Coordination and deprotonation

The coordinated and deprotonated nucleophile will show a reactivity intermediate between that of the ionised and unionised nucleophile. At a given pH, if the increase in concentration of deprotonated nucleophile which results from coordination to a metal ion more than compensates for its decreased reactivity a rate enhancement will be obtained for a reaction of the coordinated nucleophile compared with the uncoordinated species. For example, at pH 7 there is 10 M hydroxide ion but as... 

The mechanism involves a Pd(0) monocoordinate complex as the active species that undergoes oxidative addition to the aryl halide [141]. Thereafter, coordination of the amine to the palladium centre and deprotonation by the external base results in halide abstraction. After reductive elimination, the coupling product is obtained and the catalytic active species regenerated (Scheme 6.45). 

An alternative route for stabilization of quinone methides by metal coordination involves deprotonation of a ri5-coordinated oxo-dienyl ligand. This approach was introduced by Amouri and coworkers, who showed that treatment of the [Cp Ir(oxo-ri5-dienyl)]+ B1, 22 with a base (i-BuOK was the most effective) resulted in formation of stable Cp Ir(r 4-o-QM) complexes 23 (Scheme 3.14).25 Using the same approach, a series of r 4-o-QM complexes of rhodium was prepared (Scheme 3.14)26 Structural data of these complexes and a comparison of their reactivity indicated that the o-QM ligand is more stabilized by iridium than by rhodium. 

The titration coordinates evolve along with the dynamics of the conformational degrees of freedom, r, in simulations with GB implicit solvent models [37, 57], An extended Hamiltonian formalism, in analogy to the A dynamics technique developed for free energy calculations [50], is used to propagate the titration coordinates. The deprotonated and protonated states are those, for which the A value is approximately 1 or 0 (end-point states), respectively. Thus, in contrast to the acidostat method, where A represents the extent of deprotonation, is estimated from the relative occupancy of the states with A 1 (see later discussions). The extended Hamiltonian in the CPHMD method is a sum of the following terms [42],... 

The inability of amines to deprotonate upon coordination, and thus to compensate the charge of the MOs+ core, can be overcome by combination with other types of donors-including N-donor groups-in the ligand, as has already been discussed above for oxocyclam. A prominent example is provided by technetium complexes of tetradentate amine oxime ligands. 

Fig. 4 Free energy reaction coordinate profiles that illustrate a change in the relative kinetic barriers for partitioning of carbocations between nucleophilic addition of solvent and deprotonation resulting from a change in the curvature of the potential energy surface for the nucleophile addition reaction. This would correspond to an increase in the intrinsic barrier for the thermoneutral carbocation-nucleophile addition reaction.

With aquated Pt(II) compounds, numerous studies have revealed the kinetic preference of the 6-oxopurine N7 site [15,35]. In addition to the favorable electrostatic potential mentioned above [23] also steric factors seem to favor coordination to the guanine N7 site, in particular [36]. Estimated relative steric parameters (in parenthesis) suggest that the guanine N7 (1.00) and hypoxanthine N7 (1.03) atoms are the least sterically hindered binding sites in alkylated nucleobases, followed by the adenine N7 (1.17) and deprotonated hypoxanthine N1 (1.17) sites and the deprotonated N3 atoms of the different pyrimidine bases (1.39 for U, 1.44 for T, and 1.56 for C), while the adenine N1 (1.58) and... 

The catalytic Pd complex and the aryl bromide together suggest the first step is oxidative addition of Pd(0) to the C5-Br bond. (The reduction of Pd(II) to Pd(0) can occur by coordination to the amine, p-hydride elimination to give a Pd(II)-H complex and an iminium ion, and deprotonation of Pd(IE)-H to give Pd(0).) The C10-C11 k bond can then insert into the C5-Pd bond to give the C5-C10 bond. P-Hydride elimination then gives the Cl 1-C12 n bond and a Pd(II)-H, which is deprotonated by the base to regenerate Pd(0). The overall reaction is a Heck reaction. 

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