N-Heterocyclic carbene boranes are good hydride donors

The nucleophilicity parameters (N) of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene borane and 1,3-dimethylimidazol-2-ylidene borane are 9.55 and 11.88. This places N-heterocyclic carbene boranes (NHC-boranes) among the most nucleophilic classes of neutral hydride donors. Reductions of highly electron-poor C═N and C═C bonds provide hydrogenation products along with new, stable borylated products. The results suggest that NHC-boranes have considerable untapped potential as neutral organic reductants.     

Synthesis and characterization of N-heterocyclic carbene complexes of 1,3,2-dioxaborolane-4,5-dione (NHC-boryl oxalates)

NHC-boranes react with oxalic acid in acetonitrile slowly at room temperature and rapidly at 80?°C to form NHC-boryl oxalates. For example, heating of 1,3-dimethylimidazol-2-ylidene borane with oxalic acid in acetonitrile (80?°C, 2?h) provides the cyclic NHC-boryl oxalate derived from two sequential acid/base reactions in 59% isolated yield. The new NHC-boryl oxalates are robust white solids that are formally complexes of 1,3,2-dioxaborolane-4,5-dione, itself an unknown heterocycle.     

Gold‐Catalyzed Highly Selective Photoredox C(sp2)−H Difluoroalkylation and Perfluoroalkylation of Hydrazones

The first gold‐catalyzed photoredox C(sp²)?H difluoroalkylation and perfluoroalkylation of hydrazones with readily available R_F?Br reagents is reported. The resulting gem‐difluoromethylated and perfluoroalkylated hydrazones are highly functionalized, versatile molecules. A mild reduction of the coupling products can efficiently produce gem‐difluoromethylated β‐amino phosphonic acids and β‐amino acid derivatives. In mechanistic studies, a difluoroalkyl radical intermediate was detected by an EPR spin‐trapping experiment, indicating that a gold‐catalyzed radical pathway is operating.     

Evidence for Formation of a Co-H Bond from (H(2)O)(2)Co(dmgBF(2))(2) under H(2): Application to Radical Cyclizations

Under H(2), the radical cyclization of appropriate dienes can be catalyzed by cobaloximes. H? can be abstracted from an intermediate (presumably a cobalt hydride) by trityl radicals (Ar(3)C?) or by TEMPO. The rate-determining step in these reactions is the uptake of H(2), which is second order in cobalt and first order in hydrogen; the third-order rate constant is 106(3) M(-2)·s(-1).     

Phosphine‐Catalyzed Remote β‐CH Functionalization of Amines Triggered by Trifluoromethylation of Alkenes: One‐Pot Synthesis of Bistrifluoromethylated Enamides and Oxazoles

An unprecedented phosphine‐catalyzed remote β‐C? H functionalization of amine derivatives triggered by trifluoromethylation of an alkene with Togni’s reagent was disclosed. This reaction proceeded through the highly selective and concomitant activation of an unactivated alkene and the β‐C? H bond of an amine derivative, providing bistrifluoromethylated enamides in excellent yields with good regio‐, chemo‐, and stereoselectivity. Furthermore, the newly developed one‐pot protocol provides a facile and step‐economical access to valuable trisubstituted 5‐(trifluoromethyl)oxazoles. Mechanistic studies showed that this reaction may initiate with a novel phosphine‐catalyzed radical trifluoromethylation of unactivated alkene via a phosphorus radical cation.     

Rücktitelbild: Phosphine‐Catalyzed Remote β‐C?H Functionalization of Amines Triggered by Trifluoromethylation of Alkenes: One‐Pot Synthesis of Bistrifluoromethylated Enamides and Oxazoles (Angew. Chem. 13/2015)

An unprecedented phosphine‐catalyzed remote β‐C H functionalization of amine derivatives triggered by trifluoromethylation of an alkene with Togni’s reagent was disclosed. This reaction proceeded through the highly selective and concomitant activation of an unactivated alkene and the β‐C H bond of an amine derivative, providing bistrifluoromethylated enamides in excellent yields with good regio‐, chemo‐, and stereoselectivity. Furthermore, the newly developed one‐pot protocol provides a facile and step‐economical access to valuable trisubstituted 5‐(trifluoromethyl)oxazoles. Mechanistic studies showed that this reaction may initiate with a novel phosphine‐catalyzed radical trifluoromethylation of unactivated alkene via a phosphorus radical cation.     

C−C Bond‐Forming Strategy by Manganese‐Catalyzed Oxidative Ring‐Opening Cyanation and Ethynylation of Cyclobutanol Derivatives

A novel C?C bond‐forming strategy employing manganese‐catalyzed ring‐opening of cyclobutanol substrates, followed by cyanation or ethynylation, is described. A cyano C1 unit and ethynyl C2 unit are regiospecifically introduced to the γ‐position of ketones at room temperature, providing a mild yet powerful method for production of elusive aliphatic nitriles and alkynes. All transformations described are based on a common sequence: 1) oxidative ring‐opening of cyclobutanol substrates by C?C bond cleavage; 2) radical addition to triple bonds bearing an arylsulfonyl group; and 3) radical‐mediated C?S bond cleavage.     

Stereoselective synthesis of stannyl enones via palladium-catalyzed and free radical hydrostannation of alkynyl ketones with trineophyltin hydride

A study on the addition of trineophyltin hydride (1) to alkynones under free radical (AIBN and Et3B) and palladium-catalyzed [(PPh3)2PdCl2] conditions is reported. The results obtained indicate that the addition of 1 to eight ynones catalyzed by bis(triphenylphosphine)palladium(II) chloride led in all cases to addition products in very high yields (80-96%). These additions take place with excellent regio- and stereochemistry, leading to the alpha adducts as major products in seven out of the eight cases studied. Also the E adducts, resulting from a syn attack, were the only (seven cases) or the predominant (one case) products. The radical hydrostannations initiated by AIBN of ynones 2-5 with 1 led to addition products in good yields (60-88%); with the more hindered ketones 6 and 7-9 the yields obtained were lower. The radical additions initiated by triethylboron to ynones 2-6 follow a similar pattern but with lower yields; no addition products in the hydrostannation of ynones 7-9 were detected. The new acyl-substituted vinylstannanes, owing to their greater stability compared with that of their tributyl- and trimethylstannyl analogues, can be purified by column chromatography using neutral alumina (in all cases) or silica gel 60 (in most cases) as adsorbents. Full 1H, 13C, and 119Sn NMR data are given.     

*H atom and *OH radical reactions with 5-methylcytosine

The reactions between either a hydrogen atom or a hydroxyl radical and 5-methylcytosine (5-MeCyt) are studied by using the hybrid kinetic energy meta-GGA functional MPW1B95. *H atom and *OH radical addition to positions C5 and C6 of 5-MeCyt, or *OH radical induced H-abstraction from the C5 methyl group, are explored. All systems are optimized in bulk solvent. The data presented show that the barriers to reaction are very low: ca. 7 kcal/mol for the *H atom additions and 1 kcal/mol for the reactions involving the *OH radical. Thermodynamically, the two C6 radical adducts and the *H-abstraction product are the most stable ones. The proton hyperfine coupling constants (HFCC), computed at the IEFPCM/MPW1B95/6-311++G(2d,2p) level, agree well with B3LYP results and available experimental and theoretical data on related thymine and cytosine radicals.     

Electron paramagnetic resonance and computational studies of radicals derived from boron-substituted N-heterocyclic carbene boranes

Fifteen second-generation NHC-ligated boranes with aryl and alkyl substituents on boron were prepared, and their radical chemistry was explored by electron paramagnetic resonance (EPR) spectroscopy and calculations. Hydrogen atom abstraction from NHC-BH(2)Ar groups produced boryl radicals akin to diphenylmethyl with spin extensively delocalized across the NHC, BH, and aryl units. All of the NHC-B·HAr radicals studied abstracted Br-atoms from alkyl bromides. Radicals with bulky N,N'-dipp substituents underwent dimerization about 2 orders of magnitude more slowly than first-generation NHC-ligated trihydroborates. The evidence favored head-to-head coupling yielding ligated diboranes. The first ligated diboranyl radical, with a structure intermediate between that of ligated diboranes and diborenes, was spectroscopically characterized during photolysis of di-t-butyl peroxide with N,N'-di-t-butyl-imidazol-2-ylidene phenylborane. The reactive site of B-alkyl-substituted NHC-boranes switched from the boron center to the alkyl substituent for both linear and branched alkyl groups. The β-borylalkyl radicals obtained from N,N'-dipp-substituted boranes underwent exothermic β-scissions with production of dipp-Imd-BH(2)· radicals and alkenes. The reverse additions of NHC-boryl radicals to alkenes are probably endothermic for alkyl-substituted alkenes, but exothermic for conjugated alkenes (addition of an NHC-boryl radical to 1,1-diphenylethene was observed). A cyclopropylboryl radical was observed, but, unlike other α-cyclopropyl-substituted radicals, this showed no propensity for ring-opening.     

Visible‐Light‐Induced Palladium‐Catalyzed Generation of Aryl Radicals from Aryl Triflates

A mild visible‐light‐induced Pd‐catalyzed intramolecular C?H arylation of amides is reported. The method operates by cleavage of a C(sp²)?O bond, leading to hybrid aryl Pd‐radical intermediates. The following 1,5‐hydrogen atom translocation, intramolecular cyclization, and rearomatization steps lead to valuable oxindole and isoindoline‐1‐one motifs. Notably, this method provides access to products with readily enolizable functional groups that are incompatible with traditional Pd‐catalyzed conditions.     

Direct ab initio dynamics study of radical C4H (X̃2Σ+) + CH4 reaction

The methane (CH(4)) hydrogen abstraction reaction by linear butadiynyl radical C(4)H (CCCCH) has been investigated by direct ab initio dynamics over a wide temperature range of 100-3000 K, theoretically. The potential energy surfaces (PESs) have been constructed at the CCSD(T)/aug-cc-pVTZ//BB1K/6-311G(d,p) levels of theory. Two different hydrogen abstraction channels by C(1) and C(4) of C(4)H (C(1)C(2)C(3)C(4)H) have been considered. The results indicate that the C(1) position of C(4)H is a more reactive site. The electron transfer behaviors of two possible channels are also analyzed by quasi-restricted orbital (QRO) in detail. The rate constants calculated by canonical variational transition-state theory (CVT) with the small-curvature tunneling correction (SCT) are in excellent agreement with available experimental values. The normal and three-parameter expressions of Arrhenius rate constants are also provided within 100-3000 K. It is expected to be helpful for further studies on the reaction dynamics behaviors over a wide temperature range where no experimental data is available so far.     

Development of an effective chiral auxiliary for hydroxyalkyl radicals

The development of an effective chiral auxiliary for hydroxyalkyl radicals is delineated. Both the 2-tetrahydropyranyl (THP) and tri-O-benzyl-2-deoxy-alpha-D-glucopyranosyl (GLU) auxiliaries resulted in diastereoselective radical additions to methyl acrylate at -78 degrees C (ds = 6/1 and 11/1, respectively). The developing stereochemistry at the radical center was completely under auxiliary control. Correlation experiments showed that the D-GLU auxiliary led to attack on the radical Si-face. The selectivity of these radical additions dropped-off considerably when the more reactive 2-nitropropene trap was employed. Computational studies suggested that the observed facial selectivity was due primarily to entropic factors in the transition state but that a smaller temperature-dependent enthalpic contribution was also involved. It was hypothesized that incorporation of a quaternary center at C-6 (THP numbering) would restore the facial selectivity with more reactive radical traps by restricting the orientations available to the incoming alkene. In the event, the trans-6-tert-butyltetrahydropyranyl (tBu-THP) auxiliary resulted in very good diastereoselection with 2-nitropropene (ds = 35/1 at -78 degrees C, 15/1 at 0 degrees C, and 8/1 at RT) as did the tri-O-benzyl-6,6-dimethyl-2-alpha-D-deoxyglucopyranosyl (diMe-GLU) auxiliary during additions to ethyl alpha-trifluoroacetoxyacrylate (ds = 10/1 at 0 degrees C). A protocol for recovery of the sugar-derived chiral auxiliaries was also established. This work sets the stage for the development of a novel approach to 1, 3, 5.(2n + 1) polyols based on iterative radical homologation as well as the application of these pyranosidic auxiliaries to other synthetically important reactions.     

Equilibrium and kinetics studies of reactions of manganese acetate, cobalt acetate, and bromide salts in acetic acid solutions

The oxidation of hydrogen bromide and alkali metal bromide salts to bromine in acetic acid by cobalt(III) acetate has been studied. The oxidation is inhibited by Mn(OAc)(2) and Co(OAc)(2), which lower the bromide concentration through complexation. Stability constants for Co(II)Br(n)() were redetermined in acetic acid containing 0.1% water as a function of temperature. This amount of water lowers the stability constant values as compared to glacial acetic acid. Mn(II)Br(n)() complexes were identified by UV-visible spectroscopy, and the stability constants for Mn(II)Br(n)() were determined by electrochemical methods. The kinetics of HBr oxidation shows that there is a new pathway in the presence of M(II)Br(n)(). Analysis of the concentration dependences shows that CoBr(2) and MnBr(2) are the principal and perhaps sole forms of the divalent metals that react with Co(III) and Mn(III). The interpretation of these data is in terms of this step (M, N = Mn or Co): M(OAc)(3) + N(II)Br(2) + HOAc --> M(OAc)(2) + N(III)Br(2)OAc. The second-order rate constants (L mol(-)(1) s(-)(1)) for different M, N pairs in glacial acetic acid are 4.8 (Co, Co at 40 degrees C), 0.96 (Mn, Co at 20 degrees C), 0.15 (Mn(III).Co(II), Co at 20 degrees C), and 0.07 (Mn, Mn at 20 degrees C). Following that, reductive elimination of the dibromide radical is proposed to occur: N(III)Br(2)OAc + HOAc --> N(OAc)(2) + HBr(2)(*). This finding implicates the dibromide radical as a key intermediate in this chemistry, and indeed in the cobalt-bromide catalyzed autoxidation of methylarenes, for which some form of zerovalent bromine has been identified. The selectivity for CoBr(2) and MnBr(2) is consistent with a pathway that forms this radical rather than bromine atoms which are at a considerably higher Gibbs energy. Mn(OAc)(3) oxidizes PhCH(2)Br, k = 1.3 L mol(-)(1) s(-)(1) at 50.0 degrees C in HOAc.     

Reactions of hydrated electron with various radicals: spin factor in diffusion-controlled reactions

The reactions of hydrated electron (eaq-) with various radicals have been studied in pulse radiolysis experiments. These radicals are hydroxyl radical (*OH), sulfite radical anion (*SO3-), carbonate radical anion (CO3*-), carbon dioxide radical anion (*CO2-), azidyl radical (*N3), dibromine radical anion (Br2*-), diiodine radical anion (I2*-), 2-hydroxy-2-propyl radical (*C(CH3)2OH), 2-hydroxy-2-methyl-1-propyl radical ((*CH2)(CH3)2COH), hydroxycyclohexadienyl radical (*C6H6OH), phenoxyl radical (C6H5O*), p-methylphenoxyl radical (p-(H3C)C6H4O*), p-benzosemiquinone radical anion (p-OC6H4O*-), and phenylthiyl radical (C6H5S*). The kinetics of eaq- was followed in the presence of the counter radicals in transient optical absorption measurements. The rate constants of the eaq- reactions with radicals have been determined over a temperature range of 5-75 degrees C from the kinetic analysis of systems of multiple second-order reactions. The observed high rate constants for all the eaq- + radical reactions have been analyzed with the Smoluchowski equation. This analysis suggests that many of the eaq- + radical reactions are diffusion-controlled with a spin factor of 1/4, while other reactions with *OH, *N3, Br2*-, I2*-, and C6H5S* have spin factors significantly larger than 1/4. Spin dynamics for the eaq-/radical pairs is discussed to explain the different spin factors. The reactions with *OH, *N3, Br2*-, and I2*- have also been found to have apparent activation energies less than that for diffusion control, and it is suggested that the spin factors for these reactions decrease with increasing temperature. Such a decrease in spin factor may reflect a changing competition between spin relaxation/conversion and diffusive escape from the radical pairs.     

Dual Photoredox/Copper Catalysis for the Remote C(sp3)−H Functionalization of Alcohols and Alkyl Halides by N‐Alkoxypyridinium Salts

Under mild dual photoredox/copper catalysis, the reaction of N‐alkoxypyridinium salts with readily available silyl reagents (TMSN₃, TMSCN, TMSNCS) afforded δ‐azido, δ‐cyano, and δ‐thiocyanato alcohols in high yields. The reaction went through a domino process involving alkoxy radical generation, 1,5‐hydrogen atom transfer (1,5‐HAT) and copper‐catalyzed functionalization of the resulting C‐centered radical. Conditions for catalytic enantioselective δ‐C(sp³)?H cyanation were also documented.     

Adsorption of C2H radical on cobalt clusters: anion photoelectron spectroscopy and density functional calculations

We investigated the adsorption of C(2)H radical on small cobalt clusters by mass spectrometry and by measuring the photoelectron spectra of Co(n)C(2)H(-) (n = 1-5) cluster anions. The most stable structures of Co(n)C(2)H(-) (n = 1-5) and their neutrals were determined by comparing the experimental results with theoretical calculations. Our studies show that C(2)H radical still maintains its integrity as a structural unit in Co(n)C(2)H(-) clusters, rather than being divided by Co(n) clusters. The most stable isomers of Co(1-2)C(2)H(-) clusters are linear with the C(2)H interacting with only one Co atom, while those of Co(3-5)C(2)H(-) cluster anions are quasi-planar structures with the carbon-carbon bonds bending slightly toward the Co(3-5) clusters. The carbon-carbon bond of C(2)H is lengthened more in Co(3-5)C(2)H(-) clusters than in Co(1-2)C(2)H(-).     

Double-responsive polymer brushes on the surface of colloid particles

Well-defined poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes were synthesized on the surface of polystyrene latex particles by atom transfer radical polymerization (ATRP). It was found that the surface-initiated polymerization of DMAEMA catalyzed by CuCl/CuCl(2)/bpy was under good control in the solvent of acetone/water at ambient temperature (35 degrees C). High-density PDMAEMA brushes with low polydispersity (PDI 1.21) were obtained. TEM results demonstrate that the PDMAEMA-grafted particles have core-shell structure. Dynamic light scattering studies indicate that the particles with PDMAEMA brushes are both pH and temperature responsive.     

Unexpected catalyzed C=C bond cleavage by molecular oxygen promoted by a thiyl radical.

Olefin oxidation with molecular oxygen, promoted by a transition metal catalyst and a thiophenol, involved C=C bond cleavage into the corresponding carbonyl derivatives. This new reaction proceeds under one atmosphere of oxygen, at room temperature, in the presence of an excess of thiophenol and a catalyst such as MnL(2) 3a or VClL(2) 3c. It was applied to aromatic and aliphatic olefins, as well as to functionalized or unfunctionalized acyclic compounds, providing the corresponding ketones and aldehydes in up to 98% yield. The synthetic interest of this catalytic oxidation was illustrated by a one-step preparation of the fragrance (-)-4-acetyl-1-methylcyclohexene 7e in 73% isolated yield. The C=C bond cleavage probably results from a catalyzed decomposition of the beta-hydroperoxysulfide intermediate 12 that is formed by the radical addition of thiophenol to the olefin in the presence of oxygen. Although an excess of the thiophenol was used, it was transformed into the disulfide which could then be reduced without purification in 83% overall yield, thereby allowing for recycling. In addition, the C=C bond cleavage under oxygen could be promoted by catalytic quantities of the thiyl radical, generated by photolysis of the disulfide; thus, in the presence of 0.1 equiv of bis(4-chlorophenyl) disulfide 4b and 5% of the manganese complex 3a, trans-methylstilbene 1b gave, under radiation, benzaldehyde 6a and acetophenone 7a in up to 95% yield. This new reaction offers an alternative to the classical C=C bond cleavage procedures, and further developments in the fields of bioinorganic and environmental chemistry are likely.     

The mechanism of the elimination kinetics of 2-bromo-2-butene in the gas phase

The kinetics of the gas phase elimination of 2-bromo-2-butene were determined in a static system over the temperature range of 340–380°C and pressure range of 37–134 torr. The reaction in seasoned vessels, even in the presence of a free radical inhibitor, is catalyzed by hydrogen bromide. Under maximum catalysis of HBr, the kinetics were found to be of order 1.0. The reaction, when maximally catalyzed with HBr, appears to undergo a molecular elimination of HBr which follows first-order kinetics. The products are 1,2-butadiene and hydrogen bromide. The rate coefficients. under maximum catalysis, are given by the Arrhenius equation log ??₁(s^?1) = (13.57 ± 0.56) ? (200.4 ± 6.8) kJ mol^?1 (2.303RT)^?1. The catalyzed pyrolysis of 2-bromo-2-butene appears to proceed through a six-membered cyclic transition-state type of mechanism.