Geok Kheng Tan

Anion-controlled stereoselective synthesis of cyclobutane derivatives by solid-state [2 + 2] cycloaddition reaction of the salts of trans-3-(4-pyridyl) acrylic acid.

Two stereoisomers of cyclobutane derivatives with pyridyl and carboxylic acid functionalities have been stereoselectively synthesized by a solid-state photochemical [2 + 2] cycloaddition reaction in quantitative yields. The head-to-head and head-to-tail parallel orientations of the monomers, required to obtain these two isomers, have been controlled by the anions present in the salts. The photoinert behavior of these salts in solution signifies the importance of the solid-state synthesis of these cyclobutane derivatives.

Crystal engineering studies on the salts of trans-4,4′-stilbenedicarboxylic acid in the context of solid state [2 + 2] cycloaddition reaction

A series of molecular salts of trans-4,4′-stilbenedicarboxylic acid (H2SDC) with various amines (cyclohexylamine 1, ethylenediamine 2, 1,3-diaminopropane 3, 1,4-diaminobutane 4, cystamine 5, guanidine 6, 4-aminopyridine 7, piperizine 8) have been synthesized and their structures have been determined by X-ray crystallography to understand the influence of various dominant non-covalent interactions in their crystal packing to attain energy minima in the solid state architectures. The well-defined patterns of these interactions have been analysed to assign the corresponding graph set notations and the concept of ‘supramolecular synthon’ has been justified. The presence of olefinic double bond and its orientations in these molecular salts make it interesting to study their photoreactivity towards UV light for [2 + 2] cycloaddition reaction. In the salt of 1,3-diaminopropane, the dianion was found to align perfectly parallel, congenial for solid state [2 + 2] cycloaddition reaction, undergoing quantitative dimerization under a UV lamp. Although the dianion was found to align perfectly parallel in salt 1, the photostability of this salt justifies Schmidts distance criterion. The slip-stacked orientation of the dianion makes the molecular salt 4 photostable. A zigzag water cluster chain stabilised by various H-bonding interactions was identified in the molecular salt 6. The photoreactivity of the salts with 3,3′-dipropylamino amine 9 and with ammonia 10 are also accounted to discuss the possible relation between the chain length of the dications.

η1 and η2 complexes of λ3-1,2,4,6-thiatriazinyls with CpCr(CO)x

The title heterocyclic radicals coordinate to either 17e CpCr(CO)3 or 15e CpCr(CO)2 moieties as one-electron or as three-electron donors, respectively; in the former the bonding is via the perpendicular p orbital of the sulfur atom, while in the latter bonding is via p(pi) orbitals on both sulfur and nitrogen.

Hemilabile behavior of a thioether-functionalized N-heterocyclic carbene ligand

The truly hemilabile nature of a novel thioether-functionalized N-heterocyclic carbene ligand is demonstrated in a range of Pd(II) complexes.

Solid state photodimerization of trans-2-(4-pyridyl)-4-vinylbenzoic acid via salt formation and isomerisation of cyclobutane compounds in solution

Several salts of trans-2-(4-pyridyl)-4-vinylbenzoic acid (HPVBA) obtained by reacting with inorganic acids and organic diamines are discussed in the context of their solid state packing and photoreactivity. The acids CF3CO2H, HClO4, HNO3, H2SO4 produced salts where H2PVBA+ cations are oriented parallel in head-to-tail (HT) fashion, however, the trifluoroacetate salt did not undergo photodimerization. The head-to-tail (HT) parallel orientation was inversed to head-to-head (HH) orientation by incorporating excess H2SO4 during salt formation. Thus two stereoisomers viz. HT- and HH-dimers were obtained from salts derived from same salt-former. Salts were also made exploiting carboxylic acid functionality by employing ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane and it was found that only the salt obtained with 1,4-diaminobutane undergo photodimerization. In an analogy, it is shown that in spite of poor predictability, the strategy of salt formation works well and has significant impact in the synthesis of functional cyclobutane derivatives.

Curved crystal morphology, photoreactivity and photosalient behaviour of mononuclear Zn(II) complexes

Solid-state [2 + 2] photocycloaddition reactivity, mechanical motion during cycloaddition reaction under UV light and naturally curved single crystals are reported in structurally similar mononuclear Zn(II) coordination complexes containing fluoro derivatives of 4-styryl pyridine ligands.

8-Methyl-2-[4-(trifluoro­meth­yl)phen­yl]-8H-pyrazolo­[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine methanol disolvate

In the title compound, C14H10F3N7·2CH4O, the heterocyclic ring system is essentially planar (r.m.s. deviation = 0.009 Å) and makes a dihedral angle of 6.91 (8)° with the attached benzene ring. In the crystal, the main molecules form centrosymmetric R 2 2(8) dimers via pairs of N—H⋯N hydrogen bonds between the amino groups and pyrimidine N atoms. One of the independent methanol molecules and its inversion equivalent are linked to the dimers via O—H⋯N and N—H⋯O hydrogen bonds, forming R 4 4(16) graph-set motifs. The dimers along with the hydrogen-bonded methanol molecules are stacked along the a axis, with π–π interactions between the pyrazole and triazole rings [centroid–centroid distance = 3.4953 (10) Å].

Microbial transformation of the sesquiterpenoid (-)-maaliol by Mucor plumbeus.

Microbial transformation of the sesquiterpenoid (-)-maalioxide by the fungus Mucor plumbeus gave three metabolites, 9beta-hydroxymaalioxide, 1beta-hydroxymaalioxide and 7beta-hydroxymaalioxide. 9beta-hydroxymaalioxide and its structure was established on the basis of its spectroscopic properties and chemical reactions.

3-Pyridin-2-yl-1H-1,2,4-triazol-5-amine.

In the title compound, C7H7N5, the non-H atoms are almost coplanar (r.m.s. deviation = 0.050 Å), with the N atom of pyridine ring oriented to the N—N(H) side of the 1,2,4-triazole ring. The mean planes of the pyridine and 1,2,4-triazole rings form a dihedral angle of 5.58 (7)°. The N atom of the amino group adopts a pyramidal configuration. The molecules are linked into a two-dimensional network parallel to (10) by N—H⋯N hydrogen bonds.

PdCl[PPh2CH2CH2CHCH2CH2PPh2] as a precursor to homo- and hetero-metallic species directed by ESMS (electrospray ionisation mass spectrometry)

The cyclometallated Pd compound [upper bond 1 start]PdCl[PPh(2)CH(2)CH(2)C[upper bond 1 end]HCH(2)CH(2)PPh(2)], obtained from PdCl(2)(Ph(2)P(CH(2))(5)PPh(2)) in refluxing DMF (N,N-dimethylformamide), was characterized by NMR and X-ray single-crystal diffraction analysis. The cyclometallation of MCl(2)(Ph(2)P(CH(2))(5)PPh(2))(M = Pd, Pt) to give and its Pt(ii) analogue was probed using electrospray ionisation mass spectrometry (ESMS). The reactivity of towards neutral ligands such as phosphines and pyridines as well as basic metal complexes such as pyridine-thiolate compounds of Au(i), Hg(ii) and Pt(ii) in solution was also investigated. The results showed that the chloride trans to the Pd-C bond is susceptible to ligand replacement. A number of entry metalloligands was examined in an attempt to establish a route to cyclometallated aggregates and clusters.