tuned by variation of the size and number of o-aryl
substituents.4 These attractive features have led to applica-
tions as a chiral synthetic intermediate,5 molecular switch,6
chiral switch,7 and molecular torsional balance.8 In these
examples, the rates of the Caryl-Nimide bond rotation were
controlled by raising or lowering the temperature. A key
limitation of this approach is that elevated temperatures also
disrupt the supramolecular interactions that are used to
control the rotamer equilibrium. Thus, we began to explore
other methods of attenuating the N-arylimide rotational
barrier. In this paper, we report the unusual ability of
carboxylate guests to catalyze bond rotations in N-arylsuc-
cinimides containing urea recognition groups. The magnitude
of the effect was characterized and possible mechanisms for
the guest-accelerated bond rotation were explored.
into a urea group via a two-step process. A Curtis rearrange-
ment using diphenylphosphoryl azide (DPPA) in the presence
of benzyl alcohol yielded benzyl carbamate 5. Deprotection
of 5 gave an intermediate amine, which was directly
converted to the desired atropisomeric ureas 1a and 1b via
treatment with benzyl isocyanate.
Confirmation of the structure and restricted rotation of urea
1
1a was provided by X-ray crystallographic and H NMR
analyses. Two different crystal polymorphs of 1a were
isolated from slow evaporation in ethyl acetate and recrys-
tallization from hot acetonitrile (Figure 2).10 Both X-ray
N-Arylsuccinimides 1a and 1b containing urea recognition
groups were synthesized by identical routes (Scheme 1). The
Scheme 1. Synthesis of Atropisomeric Ureas 1a and 1b
Figure 2. Selected molecular units from the two crystal polymorphs
of urea 1a. The polymorphs have three and two different molecular
units, respectively. The conformations of the ureas in each crystal
were very similar. Therefore, a single representative structure is
shown for each X-ray structure.
structures confirmed the formation of the N-arylsuccinimide
ring.
In addition, the steric hindrance about the Caryl-Nimide
single bond was evident from the nonplanar conformation
of the N-aryl and succinimide rings, which were twisted out
of plane by 70° to 82°. The axial chirality generated by the
nonplanar conformation was also evident from the 1H NMR
spectrum of 1a and 1b. The methylene protons of the
succinimide ring are diastereotopic due to their proximity
to the Caryl-Nimide chiral axis. The protons are well resolved
at rt with geminal coupling of 18 Hz.
Restricted rotation about the Caryl-Nimide bond was char-
acterized by measuring the rates of isomerization or the rates
of interconversion of the rotamers (Table 1). In the case of
the higher barrier 1a, the enantiomeric rotamers were
sufficiently stable at rt to allow separation by chiral HPLC
(Chiralpak IC (250 × 4.6 mm) 1% v/v IPA:CH2Cl2). The
rotational barriers were calculated from the measured rates
of isomerization (kisom) of enantiomerically enriched samples
of 1a. For example, in hot xylenes (130 °C), the measured
kisom of 9.9 × 10-5 s-1 corresponds to a rotational barrier
(∆Gq) of 31.7 kcal/mol. In the case of the lower barrier 1b,
the rotational barriers were calculated from the rates of
interconversion as measured by variable-temperature 1H
higher barrier 1a had two o-aryl substituents (urea and methyl
groups), whereas the lower barrier 1b had only one o-aryl
substituent (a urea group). The syntheses began with the
thermal neat condensation of diphenyl succinic anhydride 2
with an anthranilic acid (3a or 3b) to cleanly yield N-
arylsuccinimide 4.9 The carboxylic acid in 4 was transformed
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