Isolation and synthesis of (-)-(5S)-2-imino-1-methylpyrrolidine-5- carboxylic acid from Cliona tenuis: structure revision of pyrostatins

[reaction: see text] (-)-(5S)-2-Imino-1-methylpyrrolidine-5-carboxylic acid (1), previously reported as the N-acetyl-beta-d-glucosaminidase inhibitor pyrostatin B, has been isolated from the organic extracts of the burrowing sponge Cliona tenuis. The structure of 1, including its absolute stereochemistry, was characterized from its spectral data and chemical transformations and confirmed by total synthesis. The synthesis of 1 reveals that the structure of pyrostatin B has been incorrectly assigned. Comparison of NMR spectral data strongly suggests that pyrostatins A and B are identical to 5-hydroxyectoine and ectoine, respectively.     

Use of alpha-chlorinated N-(tert-butanesulfinyl)imines in the synthesis of chiral aziridines

[reaction: see text] Reaction of chiral alpha-chloro tert-butanesulfinyl aldimines with Grignard reagents efficiently afforded beta-chloro N-sulfinamides in high diastereomeric excess. The latter compounds were cyclized toward the corresponding chiral aziridines in a high-yielding one-pot reaction or after separate treatment with base. The diastereoselectivity obtained in the newly synthesized beta-chloro sulfinamides is explained via the coordinating ability of the alpha-chloro atom with magnesium resulting in the opposite stereochemical outcome as generally observed for nonfunctionalized N-sulfinyl imines.     

Spatial sign preprocessing: a simple way to impart moderate robustness to multivariate estimators

The spatial sign is a multivariate extension of the concept of sign. Recently multivariate estimators of covariance structures based on spatial signs have been examined by various authors. These new estimators are found to be robust to outlying observations. From a computational point of view, estimators based on spatial sign are very easy to implement as they boil down to a transformation of the data to their spatial signs, from which the classical estimator is then computed. Hence, one can also consider the transformation to spatial signs to be a preprocessing technique, which ensures that the calibration procedure as a whole is robust. In this paper, we examine the special case of spatial sign preprocessing in combination with partial least squares regression as the latter technique is frequently applied in the context of chemical data analysis. In a simulation study, we compare the performance of the spatial sign transformation to nontransformed data as well as to two robust counterparts of partial least squares regression. It turns out that the spatial sign transform is fairly efficient but has some undesirable bias properties. The method is applied to a recently published data set in the field of quantitative structure-activity relationships, where it is seen to perform equally well as the previously described best linear model for these data.     

Cyclopropanation of poly(isoprene) using NHC-Cu(I) catalysts: Introducing carboxylates

The incorporation of functional groups into unsaturated polyolefine-polymers often represent a challenging task. Based on the known cyclopropanation of double bonds with diazoesters in the presence of metal-catalysts of low molecular weight compounds, we in this article develop an approach to decorate the polymer backbone of poly(diene)s with ester as well as carboxylic groups via cyclopropanation. Therefore, predominantly cis-1,4-poly(isoprene)s are converted with ethyl or tert-butyl diazoacetate using copper(I) N-heterocyclic carbene (NHC) catalysts, while focusing on the technically relevant cyclohexane as solvent. The application of commercially available NHC-Cu(I) catalysts results in modification degrees of 4–5%, while an increased solvent polarity, like dichloromethane, results in up to 17% modification. The resulting esters were further converted to the corresponding free carboxylic groups by deprotection using trifluoroacetic acid. Thus, an introduction of functional groups along the polymer backbone with a wide variety of application, like ionic interaction or hydrogen bonding motifs, was successfully demonstrated. Its potential for upscaling makes this approach feasible for an application in large-scale production processes, such as for manufacturing of modified synthetic rubbers.     

Promoting the Electrocatalytic Performance of Noble Metal Aerogels by Ligand-Directed Modulation

Noble metal aerogels (NMAs) are an emerging class of porous materials. Embracing nano-sized highly-active noble metals and porous structures, they display unprecedented performance in diverse electrocatalytic processes. However, various impurities, particularly organic ligands, are often involved in the synthesis and remain in the corresponding products, hindering the investigation of the intrinsic electrocatalytic properties of NMAs. Here, starting from laser-generated inorganic-salt-stabilized metal nanoparticles, various impurity-free NMAs (Au, Pd, and Au-Pd aerogels) were fabricated. In this light, we demonstrate not only the intrinsic electrocatalytic properties of NMAs, but also the prominent roles played by ligands in tuning electrocatalysis through modulating the electron density of catalysts. These findings may offer a new dimension to engineer and optimize the electrocatalytic performance for various NMAs and beyond.     

Fast Titanium-Catalyzed Hydroaminomethylation of Alkenes and the Formal Conversion of Methylamine

The scientific interest in catalytic hydroaminoalkylation reactions of alkenes has vastly increased over the past decade, but these reactions have struggled to become a viable option for general laboratory or industrial use because of reaction times of several days. The titanium-based catalytic system introduced in this work not only reduces the reaction time by several orders of magnitude, into the range of minutes, but the catalyst is also demonstrated to be easily available from common starting materials, at a cost of approximately 1 € per millimole of catalyst. We were also able to formally perform C−H activation of methylamine and achieve coupling to a broad variety of alkenes, through silyl protection of the amine and simple deprotection by water.     

A Detailed investigation of the experimental conditions for the reversible addition fragmentation chain transfer‐mediated copolymerization of acrylonitrile and butadiene

The synthesis of acrylonitrile‐butadiene rubbers (NBRs) via trithiocarbonate‐mediated reversible addition fragmentation chain transfer (RAFT) polymerization of acrylonitrile (ACN) and 1,3‐butadiene (BD) in solution under azeotropic conditions (38/62) was investigated for a broad range of common solvents: N,N‐dimethylacetamide (DMAc), chlorobenzene, 1,4‐dioxane, tert‐butanol, isobutyronitrile, toluene, trimethylacetonitrile, dimethyl carbonate, acetonitrile, methyl acetate, acetone, and tert‐butyl methyl ether. The gravimetrically determined conversions for the free radical polymerizations of ACN/BD after 22 h at 100 °C were in the range of 15% for methyl acetate to 35% for DMAc. The origin of the differences in conversion is attributed to the unequal decomposition behavior of the employed azo initiator 2,2′‐azobis(N‐butyl‐2‐methylpropionamide) ( 1 ) in the solvents under investigation, as determined by ultraviolet–visible (UV–vis) spectroscopy. Relative decomposition of 1 in solution (0.1 mol L^?1) at 100 °C was calculated from the UV–vis spectra for selected solvents. 90% of 1 in DMAc was decomposed after 22 h, 83% in tert‐butanol, 57% in 1,4‐dioxane, 53% in isobutyronitrile, 45% in chlorobenzene, and 21% in toluene. The evolution of molecular weight with conversion using the initiator 1 was in accordance with the theoretically expected values, regardless of the solvent studied. Moreover, the RAFT‐mediated copolymerization of ACN/BD in DMAc with azo initiators 1 , 1‐[(1‐cyano‐1‐methylethyl)azo]formamide ( 2 ) and 1,1′‐azobis(cyclohexanecarbonitrile) ( 3 ) was investigated. A strong deviation from the linear evolution of molecular weight due to a fast decomposition of these initiators – congruent with high primary radical delivery rates – at the selected temperature was observed when using 2 and 3 . The deviation was not observed when using 1 . © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Nanofibrillated cellulose/nanographite composite films

Though research into nanofibrillated cellulose (NFC) has recently increased, few studies have considered co-utilising NFC and nanographite (NG) in composite films, and, it has, however been a challenge to use high-yield pulp fibres (mechanical pulps) to produce this nanofibrillar material. It is worth noting that there is a significant difference between chemical pulp fibres and high-yield pulp fibres, as the former is composed mainly of cellulose and has a yield of approximately 50 % while the latter is consist of cellulose, hemicellulose and lignin, and has a yield of approximately 90 %. NFC was produced by combining TEMPO (2,2,6,6-tetramethypiperidine-1-oxyl)-mediated oxidation with the mechanical shearing of chemi-thermomechanical pulp (CTMP) and sulphite pulp (SP); the NG was produced by mechanically exfoliating graphite. The different NaClO dosages in the TEMPO system differently oxidised the fibres, altering their fibrillation efficiency. NFC–NG films were produced by casting in a Petri dish. We examine the effect of NG on the sheet-resistance and mechanical properties of NFC films. Addition of 10 wt% NG to 90 wt% NFC of sample CC2 (5 mmol NaClO CTMP-NFC homogenised for 60 min) improved the sheet resistance, i.e. from that of an insulating pure NFC film to 180 Ω/sq. Further addition of 20 (CC3) and 25 wt% (CC4) of NG to 80 and 75 wt% respectively, lowered the sheet resistance to 17 and 9 Ω/sq, respectively. For the mechanical properties, we found that adding 10 wt% NG to 90 wt% NFC of sample HH2 (5 mmol NaClO SP-NFC homogenised for 60 min) improved the tensile index by 28 %, tensile stiffness index by 20 %, and peak load by 28 %. The film’s surface morphology was visualised using scanning electron microscopy, revealing the fibrillated structure of NFC and NG. This methodology yields NFC–NG films that are mechanically stable, bendable, and flexible.     

CuH‐Catalysed Hydroamination of Styrene with Hydroxylamine Esters: A Coupled Cluster Scrutiny of Mechanistic Pathways

A detailed computational exploration of mechanistic intricacies of the copper(I) hydride (CuH)‐catalysed hydroamination of styrene with a prototype hydroxylamine ester by a recently reported [(dppbz)CuH] catalyst (dppbz≡{P^P}≡1,2‐bis(diphenylphosphino)‐benzene) is presented. A variety of plausible mechanistic avenues have been pursued by means of a sophisticated computational methodology, from which a general understanding of the factors controlling hydroamination catalysis emerged. The catalytically competent {P^P}Cu^I hydride, which is predominantly present as its dimer, involves in irreversible hydrocupration proceeding with complete 2,1 regioselectivity to form a secondary {P^P}Cu^I benzyl intermediate. Its interception with benzylamine ester produces the branched tertiary amine product and {P^P}Cu^I benzoate upon intramolecular S_N2 disruption of the amine electrophile′s N?O linkage, to precede a highly rapid, strongly exergonic C?N bond‐forming reductive elimination. The {P^P}Cu^I benzoate corresponds to the catalyst resting state and its conversion back into the {P^P}Cu^I hydride upon transmetalation with a hydrosilane is turnover limiting. The effect of electronic perturbations at the amine electrophile upon the reaction rate for productive hydroamination catalysis and also non‐productive reduction of the hydroxylamine ester has been gauged, which unveiled a more fundamental insight into catalytic structure‐performance relationships.     

Organic nanoparticles as fragmentable support for Ziegler–Natta catalysts

A fragmentable support material for Ziegler–Natta catalysts is presented based on micrometer‐sized aggregates of polystyrene nanoparticles. Hydroxyl anchoring groups are introduced by copolymerization of hydroxymethylstyrene in emulsion process to immobilize the catalysts. The catalytic activity in ethylene slurry polymerizations is found to be directly correlated to the hydroxyl group content of the supports. Furthermore, the fragmentation behavior of dye‐labeled support aggregates into the initial nanoparticles is demonstrated using laser scanning confocal fluorescence microscopy as a nondestructive method. These supported catalysts fulfill two important design criteria, high fragmentability and high catalyst loading, and produce high‐density polyethylene with medium molecular weight distributions (MWDs = 3–4). These values lie between those obtained using single‐site metallocene‐based (narrow MWD < 3) or inorganic supported multi‐site Ziegler–Natta‐based (broad MWD = 4–12) polymerizations without the need of blending. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 15–22