Thermal bond activation

Figure 4.11 Experimental data for the energy transfer rate constants from donors to c/s-stilbene (from dotted line Sandros plot plain line thermal bond activation model fitted to experimental data [46]. (Reproduced with permission from Elsevier.)...

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

The most important contributions in this area, however, directly related to bond activation chemistry, and, undoubtedly triggered by theoretical considerations along the lines of Figure 1, were reported by Whitesides and coworkers in 1986 and 1988 [11]. It was shown that the bent, bisphosphine-coordinated platinum chelate complex [(dcpe)Pt(O)] (9) (dcpe = bis(dicyclohexylphosphino)ethane), which could be generated thermally as a "hot" reactive intermediate by reductive elimination of neopentane from its ris-neopentylhydride Pt(II) precursor at around 60-70°C in solution, was able to activate C-H bonds, even of unactivated alkanes. 

We see, at first, that the reaction enthalpy for quinone abstraction reactions with the C—H bond of alkylperoxyl radicals is higher than with the O—H bond of the hydroperoxyl radical. The second important factor is different triplet repulsions in these two types of abstraction reactions. Indeed, the reaction with R02 proceeds via TS of the C H O type. Such reaction is characterized by the high thermally neutral activation energies Eeo = 62.9 kJ mol-1. The value of Ee0 for the reaction involving the O H O TS reaction center is much lower (27.3 kJ mol-1). With the rate constants have a very low value, the reaction Q + R02 cannot influence the oxidative chain termination in comparison with the interaction of two R02 radicals. Indeed, the rate constant for the latter is 105—107 L mol-1 s-1 and, in these cases, the inequality (2k6v )1/2 2k[Q] always holds. The reason for such high Ee0 values and, hence,... 

The micro mixer was made by standard photolithographic and etching methods on an ultra-flat quartz substrate [160], The microstructure created in this way was shielded by a cover plate by thermal bonding. This cover plate contained access holes for fluid supply and withdrawal. Internal surfaces of the chip were activated to have a high density of silanol groups needed for the EOF transport... 

Scheme 11. Thermal C-H bond activation by the rhodium ethylene complex.

Therefore the bond enthalpy HD and the activation enthalpy AH 6 for the dissociation process are generally identical6a). Consequently, bond enthalpies HD can be deduced from the temperature dependence of the rate constants k of thermal bond cleavage reactions with the aid of the Eyring equation... 

An excellent short review of C-H bond activation including both thermal and photochemical mechanisms has been provided by Labinger and Bercaw. Lees has reviewed the photochemical and photophysical features of these reactions. Bergman has described the contributions of his group toward the understanding of the mechanisms of C-H activation up to 1992, and within the context of physical organic chemical methods. Schneider has examined the similarities between Si-H and C-H activation. Shilov and ShuFpin have provided extensive reviews of C-H activation. As with Si-H oxidative addition, the spin state of the photochemical intermediate plays a major role in the probability of insertion. This has been reviewed for the specific case of methane activation. 

In 6-dimethylaminofulvene, the parent compound of (7), and in 6-dimethylamino-6-methylfulvene, for the rotation about the A5 6 double bond, activation energies of 92.5 kJ (22.1 kcal) and 68.6 kJ (16.4 kcal) respectively were measured5. Starting from (la), the adequate form (7 b) should be reached in a thermal equilibrium quite likely on heating. 

The bonding of PEG to the active adsorptive sites on the support can take place 1n two different ways. First, some chemical bonding may occur when the packing is used at higher temperatures. This effect is comparable with the thermal bonding of PEG as described by Aue... 

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