Cyclobutadienes metal complexes

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cyclobutadiene system is stable in complexes with metals (see Chapter 3), but in these cases electron density is withdrawn from the ring by the metal and there is no aromatic quartet.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. ... 

Catenated Organic Compounds of the Group IV Elements, 4,1 Conjugate Addition of Grignard Reagents to Aromatic Systems, 1, 221 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365 Diene-Iron Carbonyl Complexes, 1, 1... 

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. 

While one of the first preparations of a cyclobutadiene-metal complex involved the cyclodimerization of diphenylacetylene in the presence of Fe(CO)5 at high temperature212, the thermal reaction of alkynes with Fe(CO)s gives predominantly cyclopentadienone complexes (Section IV.E.l.b). The cyclization of alkynes by a wide variety of metal complexes has been reported (Scheme 59)l 5-21 A—222... 

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. 

Catenated Organic Compounds of Silicon, Germanium, Tin, and Lead Henry Gilman, William H. Atwell, and Frank K. Cartledge Cyclobutadiene-Metal Complexes P. M. Maitlis Organoarsenic Chemistry W. R. Cullen... 

P. M. Maitlis, Cyclobutadiene-Metal Complexes, Adv. Organomet. Chem. 4,95-143 (1966). 

Return now to transformation 28 - 29. 28, as it traverses the reaction coordinate to 29, clearly does not represent a bonding state of the cyclobutadiene metal complex. In fact, 29 represents the situation where both ei and its antibonding counterpart (not represented in Fig. 7) are occupied and 62 is empty. The ligand-to-metal bonding, then, associated with two important molecular orbitals in the cyclobutadiene hgand system (ei and 2) is not realized with n bond fusion of a bisacetylene ligand system. 

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. 

A53. M. P. CavaandM. J. Mitchell, Cyclobutadiene and Related Compounds. Academic Press, New York, 1967. Chapter 2, pp. 88-121 The cyclobutadiene-metal complexes. A54. F. Basolo and R. G. Pearson, Mechanisms of Inorganic Reactions. Wiley, New Yor, 1958. Chapter 8, pp. 332-367 Catalytic effects of coordination compounds (105). This section was considerably expanded in the second edition (1967) as Chapter 7, pp. 526-627 Reactions of transition metal organometallics (250). 

P. M. Maitlis Advan. Organometal. Cyclobutadiene-metal complexes 49 ... 

Cyclic hydrocarbons, syntheses, 8, 48-74 >r-Cyclobutadiene-metal complexes, 6,... 

Since these two initial reports, several other complexes possessing tetra-substituted, cyclobutadiene ligands have been reported, notably by Maitlis and co-workers ( ), who have demonstrated that one can transfer a cyclobutadiene ligand from one metal to another to produce new cyclobutadiene-metal complexes. 

Fritch, J. R., Vollhardt, K. P. C. Cyclobutadiene-metal complexes as potential intermediates of alkyne metathesis flash thermolysis of substituted 4-cyclobutadienyl- 5-cyclopentadienylcobalt complexes. Angew. Chem. 1979,91,439-440. 

---