1. Department of Chemistry and Department of Chemical Engineering, Columbia University, 3000 Broadway, Mail Code 3119, New York, New York 10027, Joint Science Department, W.M. Keck Science Center, 925 N. Mills Ave., Claremont McKenna, Pitzer, and Scripps Colleges, Claremont, California 91711, and Institut für Organische Chemie, Universität Würzburg, 97074 Würzburg, Germany