ca01 only detailed ToC 1..24

1.1.3 Nickel–Alkyne Complexes 61
of (2,2¢-bipyridyl)diethylnickel(II) leads to the production of a mixture of both of these
product classes (Scheme 52). The mechanism for the formation of the unusual disilyl substituted
ethenes may involve a novel nickel disilyl species. [106] Hydrosilylations of bis(alkynes)
are described in the section on the couplings of two alkynes (Section 1.1.3.5).
Scheme 52 Hydrosilylation of Alkynes
R 1 R 1 + HSiCl 3
1.1.3.11 Method 11:
Alkyne Carbozincation
NiEt 2(bipy)
R1 Cl3Si R1 SiCl3 R1 H
R1 SiCl3 +
major minor
Knochel has demonstrated that phenylacetylenes undergo a highly stereoselective syncarbozincation
reaction by treatment with dialkylzincs or diphenylzinc in the presence
of catalytic bis(acetylacetonato)nickel(II) (1). [107,108] With alkynes that possess one aromatic
and one aliphatic substituent, the regioselectivity is very high, favoring the addition of
the organozinc substituent to the carbon that bears the aliphatic alkyne substituent,
whereas the regiochemical outcome reverses with aryl(silyl)alkynes (Scheme 53). The intermediate
alkenylzinc reagents may be quenched with a proton, iodine, or a variety of
other electrophiles in copper-catalyzed alkylations (Scheme 54). The combination of the
above methods provides a very versatile entry to tetrasubstituted alkenes.
Scheme 53 Nickel-Catalyzed Alkyne Carbozincation [107,108]
Ar 1 R 1 + R 2 2Zn
Ni(acac) 2 1
Ar1 H
R1 R2 Ar1 R2 R1 H
R1 = aliphatic R1 or
= TMS
Scheme 54 Sequential Alkyne Carbozincation and Electrophilic Trapping [107,108]
Ph Ph + R 1 2Zn
R 1
Ph
Ni(acac) 2 1
I
Ph
I 2
R 1
Ph
ZnR 1
Ph
1. CuCN 2LiCl
Br
2.
CO2Et
1. CuCN 2LiCl
2. R2COCl R 1
Ph
O
Ph
R 2
R 1
Ph
CO2Et
Ph
68 71%
Ethyl (Z)-2-Methylene-4,5-diphenylhept-4-enoate (68,R 1 = Et); Typical Procedure: [108]
[Ni(acac) 2](1; 320 mg, 1.25 mmol, 25%) and PhC”CPh (0.89 g, 5 mmol, 1 equiv) were dissolved
in THF (3.8mL) and NMP (1.3 mL) at –408C under argon. Et 2Zn (1.0 mL, 10 mmol,
2 equiv) was carefully added via syringe at –78 8C. The mixture was allowed to warm to
–35 8C and was stirred for 2.5 h. Meanwhile, a mixture of CuCN (1.79 g, 20 mmol, 4 equiv)
and LiCl (1.69 g, 40 mmol, 8equiv) was dried in vacuo at 1308C for 2 h and then dissolved in
THF (10 mL). The soln was cooled to –608C and added by syringe to the mixture at –788C.
The resulting dark soln was warmed to 0 8C for a few min and then cooled again to –788C.
Ethyl (2-bromomethyl)acrylate (4.82 g, 25 mmol, 5 equiv) was added and the mixture was
for references see p 79