Chemistry Reference
In-Depth Information
simultaneous regio- and enantioselectivities. Palladium-catalyzed carbonylation offers,
however, the interest of its application in CO/olefi ns copolymerization and terpoly-
merization, where the use of Pd catalyst modifi ed with chiral ligands allows the control
of the steroselectivity. This chapter deals with all these topics, describing the catalytic
cycles proposed in the literature, the important facts concerning these reactions, and the
catalytic systems used, together with the most important applications. The reactions
are studied in separated sections, namely, the Rh-catalyzed hydroformylation, the Pd-
catalyzed hydroxy - and alkoxycarbonylations, and co - and terpolymerization reactions
due to the different features of such processes.
10.2. ASYMMETRIC HYDROFORMYLATION
10.2.1. Development of Chiral Hydroformylation Catalysts
The hydroformylation reaction, which converts olefi ns into aldehydes, is the largest
volume homogeneous transition metal-catalyzed reaction. This reaction has been exten-
sively studied, and, nowadays, a number of effi cient catalysts allow to control the regi-
oselectivity of the reaction in terminal or internal aldehydes (Scheme 10.1) [1].
H
2
/CO
CHO
+
R
CHO
R
R
*
[Catalyst]
Scheme 10.1.
Hydroformylation has been applied in the synthesis of intermediates and in fi ne
chemicals in particular in vitamins and in fl avors and fragrances [2]. Due to the versatile
chemistry of the aldehydes obtained through the hydroformylation reaction, further
conversions into alcohols, amines, carboxylic acid derivatives, and other products are
available. For this reason, “tandem reaction” sequences under hydroformylation condi-
tions appear as a clean and economical method of obtaining functionalized organic
molecules and constructing complex structures [2b,c]. The asymmetric version of the
hydroformylation of olefi ns is one of the most straightforward synthetic strategies for
the preparation of optically active aldehydes, which are versatile intermediates for the
synthesis of many biologically active compounds and for other synthetic transformations
[3]. However, the simultaneous control of chemo-, regio-, and enantioselectivity along
the reaction makes the hydroformylation process one of the most challenging reactions
from both academic and industrial points of view. To date, much effort in this fi eld has
been concentrated on the hydroformylation of vinylarenes as a route to obtain enantio-
merically enriched 2-aryl propionic acids, the profen class of nonsteroidal drugs.
Although less studied, the application of the asymmetric hydroformylation of other
substrates shows remarkable potential for the production of fi ne chemicals [4].
Although the fi rst highly enantioselective examples of asymmetric hydroformylation
of styrene were reported by Consiglio et al. in 1991 and used Pt-Sn systems achieving
an ee of 86%, the most successful asymmetric hydroformylation catalysts for this sub-
strate were all based on rhodium [5]. Platinum catalysts have several disadvantages: low
reaction rates, hydrogenation of the substrate, and low regioselectivity to the branched
