22 Molecular Recognition by Inclusion …
463
on two other inclusion crystals with methyl 3-tolyl sulfoxide (space group P1) and
ethyl 4-tolyl sulfoxide (P2 1 ) (Fig. 22.4b and c). Both crystal structures had a similar
one-dimensional hydrogen bonding network, and the network made a column structure from the host–guest complex. The two crystal structures differed in the direction
these columns were assembled, space group P1 arranges them in a parallel direction,
P2 1 arranges them in an antiparallel direction.
The hydrogen bond of phenyl-urea moiety with an oxygen atom of the carboxylate
made a rigid L-shaped conformation of the host, which is essential to creating the
asymmetric cavity for one preferable enantiomer of sulfoxides.
When triphenylmethylamine used an amine part instead of diphenylmethylamine,
the urea-modified phenylalanine salt crystallized without guest molecules. The amine
part was also necessary to form inclusion crystals.
More recently, Kodama reported on enantioseparation of sulfoxides and nitriles
by inclusion crystallization with chiral organic salts based on phenylalanine [16].
The host also has a binary structure composed of a hydrazide functional moiety and
a salt between a 4-hydroxybenzoic acid and chiral amine part of a phenylalanine
derivative (Fig. 22.5a).
The salt forms a one-dimensional columnar structure with a 2 1 -helical hydrogen
bonding network (Fig. 22.5b). In inclusion crystals, (S)-form (98% ee) of ethyl phenyl
sulfoxide was captured by hydrogen bonding with the phenolic hydroxy group of
the host and settling in the inclusion cavity formed between these host columns.
The phenyl hydrazide moiety needs to bind with an oxygen atom of carboxylate and
become an inclusion cavity wall to recognize guest shapes.
Similarly, several nitriles with high enantioselectivity up to 92% ee (S) were also
included in this host.
Many recent asymmetric syntheses in organic chemistry achieved high enantioselectivity. However, insufficient results give non-racemic compounds and might
require additional optical resolution. Such non-racemic mixtures of enantiomers
are base materials for optical resolution. Fogassy considers the enrichment of nonracemic mixtures of enantiomers an important part of the resolution processes
[18].
Hu reported that (3S,6S)-dipeptide derived from (S)-proline includes both enantiomers of BINOL to form a 1:1 complex of (3S,6S)-dipeptide and rac-BINOL
(Fig. 22.6) [17]. As the result, the enantiomer excess of the BINOL guest in inclusion crystals is zero, making it impossible to separate racemic BINOL. However,
this inclusion phenomenon of racemic compounds can still use the method of enantiomeric enrichment in solutions. This is a unique example of optical resolution for
non-racemic compounds. The rac-BINOL was deposited as a 1:1 complex, and enantiopure (R)- or (S)-BINOL was obtained from the filtrate. When 10% ee and 80% ee
of (R)-BINOL were applied for enantiomeric enrichment, yields of 90% and 96%
were obtained, respectively, based on the amount of enantiomeric pure BINOL (>
99% ee).
463
on two other inclusion crystals with methyl 3-tolyl sulfoxide (space group P1) and
ethyl 4-tolyl sulfoxide (P2 1 ) (Fig. 22.4b and c). Both crystal structures had a similar
one-dimensional hydrogen bonding network, and the network made a column structure from the host–guest complex. The two crystal structures differed in the direction
these columns were assembled, space group P1 arranges them in a parallel direction,
P2 1 arranges them in an antiparallel direction.
The hydrogen bond of phenyl-urea moiety with an oxygen atom of the carboxylate
made a rigid L-shaped conformation of the host, which is essential to creating the
asymmetric cavity for one preferable enantiomer of sulfoxides.
When triphenylmethylamine used an amine part instead of diphenylmethylamine,
the urea-modified phenylalanine salt crystallized without guest molecules. The amine
part was also necessary to form inclusion crystals.
More recently, Kodama reported on enantioseparation of sulfoxides and nitriles
by inclusion crystallization with chiral organic salts based on phenylalanine [16].
The host also has a binary structure composed of a hydrazide functional moiety and
a salt between a 4-hydroxybenzoic acid and chiral amine part of a phenylalanine
derivative (Fig. 22.5a).
The salt forms a one-dimensional columnar structure with a 2 1 -helical hydrogen
bonding network (Fig. 22.5b). In inclusion crystals, (S)-form (98% ee) of ethyl phenyl
sulfoxide was captured by hydrogen bonding with the phenolic hydroxy group of
the host and settling in the inclusion cavity formed between these host columns.
The phenyl hydrazide moiety needs to bind with an oxygen atom of carboxylate and
become an inclusion cavity wall to recognize guest shapes.
Similarly, several nitriles with high enantioselectivity up to 92% ee (S) were also
included in this host.
Many recent asymmetric syntheses in organic chemistry achieved high enantioselectivity. However, insufficient results give non-racemic compounds and might
require additional optical resolution. Such non-racemic mixtures of enantiomers
are base materials for optical resolution. Fogassy considers the enrichment of nonracemic mixtures of enantiomers an important part of the resolution processes
[18].
Hu reported that (3S,6S)-dipeptide derived from (S)-proline includes both enantiomers of BINOL to form a 1:1 complex of (3S,6S)-dipeptide and rac-BINOL
(Fig. 22.6) [17]. As the result, the enantiomer excess of the BINOL guest in inclusion crystals is zero, making it impossible to separate racemic BINOL. However,
this inclusion phenomenon of racemic compounds can still use the method of enantiomeric enrichment in solutions. This is a unique example of optical resolution for
non-racemic compounds. The rac-BINOL was deposited as a 1:1 complex, and enantiopure (R)- or (S)-BINOL was obtained from the filtrate. When 10% ee and 80% ee
of (R)-BINOL were applied for enantiomeric enrichment, yields of 90% and 96%
were obtained, respectively, based on the amount of enantiomeric pure BINOL (>
99% ee).