Influence of Substituents on the Rotational Energy Barrier of Axially Chiral Biphenyls, II

Abstract

The influence of substituents on the rotational energy barrier of a series of 2,2'-bis(trifluoromethy1)-and 2,2'-diisopropylbiphenyl derivatives with various substitution patterns was investigated by dynamic gas chromatography (DGC) with selectively modified cyclodextrins and by high-performance liquid chromatography (HPLC) on microcrystalline cellulose triacetate combined with polarimetry. Investigation of the steric effect of substituents was possible by gradually increasing the bulkiness of substituents in the ortho positions of 2,2'-disubstituted biphenyls and determination of the corresponding energy barriers. The buttressing effect of alkyl groups adjacent to the ortho substituents is discussed. Comparison of the energy barriers of 2,2'-bis(trifluoromethy1)biphenyl (1) with its derivatives possessing alkyl groups in positions 3 and 5, respectively, allows a discrimination between the electronic and steric contributions of the meta substituents and thus the determination of the influence on the energy barrier. The electronic effect on the rate of rotation, i.e. the rate of enantiomerization, of axially chiral biphenyls bearing substituents in the para positions depends on the aromatic system to which they are attached. Electron-donating groups decrease the rotational energy barrier of 2,2'-bis(trifluoromethy1)biphenyl derivatives but increase the barrier of 2,2'-diisopropylbiphenyl derivatives. Electron-accepting groups exhibit the opposite behavior. The unexpected effects in the case of para-substituted 2,2'-diisopropylbiphenyl derivatives are possibly related to C-H/n interactions.Atropisomeric biphenyls were successfully separated by high-performance liquid chromatography (HPLC) on microcrystalline cellulose triacetate on a preparative scale ['] and by capillary gas chromatography using modified cyclodextrins as chiral stationary phases[2,3]l, respectively. Rotation about the central bond of an axially chiral biphenyl causes enantiomerization. Accordingly, enantiomer separation requires configurational stability of at least 90 kJ mol-I. Rotamers possessing suitable configurational stability and volatility for dynamic gas chromatographic (DGC) investigations show temperature-dependent interconversion profiles and coalescence phenomena as a result of competition between resolution and enantiomerization in the gas chromatographic column. Computer simulation of experimentally obtained elution profiles provides rate constants and energy barriers. In this way, dynamic The configurational stability of atropisomeric biphenyls depends on nature, position, and number of the substituents. The bulkiness of the substituents in the ortho positions plays the most important role with respect to conformational stability of biphenyls. Similarly, steric interactions are decisive for the buttressing effect of meta substituents. They reduce the flexiblity of the adjacent ortho substituents and therefore support their steric interactions in the almost coplanar transition state. Nevertheless, electronic contributions to the i...

Atropselektive Synthese axial‐chiraler Biaryle

“…[46,56,57] So ist die Rotation im Tetraiod-Biphenyl 31 b (Abbildung 6) merklich schwieriger als in seinem Diiod-Analogon 31 a (DG°2 98 K = 126 kJ mol À1 bzw. DG°2 98 K = 98 kJ mol À1 ).…”

“…[56] para-Substituenten beeinflussen die Rotationsbarriere vor allem durch elektronische Effekte, [43,57] wie anhand der 4,4'-substituierten Biphenyle 24 deutlich wird. [57] Elektronendruck durch Resonanz seitens des Substituenten an C4 erhöht den sp 3 -Charakter an C1, was die "Out-of-plane"-Schwingung an diesen Positionen erleichtert. Dies reduziert die Spannung im Übergangszustand und senkt so die Rotationsbarriere (24 b-e).…”

Quantifizierung der Stationärphasen‐ und Lösungsmitteleinflüsse auf die Stereodynamik von biphep‐Liganden durch dynamische Drei‐Säulen‐HPLC

“…Eine geeignete Verbindungsreihe, um diese Effekte aufzuzeigen, sind fluxionale axial-chirale Biaryle, bei denen die Barriere stark durch das Substitutionsmuster beeinflusst wird. [4,5] Aus diesem Grunde lag der Schwerpunkt der Ligandensynthese bisher auf der Anwendung atroper o-tetrasubstituierter Biphenyle, die über einen großen Temperaturbereich stereochemisch stabil sind. [2] o-Substituierte Biphenyle sind bei Raumtemperatur stereochemisch instabil ("tropos"), [3] ebenso wie auch o,o'-disubstituierte Biphenyle, außer im Falle sperriger Substituenten.…”

Heteroacene Synthesis through C−S Cross‐Coupling/5‐endo‐digCyclization

“…Surprisingly, the configurational stability of terphenyls has been far less investigated than that of biphenyls . In particular, the isomerization of 2,2′,5′,2′′‐tetrabromoterphenyl or even 2,2′‐dibromobiphenyl has not yet been determined . At slightly elevated temperatures (60 °C in [D 8 ]toluene, 500 MHz) six sharp resonances were observed in the aromatic region corresponding to the two sets of three resonances of meso ‐ 6 and rac ‐ 6 (Figure ).…”