Chemistry Reference
In-Depth Information
O
O
N
O
O
Ph
2
Ph
2
P
N
PPh
2
N
P
PPh
2
N
Ph
2
P
Pt
Pt
Pt
Ph
Ph
Ph
Ph
Ph
Ph
A
B
C
Figure 8B.21.
In situ
- formed (allyl)Pt complexes.
dppe
Ph
2
P
PPh
2
O
CO
CO
CO
2
Me
Cl
bipy
Mo
PPh
2
Ph
2
P
N
N
ONa
70-75%
CO
2
Me
THF
70%
O
O
CO
2
Me
CO
2
Me
+
OC
OC
N
Mo
Cl
N
1
:
1
Scheme 8B.51.
Allylic alkylations with stoichiometric Mo-allyl complexes.
compared with Pd catalysts. Nowadays, many other metals, such as W, Rh, Ir, Ni, Pt, and
Cu were found to react in a similar behavior. The development of the asymmetric
molybdenum-catalyzed reactions was reviewed by Belda and Moberg in 2004 [207].
8B.4.1.1. Fundamentals of Mo-Catalyzed Allylic Substitutions
Molybdenum is a
very versatile metal with a vast range of oxidation (
4 to +6) and coordination numbers
(4-8) in its complexes, which makes Mo-catalyzed reactions relatively complicated to
understand [208]. But its ability to form
−
-allyl complexes easily makes it a good candi-
date for allylic alkylations. Trost and Lautens were the fi rst to investigate the reaction
behavior of
π
-allyl molybdenum complexes [209]. In their fi rst experiments, they com-
pared the stoichiometric reaction of a deprotonated cyclic
π
- keto ester with crotyl - Mo
complexes bearing either dppe or bipy as a bidentate ligand (Scheme 8B.51). Both com-
plexes showed a similar reactivity but interestingly different regioselectivity. While the
dppe complex gave rise to the linear product preferentially (
b
/
l
5:95) comparable to the
Pd-catalyzed reaction, the bipy complex provided a 1:1 mixture of linear and branched
product. This difference in selectivity was explained by different complex geometries
and by the fact that bipy is a stronger
β
σ
- donor ligand compared with dppe.
