Cyclobutadiene complexes with metals

The unfused cyclobutadiene system is stable in complexes with metals (see Chapter 3), but in these cases electron density is withdrawn from the ring by the metal and there is no aromatic quartet. In fact, these cyclobutadiene-metal complexes can be looked upon as systems containing an aromatic duet. The ring is square planar, the compounds undergo aromatic substitution, and NMR spectra of monosubstituted derivatives show that the C-2 and C-4 protons are equivalent. ... 

The course of the reaction, and the nature of the product formed in these reactions are not readily predictable. It appears, however, that there is a high tendency for cyclobutadiene complexes with a central metal ion [(Fe(0), Co(I), Ni(II), and Pd(II)] to exist. This may serve as a useful guide to future work. 

Variously substituted tricarbonyliron-cyclobutadiene complexes are readily prepared (Fitzpatrick et ah, 1965 Roberts et al., 1969 Agar et al., 1974) and cyclobutadiene complexes with other transition metals are known (Maitlis, 1966), but few of these have been used in organic synthesis. 

The reactions of the palladium-cyclobutadiene complexes with excess alkyne produced persubstituted cyclooctatetraenes in analogy to Reppe s catalytic cyclooctatetraene synthesis (Pollock and Maitlis, 1966 Reppe and Schweckendiek, 1948). Although the cyclobutadiene ligand can be transferred between metals [Eq. (76)] (Maitlis, 1971b), liberation of the ligand... 

Crystal structure data15 indicate that in the vast majority of (cyclobutadiene)metal complexes (4) the cyclobutadiene ligand is approximately square-planar with nearly equal C—C bond distances (ca 1.46 A) and bond angles of ca 90°. Within a given complex the cyclobutadiene carbon-to-metal distances are roughly equal. 

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. 

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

Cyclobutadiene complexes can also be made by metal atom reactions. For example, the reaction of 3,4-dichloro-l,2,3,4-tetramethylcyclo-but-I-ene with nickel or palladium atoms (133) is... 

These compounds have been obtained indirectly by reactions of silylated acetylenes with metal carbonyls or olefin complexes. Thus, trimethylsilylphenylacetylene reacts with rj5-cyclopentadienylcobalt dicarbonyl, cobaltocene, or rjs-cyclopentadienyl-(l,3-cyclooctadiene) cobalt, in refluxing xylene, to give a mixture of cis- and trans-bis-(trimethylsilyl)cyclobutadiene complexes (R = Me, R = Ph) 68, 127, 137) ... 

The unfused cyclobutadiene system is stable in complexes with metals155 (see Chapter 3), but in these cases electron density is withdrawn from the ring by the metal and there is... 

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... 

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. 

Like its cyclobutadiene analog 39, the 7j4-l,3-diphosphacyclobutadiene complex 38 has high thermal stability, is stable to air, and generally has low reactivity. Reagents that normally release cyclobutadiene from transition metal-cyclobutadiene complexes by oxidative demetallation [i.e., HCN, (NH4)2Ce(N01)6, and alkali metals] do not react with compound 38. 

Studies of rc-complexcs containing ligands other than the Cp family of ligands have also appeared in the literature in 20 08.23 25 Of particular note is the synthesis of Mg and Ca complexes of the tetrakis(trimethylsilyl)cyclo-butadiene dianion, [Mg(thf)3][(Me3Si)4C4] (19) and [Ca(thf)J[(Me3Si)4C4] (20), via the reduction of tetrakis(trimethylsilyl)cyclobutadiene [(Me3Si)4C4] with metallic Mg or Ca. The structure of the half-sandwich 19 shows the presence of an -bonded [(Me3Si)4C4]2 671-dianion to the Mg centre.23... 

Cyclobutadiene-metal complexes were obtained from reactions of the corresponding acetylenes with metal complexes i9U,2i) Orbital symmetry principles would suggest that these complexes are either formed via stepwise processes or involved the intervention of bimetallic species (32). The stepwise routes are particularly attractive. Acetylene ligands can reasonably be expected to undergo a [( 2s 2 -f-ji2s] cycloaddition with the metal center generating the metalocyclodiene intermediate 33. Cyclobutadiene can then be extruded from the metal center with the aid of another metal. 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

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. 

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. 

It should be noted that cyclobutadiene always replaces carbon monoxide in reactions with metal carbonyl derivatives. Yields of product parallel the known rate of exchange of CO in the starting carbonyl 184). Highest yields of ligand transfer products are attained with nickel and cobalt carbonyls which are known to very rapidly exchange their CO groups by a D-type mechanism 185-188). Lowest yields have been reported with Mo and W complexes, the carbonyls of which exchange with CO very slowly 188). 

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 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... 

The reduction of 3,4-dichlorocyclobutene (222) in the presence of metal carbonyls has been utilized to prepare the parent complex [223, MLn = Cr(CO)4, Mo(CO)3, W(CO)3, Fe(CO)3, Ru(CO)3 orCo2(CO)6] (equation 32) .Morerecently, reaction ofNi(CO)4 with 3,4-dihalocyclobutenes (X = Br or I) or with 222 in the presence of AICI3 produced the corresponding (cyclobutadiene)nickel dihalides . Methodology for the preparation of 1,2- or 1,3-disubstituted (cyclobutadiene)Fe(CO)3 complexes from 1,2- or 1,3-disubsli-tuted-3,4-dibromocyclobutenes has been presented - In turn, the substituted dibromo-cyclobutenes are prepared from squaric esters. The reaction of cz5-3,4-carbonyldioxycy-clobutene and substituted variants with l c2(CO)9 orNa2Fe(CO)4 also produces (cyclobu-tadiene)Fe(CO)3 complexes . Photolysis of a-pyrone generates 3-oxo-2-oxabicyclo [2.2.0]hex-5-ene (224) which undergoes photolysis with a variety of metal carbonyls to afford the parent cyclobutadiene complex 223 [MLn = CpV(CO)2, Fe(CO)3, CoCp. or RhCp] (equation 33) 2 0. 

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