subchapters, we will focus on completely different processes where the transition
metal fragment can be tuned to efficiently modify the reactivity.
3.2 Oxidative Addition Reactions
Oxidative addition typically constitutes the first and also the rate-limiting step in the
catalytic cycle of a good number of cross-coupling reactions [23]. This process
involves the cleaving of a C–X bond and forming of two new coordination bonds at
the transition metal (TM), which results in an increase of its oxidation state by
2 units. In principle, two main mechanisms can be envisaged for this fundamental
reaction: (a) the concerted pathway, which involves the simultaneous formation of
the TM–C and TM–X bonds in the corresponding transition state, and (b) an
S N 2-type mechanism, where the central carbon atom is attacked by the transition
metal fragment forming a cationic [TM–R]
+ species and an X
À leaving group [23–
25]. The competition between both mechanisms has been thoroughly studied by
Bickelhaupt and co-workers with the help of the ASM method on reactions involving Pd(0)-complexes and different alkyl and aryl halides [26, 27]. In addition, the
activation of other bonds, such as C–H and C–C bonds, has been also quantitatively
analyzed in detail [28]. Very recently, this research group has also explored similar
oxidative addition reactions mediated by iron complexes, finding that whereas
palladium complexes favor C–Cl activations,
1 Fe(CO) 4 shows a strong preference
for activating C–H bonds [29].
Herein, we have selected the oxidative addition of aryl C–X (X ¼ halide) bonds
mediated by gold(I) complexes [30]. Compared to Pd(0)-mediated processes, the
analogous transformation involving Au(I) ! Au(III) is considered to be kinetically
very sluggish despite being thermodynamically feasible [31]. However, the reasons
behind the reluctance of gold(I) to activate C–X bonds were not fully understood.
For this reason, we decided to apply the ASM method to understand in detail the
factors governing the oxidative addition of aryl halides to Au(I) [32].
To this end, we selected the parent reaction involving phenyl iodide and
(Me 3 P)AuCl, a representative species widely used in gold(I)-mediated transformations. As readily seen in Fig. 6, which shows the corresponding ASD from the initial
stages of the transformation up to the respective concerted transition state, although
the interaction energy between the deformed reactants is clearly stabilizing from the
beginning of the process, it cannot compensate for the strong destabilizing effect of
the strain energy. Therefore, it becomes clear that the high energy required to deform
the reactants from their equilibrium geometries to the geometries they adopt at the
transition state is the main reason behind the high barriers computed for the gold(I)mediated oxidative additions.
The partitioning of the strain energy into contributions coming from each reactant
clearly suggests that the major contributor to the total ΔE strain term is the distortion
A Quantitative Approach to Understanding Reactivity in Organometallic Chemistry
115
metal fragment can be tuned to efficiently modify the reactivity.
3.2 Oxidative Addition Reactions
Oxidative addition typically constitutes the first and also the rate-limiting step in the
catalytic cycle of a good number of cross-coupling reactions [23]. This process
involves the cleaving of a C–X bond and forming of two new coordination bonds at
the transition metal (TM), which results in an increase of its oxidation state by
2 units. In principle, two main mechanisms can be envisaged for this fundamental
reaction: (a) the concerted pathway, which involves the simultaneous formation of
the TM–C and TM–X bonds in the corresponding transition state, and (b) an
S N 2-type mechanism, where the central carbon atom is attacked by the transition
metal fragment forming a cationic [TM–R]
+ species and an X
À leaving group [23–
25]. The competition between both mechanisms has been thoroughly studied by
Bickelhaupt and co-workers with the help of the ASM method on reactions involving Pd(0)-complexes and different alkyl and aryl halides [26, 27]. In addition, the
activation of other bonds, such as C–H and C–C bonds, has been also quantitatively
analyzed in detail [28]. Very recently, this research group has also explored similar
oxidative addition reactions mediated by iron complexes, finding that whereas
palladium complexes favor C–Cl activations,
1 Fe(CO) 4 shows a strong preference
for activating C–H bonds [29].
Herein, we have selected the oxidative addition of aryl C–X (X ¼ halide) bonds
mediated by gold(I) complexes [30]. Compared to Pd(0)-mediated processes, the
analogous transformation involving Au(I) ! Au(III) is considered to be kinetically
very sluggish despite being thermodynamically feasible [31]. However, the reasons
behind the reluctance of gold(I) to activate C–X bonds were not fully understood.
For this reason, we decided to apply the ASM method to understand in detail the
factors governing the oxidative addition of aryl halides to Au(I) [32].
To this end, we selected the parent reaction involving phenyl iodide and
(Me 3 P)AuCl, a representative species widely used in gold(I)-mediated transformations. As readily seen in Fig. 6, which shows the corresponding ASD from the initial
stages of the transformation up to the respective concerted transition state, although
the interaction energy between the deformed reactants is clearly stabilizing from the
beginning of the process, it cannot compensate for the strong destabilizing effect of
the strain energy. Therefore, it becomes clear that the high energy required to deform
the reactants from their equilibrium geometries to the geometries they adopt at the
transition state is the main reason behind the high barriers computed for the gold(I)mediated oxidative additions.
The partitioning of the strain energy into contributions coming from each reactant
clearly suggests that the major contributor to the total ΔE strain term is the distortion
A Quantitative Approach to Understanding Reactivity in Organometallic Chemistry
115