ca01 only detailed ToC 1..24

1.1.4 Nickel–Alkene Complexes 77
1.1.4.10 Method 10:
Homo-Diels–Alder Cycloadditions
The nickel-catalyzed homo-Diels–Alder cycloaddition with norbornadienes and electrondeficient
alkenes is an effective method for generating strained polycyclic compounds. [181]
At the time of writing, this method is the only strategy for carrying out a nickel-catalyzed
[2+2+2] cycloaddition with three alkene ð-systems such that six contiguous stereocenters
may be generated. Both acyclic and cyclic enones participate in the process (Scheme
80).
Scheme 80 Nickel-Catalyzed Homo-Diels–Alder Reactions [181]
+
O
5−10 mol% Ni(cod) 2 2
10−20 mol% Ph3P 56% H H
Nickel-Catalyzed Diels–Alder Cycloaddition; General Procedure: [181]
[Ni(cod) 2](2; 10–25 mol%) was added to a flame-dried flask equipped with a magnetic stirring
bar and a rubber septum in the glovebox. Ph 3P (2 equiv, with respect to Ni) was introduced
against a positive flow of N 2, and the diene (1 mmol) was added in 1,2-dichloroethane
or toluene followed by the dienophile (2 mmol) as a neat liquid. The mixture was
stirred at the desired temperature under N 2 for 16–48h. The workup was as follows: The
catalyst was oxidized by stirring with the flask open to the air for 1–2 h. The reaction mixture
was filtered through a plug of silica gel using CH 2Cl 2 (100 mL) as the eluant. Evaporation
of the solvent gave a crude product, which was purified by Kugelrohr (bulb-to-bulb)
distillation or flash chromatography on silica gel.
1.1.4.11 Method 11:
Alkene Polymerization
While not emphasized in this review, the oligomerization and polymerization of ethene
forms the basis of some of the most significant industrial processes involving nickel catalysis
(see also Houben–Weyl, Vol. E 18, pp 847–863; E 20/II, pp 807–813). The Shell Higher
Order Process (SHOP), nicely described in a review by Keim, involves the oligomerization
of ethene to produce Æ-alkenes (Scheme 81). [7] Nickel polymerization catalysts were first
reported in the 1950s; [7] however, cationic nickel diimine complexes have been identified
by Brookhart as highly efficient ethene polymerization catalysts (Scheme 82). [182–185] The
unique ligand class with highly hindered ortho-substituted aryl rings on the nitrogens of
the diimine ligand results in rates of chain propagation which are much greater than
chain transfer rates, thus allowing the formation of high-molecular-weight polymers.
Scheme 81 Shell Higher Order Process [7]
H2C CH2 LnNi H LnNi C2H5 O
L =
−
O
PPh2
O
L nNi CH 2CH 2R 1
L nNi C 4H 9
R 1
L nNi H
for references see p 79