Coordination deprotonation

The reactivity of the coordinated, deprotonated nucleophile is typically intermediate between that of the un-ionized and ionized forms of the nucleophile. Carboxypeptidase (Chapter 5) contains an active site Zn, which facilitates deprotonation of a water molecule in this manner. 

The model studies imply mechanistic paths in the enzyme that would accommodate much of the rate enhancement and are consistent with the observations above. These are displayed in Scheme 14. The phosphate ester binds to both Zn ions, which activates the P center to attack by the coordinated deprotonated serine nucleophile. Binding the serine hydroxyl to the metal ion enhances its deprotonation and gives an efficient intramolecular nucleophile, as we have seen from model studies. Such a process would also be assisted by an enzyme base to remove the proton intramolecularly and protonation of the alkoxide leaving group by an enzyme acid would be helpful. In addition, inversion of configuration would ensue at the P center. A coordinated water at the second Zn + ion could then be deprotonated to function as the intramolecular nucleophile which eliminates the serine alkoxide ion. Both aspects would be assisted by an enzyme base and acid, respectively. The stereochemistry of the processes would lead to inversion and therefore to net retention overall consistent with the expectation from the experiments by Jones et al. (89). 

This example shows the extreme influence of the nature of each of the substituents at P on the stabilization of the gem-dianion as well as on the formation of carbene complexes. Most recently, we were able to synthesize either the monocarbene or the homoleptic biscarbene of titanium using dianion 7Li2 (route A) [93]. In 2009, a related class of pincer ligand was developed by Martin-Vaca, Bourissou et al. who used route B to obtain the first example of indenylidene complexes of Zr (Scheme 29) [94]. Here also, the corresponding isolable dianionic ligands are not accessible. It is to be noted here that the in situ coordination/deprotonation required forcing conditions (ca. 100°C for several hours). 

Like mthenium, amines coordinated to osmium in higher oxidation states such as Os(IV) ate readily deprotonated, as in [Os(en) (NHCH2CH2NH2)] [111614-75-6], This complex is subject to oxidative dehydrogenation to form an imine complex (105). An unusual Os(IV) hydride, [OsH2(en)2] [57345-94-5] has been isolated and characterized. The complexes of aromatic heterocycHc amines such as pyridine, bipytidine, phenanthroline, and terpyridine ate similar to those of mthenium. Examples include [Os(bipy )3 [23648-06-8], [Os(bipy)2acac] [47691-08-7],... 

Deprotonation of enols of P-diketones, not considered unusual at moderate pH because of their acidity, is faciUtated at lower pH by chelate formation. Chelation can lead to the dissociation of a proton from as weak an acid as an aUphatic amino alcohol in aqueous alkaU. Coordination of the O atom of triethanolamine to Fe(III) is an example of this effect and results in the sequestration of iron in 1 to 18% sodium hydroxide solution (Fig. 7). Even more striking is the loss of a proton from the amino group of a gold chelate of ethylenediamine in aqueous solution (17). 

Discernible associative character is operative for divalent 3t5 ions through manganese and the trivalent ions through iron, as is evident from the volumes of activation in Table 4. However, deprotonation of a water molecule enhances the reaction rates by utilising a conjugate base 7T- donation dissociative pathway. As can be seen from Table 4, there is a change in sign of the volume of activation AH. Four-coordinate square-planar molecules also show associative behavior in their reactions. 

The deprotonation of 132 is favored at Ni and the coordination of 135 occurs preferentially at 82- A second entity of 135 coordinates at N3. A computational study of thiouracil derivatives of the tungsten(O) hexacarbonyl shows that the sulfur-bound thiouracil is serving as a ir-donor during the CO dissociation (Scheme 91) [99IC4715]. DFT calculations show that 137 is significantly stabilized with respect to the alternative reaction product 138. 

Trimethylsiloxyphenyl isocyanide enters the cyclization reaction with [MCl2(NCPh)2] (M = Pt, Pd) to yield the homoleptic tetracarbenes 77 (M=Pt, Pd) (97JOM(541)51). Complex 77 (M = Pd) enters an interesting reaction with ammonia to yield the species 78 where two of this benzoxazol-2-ylidene ligands are deprotonated and become C-coordinated benzoxazole moieties, while the other two remain intact. Palladium(II) iodide in these conditions behaves differently yielding the di-Mo-cyanide complex, which in the presence of tetra- -butyl ammonium fluoride gives the dicarbene 79. 

When the catalyst coordinates to the pyrazoline nitrogen and carbonyl oxygen at the step of 1-pyrazoline formation, desilylation or deprotonation takes place at the same position to give either Na or Nb, respectively. On the other hand, when the catalyst coordinates to the two carbonyl oxygens, the methine hydrogen derived from the acceptor molecule is deprotonated to give Nc. In the reaction using a Le-... 

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