Revealing the unusual role of bases in activation/deactivation of catalytic systems: O–NHC coupling in M/NHC catalysis

Abstract

Usual oxygen bases decompose M/NHC complexes to give “NHC-free” metal species and azolones.

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“…alkali metal alkoxides and hydroxides used in many catalytic systems, may trigger O–NHC coupling. 97 The effect of O–NHC coupling on the catalytic performance of PEPPSI–IPr precatalyst was assessed in the reaction of acetophenone CH-arylation with chlorobenzene. Simple preheating of PEPPSI–IPr 2j with t -BuOK for 10 min and 3 h at 100 °C led to, respectively, 26% and 56% decrease in the yields of CH–arylation product ( Scheme 12 ).…”

“…Simple preheating of PEPPSI–IPr 2j with t -BuOK for 10 min and 3 h at 100 °C led to, respectively, 26% and 56% decrease in the yields of CH–arylation product ( Scheme 12 ). 97 It should be remembered that M/NHC complexes with metals in the highest oxidation states, e.g. Ni IV and Pd IV , are susceptible to the reductive elimination of NHC, and the use of such complexes in catalytic systems with assumed participation of M IV species may be inefficient due to their low stability.…”

“…This rate strongly depends on the rate of the M–NHC bond cleavage which, in turn, depends on the structure of the NHC ligand and co-ligands. 27 , 54 , 58 – 60 , 62 , 63 , 97 M/NHC complexes with bulky NHC ligands (or bis-NHC complexes, 59 especially chelated 250 ) are usually more resistant to R–NHC coupling and therefore decompose slower than NHC complexes with non-bulky NHC ligands. For example, the rate of Mizoroki–Heck reactions shows inverse relationship with the stability of Pd/NHC precatalyst.…”

“…Decomposition of Pd–PEPPSI–IPr precatalyst via O–NHC coupling by preheating with t -BuOK enhances its catalytic performance significantly ( Scheme 12 ). 97 Moreover, catalysis of Mizoroki–Heck reactions by Pd–PEPPSI complexes in the presence of aliphatic amine bases has been shown to proceed by a previously unknown mechanism of the active species generation that provides enhanced robustness of the formed catalytic systems ( Scheme 13 ). 59 Heated with tertiary aliphatic amines ( e.g.…”

Oxidative Addition of Water, Alcohols, and Amines in Palladium Catalysis

“…[12] Nevertheless,t he oxidative addition of adventitious water or alcohols in general seems to not have been considered in this context. In fact, water or alcohols have been reported to instead mediate the reductive generation of palladium(0) from palladium(II) precursors [13] with phosphane-or N-heterocyclic carbene (NHC) [14] ligands.…”