Racemisierungsversuche an optisch aktiven Diphensäuren. (Zur Stereochemie aromatischer Verbindungen, V)

Abstract

Die optisch aktive o,o'-Dinitrodiphensaure zeichnet sich nach unserer letzten Mitteilung l) durch eine aufierordentliche Stabilitat aus. Unter Bedingungen, die bis an den Eeginn tiefgreifender Zersetzung gewahlt wurden, widerstand sie allen Versuchen der Racemisierung. Es iiberraschte uiis daher, dafi nach G. H. C h r i s t i e nnd J. K e n n e r 2 ) die noch haher substituierte o-p-0'-p'-Tetranitrodiphensaure als Natriumsalz in waflriger Losung schon bei Zimmertemperatur in einigen Tagen die Aktivitat vollstandig einbiiflen sollte. Man hat j a die Ursache fur die Spaltbarkeit substituierter Diphensauren in der Wechselwirkung zwischen den Substituenten zu snchen, welche der raumlichen Einstellung der Benzolkerne mehr oder minder groSe Stabilitat verleiht.InUbereinstimmung mit G. H. C h r i s t i e und J. K e n n e r konnte die o-p-or-p'-Tetranitrodiphensaure (I) leicht in optisch aktiver Form erhalten werden. Sie widersetzte sich dagegen auch unter den scharfsten Bedingungen allen Racemisierungsversuchen. Da sie leichter zersetzlich ist als die o-0'-Dinitrodiphensaure, konnten allerdings die Versuchsbedingungen nicht so weitgehend variiert werden wie dort.

Artificial Molecular Rotors

“…[760][761][762] These belts (207)(208)(209) were shown to complex small organic molecules 762 and fullerenes, 763,764 which showed interesting dynamic properties. A variable-temperature NMR analysis showed that C 60 (210) rotates rapidly inside ring 207a, even at -100°C…”

Stereoisomerism in Nanohoops with Heterogeneous Biaryl Linkages of E/Z- and R/S-Geometries

“…The racemization proceeded as the first-order reaction and by fitting the racemization curves to k = t –1 ·ln(CD 0 /CD), 28,29 the rate constant and half-life were estimated as k = 1.6 × 10 –5 s –1 and t 1/2 = 12 h at 30 °C, respectively. 29,43 The temperature-dependence of the rate constant was analyzed using the Eyring equation to obtain the energy barriers of Δ H ‡ = +23 kcal/mol and Δ S ‡ = −3.7 cal/mol·K (Figure S10). As expected, the energetics of the racemization were similar to those observed with 1,1′-binaphthyl ( Δ H ‡ = +22 kcal/mol, Δ S ‡ = −5.2 cal/mol·K).…”

“…At this linkage, on the other hand, the energy barrier at R/S transitions becomes substantially high (+20 kcal/mol) and divides R - and S -geometries as two distinct species. This barrier at the R/S transitions is experimentally confirmed by the separation of enantiomers (see also below). − Hence, the descriptors of biaryl linkages are appropriately selected by considering the energy profiles of the biaryl rotations. In the case of 1,1′-binaphthyl, for instance, we should select one of the R/S descriptors and describe the specific geometry situated in the low-energy valley between two energy barriers at the R/S transitions. , …”

“…As a reference, we first performed the racemization analysis of an enantiomer of 9,9′-phenanthrenyl, (−)- 4 . According to the methods established for biaryls, − the racemization process was followed by the decay of the maximum CD signal at 302 nm for the 30–60 °C temperature range (Figures S6–S9). Representative data at 30 °C are shown in Figure a.…”

Hindered rotation in a 1-alkylnapthalene. A dynamic nuclear magnetic resonance, difference nuclear overhauser enhancement, and molecular mechanics investigation of 1-neopentylnaphthalene. The t-butyl group as a probe of chirality

“…( 3),3b and the rotational barrier is low when both substituents are tetrahedral, e.g (4).3u*b*d Planar substituents can adopt stable ground states with few interactions, e.g. (5) for (2). In contrast, tetrahedral substituents interfere drastically with each other so that any ground state is destabilised; further, interconverting ground states may be structurally close one to another as in (4), which involves a six-fold barrier.…”