Unimolecular clock reactions

Some unimolecular clock reactions are given in Table 7.1 and bimolecular clock reactions in Table 7.2, together with their kinetics. Rate constants at any... 

One-electron Chemistry of Carbohydrates Table 7.1 Unimolecular clock reactions. 

Phenyl-substituted radical clocks (Fig. 6) display definite enthalpy effects that one expects for strong radical-stabilizing groups. The result is that unimolecular clock reactions are orders of magnitude less rapid than their non-substituted counterparts as evidenced in the rate constants at ambient temperatures for 5-exo cyclization of radical 9 [39] and ring openings of radicals 10 [4, 40], 11 [4], and 12 [41]. Note that... 

The kinetic data for the reaction of primary alkyl radicals (RCH2 ) with a variety of silanes are numerous and were obtained by applying the free-radical clock methodology. The term free-radical clock or timing device is used to describe a unimolecular radical reaction in a competitive study [2-4]. Three types of unimolecular reactions are used as clocks for the determination of rate constants for this class of reactions. The neophyl radical rearrangement (Reaction 3.1) has been used for the majority of the kinetic data, but the ring expansion rearrangement (Reaction 3.2) and the cyclization of 5-hexenyl radical (Reaction 3.3) have also been employed. 

The concept of this method is illustrated in Scheme 3.1, where the clock reaction (U R ) is the unimolecular radical rearrangement with a known rate constant ( r)- The rate constant for the H atom abstraction from RsSiH by a primary alkyl radical U can be obtained, provided that conditions are found in which the unrearranged radical U is partitioned between the two reaction channels, i.e., the reaction with RsSiH and the rearrangement to R. At the end of the reaction, the yields of unrearranged (UH) and rearranged (RH) products can be determined by GC or NMR analysis. Under pseudo-first-order conditions of silane concentration, the following relation holds UH/RH = (A H/A r)[R3SiH]. A number of reviews describe the radical clock approach in detail [3,4]. 

Another common scenario in competition kinetics utilizes unimolecular radical reactions as a clock against which other reactions can be timed. Among the most commonly used free radical clocks are the cyclization of 1 -hexenyl and other radicals with double or triple bonds in the chain,33 ring opening,34 and p-elimination from alkoxyl radicals.35... 

The term radical clock is used to describe a unimolecular radical reaction that is kinetically calibrated and, thus, can be applied in a competition study to time a particular radical reaction of interest [1], Such kinetic information is necessary for mechanistic studies where a radical might be formed as a transient. It is also important for synthetic applications because most radical-based methods involve chain reactions that commonly have several competing reaction steps with absolute kinetic values available, one can calculate the concentrations of reagents necessary for a high-yield synthetic conversion. Because lifetimes of simple radicals are usually in the microsecond range, direct kinetic measurements require sophisticated instrumentation. Radical clocks provide an inexpensive alternative for kinetic studies because the rate constants for the competing reactions are determined from the product mixtures present at the end of the reaction, usually with common organic laboratory instruments. 

The evolution of kinetic scales has been highly dependent on radical clock and, more generally, indirect competition kinetic studies [6], These types of studies provide ratios of rate constants as discussed above. One can build an extensive series of relative rate constants for unimolecular clocks and bimolecular reactions, and the relative rate constants often are determined with very good to excellent precision. At some point, however, absolute rate constants are necessary to provide real values for the entire kinetic scale. These absolute kinetic values are the major source of error in the kinetics, but the absolute values are becoming more precise and, one certainly hopes, more accurate as increasingly refined techniques are introduced and multiple methods are applied in studies of specific reactions. 

The radicals formed by unimolecular rearrangement or fragmentation of the primary radicals arc often termed secondary radicals. Often the absolute rate constants for secondary radical fonnation are known or can be accurately determined. These reactions may then be used as "radical clocks", to calibrate the absolute rate constants for the bimolccular reactions of the primary radicals (c.g. addition to monomers - see 3.4). However, care must be taken since the rate constants of some clock reactions (e.g. /-butoxy fi-scissiom ) are medium dependent (see 3.4.2.1.1). 

Radical chain processes 772, 1063 Radical clock 1059 Radical reactions radical-molecule 1102-1111 radical-radical 1099-1102 unimolecular 1098, 1099 Radicals, formation during radiolysis 891-922... 

In these circumstances, where routine kinetic measurements are uninformative and direct measurements of the product-forming steps difficult, comparative methods, involving competition between a calibrated and a non-calibrated reaction, come into their own. Experimentally, ratios of products from reaction cascades involving a key competition between a first-order and a second-order processes are measured as a function of trapping agent concentration. Relative rates are converted to absolute rates from the rate of the known reaction. The principle is much the same as the Jencks clock for carbenium ion lifetimes (see Section 3.2.1). However, in radical chemistry Newcomb prefers to restrict the term clock to a calibrated unimolecular reaction of a radical, but such restriction obscures the parallel with the Jencks clock, where the calibrated reaction is a bimolecular diffusional combination with and the unknown reaction a pseudounimolecular reaction of carbenium ion with solvent. Whatever the terminology, the practical usefulness of the method stems from the possibility of applying the same absolute rate data to all reactions of the same chemical type, as discussed in Section 7.1. 

When the probe reaction being calibrated is a unimolecular process, one measures the rate constant of a radical clock directly for the initial absolute kinetic values, and, thus, the method is inverted in approach from that used for alkyl radical kinetics. LFP studies of unimolecular process give more precise data than those of bimolecular processes, and the approach typically starts with inherently good kinetic data. The synthetic efforts necessary for production of appropriate radical precursors are a drawback to this method, but it is, nonetheless, useful for establishing absolute kinetics for some classes of radicals where little kinetic information was available, such as nitrogen-centered radicals discussed later. 

Section 8.1.4 on the competition of radical-molecule reactions with unimolecular processes refers almost exclusively to competitions between hydrogen abstraction or double bond addition with the fl-cleavage of the radical, which is used as a clock . In the case of f-butoxyl radicals, these studies involve the measurement of t-butanol-to-acetone ratios. So in the case of subsection 8.I.3.1, these studies do not provide information on the site of attack, which needs to be derived from infrared or other experiments. The ratios of reactivity reported in this... 

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