External reagents reaction mechanisms

For reactions of these types, the identity of the ligand, its environment within the complex—e.g., the nature of the central metal and the properties and positions of the other ligands—and the character of any external redox reagent are important variables which will influence the thermodynamic feasibility of processes, the products formed under given experimental conditions, and the rates and mechanisms of redox processes. Within this framework some significant problems await solution. 

Two cw chemical lasers have been reported that operate without any external source of power to produce atomic reagents. Cool, Shirley, and Stephens [250-252] achieved oscillation on the (00°1) —(10°0) transition of C02. The mechanism was comprised of three important stages (a) production of F atoms by the reaction NO + F2 -> NOF + F (b) reaction between F and D2 to produce DF1 (c) vibrational-vibrational energy exchange from DFf to C02 to produce C02 (00°1). Meinzer [259] was able to obtain stimulated emission from DFf itself using F atoms generated in an H2-F2 flame. 

Since A and 8 are difficult to measure for a hquid membrane system, DA/b can be replaced by D ( 14/ I c) where D is an effective diffusivity and VJ Tc is the treat ratio or holdup ratio (volume ratio of emulsion to external phase). In the case of type-I facilitated transport mechanism, where the solute is removed by reaction with internal stripping reagent, the solute concentration in the internal phase can be considered to be zero and hence Eq. (1) becomes... 

Such a mechanism was unlikely as addition of an external trap, 1,1-diphenylethylene, had no effect on the course of the arylation of p-ketoesters.i l A second approach involved the use of an internal trapping system which had been successfully used in the study of the radical reactions of arenediazonium salts.The internal trap containing reagent, ( rf/io-allyloxyphenyl)lead triacetate (94), can be easily prepared from the corresponding boronic acid. 2 Reaction with various types of nucleophiles, such as ethyl 2-oxocyclopentanecarboxylate (86), mesitol (36), the sodium salt of nitropropane, iodide and azide always afforded the C-arylation products in high yield. No trace of the 3-substituted dihydrobenzofurans, expected in a mechanism involving the intermediacy of free radicals, could be detected. 

These results, along with quantum mechanical calculations, supported the conclusion that a six-membered chelated flat-chairlike transition state operated under the kinetic conditions. The diastereoselectivity of reactions between Li-salts of the (/ )-methyl p-tolyl sulfoxide formed by a-deprotonation with -BuLi, could be further enhanced by the addition of external C2-synunetrical bidentate ligand (7 ,7 )-l,2-M,M-bis(trifluoromethanesulfonyla-mino)cyclohexane in the form of the lithium M,M-dianion providing the amine in 80% yield with a diastereoselectivity of 99 1, apparently achieving a match between the chirality of the additive and the chirality of the sulfoxide reagent. 

The reactions of HTIB with alkenes (Scheme 3.73) can be rationalized by a polar addition-substitution mechanism similar to the one shown in Scheme 3.70. The first step in this mechanism involves electrophilic flnfi-addition of the reagent to the double bond and the second step is nucleophilic substitution of the iodonium fragment by tosylate anion with inversion of configuration. Such a polar mechanism also explains the skeletal rearrangements in the reactions of HTIB with polycyclic alkenes [227], the participation of external nucleophiles [228] and the intramolecular participation of a nucleophilic functional group with the formation of lactones and other cyclic products [229-231]. An analogous reactivity pattern is also typical of [hydroxy(methanesulfonyloxy)iodo]benzene [232] and other [hydroxy(organosulfonyloxy)iodo]arenes. 

We found that an external chlorinating reagent preferentially passed the chlorine to the template cyclodextrin first, and that the cyclodextrin then relayed the chlorine on to the substrate. Furthermore, this was a catalytic process, and occurred faster than chlorination in the absence of the template. The mechanism involved was established by detailed studies, including reaction kinetics. Modification of the cyclodextrin, and its incorporation into a polymer, have led to the production of highly selective catalysts for this aromatic substitution reaction [22]. In other laboratories an electrochemical adaptation of our reaction has also been made, in which the cyclodextrin molecule is attached to the electrodes [23]. 

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