Ernst Schering Foundation Symposium Proceedings, Vol. 2, pp. 255–279
DOI 10.1007/2789_2007_065
© Springer-Verlag Berlin Heidelberg
Published Online: 18 December 2007
Chiral Organocatalysts for
Enantioselective Photochemical Reactions
S. Breitenlechner, P. Selig, T. Bach
(u)
Lehrstuhl für Organische Chemie I, Technische Universität München, Lichtenbergstr. 4,
85747 Garching, Germany
email: thorsten.bach@ch.tum.de
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
2
Stoichiometric Template-Based Enantioselectivity . . . . . . . . . 260
2.1
Properties and Synthesis of the Template . . . . . . . . . . . . . . 260
2.2
[2+2]-Photocycloaddition Reactions . . . . . . . . . . . . . . . . 262
2.3
Other Cycloaddition Reactions . . . . . . . . . . . . . . . . . . . 262
2.4
Photocyclisation Reactions . . . . . . . . . . . . . . . . . . . . . 264
3
Radical Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 265
3.1
Enantioselective Hydrogen Abstraction . . . . . . . . . . . . . . . 266
3.2
Cyclisation Reactions . . . . . . . . . . . . . . . . . . . . . . . . 267
3.3
Norrish–Yang Cyclisation . . . . . . . . . . . . . . . . . . . . . . 269
4
Sensitised Reactions . . . . . . . . . . . . . . . . . . . . . . . . . 271
4.1
Possible Templates and Substrates . . . . . . . . . . . . . . . . . 271
4.2
PET-Catalysed Reaction . . . . . . . . . . . . . . . . . . . . . . . 273
5
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
Abstract. In this account the development of chiral organocatalysts for enantioselective photochemical reactions is described from a personal perspective.
The need for enantioselective photochemical reactions is immediately linked
to the challenge of establishing the first stereogenic centre in a molecule by organic photochemistry. Hydrogen bonds were identified as ideal means to embed
a prochiral substrate into a chiral environment. The combination of a binding
site with a suitable chiral architecture led to the generally applicable chiral template 12, which has been amply employed in photochemical reactions. Exam-
DOI 10.1007/2789_2007_065
© Springer-Verlag Berlin Heidelberg
Published Online: 18 December 2007
Chiral Organocatalysts for
Enantioselective Photochemical Reactions
S. Breitenlechner, P. Selig, T. Bach
(u)
Lehrstuhl für Organische Chemie I, Technische Universität München, Lichtenbergstr. 4,
85747 Garching, Germany
email: thorsten.bach@ch.tum.de
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
2
Stoichiometric Template-Based Enantioselectivity . . . . . . . . . 260
2.1
Properties and Synthesis of the Template . . . . . . . . . . . . . . 260
2.2
[2+2]-Photocycloaddition Reactions . . . . . . . . . . . . . . . . 262
2.3
Other Cycloaddition Reactions . . . . . . . . . . . . . . . . . . . 262
2.4
Photocyclisation Reactions . . . . . . . . . . . . . . . . . . . . . 264
3
Radical Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . 265
3.1
Enantioselective Hydrogen Abstraction . . . . . . . . . . . . . . . 266
3.2
Cyclisation Reactions . . . . . . . . . . . . . . . . . . . . . . . . 267
3.3
Norrish–Yang Cyclisation . . . . . . . . . . . . . . . . . . . . . . 269
4
Sensitised Reactions . . . . . . . . . . . . . . . . . . . . . . . . . 271
4.1
Possible Templates and Substrates . . . . . . . . . . . . . . . . . 271
4.2
PET-Catalysed Reaction . . . . . . . . . . . . . . . . . . . . . . . 273
5
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
Abstract. In this account the development of chiral organocatalysts for enantioselective photochemical reactions is described from a personal perspective.
The need for enantioselective photochemical reactions is immediately linked
to the challenge of establishing the first stereogenic centre in a molecule by organic photochemistry. Hydrogen bonds were identified as ideal means to embed
a prochiral substrate into a chiral environment. The combination of a binding
site with a suitable chiral architecture led to the generally applicable chiral template 12, which has been amply employed in photochemical reactions. Exam-