30
Cahiez and co-workers disclosed the manganese-catalyzed oxidative crosscoupling of alkynyl, alkenyl, and aryl magnesium reagents in the presence of O 2 as
the oxidant under mild conditions (Scheme 2.43) [95]. Various carbon–carbon
bonds including aryl–alkynyl, aryl–aryl, alkynyl–alkynyl, alkynyl–alkenyl, and
alkenyl–alkenyl were formed. The selectivity between cross- and homo-coupling
reactions is highly dependent on both steric factors and the electronic factors of the
Grignard reagents. A plausible mechanism of this Mn-catalyzed oxidative crosscoupling is proposed in Scheme 2.44. The initial step involves the reaction of MnCl 2
with Grignard reagents and O 2 to afford the peroxo complex (η
2
-O 2 )Mn
II
. The double transmetalation of (η
2
-O 2 )Mn
II
with two different Grignard reagents generates
the diorganomanganese intermediate R
1
Mn
II
R
2
, which is oxidized by O 2 to afford a
manganese(IV) peroxo complex. The following reductive elimination releases the
cross-coupling product and regenerates (η
2
-O 2 )Mn
II
.
Using air as the oxidant, Lei and co-workers achieved the cross-coupling of
terminal alkynes with alkylzinc reagents in the presence of palladium catalysis at
room temperature (Scheme 2.45) [96]. Pd(dba) 2 was employed as catalyst without
any phosphine ligand. CO played a critical role in gaining both high yield and
selectivity. The authors speculated that CO as well as DBA (dibenzylideneacetone)
might act as π-acidic ligands to promote the reductive elimination step to form the
alkynyl- alkyl cross-coupling products. Alkynyl zinc reagents generated in situ
RM
[catalyst]/oxidant
R R
catalyst
oxidant
RM
ArB(OH) 2
ArB(OH) 2
ArB(OH) 2
air
[{(phen)Cu( -OH)} 2 Cl 2 ] 3 H 2 O
(2
m
b
mol%)
CuCl (2 mol%)
a ir
Fe 3 O 4 -Cu 2 - -CD (10 mol%)
CuLi 2 Cl 4 (5 mol%)
O 2
air
RMgX
Scheme 2.38 Cu-catalyzed oxidative homo-coupling of organoboron and organomagnesium
reagents
RMgBr
MnCl 2 2LiCl (5 mol%)
or FeCl 3 (5 mol%)
air, THF, r.t., 45 min
R R
R = Aryl, Alkenyl, Alkynyl, Alkyl
Scheme 2.39 Mn- and Fe-catalyzed oxidative homo-coupling of aryl, alkenyl, and alkynyl
Grignard reagents
ArMgBr
FeCl 3 (6 mol%)
Bipy (12 mol%)
O 2 , THF, r.t., 10 min
Ar Ar
N
N
Bipy
Scheme 2.40 Fe-catalyzed
oxidative homo-coupling
of aryl Grignard reagents
H. Zhang
Cahiez and co-workers disclosed the manganese-catalyzed oxidative crosscoupling of alkynyl, alkenyl, and aryl magnesium reagents in the presence of O 2 as
the oxidant under mild conditions (Scheme 2.43) [95]. Various carbon–carbon
bonds including aryl–alkynyl, aryl–aryl, alkynyl–alkynyl, alkynyl–alkenyl, and
alkenyl–alkenyl were formed. The selectivity between cross- and homo-coupling
reactions is highly dependent on both steric factors and the electronic factors of the
Grignard reagents. A plausible mechanism of this Mn-catalyzed oxidative crosscoupling is proposed in Scheme 2.44. The initial step involves the reaction of MnCl 2
with Grignard reagents and O 2 to afford the peroxo complex (η
2
-O 2 )Mn
II
. The double transmetalation of (η
2
-O 2 )Mn
II
with two different Grignard reagents generates
the diorganomanganese intermediate R
1
Mn
II
R
2
, which is oxidized by O 2 to afford a
manganese(IV) peroxo complex. The following reductive elimination releases the
cross-coupling product and regenerates (η
2
-O 2 )Mn
II
.
Using air as the oxidant, Lei and co-workers achieved the cross-coupling of
terminal alkynes with alkylzinc reagents in the presence of palladium catalysis at
room temperature (Scheme 2.45) [96]. Pd(dba) 2 was employed as catalyst without
any phosphine ligand. CO played a critical role in gaining both high yield and
selectivity. The authors speculated that CO as well as DBA (dibenzylideneacetone)
might act as π-acidic ligands to promote the reductive elimination step to form the
alkynyl- alkyl cross-coupling products. Alkynyl zinc reagents generated in situ
RM
[catalyst]/oxidant
R R
catalyst
oxidant
RM
ArB(OH) 2
ArB(OH) 2
ArB(OH) 2
air
[{(phen)Cu( -OH)} 2 Cl 2 ] 3 H 2 O
(2
m
b
mol%)
CuCl (2 mol%)
a ir
Fe 3 O 4 -Cu 2 - -CD (10 mol%)
CuLi 2 Cl 4 (5 mol%)
O 2
air
RMgX
Scheme 2.38 Cu-catalyzed oxidative homo-coupling of organoboron and organomagnesium
reagents
RMgBr
MnCl 2 2LiCl (5 mol%)
or FeCl 3 (5 mol%)
air, THF, r.t., 45 min
R R
R = Aryl, Alkenyl, Alkynyl, Alkyl
Scheme 2.39 Mn- and Fe-catalyzed oxidative homo-coupling of aryl, alkenyl, and alkynyl
Grignard reagents
ArMgBr
FeCl 3 (6 mol%)
Bipy (12 mol%)
O 2 , THF, r.t., 10 min
Ar Ar
N
N
Bipy
Scheme 2.40 Fe-catalyzed
oxidative homo-coupling
of aryl Grignard reagents
H. Zhang