Synthesis and Spin‐Trapping Properties of a Trifluoromethyl Analogue of DMPO: 5‐Methyl‐5‐trifluoromethyl‐1‐pyrroline N‐Oxide (5‐TFDMPO)

The 5‐diethoxyphosphonyl‐5‐methyl‐1‐pyrroline N‐oxide superoxide spin adduct (DEPMPO?OOH) is much more persistent (about 15 times) than the 5,5‐dimethyl‐1‐pyrroline N‐oxide superoxide spin adduct (DMPO?OOH). The diethoxyphosphonyl group is bulkier than the methyl group and its electron‐withdrawing effect is much stronger. These two factors could play a role in explaining the different half‐lifetimes of DMPO?OOH and DEPMPO?OOH. The trifluoromethyl and the diethoxyphosphonyl groups show similar electron‐withdrawing effects but have different sizes. We have thus synthesized and studied 5‐methyl‐5‐trifluoromethyl‐1‐pyrroline N‐oxide (5‐TFDMPO), a new trifluoromethyl analogue of DMPO, to compare its spin‐trapping performance with those of DMPO and DEPMPO. 5‐TFDMPO was prepared in a five‐step sequence by means of the Zn/AcOH reductive cyclization of 5,5,5‐trifluoro‐4‐methyl‐4‐nitropentanal, and the geometry of the molecule was estimated by using DFT calculations. The spin‐trapping properties were investigated both in toluene and in aqueous buffer solutions for oxygen‐, sulfur‐, and carbon‐centered radicals. All the spin adducts exhibit slightly different fluorine hyperfine coupling constants, thereby suggesting a hindered rotation of the trifluoromethyl group, which was confirmed by variable‐temperature EPR studies and DFT calculations. In phosphate buffer at pH 7.4, the half‐life of 5‐TFDMPO?OOH is about three times shorter than for DEPMPO?OOH and five times longer than for DMPO?OOH. Our results suggest that the stabilization of the superoxide adducts comes from a delicate balance between steric, electronic, and hydrogen‐bonding effects that involve the β group, the hydroperoxyl moiety, and the nitroxide.     

Absolute Rate Constants for the Addition of the 1‐(tert‐Butoxy)carbonylethyl Radical to Alkenes in Solution

Absolute rate constants and some of their Arrhenius parameters are reported for the addition of the 1‐[(tert‐butoxy)carbonyl]ethyl radical (MeC . HCO₂Me₃) to several mono‐ or 1,1‐disubstituted alkenes in acetonitrile as obtained by time‐resolved electron spin resonance spectroscopy. At 295 K, the rate constants range from 470 M ⁻¹ s⁻¹ (but‐1‐ene) to 2.4⋅10⁵ M ⁻¹ s⁻¹ (1,1‐diphenylethene), the experimental activation energies range from 26.8 kJ/mol (but‐1‐ene) to 14.7 kJ/mol (styrene), and the frequency factors obey on the average log (A/M ⁻¹ s⁻¹)=7.9±0.5. The rate constants of the secondary 1‐[(tert‐butoxy)carbonyl]ethyl radical are close to the geometric mean of those of the related primary [(tert‐butoxy)carbonyl]methyl and the tertiary 2‐(methoxycarbonyl)propan‐2‐yl radicals. The activation energies for addition of these three carboxy‐substituted alkyl radicals are mainly governed by the addition enthalpy but are also substantially lowered by ambiphilic polar effects. The results support a previously derived predictive analysis, and relations to rate constants of acrylate polymerizations are discussed.     

Seeking Hidden Magnetic Phenomena by Theoretical Means in a Thiooxoverdazyl Adduct

The oxidation of 1,5‐dimethyl‐3‐(2′‐pyridyl)‐6‐thiooxotetrazane (SvdH₃py) by benzoquinone leads to a 1:1 adduct of 1,5‐dimethyl‐3‐(2′‐pyridyl)‐6‐thiooxoverdazyl radical (Svdpy) with hydroquinone (hq). The single‐crystal X‐ray diffraction of this adduct at room temperature (RT) shows that the radicals exhibit a slight curvature that leads to the formation of alternating head‐to‐tail (antiparallel) stacked 1D chains. Moreover, temperature‐dependent X‐ray measurements at 100, 200, and 303 K reveal that the lateral slippages between the radicals of the stacks |δ₁| and |δ₂| vary from 0.64 to 0.78 Å and 0.54 to 0.40 Å between 100 and 303 K. Despite the alternation of the inter‐radical distances and lateral slippages, the magnetic susceptibility data can be fitted with excellent agreement using a regular one‐dimensional antiferromagnetic chain model with J=?5.9 cm^?1. Wavefunction‐based calculations indicate an alternation of the magnetic interaction parameters correlated with the structural analysis at RT. Moreover, they demonstrate that the thermal slippage of the radicals induces a switching of the physical behavior, since the exchange interaction changes from antiferromagnetic (?0.9 cm^?1) at 100 K to ferromagnetic (1.4 cm^?1) at 303 K. The theoretical approach thus reveals a much richer magnetic behavior than the analysis of the magnetic susceptibility data and ultimately questions the relevance of a spin‐coupled picture based on temperature‐independent parameters.     

NHC–Copper(I) Halide‐Catalyzed Direct Alkynylation of Trifluoromethyl Ketones on Water

An efficient and easily scalable NHC–copper(I) halide‐catalyzed addition of terminal alkynes to 1,1,1‐trifluoromethyl ketones, carried out on water for the first time, is reported. A series of addition reactions were performed with as little as 0.1–2.0 mol % of [(NHC)CuX] (X=Cl, Br, I, OAc, OTf) complexes, providing tertiary propargylic trifluoromethyl alcohols in high yields and with excellent chemoselectivity from a broad range of aryl‐ and more challenging alkyl‐substituted trifluoromethyl ketones (TFMKs). DFT calculations were performed to rationalize the correlation between the yield of catalytic alkynylation and the sterics of N‐heterocyclic carbenes (NHCs), expressed as buried volume (%VBur), indicating that steric effects dominate the yield of the reaction. Additional DFT calculations shed some light on the differential reactivity of [(NHC)CuX] complexes in the alkynylation of TFMKs. The first enantioselective version of a direct alkynylation in the presence of C₁‐symmetric NHC–copper(I) complexes is also presented.     

THE BAYLIS—HILLMAN REACTION: TiCl4 MEDIATED COUPLING OF ALKYL VINYL KETONES WITH α-KETO ESTERS AND ALDEHYDES

TiCl₄ mediated coupling of alkyl vinyl ketones with α-keto esters and aldehydes provides respectively 2-aryl-2-hydroxy-3-methylene-4-oxoalkanoates and (Z)-keto allyl chlorides in 1 h time at room temperature. Similar coupling of trifluoromethyl phenyl ketone with methyl vinyl ketone produces 1,1,1-trifluoro-2-hydroxy-2-phenyl-3-methylenepentan-4-one.     

Structure and Properties of 2,2-Dibromovinyl Trifluoromethyl Ketone

It was established by IR spectroscopy and quantum-chemical calculations along nonempirical DFT method in B3LYP version with the basis set 6-311 G(d,p) that 2,2-dibromovinyl trifluoromethyl ketone consisted of a mixture of s-cis planar conformer and s-trans-form deviating from a plane by 13°, whereas the s-cis-form is more energetically stable than the s-trans one (E -5.07 kcal mol^{- 1}). Also in 2,2-dibromovinyl methyl ketone the planar s-cis conformer is more stable. Chlorine-containing analogs, 2,2-dichlorovinyl trifluoromethyl ketone and 2,2-dichlorovinyl methyl ketone, are more stable in the planar s-trans-conformation. Charge distribution and polarization in the dibromovinyl ketones are analogous to those in dichlorovinyl ketones in agreement with the established reactivity of dibromovinyl trifluoromethyl ketone. By reaction of 2,2-dibromovinyl trifluoromethyl ketone with 2,4-dinitrophenyl-, alkylhydrazines, N,N-dimethylhydrazine, N,N-, N,O-, N,S-binucleophiles were respectively obtained hydrazone, derivatives of pyrazole, imidazole, oxazole, and 1,3-thiazine containing a trifluoromethyl group.     

Spin Trapping of Radical Intermediates Generated by the Oxidation of Substituted 4‐Methylphenols

A series of substituted 4‐methylphenols 1 and 2 was oxidized with PbO₂ in the presence of nitroso compounds 3 – 10 . The formation of adducts of benzyl radicals with the nitroso spin traps in the reaction mixture was established, suggesting the abstraction of an H‐atom from the methyl substituent of 1 or 2 . In the consecutive steps, the adducts underwent a further rearrangement to the corresponding nitrones. When the starting phenol contained bulky ^tBu groups in ortho‐position (see 2,6‐di(tert‐butyl)‐4‐methylphenol ( 1a )), the stable 2,6‐di(tert‐butyl)‐4R‐phenoxy radicals (R=? CH?N⁺(O^?)? X) were detected as the final radical products. The indirect evidence of nitrones in the reaction mixture was performed in one case by the reaction with a RO radicals.     

Highly selective trifluoroacetic ester/ketone metathesis: an efficient approach to trifluoromethyl ketones and esters

A highly selective and atom efficient ‘trifluoroacetic ester/ketone metathesis’ has been sincerely witnessed. Enolizable alkyl (at least two non-hydrogen atoms) aryl ketones were found to react readily with ethyl trifluoroacetate under the promotion of NaH to afford trifluoroacetic ester/ketone exchange products, trifluoromethyl ketones (TFMKs), and aromatic acid esters, which were quite different from the general Claisen condensation products, 1,3-diketones. The outcome of the reaction between ketone and ethyl trifluoroacetate is strongly related to the structures of substrates, the steric congestion caused by alkyl group is in favor of the C–C bond cleavage. DFT investigation further disclosed that the metathesis reaction was a kinetically favored pathway. Using only a slight excess of cheap trifluoromethylation reagent, simple operation and mild conditions make it a practical method for preparation of TFMKs on large scale, as well as a new choice of converting aryl alkyl ketones to aromatic acid esters.     

Iron Borohydride Pincer Complexes for the Efficient Hydrogenation of Ketones under Mild,Base‐Free Conditions: Synthesis and Mechanistic Insight

The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr‐PNP)Fe(H)(CO)(η¹‐BH₄)] ( 1 ) catalyzes the base‐free hydrogenation of ketones to their corresponding alcohols employing only 4.1 atm hydrogen pressure. Turnover numbers up to 1980 at complete conversion of ketone were reached with this system. Treatment of 1 with aniline (as a BH₃ scavenger) resulted in a mixture of trans‐[(iPr‐PNP)Fe(H)₂(CO)] ( 4 a ) and cis‐[(iPr‐PNP)Fe(H)₂(CO)] ( 4 b ). The dihydrido complexes 4 a and 4 b do not react with acetophenone or benzaldehyde, indicating that these complexes are not intermediates in the catalytic reduction of ketones. NMR studies indicate that the tetrahydridoborate ligand in 1 dissociates prior to ketone reduction. DFT calculations show that the mechanism of the iron‐catalyzed hydrogenation of ketones involves alcohol‐assisted aromatization of the dearomatized complex [(iPr‐PNP*)Fe(H)(CO)] ( 7 ) to initially give the Fe⁰ complex [(iPr‐PNP)Fe(CO)] ( 21 ) and subsequently [(iPr‐PNP)Fe(CO)(EtOH)] ( 38 ). Concerted coordination of acetophenone and dual hydrogen‐atom transfer from the PNP arm and the coordinated ethanol to, respectively, the carbonyl carbon and oxygen atoms, leads to the dearomatized complex [(iPr‐PNP*)Fe(CO)(EtO)(MeCH(OH)Ph)] ( 32 ). The catalyst is regenerated by release of 1‐phenylethanol, followed by dihydrogen coordination and proton transfer to the coordinated ethoxide ligand.     

Polyolefin Macroradical Properties: A Semiempirical Quantum Chemistry Study

Summary: Semiempirical quantum chemistry simulations have been used to estimate the properties of mid‐chain alkyl radicals of ethylene, propylene and styrene oligomers depending on the chain length. The values of spin density and charge on the radical site proved to be almost unchanged for oligomer radicals having from 2 to 11 repeated units. Bond strength parameters of bonds neighboring the radical site demonstrate stable values starting from pentamers. The reliable inference is that the electron structure of polyethylene, poly(propylene) and polystyrene (PS) macroradicals may be simulated by means of relatively short oligomers. The obtained data indicate clearly that the polystyrene tertiary alkyl radical has noticeably decreased values of both spin and charge onto the radical site as well as very decreased strength of β‐bonds. For that reason the PS tert‐alkyl radical is estimated as that having decreased activity and increased susceptibility to chain scission. The most probable cause is delocalization of the free electron onto the neighboring aromatic ring.

Spin density distribution in tertiary radical of polystyrene.     

A First Principle Characterization of Polyethylene,Poly(propylene) and Polystyrene Macroradicals Properties

Summary: Structures of polyethylene, poly(propylene) and polystyrene macroradicals have been studied by means of the B3LYP/6‐31G** method. The polystyrene tertiary alkyl radical has been found to be less active as having spin density partially delocalized on the aromatic ring neighboring to the radical site. Considering the main chain bond orders, the following radicals have been found to be susceptible to main chain scission: the sec‐ and tert‐oxyl poly(propylene) ones, the sec‐ and tert‐oxyl polystyrene ones as well as the tertiary alkyl polystyrene one. The energy levels of the last frontier molecular orbitals in the polyolefin macroradicals have been found to be split up and moved—up and down for HOMOs and LUMOs, respectively. The alkyl macroradicals have single occupied molecular orbital with increased energy. In the oxyl and peroxyl radicals the spin density comes from two or three last orbitals that are spread over a broad molecular region and no single occupied molecular orbital (SOMO) may be identified.

Localization of the spin density.     

Influence of intramolecular interaction on the enthalpy and entropy of methylene unit sorption for the homologous series of carbonyl-containing compounds analyzed by capillary GC

The enthalpy and entropy of sorption of methylene units in the homologous series of n-alkyl acetates; methyl, n-butyl, and phenyl n-alkyl ketones; and the methyl esters and chloroanhydrides of n-aliphatic carboxylic acids have been determined on an SE-54 capillary column. The enthalpy of sorption of the first methylene unit is anomalously high when the growing n-alkyl chain is connected directly to a carbonyl group. This effect is a result of intramolecular interaction between neighboring methyl and carbonyl groups. It has been shown by measurement of the enthalpy and entropy of sorption of the difluoromethylene unit in the series of n-hexyl esters of perfluorinated carboxylic acids that for these compounds the effect is absent. The intramolecular interaction was found to increase in the order methyl butyl ketone < methyl phenyl ketone < methyl acetate < acetyl chloride < acetone.     

Copper‐Catalyzed Radical/Radical CH/PH Cross‐Coupling: α‐Phosphorylation of Aryl Ketone O‐Acetyloximes

The selective radical/radical cross‐coupling of two different organic radicals is a great challenge due to the inherent activity of radicals. In this paper, a copper‐catalyzed radical/radical C? H/P? H cross‐coupling has been developed. It provides a radical/radical cross‐coupling in a selective manner. This work offers a simple way toward β‐ketophosphonates by oxidative coupling of aryl ketone o‐acetyloximes with phosphine oxides using CuCl as catalyst and PCy₃ as ligand in dioxane under N₂ atmosphere at 130 °C for 5 h, and yields ranging from 47 % to 86 %. The preliminary mechanistic studies by electron paramagnetic resonance (EPR) showed that, 1) the reduction of ketone o‐acetyloximes generates iminium radicals, which could isomerize to α‐sp³‐carbon radical species; 2) phosphorus radicals were generated from the oxidation of phosphine oxides. Various aryl ketone o‐acetyloximes and phosphine oxides were suitable for this transformation.     

Free Radical Chemistry of Phosphasilenes

Understanding the characteristics of radicals formed from silicon‐containing heavy analogues of alkenes is of great importance for their application in radical polymerization. Steric and electronic substituent effects in compounds such as phosphasilenes not only stabilize the Si=P double bond, but also influence the structure and species of the formed radicals. Herein we report our first investigations of radicals derived from phosphasilenes with Mes, Tip, Dur, and NMe₂ substituents on the P atom, using muon spin spectroscopy and DFT calculations. Adding muonium (a light isotope of hydrogen) to phosphasilenes reveals that: a) the electron‐donor NMe₂ and the bulkiest Tip‐substituted phosphasilenes form several muoniated radicals with different rotamer conformations; b) bulky Dur‐substituted phosphasilene forms two radicals (Si‐ and P‐centred); and c) Mes‐substituted phosphasilene mainly forms one species of radical, at the P centre. These significant differences result from intramolecular substituent effects.     

Solvent‐Free Photooxidation of Alkanes by Dioxygen with 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone via Photoinduced Electron Transfer

Photooxidation of alkanes by dioxygen occurred under visible light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) which acts as a super photooxidant. Solvent‐free hydroxylation of cyclohexane and alkanes is initiated by electron transfer from alkanes to the singlet and triplet excited states of DDQ to afford the corresponding radical cations and DDQ^??, as revealed by femtosecond laser‐induced transient absorption measurements. Alkane radical cations readily deprotonate to produce alkyl radicals, which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl radicals abstract hydrogen atoms from alkanes to yield alkyl hydroperoxides, accompanied by regeneration of alkyl radicals to constitute the radical chain reactions, so called autoxidation. The radical chain is terminated in the bimolecular reactions of alkylperoxyl radicals to yield the corresponding alcohols and ketones. DDQ^??, produced by the photoinduced electron transfer from alkanes to the excited state of DDQ, disproportionates with protons to yield DDQH₂.     

Decomposition of model alkoxyamines in simple and polymerizing systems. I. 2,2,6,6‐tetramethylpiperidinyl‐ N‐oxyl‐based compounds

The thermal decomposition of five alkoxyamines labeled TEMPO–R, where TEMPO was 2,2,6,6‐tetramethylpiperidinyl‐N‐oxyl and R was cumyl (Cum), 2‐tert‐butoxy‐carbonyl‐2‐propyl (PEst), phenylethyl (PhEt), 1‐tert‐butoxy‐carbonylethyl (EEst), or 1‐methoxycarbonyl‐3‐methyl‐3‐phenylbutyl (Acrylate‐Cum), was studied with ¹H NMR in the absence and presence of styrene and methyl methacrylate. The major products were alkenes and the hydroxylamine 1‐hydroxy‐2,2,6,6‐tetramethyl‐ piperidine (TEMPOH), and in monomer‐containing solutions, unimeric and polymeric alkoxyamines and alkenes were also found. Furthermore, the reactions between TEMPO and the radicals EEst and PEst were studied with chemically induced dynamic nuclear polarization. In comparison with coupling, TEMPO reacted with the radicals Cum, PEst, PhEt, and EEst and their unimeric styrene adducts by disproportionation to alkenes and TEMPOH only to a minor extent (0.6–3%) but with the radical adducts to methyl methacrylate to a considerable degree (≥20%). Parallel to the radical cleavage, TEMPO–EEst (but not the other alkoxyamines or TEMPO–Acrylate‐Cum) underwent substantial nonradical decay. The consequences for TEMPO‐mediated living radical polymerizations are discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3604–3621, 2001     

Photochemical Reactions. 153th communication. Photochemistry of acylsilanes: Photolyses and thermolyses of α,β-epoxy silyl ketones

The photolyses and thermolyses of the α,β-epoxy silyl ketones 5 and 6 are described. On n,π*-excitation, the silyl ketones 5 and 6 were transformed to the ketone 7 and the ketene 8 in quantitative yield. The formation of 8 may be explained by initial cleavage of the C(α)? O bond and subsequent C(1)→C(2) migration of the (t-Bu)Me₂Si group. In contrast to the acylsilanes 5 and 6 , the photolyses of the analogous methyl ketones 11 and 12 gave a very complex mixture of products. On thermolysis, 5 and 6 yielded the ketone 7 and the acetylenic compound 9 , which were probably formed via a siloxycarbene intermediate. In addition, the 1,3-dioxle 10 was formed via an initial C(α)? C(β) bond cleavage leading to the ylide g and subsequent intramolecular addition of the carbonyl group. The analogous 1,3-dioxole 13 was obtained on pyrolysis of the methyl ketones 11 and 12 .     

Synergistic Effects of Lewis Bases and Substituents on the Electronic Structure and Reactivity of Boryl Radicals

Boryl radicals have the potential for the development of new molecular entities and for application in new radical reactions. However, the effects of the substituents and coordinating Lewis bases on the reactivity of boryl radicals are not fully understood. By using first‐principles methods, we investigated the spin‐density distribution and reactivity of a series of boryl radicals with various substituents and Lewis bases. The substituents, along with the Lewis bases, only affect the radical reactivity when an unpaired electron is in the boron p_z orbital, that is, for three‐coordinate radicals. We found evidence of synergistic effects between the substituents and the Lewis bases that can substantially broaden the tunability of the reactivity of the boryl radicals. Among Lewis bases, pyridine and imidazol‐2‐ylidene show a similar capacity for stabilization by delocalizing the spin density. Electron‐donating substituents, such as nitrogen, more efficiently stabilize boryl radicals than oxygen and carbon atoms. The reactivity of a boryl radical is always boron based, irrespective of the spin density on boron.     

Hydrindanone Synthesis: An Incisterol Model

Ethyl 1‐methyl‐2‐oxocyclohexanecarboxylate ( 1a ) and its homologue 1b were converted to hydroisobenzofuran acids 7 (via 6‐[(butylsulfanyl)methylene] and epoxide derivatives), one of which furnished hexalone derivative 11 (via an intermediate diazomethyl ketone derivative). The above‐mentioned starting esters were converted to ethylene ketals, the free‐radical oxidations of which led to hydrobenzofuran acids. One of the latter led to a hydrindanone (via a diazomethyl ketone), whose further chemical elaboration yielded an incisterol model. A second hydrobenzofuran acid gave a cyclobutenone (via the diazomethyl ketone), which was transformed into a more‐stable cyclopentenone isomer by treatment with Lewis acid.     

Spin trapping studies of poly(methyl methacrylate) degradation in solution

Free radicals produced either by γ or ultrasonic irradiation of poly(methyl methacrylate) (PMMA) in benzene solution were stabilized by spin trapping; they were identified by analysis of ESR spectra of the trapped radicals (the spin adducts). The radical species identified after γ-irradiation were methyl, ester (COOCH₃), a pair of the chain scission radicals, ～CH₂C(CH₃)(COOCH₃) and CH₂C(CH₃)(COOCH₃)～, and phenyl radical originating from the solvent. The chain scission radicals were also detected by spin trapping after ultrasonic irradiation of the benzene solution. Taking account of the difference in the trapping rate for two spin trapping agents, 2,4,6-tri-t-butylnitrosobenzene (BNB) and penta-methyl-nitrosobenzene (PMNB) the radical species trapped by PMNB are assumed to be precursors of those trapped by BNB. Based on the radical species found by the spin trapping method, plausible degradation processes for PMMA in benzene solution are proposed.