Organic reaction mechanisms - 1998

3 Radical Reactions: Part 1

147

O

H

O•

OH

•

OH

H

O

OH

(56)

SCHEME 44

•

CO2 via initial • OH-induced oxidation to both H2N+• CH2CO2− and HNCH2CO2

− in

63 and 37% yields, respectively. The fragmentation of the radical cation to CO2 and

• CH2NH2 was found to be fairly fast (<100 ns).178

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Organic Reaction Mechanisms - 1998: An Annual Survey Covering the Literature Dated December 1997 to November 1998. Edited by A. C. Knipe and W. E. Watts Copyright ∂ 2003 John Wiley & Sons, Ltd.

ISBN: 0-471-49017-2

CHAPTER 4
Radical Reactions: Part 2
A. P. DOBBS and P. DIMOPOULOS
Chemistry Department, The Open University, Milton Keynes, UK
Structure and Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	153
Carbon-centred Radicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	153
Miscellaneous Radicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	157
Nitroxides and Spin Trapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	163
Oxidation and Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	166
Electron-transfer Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	172
Photo-induced Electron Transfers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	172
Other Electron Transfers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	174
Radical Cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	177
Radical Anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	183
Biradicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	186
Pyrolysis and Thermolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	191
Photolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	200
Radioloysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	202
Autoxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	209
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	209

Structure and Stability

Carbon-centred Radicals

The Whiffen effect in EPR spectroscopy and the relevant aspects of the H−C and M−C (M = metal) hyperconjugation in radicals and spin-paired molecules has been reviewed.1 A paper on the resonance structures of benzoid conjugated radicals has been published. The number of resonance structures (SC) in radical benzoid hydrocarbons is larger than the numbers of resonance structures in Kekulean´ (closed-shell) benzoid hydrocarbons. Analytical expressions for the SC of benzoid radicals have been derived.2 Enthalpies of formation of 20 cyclic and conjugated hydrocarbon radicals were calculated.3 Proton and electron affinities were calculated for a series of aliphatic and alicyclic carbanions and radicals using MP2/6–31 and B3LYP methods. Structural and electronic factors that control anion and radical stabilization were examined by natural bond orbital analysis. Structural analyses of several systems showed the importance of C−H and C−C hyperconjugation effects in both radicals and anions. No correlations between the % s-character of ionizing C−H bonds and proton affinity existed.4 The mechanisms of decomposition of primary arsines and dialkyl R2E,

153

154	Organic Reaction Mechanisms 1998
where E =5	S, Se, Te, as precursors for metal organic vapour-phase epitaxy has been
reviewed.

A CAS-MCSCF and CCSD(T) theoretical study on the mesolytic dissociations of methyland silyl-cyclopropenyl radical cations and anions has shown that the radical cations dissociate into c-C3H3+ and XH3• (X = C, Si). The radical anions fragment into c-C3H3• and XH3− (X = C, Si).6 The cleavage reactions of but-1-ene and 4,4-dimethyl pent-1-ene molecules and their cations to form neutral and charged hydrocarbon products were investigated using Hartree–Fock/density-functional theory. For the radicals studied, the isotropic coupling constants are reported and are comparable to experimental data. It was found that the experimental hyperfine properties of the but-1-ene cation could be explained by rotational averaging caused by the flat potential surface for the rotation about the C(2)−C(3) bond.7

Ab initio MO and B3LYP hybrid Hartree–Fock/density-functional (HF/DF) calculations of benzene and toluene nitrosation confirm that nitrosation proceeds through initial formation of intermediate electron-acceptor π -complexes.8 Transformation of (benzene–NO)+ and (toluene–NO)+ π -complexes into N -protonated nitroso derivatives in B3LYP and MP2 calculations occur by a novel migratory insertion of nitrogen into the aromatic C−H bond. The IMOMO (integrated MO + MO) method has been used to calculate single-bond dissociation energies for the C−H bond of benzene, the C−F bond of fluorobenzene, the C−CH3 bond of toluene, the Si−H bond of phenylsilane, the O−H bond of n-propanol, isopropanol, n-butanol and t -butanol, the C−S bond of PhCH2-SCH2, and the O−O bond of SF5O−OSF3.9 The PM3 method has been used to study the ‘breakage mechanism’ of N−NO2 and the ‘cleavage mechanism’ of C-NO2 for the decomposition of o-nitroazidobenzene.10 Ab initio calculations on the gas-phase decomposition of nitroethylene have revealed that the first stage involved the formation of a four-membered cyclic intermediate, the decomposition of which to H2CO and HCNO proceeds via a biradical intermediate.11 A coupled-cluster analysis of the electronic states of 4-aminobenzonitrile and 4-(N ,N - dimethylamino)benzonitrile has been reported.12

Polarity reversal catalysis by tri-t -butoxysilanethiol has been applied to promote

radical-chain epimerization selectively at carbon centres of the type R1R2C HOR.13

+

B3LYP and post-HF computations performed on α-substituted carbocations CH3CHR, (R = H, CH3, CH=CH2, C≡CH, F, and Cl) revealed that the substituents stabilize the cations compared with R = H in the order CH=CH2 > CH2 > C≡CH > Cl > F.14 The interactions of acetone with liquid sulfuric acid solutions have been described.15

A theoretical analysis of the reaction of H with C2H5 has shown three barrierless pathways, two leading to association and one for abstraction.16 Similarly, the reaction H + CH2CO → CH3 + CO has been studied at high temperatures and pressures.17 An ESR study of the radical species formed by pyrrole reaction with cyanoacetylene in the system KOH–DMSO was carried out.18 The effect of bridgehead substituents on the stability of 1-norbornyl radical (2) generated by electrochemical reduction of a series 4-X-substituted bicyclo[2.2.1]heptan-1-yl bromides and iodides (1) (X = H, F, Cl, Br, I, SnMe3) has been investigated by cyclic voltammetry.19 The variations in the peak reduction can be translated to values for the weakening of the C−Br and C−I bond dissociation energies upon replacement of X = H by all the substituents

4 Radical Reactions: Part 2

155

Y

•

X	X
(1)	(2)
X = H, F, Cl, Br, I, SnMe3
Y = Br, I

X, using dissociative electron-transfer theory. A through-space stabilizing interaction (homohyperconjugation) in the 4-X-substituted bicyclo[2.2.1]heptane radical species has been shown to exist.

The reactions of sodium dimethyl and diisopropyl phosphite with 4-nitrobenzyl chloride, 9-chlorofluorene, and diphenylchloromethane provided information that supported the proposed reaction mechanism. The R2PO− anion acts towards an arylmethyl chloride as a base and abstracts a proton to form a carbanion, which can then participate

in single-electron transfer processes to produce carbon-centred radicals.20

•

The 2-glycyl radical H2NCHCO2H has been generated by collisional neutralization

+

of the stable 2-glycyl cation (H2NCHCO2H) and is stable on the microsecond timescale.21 Losses of CO, water, and an amine hydrogen were calculated to be the lowest energy dissociations by combined density-functional theory and ab initio calculations. The authors concluded that depletion of the glycyl radicals in biological systems most likely occur via a bimolecular reaction.

A samarium(II) iodide-mediated cascade sequence, that leads to a highly stereoselective dimerization, has been reported. This sequence begins with an SmI2-mediated formation of a ketyl radical and leads to an alkyl radical which appears to be partially protected from further reduction to the organosamarium by ligation to the ketyl oxygen-bound samarium. This radical instead undergoes dimerization (6) and reduction to a smaller extent (7).22 The cyclization of the initially formed ketyl radical may proceed via a chair-like transition state to give the intermediate radical as a single diastereoisomer which then cyclizes to give the alkyl radical.

ω-Iodo-aldehydes (8) or -ketones in the presence of triethylborane as a radical initiator and in the presence of oxygen or light as terminator undergo 5-exo-trig cyclization to give (9).23 In these conditions a 5-exo-trig cyclization on an aldehyde is faster than on an alkene. The high reactivity of carbonyl derivatives may be attributed to the Lewis acidity of triethylborane.

The kinetics of the reaction of 2 -deoxyuridin-1 -yl radicals (11) with thiols, with superoxide release from the peroxyl radical (13) generated, have been reported.24 Radical (11) is produced by photolysis of precursor (10). When the radical is produced in the presence of thiols, (12) is formed. Second-order kinetics were found for the reactions with thiols. Peroxyl radical (13) is formed in the presence of oxygen. This

−

undergoes heterolytic fragmentation to the superoxide anion O2 • and cation (14), which ultimately leads to 2-deoxyribonolactone (15).

156	Organic Reaction Mechanisms 1998

O

LnSmO

HO

SmI2

H

t-BuOH

•

O H

HMPA/THF

•

H

H

O

O

(3)

(4)

(5)

HO

HO

H

H +

O

H

H

O

H

H

O

OH

O

(7)

Et3B, hν

(6)

HO

H

Tol, 10 οC

I

(8)

(9)

HO

Ura

HO

Ura

HO

Ura

O

O

O

CMe3

•

RSH

H

OH

O

OH

OH

(10)

(11)

(12)

HO

HO

Ura

HO

Ura

O

O

O

O

−H2O

+

−

−(O2) •

OO

OH

OH

OH

(15)

(14)

(13)

4 Radical Reactions: Part 2

157

A kinetic and mechanistic study of the reaction between toluidine blue (TB) and sulfite has shown a first-order dependence on both reactants, a stoichiometric ratio of 1:1 and pH dependence.25 The reactive species are TB+ and SO32− ions and Cu(II) acted as a promoter by facilitating the formation of a ternary complex with TB+ and SO32−.

Miscellaneous Radicals

The spin-density distribution in carbon-based peroxyl radicals was studied by densityfunctional theory at the B3LYP level. Electronegative substitution at the carbon α to the peroxyl group results in an increase of terminal hyperfine coupling and spindensity shortening of the C−O bond and lengthening of the O−O bond. In cases of steric hindrance, the C−O bond-shortening was prevented. Thiyl peroxyl radicals were reinvestigated and it was confirmed that the addition of an electron-pair donor (hydroxide) to CH3SOO• alters the spin-density contribution in the peroxyl group.26

The reaction of HO• radical with a number of dialkyl sulfides was reported to be affected by the pH, the nature of the functional group, and the chain length.27 The presence of the CH2CH2OH group results in the formation of α-thio radicals and dimer radical cation in neutral and acidic conditions. In the case of the CH2CH2CH2OH group, an intramolecular radical cation, with p-orbital overlap between oxidized sulfur and O, is observed that forms a five-membered ring. The reaction with 2,2 - thiodiethanoyl chloride leads to the formation of α-thio radicals in neutral conditions and in acidic conditions an intramolecular cation forms a four-membered ring between the oxidized sulfur and chlorine. The hydroxyl radical-induced decomposition of 2 - deoxycytidine has been reported and the products were identified.28 An explanation of the decomposition mechanism was provided. The Bell–Evans–Polanyi principle and CASSCF wavefunctions were used to locate transition structures for the unimolecular decomposition of methyldioxirane into MeCH(O• )2 and of MeCH(O• )2 into AcOH, HCO2Ac, and CO2 + CH4.29 Semiempirical UHF/PM3 calculations examined three possible mechanisms for the diphenylcarbonyl oxide (Ph2COO) bimolecular decay.30 The ‘head-to-tail’ interaction of two Ph2COO molecules has been found to be the most favourable pathway.

A report considers the reactions of 1-butoxy and 1-pentoxy radicals with oxygen (eqs 1 and 2) and of their isomerizations by 1,5-H-shift (eqs 3 and 4) using direct and time-resolved monitoring of the formation of NO2 and HO radicals in the laser flash-initiated oxidation of 1-butyl and 1-pentyl radicals.31

CH3CH2CH2CH2O + O2	−−−→ CH3CH2CH2CHO + HO2	(1)
CH3CH2CH2O + O2	−−−→ CH3CH2CHO + HO2	(2)
CH3CH2CH2CH2O −−−→ CH2CH2CH2CHOH		(3)
CH3CH2CH2CH2O −−−→ CH3CHCH2CHOH		(4)

Their rate coefficients were determined and showed that the primary alkoxy radicals have slightly higher rate coefficients for the reaction with O2 than the secondary