Cyclobutadiene complexes structure

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. 

A silylated cyclobutadiene complex of unusual structure was formed, along with silylated bicyclic compounds, in the reaction of terminal alkydiynes with bis(trimethylsilyl)acetylene and cyclopentadienyldicar-bonylcobalt in refluxing octane (R = Me) (69) ... 

To highlight what one would expect in reactions of the diphosphazirco-nole 37, it is instructive to examine the rj4-l,3-diphosphacyclobutadiene complex (38) (94,95), whose X-ray structure is compared in Fig. 15 with that of the isoelectronic rj4-cyclobutadiene complex 39 (96). Compound 38 is readily obtained from reaction of (Cp)Co(T/2-C2H4)2 and 2 equiv of Bu CP. The same reaction with a pure alkyne does not stop at a cyclodimer but leads to cyclotrimerization (97). In fact, transition metal-cyclobutadiene complexes normally form only at temperatures above 80°C, presumably from a metallole intermediate, by a double reductive elimination process. It is noteworthy how readily this cyclodimerization to complex 38 takes place with phosphaalkynes. 

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. 

Diphenylacetylene with Mo(CO)a (298) in a sealed tube at 160°-170°C produces, in addition to two cyclobutadiene complexes, a yellow compound with the empirical formula [C9(CgH5)gO]Mo(CO)2, the infrared spectrum and chemical properties of which suit the tetraphenyl-cyclopentadienone complex of structure (11). On the other hand, the interaction of 3-hexyne with (CH3CN)gMo(CO)3 yields only the alkyne complex (570). 

Cyclobutadiene-metal complexes have also been suggested as intermediates in a number of other reactions, notably in the formation of benzenes by trimerization of acetylenes 2, 18, 57, 90) and also in other reactions. Conclusive evidence is still lacking, but the inertness of the known cyclobutadiene complexes towards acetylenes in Diels-Alder type reactions makes this rather unlikely. Intermediates with open-chain structures such as (XCVI) appear more attractive. Arnett and Bollinger (2) have isolated by-products from the dicobalt-octacarbonyl-catalyzed trimerization of diisopropylacetylene which are very similar to some, e.g., (LI), obtained in the thermal decomposition of tetramethylcyclobutadienenickel chloride complexes (29). Again, here, however, the evidence is by no means conclusive and a variety of intermediates other than a cyclobutadiene-metal complex can be postulated to explain the observed products however, see also the Appendix. 

Cyclobutadiene complexes are also relatively stable, but many of them undergo decomposition in air. Such complexes are formed by metals of groups 4-10 containing central atoms of d" -d electronic structures. Recently, the first biscyclobutadiene compound was obtained it is the nickel complex [Ni(C4Ph4)2] which is isoelectronic with ferrocene, Fecp2- The molecular orbital scheme for this compound is presented in Figure 8.8. 

The first cyclobutadiene complexes were obtained either from alkynes (p. 246) or by reaction of cis-dichlorocyclobutenes with metal carbonyls. Many more derivatives were then prepared by transfer of the cyclobutadiene ligand from one transition metal to another, especially from palladium in [(Ph4C4)PdCl2]2. In their complexes cyclobutadienes adopt a planar square structure. 

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. 

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... 

A rhodium-catalyzed cycloisomerization reaction of triyne 137 to 141 involves cleavage of the C=C triple bond (Scheme 7.49) [68]. The following reaction pathway is proposed initially, oxidative cyclization produces the rhodacycle 138, which then undergoes reductive elimination. The rhodium cyclobutadiene complex 139 is thus generated, and then undergoes oxidative addition to produce the rhodacycle 140. This isomerization from 138 to 140 would reduce the steric congestion of the heUcal structure. Subsequently, a cycloaddition reaction between the rhodacycle and the pendant alkyne moiety takes place to afford 141. 

Similar to the first syntheses of cyclobutadiene complexes [2, 9], the first synthesis of a trimethylenemethane complex started from dichloride 2, which was treated with diironenneacarbonyl to give tricarbonyl(trimethylenemethane) iron(O) (3) in 30% yield In addition to iron(II) chloride (Scheme 10.1) [10]. The r -coordination has been confirmed by crystal structure analyses [11, 12]. Very recently, Frenking et al. published a detailed theoretical bonding analysis of some late transition metal sandwich trimethylenemethane complexes [13]. 

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