And radical clocks

A major source of error in any indirect method is inaccuracy of the basis rate constants. Errors can result from determinations of rate constants by a sequence of several indirect studies or by an unanticipated solvent effect on the kinetics of a basis reaction. An error can also result in calibration of a radical clock if the requisite assumption that the clock radical will react with a rate constant equal to that of a simple model radical is not correct. Nevertheless, indirect methods in general, and radical clock studies in particular, have been the workhorse of radical kinetic determinations. 

The mechanism of insertion of 2-alkylphenylnitrenes into a 1,5-related CH bond was studied by three methods 80 determination of isotope effects, stereochemistry, and radical clock. During the formation of indolines, a kn/kD of 12.6-14.7 was observed coupled with complete loss of stereochemical integrity at the CH carbon. When the CH insertion carbon bore a cyclopropane group, ring-opening products were observed. These observations suggest a mainly radical H-atom abstraction mechanism. The sensitivity of the isotope effects to solvent was taken to imply a small concerted nitrene insertion contribution. 

Substrate probes have aided mechanistic understanding of the key C— H activation step in the MMOH reaction cycle. Chiral alkanes and radical-clock substrate probes " " were used to discriminate between radical recoil/rebound and nonsynchronous concerted insertion pathways. A short lifetime (< 150 fs) estimated for the putative radical species derived from cyclopropane-based radical-clock substrates favors the latter process,whereas partial racemization of chiral ethane substrate is consistent with the former scenario. A unifying model was proposed, in which both recoil/rebound and concerted reaction channels are available for a bound radical intermediate and the partitioning between each trajectory is dependent on the substrate. Formation of carboca-tion-derived products from certain probes implicates yet another route involving a formal OH+ insertion.Participation of multiple species capable of oxygen transfer is an emerging mechanistic view in both heme and nonheme systems, as exemplified by the studies of cP450s and their synthetic models.Scheme 3 depicts various density functional theory (DFT) models of MMOHq and their computed reaction pathways, which are reviewed in detail elsewhere. 

A concerted [2 + 2] cycloaddition pathway in which an oxametallocycle intermediate is generated upon reaction of the substrate olefin with the Mn(V)oxo salen complex 8 has also been proposed (Scheme 1.4.5). Indeed, early computational calculations coupled with initial results from radical clock experiments supported the notion.More recently, however, experimental and computational evidence dismissing the oxametallocycle as a viable intermediate have emerged. In addition, epoxidation of highly substituted olefins in the presence of an axial ligand would require a seven-coordinate Mn(salen) intermediate, which, in turn, would incur severe steric interactions. " The presence of an oxametallocycle intermediate would also require an extra bond breaking and bond making step to rationalize the observation of trans-epoxides from dy-olefms (Scheme 1.4.5). 

By using an identical radical clock substrate probe which did not rearrange upon hydroxylation with M. capsulatus (Bath), rearranged product was detected with MMO from AT. trichosporium 0B3b (59). From the ratio of unrearranged to rearranged products, a rebound rate constant was calculated to be 6 x 1012 s-1 at 30°C for this system. A separate study with another radical clock substrate probe with MMO from M. trichosporium 0B3b reported products consistent with both radical and cationic substrate intermediates (88). 

Since work with the radical clock substrate probes indicated important differences in the hydroxylation mechanisms for M. capsulatus (Bath) and M. trickosporium OB3b, work with (R) and (S)-[1-2H,1-3H]ethane with both enzymes was carried out (93, 94). With M. tri-chosporium OB3b, approximately 65% of the product displays retention of stereochemistry (93). A rebound rate constant of 2 - 6 x 1012 s-1 was calculated, assuming a free energy change of 0.5 kcal mole-1 for rotation about the C-C bond (94). This estimate approaches the value obtained from the radical clock substrate probe analysis (59). 

In summary, mechanistic studies have revealed intriguing differences between MMO from M. capsulatus (Bath) and MMO from M. triehosporium OB3b. With M. capsulatus (Bath), radical clock substrate probes indicated either that a substrate radical is not produced or that it reacts with a rate constant > 1013 s-1. With MMO from M. triehosporium OB3b, radical involvement was suggested from several experiments, and a rebound rate constant of 6 x 1012 s"1 was calculated for this system. 

He, X. and Ortiz de Montellano, P.R. (2004) Radical rebound mechanism in cytochrome P-450 catalyzed hydroxylation of multifaceted radical clocks a-and p-thujone. The Journal of Biological Chemistry, 279, 39479-39484. 

LFP-Clock Method. In this method, rate constants for the radical clock reactions are measured directly by LFP, and the clocks are used in conventional competition kinetic studies for the determination of second-order rate constants. The advantages are that the clock can be calibrated with good accuracy and precision in the solvent of interest, and light-absorbing reagents can be studied in the competition reactions. The method is especially useful when limited kinetic information is available for a class of radicals. 

Table II. Most of the data was obtained from radical clock studies. The neophyl radical rearrangement24 [Eq. (2)] was used for the majority of the kinetic data in Table II, but the ring expansion rearrangement reactions25-27 of radicals 7 and 8, cyclizations of 5-hexenyl type radicals,...

The kinetic data for these reactions are numerous, as shown in Table VI. Most of values were obtained by radical clock methods. The ring expansion of radical 7 has been employed as the clock in a study that provided much of the data in Table VI.74 Cyclizations of 5-hexenyl-type radicals also have been used as clocks,75-77 and other competition reactions have been used.78 Hydrogen atom abstraction from n-Bu3GeH by primary alkyl radicals containing a trimethylsilyl group in the a-, >8-, or y-position were obtained by the indirect method in competition with alkyl radical recombi-... 

Rate constants for reactions of Bu3SnH with some a-substituted carbon-centered radicals have been determined. These values were obtained by initially calibrating a substituted radical clock on an absolute kinetic scale and then using the clock in competition kinetic studies with Bu3SnH. Radical clocks 24 and 25 were calibrated by kinetic ESR spectroscopy,88 whereas rate constants for clocks 26-31 were measured directly by LFP.19,89 90 For one case, reaction of Bu3SnH with radical 29, a rate constant was measured directly by LFP using the cyclization of 29 as the probe reaction.19... 

The rate constants for reaction of Bu3SnH with the primary a-alkoxy radical 24 and the secondary ce-alkoxy radical 29 are in reasonably good agreement. However, one would not expect the primary radical to react less rapidly than the secondary radical. The kinetic ESR method used to calibrate 24 involved a competition method wherein the cyclization reactions competed with diffusion-controlled radical termination reactions, and diffusional rate constants were determined to obtain the absolute rate constants for the clock reactions.88 The LFP calibrations of radical clocks... 

The tertiary a-ester (26) and a-cyano (27) radicals react about an order of magnitude less rapidly with Bu3SnH than do tertiary alkyl radicals. On the basis of the results with secondary radicals 28-31, the kinetic effect is unlikely to be due to electronics. The radical clocks 26 and 27 also cyclize considerably less rapidly than a secondary radical counterpart (26 with R = H) or their tertiary alkyl radical analogue (i.e., 26 with R = X = CH3), and the slow cyclization rates for 26 and 27 were ascribed to an enforced planarity in ester- and cyano-substituted radicals that, in the case of tertiary species, results in a steric interaction in the transition states for cyclization.89 It is possible that a steric effect due to an enforced planar tertiary radical center also is involved in the kinetic effect on the tin hydride reaction rate constants. 

The reaction of bornyl and isobornyl bromides with the nucleophile (Scheme 18) is another case where the amount of inversion is small and the rate constant close to that observed with an aromatic anion radical of the same standard potential (Daasbjerg et al., 1989) it can therefore be rationalized along the same lines. Cyclizable radical-probe experiments carried out with the same nucleophile and 6-bromo-6-methyl-1-heptene, a radical clock presumably slower than the preceding one, showed no cyclized coupling product. It should be noted, on the other hand, that, unlike the case... 

The one-carbon ring expansion of (17) to (18) has been accurately measured and proposed as an alternative radical clock to the 5-hexenyl radical to help determine rates in the middle regions of the kinetic scale (Scheme 8). Ab initio calculations have indicated that the isomerization of the 3-oxocyclopentylmethyl radical to the 3-oxocyclohexyl radical is energetically more favourable than the process leading to the ring-opened 5-hexenoyl radical. " ... 

Cyclizations of amidyl radicals have been studied both synthetically and kinetically. A detailed study on the rates of a variety of amidyl radical reactions was determined by both LFP and indirect competition methods (Table l) In addition, the rate constants for reactions with BusSnH and PhSH were also reported (thus giving a range of simple amidyl radical clocks). The results obtained will be useful in synthetic sequenceplanning involving amidyl radicals. 

The kinetic data reported in this chapter have been determined either by direct measurements, using for example kinetic EPR spectroscopy and laser flash photolysis techniques or by competitive kinetics like the radical clock methodology (see below). The method for each given rate constant will be indicated as well as the solvent used. An extensive compilation of the kinetics of reaction of Group 14 hydrides (RsSiH, RsGeH and RsSnH) with radicals is available [1]. 

PRIMARY ALKYL RADICALS AND FREE-RADICAL CLOCK METHODOLOGY... 

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]. 

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