Enantioselective Photochemistry
271
obtained at lower temperature (60% ee at –45 °C compared to 5% ee at
30 °C).
4
Sensitised Reactions
Both energy transfer and electron transfer from a photoexcited compound to a given substrate are distance dependent. This property allows
one to delineate—at least on paper—a catalytic cycle for a sensitised
process with an appropriately modified template (Scheme 14). If the
passive tetrahydronaphthalene shield in 12 is replaced by a photoactive moiety, this part of the compound can, after excitation, facilitate
an energy or electron transfer significantly faster at a bound than at an
unbound substrate.
The second property of the photoactive moiety is identical to the previously used shield, that is it must be bulky and rigid enough to guarantee the desired enantioface differentiation. After the reaction, the binding site at the sensitiser can be occupied by another substrate molecule
unless product dissociation is disfavoured. In most photochemical processes the product is more space demanding than the substrate so that
the equilibrium is shifted favouring substrate association. From our experience there are three major constraints to the ideal picture depicted
in Scheme 14. First, intermolecular sensitisation becomes increasingly
important if the substrate–template association is not sufficiently strong.
Second, most substrates are not UV transparent but have a significant
absorption band, frequently at shorter wavelengths than the sensitiser.
Direct excitation can compete with sensitisation. Third, the lifetime of
the sensitiser is limited by intra- and intermolecular decomposition processes. Commonly used carbonyl sensitisers absorb light with an energy
content of 300–400 kJ Es
–1 . If this energy is not quickly distributed
bond cleavage reactions are unavoidable.
4.1
Possible Templates and Substrates
The ideal substrate for a sensitised reaction should absorb light at
a wavelength, which is at least 50 nm shorter than the absorption maximum of the sensitiser. It should coordinate with an association constant, which is significantly higher than its dimerisation constant, and it
271
obtained at lower temperature (60% ee at –45 °C compared to 5% ee at
30 °C).
4
Sensitised Reactions
Both energy transfer and electron transfer from a photoexcited compound to a given substrate are distance dependent. This property allows
one to delineate—at least on paper—a catalytic cycle for a sensitised
process with an appropriately modified template (Scheme 14). If the
passive tetrahydronaphthalene shield in 12 is replaced by a photoactive moiety, this part of the compound can, after excitation, facilitate
an energy or electron transfer significantly faster at a bound than at an
unbound substrate.
The second property of the photoactive moiety is identical to the previously used shield, that is it must be bulky and rigid enough to guarantee the desired enantioface differentiation. After the reaction, the binding site at the sensitiser can be occupied by another substrate molecule
unless product dissociation is disfavoured. In most photochemical processes the product is more space demanding than the substrate so that
the equilibrium is shifted favouring substrate association. From our experience there are three major constraints to the ideal picture depicted
in Scheme 14. First, intermolecular sensitisation becomes increasingly
important if the substrate–template association is not sufficiently strong.
Second, most substrates are not UV transparent but have a significant
absorption band, frequently at shorter wavelengths than the sensitiser.
Direct excitation can compete with sensitisation. Third, the lifetime of
the sensitiser is limited by intra- and intermolecular decomposition processes. Commonly used carbonyl sensitisers absorb light with an energy
content of 300–400 kJ Es
–1 . If this energy is not quickly distributed
bond cleavage reactions are unavoidable.
4.1
Possible Templates and Substrates
The ideal substrate for a sensitised reaction should absorb light at
a wavelength, which is at least 50 nm shorter than the absorption maximum of the sensitiser. It should coordinate with an association constant, which is significantly higher than its dimerisation constant, and it