Thermolabile Kohlenwasserstoffe, XII. Konformationsanalyse von 1,2‐Dialkyl‐1,2‐diarylethanen

Abstract

Die quantitative Konformationsanalyse von 1,2-Dialkyl-1,2-diarylethanen 1 durch Rechnungen mit EFF-und EHMO-Methoden zeigt, da8 alle meso-Diastereomeren von 1 und die DL-Formen la-c und e die anti-Konformation einnehmen; die DL-Formen I d , f und g dagegen bevorzugen wegen der starken geminalen Repulsion ihrer voluminosen tert-Butyl-und Si(CH,),-Reste das Rotamere vom gauche-I-Typ. -Diese Ergebnisse werden durch Rontgenstrukturanalysen von mesound DL-Id sowie NMR-Daten und Dipolmomentmessungen6b) bestatigt. Einheitlich gedeutet werden die Unterschiede des konformativen Verhaltens dieser 1,2-Dialkyl-l,f-diarylethane im Vergleich zu symm. Tetraalkyl-2) und Tetraar~lethanen~) (1) durch das Ausman der Repulsion zwischen den geminalen Substituenten R. Arylreste erzeugen wegen ihrer Form einen deutlich geringeren geminalen Druck als Alkylreste.Thermolabile Hydrocarbons, XII1a,b) Conformational Analysis of 1,2-Dialkyl-l,2-diarylethanes Quantitative conformational analysis by means of EFF and EHMO calculations shows that the meso-diastereomers of 1 ,Zdialkyl-I ,Zdiarylethanes 1 and the DL-diastereomers la-c, e adopt the anti-conformation. In contrast, the DL-diastereomers I d , f , g prefer the gauche-I-conformation because of the geminal repulsion of the large C(CH,), and Si(CH3), groups. -These computional results are confirmed by the X-ray structural analysis of meso-and DL-Id and by NMR and dipole moment") measurements. -The differences in conformational behavior between these 1,2-dialkyl-l,2-diarylethanes and the sym. tetraalkyl-2) and tetraarylethanes3) (1) is uniformly explained by the magnitude of repulsion between geminal substituents R. Because of their shape, aryl groups produce less geminal hindrance than alkyl groups.Geometrie und Energieinhalt der Konformationen 1 ,I ,2,2-tetrasubstituierter Ethane 1 standen im Mittelpunkt einer Reihe neuerer Untersuchungenz,3).

Conformational behaviour of highly hindered ethanes: meso- and racemic-2,2,2′,2′-tetramethyl-1,1′-biindanyl

“…ptyl derivative. The central C-C bond lengths in 9 and 10 were 1.573-1.589 A (15), considerably longer than those in 1.…”

“…Agreement between the calculated and experimental C(1)-C(1') bond lengths was also reasonably good, 1.565 A in c l , compared to 1.554 (5) A in the X-ray results and 1.555 and 1.520 A by MM3 and AM1, respectively. In meso-l,2-di-tert-butyl-l,2-diphenylethane, the central C-C bond length varies from 1.573 to 1.577 A in different molecules in the unit cell, while it is 1.589 A in the racemic isomer (15), suggesting that the current molecules are less strained. In addition, the critical C(2)-C(1)-C(1')-C(2') angle is only 1.6" larger in the STO-3G results than measured by X-ray diffraction.…”

“…The conformational properties of 9 and 10, the noncyclized analogues of 1 and 2, have been investigated by means of molecular mechanics calculations, X-ray crystallography, and NMR measurements (15). In contrast to the current observations, in the solid state and solution, the meso compound 9 was present in the anti conformation, whereas the racemic compound 10 had the phenyl groups gauche in the solid state and included 1,l'-diphenyl-1,I'-bicyclopentyl (17,28).…”

“…Barriers about bonds between two tertiary carbons are normally much lower; for instance, the barrier for rotation about the bond between the two isopropyl groups in 2,3-dimethylbutane is about 18 kJ mol-' (10). Where higher barriers were observed about bonds between two tertiary carbons, several very large groups have always been present at both ends of the bond and the barriers observed have often resulted from correlated motion of several groups (8, 1 1,[13][14][15]. There has also been considerable interest in bond lengths (16,17), in bond dissociation energies (1 1, 15), and in the applicability of molecular mechanics calculations (1 8) to compounds containing sterically congested bonds of this type.…”

ESR study of persistent thioaminyls, N-(arylthio)-3,5-di-tert-butylphenylaminyls

“…The hyperfine splitting (hfs) constants, g values, and Xm" obtained for 2 are summarized in Table I, together with those for the unsubstituted thioaminyl 4, which was previously re--NS 4 ported. 8 The aminyls 2 were also generated by hydrogen abstraction from 1 using fert-butoxyl radicals which were obtained on thermolysis of di-fert-butyl diperoxyoxalate or by photolysis of 1 using a high-pressure mercury lamp. However, the first procedure, the oxidation of 1 with lead dioxide, was found to be the most effective for generation (8) Y. Miura and M. Kinoshita, Bull.…”