Mechanistic Study of an Improved Ni Precatalyst for Suzuki–Miyaura Reactions of Aryl Sulfamates: Understanding the Role of Ni(I) Species

Nickel
precatalysts are potentially a more sustainable alternative
to traditional palladium precatalysts for the Suzuki–Miyaura
coupling reaction. Currently, there is significant interest in Suzuki–Miyaura
coupling reactions involving readily accessible phenolic derivatives
such as aryl sulfamates, as the sulfamate moiety can act as a directing
group for the prefunctionalization of the aromatic backbone of the
electrophile prior to cross-coupling. By evaluating complexes in the
Ni(0), (I), and (II) oxidation states we report a precatalyst, (dppf)­Ni­(o-tolyl)­(Cl) (dppf = 1,1′-bis­(diphenyl­phosphino)­ferrocene),
for Suzuki–Miyaura coupling reactions involving aryl sulfamates
and boronic acids, which operates at a significantly lower catalyst
loading and at milder reaction conditions than other reported systems.
In some cases it can even function at room temperature. Mechanistic
studies on precatalyst activation and the speciation of nickel during
catalysis reveal that Ni­(I) species are formed in the catalytic reaction
via two different pathways: (i) the precatalyst (dppf)­Ni­(o-tolyl)­(Cl) undergoes comproportionation with the active
Ni(0) species; and (ii) the catalytic intermediate (dppf)­Ni­(Ar)­(sulfamate)
(Ar = aryl) undergoes comproportionation with the active Ni(0) species.
In both cases the formation of Ni­(I) is detrimental to catalysis,
which is proposed to proceed via a Ni(0)/Ni­(II) cycle. DFT calculations
are used to support experimental observations and provide insight
about the elementary steps involved in reactions directly on the catalytic
cycle, as well as off-cycle processes. Our mechanistic investigation
provides guidelines for designing even more active nickel catalysts.