Trimerization catalysts Subject

Representatives of the bridged sulfone system 70 have been subjected to ruthenium catalysed ring-closing metathesis reactions (Grubbs catalyst) and shown to afford, in low yields, a few selected cyclic dimers and trimers, of all the possibilities available. The diastereoselectivities observed were rationalised in terms of kinetic control involved with internal ruthenium/sulfonyl oxygen coordination . 

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. 

Nylon 12 first became available on a semicommercial scale in 1963. The monomer, dodecanelactam, is prepared from butadiene by a multistaged reaction. In one process butadiene is treated with a Ziegler-type catalyst system to yield the cyclic trimer, cyclododeca-1, 5, 9-triene. This may then be hydrogenated to give cyclododecane, which is then subjected to direct air oxidation to give a mixture of cyclododecanol and cyclododecanone. Treatment of the mixture with... 

It remains unknown how GTP-bound a or py of G-proteins interacts with the effector system such as phospholipase C or ion chaimels. In the case of adenylate cyclase, the activation involves direct interaction of the GTP-bound a-subunit of G s with the adenylate cyclase catalyst. The inhibition of adenylate cyclase, however, results from combined effects of direct and indirect interactions between some a-subunits, Py -subunits and the catalytic protein of adenylate cyclase as follows. Firstly, Py liberated from Gi, Go and Go may form a trimeric complex with the a-subunit of Gs, thereby decreasing the concentration of free as, the direct activator of the cyclase catalyst (31). The inhibition by this mechanism could be expected to occur in a number of mammalian cell types, since these lAP substrates are much more abundant than Gs in these cells (37). Secondly, the a-subunit of Gi competes with the a-subunit of Gs for the activation site on the cyclase catalyst, though the affinity of Gi was much lower than the affinity of Gs for this site (38). No competition was observed, however, between the a-subunit of G s and a-subimits of Go, Go and G hl. Thirdly, py is capable of direct interaction with the adenylate cyclase catalyst in such a manner as to lower the cyclase activity (38). The interaction was observed at rather higher concentrations of Py. Fourthly, Py binds to calmodulin with a high affinity (39). Calmodulin is a potent activator of the adenylate cyclase catalyst as such, but is not so after it is bound by Py of G-proteins. Thus, the inhibition of adenylate cyclase by Py of G-proteins was biphasic in the presence of calmodulin the inhibition by lower concentrations of Py was due to prevention of calmodulin activation of the cyclase and the inhibition by the higher concentrations reflected the direct interaction with the cyclase. The relative importance of these multiple mechanisms for adenylate cyclase inhibition will be the subject of future investigations. 

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