Cl-atom transfer reactions

Table II. Rate Constants and Arrhenius Parameters of Cl Atom Transfer Reactions of Cyclohexyl Radicals Ce)...

In trichlorosilane the chain decomposition of chloromethanes, CM, proceeds by reactions 52 and 53. In the ClsSiH-ClM systems the relative Arrhenius parameters of the Cl atom transfer, reaction 52, were determined in competitive studies (41). The same method was... 

In consideration of the above experimental and theoretical evidence, it is concluded that the reaction, at least the ones that have been investigated, be viewed as proceeding through a weakly solvated 17-electron Re(CO)5 radical instead of a 19-electron or a charge-transfer intermediate. The fact that no other intermediates are detected prior to the product formation suggests that the reaction involve only the rate-limiting Cl atom transfer step. 

Fig. 7.6 Formation of the transition state in an atom transfer reaction between Cl and CH3I.

Extensive studies have been made of solvent effects on atom transfer reactions involving ions [12]. In the case of reaction (7.3.23), the rate constant decreases from 250M s in A-methylpyrrolidinone to 3 x 10 M s in methanol. This effect can be attributed to solvation of the anionic reactant Cl and the anionic transition state [12]. Since the reactant is monoatomic, its solvation is much more important. It increases significantly with solvent acidity leading to considerable stabilization of the reactants. As a result the potential energy barrier increases and the rate decreases with increase in solvent acidity. As shown in fig. 7.7, this leads to an approximate linear relationship between the logarithm of the rate constant and the solvent s acceptor number AN, an empirical measure of solvent acidity (see section 4.9). Most of the results were obtained in aprotic solvents which have lower values of AN. The three data points at higher values of AN are for protic solvents. 

The kinetics of reaction (29) have been investigated by rapid-scan i.r. spectroscopy. The experimental rate constant is in reasonable agreement with an earlier calorimetric value, but the use of isotopic NO has now established the reaction mechanism to be a Cl atom transfer. Rate constants for the nitrogen isotope exchange reactions (30) and (31) have also been determined. ... 

While C-C bond-forming processes involving Cl transfer from polyhalomethanes comprise the earliest observed class of atom transfer reactions, they are typically unsuccessful when performed on olefins prone to polymerization [40]. As a result, there are only a modest number of examples of Cl atom transfers in other typical organic systems because of their slow rate relative to other radical processes. An example of a successful reaction of this type is illustrated in the addition of CCI4 to germacrene, illustrated in Scheme 13 [41]. 

Reaction 11 (63). Our (O2/CI2) mixed scavenger studies support a simikr interpretation. In experiments with Cl2-scavenged CF4 the CCl2F F yield is not diminished in the presence of O2. Therefore, a two-step sequential mechanism for Reaction 11 initiated by Cl-atom transfer can probably be discoimted. 

The gas-phase stoichiometric Reaction 12 necessarily involves a sequential mechanism. Because CClg F is formed in the above noted (O2/CI2) mixed scavenger experiments, a two-step sequence initiated by Cl-atom transfer and forming a singlet carbene product is probably important (Reactions 14 and 15). Further research is needed on these... 

The cupric halide-catalyzed reaction of an aryl diazonium salt with an olefin-known as the Meerwein arylation-is a powerful and well-developed method for the arylation of olefins (136, 137). Good results have been obtained for olefins bearing electron-withdrawing groups such as a carbonyl, a cyano, an aryl or an alkenyl group. Depending on the nature of the olefin and the diazonium salt, the reaction can lead either to the oxidation product 88 or to the Cl-atom transfer product 89 (Equation 13.10) [138]. 

The alkyl radical R and the polymer radical Pi are reversible with respect to the corresponding halides R-Cl and Pi-Cl, respectively. This process also involves an atom transfer reaction. The concentration of the growing radical is low. This means that the equilibrium is towards the halides. This also implies that the bimolecular reactions between radicals are low and therefore termination reactions are minimized. This virtually means that a living polymerization is achieved. 

The radical 147 may also react with a neutral species such as diatomic chlorine, Cl-Cl, in another atom transfer reaction. The carbon radical 147 donates one electron, and one electron is transferred from the Cl-Cl covalent bond to form 149, but the second electron of the Cl-Cl bond is transferred to the other chlorine atom to form a new chlorine radical. Cl. In this case, a radical reacts with a neutral molecule to form a neutral product and another radical. This process is known as radical substitution, or atom transfer. 

Chlorine elimination reactions. Chloroethyl radicals CClEt) formed by Cl or Br transfer reactions in cyclohexane can subsequently eliminate a Cl atom by reaction 34 or abstract a H atom from the solvent by reaction 35. The rate constant ratio 34/ 35 el/ H related Arrhenius parameters can be... 

The ability of triethyl silyl radicals to remove the strongly bound vinylic chlorine from trichloroethylene (reaction 54) is a characteristic feature of the reactions of triethylsilyl radicals with other chloroethylenes. Radical addition to acetylenes is the method used most frequently to generate vinyl radicals. The formation of vinyl radicals by Cl atom transfer from chloroethylenes to silyl radicals thus offers an interesting alternative to this oaethod. 

The BEBO method has been quite successful in calculating the activation energies of a large number of atom-transfer reactions, but reveals some difficulties with reactions involving dramatic ZPE differences. For example, it predicts the structure Cl-H-Cl to be a stable molecule rather than a transition state of the Cl+HCl atom exchange. We will not pursue the BEBO method any further, but make use of its definition of the reaction coordinate to express the sum of bond extensions as... 

The N-atom transfer reactions using A-tosyliminoiodinane (PhI=NTs), which was a famous nitrene source discovered by Okawara and coworkers [20], with Fe(TPP)(Cl) or Mn(TPP)(Cl) complex as a catalyst were first reported by Breslow and Gellman [21]. Their work demonstrated that intermolecular and intramolecular nitrene insertion reactions into C-H bond. In contrast to the nitrene insertion reactions. 

For reasons given in Section 1.2.2, there have been few direct experiments on bimolecular reactions involving molecules with more than one quantum of vibrational excitation. However, the energies associated with single quanta are comparable with the activation energies of many elementary atom-transfer reactions, so the resultant rate enhancement can be considerable and revealing. In this section, data on the reactions (and parallel relaxations) of diatomic hydrides such as Hg, HX (X = F, Cl, Br, I), and OH, are reviewed first, and then some examples are provided of measurements on reagents excited by CO2 laser photons. 

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