Ruthenium complexes cyclobutadiene

Computational studies performed on both reactions, leading either to dienyl esters or to alkenylidenecyclobutenes, show that the biscarbene ruthenium intermediate requires a high activation energy to produce the cyclobutadiene complex from terminal alkynes whereas with propargyl alcohols this step occurs readily [25],... 

In contrast to these linear couplings of propargyl alcohol derivatives, a three-component cyclo-coupling of 72 proceeded in the presence of 17 and acetic acid to give rise to an alkylidenecyclobutene 73 in 66% yield (Scheme 4.27) [61]. The four-mem-bered ring skeleton vas considered to be derived from the ruthenium(II) cyclobutadiene complex, vhich might be formed via the corresponding ruthenacyclopenta-diene. 

The cyclobutadiene complex Ru6(/r6-C)(/r-CO)3(CO)i2( -C4Ph4) 138 is formed from redox-mediated ligand transfer between [Pd(acetone)2( 7 -C4Ph4)] and [Ru6(/r6-C)(CO)i6]. The metal core consists of a bond-opened octahedron, with the 77 -organic ligand bound to a basal ruthenium atom." ... 

Yamamoto, Y., Arakawa, T. and Itoh, K. (2004) Synthesis of naphthoquinone-fused cyclobutadiene ruthenium complexes. Organometallics, 23, 3610. 

In the synthesis of alkylidenecyclobutenes from propargyl alcohols, stoichiometric experiments show that the first step involves [2+2] oxidative head-to-head coupling of the alkynes, leading to an isolable cyclobutadiene-ruthenium complex. Addition of acid generates a cyclobutenyl metal intermediate which undergoes carboxylate addition on the less substituted allylic carbon atom (Scheme 7). 

Cyclopentadienyl dicarbonyl ruthenium dimer 132 reacts with silver tetrafluoroborate and diphenylacetylene to afford the cyclobutadiene ruthenium complex 133 (Scheme 12). Irradiation of 133 in dichloro-methane in the presence of several alkynes leads to the arene cyclopentadienyl ruthenium complexes 125 in high yield. This reaction appears to be a general route to sterically crowded ruthenium arene cations (55). 

The ruthenium example probably involves oxidative addition of the di-halide to two Ru(CO)3 fragments derived from the photolysis of the cluster then the metals probably disproportionate, so that one becomes the observed product and the other carries away the halides in the form of undefined Ru(II) halo complexes. The reaction of Eq. 5.32 probably goes by an oxidative coupling to give 5.20, which is very favorable for alkynes, followed by a reductive elimination of the cyclobutadiene ligand. 

---