Jan Sandström

Recent advances in the chemistry of 1,3,4-thiadiazoles.

Publisher Summary This chapter discusses homologs, functional groups, physical properties, and the uses of 1, 3, 4-thiadiazole. The development of 1, 3, 5- thiadiazole chemistry is linked to the discovery of phenylhydrazine and hydrazine. The numbering of the 1, 3, 4-thiadiazole ring follows from 1. Clockwise or anticlockwise numbering is used according to convenience. In 2- or 5-imino-, -oxo-, or -thiono-1, 3, 4- 4-thiadiazoles, the position of the “extra” hydrogen atom is shown by the corresponding number in parentheses, e.g., 1, 3, 4-thiadiazolin-2(3)-one(2). Analysis of the microwave spectra of 1, 3, 4-thiadiazole and three isotopically substituted species can determine the structure of the molecule with an uncertainty of 0.03 A in the coordinates of the hydrogen atoms and of less than 0.003 A in the coordinates of the other atoms. By an analysis of differences between the measured bond lengths and the covalent radii, the authors came to the conclusion that the aromatic character, as measured by the r-electron delocalization, decreases in the order 1, 2, 5-thiadiazole > thiophene > 1, 3, 4-thiadiazole > 1, 2, 5- oxadiazole.

Barriers to topomerization* in iminodithiocarbonates : A hammett correlation and complete lineshape study☆

Abstract The rate of topomerization at the CN bond in a series of p-substituted dimethyl N-phenyl-iminodithiocarbonates has been studied by the NMR technique at the coalescence temperature. For the p-methoxy compound the rate has been measured over a larger temperature interval by the complete lineshape method, and the enthalpy and entropy of activation have been found to be 11·8 ± 0·4 kcal/mole and − 11·1 ± 1·7 e.u. The entropy of activation is more negative than expected for an inversion path, whereas the low reaction constant in the Hammett correlation supports this mechanism.

Determination of absolute configurations and conformations of organic compounds by theoretical calculations of CD spectra

The usefulness and limitations of the CNDO/S method for calculations of rotational strengths of inherently chiral chromophores are illustrated with examples from bridged biphenyls. The more empirical Schellman matrix method is employed to calculate CD spectra of compounds containing two chirally disposed chromophores. Analysis of the CD spectra of a compound containing two interacting thioamide groups gives detailed conformational information. Analysis of the CD spectra of two closely analogous 1,2,3,4-tetrahydro-2-aryl-N-formyl-pyridines with spiro structure shows how a small structural change can lead to the appearance of a nearly mirror image CD spectrum with retained configuration at the spiro atom. Chirality 12:162–171, 2000.

Static and dynamic stereochemistry of alkyl and analogous groups

Publisher Summary This chapter discusses the static and dynamic stereochemistry of alkyl and analogous groups. It discusses the forces that determine the conformations of organic molecules and the experimental and theoretical methods that are available for use in conformational analysis. The experimental and theoretical material is organized with respect to the central bonds, sp3-sp3 or sp2-sp3, and also with respect to the atoms involved. The selection of examples is not intended to be exhaustive, but is governed by a wish to display typical trends and by the bias created by our own research experience. The chapter discusses an example, provided by the tri-t-butyl-methanes, -silanes and -phosphines, where the same type of energy minima (C3) and the same exchange processes occur in spite of considerably different bond lengths. Two isopropyl groups bound to an sp2 hybridized atom or to vicinal atoms in a planar five-membered ring comprise another structural element with similar behavior in several different environments. This entity also illustrates the dangers of reliance on simple and rigid mechanical models. Concerted rotation is a rarer occurrence than might be expected, although this mechanism is implied by molecular mechanics calculations for tri-t-butylmethane and analogs, and is convincingly demonstrated by experiments for the ditriptycyl derivatives.

Studies of polarized ethylenes—VIII1: Conformational analysis of twisted ethylenes

Abstract Push-pull ethylenes have been prepared, which in the ground state are twisted around the double bond, and the barriers for passage through a planar transition state have been measured with DNMR. The influence of steric and electronic factors on the rotational barriers has been investigated. Compounds have also been prepared where two barriers could be measured, one for passage through the planar state, and the other for passage through a 90° twisted state.

The UV and CD spectra of a 60∘ twisted bridged biphenyl: an experimental and CNDO/S study

Abstract The UV and CD spectra of l,2,3,4-dibenzo-1,3-cyclooctadiene show six bands between 280 and 192 nm. With the aid of qualitative composite molecule arguments and CNDO/S calculations on a biphenyl model the bands are assigned in turn to the in-phase 1 L a combination (277 and 266 nm, “H” transition), the in-phase 1 L a combination (232 nm, “A” transition), the in-phase and out-of-phase combinations of the 1 B b transitions (210 and 206 nm, the latter previously denoted the “B” transition). A higher transition of uncertain origin is found at 192.5 nm. The calculated signs of the rotational strengths of the “H” and “A” transitions agree with experiments, but the calculated energies of the 1 B b combination fall in the wrong order, with the positive, in-phase transition at the higher energy. The same result, at variance with the experi- mental one, is obtained by using the coupled oscillator technique with the 1 L b and 1 B b transitions.

Interactions between CS groups in 1,2- and 1,3-bis(thiocarbonyl) compounds: a study by spectroscopy, x-ray crystallography, and CNDO/S calculations

Abstract Through-bond interactions between thiocarbonyl groups separated by one and two single bonds have been studied by UV-visible and CD spectroscopy and by CNDO/S calculations. The compounds discussed are dithion- and tetrathio-oxalates, dithiooxamides, and five- and six-membered cyclic dithioimides. The effects are most clearly seen in the n →π ∗ transitions, for which a maximum of four are expected on the basis of the splitting of the n and π ∗ levels. In most cases, two n →π ∗ bands are observed, but in 90° twisted dithiooxamides all four bands are observed in the CD spectrum. The absolute configuration of a chiral dithiooxamide is determined by X-ray crystallography.

Enantiomers of polarized alkenes: chromatographic enrichment and thermal interconversion

Abstract (+)- and (−)- 1 as well as (+)- and (−)- 2 were partially separated by liquid chromatography on triacetylcellulose; racemizations resulted in the barriers (Table 2) to rotation via planarized transition states.

Barriers to ring inversion in dibenzo[a,c]cyclooctene and its dihydro and tetrahydro derivatives by racemization and NMR bandshape studies

Abstract Dibenzo[a,c]cyclooctene and its 9,10-dihydro and 9,10,11,12-tetrahydro derivatives have been resolved by chromatography and their inversion barriers determined by thermal racemization. The 9,12-dihydro derivative has been studied by 1 H NMR bandshape technique.

The internal rotation of acyl groups in acylferrocenes. A 1H and 13C DNMR study

Abstract The barriers to acyl group rotation in a series of acylferrocenes, including 1,1′-diacetylferrocene, have been determined by 1 H and 13 C DNMR. Complete lineshape analysis over wide temperature ranges gave ΔG ≠ 173 8.79 ± 0.03 kcal mol −1 , ΔH ≠ 7.5 ± 0.3 kcal mol −1 and ΔS ≠ −8 ± 2 cal mol −1 K −1 for [ 2 H]formlyferrocene and ΔG ≠ 173 8.51 ± 0.02 kcal mol −1 , ΔH ≠ 7.1 ± 0.2 kcal mol −1 and ΔS ≠ −8 ± 2 cal mol −1 for acetylferrocene. The unexpected high barrier in diferrocenylketone (ΔG ≠ 143 6.8 ± 0.2 kcal mol −1 is interpreted in terms of strain in the transition state, and the relatively low barrier in 1,1′-diacetylferrocene (ΔG ≠ 163 7.8 ± 0.3 kcal mol −1 ) is ascribed to an interannular effect.