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1.2.3.3.2 4-Toluenesulfonates

DOI: 10.1055/sos-SD-223-00066

Martin, A. R.Science of Synthesis: N-Heterocyclic Carbenes in Catalytic Organic Synthesis, (20161176.

Similarly to trifluoromethanesulfonates, 4-toluenesulfonates (tosylates) are a class of sulfonyl derivatives that can be used as pseudohalides. They are relatively stable toward hydrolysis and can be prepared from the corresponding phenol using inexpensive 4-toluenesulfonyl chloride. However, compared to their trifluoromethanesulfonate counterparts, 4-toluenesulfonates are less prone to undergo oxidative addition to the metal catalyst.[‌51‌] Nevertheless, two procedures employing nickel-based catalysts are available. The first uses an in situ generated nickel/NHC complex[‌52‌] [40/2(IPr•HCl)] while the second employs a well-defined nickel(0) precatalyst 41.[‌53‌] Both methods afford the coupling products in short reaction times (15 min to 5 h) with 5–10 mol% catalyst loadings. Cyclic secondary amines, anilines, and N-methylaniline are successfully used in this coupling reaction (Scheme 19). ortho-Substitution is well tolerated on the amine substrate whereas it is detrimental to the yield when located on the aryl 4-toluenesulfonate. The main limitation of the proposed catalytic systems is the need for a tert-butoxide base and elevated reaction temperatures (110 °C). This implies a low functional-group tolerance. Indeed, non-enolizable ketones, methyl, and methoxy groups are the sole reported substituents.

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