Oxygen Rebound Research Articles

A theoretical study of reactivity and regioselectivity in the hydroxylation of adamantane by ferrate(VI).

The conversion of adamantane to adamantanols mediated by ferrate (FeO(4)(2)(-)), monoprotonated ferrate (HFeO(4)(-)), and diprotonated ferrate (H(2)FeO(4)) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the C-H bonds of adamantane via two reaction pathways, in which 1-adamantanol is formed by the abstraction of a tertiary hydrogen atom (3 degrees ) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2 degrees ). Each reaction pathway is initiated by a C-H bond cleavage via an H-atom abstraction that leads to a radical intermediate, followed by a C-O bond formation via an oxygen rebound step to lead to an adamantanol complex. The activation energies for the C-H cleavage step are 6.9 kcal/mol in the 1-adamantanol pathway and 8.4 kcal/mol in the 2-adamantanol pathway, respectively, at the B3LYP/6-311++G level of theory, whereas those of the second reaction step corresponding to the rebound step are relatively small. Thus, the rate-determining step in the two pathways is the C-H bond dissociation step, which is relevant to the regioselectivity for adamantane hydroxylation. The relative rate constant (3 degrees )/(2 degrees ) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 degrees C, which is fully consistent with an experimental value of 10.1.

A theoretical study on the mechanism of camphor hydroxylation by compound I of cytochrome p450.

Mechanistic and energetic aspects for the conversion of camphor to 5-exo-hydroxycamphor by the compound I iron-oxo species of cytochrome P450 are discussed from B3LYP DFT calculations. This reaction occurs in a two-step manner along the lines that the oxygen rebound mechanism suggests. The activation energy for the first transition state of the H atom abstraction at the C5 atom of camphor is computed to be more than 20 kcal/mol. This H atom abstraction is the rate-determining step in this hydroxylation reaction, leading to a reaction intermediate that involves a carbon radical species and the iron-hydroxo species. The second transition state of the rebound step that connects the reaction intermediate and the product alcohol complex lies a few kcal/mol below that for the H atom abstraction on the doublet and quartet potential energy surfaces. This energetic feature allows the virtually barrierless recombination in both spin states, being consistent with experimentally observed high stereoselectivity and brief lifetimes of the reaction intermediate. The overall energetic profile of the catalytic mechanism of camphor hydroxylation particularly with respect to why the high activation energy for the H atom abstraction is accessible under physiological conditions is also considered and calculated. According to a proton source model involving Thr252, Asp251, and two solvent water molecules (Biochemistry 1998, 37, 9211), the energetics for the conversion of the iron-peroxo species to compound I is studied. A significant energy over 50 kcal/mol is released in the course of this dioxygen activation process. The energy released in this chemical process is an important driving force in alkane hydroxylation by cytochrome P450. This energy is used for the access to the high activation energy for the H atom abstraction.

Methane Hydroxylation by Methane Monooxygenase: On the Problem of the Process Dynamics

Published data on the kinetic isotope effects of the hydroxylation of deuterium-substituted methane molecules (CHD3, CH2D2, and CH3D) by methane monooxygenase are examined in the framework of the two-step nonradical mechanism through the intermediate formation of a complex containing pentacoordinate carbon. The kinetic schemes with the first step involving one, two, and three hydrogen atoms of the oxidized substrate are considered. Contrary to the widely accepted oxygen rebound mechanism, the experimental results obtained for the oxidation of various substrates by methane monooxygenase and cytochrome P450 can be explained from the viewpoint of the dynamics of a general nonradical mechanism.

Mechanism of the oxidation of aromatic sulfides catalysed by a water soluble iron porphyrin.

The oxygen atom transfer-electron transfer (ET) mechanistic dichotomy has been investigated in the oxidation of a number of aryl sulfides by H2O2 in acidic (pH 3) aqueous medium catalysed by the water soluble iron(III) porphyrin 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p',p",p"'-tetrasulfonic acid iron(III) chloride (FeTPPSCl). Under these reaction conditions, the iron-oxo complex porphyrin radical cation, P+. Fe(IV)=O, should be the active oxidant. When the oxidation of a series of para-X substituted phenyl alkyl sulfides (X = OCH3, CH3, H, Br, CN) was studied the corresponding sulfoxides were the only observed product and the reaction yields as well as the reactivity were little influenced by the nature of X as well as by the bulkiness of the alkyl group. Labelling experiments using H(2)18O or H(2)18O2 clearly indicated that the oxygen atom in the sulfoxides comes exclusively from the oxidant. Moreover, no fragmentation products were observed in the oxidation of a benzyl phenyl sulfide whose radical cation is expected to undergo cleavage of the beta C-H and C-S bonds. These results would seem to suggest a direct oxygen atom transfer from the iron-oxo complex to the sulfide. However, competitive experiments between thioanisole (E degree = 1.49 V vs. NHE in H2O) and N,N-dimethylaniline (E degree = 0.97 V vs. NHE in H2O) resulted in exclusive N-demethylation, whereas the oxidation of N-methylphenothiazine (10, E degree = 0.95 V vs. NHE in CH3CN) and N,N-dimethyl-4-methylthioaniline (11, E degree = 0.65 V vs. NHE in H2O) produced the corresponding sulfoxide with complete oxygen incorporation from the oxidant. Since an ET mechanism must certainly hold in the reactions of 10 and 11, the oxygen incorporation experiments indicate that the intermediate radical cation, once formed, has to react with PFe(IV)=O (the reduced form of the iron-oxo complex which is formed by the ET step) in a fast oxygen rebound. Thus, an ET step followed by a fast oxygen rebound is also suggested for the other sulfides investigated in this work.

Thoughts on thiolate tethering. Tribute and thanks to a teacher.

A unique feature of P450 enzymes is in the presence of a thiolate ligand heme but its exact function in catalysis is a matter of debate. For P450 dependent monooxygenases the "active oxygen" complex seems to exist only as a transition state in which the thiolate ligand provides electron density in order to prevent pi-backbonding of the oxygen to the iron (-S-Fe-O(z.rad;)). The corresponding ground state (Compound I) would be a ferryl species (Fe(IV)z.dbnd6;O) with an electron hole either at the porphyrin or at the sulfur. Apart from this role we postulate that a second function is related to the electronic structure of Compound II as an electron acceptor and this property is shared among monooxygenases, thromboxane synthase, prostacyclin synthase, allene oxide synthase, P450(NOR(-)) and chloroperoxidase. As a common step in all P450 enzymes an extremely rapid electron uptake by Compound II allows that the primary substrate radicals are oxidized to cations which immediately combine with a neighbouring nucleophile. Thus "electron transfer" may substitute for "oxygen rebound" as the final step leading to product formation. The same principle also applies methane monooxygenases in which the role of the thiyl sulfur is replaced by a ferryl-oxyl entity.

Catalytic mono-oxygenation reactions using mono- and bi-nuclear ruthenium amide complexes as the homogeneous catalysts

Mono-nuclear amide complexes, their substituted complexes and bi-nuclear amide complexes of ruthenium, reported earlier by us, have been used as catalysts for the epoxidation of olefins. The complexes exhibit low to moderate catalytic activity. The electronic spectra and electrochemical study of the catalyst does not show any change on the addition of PhIO, ruling out the involvement of the metal-oxo intermediate and hence the oxygen rebound mechanism. The catalytic reaction has been explained on the basis of lewis acid behaviour of the ruthenium centre in these complexes.

Oxygen transfer reactions. 4. Reaction of high valent oxoruthenium compounds with sulfides.

The oxidation of methoxy substituted benzyl phenyl sulfides can be used to distinguish between oxidants that react by single electron transfer (followed by oxygen rebound) and those which react by direct oxygen atom transfer in a two-electron process. Transfer of a single electron results in the formation of an intermediate radical cation, which can undergo C-S bond cleavage and deprotonation reactions leading to the formation of methoxy substituted benzyl derivatives, methoxy substituted benzaldehydes, and diphenyl disulfide. The oxidation of 4-methoxybenzyl phenyl sulfide and 3,4,5-trimethoxybenzyl phenyl sulfide by oxidants known to participate in single electron transfers (Ce(4+), Mn(3+), and Cr(6+)) results in the formation of the corresponding benzaldehydes, benzyl alcohols, benzyl acetates, and benzyl nitrates in variable yields. However, the only products obtained from the oxidation of the same compounds with RuO(4), RuO(4-), and RuO(4)(2-) are sulfoxides and sulfones. Therefore, it is concluded that the oxidation of sulfides by oxoruthenium compounds likely proceeds by a concerted mechanism.

N-OXIDATIVE TRANSFORMATION OF FREE AND N-SUBSTITUTED AMINE FUNCTIONS BY CYTOCHROME P450 AS MEANS OF BIOACTIVATION AND DETOXICATION

Indirect evidence for the participation of cytochrome P450 (P450) in the microsomal N-oxygenation of primary and N-substituted amine functions is presented by studies employing diagnostic modifiers of the hemoprotein system as well as immunochemical approaches. Experiments with recombinant hemoproteins or isozymes purified from the tissues of various animal species support the results obtained by the inhibitor assays. Amine substrates and the redox proteins of the microsomal electron transfer chain reveal to be mutually beneficial in interactions with P450s. Numerous N-substituted amines undergo P450-catalyzed N-oxidative transformation despite the presence of accessible α-C hydrogens in these structures rather thought to favor N-dealkylation. In these instances, stabilization of the initially formed aminium radicals by the specific active site orientation of the particular P450s obviously permits oxygen rebound. Apart from common iron-oxenoid chemistry involving a (FeO)3+ species, iron-bound hydroperoxide, (FeO2H)3+, appears to act as an electrophilic oxidant with certain N-substituted amines and P450 subforms. Generally, P450-mediated N-oxygenation of amines can produce cytotoxic and mutagenic metabolites, but equally can well yield hydrophilic products, that are readily excreted and thus promote detoxication.

Intermediate Q from soluble methane monooxygenase hydroxylates the mechanistic substrate probe norcarane: evidence for a stepwise reaction.

Norcarane is a valuable mechanistic probe for enzyme-catalyzed hydrocarbon oxidation reactions because different products or product distributions result from concerted, radical, and cation based reactions. Soluble methane monooxygenase (sMMO) from Methylosinus trichosporium OB3b catalyzes the oxidation of norcarane to afford 3-hydroxymethylcyclohexene and 3-cycloheptenol, compounds characteristic of radical and cationic intermediates, respectively, in addition to 2- and 3-norcaranols. Past single turnover transient kinetic studies have identified several optically distinct intermediates from the catalytic cycle of the hydroxylase component of sMMO. Thus, the reaction between norcarane and key reaction intermediates can be directly monitored. The presence of norcarane increases the rate of decay of only one intermediate, the high-valent bis-mu-oxo Fe(IV)(2) cluster-containing species compound Q, showing that it is responsible for the majority of the oxidation chemistry. The observation of products from both radical and cationic intermediates from norcarane oxidation catalyzed by sMMO is consistent with a mechanism in which an initial substrate radical intermediate is formed by hydrogen atom abstraction. This intermediate then undergoes either oxygen rebound, intramolecular rearrangement followed by oxygen rebound, or loss of a second electron to yield a cationic intermediate to which OH(-) is transferred. The estimated lower limit of 20 ps for the lifetime of the putative radical intermediate is in accord with values determined from previous studies of sterically hindered sMMO probes.

Oxidation of 2,6-dimethylaniline by recombinant human cytochrome P450s and human liver microsomes.

2,6-Dimethylaniline (2,6-DMA) is classified as a rodent nasal cavity carcinogen and a possible human carcinogen. The major metabolite of 2,6-DMA in rats and dogs is 4-amino-3,5-dimethylphenol (DMAP) but oxidization of the amino group to produce metabolites such as N-(2,6-dimethylphenyl)hydroxylamine (DMHA) is also indicated by the occurrence of hemoglobin adducts of 2,6-DMA in human and rats. Previous studies have shown a large interindividual variability in human 2,6-DMA hemoglobin adduct levels. In the present study, 2,6-DMA oxidation in vitro by human liver microsomes and recombinant human P450 enzymes was investigated to assess whether the hemoglobin adduct variability could be attributed to metabolic differences. At micromolar concentrations, the only product detectable (UV) was DMAP, while at 10 nM, DMHA was a substantial product. 2E1 and 2A6 were identified as the major P450s in human liver microsomes responsible for the production of DMAP by using P450-specific chemical inhibitors and mouse monoclonal antibodies that selectively inhibit human P450 2E1 and 2A6. 2A6 was identified as the major P450 responsible for the N-hydroxylation. Native P450 2E1 and human liver microsomes catalyzed the rearrangement of DMHA to DMAP independent of NADPH. Consistent with a mechanism involving oxygen rebound to the heme iron center, labeled oxygen was not incorporated into DMAP from either 18O2 gas or H2 18O in this rearrangement. Results presented here suggest much of the observed interindividual variability of 2,6-DMA hemoglobin adduct levels could be due to differences in the relative amounts of hepatic 2E1 and 2A6.

Energetics for the Oxygen Rebound Mechanism of Alkane Hydroxylation by the Iron-Oxo Species of Cytochrome P450

Abstract Density-functional-theory calculational results on the reaction pathway for alkane hydroxylation by a compound I model of cytochrome P450 are discussed. Our calculations demonstrate that the transition state for the H-atom abstraction of ethane involves a linear (Fe)O…H…C array and that the resultant carbon radical is bound to the iron-hydroxo species. This comptational result is partly consistent with the oxygen rebound mechanism in that the direct H-atom abstraction by the iron-oxo species takes place in the initial stages of the reaction pathway. However, the iron-hydroxo species cannot be viewed as a stable reaction intermediate in view of the energy diagram. Our results may not be consistent with the model of a free radical species with a finite lifetime and barrier to displacement of the OH group from the iron center that is commonly assumed and typically stated for the oxygen rebound mechanism.

Biomimetical catalytic activity of iron(III) porphyrins encapsulated in the zeolite X

The approach adopted for the obtention of zeolite-encapsulated FeP led to clean syntheses of biomimetical catalyst. The catalysts were obtained through the zeolite synthesis method, where NaX zeolite was synthesised around one of the cationic FePs: iron(III) 5,10,15,20-tetrakis(4- N-methylpyridyl)porphyrin (FeP 1) or iron(III) 5-mono(2,6-dichloro-phenyl)10,15,20-tris(4- N-methylpyridyl)porphyrin (FeP 2). The syntheses yielded pure FeP 1NaX and FeP 2NaX catalysts without any by-products blocking the zeolite nanopores. FeP 1NaX and FeP 2NaX efficiently catalysed the epoxidation of ( Z)-cyclooctene by iodosylbenzene (PhIO) in DCE, giving rise to cis-epoxycyclooctane yields of 85% and 95%, respectively. Hydroxylation of adamantane shows a preferable alkane oxidation at the tertiary CH bond, indicating a hydrogen abstraction through the Fe IV(O)P ·+species in the initial step. The total adamantanol yields were 52% and 45% for FeP 1NaX and FeP 2NaX, respectively. Concerning selectivity, FeP 1NaX and FeP 2NaX gave an 1-adamantanol (Ad-1-ol)/2-adamantanol (Ad-2-ol) ratio of 20:1 and 11:1, respectively (after statistical correction). Therefore, these results indicate a free radical activation of the CH bonds of adamantane as expected for P-450 models. In the cyclohexane oxidation catalysed by FeP 1NaX in DCE, a cyclohexanol (C 6-ol) yield of 50% and an alcohol/ketone ratio of 10 was obtained. The hydroxylation occurs according to the so-called oxygen rebound mechanism, as expected for a P-450 model system. FeP 2NaX is less selective (C 6-ol yield=25% and alcohol/ketone=1.2). One possible explanation is that a Russell-type mechanism involving O 2 imprisoned within the zeolite cages may be operating parallelly, generating both C 6-ol and cyclohexanone.

Manganese Lipoxygenase: DISCOVERY OF A BIS-ALLYLIC HYDROPEROXIDE AS PRODUCT AND INTERMEDIATE IN A LIPOXYGENASE REACTION

Linoleic acid was incubated with manganese lipoxygenase (Mn-LO) from the fungus Gäumannomyces graminis. The product consisted of (13R)-hydroperoxy-(9Z,11E)-octadecadienoic acid ((13R)-HPOD) and a new hydroperoxide, (11S)-hydroperoxy-(9Z,12Z)-octadecadienoic acid ((11S)-HPOD). Incubation of (11R)-[2H]- and (11S)-[2H]linoleic acids with Mn-LO led to the formation of hydroperoxides that largely retained and lost, respectively, the deuterium label. Conversion of the (11S)-deuteriolinoleic acid was accompanied by a primary isotope effect, which manifested itself in a strongly reduced rate of formation of hydroperoxides and in a time-dependent accumulation of deuterium in the unconverted substrate. These experiments indicated that the initial step catalyzed by Mn-LO consisted of abstraction of the pro-S hydrogen of linoleic acid to produce a linoleoyl radical. (11S)-HPOD was converted into (13R)-HPOD upon incubation with Mn-LO. The mechanism of this enzyme-catalyzed hydroperoxide rearrangement was studied in experiments carried out with 18O2 gas or 18O2-labeled hydroperoxides. Incubation of [11-18O2](11S)-HPOD with Mn-LO led to the formation of (13R)-HPOD, which retained 39-44% of the 18O label, whereas (11S)-HPOD incubated with Mn-LO under 18O2 produced (13R)-HPOD, which had incorporated 57% of 18O. Furthermore, analysis of the isotope content of (11S)-HPOD remaining unconverted in such incubations demonstrated that [11-18O2](11S)-HPOD suffered a time-dependent loss of 18O when exposed to Mn-LO, whereas (11S)-HPOD incorporated 18O when incubated with Mn-LO under 18O2. On the basis of these experiments, it was proposed that the conversion of (11S)-HPOD into (13R)-HPOD occurred in a non-concerted way by deoxygenation into a linoleoyl radical. Subsequent reoxygenation of this intermediate by dioxygen attack at C-13 produced (13R)-HPOD, whereas attack at C-11 regenerated (11S)-HPOD. The hydroperoxide rearrangement occurred by oxygen rebound, although, as demonstrated by the 18O experiments, the oxygen molecule released from (11S)-HPOD exchanged with surrounding molecular oxygen prior to its reincorporation.

Oxidations of benzyl and phenethyl phenyl sulfides. Implications for the mechanism of the microsomal and biomimetic oxidation of sulfides

The study of the oxidation of 4-methoxyphenethyl phenyl sulfide and 3,4-dimethoxy-phenethyl phenyl sulfide with potassium 12-tungstocobalt(III)ate [Co(III)W] suggests that in the radical cations of 3,4,5-(MeO) 3PhCH 2SPh ( 4) and 2,4,6(MeO) 3PhCH 2SPh ( 5) the positive charge is not localized on the sulfur atom, but in the benzylic aromatic ring. Nevertheless, in the biomimetic and microsomal oxidation of 4 and 5 the products observed are exclusively sulfoxides and sulfones, which appears in contrast with a mechanism involving the formation of an intermediate sulfide radical cation followed by a fast oxygen rebound. A direct oxygen transfer mechanism seems most likely.

Origin of the Oxygen Atom in C-H Bond Oxidations Catalyzed by a Water-Soluble Metalloporphyrin.

The monopersulfate oxidation of 4-isopropylbenzoic acid performed in H(2)(18)O and catalyzed by a water-soluble metalloporphyrin indicated that half of the oxygen atoms incorporated in 4-(1-hydroxy-1-methylethyl)benzoic acid, the primary hydroxylation product, came from water. A redox tautomerism of the active high-valent hydroxo-metal-oxo porphyrin intermediate coupled with an oxygen rebound mechanism explained this result. Under similar conditions, ketoprofen was directly oxidized to 3-benzoylacetophenone, via at least two different reaction pathways. Trapping of radical intermediates by molecular oxygen competed with the oxygen rebound mechanism.

Iron–Phthalocyanine Immobilized on Activated Carbon Black: A Selective Catalyst for Alkane Oxidation

Carbon black is tested as a support for iron–phthalocyanine within the frame of the oxidation of hydrocarbons witht-butylhydroperoxide as oxygen donor. The increased hydrophobicity of the carrier surface, with respect to zeolite Y, changes the adsorption behavior of the components in the reaction mixture towards the alkane. A major improvement in the oxidation conversion and efficiency of cyclohexane has been established. Furthermore, the kinetic isotope effect and the reactivity order of secondary and tertiary carbon atoms measured with adamantane provide evidence for an “oxygen rebound” reaction mechanism, a non-free-radical oxidation pathway where the metallo-complex is responsible for the hydrogen abstraction.

Oxidation of 9-alkylanthracenes by cytochrome P450 2B1, horseradish peroxidase, and iron tetraphenylporphine/iodosylbenzene systems: anaerobic and aerobic mechanisms.

Variously substituted alkylanthracenes were studied as models for polycyclic hydrocarbon oxidations. 9-Methylanthracene was oxidized to 9-(hydroxymethyl)anthracene, 10-methyl-10-hydroxy-9-anthrone, and anthraquinone in several systems, including (i) NADPH- and O2-fortified rat liver microsomes, (ii) cytochrome P450 (P450) 2B1 Supported by either iodosylbenzene (PhIO) or a mixture of NADPH-P450 reductase, NADPH, and O2, (iii) horseradish peroxidase and either H2O2 or ethyl hydroperoxide, and (iv) a mixture of iron tetraphenylporphine (FeTPP) and PhIO (in anhydrous CH2Cl2/MeOH). The microsomal system also formed dihydrodiols from 9-methyl- and 9-ethylanthracenes. The formation of the three oxidized products by the P450/NADPH/O2 system was dependent upon O2 label from 18O2 was incorporated into the products, and no label from H2(18O) was incorporated. No label from 18O2 was incorporated into the three products in the FeTPP/PhIO system. In the horseradish peroxidase/H2O2 system, the formation of the three products was decreased when O2 was omitted, and label from both H2(18)O and 18O2 was incorporated into all three products. The results are interpreted in terms of three mechanisms. One is used by the FeTPP and P450 systems, with all oxygen transfers coming from an FeO entity. The other two pathways are utilized by horseradish peroxidase and begin with formation of a radical cation, which can undergo reactions either with H2O or with O2 to form the products detected here. The involvement of a 9-methylanthracene radical cation in the P450 and FeTPP pathways is a possibility, but rapid rearrangement and oxygen rebound must be invoked. Comparisons of products from various 9-alkylanthracenes do not provide evidence that one-electron oxidation is an integral part of the epoxidation process with these compounds. The significance of the lack of trapping of radical (by H2(18O) in the P450 reactions to DNA adduct formation is considered.

Oxidation of Sulfides by Peroxidases. Involvement of Radical Cations and the Rate of the Oxygen Rebound Step

Oxidation of Sulfides by Peroxidases. Involvement of Radical Cations and the Rate of the Oxygen Rebound Step

Five-coordinate carbon hydroxylation mechanism

Oxygen rebound mechanism for hydrocarbon hydroxylation by high-valent metal complexes is critically analyzed and is shown to be contradictory in certain aspects. The synchronous insertion of O atom mechanism is found to be significantly less contradictory (at least for the most interesting cases of strong CH bonds, for example in methane) if it is supplemented by the stage of formation of the five-coordinate carbon intermediate complex of a hydrocarbon molecule via oxygen atom of an active center.

Theoretical model for an alternate mechanism for the cytochrome P-450 hydroxylation of quadricyclane

During the past decade the mechanism of enzymatic hydroxylation of saturated hydrocarbons has come under increased scrutiny.’ The widely accepted mechanism for the catalytic cycle for cytochrome P-450 monooxygenase hydroxylation of alkanes is thought to involve the recombination of an iron-bound hydroxyl radical with a carbon-centered free radical in a process termed “oxygen rebound”.2 Another related area of chemistry that has developed simultaneously is the use of “radical clock’ substrates to estimate the magnitude of the rate constant for the oxygen rebound step ( k o ~ ) . If alkane hydroxylation is initiated by hydrogen abstraction, then the free-radical intermediate may be hydroxylated directly (&) or it may undergo rearrangement (k,) and subsequent recombination to afford a rearranged alcohol as exemplified for the P-450 hydroxylation of bicyclo[2.1 .O]pentane (Scheme 1). Ortiz de Montellano and reported a 7:l ratio of unrearranged (4) to rearranged (5) alcohols, implying that the oxygen rebound step is about seven times faster than the ring-opening of bicyclo[2.1 .O]pent2-yl radical (2) in Scheme 1. We predict a classical activation barrier for rearrangement of 2 to cyclopenten-4-yl radical 3 of 6.8 kcdmol (AG*298 = 6.23 kcaVmol) at the PMP4 level and a rate constant at 25 “C of k, = 1.7 x lo8 s ’ . ~ A recent estimate of the rate constant for radical recombination to form unrearranged alcohol 4 is OH = 2.2 x 1O’O During the past several years the kinetic scale for these very rapid radical reactions has approached the theoretical limit for the frequency factor kTlh = Measured unimolecular rate constants for rearrangement of aryl-substituted cyclopropylcarbinyl radical of 5 x 10’’ s-’ have been r e p ~ r t e d ~ ~ . ~ requiring an estimated rebound rate constant for enzymatic hydroxylation8 with methane monooxygenase of kOH > 1OI3 SKI.