Organic chemistry is constantly concerned with effecting reactions at a
particular centre in a complex molecule, and if possible with a high and
predictable level of stereoselectivity. In the light of much accumulated
ex- perience within organic chemistry it is usually possible to assess
the likeli- hood of alternative reaction pathways at least
qualitatively. However, well based expectations can be falsified, and
the experiments directed to the synthesis of vitamin B12 which led to
Woodward's recognition of orbital symmetry control in organic chemistry
are an instructive example. Our limi- tations in this respect are very
much accentuated in the case of hetero- geneous reactions, which present
additional problems, and except for very well studied instances,
heterogeneous catalysis has remained a relatively empirical area of
chemistry. Knowledge in this area has, however, been greatly improved by
the development of transition metal complexes which replicate the
catalytic properties of the metals, and are effective in a homo- geneous
reaction system. This development has advanced our understanding of
catalysis by making it possible to interpret reactions in strictly
molecular terms. In addition, these homogeneously active complexes are
frequently more selective than their heterogeneous metallic counterparts
either in discriminating between different functional centres in a
molecule or in of- fering better stereoselectivity. Homogeneous
catalysts have now been devised for a number of organic chemical
reactions, including hydrogenation, carbonylation, polymerisa- tion, and
isomerisation and dismutation of alkenes.
