ca01 only detailed ToC 1..24

2.6.1 Metal–Carbene Complexes 101
purified by flash chromatography (0–7% EtOAc/hexane) to yield the substituted cyclohexene
as a colorless oil; yield: 27 mg (84%); 1 H NMR (benzene-d 6, ä): 5.72 (br s, CHCH 2),
2.05–1.45 (m, 8H, CH 2CH 2CH 2CH 2).
2.6.1.5.4 Variation 4:
Other Selective Metathesis Processes
These processes are usually restrained to monosubstituted alkenes, yielding ethene as the
byproduct. Fairly selective cross-coupling processes have been reported from a combination
of an electron-poor alkene, especially one containing a ð-substituent (e.g., styrenes,
acrylonitrile), [56,57] and a more nucleophilic one containing a small, electron-rich, nonconjugated
substituent (see, for example, the synthesis of 31 in Scheme 12), [56] or with allylsilanes.
[58] The electron-poor alkene self-metathesizes only slowly and, in addition, inhibits
the self-metathesis of the nucleophilic alkene. Greater than 95% trans selectivity is observed
in the reactions with styrene substrates, [56] while the cis-product is highly favored
for cross-metatheses with acrylonitrile. [57]
The formation of the homo dimer may also be significantly slowed down by steric
bulk. For example, compounds 32 and 33 yield the cross-coupling product 34 without
any self-metathesis of 32 and with only 26% yield of the 33 homo dimer. [59] The E/Z selectivities,
however, are usually low for nonconjugated alkene products.
Scheme 12 Cross-Coupling Alkene Metathesis [56,59]
Ph
O
O
+
CO 2Me
( ) 5
+
1.5
32 33
Pr i
N Pri (F3C) 2MeCO
Mo
(F3C)2MeCO CHCMe2Ph 26 (cat.)
CH2Cl2, rt, 1 h
CO2Me
Ph
( )
5
+
( ) 5
31 89%; E 100% 2%
Pr i
N Pri (F3C)2MeCO
Mo
(F3C) 2MeCO CHCMe2Ph 26 (cat.)
83%
O
O
CO 2Me
( ) 5
34 (E/Z) 2:1
CO2Me
for references see p 135