ca01 only detailed ToC 1..24

56 Science of Synthesis 1.1 Organometallic Complexes of Nickel
Scheme 41 Iodoalkene–Alkyne Couplings [85]
O
I
+
Et Et
NiBr 2, Zn
85%
6,7,8,9-Tetraethylspiro[4.4]nona-6,8-dien-2-one (61): [85]
To a mixture of NiBr 2 (0.080 g, 0.36 mmol), Zn powder (0.14 g, 2.2 mmol), and 3-iodocyclopent-2-enone
(0.40 g, 1.8mmol) in MeCN (1.0 mL) was added hex-3-yne (0.30 g, 3.6 mmol).
The system was then heated under N 2 at 608C with stirring for 20 h. During the course of
catalysis, the color of the soln changed from light green to a persistent dark red in the
first few hours. The MeCN soln was cooled, the soln was concentrated, and the resulting
compounds were separated by column chromatography (silica gel, hexane/EtOAc 10:1) to
give the desired product 61; yield: 0.40 g (85%).
1.1.3.6 Method 6:
Coupling of Alkynes with Alkenes
The couplings of enynes with a third unsaturated component are described in Sections
1.1.3.3 and 1.1.3.7. However, several other processes merit discussion here that do not
fall into the other categories described. For example, Ikeda and Montgomery have extensively
investigated intermolecular couplings of enones and alkynes with organometallic
reagents such as organozincs [86,87] and organotins. [88,89] Intermolecular couplings efficiently
produce ª,ä-unsaturated ketones with highly selective tri- or tetrasubstituted alkene
formation (Scheme 42). The scope of the process is quite broad, and the mechanism is
likely to involve the formation of metallacycles as key intermediates.
Scheme 42 Enone–Alkyne Couplings with Organozincs or Organoaluminums [86–89]
O
O
L L
Ni
M = AlR 3 2 or ZnR 2
H
+
R 1
R1 H + MR2 MR 2
MO
Et
O
Et
61
Et
Et
Ni(acac) 2 1 or Ni(cod) 2 2, Ph 3P
LnNi
R1 R2 Montgomery has investigated the intramolecular variant of this process. [90–93] The addition
of triphenylphosphine promotes a â-hydride elimination process that leads to hydrogen-atom
introduction instead of alkyl-group introduction. In both cases, the exocyclic
double bond is created with complete selectivity (Scheme 43). A formal synthesis of (+)-
Æ-allokainic acid (62) was completed employing this methodology as the key step
(Scheme 44). [94]
H
MO
O
R 2
R 2
H
R 1
R 1