Catalytic and metal-free intramolecular hydroalkoxylation of alkynes

Benzyltrimethylammonium hydroxide act as an efficient metal-free catalyst for the intramolecular hydroalkoxylation of alkynes. Notably, the use of microwave irradiation allowed reaction to operate in only two minutes. Under optimized reaction conditions, linear alkynes bearing aryl and heteroaryl substituents were successfully cyclized with good level of stereoselectivity.     

Catalytic chain transfer polymerization of isobutylene: The role of nucleophilic impurities

Fast polymerization of isobutylene (IB) initiated by tert‐butyl chloride using ethylaluminum dichloride·bis(2‐chloroethyl) ether complex (T. Rajasekhar, J. Emert, R. Faust, Polym. Chem. 2017, 8, 2852) was drastically slowed down in the presence of impurities, such as propionic acid, acetone, methanol, and acetonitrile. The effect of impurities on the polymerization rate was neutralized by using two different approaches. First, addition of a small amount of iron trichloride (FeCl₃) scavenged the impurity and formed an insoluble · impurity complex in hexanes. The polymerization rate and exo‐olefin content were virtually identical to that obtained in the absence of impurities. Heterogeneous phase scavenger (FeCl₃) exhibited better performance than homogenous phase scavengers. In the second approach, conducting the polymerization in wet hexanes, the fast polymerization of IB was retained in the presence of impurities with a slight decrease in exo‐olefin content. ¹H NMR studies suggest that nucleophilic impurities are protonated in the presence of water, and thereby neutralized. Mechanistic studies suggest that the rate constant of activation (k_a), rate constant of propagation (k_p), and rate constant of β‐proton elimination (k_tr) are not affected by the presence of impurities. To account for the retardation of polymerization in the presence of impurities, delay of proton transfer to monomer in the chain transfer step is proposed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3697–3704     

Synthetic and mechanistic studies of metal-free transfer hydrogenations applying polarized olefins as hydrogen acceptors and amine borane adducts as hydrogen donors

Metal-free transfer hydrogenation of polarized olefins (RR'C=CEE': R, R' = H or organyl, E, E' = CN or CO(2)Me) using amine borane adducts RR'NH-BH(3) (R = R' = H, AB; R = Me, R' = H, MAB; R = (t)Bu, R' = H, tBAB; R = R' = Me, DMAB) as hydrogen donors, were studied by means of in situ NMR spectroscopy. Deuterium kinetic isotope effects and the traced hydroboration intermediate revealed that the double H transfer process occurred regio-specifically in two steps with hydride before proton transfer characteristics. Studies on substituent effects and Hammett correlation indicated that the rate determining step of the H(N) transfer is in agreement with a concerted transition state. The very reactive intermediate [NH(2)=BH(2)] generated from AB was trapped by addition of cyclohexene into the reaction mixture forming Cy(2)BNH(2). The final product borazine (BHNH)(3) is assumed to be formed by dehydrocoupling of [NH(2)=BH(2)] or its solvent stabilized derivative [NH(2)=BH(2)]-(solvent), rather than by dehydrogenation of cyclotriborazane (BH(2)NH(2))(3) which is the trimerization product of [NH(2)=BH(2)].     

Carbon-to-metal hydrogen atom transfer: direct observation using time-resolved infrared spectroscopy

We report the direct spectroscopic observation of hydrogen atom transfer reactions from carbon to metals, in which homolytic cleavage of a C-H bond is accomplished at a single metal center. Laser flash photolysis (355 nm) of a solution of [Cp(CO)2Os]2 leads to homolysis of the Os-Os bond and formation of the osmium-centered radical, Cp(CO)2Os*, as observed by time-resolved infrared (TRIR) spectroscopy. DFT computations on Cp(CO)2Os* support this assignment. Continuous photolysis (lambda > 300 nm) of [Cp(CO)2Os]2 in the presence of excess 1,4-cyclohexadiene produces the osmium hydride Cp(CO)2OsH. The kinetics of this carbon-to-metal hydrogen atom transfer were examined by TRIR spectroscopy. The second-order rate constant for hydrogen atom transfer from 1,4-cyclohexadiene to Cp(CO)2Os* in hexane at 23 degrees C is kH = (2.1 +/- 0.2) x 106 M-1 s-1. The pKa of Cp(CO)2OsH was determined as 32.7 in CH3CN, and use of a thermochemical cycle provided an estimated lower limit of 82 kcal/mol for the Os-H bond dissociation energy, indicating that it is an exceptionally strong M-H bond. Photolysis of [Tp(CO)2Os]2 (Tp = hydridotris(pyrazolyl)borate) results in carbon-to-metal hydrogen atom transfers from even stronger C-H bonds (THF or toluene) and produces Tp(CO)2OsH.     

Catalytic metal-free intramolecular hydroaminations of non-activated aminoalkenes: a computational exploration

Frustrated Lewis pairs (FLPs) has been applied to catalytic metal-free hydrogenation. Can the FLP reactivity be used for catalytic hydroamination? Using density functional theory (DFT) calculations, we have explored whether the molecules cat1-cat3, which were previously designed by integrating the dearomatization-aromatization effect and the FLP reactivity, can catalyze the intramolecular hydroaminations of non-activated aminoalkenes to afford nitrogen heterocycles. The study shows that the γ-aminoalkene (am1) hydroamination catalyzed by cat1 proceeds via two steps (aminoalkene N-H bond activation and C-N bond formation) with experimentally accessible energetics, giving the five-membered nitrogen heterocycle product 1,1-dimethylpyrrolidine. The N-H bond activation is reversible. The C-N bond formation step undergoes a concerted mechanism and complies with the Markovnikov addition rule. Possible side reactions which may cause catalyst deactivation were confirmed to be energetically unfavorable. The molecules cat2 and cat3 are less effective than cat1 in catalyzing the am1 hydroamination, but the barriers are not too high. By following the most favorable pathway of the cat1-mediated am1 hydroamination, we further extended the substrate (am1) to other aminoalkenes, including the methyl and phenyl β-substituted am1 (i.e. am2 and am3, respectively), the benzyl-protected primary aminoalkene (am4), and the δ-aminoalkene (am5). The hydroaminations of am2 and am3 have energetics comparable with am1 hydroamination, the am5 hydroamination is energetically less favorable, and the am4 hydroamination is least favorable but could be realizable by elevating the temperature and pressure. We call experimental efforts to synthesize cat1-cat3 or similar new molecules on the basis of the design strategy.     

Influence of gaseous hydrogen on the hydrogen transfer reaction between phenanthrene and hydrogen donors

Transfer hydrogenation of phenanthrene was performed in the presence of superbases or strong acids and gaseous hydrogen. The influence of hydrogen on the yield of these reactions was discussed with respect to the mechanism of hydrogen transfer.     

Catalytic polymerization of olefins

Russian Chemical Bulletin -     

Catalytic hydrogen transfer over MgO, VII. Dehydrogenation of pinocarveol and pinocampheol

The dehydrogenation of pinocarveol to pinocarvone and pinocampheol to pinocamphone over MgO by hydrogen transfer to aldehydes and ketones as acceptors has been studied. The yields of pinocarvone formed from the unsaturated alcohol did not exceed 10 mol%, while pinocampheol was dehydrogenated to the desired ketone with yields in the range of 40 mol%.     

Surface-initiated PET-RAFT polymerization under metal-free and ambient conditions using enzyme degassing

An open-to-air method for the efficient synthesis of surface-tethered polymer brushes based on photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization is reported. Key to this approach is an enzyme-assisted strategy using glucose oxidase to facilitate the in situ removal of oxygen during the polymerization process. Control experiments in the absence of glucose oxidase confirm the importance of enzymatic deoxygenation for successful polymerization of a variety of acrylamide, methacrylate, and acrylate monomers. In accordance with controlled polymerization kinetics, a linear increase in brush height as a function of irradiation time for a range of light intensities is demonstrated. Importantly, the use of light to mediate growth and the inherent monomer versatility of PET-RAFT allow for the facile fabrication of well-defined polymer brushes under aqueous conditions. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 70–76     

Formation and crystal structure of an unexpected inclusion complex of a metal-free phthalocyanine and oxalic acid

Treatment of 3-(2,4-dimethyl-3-pentyloxy)phthalonitrile (2) with CeCl3 in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in n-pentanol gives the corresponding metal-free phthalocyanine 3, which unexpectedly traps the oxalic acid in the crystal lattice forming a 1:1 inclusion complex.     

Catalytic formation of hydrogen peroxide from H2 and O2 over hydrogen transfer agent-anchored Pd°/beta zeolites

Hydrogen transfer agents (HTAs) have been successfully anchored on Pd°/H-beta zeolite and exhibit enhanced catalytic activity in the formation of H₂O₂ from H₂ and O₂ at atmospheric pressure compared with that of the Pd°/H-beta and the HTA included catalysts.     

Catalytic hydrogen transfer over MgO, VI dehydrogenation of 2-ethyl-1-hexanol

The dehydrogenation of 2-ethyl-l-hexanol to 2-ethylhexanal by hydrogen exchange with aliphatic aldehydes has been studied over MgO. As hydrogen acceptors acetaldehyde, propionaldehyde and isobutyraldehyde were used. Reaction with propionaldehyde was found to be an effective synthetic route for 2-ethylhexanal preparation, whereas during reactions with acetaldehyde and isobutyraldehyde a gradual catalyst deactivation vs. time-on-stream was observed.Part V in Stud. Surf. Sci. Catal., 78, 631 (1993) Fine Chemicals III), and references therein.     

Interaction between biimidazole complexes of ruthenium and acetate: hydrogen bonding and proton transfer

The hydrogen bonding and deprotonation processes between four ruthenium biimidazole complexes, namely [Ru(bpy)(2)(BiimH(2))](PF(6))(2) (1, bpy is bipyridine, BiimH(2) is 2,2'-biimidazole), [Ru(bpy)(2)-(BbimH(2))](PF(6))(2) (2, BbimH(2) is 2,2'-bibenzimidazole), and [Ru(bpy)(2)(DMBbimH(2))](PF(6))(2) (3, DMBbimH(2) is 7,7'-dimethyl-2,2'-bibenzimidazole) and [Ru(bpy)(2)(TMBbimH(2))](2+) (4, TMBbimH(2) is 5,6,5',6'-tetramethyl-2,2'-bibenzimidazole), and acetate are investigated. Their hydrogen bonded adducts are indeed trapped and observed by absorption spectra and electrochemical experiments in acetonitrile solution in the presence of an excess of acetic acid for the first time. The binding constants log K(B) for these adducts are 6.74 for 1·OAc, 7.11 for 2·OAc, 7.26 for 3·OAc, and 6.99 for 4·OAc. A new approach to calculate the deprotonation constant is also developed by establishing a set of circular equilibria. The equilibrium constants for the first deprotonation step of the complexes log K(A) are 2.74 for 1, 5.19 for 2, 4.54 for 3, and 3.78 for 4. The pK(a1) values of the complexes in acetonitrile solution are calculated by subtracting log K(A) from pK(a) (HOAc in acetonitrile), giving 19.6 for 1, 17.1 for 2, 17.8 for 3, and 18.5 for 4. The degree of proton transfer (D(PT)) can be quantified by the calculation of absorption spectral and redox data, which is 0.41 for 1·OAc, 0.53 for 2·OAc, 0.57 for 3·OAc, and 0.47 for 4·OAc. Interestingly, the binding constant log K(B) (7.26) and D(PT) value (0.57) both reach their maxima at a critical point, where pK(a1) for the complex is 17.8 and ΔpK(a) for the adduct is 4.5 (ΔpK(a) = pK(a)(HOAc) - pK(a1), in acetonitrile solution). Moreover, the binding constant log K(B) shows linear correlation with the degree of proton transfer D(PT).     

Catalytic intermolecular hydrogen transfer in the hydrogenation of heterocyclic aldehydes and ketones. A review

Literature data and the investigation results of the present authors on the reduction of heterocyclic aldehydes and ketones using catalytic hydrogen transfer have been analyzed.Riga Institute of Organic Synthesis, Riga LV-1006, Latvia. Translated from Khimiya Geterotskilicheskikh Soedinenii, No. 4, pp. 435–449, April, 1994. Original articla submitted November 29, 1993.