33
cross-coupling product in the presence of O 2 as oxidant. In an earlier report, Hirao
and co-workers described a vanadium-catalyzed oxidative ligand coupling of
organoborates under oxygen atmosphere to provide symmetrical or unsymmetrical
biaryls (Scheme 2.47) [98]. V(OEt)Cl 2 was used as catalyst and moderate-to-good
selectivities were achieved.
R 1 Mn II OOMgX
R 1 Mn II R 2
R 1 MgX
R 2 MgX
XMgOOMgX
R 1 R 2
O 2
Mn II
O
O
Mn IV
O
O
R 1
R 2
MnCl 2
RMgX
O 2
Scheme 2.44 Proposed
mechanism for
Mn-catalyzed oxidative
cross-coupling of two
organometallic compounds
involving O 2 as oxidant
R 1
+ R 2 ZnX
Pd(dba) 2 (5 mol%)
air, CO, r.t., 24 h
R 1
R 2
MeO
Me
Me
Ph
n Bu
OMe
n Bu
Cl
n Bu
Br
n Bu
C 5 H 11
n Bu
n C 6 H 13
C 5 H 11
n C 8 H 17
i Bu
Ph
Ph
O
OEt
C 5 H 11
C 5 H 11
O
OEt
71%
73%
8 6%
80%
83%
7 5%
82%
88%
8 6%
93%
79%
9 3%
85%
72%
7 0%
Scheme 2.45 Pd-catalyzed cross-coupling of terminal alkynes with alkylzinc reagents
2 Transition Metal-Catalyzed Oxidative Coupling Involving Two Organometallic…