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.

Yoshikai+Glorius_1

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.

 Yoshikai+Glorius_2

Specific examples are shown below:

Yoshikai+Glorius_3

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2 responses to “Pd(II)-Catalyzed Dehydrogenative Cyclizations of N-Allylimines”

  1. Albert Padwa says :

    excellent choice for us heterocyclic chemists

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