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Sulfination by Using Pd-PEPPSI Complexes: Studies into Precatalyst Activation, Cationic and Solvent Effects and the Role of Butoxide Base

Authors

  • Mahmoud Sayah,

    1. Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3 (Canada), Fax: (+1) 416-736-5936
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  • Dr. Alan J. Lough,

    1. Department of Chemistry, University of Toronto 80 St. George Street, Toronto, Ontario M5S 3H6 (Canada)
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  • Prof. Michael G. Organ

    Corresponding author
    1. Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3 (Canada), Fax: (+1) 416-736-5936
    • Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3 (Canada), Fax: (+1) 416-736-5936
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  • PEPPSI=Pyridine-enhanced precatalyst preparation stabilisation and initiation.

Abstract

The activation of PEPPSI precatalysts has been systematically studied in Pd-catalysed sulfination. Under the reactions conditions of the sulfide and KOtBu in toluene, the first thing that happens is exchange of the two chlorides on the PEPPSI precatalyst with the corresponding sulfides, creating the first resting state; it is via this complex that all Pd enters the catalytic cycle. However, it is also from this same complex that a tri-Pd complex forms, which is a more persistent resting state. Under standard reaction conditions, this complex is catalytically inactive. However, if additional pyridine or a smaller base (i.e., KOEt) is added, this complex is broken down, presumably initially back to the first resting state and it is again capable of entering the catalytic cycle and completing the sulfination. Of note, once the tri-Pd complex forms, one equivalent of Pd is lost to the transformation. Related to this, the nature of the cation of the sulfide salt and solvent dielectric is very important to the success of this transformation. That is, the less soluble the salt the better the performance, which can be attributed to lowering sulfide concentration to avoid the movement of the Pd-NHC complex into the above described off-cycle sulfinated resting states.

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