain which properties are due to the presence of a cyclobutadiene group. Thus while cyclobutadieneiron tricarbonyl (XVIII) is easily oxidized by ferric chloride in ethanol (as are other diene-iron tri-carbonyl complexes, albeit to different types of product), tetraphenyl-cyclobutadieneiron tricarbonyl (XIII) is very resistant to this reagent, and indeed to most others, presumably mainly due to the steric hindrance of the phenyls. 

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