[M] as shown in Fig. 1c. Since the present book is dedicated to cooperativity or
synergism [7] in metal-mediated homogeneous catalysis, the term [M] in this
chapter will possess four interrelated characteristics, and [M] will:
(1) Denote a reaction mechanism.
(2) This reaction mechanism will arise from the application of one or two metallic
elements in the catalytic system.
(3) The system will exhibit either an unusual rate dependence and/or an unusual
selectivity pattern that has its origin(s) in the structure of the reaction mechanism and not in some secondary effect due to physico-chemical issues such as
transport [8].
(4) Finally, all overall reactants for the organic reaction and all organometallic
intermediates for this organic reaction are solvated during all individual reaction steps present in the catalytic reaction mechanism.
The motivation for stating point (4) arises from the fact that many so-called
homogeneous catalytic reactions are actually present in multiphase slurries and
these slurries arise, for example, and, in some cases, due to the addition of various
insoluble additives or auxiliaries or the formation of precipitates. For the present
chapter, point (4) explicitly excludes surface-mediated reaction steps in the homogeneous catalytic reaction mechanism.
The expression unusual has been deliberately been left open ended or ill defined
since the effect will almost certainly differ between systems having different
phenomenological bases for cooperativity or synergism. Nevertheless, a rather
useful working definition might be a rate or selectivity dependence which cannot
be explained as a strictly additive effect of the metal(s) used. Having said that, the
homometallic and heterobimetallic catalytic binuclear elimination reactions
(CBERs) which are the focus of this chapter have very well-defined rate dependences which can be traced back to the topology of the reaction mechanisms.
1.2 Stoichiometric Binuclear Elimination
Heck and Breslow [9] observed that the addition of HCo(CO) 4 to a solution of
CH 3 COCo(CO) 4 resulted in the formation of the corresponding aldehyde CH 3 CHO
and the homometallic dinuclear carbonyl Co 2 (CO) 8 . They postulated that such a
reaction might be operating under alkene cobalt-mediated hydroformylation reaction conditions and thus contribute to aldehyde product formation. This observed
stoichiometric reaction is apparently the first documented case of a well-defined
reaction between two organometallic complexes RML n and R
0 M
0 L m , leading to the
formation of an organic product and a dinuclear complex. This class of stoichiometric reactions became known as binuclear eliminations [10]. The generic representation for a stoichiometric homometallic binuclear elimination is shown in
(Eq. 1) where M ¼ M
0 and where the Greek letter α will be used to denote the
bimolecular reaction between mononuclear organometallics throughout the remainder of this chapter. Clearly Eq. 1 is not in general elementary, and it is not
190
M. Garland
synergism [7] in metal-mediated homogeneous catalysis, the term [M] in this
chapter will possess four interrelated characteristics, and [M] will:
(1) Denote a reaction mechanism.
(2) This reaction mechanism will arise from the application of one or two metallic
elements in the catalytic system.
(3) The system will exhibit either an unusual rate dependence and/or an unusual
selectivity pattern that has its origin(s) in the structure of the reaction mechanism and not in some secondary effect due to physico-chemical issues such as
transport [8].
(4) Finally, all overall reactants for the organic reaction and all organometallic
intermediates for this organic reaction are solvated during all individual reaction steps present in the catalytic reaction mechanism.
The motivation for stating point (4) arises from the fact that many so-called
homogeneous catalytic reactions are actually present in multiphase slurries and
these slurries arise, for example, and, in some cases, due to the addition of various
insoluble additives or auxiliaries or the formation of precipitates. For the present
chapter, point (4) explicitly excludes surface-mediated reaction steps in the homogeneous catalytic reaction mechanism.
The expression unusual has been deliberately been left open ended or ill defined
since the effect will almost certainly differ between systems having different
phenomenological bases for cooperativity or synergism. Nevertheless, a rather
useful working definition might be a rate or selectivity dependence which cannot
be explained as a strictly additive effect of the metal(s) used. Having said that, the
homometallic and heterobimetallic catalytic binuclear elimination reactions
(CBERs) which are the focus of this chapter have very well-defined rate dependences which can be traced back to the topology of the reaction mechanisms.
1.2 Stoichiometric Binuclear Elimination
Heck and Breslow [9] observed that the addition of HCo(CO) 4 to a solution of
CH 3 COCo(CO) 4 resulted in the formation of the corresponding aldehyde CH 3 CHO
and the homometallic dinuclear carbonyl Co 2 (CO) 8 . They postulated that such a
reaction might be operating under alkene cobalt-mediated hydroformylation reaction conditions and thus contribute to aldehyde product formation. This observed
stoichiometric reaction is apparently the first documented case of a well-defined
reaction between two organometallic complexes RML n and R
0 M
0 L m , leading to the
formation of an organic product and a dinuclear complex. This class of stoichiometric reactions became known as binuclear eliminations [10]. The generic representation for a stoichiometric homometallic binuclear elimination is shown in
(Eq. 1) where M ¼ M
0 and where the Greek letter α will be used to denote the
bimolecular reaction between mononuclear organometallics throughout the remainder of this chapter. Clearly Eq. 1 is not in general elementary, and it is not
190
M. Garland