Pd(II)-Catalyzed Dehydrogenative Cyclizations of N-Allylimines
Near-simultaneous reports of an interesting new pyrrole (and dihydropyrrole) synthesis have appeared recently from the labs of Frank Glorius (Westfälische Wilhelms-Universität Münster) and Naohiko Yoshikai (Nanyang Technologial University, Singapore). Optimization of the additives, solvent, and temperature led to essentially the same conditions from each laboratory. A balloon of oxygen is preferred: yields are much lower when air is used. A Heck-type mechanism is shown, but a Wacker-type mechanism involving nucleophilic attack of an enamine onto a Pd-complexed alkene could not be ruled out.
The scope of the reaction is decent, as indicated below. Failures include R3 = CF3 or cyclopropyl and R2 = Ph. N-Homoallylic and N-cinnamyl imines were also unsuccessful.
Specific examples are shown below: