Cross metathese, alkene

Cross-metathesis, however, is usually a nonselective reaction. Transformation of two terminal alkenes in the presence of a metathesis catalyst, for instance, can give six possible products (three pairs of cis/trans isomers) since self-metathesis of each alkene and cross-metathesis occur in parallel. It has been observed, however, that terminal olefins when cross-metathesized with styrene yield trans-P-alkylstyrenes with high selectivity.5 A useful synthetic application of cross-metathesis is the cleavage of internal alkenes with ethylene called ethenolysis to yield terminal olefins ... 

Kim et al. used the relay-CM strategy as a key step in the construction of the conjugated ene-yne in (+)-scanloenyne (79) (Scheme 9.19) [25], The relay-activated partner 77, under the action of [Ru]-XII, was successfully cross-metathesized with the terminal alkene in 78 to give the ene-yne in 79 with high Z-selectivity, a type of stereocontrol first demonstrated by Lee [7a]. 

The Grubbs pyridine solvates are the fastest initiators of alkene metathesis and are valuable as synthetic intermediates to prepare other ruthenium carbene complexes. In particular, the 18-electron pyridine solvates 4a,b are very fast initiators that were developed to catalyze difficult alkene metatheses (e.g., the cross metathesis of acrylonitrile) [6]. The rates of initiation for several complexes are provided in Table 9.9. The pyridine solvate 4a has been found to initiate about 105 times faster than the parent Grubbs complex 2 and at least 100 times faster than the second-generation triphenylphosphine variant 26. When compared with the Hoveyda-Blechert complex 3a, 4a initiated about 100 times faster (c entry 3 vs. entry 5). The bromopyridine solvate 4b exceeded all of these in its initiation rate it was at least 20 times more reactive than 4a. 

Metathesis can usefully be divided into a number of types, depending on the nature of the substrates and products in the catalytic reaction. A reaction such as Eq. 12.1 is a sinq>te metathesis. With two substrates we have the reverse version, a cross metathesis (Eq. 12.3). With some choices of R and R the cross product can be favored kinetically. This happens in Eq. 12.4 wh e one alkene, r-BuCH=CH2, present in excess, is too bulky to metathesize with itself, and the cross product is formed in 93% yield. ... 

Metatheses naturally divide into types, depending on the substrates and products. Beyond simple metathesis (Eq. 12.1) involving a single alkene as reactant, comes cross metathesis (CM, Eq. 12.2), where two different alkenes react. In a common variant of CM, one product is removed, such as volatile C2H4 in Eq. 12.2, to drive the reaction to the... 

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