Nature of the chain propagation in the photostimulated reaction of 1-bromonaphthalene with sulfur-centered nucleophiles

The reactivity of -SC(NH)NH2 (1), MeCOS- (2), and PhCOS- (3) toward 1-naphthyl radicals was studied in DMSO. The photostimulated reaction of anions 1, 2, and 3 with 1-bromonaphthalene (4) after quenching with MeI renders 1-(methylthio)naphthalene (6) as a main product together with bis(1-naphthyl) sulfide (7) and naphthalene (5). The thioacetate ion (2) and thiobenzoate ion (3) were unreactive toward 4 as electron-donor under photostimulation; however, in the presence of potassium tert-butoxide anion (entrainment conditions), they gave the mentioned products 5, 6, and 7, after the addition of MeI. Quenching of the triplet state of 4 was assigned as the photoinduced initiation step, with a rate constant value of (4.6+/-0.5)x10(8) M-1 s-1 for tert-butoxide anion and a rough estimated value of (8+/-7)x10(7) M-1 s-1 for anion 1. By using hydrogen abstraction from DMSO as the competitive reaction, the absolute rate constants for the addition of anions 1, 2, and 3 to 1-naphthyl radicals have been determined to be 1.0x10(9), 1.2x10(9), and 3.5x10(9) M-1 s-1, respectively. This reactivity order is in agreement with the stability of the resulting radical anions (ArNu)*- (10-12)*-. The inhibition experiments of the photoinduced substitution reaction in the presence of radical scavengers and the global quantum yield higher than the unity are evidence of a radical chain mechanism for these substitution reactions by anions 1 and 2. Anion 3 adds to the 1-naphthyl radical, but is neither able to initiate nor to keep the propagation cycle. Evaluation of the electron-transfer driving forces for the reaction between (ArNu)*- and 4 together with the absence of a chain reaction for the anion 3 indicate that the propagation in the proposed mechanism is given by an acid-base reaction between the radical .C(O)Me or .C(NH)NH2 (13) and a base.     

S(RN)1 Reactions of 7-Iodobicyclo[4.1.0]heptane, 1-Iodoadamantane, and Neopentyl Iodide with Carbanions Induced by FeBr(2) in DMSO

There was no reaction of 7-iodobicyclo[4.1.0]heptane (7-iodonorcarane, 1) (exo-endo ratio of ca. 1) with acetophenone enolate ions 2 in DMSO at 25 degrees C; however, with the addition of SmI(2) or FeBr(2) and under the same experimental conditions, the substitution product 3 was obtained in 9% and 72% yields, respectively, with an exo-endo ratio of ca. 16 similar to the product ratio from photostimulated reactions. Thus, it seems that 7-norcaranyl radicals are intermediates of these reactions. With FeBr(2) at 60 degrees C the yield of 3 was as high as 90%. Reactions of 1 with the enolate ion of 2-naphthyl methyl ketone 4 induced by FeBr(2) gave substitution product 5 in 60% yield (96% of it the exo isomer). In competition experiments, 4 was 1.7 times more reactive than 2, and the anion of nitromethane (7) was 6.5 times more reactive than 2 toward 7-norcaranyl radicals. The reactions of 1-iodoadamantane (9) and neopentyl iodide (11) with carbanion 2 induced by FeBr(2) gave the substitution products in 85% and 92% yields, respectively. These observations indicate that all these reactions induced by FeBr(2) occur by the S(RN)1 mechanism.     

Quantum yields of the initiation step and chain propagation turnovers in S(RN)1 reactions: photostimulated reaction of 1-iodo-2-methyl-2-phenyl propane with carbanions in DMSO

Neophyl radicals were generated by photoinduced electron transfer (PET) from a suitable donor to the neophyl iodide (1, 1-iodo-2-methyl-2-phenylpropane). The PET reaction of 1 with the enolate anion of cyclohexenone (2) afforded mainly the reduction products tert-butylbenzene (5) and the rearranged isobutylbenzene (6), arising from hydrogen abstraction of the neophyl radical (15) and the rearranged radical 16 intermediates, respectively. The photostimulated reaction of 1 with 2 in the presence of di-tert-butylnitroxide, as a radical trap, afforded adduct 10 in 57% yield. The photoinduced reaction of the enolate anion of acetophenone (3) with 1 gave the substitution products 11 (50%) and 12 (16%), which arise from the coupling of 3 with radicals 15 and 16, respectively. The rate constant obtained for the addition of anion 3 to radical 15 was 1.2 x 10(5) M(-)(1) s(-)(1), by the use of the rearrangement of this radical as a clock reaction. The anion of nitromethane (4) was almost unreactive at the initiation step, but in the presence of 2 under irradiation, it gave high yields (67%) of the substitution product 13 and only 2% of the rearranged product 14. When the ratio of 4 to 1 was diminished, it was possible to observe both substitution products 13 and 14 in 16% and 6.4% yields, respectively. These last results allowed us to estimate the coupling rate constant of neophyl radicals 15 with anion 4 to be at least of the order of 10(6) M(-)(1) s(-)(1). Although the overall quantum yield determined (lambda = 350 nm) for the studied reactions is below 1, the chain lengths (Phi(propagation)) for the reaction of 1 with anions 3 and 4 are 127 and 2, respectively.     

Mechanism of carbon-halogen bond reductive cleavage in activated alkyl halide initiators relevant to living radical polymerization: theoretical and experimental study

The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX(?-) through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX(?-), which is shown here to be nonexistent.     

Stepwise and concerted pathways in photoinduced and thermal electron-transfer/bond-breaking reactions. experimental illustration of similarities and contrasts.

The electrochemical (cyclic voltammetry) and photoinduced (fluorescence quenching, quantum yields) reductive cleavages of four compounds, 4-cyano-alpha-trifluorotoluene (1), dimethylphenyl sulfonium (2), 4-cyanobenzylmethylphenyl sulfonium (3), and 4-cyanobenzyl chloride (4), are investigated and compared in terms of concerted vs stepwise mechanisms. Bearing in mind that an increase of the thermodynamic driving force shifts the mechanism from concerted to stepwise and that the driving force is larger under photochemical than under electrochemical conditions, 1 and 2 are typical examples where a stepwise mechanism is followed with compatible kinetic characteristics under both regimes. 4 undergoes a concerted electrochemical reductive cleavage, and the same mechanism is followed in the photoinduced reaction with consistent kinetic characteristics. The case of 3 is of particular interest, since a trend of passing from a concerted to a stepwise mechanism when going from the electrochemical to the photochemical conditions indeed appears upon analysis of the experimental results. The change of mechanism is, however, not complete since, in the photoinduced reaction, there is a balanced competition between the two pathways. In the same families of compounds, the unsubstituted benzylmethylphenyl sulfonium cations shows such a borderline behavior during the electrochemical reaction. In the photoinduced reaction, it is the 4-cyano derivative which behaves in a borderline manner, in line with the fact that it gives rise more readily to a concerted mechanism than the unsubstituted compound.     

Regiochemistry of the photostimulated reaction of the phthalimide anion with 1-iodoadamantane and tert-butylmercury chloride by the S(RN)1 mechanism

The photostimulated reaction of the phthalimide anion (1) with 1-iodoadamantane (2) gave 3-(1-adamantyl) phthalimide (3) (12%) and 4-(1-adamantyl) phthalimide (4) (45%), together with the reduction product adamantane (AdH) (21%). The lack of reaction in the dark and inhibition of the photoinduced reaction by p-dinitrobenzene, 1,4-cyclohexadiene, and di-tert-butylnitroxide indicated that 1 reacts with 2 by an S(RN)1 mechanism. Formation of products 3 and 4 occurs with distonic radical anions as intermediates. The photoinduced reaction of anion 1 with tert-butylmercury chloride (10) affords 4-tert-butylphthalimide (11) as a unique product. By competition experiments toward 1, 1-iodoadamantane was found to be ca. 10 times more reactive than tert-butylmercury chloride.     

Methyl and silyl mesolytic dissociations in the radical cations and radical anions of but-1-ene,allylsilane, hexa-1,3-diene,and penta-2,4-dienylsilane. CAS-MCSCF and coupled cluster theoretical study

Methyl or silyl dissociation in the CH(2)=CHCH(2)-XH(3) (a-XH(3)(*)(+)) and CH(2)=CHCH=CHCH(2)-XH(3) (p-XH(3)(*) (+)) radical cations (X = C, Si) yields a(+) or p(+) and XH(3)(*). Similarly, the radical anions a-CH(3)(*) (-) and p-CH(3)(*) (-) give the pi-delocalized anion and CH(3)(*) preferentially. In contrast, a-SiH(3)(*) (-) and p-SiH(3)(*-) prefer to dissociate into the pi-delocalized radical and silide. All reactions are endoergic: by 43-50 kcal mol(-)(1) in the radical cations, and easier to some extent in the radical anions, that require 29-33 (X = C) and 13-14 kcal mol(-)(1) (X = Si). The fragmentation energy profiles do not present significant barriers for the backward process in the case of the radical cations. All radical anions exhibit an energy maximum along the dissociation pathway, but the barrier is lower than the dissociation limit. Fragmentation is "activated" more in the anions than in the cations with respect to homolysis in the corresponding neutrals (that requires 72-81 kcal mol(-)(1)). Wave function analysis indicates that the C-X bond cleavage in the hydrocarbon radical ions, although formally comparable to a homolytic process, is at variance with this model, due to the spin recoupling of one of the two C-X bond electrons with the originally unpaired electron. This is basically true also for the silyl-substituted radical anions, in which the initial more delocalized charge distribution might suggest some heterolytic character of the bond cleavage.     

Scandium ion-promoted photoinduced electron transfer from electron donors to acridine and pyrene. Essential role of scandium ion in photocatalytic oxygenation of hexamethylbenzene

Photoinduced electron transfer from a variety of electron donors including alkylbenzenes to the singlet excited state of acridine and pyrene is accelerated significantly by the presence of scandium triflate [Sc(OTf)(3)] in acetonitrile, whereas no photoinduced electron transfer from alkylbenzenes to the singlet excited state of acridine or pyrene takes place in the absence of Sc(OTf)(3). The rate constants of the Sc(OTf)(3)-promoted photoinduced electron-transfer reactions (k(et)) of acridine to afford the complex between acridine radical anion and Sc(OTf)(3) remain constant under the conditions such that all the acridine molecules form the complex with Sc(OTf)(3). In contrast to the case of acridine, the k(et) value of the Sc(OTf)(3)-promoted photoinduced electron transfer of pyrene increases with an increase in concentration of Sc(OTf)(3) to exhibit first-order dependence on [Sc(OTf)(3)] at low concentrations, changing to second-order dependence at high concentrations. The first-order and second-order dependence of k(et) on [Sc(OTf)(3)] is ascribed to the 1:1 and 1:2 complexes formation between pyrene radical anion and Sc(OTf)(3). The positive shifts of the one-electron redox potentials for the couple between the singlet excited state and the ground-state radical anion of acridine and pyrene in the presence of Sc(OTf)(3) as compared to those in the absence of Sc(OTf)(3) have been determined by adapting the free energy relationship for the photoinduced electron-transfer reactions. The Sc(OTf)(3)-promoted photoinduced electron transfer from hexamethylbenzene to the singlet excited state of acridine or pyrene leads to efficient oxygenation of hexamethylbenzene to produce pentamethylbenzyl alcohol which is further oxygenated under prolonged photoirradiation of an O(2)-saturated acetonitrile solution of hexamethylbenzene in the presence of acridine or pyrene which acts as a photocatalyst together with Sc(OTf)(3). The photocatalytic oxygenation mechanism has been proposed based on the studies on the quantum yields, the fluorescence quenching, and direct detection of the reaction intermediates by ESR and laser flash photolysis.     

Mechanisms of photoinduced electron transfer reactions of lappaconitine with aromatic amino acids. Time-resolved CIDNP study

CIDNP techniques were applied to the investigation of the elementary mechanism of photoinduced interaction between anti-arrhythmic drug lappaconitine and amino acids tyrosine and tryptophan. It has been shown that the reactions involve the formation of lappaconitine radical anion. Lappaconitine radical anion is unstable and rapidly eliminates N-acetyl anthranilic acid via protonation and ether bond cleavage. The rate constant of ether bond cleavage was estimated to be equal to 4 x 10(5) s(-1). The role of single electron transfer is discussed in the light of the model of drug-receptor interactions.     

Direct detection of products from the pyrolysis of 2-phenethyl phenyl ether

The pyrolysis of 2-phenethyl phenyl ether (PPE, C(6)H(5)C(2)H(4)OC(6)H(5)) in a hyperthermal nozzle (300-1350 °C) was studied to determine the importance of concerted and homolytic unimolecular decomposition pathways. Short residence times (<100 μs) and low concentrations in this reactor allowed the direct detection of the initial reaction products from thermolysis. Reactants, radicals, and most products were detected with photoionization (10.5 eV) time-of-flight mass spectrometry (PIMS). Detection of phenoxy radical, cyclopentadienyl radical, benzyl radical, and benzene suggest the formation of product by the homolytic scission of the C(6)H(5)C(2)H(4)-OC(6)H(5) and C(6)H(5)CH(2)-CH(2)OC(6)H(5) bonds. The detection of phenol and styrene suggests decomposition by a concerted reaction mechanism. Phenyl ethyl ether (PEE, C(6)H(5)OC(2)H(5)) pyrolysis was also studied using PIMS and using cryogenic matrix-isolated infrared spectroscopy (matrix-IR). The results for PEE also indicate the presence of both homolytic bond breaking and concerted decomposition reactions. Quantum mechanical calculations using CBS-QB3 were conducted, and the results were used with transition state theory (TST) to estimate the rate constants for the different reaction pathways. The results are consistent with the experimental measurements and suggest that the concerted retro-ene and Maccoll reactions are dominant at low temperatures (below 1000 °C), whereas the contribution of the C(6)H(5)C(2)H(4)-OC(6)H(5) homolytic bond scission reaction increases at higher temperatures (above 1000 °C).     

High-yielding synthesis of Weinreb amides via homogeneous catalytic carbonylation of iodoalkenes and iodoarenes

Iodoarenes (iodobenzene and 2-iodothiophene) and iodoalkenes (1-iodocyclohexene, 1-iodo-4-tert-butylcyclohexene, 1-iodo-2-methylcyclohexene and 1-iodo-1-(1-naphthyl)ethene) were used as substrates in palladium-catalysed aminocarbonylation with N,O-dimethylhydroxylamine. The corresponding Weinreb amides were prepared in high isolated yields (up to 87%) when forcing conditions (40-60 bar of CO, 50 °C) were used. The aminocarbonylation provides the Weinreb amides as pure products in a chemoselective reaction. No formation of ketocarboxamides, due to double CO insertion, except for 2-iodothiophene, was observed even at 60 bar of CO pressure.     

Ab-inito Calculations in Reductive Bond-breaking Reaction of C-X Bond in CH3X and CH2X2 with X = F and CI

MP4/6-31+G* level calculations are performed to study the reductive bond-breaking reaction of the C-X bond in halomethanes, CH₃X and CH₂X₂ where X is a fluorine atom or chlorine atom. This type of reaction involves a radical anion, after attaching an extra electron to the halomethane molecule, in which a C-X bond-breaking takes place. Products are a radical and a halogen anion. The equilibrium geometry and bond dissociation energy of the C-X bond thus found are in good agreement with previous theoretical and experimental results. The anomeric effect, electrostatic effect, and radical re-stabilization effect, are investigated to find their influences on bond length and bond dissociation energy in CH₃X and CH₂X₂. Potential energy curves are calculated for the reductive bond-cleavage process, and trends in activation energy for various cases are discussed.     

A different route to 3-aryl-4-hydroxycoumarins

We herein report the simple and direct arylation of 4-hydroxycoumarins by photoinduced reaction with aryl halides (iodobenzene, iodonaphthalene, 4-iodoanisole, 2-iodoanisole). Good yields of 3,4-disubstituted coumarins were obtained in these reactions (>60%). Extension of the procedure to the reaction with o-dihalobenzenes leads to the synthesis of ring closure products which bear a tetracyclic aromatic-condensed ring system, although in lower overall yields (≈45%).     

The Reaction of o-Benzyne with Vinylacetylene: An Unexplored Way to Produce Naphthalene

The mechanism and kinetics of the reaction of ortho-benzyne with vinylacetylene have been studied by ab initio and density functional CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) calculations of the pertinent potential energy surface combined with Rice-Ramsperger-Kassel-Marcus - Master Equation calculations of reaction rate constants at various temperatures and pressures. Under prevailing combustion conditions, the reaction has been shown to predominantly proceed by the biradical acetylenic mechanism initiated by the addition of C₄H₄ to one of the C atoms of the triple bond in ortho-benzyne by the acetylenic end, with a significant contribution of the concerted addition mechanism. Following the initial reaction steps, an extra six-membered ring is produced and the rearrangement of H atoms in this new ring leads to the formation of naphthalene, which can further dissociate to 1- or 2-naphthyl radicals. The o-C₆H₄+C₄H₄ reaction is highly exothermic, by ∼143 kcal/mol to form naphthalene and by 31–32 kcal mol⁻¹ to produce naphthyl radicals plus H, but features relatively high entrance barriers of 9–11 kcal mol⁻¹. Although the reaction is rather slow, much slower than the reaction of phenyl radical with vinylacetylene, it forms naphthalene and 1- and 2-naphthyl radicals directly, with their relative yields controlled by the temperature and pressure, and thus represents a viable source of the naphthalene core under conditions where ortho-benzyne and vinylacetylene are available.     

An efficient photoinduced iodoperfluoroalkylation of carbon-carbon unsaturated compounds with perfluoroalkyl iodides

Dependent on the selection of the light sources employed, the photoinduced iodoperfluoroalkylation of a variety of unsaturated compounds takes place efficiently via a radical mechanism. Upon irradiation with a xenon lamp through Pyrex (hnu >300 nm), terminal alkenes (R-CH=CH2) and alkynes (R-C triple bond CH) undergo iodoperfluoroalkylation with perfluoroalkyl iodides (RF-I) regioselectively, providing R-CH(I)-CH2-RF and R-C(I)=CH-RF, respectively. In the case of terminal allenes (R-CH=C=CH2), the photoinduced iodoperfluoroalkylation occurs selectively at the terminal double bond, giving the corresponding beta-perfluoroalkylated vinylic iodides (R-CH=C(I)-CH2-RF) in good yields. The photoinitiated reaction of vinylcyclopropanes (c-C3H5-C(R)=CH2) with RF-I proceeds via the rearrangement of cyclopropylcarbinyl radical intermediates to the homoallylic radical intermediates, and the corresponding 1,5-iodoperfluoroalkylated products (I-(CH2)2CH=C(R)-CH2-RF) are obtained in high yields. Isocyanides (R-NC), as C-N unsaturated compounds, also undergo the xenon-lamp-irradiated iodoperfluoroalkylation to provide the corresponding 1,1-adducts (R-N=C(I)-RF) in good yields. Furthermore, the present photoinitiation procedure can be applied to the iodotrifluoromethylation of unsaturated compounds, when the xenon-lamp-irradiated reactions are conducted under the refluxing conditions of excess CF3-I.     

Use of aromatic radical-anions in the absence of THF. Tandem formation and cyclization of benzyllithiums derived from the attack of homo- and bishomoallyllithiums on alpha-methylstyrenes: two-pot synthesis of cuparene

When a homo- or bishomoallyllithium, generated by reductive lithiation of the corresponding phenyl thioether by the radical anion lithium 1-(dimethylamino)naphthalenide (LDMAN), is added to alpha-methylstyrene, a tandem addition/cyclization to a phenyl-substituted five- or six-membered-ring occurs. The yields are compromised by polymerization of the alpha-methylstyrene, a process favored by tetrahydrofuran (THF), the solvent used to generate lithium aromatic radical anions. Thus, a new method of generating LDMAN (unsuccessful for other common radical anions) in the absence of THF has been developed. The radical anion can be generated and the reductive lithiation performed in dimethyl ether at -70 degrees C. After the addition of diethyl ether or other solvent, and evaporation of the dimethyl ether in vacuo, the alpha-methylstyrene is added and the solution is warmed to -30 degrees C. When the unsaturated alkyllithium is primary, no adduct forms in THF due to polymerization of the alpha-methylstyrene, but moderate yields are attained in a solvent containing mainly hexanes. It was also found that the cyclized organolithiums, which would have become protonated in the presence of THF, can be captured by an electrophile, even at ambient temperature. A two-pot synthesis, the most efficient reported, of the sesquiterpene (+/-)-cuparene in 46% yield, using this technology is reported.     

Reactions of 1-hydroxy-1-methylethyl radicals with NO2-: time-resolved electron spin resonance

The reaction of the alpha-hydroxyalkyl radical of 2-propanol (1-hydroxy-1-methylethyl radical) with nitrite ions was characterized. A product of the reaction was assigned as the adduct nitro radical anion, [HO-C(CH(3))(2)NO(2)](*-). This radical was identified using time-resolved electron spin resonance (TRESR). The radical's magnetic parameters, the nitrogen hyperfine coupling constant (a(N) = 26.39 G), and its g-factor (2.0052) were the same as those of the nitro radical anion previously discovered in (*)OH spin-trapping experiments with the aci-anion of (CH(3))(2)CHNO(2). Production of [HO-C(CH(3))(2)NO(2)](*-) was determined to be 38% +/- 4% of the reaction of (CH(3))(2)C(*)-OH with nitrite. The reason why this fraction was less than 100% was rationalized by invoking the competitive addition at oxygen, which forms [HO-C(CH(3))(2)ONO](*-), followed by a rapid loss of (*)NO. Furthermore, by taking this mechanism into account, the bimolecular rate constant for the total reaction of (CH(3))(2)C(*)-OH with nitrite at reaction pH 7 was determined to be 1.6 x 10(6) M(-1) s(-1), using both decay traces of (CH(3))(2)C(*)-OH and growth traces of [HO-C(CH(3))(2)NO(2)](*-). This correspondence further confirms the nature of the reaction. The reaction mechanism is discussed with guidance by computations using density functional theory.     

Competitive ArC-H and ArC-X (X = Cl, Br) activation in halobenzenes at cationic titanium centers

The titanium methyl cation [Cp*((tBu3P=N)TiCH3]+ [B(C6F5)4]- reacts rapidly with H2 to give the analogous cationic hydride [Cp*((tBu3P=N)TiH(THF)n]+ [B(C6F5)4]- (n = 0, 1), which can be trapped and isolated as its THF adduct 1 x THF (n = 1). When generated in the presence of chloro or bromobenzene, 1 undergoes C-X activation or ortho-C-H activation, depending on the amount of dihydrogen present in the reaction medium. At approximately 4 atm of H2, C-X activation is preferred, giving the halocations [Cp*((tBu3P= N)TiX]+ [B(C6F5)4]- (2X) and C6H6/biphenyl mixtures. At lower pressures of H2 (>1 atm), the beta-halophenyl cations [Cp*((tBu3P=N)Ti(2-X-C6H4)]+ [B(C6F5)4]- (3X) are the products isolated. In the absence of H2, these compounds are quite thermally stable, but undergo beta-halogen elimination upon moderate heating, to give 2X (approximately 20%) and compounds 4X which are the result of reaction between 2X and benzyne via addition of the benzyne C-C triple bond across the Ti-N bond of the phosphinimide ligand. Thus, three separate bond activation processes are operative in this system: direct C-X activation, ortho-C-H activation, and indirect C-X activation via beta-halogen elimination. Mechanistic studies on all three processes have been done and support a radical pathway for direct C-X cleavage, sigma-bond metathesis of the ortho-C-H bond of eta(1)-coordinated C6H5X, and beta-halogen elimination from base-free compound 3X.     

Ab initio evaluation of the thermodynamic and electrochemical properties of alkyl halides and radicals and their mechanistic implications for atom transfer radical polymerization

High-level ab initio molecular orbital calculations are used to study the thermodynamics and electrochemistry relevant to the mechanism of atom transfer radical polymerization (ATRP). Homolytic bond dissociation energies (BDEs) and standard reduction potentials (SRPs) are reported for a series of alkyl halides (R-X; R = CH 2CN, CH(CH 3)CN, C(CH 3) 2CN, CH 2COOC 2H 5, CH(CH 3)COOCH 3, C(CH 3) 2COOCH 3, C(CH 3) 2COOC 2H 5, CH 2Ph, CH(CH 3)Ph, CH(CH 3)Cl, CH(CH 3)OCOCH 3, CH(Ph)COOCH 3, SO 2Ph, Ph; X = Cl, Br, I) both in the gas phase and in two common organic solvents, acetonitrile and dimethylformamide. The SRPs of the corresponding alkyl radicals, R (*), are also examined. The computational results are in a very good agreement with the experimental data. For all alkyl halides examined, it is found that, in the solution phase, one-electron reduction results in the fragmentation of the R-X bond to the corresponding alkyl radical and halide anion; hence it may be concluded that a hypothetical outer-sphere electron transfer (OSET) in ATRP should occur via concerted dissociative electron transfer rather than a two-step process with radical anion intermediates. Both the homolytic and heterolytic reactions are favored by electron-withdrawing substituents and/or those that stabilize the product alkyl radical, which explains why monomers such as acrylonitrile and styrene require less active ATRP catalysts than vinyl chloride and vinyl acetate. The rate constant of the hypothetical OSET reaction between bromoacetonitrile and Cu (I)/TPMA complex was estimated using Marcus theory for the electron-transfer processes. The estimated rate constant k OSET = approximately 10 (-11) M (-1) s (-1) is significantly smaller than the experimentally measured activation rate constant ( k ISET = approximately 82 M (-1) s (-1) at 25 degrees C in acetonitrile) for the concerted atom transfer mechanism (inner-sphere electron transfer, ISET), implying that the ISET mechanism is preferred. For monomers bearing electron-withdrawing groups, the one-electron reduction of the propagating alkyl radical to the carbanion is thermodynamically and kinetically favored over the one-electron reduction of the corresponding alkyl halide unless the monomer bears strong radical-stabilizing groups. Thus, for monomers such as acrylates, catalysts favoring ISET over OSET are required in order to avoid chain-breaking side reactions.     

alpha-Nitration of Ketones via Enol Silyl Ethers. Radical Cations as Reactive Intermediates in Thermal and Photochemical Processes

Highly colored (red) solutions of various enol silyl ethers and tetranitromethane (TNM) are readily bleached to afford good yields of alpha-nitro ketones in the dark at room temperature or below. Spectral analysis show the red colors to be associated with the intermolecular 1:1 electron donor-acceptor (EDA) complexes between the enol silyl ether and TNM. The formation of similar vividly colored EDA complexes with other electron acceptors (such as chloranil, tetracyanobenzene, tetracyanoquinodimethane, etc.) readily establish enol silyl ethers to be excellent electron donors. The deliberate irradiation of the diagnostic (red) charge-transfer absorption bands of the EDA complexes of enol silyl ethers and TNM at -40 degrees C affords directly the same alpha-nitro ketones, under conditions in which the thermal reaction is too slow to compete. A common pathway is discussed in which the electron transfer from the enol silyl ether (ESE) to TNM results in the radical ion triad [ESE(*)(+), NO(2)(*), C(NO(2))(3)(-)]. A subsequent fast homolytic coupling of the cation radical of the enol silyl ether with NO(2)(*)() leads to the alpha-nitro ketones. The use of time-resolved spectroscopy and the disparate behavior of the isomeric enol silyl ethers of alpha- and beta-tetralones as well as of 2-methylcyclohexanone strongly support cation radicals (ESE(*)(+)) as the critical intermediate in thermal and photoinduced electron-transfer as described in Schemes 1 and 2, respectively.