biphenyl A band appearing between 240 and 250 nm.9 It is
well documented that the sign of the Cotton effect corres-
ponding to the biphenyl A band is related to the twist of the
biphenyl system: the positive effect corresponds to M twist and
the negative one to P twist of biphenyl.10,11 However, in the gel
CD spectra of some dioxalamide gelators lacking any biphenyl
group a strong positive signal close to 240 nm possessing a
characteristic tail could be observed (ESIw, Fig. S3a). The
latter may originate from the circular differential scattering effect
which is also observed in the CD spectra of macromolecules with
sizes larger than l/20 nm.12 Hence, a strong positive signal at
241 nm in the CD spectrum of (S,S)-4 1-octanol gel may be
the consequence of the differential scattering effect. Recent
experimental and theoretical study of chiral 2,20-(2-butoxy)-
biphenyls showed the appearance of a positive–negative–
positive sequence of Cotton effects at around 280, 240 and
210 nm, respectively, for the (aS)-biphenyl atropisomer and
the opposite sequence for the (aR)-isomer.13 Based on the
presence of the positive Cotton effect at 281 nm in the CD
spectrum of (S,S)-4 1-octanol gel (Fig. 1a) the (aS)-configu-
ration could be ascribed to the biphenyl of assembled gelator
molecules; the expected negative (B240 nm) and positive
(B210 nm) Cotton effects of (aS)-biphenyl are masked by
strong differential scattering of opposite sign at 240 nm and
the opposite signed strong effect at 212 nm (Fig. 1a) corres-
ponding to overlapped electronic absorptions of biphenyl
(208 nm) and oxalamide (212 nm) (ESIw, Fig. S2).14 In the
CD spectrum of (S,S)-4 ethanol solution (ESIw, Fig. S3b) the
positive signals at 241 and 281 nm observed in the gel
spectrum are absent. This together with the observed increase
of the CD signals with increasing (S,S)-4 concentration (ESIw,
Fig. S4) and their decrease with increasing temperature
(Fig. 1a) points to their aggregation origin. CD spectra of
(S,S)-4 and (R,R)-4 1-octanol gels (Fig. 1b) are in the mirror
image relationship showing that for the gelators of opposite
central chirality also the opposite axial chirality of biphenyl
units is induced by gelation. The latter results taken together
consistently support the conclusion that central to axial chirality
transfer is the consequence of gelator self-assembly.
Crystal structure of rac-4 crystallized from the
DMF : dichloromethane 5 : 1 mixture provides the evidence
of central to axial chirality transfer that occurred also in the
solid state. The structure is composed of the homochiral
(S,aS,S)-4 and (R,aR,R)-4 bands assembled by hydrogen bonds
between oxalamide units in the direction of c axes (Fig. 2 and
ESIw, Fig. S7–S9). Formation of separated homo-chiral bands
represents the example of the self-sorting process during the
crystallization.1a,15 It starts with the fast interconverting
(S,aS,S)-, (S,aR,S)- and (R,aR,R)-, (R,aS,S)-diastereoisomeric
pairs in the solution followed by the selection and co-crystallization
of (S,aS,S)- and (R,aR,R)-enantiomers assembled into homochiral
bands. In the latter process the (S,S)- and (R,R)-configurations of
chiral centres are transcribed into (aS)- and (aR)-configurations of
biphenyls, respectively, as in the case of gelation. Hence, a similar
type of self-assembly and diastereomeric self-sorting are likely
to occur in the gel and in the solid state.
The analysis of the (S,aS,S)-4 hydrogen bonded dimer
found in the crystal structure of rac-4 explains the origin of
chirality transfer in the assembled molecules. In accord with
other crystal structures of bis(amino acid or amino alcohol)
oxalamide gelators16 in the Leu-oxalamide fragment the M
(medium; methyl ester) and L (large; iBu) groups are oriented
above and below the oxalamide plane with the S (small; H)
group being close to the syn-periplanar position to the oxalamide
carbonyl group (Fig. 3, inset). In the dimer, the chiral centre of
the first molecule comes in the close proximity of the biphenyl
unit of the second one (Fig. 3). In the (S,aS,S)-4 dimer the
(aS)-biphenyl configuration allows accommodation of the M
group (Fig. 3a) while the (aR)-biphenyl configuration of the
(S,aR,S)-4 dimer prevents it (Fig. 3b). The latter provides
plausible rationale for the observed self-sorting processes in
gelation and crystallization of the racemate.
The amino acid derived oxalamide gelators 4–7 incorporat-
ing 2,20-substituted proatropoisomeric biphenyl are good
gelators of aromatic solvents and various alcohols. Efficient
central to axial chirality transfer was observed for such
molecules when assembled in gel or solid state aggregates. In
contrast to molecular systems where a short 3–4 bond intra-
molecular distance between chiral centres and biphenyl units is
critical for chirality transfer to occur here we show that for the
self-assembled enantiomers of 4 it occurs despite the large, 9
bond intramolecular distance between biphenyl and chiral
centres. Crystal structure of rac-4 reveals that chirality transfer
may occur by intermolecular interactions between chiral
centres and biphenyl located in the close spatial proximity in
Temperature dependent FTIR and 1H-NMR investigations
of (S,S)-4 1-octanol gel and its CDCl3 solution show strong
tendency of intermolecular hydrogen bonding by oxalamide
units in both systems (ESIw, Fig. S5 and S6a). In the 1H-NMR
spectrum at ꢀ40 1C the appearance of doubled CH2–O and
CH2N methylene resonances is in accord with formation and slow
interconversion of (S,aS,S)- and (S,aR,S)-4 diastereoisomers
(ESIw, Fig. S6b).
Fig. 1 (a) Temperature dependent CD spectra of (S,S)-4 1-octanol
gel (temp. range 20–45 1C) and (b) the mirror image relationship of
(S,S)-4 and (R,R)-4 1-octanol gel CD spectra.
Fig. 2 Crystal structure of rac-4 (hydrogens omitted for clarity)
showing stacked corrugated bilayers of molecules of single chirality
along the crystallographic c axis.
c
7408 Chem. Commun., 2012, 48, 7407–7409
This journal is The Royal Society of Chemistry 2012
Makarevic, Janja
