Determination of distribution coefficients of polycyclic aromatic nitrogen heterocycles using solid-phase microextraction

The possibility and the efficiency of solid-phase microextraction (SPME) for determination of distribution coefficients of polycyclic aromatic nitrogen heterocycles (PANHs) in aqueous samples are presented in this study. Influence of sample extraction conditions, extraction time, stirring rate, pH, salinity and temperature were tested to optimize extraction efficiency. Three types of fibre sorbent layers, PDMS, PDMS/DVB and PA, were tested and fibre/water distribution coefficients, K_{f s} , were calculated. The extraction efficiency for PANHs increased in the order PDMS?<?PA?<?PDMS/DVB. Values of log?K_fs ranged from 0.2 to 2.7 for PDMS, from 0.4 to 3.6 for PA and from 1.4 to 4.4 for PDMS/DVB. The values of log?K_fs were correlated with octanol/water distribution coefficients log?K_ow . The relationship between log?K_fs and log?K_ow shows linear response with regression coefficient in the range of 0.985 and 0.997.     

Synergistic Catalysis: Enantioselective Addition of Alkylbenzoxazoles to Enals

A novel catalytic enantioselective methodology based on synergistic catalysis is reported. The strategy involves: 1) the metal‐Lewis‐acid activation of alkylazaarenes, and 2) the secondary‐amine activation of enals. Consequently, highly functionalized chiral alkylazaarenes were obtained in good yields and with reasonable stereoselectivity.     

Determination of basic azaarenes in aviation kerosene by solid-phase extraction and HPLC-fluorescence detection

SPE in combination with HPLC and fluorescence detection has been used for sensitive determination of six basic azaarenes (7,8-benzoquinoline, 7,9-dimethylbenz[c]acridine, 9-amino-1,2,3,4-tetrahydroacridine, 9-methylacridine, acridine, and dibenz[a,j]acridine) in aviation kerosene (jet fuel). SPE was performed in a single step using a strong cation exchange sorbent. The HPLC system consisted of C18 column with a selected detection program of optimal λ_exc and λ_em. A gradient elution with ACN and phosphate buffer (pH 6.5) at a flow rate of 1 mL/min allowed efficient and fast separation of azaarenes within 15 min. The LOD and LOQ values (S/N ratio 3:1 and 10:1, respectively) were between 0.0013 and 0.021 and from 0.0044 to 0.072 ng per injection. The calibration curves showed linear behavior from the LOQ to 250 μg/L (r² >0.99). For the spiked concentration of 6.0 μg/L, recoveries were from 92 to 107% for jet fuel samples, except for 9-amino-1,2,3,4-tetrahydroacridine, which presented 68% recovery. The proposed method was applied to the quantification of those six basic azaarenes in one commercial kerosene and in three aviation kerosene samples. The presence of 7,8-benzoquinoline (up to 3.2 μg/L) and dibenzo[a,j]acridine (up to 6.3 μg/L) was confirmed in aviation kerosene.     

Asymmetric Synthesis of Adjacent Tri- and Tetrasubstituted Carbon Stereocenters: Organocatalytic Aldol Reaction of an Hydantoin Surrogate with Azaarene 2-Carbaldehydes

A bifunctional amine/squaramide catalyst promoted direct aldol addition of an hydantoin surrogate to pyridine 2-carbaldehyde N-oxides to afford adducts bearing two vicinal tertiary/quaternary carbons in high diastereo- and enantioselectivity (d.r. up to >20:1; ee up to 98 %) is reported. Acid hydrolysis of adducts followed by reduction of the N-oxide group yields enantiopure carbinol-tethered quaternary hydantoin-azaarene conjugates with densely functionalized skeletons. DFT studies of the potential energy surface (B3LYP/6–31+G(d)+CPCM (dichloromethane)) of the reaction correlate the activity of different catalysts and support an intramolecular hydrogen-bond-assisted activation of the squaramide moiety in the transition state of the catalytic reaction.     

Azaarene Dimers

Binaphthyl-3,3′,4,4′-tetraone was prepared and coupled to different bis(TIPS-ethynyl)-substituted (TIPS=triisopropyl silane) aromatic diamines, resulting in the formation of dimeric benzo-fused azaacenes, centrally connected by a single bond. The two halves of the molecules are highly twisted with respect to each other and showed limited electronic interaction in the ground state because their absorption spectra remained very similar to those of the constituting monomers. The dimers displayed greatly reduced fluorescence when compared to the monomers, suggesting that there is a significant interaction of the two azarene units in the excited state. Preliminary investigations showed that the dimers are attractive for application as acceptors in organic photovoltaic because they significantly outperform their monomeric counterparts.     

Iron-catalyzed C(sp)–H functionalization of methyl azaarenes with α-oxoesters: a facile approach to lactic acid derivatives

A highly efficient method for the C(sp³)–H functionalization of methyl azaarenes to α-oxoesters in the presence of iron(II) acetate as an inexpensive, nontoxic catalyst with moderate-to-excellent yields has been developed. This transformation represents a facile approach to medicinally important lactic acid derivatives.     

Chemoselective arylamination of β-bromovinylaldehydes followed by acid catalyzed cyclization: a general method for polycyclic quinolines

A synthesis of polycyclic quinolines is described via palladium-catalyzed chemoselective arylamination of β-bromovinylaldehydes with aromatic amines followed by acid catalyzed cyclization.     

Cyclopentannulated Dihydrotetraazapentacenes

The transition-metal-catalyzed cyclization of bissilylethynylated N,N’-dihydrotetraazapentacene (TIPS-TAP-H₂) into bissilylated cyclopenta[fg,qr]pentacenes is reported. Depending on the catalyst either none, one or two silyl groups migrate and change their positions in the formed five-membered rings. The optoelectronic properties are quite similar, whereas the packing motifs differ dramatically. Control experiments and quantum chemical calculations were performed to investigate the mechanism of the reaction and the selectivity of the silyl shift.