Metathesis, chelation effects

The polymerization of ether and thioether monomers was also studied, and it was found that the rate of polymerization was a great deal slower with the functionalized monomers. The number of methylene units between the olefin and the heteroatom greatly affected the rates observed, giving credence to the chelation effect shown in Fig. 6.1. In addition, catalyst 2 polymerizes 1,5-hexadiene, whereas catalyst 6 mainly cyclizes the metathesis dimer to cyclo-l,5-octadiene. At this point there is no clear explanation for this result, and, furthermore, the reason that the COD generated did not undergo ROMP in these reactions is unclear. The data from these experiments clearly shows that Lewis basic functionality retards the rate of metathesis with complex 6 more than with complex 2, although 6 is clearly the more functional group-tolerant complex overall [35]. 

In the example first cited, the enthalpies make a slight favorable contribution, but the main source of the chelate effect is still to be found in the entropies. We may look at this case in terms of the following metathesis [Ni(NH3)6]2 + (aq) + 3 en(aq) = [Ni en3]2+(aq) + 6NH3(aq) logy = 9.67... 

Catalyst initiation and overall catalytic activity are not always opposing effects. For example, complexes that contain chelating ligands, such as bidentate phosphines [90], Schiff bases [91], and tris(pyrazolyl)borates [92], generally show both low rates of initiation and low overall olefin metathesis activity (Fig. 4.34). Both effects appear to result from slow rates of ligand dissociation from the starting complexes, which leads to low concentrations of the catalytically active 14-electron species in solution. 

Various functional groups have been proposed to coordinate the metal center in intermediates on metathesis pathways. Beneficial and detrimental effects have been proposed in different studies in the Hterature to result from the coordinating effects of Lewis basic functionality. Typically, this functional group is positioned such that a 5- or 6-membered chelate is formed (e.g. Figure 2.17). 

Researchers at Boehringer Ingelheim studied the effects of amide protecting group on the RCM reactions of 56—59 (Figure 2.18). Exposure of 56-59 to 30 mol% G1 in DCM-d2 allowed the relative proportions of carbene at each terminus to be evaluated by NMR. The catalyst underwent metathesis with the unprotected amide 56 or N-benzylated amide (58) at terminus A preferentially, while BOG protection (57) or N-acylation (59) favored initiation at terminus B (BOG = ferf-butyloxycarbonyl). The latter terminus is remote from the modification and so serves as a control terminus. Protection of the amide was proposed to disrupt 1,6-chelation of the metal center by the ester through A13 strain. 

Alternatively, Dorta and co-workers used SINap ligands in order to improve the activity and stability of second-generation ruthenium metathesis catalysts (14). Effects of NHC backbone substitution on imidazolinylidene-derived ligands were studied by Grubbs et who also replaced the imidazole-based N-heterocycles with their thiazole analogues.Because these modifications were also applied to oxygen-chelated alkylidene catalysts, they are discussed in more details in the next Section. 

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