ca01 only detailed ToC 1..24

1.1.3 Nickel–Alkyne Complexes 49
Carbonylative Cyclizations to Cyclopentenes 44 and 45; General Procedure: [63]
To a 100-mL Schlenk flask filled with argon was added [Ni(cod) 2](2; 1.24 g, 4.51 mmol) and
anhyd toluene (20 mL). The temperature was maintained at –40 8C and propenal (0.50 g,
9.02 mmol) was added dropwise. Soon, a red insoluble complex of Ni(CH 2=CHCHO) 2 was
formed. The temperature was allowed to warm to –20 8C, when TMSCl (0.49 g, 4.51 mmol)
was added dropwise. The original red solid gradually dissolved. At –108C the oxidative addition
was complete and a deep red homogeneous soln resulted. The temperature was
lowered again to –788C. At this point, the alkyne (4.51 mmol) was added and the argon
was replaced by CO by means of a vacuum line. The temperature was again allowed to
rise and an excess of MeOH was added at –158C for activated alkynes and at rt for unactivated
alkynes. The reaction was allowed to proceed for 4 h, and the solvent was completely
evaporated in vacuo. The residue was treated with H 2O and extracted several times with
Et 2O. The combined organic phases were dried (Na 2SO 4) and evaporated to dryness. The
crude oil was chromatographed (hexane/EtOAc) on silica gel that had been treated with
a soln of Et 3N, and the solvent was removed to obtain the adducts 44 and 45.
1.1.2.10 Method 10:
Alkene Insertions with Nickel–Allyl Complexes
The cyclization of allylic acetates tethered with alkenes has been extensively investigated
by Oppolzer, employing a number of transition metals including nickel. The process has
been termed a “metallo-ene” reaction; however, the mechanism likely involves a sequence
of oxidative addition to generate a ð-allyl complex, alkene insertion, and â-hydride
elimination. [64] The process is quite general in scope and provides a very useful
method for the preparation of 1,4-dienes such as 46 (Scheme 28).
Scheme 28 Intramolecular Nickel-Catalyzed Alkene–Allylic Acetate Couplings [64]
AcO
Ts
N
1.1.3 Product Subclass 3:
Nickel–Alkyne Complexes
Ni(cod)2 2, dppb
Ts
N
+
Ni
Ln +
H Ni
Ln
Nickel complexes of alkynes are involved in many important catalytic transformations. A
fundamental transformation of nickel(0)–alkyne complexes that forms the basis of a
number of useful stoichiometric and catalytic reactions is the oxidative cyclization of
one alkyne and a second unsaturated unit to form a five-membered metallacyclopentene
47 (Scheme 29). If the second unsaturated unit is also an alkyne, linear oligomerizations
or cyclooligomerizations result. The catalytic tetramerization of acetylene to octatetraene
was discovered more than 50 years ago by Reppe, [65] and an excellent review on the
historical development of this area has appeared. [5] The complexities of the mechanistic
Ts
N
88%
LnNi +
Ts
N
46
Ts
N
for references see p 79