ca01 only detailed ToC 1..24

1.1.2 Nickel–Allyl Complexes 39
Bis(ç 3 -2-methylallyl)nickel (23): [8]
Following the general procedure of Wilke, [37,190] anhyd NiBr 2 (18.7 g, 0.086 mmol) was suspended
in anhyd Et 2O (100 mL) in a 1-L three-necked flask fitted with a 500-mL dropping
funnel, a mechanical stirrer, and a Claisen head containing a low-temperature thermometer
and a three-way stopcock. The system was alternately evacuated to aspirator vacuum
and filled with inert gas (N 2 or argon) several times, and cooled to –108C. 2-Methylallylmagnesium
bromide (27.1 g, 0.17 mol) in Et 2O (300 mL) was added dropwise to the vigorously
stirred soln over 2 h. The mixture was stirred for an additional 0.4 h at –108C,
warmed to 258C, and filtered under inert gas. (The simplest technique is to use a reaction
flask with a fritted disk and two-way stopcock attached to the bottom; the soln can be
forced through the frit with positive pressure into a second flask which is also under inert
gas.) The residue was washed with Et 2O (50 mL), filtered, and the combined Et 2O filtrates
were concentrated under aspirator vacuum at –308C. Pentane (125 mL) was added, the resulting
slurry was filtered under inert gas (most of the pentane was removed from the filtrate
at aspirator vacuum), and the remaining brown soln was cooled to –788C. Impure
brown crystals (mp 358C) of 23 were isolated by decanting the supernatant liquid under
inert gas. (It is convenient to store the complex at this stage at –208C and then sublime the
requisite quantity into the vessel for further reaction.) Sublimation at 258C/0.1 Torr with
a –788C receiver afforded yellow crystals. No precise yield has been reported for this
method.
1.1.2.3 Method 3:
Oxidative Addition of Nickel(0) with Enones in the Presence of Lewis Acids
An advance by Mackenzie has made nickel–ð-allyl complexes accessible from enals and
enones. [38,39] In a reaction that is mechanistically analogous to Method 1 in Section
1.1.2.1, enals, when treated with bis(ç 4 -cycloocta-1,5-diene)nickel(0) (2) in the presence
of chlorosilanes, afford chloro-bridged dimeric ç 3 -allylnickel complexes such as 25
(Scheme 14). Enones are less reactive in the process and require pyridine to facilitate the
oxidative addition. Rather than using bis(ç 4 -cycloocta-1,5-diene)nickel(0) (2), a more convenient
and less expensive alternative involves the in situ reduction of dichlorotetrakis(pyridine)nickel(II)
(26) with sodium metal in the presence of cyclooctadiene to give
enone-derived ç 3 -allylnickel complexes (e.g., 27).
Scheme 14 Preparation of Enone-Derived ç 3 -Allylnickel Complexes [38,39]
H
O
NiCl2 6H2O
+
Ni(cod) 2
2
py
+
TMSCl
NiCl2(py)4
26
86%
TMSO
1. 2Na, excess cod
2. cyclopent-2-enone
TBDMSCl
Cl
Ni Ni
Cl
25
OTMS
NiCl(py) 2
27
OTBDMS
Di-ì-chlorobis{[ç 3 -1-(trimethylsiloxy)allyl]nickel(II)} (25): [38]
In a drybox, a 50-mL Schlenk tube was charged with [Ni(cod) 2](2; 500 mg, 1.82 mmol,
1.00 equiv). On a Schlenk line, a soln of propenal (244 ìL, 3.64 mmol, 2.00 equiv) in benzene
(8mL) in a 25-mL Schlenk vessel was treated with (MeO)(TMSO)C=CMe 2 (185 ìL,
0.909 mmol, 0.500 equiv; added as a proton-scavenging reagent), stirred for 5 min, and
for references see p 79