Synthesis and isomerism of hydrazones of 2-(5-thioxo-4,5-dihydro-1,3,4-thiadiazol-2-ylthio)acetohydrazide

New hydrazones of 2-(5-thioxo-4,5-dihydro-1,3,4-thiadiazol-2-ylthio)acetohydrazide have been obtained and the percentages of anti/syn - conformers were determined. Based on the analyses of 1H NMR spectra, it was concluded that for hydrazones obtained from the 2- hydroxybenzaldehydes and 2’-hydroxycetophenones the ratio between the anti-and syn-conformers depends on the strength of intramolecular hydrogen bond (IMHB) between the nitrogen atom of the imino group and the proton of the 2-hydroxy group. It was shown that increase in IMHB strength results in stabilization of the anti-conformer in solution.

Synthesis and Characterization of Cl2HSi-O-NMe2

O-(Dichlorosilyl)-N,N-dimethylhydroxylamine, Cl2HSiONMe2, was synthesised by the reaction of O-lithio-N,N-dimethylhydroxylamine with an excess of trichlorosilane. The compound was characterised by multinuclear NMR spectroscopy (1H, 13C, 15N, 29Si), gas-phase IR spectroscopy and mass spectrometry. The structure of Cl2HSiONMe2 was determined by gas-phase electron diffraction. It exists as two conformers in the vapour, named anti and gauche after the position of the SiH hydrogen atom relative to the SiON skeleton. The anti:gauche ratio in the vapour was determined to be 40:60. The anti conformer shows only a weak attractive interaction between the geminal donor and acceptor centres N and Si [angle Si-O-N 111.1(20)­°], whereas these interactions are stronger in the gauche conformer [angle Si-O-N 98.8(12)°]. Further structure-determining forces from weak hydrogen bridges of the Si-Cl ···H-C type and van der Waals repulsive forces are also discussed. The experimental results are in reasonable agreement with ab initio calculations at the MP2/6-311++G** level of theory.

The Low-Temperature Polymorph of Tetramethyldiphosphinebis( monoborane): Insight into the Stabilization of Different Rotational Isomers in the Solid State

The low-temperature (LT) polymorph of tetramethyldiphosphine-bis(monoborane), Me2(H3B)PP( BH3)Me2, was obtained by crystallization from diethyl ether at 4 °C. It crystallizes in the monoclinic space group P21/c, a = 6.464(1), b = 7.605(1), c = 11.867(2) Å , β = 119.99(1)° (at 153 K) with 2 molecules per unit cell. This implies that the individual molecules have crystallographic inversion symmetry and a strict anti arrangement with respect to the central B-P-P-B skeleton. At 87.6 °C (DTA) the LT polymorph transforms to the high-temperature (HT) modification which contains the anti and gauche conformers in a 1:2 ratio (P21/c, Z = 6; H. L. Carrell, J. Donohue, Acta Crystallogr. B24, 699 (1968)). This strongly suggests that the gauche conformer is higher in energy and stabilized by the crystal packing of the HT modification. The P-P-B angle in the anti LT form (113.91(6)°) ascertains the value of the anti conformer in the HT form (114.4(6)°) which was found to be significantly different from the gauche conformer which centered around 110 °C.

Three-Membered Ring Formation by Si···N Interactions in Aminosulfenylsilanes

The reaction of dimethylaminosulfenylchloride Me2NSCl with trichlorosilane HSiCl3 and triethylamine gives elemental sulphur and Me2NSiCl3. The aminosulfenylsilane Me2NSSiCl3 is postulated to be an intermediate of this reaction. Ab initio calculations (MP2/6D311G(d,p)) on Me2N-S-SiH3, Me2N-S-SiH2Cl, Me2N-S-SiH2F, and Me2N-S-SiCl3 have been carried out, demonstrating the occurrence of acute valence angles at sulphur and short Si···N distances, which are indicative of NSSi three-membered rings. The strength of the Si · · · N interactions depends on the electronegativity of the substituent at silicon in anti-position to the nitrogen atom and is strongest in the anti conformer of Me2N-S-SiH2F (< NSSi 68°, Si···N 2.208 Å ). The coordination spheres of the nitrogen atoms in the Me2N-S-SiR3 molecules are steeply pyramidal, which is in contrast to the planarised N atoms in other SNMe2 compounds, such as Me2NSCl. The crystal structure of this compound has been determined, as well as that of the product of the above reaction, Me2NSiCl3. Both have planar coordination geometries at the nitrogen atoms.

Medium-sized cyclophanes. Part 53.1 Synthesis and conformational studies, and photoinduced cyclization of syn-[n.2]metacyclophanenes

A series of [n.2]metacyclophanenes (3) and (6) were prepared in good yields by a McMurry cyclization of 1,n-bis(3-acetyl-4-methoxyphenyl)alkanes (2) and 1,3-bis(3-formyl-4-methoxyphenyl)propane (5), respectively. Compounds 3b, 3c exist in the syn-conformation due to the steric repulsion between the methyl groups at the ethano bridge and the methoxy groups at the aromatic rings while compound 6 prefers the anti-conformation typical of [3.2]metacyclophanes. The assignment of syn-conformations has been confirmed by 1H NMR analyses and X-ray diffraction studies. Photoinduced transannular cyclization of [n.2]metacyclophanenes (3) and (6) in the presence of iodine as an oxidant afforded phenanthrene-anellated polycyclic aromatic hydrocarbons. Apparently, the rate of the photocyclization of anti-6 was found to be much faster than that of syn-3b and almost completed within 1 h. Thus, the different reactivities for the irradiation of syn- and anti-conformer were observed. The reason for the present preference for the formation of trans-dihydrophenanthrene rather than cis-dihydrophenanthrene as the intermediate might be attributable to the more stable chair form transition state than boat one and the conformational fixation to the chair form in the ground and transition state is possible in the anti-conformer.Key words: cyclophanes, strained molecule, McMurry reaction, C—C coupling, conformation analysis, cyclizations, photolysis, transannular reactions, transition states.

ipso-Nitration of tert-butyl[n.2]metacyclophanes; through-space electronic interactions between two benzene rings

The selective introduction of one or two nitro groups by direct replacement of tert-butyl groups via the ipso aromatic nitration of the meta-bridged aromatic compounds having two arene rings is described. The ipso-nitration at the tert-butyl groups of syn- and anti-di-tert-butyl-dimethoxy[n.2]metacyclophanes 1 is attributed to the highly activated character of the aryl ring and the increased stabilization of a σ-complex intermediate arising from the dienone-type σ-complex intermediate possible in the case of an internally methoxy substituent. However, only one tert-butyl group is ipso-nitrated in the mild reaction conditions such as copper(II) nitrate in an acetic anhydride solution because of deactivation of the second aromatic ring by the introduced nitro group. A first σ-complex intermediate would be stabilized by two types of the through-space electronic interaction, i.e., face-to-face overlapping and intra-annular interaction with the opposing benzene ring, thus accelerating the reaction. The higher yields of mononitrated product are obtained in syn-conformer than in anti-conformer. Therefore, face-to-face overlapping might be much more favourable to stabilize the first σ-complex intermediate than the intra-annular interaction. On the other hand, nitration of 1 with fuming nitric acid afforded the two-fold ipso-nitration product 3 in quantitative yield. Thus, the number of ipso-nitration at the tert-butyl groups of 1 was strongly affected by the reactivity of the nitration reagent.Key words: cyclophanes, electrophilic aromatic substitutions, ipso-nitration, σ-complex intermediates, through-space electronic interactions.

The Molecular Structures and Conformational Preferences of Bis(dimethylstibyl)-Sulfane and -Tellurane, E(SbMe2)2, E = S or Te, Me = CH3, by Density Functional Theory Calculations and Gas Electron Diffraction

Density functional theory calculations on E(SbMe2)2, E = S or Te, Me = CH3, indicate that the equilibrium structures are syn-syn or near syn-syn conformers with overall C2v or C2 symmetry. The calculations further indicate the existence of syn-anti conformers about 4 kJ mol-1 (E = S) or 1 kJ m ol-1 (E = Te) above the equilibrium structure. G as electron diffraction data show that both conform ers are present in gaseous S(SbMe2)2, while the presence of the syn-anti conformer in gaseous Te(SbMe2)2 is uncertain. The Sb - S and Sb - Te bond distances are 241.4(4) and 278.1(3) pm, respectively, the valence angles of the synsyn conformers are <SbSSb = 9 8.7(5)° and <SbTeSb = 91(2)°.