Titanium complexes cyclobutadiene

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. 

Different polystyrene-bound biscyclopentadienyl complexes have been prepared. One example is shown as 69 in Eq. 5-9 [180]. Treatment of soluble copolymers of styrene and methacrylic acid with dichlorotitanocene resulted in soluble titanium-containing copolymers 70 (Eq. 5-10] [181]. Finally polymeric organometallic mesogens 71 with [l,3-(diethynyl)cyclobutadiene]cyclopentadi-enyl Co(II) moieties were prepared by Heck coupling of a corresponding cyclobutadiene monomer with 2,5-diiodothiophene [182],... 

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