yield of dimerization and stereoselectivity in favor of the syn-head-to-head (syn-HH,
Fig. 36) were observed for CAs irrespective of the substituent on the aromatic ring.
Inferences regarding the complex stability were derived from this work as high
dimerization efficiency corresponded to the proximity of the alkene bond. However,
at that time, no inferences regarding the complex structure (i.e., electronic/steric
origins of HT vs. HH arrangement, Fig. 36) based on stereoselectivity were derived.
Later studies, especially in the last decade, including our group [92], have delved
into the stereoselectivity aspect of these reactions and deduced their supramolecular
implications.
For instance, photoactivation of alkyl cinnamates (Fig. 37, R 1 ¼ ÀMe, ÀEt,
ÀiPr) within cavitands yielded anti-HT dimers [92]. The dominance of the HT
dimer, with respect to the syn-HH dimer, was directly proportional to the size of
the alkyl group; methyl cinnamate afforded approximately equal amounts of HH and
HT dimers; and isopropyl cinnamates yielded exclusively anti-HT dimers, while
CAs yielded exclusively syn-HH dimers in all cases. This progressive inversion in
dimer stereoselectivity from free carboxylic acid to isopropyl ester indicated the
impact of supramolecular interaction (alkyl. . .alkyl sterics) on the relative alignment
of alkenes within the cavitand. However, a more important, previously unstated,
inference was also derived from this work. The fact that CAs exclusively afforded
syn-HH dimer suggests that HH arrangement is favored within the cavity, which
could be attributed to two directing interactions, π-π (aromatic rings) and hydrogenbonding interaction (carboxylic acids). However, whether either or both interactions
Fig. 36 Product distribution in photochemistry of cinnamic acids (CAs). Four isomeric dimers are
possible syn-HH (syn-head-to-head); anti HH (anti head-to-head); syn HT (head-to-tail); and antiHT (anti head-to-tail)
Photophysicochemical Processes Directed Within Nano-Containers
357
Fig. 36) were observed for CAs irrespective of the substituent on the aromatic ring.
Inferences regarding the complex stability were derived from this work as high
dimerization efficiency corresponded to the proximity of the alkene bond. However,
at that time, no inferences regarding the complex structure (i.e., electronic/steric
origins of HT vs. HH arrangement, Fig. 36) based on stereoselectivity were derived.
Later studies, especially in the last decade, including our group [92], have delved
into the stereoselectivity aspect of these reactions and deduced their supramolecular
implications.
For instance, photoactivation of alkyl cinnamates (Fig. 37, R 1 ¼ ÀMe, ÀEt,
ÀiPr) within cavitands yielded anti-HT dimers [92]. The dominance of the HT
dimer, with respect to the syn-HH dimer, was directly proportional to the size of
the alkyl group; methyl cinnamate afforded approximately equal amounts of HH and
HT dimers; and isopropyl cinnamates yielded exclusively anti-HT dimers, while
CAs yielded exclusively syn-HH dimers in all cases. This progressive inversion in
dimer stereoselectivity from free carboxylic acid to isopropyl ester indicated the
impact of supramolecular interaction (alkyl. . .alkyl sterics) on the relative alignment
of alkenes within the cavitand. However, a more important, previously unstated,
inference was also derived from this work. The fact that CAs exclusively afforded
syn-HH dimer suggests that HH arrangement is favored within the cavity, which
could be attributed to two directing interactions, π-π (aromatic rings) and hydrogenbonding interaction (carboxylic acids). However, whether either or both interactions
Fig. 36 Product distribution in photochemistry of cinnamic acids (CAs). Four isomeric dimers are
possible syn-HH (syn-head-to-head); anti HH (anti head-to-head); syn HT (head-to-tail); and antiHT (anti head-to-tail)
Photophysicochemical Processes Directed Within Nano-Containers
357