Cyclopentadienyl rings cyclobutadiene complexes

Cycloalkenes, into if-allyl palladium complexes, 8, 363 Cycloalkenyl rings, metal complex conformational interconversions, 1, 414 Cycloalkynes, in nickel complexes, 8, 147 (Cyclobutadiene)cyclopentadienyl complexes, with cobalt, polymercuration, 2, 435 Cyclobutadienes... 

The reaction of (cyclobutadiene)metal complexes with X2 results in the oxidative decomplexation to generate either dihalocyclobutenes or tetrahalocyclobutanes. In comparison, substitution of (cyclobutadiene)MLn complexes 223 [MLn = Fe(CO)3, CoCp, and RhCp] with a variety of carbon electrophiles has been observed (equation 34)15. Electrophilic acylation of 1-substituted (cyclobutadiene)Fe(CO)3 complexes gives a mixture of regioisomers predominating in the 1,3-disubstituted product and this has been utilized for the preparation of a cyclobutadiene cyclophane complex 272 (equation 35)246. For (cyclobutadiene)CoCp complexes, in which all of the ring carbons are substituted, electrophilic acylation occurs at the cyclopentadienyl ligand. 

POLYMERS CONTAINING METALLACYCLOPENTADIENE RINGS AND CYCLOBUTADIENE RINGS COORDINATED TO CYCLOPENTADIENYL METAL COMPLEXES... 

The proton NMR spectra of some of these complexes have been determined. The spectrum of cyclobutadieneiron tricarbonyl (XVIII) shows a singlet at 6.09r and that of benzocyclobutadieneiron tricarbonyl (XIX) a singlet at 5.98r due to the cyclobutadiene protons, as well as a multiplet due to the aromatic protons at 3.05r (3S). The NMR spectra of monosubstituted cyclobutadieneiron tricarbonyls (see Appendix) show the equivalence of the two cyclobutadiene ring protons adjacent to the substituent. This implies that the four-membered ring must be square (39a). Tetramethylcyclo-butadienenickel chloride in water shows only a single resonance due to the 12 equivalent methyl protons (32). The spectra of the tetraphenylcyclo-butadiene-metal complexes are those due to phenyl protons and are usually complex. In the (cyclopentadienyl)(tetraphenylcyclobutadiene)nickel and -palladium bromides (XLIV), however, sharp single phenyl proton resonances are obtained at 2.39r (65). The reason for the apparent equivalence of all the phenyl protons in (XLIV) is not clear. 

Endocyclic dienes ranging in ring size from four to seven have also been used in the intramolecular Diels-Alder reaction. Cyclobutadienes are prepared from the corresponding metal complexes [37]. Cyclopentadienes are most readily prepared by alkylation of cyclopentadienyl anion [38, 39]. They are also available by Michael addition to fulvene derivatives and by aminal exchange [40, 41]. 

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