Niobium phosphates as new highly selective catalysts for the oxidative dehydrogenation of ethane

Several niobium phosphate phases have been prepared, fully characterized and tested as catalysts for the selective oxidation of ethane to ethylene. Three distinct niobium phosphate catalysts were prepared, and each was comprised predominantly of a different bulk phase, namely Nb(2)P(4)O(15), NbOPO(4) and Nb(1.91)P(2.82)O(12). All of the niobium phosphate catalysts showed high selectivity towards ethylene, but the best catalyst was Nb(1.91)P(2.82)O(12), which was produced from the reduction of niobium oxide phosphate (NbOPO(4)) by hydrogen. It was particularly selective for ethylene, giving ca. 95% selectivity at 5% conversion, decreasing to ca. 90% at 15% conversion, and only produced low levels of carbon oxides. It was also determined that the only primary product from ethane oxidation over this catalyst was ethylene. Catalyst activity also increased with time-on-line, and this behaviour was ascribed to an increase of the concentration of the Nb(1.91)P(2.82)O(12) phase, as partially transformed NbOPO(4), formed during preparation, was converted to Nb(1.91)P(2.82)O(12) during use. Catalysts with predominant phases of Nb(2)P(4)O(15) and NbOPO(4) also showed appreciable activity and selectivities to ethylene with values around 75% and 85% respectively at 5% ethane conversion. The presence of phosphorous is required to achieve high ethylene selectivity, as orthorhombic and monoclinic Nb(2)O(5) catalysts showed similar activity, but displayed selectivities to ethylene that were <20% under the same reaction conditions. To the best of our knowledge, this is the first time that niobium phosphates have been shown to be highly selective catalysts for the oxidation of ethane to ethylene, and demonstrates that they are worthy candidates for further study.     

A stereoselective, Sm(II)-mediated approach to decorated cis-hydrindanes: synthetic studies on faurinone and pleuromutilin

The cis-hydrindane motif is found in a number of natural products that display important biological activity. A flexible, stereoselective approach to the framework has been developed that features highly diastereoselective, SmI(2)-mediated cyclisations. The strategy has been exploited in the first synthesis of the proposed structure of faurinone and an approach to the skeleton of the antibacterial natural product, pleuromutilin.     

The immobilisation of chiral organocatalysts on magnetic nanoparticles: the support particle cannot always be considered inert

A systematic study concerning the immobilisation onto magnetic nanoparticles of three useful classes of chiral organocatalyst which rely on a confluence of weak, easily perturbed van der Waals and hydrogen bonding interactions to promote enantioselective reactions has been undertaken for the first time. The catalysts were evaluated in three different synthetically useful reaction classes: the kinetic resolution of sec-alcohols, the conjugate addition of dimethyl malonate to a nitroolefin and the desymmetrisation of meso anhydrides. A chiral bifunctional 4-N,N-dialkylaminopyridine derivative could be readily immobilised; the resulting heterogeneous catalyst is highly active and is capable of promoting the kinetic resolution of sec-alcohols with synthetically useful selectivity under process-scale friendly conditions and has been demonstrated to be reusable in a minimum of 32 consecutive cycles. The immobilisation of a cinchona alkaloid-derived urea-substituted catalyst proved considerably less successful in terms of both catalyst stability and product levels of enantiomeric excess. An immobilised cinchona alkaloid-derived sulfonamide catalyst was also prepared, with mixed results: the catalyst exhibits outstanding recyclability on a par with that associated with the successful N,N-dialkylaminopyridine analogue, however product enantiomeric excess is consistently lower than that obtained using the corresponding homogeneous catalyst. While no physical deterioration of the heterogeneous catalysts was detected on analysis after multiple recycles, in the cases of both the conjugate addition to nitroolefins and the desymmetrisation of meso anhydrides, significant levels of background catalysis by the nanoparticles in the absence of the organocatalyst was detected, which explains in part the poor performance of the immobilised organocatalysts in these reactions from a stereoselectivity standpoint. It seems clear that the immobilisation of sensitive chiral organocatalysts onto magnetite nanoparticles does not always result in heterogeneous catalysts with acceptable activity and selectivity profiles, and that consequently the applicability of the strategy must be ascertained (until more data is available) on a case-by-case basis.     

Tuning emission wavelength and redox properties through position of the substituent in iridium(III) cyclometallated complexes

Cyclometallated phenyls with substituents para to the metal have a larger impact on the redox potentials and emission of complexes [Ir(R-ppz)(2)(bipy)][PF(6)] than substituents at the meta position and hence enable tuning of emission wavelength over a wider range using the same substituent.     

Asymmetric synthesis of polyhydroxylated pyrrolizidines via transannular iodoamination with concomitant N-debenzylation

The doubly diastereoselective "matched" conjugate addition of lithium (R)-N-but-3-enyl-N-(α-methyl-p-methoxybenzyl)amide to tert-butyl (4S,5R,E)-4,5-O-isopropylidene-2,7-dienoate (derived from d-ribose in 3 steps) and in situ enolate oxidation with (-)-camphorsulfonyloxaziridine was followed by ring-closing metathesis with Grubbs I to give a hexahydroazocine scaffold. Subsequent treatment with I(2) resulted in transannular iodoamination accompanied by loss of the α-methyl-p-methoxybenzyl group to give the corresponding pyrrolizidine scaffold as a single diastereoisomer upon direct crystallization from the crude reaction mixture. Further functional group manipulations enabled the preparation of (-)-7a-epi-hyacinthacine A1.     

Marrying iterative synthesis to cascading radical cyclization: 6-endo/5-exo radical cascade across bis-vinyl ethers

Application of iterative protocols to the synthesis of functionally and stereochemically complex small molecules is an emerging area of research with the potential to create new efficiencies in complex molecule synthesis. Similarly, the discovery of tandem or cascade reactions can aid in the rapid generation of new structures for biological screening programs. This report describes a cascading 6-endo-trig/5-exo-trig radical cyclization across bis-vinyl ether substrates, which are themselves iteratively synthesized from simple building blocks.     

Highly diastereoselective and stereodivergent dihydroxylations of acyclic allylic amines: application to the asymmetric synthesis of 3,6-dideoxy-3-amino-L-talose

Aminohydroxylation of tert-butyl sorbate [tert-butyl (E,E)-hexa-2,4-dienoate] using enantiopure lithium (R)-N-benzyl-N-(α-methylbenzyl)amide and (-)-camphorsulfonyloxaziridine gives tert-butyl (R,R,R,E)-2-hydroxy-3-[N-benzyl-N-(α-methylbenzyl)amino]hex-4-enoate in >99:1 dr. Subsequent dihydroxylation under Upjohn conditions (OsO(4)/NMO) gives tert-butyl (2R,3R,4S,5S,αR)-2,4,5-trihydroxy-3-[N-benzyl-N-(α-methylbenzyl)amino]hexanoate (in 95:5 dr) while dihydroxylation under Donohoe conditions (OsO(4)/TMEDA) proceeds with antipodal diastereofacial selectivity to give the (R,R,R,R,R)-diastereoisomer (in 95:5 dr). The amino triols resulting from these dihydroxylation reactions are useful for further elaboration, as demonstrated by the asymmetric synthesis of 3,6-dideoxy-3-amino-L-talose.     

D2-symmetric dirhodium catalyst derived from a 1,2,2-triarylcyclopropanecarboxylate ligand: design, synthesis and application

Dirhodium tetrakis-(R)-(1-(4-bromophenyl)-2,2-diphenylcyclopropanecarboxylate) (Rh(2)(R-BTPCP)(4)) was found to be an effective chiral catalyst for enantioselective reactions of aryl- and styryldiazoacetates. Highly enantioselective cyclopropanations, tandem cyclopropanation/Cope rearrangements and a combined C-H functionalization/Cope rearrangement were achieved using Rh(2)(R-BTPCP)(4) as catalyst. The advantages of Rh(2)(R-BTPCP)(4) include its ease of synthesis, its tolerance to the size of the ester group in the styryldiazoacetates, and its compatibility with dichloromethane as solvent. Computational studies suggest that the catalyst adopts a D(2)-symmetric arrangement, but when the carbenoid binds to the catalyst, two of the p-bromophenyl groups on the ligands rotate outward to make room for the carbenoid and the approach of the substrate to the carbenoid.     

The effect of dipole moment and electron deficiency of analytes on the chemiresistive response of TiO2(B) nanowires

Nanostructured TiO(2)(B) thin films were found to have strong and fast chemiresistive response to nitro-aromatic and nitro-amino explosives recently. In this study, the effects of dipole moment and electron deficiency of the analyte molecules on the chemiresistive response are explored to understand the details of molecular interactions of analytes with the sensor surface which lead to charge depletion and the chemiresistive effect. It was found that the speed of the response is dominated by the polarity of the analytes and molecules with larger dipole moments produce faster responses. The degree of the response was found to be dominated by the electron deficiency of the analytes and molecules with greater electron deficiency produce stronger chemiresistive responses.     

Rh2(S-PTAD)4-catalyzed asymmetric cyclopropenation of aryl alkynes

Rh₂(S-PTAD)₄ is an effective catalyst for the asymmetric cyclopropenation of aryl alkynes using a siloxyvinyldiazoacetate as the carbenoid precursor. Upon deprotection of the silyl protecting group, highly enantioenriched cyclopropenes bearing geminal acceptor groups can be accessed. These cyclopropenes undergo regioselective rhodium(II)-catalyzed ring expansion to furans.