Ap Conformation Research Articles

Dipole moments and conformation of α-disulphones. The gauche effect

The dipole moments of substituted diphenyl disulphones I-III were measured in benzene solution and interpreted by the previously described graphical method. The results suggest an equilibrium of the ap and sc conformations in the ratio 3 : 1, at variance with various empirical rules and theories of the so-called gauche effect. A statistical treatment of X-ray data of various sulphonyl derivatives revealed essentially two preferred conformations: the more stable sc, with a broadened dihedral angle, and the less stable ap.

9-DIMETHYLAMINO-9,10-DIHYDRO-9,10-ETHENOANTHRACENE N-OXIDE: AN EXAMPLE WHICH SHOWS SLOW INTERNAL ROTATION ABOUT A Nsp3–Csp3 BOND

Abstract 9-Dimethylamino-9,10-dihydro-9,10-ethenoanthracene was prepared by reductive dechlorination of its tetrachloro derivative and was oxidized to produce the title compound. The compound crystallized as an ap conformer. On dissolution, it isomerized to its ±sc conformer with a rate constant 2.52×10−5 s−1 at 30 °C until the equilibrium (±sc/ap=0.16) was reached. The following activation parameters were obtained: ΔH≠ 23.1±4.4 kcal mol−1, ΔS≠ −3.9±14.1 e. u.

Reactivities of Stable Rotamers. XI. Rotamer Distributions and Some Addition Reactions to the Carbonyl in 9-[2-(Substituted carbonyl)-1-naphthyl]fluorene Rotamers

Abstract Rotational isomers (sp and ap) of several 9-[2-(substituted carbonyl)-1-naphthyl]fluorenes where the substituent is methyl, phenyl, hydroxyl, methoxyl, 1-imidazolyl, or others have been obtained as stable entities. Barriers to rotation in these compounds were ca. 26.5 kcal for the process sp→ap but those for the reverse (ap→sp) were dependent on the substituent, ranging from 23.8 to 25.3 kcal/mol at 55 °C. The difference is reflected in the populations of rotamers: if the substituent is a hydroxyl, the sp/ap value becomes as large as ca. 20. The relative instability of the ap form was attributed to twisting of the carbonyl plane from that of naphthalene. Infrared spectra support the twisting. Addition reactions to the carbonyl occur smoothly in the sp conformation but they do not occur to a detectable extent in the ap conformation, if the substituent on the carbonyl is larger than hydrogen. The results are attributed to the steric effect of the fluorene ring.

ChemInform Abstract: ATTRACTIVE INTERACTIONS BETWEEN CARBONYLS AND GROUPS BEARING LONE‐PAIR ELECTRONS IN TRIPTYCENE SYSTEMS

AbstractNach 4 verschiedenen Methoden werden die Triptycen‐Derivate′(III) [aus den Komponenten (Ia), (Ib) + (II)], (VI) [aus den Komponenten (I)+(IV) über die Stufen (V)], (VIII) [durch Ozonolyse entsprechender Olefine (VII)] bzw. (IX) [aus (VIIId)] synthetisiert (z.T.

Reactivities of Stable Rotamers. VIII. Difference in Reactivities of 1,2,3,4-Tetrachloro-9-(2-halo-1,1-dimethylethyl)triptycene Rotamers in Lewis Acid-catalyzed Reactions

Abstract ±sc-1,2,3,4-Tetrachloro-9-(2-chloro-1,1-dimethylethyl)triptycene was found to react in the presence of titanium(IV) chloride at room temperature to give an isomeric mixture of 1,2-dimethyl-7,8,9,10-tetrachloro-1,2,6,10b-tetrahydro-6,10b-o-benzenoaceanthrylenes. The structure of the latter was confirmed by independent syntheses. In contrast, the ap conformer of the former did not give any sign of the reaction under the same conditions but reacted when antimony(V) chloride was added to give the same mixture. Careful examination of the reaction mixture suggests that the starting material was rapidly converted to (E)-1,2,3,4-tetrachloro-9-(1-methyl-1-propenyl)triptycene and cyclization followed. Action of silver nitrate on ±sc-9-(2-bromo-1,1-dimethylethyl)-1,2,3,4-tetrachlorotriptycene afforded a mixture of the stereoisomers of the olefin while the ap form remained intact.

Attractive Interactions between Carbonyls and Groups Bearing Lone-pair Electrons in Triptycene Systems

Abstract Compounds which carry a methoxyl or a chloro group in 1-position and a substituted carbonylmethyl group in 9-position of the triptycene system have been prepared to examine populations of rotamers by 1H NMR spectra. The methoxy compounds favored the ±sc conformations, whereas the ap conformations were relatively favored in the chloro compounds. The results indicate that there are attractive interactions between the carbonyls and a methoxyl or a chloro group and are discussed from the electron-accepting ability of the carbonyl group and the electron-donating ability of the methoxyl and the chloro groups. An aldehyde group was apparently less able to interact with electron-donors than a ketone. This apparent anomaly was discussed from the intramolecular dipolar interactions.

INTRAMOLECULAR INTERACTIONS BETWEEN CARBONYL MOIETIES AND CHLORO OR METHOXYL GROUP IN TRIPTYCENE SYSTEMS

Abstract 9-Substituted 1,4-dimethoxytriptycenes, where the substituents contain a carbonyl group, favor ±sc conformations relative to the ap. 9-Substituted 1,2,3,4-tetrachlorotriptycenes exist as mixtures of ±sc and ap conformations of which ratios vary according to steric and electronic effects. Some anomalies involving aldehydes and ketones are discussed with the use of 1H NMR and IR spectra.

Calculation of solvent effect on conformation stability and anomeric effect in dimethoxymethane

Solvent effect has been investigated on relative stability of the dimethoxymethane (DMM) conformers formed by rotation about the central C-O bonds. Potential energy of the isolated molecule in the whole conformation space determined by torsion angles (φ1, φ2) has been calculated by the PCILO semiempirical quantum-chemical method. For determination of potential energy of DMM in solvent environment the procedure by Sinanoglu has been used in which the solvation energy is divided into electrostatic, dispersion and cavity terms. The calculation predicts that DMM in CCl4 solution as well as in vapour phase exists almost exclusively in its most stable(sc, sc) conformation with slight population of the (ap, sc) conformations. On the contrary, in aqueous solution an almost exclusive occurrence of the (ap, ap) conformation can be expected. The predicted abundances of the DMM conformers agree well with the available experimental data for DMM and similar molecules. Also the role of environmental factors on the anomeric (gauche) effect has been analyzed which makes itself in preference for sc to ap conformations in a number of polar molecules. It has been shown that the original intramolecular preference for the sc conformation can be compensated by intermolecular solute-solvent interactions, and the anomeric effect can disappear with increasing solvent polarity. The calculated solvation energies have been used for estimation of dilution enthalpies of DMM, too.