Cyclobutadienes transition metal complexes

NMR, 3, 542 oxidation, 3, 546 phosphorescence, 3, 543 photoelectron spectra, 3, 542 photolysis, 3, 549 reactions, 3, 543-555 with alkenes, 3, 50 with alkynes, 3, 50 with IH-azepines, 3, 552 with azirines, 3, 554 with cyclobutadiene, 3, 551 with cyclopropenes, 3, 550 with dimethylbicyclopropenyl, 3, 551 with heterocyclic transition metal complexes, 7, 28 29... 

Cyclobutadiene-transition metal complexes predicted by H. C. Longuet-Higgins and L. E. Orgel 3 y before they... 

Compounds with a narrow HOMO-LUMO gap (Figure 5.5d) are kinetically reactive and subject to dimerization (e.g., cyclopentadiene) or reaction with Lewis acids or bases. Polyenes are the dominant organic examples of this group. The difficulty in isolation of cyclobutadiene lies not with any intrinsic instability of the molecule but with the self-reactivity which arises from an extremely narrow HOMO-LUMO gap. A second class of compounds also falls in this category, coordinatively unsaturated transition metal complexes. In transition metals, the atomic n d orbital set may be partially occupied and/or nearly degenerate with the partially occupied n + 1 spn set. Such a configuration permits exceptional reactivity, even toward C—H and C—C bonds. These systems are treated separately in Chapter 13. 

Several transition-metal complexes of cyclobutadiene have been prepared, and this is all the more remarkable because of the instability of the parent hydrocarbon. Reactions that logically should lead to cyclobutadiene give dimeric products instead. Thus, 3,4-dichlorocyclobutene has been de-chlorinated with lithium amalgam in ether, and the hydrocarbon product is a dimer of cyclobutadiene, 5. However, 3,4-dichlorocyclobutene reacts with diiron nonacarbonyl, Fe2(CO)9, to give a stable iron tricarbonyl complex of cyclobutadiene, 6, whose structure has been established by x-ray analysis. The 7r-electron system of cyclobutadiene is considerably stabilized by complex formation with iron, which again attains the electronic configuration of krypton. 

The first attempts to prepare cyclobutadiene (CB) were made around 1870,33 but its antiaromaticity was only understood by Hiickel in 1932. The prediction that CB could be stabilized by complexation was published in 195634 and the first cyclobutadienes were made - as transition metal complexes - 3 years later.35... 

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. 

To date there are no examples of the very highly strained cyclopropadiene (288), 1,2-cyclobutadiene (289), or 1,2-cyclopentadiene (290), either free or complexed to transition metals. Complexes of a valence isomeric form of cyclopropadiene (291) have been prepared,110 but as with the free hydrocarbon,111112 there is no evidence for any converting to the allene form [Eq. (46)]. 

G. M. Whitesides and W. J. Ehmann, 7. Am. Chem. Soc., 1969, 91, 3800 see also G. A. Ville, K. P. C. Vollhardt and M. J. Winter, Organometallics, 1984, 3, 1177 for further studies along these lines, and J. R. Strickler, P. A. Wexler and D. E. Wigley, Organometallics, 1988, 7, 2067 for studies of relevant model systems associated with alkyne trimerizations catalyzed by early transition metals. Note that although cyclobutadiene-like structures are not involved in most alkyne trimerization mechanisms, metal-complexed cyclobutadienes are fairly common side products in alkyne trimerization reactions. As their yields are usually low, their formation does not present practical problems in arene synthesis. 

An attractive transformation of cyclic diacetylenes (4) to cyclobutadienes (92) was attempted considering the proximity interaction, but it has been unsuccessful so far. However, transition metal complexes such as the u-cyclopenladienyl cobalt... 

Syntheses and properties of cyclobutadiene-transition metal complexes have been discussed in detail by Maitlis 167). Brown 168), and others 169) have reviewed the metal-ligand bond in terms of the MO approximation. The main bonding in these complexes is due to an overlap of the two degenerate nonbonding cyclobutadiene orbitals with spd hybrid metal atomic orbitals. 

Transition-metal complexes containing cyclopentadiene and cyclobutadiene ligands are mercurated readily, e.g., ferrocene , (h -C5Hj)2Fe, reacts with Hg(02CCHj)2 in HO2CCH3, EtjO-EtOH, or PhH-EtOH to give mixtures of mono- and dimercurated ferrocenes in proportions dependent on the reagents ratio ... 

Cyclobutadiene transition metal complexes were actually synthesized long before cyclobutadiene itself.14 Interestingly, one of these complexes was also a halogen-bridged dimeric nickel complex, as shown on the next page. 

Transition-metal complex of benzyne, It is well known that some very unstable compounds (e.g., cyclobutadiene) can be isolated as complexes with transition... 

Other cycloaddition reactions are the dimerization and oligomerization of strained cycloal-kynes, which proceed via cyclobutadiene systems (Scheme 8-24) [1, 3 b]. Usually, however, these reactions require the assistance of transition metal complexes, which are dealt with in the following section. 

Dihapto-Cyclobutadienoid Transition Metal Complexes. The Preparation and Characterization of a Binuclear Complex Possessing a Bridging Cyclobutadiene Ligand, A. Sanders and W. D. Giering, J. Amer. Chem. Soc., 96, 5247 (1974). 

The reactions of alkynes with transition metal complexes often lead to bewildering mixtures of products. Substituted alkynes often produce rj -cyclobutadiene derivatives (p. 269) metal carbonyls also afford products containing cyclopentadienone and quinone ligands, which incorporate two alkyne units together with one or two carbonyl groups. Some of these systems catalyse the cyclotri- or tetra-merization of alkynes to benzenes or cyclooctatetraenes respectively (Fig. 7.12). 

As we saw in Section 11.17, cyclobutadiene is antiaromatic and exceedingly difficult to prepare and study. Its successful preparation by Rowland Pettit (University of Texas) in 1965 demonstrated how transition-metal organometallic cbemistry can provide access to novel reactions and structures. His approach was to prepare cyclobutadiene as a transition-metal complex, then destabilize the complex to trigger its dissociation. The sequence for cyclobutadiene begins with the reaction of cis-3,4-dichlorocyclobutene with diiron nonacarbonyl [Fe2(CO)9]. The resulting iron-cyclobutadiene complex satisfies the 18-electron rale, is stable, and undo-goes a variety of reactions. Most importantly, oxidation with ceric ammonium nitrate (a source of Ce ) lowers the electron count Irom 18 to 16, causing the complex to dissociate and Ubo-ate free cyclobutadiene. 

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