To date, there are very few examples of early–late heterobimetallic catalysts
with a group 5 element. This might be explained considering the weaker Lewis acid
character of group 5 vs group 4 metal complexes which limits the possibility of dual
activation of polar substrates. First examples appeared in the early 1990s in the
group of Bergman who described the synthesis of a series of bis-bridging methylene
Ir/Ta complexes (see Sect. 3.2.3) and Rh/Ta complexes and reported their catalytic
performances for alkene hydrogenation, isomerization, and hydrosilylation
[134]. The Rh/Ta complexes 81 and 82 were synthesized by reaction of [Cp 2 Ta
(CH 2 )(CH 3 )] with LiN(TMS) 2 and addition of the resulting deprotonated complex
to [Rh(CO) 2 Cl] 2 or [RhCl(CO)(PPh 3 ) 2 ] [135]. The Rh/Ta complexes 81 and 82
were tested for the catalytic hydrogenation in a variety of alkenes in parallel with a
phosphorus-ylide analogue 83 for comparative purposes (Scheme 46). Both complexes catalyzed hydrogenation of ethylene under atmospheric pressure with a TOF
of 24 h
À1 for complex 81 (at 45
C) and 60 h
À1 for complex 82 (at 25
C). With
1-butene as substrate, butane (noted B) and cis- and trans-2-butene (noted C and T)
were formed at much slower rates (complex 81, TOF ¼ 0.5 h
À1 ; complex 82,
TOF ¼ 6.6 h
À1 ). These performances are much better than the ones with the parent
Ir/Ta complexes (vide infra) but comparable to those reported with the phosphorusO
N
O
Ph 2 P
[Ti(OiPr)4]
HO
OH
*
1)
2)
HO
N
PPh 2
2
O
O
*
Ti
O
N
PPh 2
O
N
Ph 2 P
(10 diols*)
(8 Schiff bases)
O
N
O
PPh 2
Ti
O
O
Ph 2 C
CPh 2
O
O
H
H
Rh
(nbd)
80
Ph
CO 2 Me
NHCOMe
Ph
CO 2 Me
NHCOMe
*
cat. 80 (0.1 mol%)
in situ generated
CH 2 Cl 2 , r.t., 3h
+ H 2
(3 bar)
100% conversion
92% ee
chiral ligands library
"best combination"
Scheme 45 Asymmetric hydrogenation catalyzed by Rh/Ti heterobimetallic complex
+ H 2
(P atm)
cat Rh/Ta (5 mol%)
C 6 D 6
cat. 81: TOF = 24h -1 at 45°C
cat. 82: TOF = 60h -1 at 25°C
cat. 83: TOF = 15h -1 at 25°C
Cp2Ta
C
H 2
Rh
H2
C
CO
CO
Cp2Ta
C
H 2
Rh
H2
C
PPh 3
CO
Ph 2 P
C
H 2
Rh
H2
C
PPh 3
CO
81
82
83
+ H 2
(P atm)
cat Rh/Ta (5 mol%)
C 6 D 6, 45°C
+
+
B
C
T
cat. 81: TOF = 0.5h -1 (B:C:T = 1:8:11)
cat. 82: TOF = 6.6h -1 (B:C:T = 3:1:1)
cat. 83: TOF = 5.4h -1 (B:C:T = 1:1:1)
Scheme 46 Hydrogenation and isomerization of alkenes catalyzed by Rh/Ta heterobimetallic
complexes
168
E. Bodio et al.