Sethuraman Sankararaman

Donor-acceptor substituted phenylethynyltriphenylenes – excited state intramolecular charge transfer, solvatochromic absorption and fluorescence emission

Summary Several 2-(phenylethynyl)triphenylene derivatives bearing electron donor and acceptor substituents on the phenyl rings have been synthesized. The absorption and fluorescence emission properties of these molecules have been studied in solvents of different polarity. For a given derivative, solvent polarity had minimal effect on the absorption maxima. However, for a given solvent the absorption maxima red shifted with increasing conjugation of the substituent. The fluorescence emission of these derivatives was very sensitive to solvent polarity. In the presence of strongly electron withdrawing (–CN) and strongly electron donating (–NMe2) substituents large Stokes shifts (up to 130 nm, 7828 cm−1) were observed in DMSO. In the presence of carbonyl substituents (–COMe and –COPh), the largest Stokes shift (140 nm, 8163 cm−1) was observed in ethanol. Linear correlation was observed for the Stokes shifts in a Lippert–Mataga plot. Linear correlation of Stokes shift was also observed with E T(30) scale for protic and aprotic solvents but with different slopes. These results indicate that the fluorescence emission arises from excited state intramolecular charge transfer in these molecules where the triphenylene chromophore acts either as a donor or as an acceptor depending upon the nature of the substituent on the phenyl ring. HOMO–LUMO energy gaps have been estimated from the electrochemical and spectral data for these derivatives. The HOMO and LUMO surfaces were obtained from DFT calculations.

Highly Selective Synthetic Transformations Catalyzed by Lithium Perchlorate in Organic Media

Highly concentrated solutions of lithium perchlorate in various organic solvents have been used as media for many synthetic transformations at room temperature which are otherwise difficult to carry out under ordinary conditions. The rates of several organic reactions are faster in these media compared than in pure solvents. More importantly high chemo-, regio- and stereoselectivities have been observed. The mild Lewis acidity of the lithium ion in donor solvents such as diethyl ether is responsible for these selectivities. Fine tuning of the Lewis acidity of the lithium ion, depending upon the solvent basicity and polarity, has been possible for selective activation of organic substrates in these media. The activation of carbonyl- and other oxygen-containing organic compounds in 5 M lithium perchlorate in diethyl ether (LPDE) and in nitromethane (LPNM) is presented.

Conformational Isomers from Rotation of Diacetylenic Bond in an Ethynylpyrene-Substituted Molecular Hinge

The first example of isolation and X-ray crystallographic structural characterization of two conformers arising from rotation along a diacetylenic bond is reported. In both the conformers extensive pi-pi interactions are observed in the solid state. VT-NMR and fluorescence spectroscopic studies in solution suggest that the closed and open conformers are in equilibrium and that the closed conformer is the predominant species at room temperature.

Catalytic asymmetric hydrogenation using a [2.2]paracyclophane based chiral 1,2,3-triazol-5-ylidene–Pd complex under ambient conditions and 1 atmosphere of H2

Chiral 1,2,3-triazol-5-ylidene–Pd complexes with the planar chiral [2.2]paracyclophane wing tip group have been synthesized and structurally characterized. The complex with a labile acetonitrile co-ligand is an excellent catalyst for chemoselective hydrogenation of alkynes and alkenes and enantioselective hydrogenation of prochiral alkenes at ambient conditions and 1.0 atmosphere of H2.

Hydrogen-bond network in isomeric phenylenedipropynoic acids and their DABCO salts. Water mediated helical hydrogen bond motifs

The crystal structures of isomeric phenylenedipropynoic acids (PPA) are reported. 1,2-PPA shows an intra- and intermolecular hydrogen bonded 2D layered sheet structure. 1,4-PPA also has a 2D layered sheet structure. However the hydrogen bond is mediated by water molecules in the lattice. 1,3-PPA has an interpenetrating 3D structure without any void. The lattice of 1,3-PPA consists of two types of helical hydrogen bond network mediated by water molecules. The crystal structures of the DABCO salts of the 1,2-PPA and 1,4-PPA also show an interesting 3D and 2D hydrogen bond network, respectively.

Synthesis and structure of trans-bis(1,4-dimesityl-3-methyl-1,2,3-triazol-5-ylidene)palladium(II) dichloride and diacetate. Suzuki–Miyaura coupling of polybromoarenes with high catalytic turnover efficiencies

Summary trans-Bis(1,4-dimesityl-3-methyl-1,2,3-triazol-5-ylidene)palladium(II) dichloride has been shown to be an excellent catalyst for the multiple Suzuki–Miyaura coupling reactions of polybromoarenes to the corresponding fully substituted polyarylarenes. The reactions proceeded in excellent yields and with high turnover numbers. With 1,4-dibromobenzene the catalyst was found to be active for up to 13 consecutive cycles with a turnover number of 1260. The polyarylarenes were obtained in pure form after crystallization once without recourse to chromatographic purification. The single-crystal X-ray structures of the chloro (1) as well as the corresponding acetato (2) complexes are also reported and compared with the corresponding complexes of 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene as the ligand.

Novel Chiral Metallocenophanes Derived from [2.2]Paracyclophane and Their Use in Olefin Polymerization

The 4-fulvenyl[2.2]paracyclophane (1) was transformed, by treatment with lithium aluminium hydride and methyllithium in tetrahydrofuran, to the corresponding cyclopentadienyl anions 6 and 8, respectively. The iron complex 7 was prepared from 6, while 8 was converted into the metal complexes 9 (Fe complex), 10 (Ti), 11 (Zr), 12 (Ti), and 13 (Zr). All new metallocene complexes were characterized by their spectroscopic data. In addition, a single crystal X-ray crystallographic analysis was performed for complex 11. The metallacyclic zirconocene complex 15 was synthesized by the reaction of the dichloride derivative 13 with two equivalents of n-butyllithium. After activation of complexes 10−13 and 15 with methylalumoxane (MAO) they were used for the catalytic polymerization of ethene and propene.

2,6-Bis(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine and its octahedral copper complex.

In the tridentate ligand 2,6-bis(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine, C(23)H(19)N(7), both sets of triazole N atoms are anti with respect to the pyridine N atom, while in the copper complex aqua[2,6-bis(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine](pyridine)(tetrafluoroborato)copper(II) tetrafluoroborate, [Cu(BF(4))(C(5)H(5)N)(C(23)H(19)N(7))(H(2)O)]BF(4), the triazole N atoms are in the syn-syn conformation. The coordination of the Cu(II) atom is distorted octahedral. The ligand structure is stabilized through intermolecular C-H...N interactions, while the crystal structure of the Cu complex is stabilized through water- and BF(4)-mediated hydrogen bonds. Photoluminiscence studies of the ligand and complex show that the ligand is fluorescent due to triazole-pyridine conjugation, but that the fluorescence is quenched on complexation.

Synthesis of differentially protected/functionalised acetylenic building blocks from p-benzoquinone and their use in the synthesis of new enediynesElectronic supplementary information (ESI) available: 400 MHz 1H NMR and 13C NMR spectra. See http://www.rsc.org/suppdata/ob/b3/b302323k/

Sequential addition of two different lithium acetylides to p-benzoquinone yielded diastereomeric mixtures of 1,4-diethynylcyclohexa-2,5-diene-1,4-diols wherein the two ethynyl groups bear different protective/functional groups. Selective deprotection to the terminal acetylene followed by Pd(0) mediated coupling with Z-1,2-dichloroethene yielded new enediynes bearing cyclohexa-2,5-diene units.

Chemoselective aldol type condensation of silyl enol ethers and acetals in 5 mol dm–3 lithium perchlorate–diethyl ether

Acetals are efficiently and chemoselectively converted into the corresponding aldol ethers upon treatment with 1-trimethylsilyloxycyclohexene 2 in 5 mol dm–3 lithium perchlorate–diethyl ether (LPDE) at ambient temperature with moderate diastereoselectivity, whereas under the same conditions aldehydes and ketals fail to react with 2. The present method allows the acetal substitution to be carried out under neutral reaction and work up conditions. A mechanism involving the formation of an oxocarbenium ion intermediate from the acetal followed by nucleophilic addition of the silyl enol ether is proposed. The observed chemoselectivity is attributed to the mild Lewis acidity of the lithium ion in diethyl ether.