Ring structures cyclobutadiene complexes

The structures of the silylated cyclobutadiene complexes were established by mass spectrometry and X-ray diffraction studies 13, 85, 127). It was found that the planes of the two rings are almost parallel. 

Several phosphete-containing transition metal complexes have been structurally determined. In their crystallographic structures, phosphete rings indicated their delocalized structures. Therefore, the aromaticity and antiaromaticity of these classes of compounds attract special attention, and encourages comparison to the highly antiaromatic cyclobutadienes. 

The X-ray study 170, 171) established a planar structure for the cyclobutadiene ring with C-—C distance equal to 1.46 A and angles of 90°. All the M—C distances are equivalent and close to those observed in ferrocene. The phenyl and methyl substituents are distorted from the ring plane and bent towards the metal atom. If one assumes that cyclobutadiene occupies two coordination sites then in the known tetraphenylcyclobutadiene-nickel and -palladium complexes the metal atom has a coordination number of 5. This suggests coordinative unsaturation for the metal and a priori one may expect an associative substitution for such complexes. 

The structures of both complexes were subsequently confirmed by x-ray analyses 68, 69, 72, 73) which showed that the cyclobutadiene ligand was square planar in each complex, with the metal atom located symmetrically below the ring. 

The smallest conceivable conjugated monocyclic diene is 1,3-cyclobutadiene. Several complexes involving cyclobutadiene are known. The compound itself is unstable and has not been studied by structural methods. It will therefore not be included in the present discussion. 1,3-Cyclobutadiene has, however, been isolated in argon matrices, and it has been established that the molecule has symmetry. For the tetra-tert-butyl derivative an envelope conformation (twist angle 7°) was found by X-ray methods, however the distances in the ring were obviously too similar for an antiaromatic system [1.464(3) and 1.483(3) A]. A redetermination at even lower temperatures gave more reasonable results (1.441 and 1.527 A) and a further analysis of the anisotropic parameters revealed that some residual disorder is still responsible for some equilibration and distances of 1.34 and 1.60 A were assumed to be the correct ones. ... 

The reaction of 1,2-diphenylcyclobutadiene (generated in situ by oxidation of its iron tricarbonyl complex) with /j-benzoquinone yields adduct 1-A as the exclusive product. A completely analogous structure is obtained using maleimide as the dienophile. However, with the more reactive dienophiles tetra-cyanoethylene and dicyanomalemide, two isomeric adducts of type 1-B and 1-C are found in a 1 7 ratio in each case. Discuss these results and explain how they relate to the issue of a square versus a rectangular structure for the cyclobutadiene ring. 

Cycloaddition. The photochemical reactions of 1,2-dimethylcyclobutadiene tri-carbonyliron with acetylene gives o- and p-xylene while propyne gives 1,2,3- and 1,2,4-trimethylbenzene and but-2-yne forms 1,2,3,4- and 1,2,4,5-tetramethyl-benzene. Preferential formation of these isomers is best rationalized in terms of Scheme 5 in which initial carbon-carbon bond formation occurs between the acetylene and a carbon atom of the cyclobutadiene ring bearing a methyl group. The intermediacy of complexes structurally analogous to (38) in cycloaddition reactions... 

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