3:00pm - 4:00pm
Room 209 Havemeyer
New York, NY 10027
Friday, April 29, 2016 at 3:00pm (PLEASE NOTE TIME CHANGE)
Room 209 Havemeyer
Hosted by James Leighton
[2+2] Photocycloaddition reactions belong undisputedly to the most important reaction classes in photochemistry. The creation of up to four new stereogenic centers in a single step and the further use of the formed cyclobutane rings – either directly or after appropriate ring opening – are hallmarks of this powerful transformation. In our group, we have studied two different approaches to achieve a catalytic enantioselective reaction1 course in [2+2] photocycloaddition reactions. The first approach is based on a triplet energy transfer by hydrogen-bonding chiral catalysts, which in turn are derived from the previously described template 12 successfully employed in the total synthesis of (+)-meloscine (2).3 The second approach relies on chiral Lewis acids, which change the photophysical parameters of the substrate and allow a selective excitation in the chiral environment, which they provide.
Xanthone 3a turned out to be an efficient organocatalyst providing good turnover (10 mol%) and high enantioselectivities (>90% ee) in [2+2] photocycloaddition reactions4 while thioxanthone 3b allows for enantioselective reactions promoted by visible light.5 Apart from this approach, we have also looked into the possibility of Lewis-acid mediated enantioselectivity in photochemical reactions. Lewis acid 4 was developed for [2+2] photocycloaddition reactions and is currently being further explored.6 The presentation discusses the background of the above-mentioned studies and provides the latest results of our research efforts in this area.
Department of Chemistry, Columbia University, Havemeyer Hall, 3000 Broadway, New York, NY 10027, USA | 212-854-2202 | www.columbia.edu/cu/chemistry