The effect of ring substitution on the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. A product and time-resolved kinetic study

[reaction: see text] A product and time-resolved kinetic study of the effect of ring substitution on the reactivity of 1,1-diarylalkoxyl radicals has been carried out. The radicals undergo an O-neophyl shift to give the isomeric 1-aryl-1-aryloxyalkyl radicals from which the corresponding aromatic ketones are formed. The rearrangement rate constants are influenced by ring substitution, increasing in the presence of electron-withdrawing substituents and decreasing in the presence of electron-donating ones. From the results of product and kinetic studies, the following migratory aptitudes have been obtained: 4-trifluoromethylphenyl > phenyl approximately = 4-methylphenyl > 4-methoxyphenyl. Excellent Hammett-type correlations between the sigma+ substituent constants and both the visible absorption band maxima and the rearrangement rate constants have been obtained. The experimental results indicate that the rearrangement is governed by electronic effects in the starting 1,1-diarylalkoxyl radicals, whereas the stability of the rearranged carbon-centered radical plays a minor role, in line with a reactant-like transition state, strongly supporting the hypothesis that the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals proceeds through a concerted mechanism.     

Solvent effects on the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. A laser flash photolysis study

[reaction: see text] A laser flash photolysis study has been carried out to assess solvent effects on the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. The rearrangement rate constant k decreases by increasing solvent polarity and an excellent correlation with negative slope is obtained between log k and the solvent polarity parameter E(T)N. These evidences are in full agreement with the previous indication that the extent of internal charge separation decreases on going from the starting 1,1-diarylalkoxyl radical to the transition state.     

Oxygen acidity of ring methoxylated 1,1-diarylalkanol radical cations bearing alpha-cyclopropyl groups. The competition between O-neophyl shift and C-cyclopropyl beta-scission in the intermediate 1,1-diarylalkoxyl radicals

A product and time-resolved kinetic study on the reactivity of the radical cations generated from cyclopropyl(4-methoxyphenyl)phenylmethanol (1) and cyclopropyl[bis(4-methoxyphenyl)]methanol (2) has been carried out in aqueous solution. In acidic solution, 1*+ and 2*+ display very low reactivities toward fragmentation, consistent with the presence of groups at Calpha (aryl and cyclopropyl) that after Calpha-Cbeta bond cleavage would produce relatively unstable carbon-centered radicals. In basic solution, 1*+ and 2*+ display oxygen acidity, undergoing -OH-induced deprotonation from the alpha-OH group, leading to the corresponding 1,1-diarylalkoxyl radicals 1r* and 2r*, respectively, as directly observed by time-resolved spectroscopy. The product distributions observed in the reactions of 1*+ and 2*+ under these conditions (cyclopropyl phenyl ketone, cyclopropyl(4-methoxyphenyl) ketone, and 4-methoxybenzophenone from 1*+; cyclopropyl(4-methoxyphenyl) ketone and 4,4'-dimethoxybenzophenone from 2*+) have been rationalized in terms of a water-induced competition between O-neophyl shift and C-cyclopropyl beta-scission in the intermediate 1,1-diarylalkoxyl radicals 1r* and 2r*.     

Absolute rate constants for reactions of tributylstannyl radicals with bromoalkanes, episulfides, and alpha-halomethyl-episulfides, -cyclopropanes, and -oxiranes: new rate expressions for sulfur and bromine atom abstraction

Arrhenius rate expressions were determined for the abstraction of bromine atom from 2-phenethyl bromide by tri-n-butylstannyl radical (Bu(3)Sn(*)) in benzene using transient absorption spectroscopy, (log(k(abs,Br)/M(-1) s(-1)) = (9.21 +/- 0.20) - (2.23 +/- 0.28)/theta, theta = 2.3RT kcal/mol, errors are 2sigma) and for the abstraction of sulfur atom from propylene sulfide to form propylene, (log(k(s)/M(-1) s(-1)) = (8.75 +/- 0.91) - (2.35 +/-1.33)/theta). Rate constants for reactions of organic bromides, RBr, with Bu(3)Sn(*) were found to vary as R = benzyl (15.6) > thiiranylmethyl (6.2) > oxiranylmethyl (3.1) > cyclopropylmethyl (1.3) > 2-phenethyl (1.0), with k(abs,Br) = 6.8 x 10(7) M(-1) s(-1) at 353 K for 2-phenethyl bromide. Bromine abstraction from alpha-bromomethylthiirane is about 7-fold faster than sulfur atom abstraction and is comparable to the reactivity of a secondary alkyl bromide. The potential surface for the vinylthiomethyl --> allylthiyl radical rearrangement at UB3LYP/6-31G(d) and UB3LYP/6-311+G(2d,2p) levels of theory suggests that the thiiranylmethyl radical is produced about 9 kcal/mol above the allylthiyl radical on the rearrangement surface, consistent with the observed enhancement of the Br atom abstraction from the thiirane and with synchronous C-S bond scission of the thiirane ring. The selectivities reported in this work for S vs Cl and Br abstraction provide applications for radical-based synthesis and new competition basis rate expressions for trialkylstannyl radicals.     

The O-neophyl rearrangement of 1,1-diarylalkoxyl radicals. Experimental evidence for the formation of an intermediate 1-oxaspiro[2,5]octadienyl radical

A product study on the reactivity of a 1,1-diarylalkoxyl radical bearing 2,2-diphenylcyclopropyl groups in the para-positions has been carried out. The exclusive formation of a product deriving from cyclopropyl ring-opening has been observed, indicating that 1,1-diarylalkoxyl radicals exist in equilibrium with a bridged 1-oxaspiro[2,5]octadienyl radical. This represents the first experimental evidence in support of the stepwise nature of the O-neophyl rearrangement of 1,1-diarylalkoxyl radicals.     

Room temperature and shock tube study of the reaction HCO+O2 using the photolysis of glyoxal as an efficient HCO source

The rate of the reaction 1, HCO+O2-->HO2+CO, has been determined (i) at room temperature using a slow flow reactor setup (20 mbarH2+HCO+CO, into additional HCO radicals. The rate constants of reaction 4 were determined from unperturbed photolysis experiments to be k4(295 K)=(3.6+/-0.3)x10(10) cm3 mol-1 s-1 and k4(769-1107 K)=5.4x10(13)exp(-18 kJ mol-1/RT) cm3 mol-1 s-1(Delta log k4=+/-0.12).     

Thermochemical studies of N-methylpyrazole and N-methylimidazole

The 351.1 nm photoelectron spectra of the N-methyl-5-pyrazolide anion and the N-methyl-5-imidazolide anion are reported. The photoelectron spectra of both isomers display extended vibrational progressions in the X2A' ground states of the corresponding radicals that are well reproduced by Franck-Condon simulations, based on the results of B3LYP/6-311++G(d,p) calculations. The electron affinities of the N-methyl-5-pyrazolyl radical and the N-methyl-5-imidazolyl radical are 2.054 +/- 0.006 eV and 1.987 +/- 0.008 eV, respectively. Broad vibronic features of the A(2)A' ' states are also observed in the spectra. The gas-phase acidities of N-methylpyrazole and N-methylimidazole are determined from measurements of proton-transfer rate constants using a flowing afterglow-selected ion flow tube instrument. The acidity of N-methylpyrazole is measured to be Delta(acid)G(298) = 376.9 +/- 0.7 kcal mol(-1) and Delta(acid)H(298) = 384.0 +/- 0.7 kcal mol(-1), whereas the acidity of N-methylimidazole is determined to be Delta(acid)G(298) = 380.2 +/- 1.0 kcal mol(-1) and Delta(acid)H(298)= 388.1 +/- 1.0 kcal mol(-1). The gas-phase acidities are combined with the electron affinities in a negative ion thermochemical cycle to determine the C5-H bond dissociation energies, D(0)(C5-H, N-methylpyrazole) = 116.4 +/- 0.7 kcal mol(-1) and D(0)(C5-H, N-methylimidazole) = 119.0 +/- 1.0 kcal mol(-1). The bond strengths reported here are consistent with previously reported bond strengths of pyrazole and imidazole; however, the error bars are significantly reduced.     

Thermal decomposition of the perfluorinated peroxides CF3OC(O)OOC(O)F and CF3OC(O)OOCF3

Gas phase thermal decomposition of CF(3)OC(O)OOC(O)F and CF(3)OC(O)OOCF(3) was studied at temperatures between 64 and 98 degrees C (CF(3)OC(O)OOC(O)F) and 130-165 degrees C (CF(3)OC(O)OOCF(3)) using FTIR spectroscopy to follow the course of the reaction. For both substances, the decompositions were studied with N(2) and CO as bath gases. The rate constants for the decomposition of CF(3)OC(O)OOC(O)F in nitrogen and carbon monoxide fit the Arrhenius equations k(N)2 = (3.1 +/- 0.1) x 10(15) exp[-(29.0 +/- 0.5 kcal mol(-1)/RT)] and k(CO) = (5.8 +/- 1.3) x 10(15) exp[-(29.4 +/- 0.5 kcal mol(-1)/RT)], and that for CF(3)OC(O)OOCF(3) fits the equation k = (9.0 +/- 0.9) x 10(13) exp[-(34.0 +/- 0.7 kcal mol(-1)/RT)] (all in units of inverted seconds). Rupture of the O-O bond was shown to be the rate-determining step for both peroxides, and bond energies of 29 +/- 1 and 34.0 +/- 0.7 kcal mol(-1) were obtained for CF(3)OC(O)OOC(O)F and CF(3)OC(O)OOCF(3). The heat of formation of the CF(3)OCO(2)(*) radical, which is a common product formed in both decompositions, was calculated by ab initio methods as -229 +/- 4 kcal mol(-1). With this value, the heat of formation of the title species and of CF(3)OC(O)OOC(O)OCF(3) could in turn be obtained as Delta(f) degrees (CF(3)OC(O)OOC(O)F) = -286 +/- 6 kcal mol(-1), Delta(f) degrees (CF(3)OC(O)OOCF(3)) = -341 +/- 6 kcal mol(-1), and Delta(f) degrees (CF(3)OC(O)OOC(O)OCF(3)) = -430 +/- 6 kcal mol(-1).     

Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.     

Kinetics of manganese(III) acetate in acetic acid: generation of Mn(III) with Co(III), Ce(IV), and dibromide radicals; reactions of Mn(III) with Mn(II), Co(II), hydrogen bromide,and alkali bromides

The reaction of cobalt(III) acetate with excess manganese(II) acetate in acetic acid occurs in two stages, since the two forms Co(IIIc) and Co(IIIs) are not rapidly equilibrated and thus react independently. The rate constants at 24.5 degrees C are kc = 37.1 +/- 0.6 L mol-1 s-1 and ks = 6.8 +/- 0.2 L mol-1 s-1 at 24.5 degrees C in glacial acetic acid. The Mn(III) produced forms a dinuclear complex with the excess of Mn(II). This was studied independently and is characterized by the rate constant (3.43 +/- 0.01) x 10(2) L mol-1 s-1 at 24.5 degrees C. A similar interaction between Mn(III) and Co(II) is substantially slower, with k = (3.73 +/- 0.05) x 10(-1) L mol-1 s-1 at 24.5 degrees C. Mn(II) is also oxidized by Ce(IV), according to the rate law -d[Ce(IV)]/dt = k[Mn(II)]2[Ce(IV)], where k = (6.0 +/- 0.2) x 10(4) L2 mol-2 s-1. The reaction between Mn(II) and HBr2., believed to be involved in the mechanism by which Mn(III) oxidizes HBr, was studied by laser photolysis; the rate constant is (1.48 +/- 0.04) x 10(8) L mol-1 s-1 at approximately 23 degrees C in HOAc. Oxidation of Co(II) by HBr2. has the rate constant (3.0 +/- 0.1) x 10(7) L mol-1 s-1. The oxidation of HBr by Mn(III) is second order with respect to [HBr]; k = (4.10 +/- 0.08) x 10(5) L2 mol-2 s-1 at 4.5 degrees C in 10% aqueous HOAc. Similar reactions with alkali metal bromides were studied; their rate constants are 17-23 times smaller. This noncomplementary reaction is believed to follow that rate law so that HBr2. and not Br. (higher in Gibbs energy by 0.3 V) can serve as the intermediate. The analysis of the reaction steps then requires that the oxidation of HBr2. to Br2 by Mn(III) be diffusion controlled, which is consistent with the driving force and seemingly minor reorganization.     

Bimetallo-radical carbon-hydrogen bond activation of methanol and methane

Carbon-hydrogen bond cleavage reactions of CH3OH and CH4 by a dirhodium(II) diporphyrin complex with a m-xylyl tether (.Rh(m-xylyl)Rh.(1)) are reported. Kinetic-mechanistic studies show that the substrate reactions are bimolecular and occur through the use of two Rh(II) centers in the molecular unit of 1. Second-order rate constants (T = 296 K) for the reactions of 1 with methanol (k(CH3OH) = 1.45 x 10-2 M-1 s-1) and methane (k(CH4) = 0.105 M-1 s-1) show a clear kinetic preference for the methane activation process. The methanol and methane reactions with 1 have large kinetic isotope effects (k(CH3OH)/k(CD3OD) = 9.7 +/- 0.8, k(CH4)/k(CD4) = 10.8 +/- 1.0, T = 296 K), consistent with a rate-limiting step of C-H bond homolysis through a linear transition state. Activation parameters for reaction of 1 with methanol (DeltaH = 15.6 +/- 1.0 kcal mol-1; DeltaS = -14 +/- 5 cal K-1 mol-1) and methane (DeltaH = 9.8 +/- 0.5 kcal mol-1; DeltaS = -30 +/- 3 cal K-1 mol-1) are reported.     

Enthalpies of formation, bond dissociation energies, and molecular structures of the n-aldehydes (acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal) and their radicals

Aldehydes are important intermediates and products in a variety of combustion and gas-phase oxidation processes, such as in low-temperature combustion, in the atmosphere, and in interstellar media. Despite their importance, the enthalpies of formation and bond dissociation energies (BDEs) for the aldehydes are not accurately known. We have determined enthalpies of formation for acetaldehyde, propanal, and butanal from thermodynamic cycles, using experimentally measured reaction and formation enthalpies. All enthalpy values used for reference molecules and reactions were first verified to be accurate to within around 1 kcal mol-1 using high-level ab initio calculations. Enthalpies of formation were found to be -39.72 +/- 0.16 kcal mol-1 for acetaldehyde, -45.18 +/- 1.1 kcal mol-1 for propanal, and -49.27 +/- 0.16 kcal mol-1 for butanal. Enthalpies of formation for these three aldehydes, as well as for pentanal, hexanal, and heptanal, were calculated using the G3, G3B3, and CBS-APNO theoretical methods, in conjunction with bond-isodesmic work reactions. On the basis of the results of our thermodynamic cycles, theoretical calculations using isodesmic work reactions, and existing experimental measurements, we suggest that the best available formation enthalpies for the aldehydes acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal are -39.72, -45.18, -50.0, -54.61, -59.37, and -64.2 kcal mol-1, respectively. Our calculations also identify that the literature enthalpy of formation of crotonaldehyde is in error by as much as 1 kcal mol-1, and we suggest a value of -25.1 kcal mol-1, which we calculate using isodesmic work reactions. Bond energies for each of the bonds in the aldehydes up to pentanal were calculated at the CBS-APNO level. Analysis of the BDEs reveals the R-CH(2)CH=O to be the weakest bond in all aldehydes larger than acetaldehyde, due to formation of the resonantly stabilized vinoxy radical (vinyloxy radical/formyl methyl radical). It is proposed that the vinoxy radical as well as the more commonly considered formyl and acetyl radicals are important products of aldehyde combustion and oxidation, and the reaction pathways of the vinoxy, formyl, and acetyl radicals are discussed. Group additivity values for the carbon-oxygen-hydrogen groups common to the aldehydes are also determined. Internal rotor profiles and electrostatic potential surfaces are used to study the dipole induced dipole-dipole interaction in the synperiplanar conformation of propanal. It is proposed that the loss of this dipole-dipole interaction in RC(.-)HCH(2)CH=O radicals causes a ca. 1-2 kcal mol-1 decrease in the aldehyde C-H and C-C bond energies corresponding to RC(.-)HCH(2)CH=O radical formation.     

Unimolecular reactions of chemically activated CF2BrCF2CH3 and CF2BrCF2CD3: evidence for 1,2-FBr interchange

Vibrationally excited CF2BrCF2CH3 and CF2BrCF2CD3 molecules were prepared with 96 kcal mol-1 energy at room temperature by the recombination of CF2BrCF2 and CH3 (CD3) radicals. The observed unimolecular reactions are 1,2-BrF interchange to give CF3CFBrCH3 (CD3) molecules and 2,3-FH (FD) elimination; the rate constants are 2.2 x 10(5) (1.5 x 10(5)) s(-1) and 2.0 x 105 (0.75 x 10(5)) s(-1), respectively. The CF3CFBrCH3 (CD3) molecules rapidly, relative to the reverse reaction, eliminate HBr or DBr to give the observed product CF3CF=CH2 (CD2). Density functional theory at the B3PW91/6-311+G(2d,p) level was used to obtain vibrational frequencies and moments of inertia of the molecule and transition states for subsequent calculations of statistical rate constants for CF2BrCF2CH3 and CF2BrCF2CD3. Matching experimental and calculated rate constants gave threshold energies of 62 and 66 kcal mol-1 for 1,2-BrF interchange and 2,3-FH elimination, respectively. The BrF interchange reaction is compared to ClF interchange from CF2ClCF2CH3 and CF2ClCHFCH3.     

Repair reactions of pyrimidine-derived radicals by aliphatic thiols

Pyrimidinyl radicals of various structures (Pyr*) were generated in aqueous and alcohol-containing solutions by means of pulse radiolysis to determine the rate constants of their repair reactions by different thiols (RSH = cysteamine, 2-mercaptoethanol, cysteine, and penicillamine): Pyr* + RSH --> PyrH + RS*. C5-OH and C6-OH adduct radicals of the pyrimidines react with thiols with k9 = (1.2-10.0) x 10(6) dm3 mol(-1) s(-1). Similar repair rate constants were found for uracil- and thymine-derived N1-centered radicals, k31 = (1.5-6.1) x 10(6) dm3 mol(-1) s(-1). However, pyrimidine radical anions protonated at their C6 position and C6-uracilyl radicals, with carbonyl groups at their C5 position, react with thiols faster, with k24 = (0.5-7.6) x 10(7) dm3 mol(-1) s(-1) and k14 = (1.4-4.8) x 10(7) dm3 mol(-1) s(-1), respectively. Quantum chemical calculations, at the B3LYP/6-31G(d,p) and self-consistent reaction field polarizable continuum model level point to the combined effects of the energy gap between interacting molecular orbitals, charge distribution within different pyrimidine-derived radicals, and the coefficients of the atomic orbitals as the possible reasons for the differences in the rate constants of repair.     

Rate constants for anilidyl radical cyclization reactions

[reaction: see text] N-Aryl-5,5-diphenyl-4-pentenamidyl radicals (3) were produced by 266 nm laser-flash photolysis of the corresponding N-(phenylthio) derivatives, and the rate constants for the cyclizations of these radicals were measured directly. The 5-exo cyclization reactions were fast (k(c) > 2 x 10(5) s(-1)), and radicals 3 generally behaved as electrophilic reactants with a Hammett correlation of rho = 1.9 for five of the six radicals studied. However, the p-methoxyphenyl-substituted radical 3f cyclized much faster than expected from the Hammett analysis. Variable temperature studies of parent radical 3a (aryl = phenyl) gave an Arrhenius function with log k = 9.2 - 4.4/2.3RT (kcal/mol). The rate constant for the reaction of p-ethylphenyl-substituted anilidyl radical 3b with Bu(3)SnH at 65 degrees C was k(T) = 4 x 10(5) M(-1) s(-1).     

Solvolysis of 1,1-dimethyl-4-alkenyl chlorides: evidence for pi-participation

Tertiary 1,1-dimethyl-4-alkenyl chloride (1) solvolyzes with significantly reduced secondary beta-deuterium kinetic isotope effect (substrate with two trideuteromethyl groups) and has a lower entropy and enthalpy of activation than the referent saturated analogue 4 (k(H)/k(D) = 1.30 +/- 0.03 vs k(H)/k(D) = 1.79 +/- 0.01; Delta Delta H(++) = -9 kJ mol(-1), Delta Delta S(++) = -36 J mol(-1) K(-1), in 80% v/v aqueous ethanol), indicating participation of the double bond in the rate-determining step. Transition structure 1-TS computed at the MP2(fc)/6-31G(d) level of theory revealed that the reaction proceeds through a late transition state with considerably pronounced double bond participation and a substantially cleaved C-Cl bond. The doubly unsaturated compound 3 (1,1-dimethyl-4,8-alkadienyl chloride) solvolyzes with further reduction of the isotope effect, and a drastically lower entropy of activation (k(H)/k(D) = 1.14 +/- 0.01; DeltaS(++) = -152 +/- 12 J mol(-1) K(-1), in 80% v/v aqueous ethanol), suggesting that the solvolysis of 3 proceeds by way of extended pi-participation, i.e., the assistance of both double bonds in the rate-determining step.     

One-electron oxidation of a hydrogen-bonded phenol occurs by concerted proton-coupled electron transfer

The hydrogen-bonded phenol 2-(aminodiphenylmethyl)-4,6-di-tert-butylphenol (HOAr-NH2) was prepared and oxidized in MeCN by a series of one-electron oxidants. The product is the phenoxyl radical in which the phenolic proton has transferred to the amine, *OAr-NH3+. The reaction of HOAr-NH2 and tris(p-tolyl)aminium ([N(tol)3]*+) to give *OAr-NH3+ + N(tol)3 has Keq = 2.0 +/- 0.5, follows second-order kinetics with k = (1.1 +/- 0.2) x 105 M-1 s-1 (DeltaG = 11 kcal mol-1), and has a primary isotope effect kH/kD = 2.4 +/- 0.4. Oxidation of HOAr-NH2 with [N(C6H4Br)3]*+ is faster, with k congruent with 4 x 107 M-1 s-1. The isotope effect, thermochemical arguments, and the dependence of the rate on driving force (DeltaDeltaG/DeltaDeltaG degrees = 0.53) all indicate that electron transfer from HOAr-NH2 must occur concerted with intramolecular proton transfer from the phenol to the amine (proton-coupled electron transfer, PCET). The data rule out stepwise paths that involve initial electron transfer to form the phenol radical cation *+HOAr-NH2 or that involve initial proton transfer to give the zwitterion -OAr-NH3+. The dependence of the electron-transfer rate constants on driving force can be fit with the adiabatic Marcus equation, yielding a large intrinsic barrier: lambda = 34 kcal mol-1 for reactions of HOAr-NH2 with NAr3*+.     

Molecule-induced alkane homolysis with dioxiranes.

The mechanisms of C-H and C-C bond activations with dimethyldioxirane (DMD) were studied experimentally and computationally at the B3LYP/6-311+G**//B3LYP/6-31G* density functional theory level for the propellanes 3,6-dehydrohomoadamantane (2) and 1,3-dehydroadamantane (3). The sigma(C-C) activation of 3 with DMD (Delta G(*) = 23.9 kcal mol(-1) and Delta G(r) = -5.4 kcal mol(-1)) is the first example of a molecule-induced homolytic C-C bond cleavage. The C-H bond hydroxylation observed for 2 is highly exergonic (Delta G(r) = -74.4 kcal mol(-1)) and follows a concerted pathway (Delta G(*) = 34.8 kcal mol(-1)), in contrast to its endergonic molecule-induced homolysis (Delta G(*) = 28.8 kcal mol(-1) and Delta G(r) = +9.2 kcal mol(-1)). The reactivities of 2 and 3 with CrO(2)Cl(2), which follow a molecule-induced homolytic activation mechanism, parallel the DMD results only for highly reactive 3, but differ considerably for more stable propellanes such as 4-phenyl-3,6-dehydrohomoadamantane (1) and 2.     

Polypyrroles as antioxidants: kinetic studies on reactions of bilirubin and biliverdin dimethyl esters and synthetic model compounds with peroxyl radicals in solution. Chemical calculations on selected typical structures

[reaction: see text] Rate constants for hydrogen-atom transfer (HAT) from bilirubin dimethyl ester (BRDE) and biliverdin dimethyl ester (BVDE) to peroxyl radicals during inhibited autoxidation of styrene initiated by azo-bisisobutyronitrile (AIBN) were k(inh)(BRDE) = 22.5 x 10(4) and k(inh)(BVDE) = 10.2 x 10(4) M(-1) s(-1), and the stoichiometric factors (n) were 2.0 and 2.7, respectively. A synthetic tetrapyrrole (bis(dipyrromethene)) containing the alpha-central (2,2') CH2 linkage gave k(inh) = 39.9 x 10(4) M(-1) s(-1) and n = 2.3, whereas the beta-linked (3,3') isomer was not an active antioxidant. Several dipyrrinones were synthesized as mimics of the two outer heterocyclic rings of bilirubin and biliverdin. The dipyrrinones containing N-H groups in each ring were active antioxidants, whereas those lacking two such "free" N-H groups, such as N-CH3 dipyrrinones and dipyrromethenes, did not exhibit antioxidant activity. Overall, the relative k(inh) values compared to those of phenolic antioxidants, 2,6-di-tert-butyl-4-methoxyphenol (DBHA) and 2,6-di-tert-butyl-4-methylphenol (BHT), were 2,2'-bis(dipyrromethene) > BRDE > DBHA > dipyrrinones > BVDE > BHT. This general trend in antioxidant activities was also observed for the inhibited autoxidation of cumene initiated by AIBN. Chemical calculations of the N-H bond dissociation enthalpies (BDEs) of the typical structures support a HAT mechanism from N-H groups to trap peroxyl radicals. Intramolecular hydrogen bonding of intermediate nitrogen radicals has a major influence on the antioxidant activities of all compounds studied. Indeed, chemical calculations showed that the initial nitrogen radical from a dipyrrinone is stabilized by 9.0 kcal/mol because of H-bonding between the N-H remaining on one ring and the ground-state pyrrolyl radical of the adjacent ring in the natural zusammen structure. The calculated minimum structure of bilirubin shows strong intramolecular H-bonding of the N-H groups with carbonyl groups resulting in the known "ridge-tile" structure which is not an active HAT antioxidant. The calculated minimum structure of biliverdin is planar. BRDE is readily converted into BVDE by reaction with the electron-deficient DPPH* radical under argon in chlorobenzene. An electron-transfer mechanism is proposed for the initiating step in this reaction, and this is supported by the relatively low ionizing potential of a model dipyrrole representing the two central rings of bilirubin.     

Elucidation of the electron transfer reduction mechanism of anthracene endoperoxides

The homogeneous and heterogeneous reductions of the endoperoxides 9,10-diphenyl-9,10-epidioxyanthracene (DPA-O2) and 9,10-dimethyl-9,10-epidioxyanthracene (DMA-O2) were investigated, and they were found to undergo a dissociative electron-transfer reduction of the O-O bond to yield a distonic radical anion, with no evidence for C-O bond dissociation. A number of thermochemical parameters for each were determined using Savéant's model for dissociative electron transfer (ET), including E degrees, DeltaG(o)++, and bond dissociation energies. The products of the ET are dependent on the mode of reduction, namely heterogeneous or homogeneous, and on the electrode potential or standard potential of the homogeneous donor, respectively. The dissociative reduction of DMA-O2 under heterogeneous and homogeneous conditions yields the corresponding 9,10-dihydroxyanthracene DMA-(OH)2, quantitatively, in an overall two-electron process. In the case of DPA-O2, ET reduction also yields the corresponding 9,10-dihydroxyanthracene DPA-(OH)2 from reduction of the distonic radical anion, but in competition with this reduction, an O-neophyl-type rearrangement occurs that generates a carbon radical with a minimum rate constant of 5.9 x 10(10) s(-1). In the presence of a sufficiently reducing medium, the carbon-centered radical is reduced (E degrees = -0.85 V vs SCE) and ultimately yields 9-phenoxy-10-phenyl anthracene (PPA). The observation of this product is remarkable. In the heterogeneous ET, the yield of DPA-(OH)2/PPA is 97:3 and allows an estimate of the rate constant for ET to the distonic radical anion. In homogeneous reductions, the O-neophyl rearrangement is quantitative, but the yield of PPA depends on the redox properties of the donor. A unified mechanism of reduction of DPA-O2 is presented to account for these observations.