_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Mechanisms of industrial processes
catalyst has improved stability so that the process can be operated at lower
pressures of carbon monoxide, but only about o ne fifth of the activity of the simple
carbonyl. Consequently larger reactors and slightly higher temperatures are
required. The aldehydes, which are formed initially are further catalytically
hydrogenated to alcohols. Unfortunately some of the alkene feedstock is also
converted into alkane. Nevertheless the process provides a valuable method of
converting alkenes into unbranched alcohols in a single stage.
(b) RHODIUM CATALYSTS. Work at Union Carbide and concurrently in
Wilkinson's laboratory at Imperial College, London in the 1960s led to the
discovery that the rhodium complex HRh(CO)(PPh 3 ) 3 is a very active and
selective catalyst for the hydroformylation of terminal alkenes. A pilot plant was
set up in 1972 and the process first entered commercial production in Puerto Rico
early in 1976. The great advantages of the rhodium over the cobalt catalysed
processes are apparent from Table 12.6; lower operating pressures, lower
temperatures, high n: iso ratio, lack of by-products and long catalyst life ( > 1
year).
Several rhodium complexes including RhCl(CO)L 2 , RhH( CO)L 3 , RhClL 3 or even
rhodium trichloride + L (L = Ph 3 P) ha ve been used as catalyst precursors. Under
the conditions of hydroformylation they are ali converted, apparently, into
HRh(C0) 2 L 2 • Two mechanisms have been suggested. In the first initialloss ofL
from HRh(C0) 2 L 2 gives a four-coordinate 16-electron intermediate which then
adds alkene (not shown). In the second (Fig. 12.17) the Rh-H bond of
HRh(C0) 2 L 2 adds to alkene in a concerted step yielding an alkyl complex. Alkene
cannot reasonably coordinate to HRh(C0) 2 L 2 , as a 20-electron intermediate
would then arise. Two phosphine ligands remain coordinated to rhodium
CATALYST PRECURSOR
1lslow
slo~
(fost)
synchronous
HRh(CO )2 L2(RCH=CH2)
(18)
'r-
HRh(CO)L2
RCH2CH 2CORh(CO)L2
/; (16) (16)
/Î( H2 Rh(COCH2 CH 2R)(CO)L2 \ ll
/' L (18) CQ
H
HRh(CO)L3 RCH2CH2CHO 2
RCH2CH2 CORh(COl2L2
(18)
Fig. 12.17 Possible reaction mechanism (associative) for hydroformylation catalysed by
rhodium carbonyl phosphine complexes. L = Ph,P. Formation of n (unbranched)
product only shown. 16~> complexes square planar, 18~> probably
trigonal bipyramidal.
390