ca01 only detailed ToC 1..24

1.1.4 Nickel–Alkene Complexes 69
Scheme 65 [3+2] Cycloadditions Involving Methylenecyclopropanes [154–156]
L nNi
E
R 1
+
R 2
R 2
R 1
E
E = electron-withdrawing group
Ni(cod)2 2
L nNi
E
86
R 1
R 2
R 1
E R 2
87
+
R 1
LnNi
E R 2
88
R 1
E R 2
+ L nNi
R 1
E R 2
The reductive coupling of bis(enones) is another reaction class that may involve alkenederived
metallacyclopentanes. Treatment of a symmetrical bis(enone) 89 with bis(ç 4 -cycloocta-1,5-diene)nickel(0)
(2) and butyllithium/zinc(II) chloride leads to coupling of the
two enone â-carbons followed by an intramolecular aldol addition to afford bicyclo[3.3.0]octanols
90 (Scheme 66). [91,131] Sterically hindered enones have also been shown to participate
in this reaction class, as evidenced by the spirocyclization of a substituted cyclopentenone
to produce an angular triquinane (Scheme 66). [158]
Scheme 66 Reductive Cyclization–Aldol Addition of Bis(enones) [91,131,158]
R 1
O
O
89
O
O
R 1
N
O
O
Ni(cod) 2 2
BuLi, ZnCl2 BuZnO O
R 1
R 1 = Ph 60%
O
Ni(cod)2 2
Et2Zn, ZnCl2
O H
58% H
R 1
R 1
O
HO
R 1
H H
The dimerization of alkenes has been extensively studied by Wilke (Scheme 67). [5] In the
tail-to-tail dimerization of methyl acrylate, cationic nickel hydride species have been proposed
as the active catalysts. The mechanism of this process proceeds by alkene hydrometalation,
insertion of a second equivalent of methyl acrylate, and then â-hydride elimination
to release the product and generate the nickel hydride catalyst.
90
for references see p 79