Allylbenzenes metathesis

An alternative approach to phosphine-free ruthenium precatalysts is based on pyridine complex 70 [48], which has been established by Grubbs et al. as a valuable precursor for other mixed NHC-phosphine complexes (cf. Scheme 15). Complex 70 is only moderately active in the cross metathesis of allylbenzene... 

Figure 7 Cross metathesis of allylbenzene and CDAB with common metathesis catalysts. Adapted from [22],...

Scheme 7 Cross metathesis of allylbenzene and acrylonitrile with catalysts bearing N-aryl, N -alkyl NHCs...

Bu4N[MoCl(CO)5]/MeAlCl2 does not metathesize allylbenzene but does cause metathesis of 4-phenylbut-l-ene (4, X = H, n = 2) to give good yields of 1,6-diphenylhex-3-ene (Doyle 1973). 

In a subsequent report, the CAAC-based metathesis catalysts were examined for selectivity in the formation of Z E olefins, as well as their activity for ethenolysis [12]. Both of these processes require kinetic selectivity to produce the thermodynamically less-favored Z and terminal olefins, respectively. It was discovered that the CAAC catalysts displayed improved conversion to the Z olefin EIZ= 1.5—2.5 after 70% conversion) for the cross metathesis of cis-l,4-diacetoxy-2-butene with allylbenzene, relative to that observed using the classical NHC- and phosphine-based systems ElZ = 3-4) at comparable conversion (Scheme 4.3). 

Scheme 4.3 Cross metathesis of c s-1,4-diacetoxy-2-butene with allylbenzene.

The kinetic selectivity of the CAAC-based catalysts was investigated by probing the EjZdiastereoselectivity in the cross-metathesis of cA-l,4-diaceto-2-butene with allylbenzene (Equation (5.4)). Compared to the commercially available Grubbs catalysts, 66-68 afforded lower EjZ ratios (3 1 at 70%... 

Interestingly, catalyst 4 not only demonstrated catalytic activity in the CM of allylbenzene and c -l,4-diacetoxybutene but also provided the most promising Z-selectivity (41 % Z) observed using a ruthenium-based metathesis catalyst. It was... 

---