4 The Pauson–Khand Reaction and Silylformylation . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . 116
4.1 The Pauson–Khand Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
4.2 Silylformylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5 Hydroelementation with Silanes, Alcohols, Carboxylic Acids and Amines . . . . . . . . . . . . . . 118
5.1 Hydrosilylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.2 Hydroalkoxylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
5.3 Hydrocarboxylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
5.4 Hydroamination of Alkynes, Allenes and Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.5 Chiral Bimetallic Catalysts for Hydroamination . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . 129
6 Conclusions . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . 133
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
1 Introduction
The use of bimetallic complexes as catalysts for organic transformations is of
significant interest because they can dramatically enhance the rate and selectivity
of a reaction compared to a monometallic catalysed process. Much early work on
bimetallic catalyst design was inspired by the impressive catalytic efficiency of
dinuclear metalloenzymes which contain bimetallic active sites [1]. More recently
artificial bimetallic catalysts have been developed that offer substantial advantages
compared to monometallic catalysts of similar structure [2–7]. This increase in
catalyst performance is often attributed to a synergistic or ‘cooperative’ interaction
between the two metals in activating the substrate of interest. Quite often the origin of
this cooperativity is poorly understood on a molecular level. How the two metals of a
bimetallic catalyst interact with each other and/or the reaction substrate during the
catalytic cycle is difficult to determine from direct methods. In general, the second
metal in a bimetallic catalyst may participate in a reaction via three possible means:
(a) An electronic influence, where the catalytic cycle proceeds solely via interaction of the substrate with one of the metal centres, yet the second metal
provides a beneficial electronic influence on the first metal either through
direct metal–metal bonding or through a shared ligand group.
(b) A steric influence, where the second metal acts to direct the substrate into an
advantageous alignment with the first metal centre, without playing a direct
role in the bond forming/breaking processes. This is particularly notable in
bimetallic catalysts where an enhancement of the reaction selectivity is
observed compared to a monometallic catalysed process.
(c) A concerted activation process, where both metal centres interact with the
substrate(s) to provide a lower activation barrier towards reaction than could
be obtained with a single metal centre. This may involve activation of each
substrate in a reacting pair of molecules or the combined activation of a single
substrate.
Note that these three modes of action are not mutually exclusive, and all
processes may take part in the catalytic cycle.
104
M.J. Page et al.
4.1 The Pauson–Khand Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
4.2 Silylformylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5 Hydroelementation with Silanes, Alcohols, Carboxylic Acids and Amines . . . . . . . . . . . . . . 118
5.1 Hydrosilylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.2 Hydroalkoxylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
5.3 Hydrocarboxylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
5.4 Hydroamination of Alkynes, Allenes and Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.5 Chiral Bimetallic Catalysts for Hydroamination . . . .. . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . 129
6 Conclusions . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . 133
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
1 Introduction
The use of bimetallic complexes as catalysts for organic transformations is of
significant interest because they can dramatically enhance the rate and selectivity
of a reaction compared to a monometallic catalysed process. Much early work on
bimetallic catalyst design was inspired by the impressive catalytic efficiency of
dinuclear metalloenzymes which contain bimetallic active sites [1]. More recently
artificial bimetallic catalysts have been developed that offer substantial advantages
compared to monometallic catalysts of similar structure [2–7]. This increase in
catalyst performance is often attributed to a synergistic or ‘cooperative’ interaction
between the two metals in activating the substrate of interest. Quite often the origin of
this cooperativity is poorly understood on a molecular level. How the two metals of a
bimetallic catalyst interact with each other and/or the reaction substrate during the
catalytic cycle is difficult to determine from direct methods. In general, the second
metal in a bimetallic catalyst may participate in a reaction via three possible means:
(a) An electronic influence, where the catalytic cycle proceeds solely via interaction of the substrate with one of the metal centres, yet the second metal
provides a beneficial electronic influence on the first metal either through
direct metal–metal bonding or through a shared ligand group.
(b) A steric influence, where the second metal acts to direct the substrate into an
advantageous alignment with the first metal centre, without playing a direct
role in the bond forming/breaking processes. This is particularly notable in
bimetallic catalysts where an enhancement of the reaction selectivity is
observed compared to a monometallic catalysed process.
(c) A concerted activation process, where both metal centres interact with the
substrate(s) to provide a lower activation barrier towards reaction than could
be obtained with a single metal centre. This may involve activation of each
substrate in a reacting pair of molecules or the combined activation of a single
substrate.
Note that these three modes of action are not mutually exclusive, and all
processes may take part in the catalytic cycle.
104
M.J. Page et al.