Stark effect kinetics

Borovkov V Yu, Serykh AI and Kazansky V B (2000), Diffuse reflectance IR spectra of orf/to-hydrogen and para-hydrogen adsorbed on zeolite BaX at 77 K and their interpretation in terms of the Stark effect , Kinet Catal., 41, 787. 

In contrast, Cozzi and Umani-Ronchi found the (salen)Cr-Cl complex 2 to be very effective for the desymmetrization of meso-slilbene oxide with use of substituted indoles as nucleophiles (Scheme 7.25) [49]. The reaction is high-yielding, highly enantioselective, and takes place exclusively at sp2-hybridized C3, independently of the indole substitution pattern at positions 1 and 2. The successful use of N-alkyl substrates (Scheme 7.25, entries 2 and 4) suggests that nucleophile activation does not occur in this reaction, in stark contrast with the highly enantioselective cooperative bimetallic mechanism of the (salen)Cr-Cl-catalyzed asymmetric azidolysis reaction (Scheme 7.5). However, no kinetic studies on this reaction were reported. 

Echavarren has also studied intramolecular ortho C-H functionalization of aromatics by alkylpalladium(II) species (Scheme 6). In a system where the aromatic ring is deuterated at one of the ortho positions, the production of a statistically equivalent mixture of both deuterated (19) and nondeuterated (20) products was observed. The absence of a kinetic isotope effect supports an EAS mechanism in which deprotonation is not rate limiting [24], The EAS mechanism for C-H activation by an alkylpalladium(II) species is in stark contrast to the mechanism of C-H functionalization reactions by vinyl- or arylpalladium(II) species, where proton abstraction has been determined to be rate limiting [25]... 

In order to collect more information about the mechanism of the reaction, we devised three independent experiments in which the three reactants (alkyne, silane, and catalyst) were incubated two by two at 60 °C for 3h, before addition of the third component at 20 °C [35]. The results of these experiments are presented in Figure 5.20. As can be seen, the addition of complex 45 to a mixture of alkyne and silane displays a kinetic profile identical to what was observed previously (curve A). However, when the silane was added to the alkyne, previously incubated with the precatalyst, a considerable reduction of the catalytic activity occurred (curve B). It thus transpires that the alkyne triggers somehow the deactivation of the catalyst. In stark contrast, a dramatic acceleration of the reaction rate, concomitant with the disappearance of the induction period, was observed when the catalyst was heated with the silane prior to addition of the alkyne (curve C). This last effect is reminiscent of what was observed upon repeated addition of fresh reactants during the hydrosilylation of alkenes (see Figure 5.7). Therefore, treating the precatalyst with a silane before adding the alkyne leads to a particularly active and selective catalyst. 

---