Chemistry Reference
In-Depth Information
In, THF,
AcOH
Ru
3
(CO)
12
, Et
3
N
CO (100 psi)
O
O
•
Br
N
N
H
HO
4.113
4.112
CHO
H
O
NiCl
2
, NaBH
4
,
MeOH, -30 °C
O
O
N
O
N
H
H
O
O
4.114
4.115
Scheme 4.44
CHO
H
2
, CO,
catalyst
CHO
+
R
R
R
4.117
branched
4.116
linear
HRhL
2
(CO)
2
H
± CO
O
reductive
elimination
HRhL
2
(CO)
coordination
Rh(CO)L
2
4.118
H H
4.122
O
Rh(CO)L
2
H
H
2
insertion
oxidative
addition
Rh(CO)L
2
Rh(CO)L
2
4.119
4.121
O
CO
coordination
CO insertion
Rh(CO)
2
L
2
4.120
Scheme 4.45
developed as an industrial process using initially ill-defined cobalt catalysts. Mechanistic studies came much
later with the work of Heck
48
and Wilkinson. The latter also introduced rhodium catalysis, which works
under much milder conditions. Applications to the synthesis of complex organic molecules came even later.
49
The mechanism of hydroformylation is believed to be largely the same whether cobalt or rhodium is used.
The mechanism involves coordination of the substrate alkene to a rhodium hydride complex, insertion of the
alkene to form alkyl rhodium complexes
4.119
and
4.120
, followed by CO insertion to convert them to an acyl
rhodium complex
4.121
. Oxidative addition of hydrogen and reductive elimination then gives the product and
regenerates the initial rhodium hydride. If the insertion reaction proceeds with the opposite regiochemistry
to put the rhodium at the internal, more hindered position, then the branched isomer
4.117
is formed
(Scheme 4.46).
