Kinetic isotope effects radicals

A distinction between these four possibilities can be made on the basis of the kinetic isotope effect. There is no isotope effect in the arylation of deuterated or tritiated benzenoid compounds with dibenzoyl peroxide, thereby ruling out mechanisms in which a C5— bond is broken in the rate-determining step of the substitution. Paths (ii) and (iii,b) are therefore eliminated. In path (i) the first reaction, Eq. (6), is almost certain to be rate-determining, for the union of tw o radicals, Eq. (7), is a process of very low activation energy, while the abstraction in which a C—H bond is broken would require activation. More significant evidence against this path is that dimers, Arz, should result from it, yet they are never isolated. For instance, no 4,4 -dinitrobiphenyl is formed during the phenylation of... 

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

This is in accordance with the primary kinetic isotope effect for Mn(III) sulphate With Co(III) electron abstraction may occur to give a radical-cation which suffers further oxidation. The alternative explanation of the lack of an isotope effect is that formation of the Co(III)-ketone complex is rate-determining this lacks, however, other kinetic support . ... 

Phenols decrease the intensity of CL 7chi in oxidized hydrocarbons as a result of chain termination by the reaction with peroxyl radicals. Since Icu [R02 ]2 (see Chapter 2), the ratio (/0//)12 was found to be proportional to [ArOH] [7]. The kinetic isotope effect (k0K/k0n 1) proves that the peroxyl radical abstracts a hydrogen atom from the O—H bond of phenol [2,8]. 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

The mechanistic proposal of rate-limiting hydrogen atom transfer and radical recombination is based on the observed rate law, the lack of influence of CO pressure, kinetic isotope effects [studied with DMn(CO)s] and CIDNP evidence. In all known cases, exclusive formation of the overall 1,4-addition product has been observed for reaction of butadiene, isoprene and 2,3-dimethyl-l,3-butadiene. The preferred trapping of allyl radicals at the less substituted side by other radicals has actually been so convincing that its observation has been taken as a mechanistic probe78. 

Abeywickrema and Beckwith162 have measured the primary hydrogen-deuterium kinetic isotope effect for the reaction between an aryl radical and tributyltin hydride. The actual isotope effect was determined by reacting tributyltin hydride and deuteride with the ort/ro-alkcnylphcnyl radical generated from 2-(3-butenyl)bromobenzene (equation 111). 

The exo and the endo ring closures (the kc reactions) are in competition with the aryl radical-tributyltin hydride transfer (the ks or ku reaction). These workers162 used this competition to determine the primary hydrogen-deuterium kinetic isotope effect in the hydride transfer reaction between the aryl radical and tributyltin hydride and deuteride. 

This method gave a primary hydrogen-deuterium kinetic isotope effect of 1.3 for the reaction between the aryl radical and tributyltin hydride. This isotope effect is smaller than the isotope effect of 1.9 which San Filippo and coworkers reported for the reaction between the less reactive alkyl radicals and tributyltin hydride163 (vide infra). The smaller isotope effect of 1.3 in the aryl radical reaction is reasonable, because an earlier transition state with less hydrogen transfer, and therefore a smaller isotope effect164, should be observed for the reaction with the more reactive aryl radicals. 

Several workers have measured the primary hydrogen-deuterium kinetic isotope effects for the reaction between organic radicals and tributyltin hydrides (equation 114). 

In one study, Ingold and coworkers166 measured the rate constants for the reactions of several alkyl radicals with tributyltin hydride using a laser flash photolytic technique and direct observation of the tributyltin radical. They also used this technique with tributyltin deuteride to determine the primary hydrogen-deuterium kinetic isotope effects for three of these reactions. The isotope effects were 1.9 for reaction of the ethyl radical, and 2.3 for reaction of the methyl and n -butyl radicals with tributyltin hydride at 300 K. 

Other primary hydrogen-deuterium kinetic isotope effects have been measured for radical reactions with tributyltin hydride. For example, Carlsson and Ingold167 found primary hydrogen-deuterium kinetic isotope effects of 2.7 and 2.8, respectively, for the... 

TABLE 11. The primary hydrogen-tritium kinetic isotope effects found in the reactions between various alkyl radicals and tributyltin hydride and tributyltin hydride-t... 

Finally, Franz and coworkers171 measured the rate constants and primary hydrogen-deuterium kinetic isotope effects for the radical reactions between tributyltin hydride and the neophyl and the 2-allylbenzyl radical in diphenyl ether. The isotope effect in the first reaction was 1.64 at 192.5 °C and that in the second reaction was 1.91 at 236 °C. These values compare well with those predicted from Kozuka and Lewis s primary... 

According to Scheme 11, the isomeric ortho para) product ratios are established during the collapse of the radical pair in (64) (most probably at the positions of AN+- with the highest electron density). Furthermore, the absence of a measurable kinetic isotope effect in the decay of the deuterated analogue (C6D5OCH.v) in Table 3 is predicted from Scheme 11 since the proton loss occurs in a subsequent, rapid step (65). The absence of a deuterium kinetic isotope effect also indicates that the presence of pyridine in the triad in (63) does not lead to the nitroanisoles by an alternative... 

Disproportionation (equation 13) is one of the side reactions that can occur in benzidine rearrangements. Shine and coworkers measured the nitrogen and carbon kinetic isotope effects for the disproportionation reaction of 4,4 -diiodohydrazobenzene, which only yielded disproportionation products, at 25 °C in 70% aqueous dioxane that was 0.376 M in perchloric acid29. The reaction was first order in hydrazobenzene and it has been assumed that an intermediate was involved in the disproportionation reaction. This intermediate must be one of a radical ion30 (equations 14 and 15), a jr-complex31 (equation 16) or a quinonoid structure32 (equation 17). 

The review is divided into sections according to the type of metal hydride for convenience in discussing the information systematically. At one extreme, kinetic studies have been performed with many types of silicon hydrides, and much of the data can be interpreted in terms of the electronic properties of the silanes imparted by substituents. At the other extreme, kinetic studies of tin hydrides are limited to a few stannanes, but the rate constants of reactions of a wide range of radical types with the archetypal tin hydride, tributylstannane, are available. Kinetic isotope effects for the various hydrides are collected in a short section, and this is followed by a section that compares the kinetics of reactions of silicon, germanium, and tin hydrides. 

The reaction rate is half-order in palladium and dimeric hydroxides of the type shown are very common for palladium. The reaction is first order in alcohol and a kinetic isotope effect was found for CH2 versus CD2 containing alcohols at 100 °C (1.4-2.1) showing that probably the (3-hydride elimination is rate-determining. Thus, fast pre-equilibria are involved with the dimer as the resting state. When terminal alkenes are present, Wacker oxidation of the alkene is the fastest reaction. Aldehydes are prone to autoxidation and it was found that radical scavengers such as TEMPO suppressed the side reactions and led to an increase of the selectivity [18],... 

ESR and CIDNP studies intended to detect the radical intermediates failed [63], Conjugate addition of a vinylcuprate reagent to an enone takes place with retention of the vinyl geometry indicating that no vinyl radical intermediate is involved [64, 65], Kinetic isotope effects and substituent effects in cuprate addition to benzophenone indicate that C-C bond formation is rate-determining, which is not consistent with the involvement of a radical ion pair intermediate [66]. 

A kinetic isotope effect supports C —H bond breakage as a rds. The intermediacy of Cr(V) is demonstrated by epr. The Cr(IV) species is epr-silent. Free radical formation in some systems has also been demonstrated. ... 

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