The Synthesis and Catalytic Activity of New Mixed NHC-Phosphite Nickel(0) Complexes

Abstract

Herein we describe the synthesis and isolation of the first low-valent NHC-phosphite nickel complexes of general formula Ni­(NHC)­[P­(OAr)3]2. These three-coordinate Ni(0) compounds were fully characterized, including by X-ray crystallography that highlighted their trigonal planar geometry. The representative complex Ni­(IMes)­[P­(OPh)3]2 was used to show that a phosphite ligand is readily substituted in the presence of an aldehyde or nitrile. These stoichiometric studies then led to an investigation of their catalytic properties in the Suzuki–Miyaura cross-coupling reactions between aryl tosylates and aryl boronic acids, a first for such a NHC-Ni catalyst. Finally, mechanistic investigations led to the isolation of a well-defined oxidative addition product.

Ligand Steric Effects of α-Diimine Nickel(II) and Palladium(II) Complexes in the Suzuki–Miyaura Cross-Coupling Reaction

“…15,49 Nonetheless, a major portion of the catalytic systems in SMC are lacking thermal properties as they demand high catalyst loading and temperature along with extended reaction time. 41,43,44,46,47,50,96 In contrast, our complexes' high thermal stability is evident from the data presented earlier (Figure 3). Although Ni B is comparatively less stable because of its bridge-like structure, Steric Effect Versus Catalytic Performance.…”

“…The thermal stability of a catalyst is crucial to preserve the inherent catalytic performance over prolonged reaction times, particularly for industrial applications. The supported transition metal catalytic system usually demonstrates thermal stability for SMC application. , Nonetheless, a major portion of the catalytic systems in SMC are lacking thermal properties as they demand high catalyst loading and temperature along with extended reaction time. ,,,,,, In contrast, our complexes’ high thermal stability is evident from the data presented earlier (Figure ). Although Ni B is comparatively less stable because of its bridge-like structure, the optimized temperature (80 °C) used in SMC reactions is much lower than the decomposition temperature ( T d = 120 °C at 95% weight).…”

“… 32 , 33 , 37 − 46 Nonetheless, the greater expansion of nickel-based catalytic systems in SMC is facing inherent challenges from the activation of precatalysts 41 , 43 , 44 and air and moisture sensitivity. 20 , 25 , 37 , 43 , 46 Moreover, high catalyst loading (2.5–10 mol %), 25 , 41 , 43 , 44 , 46 , 47 elevated temperature (100–130 °C), 25 , 44 , 47 − 50 and prolonged reaction time (12–48 h) 25 , 41 , 44 , 46 , 48 , 50 , 51 significantly affect their prospects in SMC compared to the Pd counterparts. Also, there is a need for a large excess of boronic acid (2.5–5 equiv) 25 , 41 , 43 , 44 , 48 for generating the desired yield because of competitive protodeboronation.…”

“…The smaller size of Ni(0)-relative Pd(0) results in higher nucleophilicity and facilitates the oxidative addition of nontraditional substrates. ,− In the early stages of nickel catalyst development, Ni­(COD) 2 (COD = 1,5-cyclooctadiene) was used as a Ni(0) precursor along with a free ligand for producing catalytically active species. ,,, Ni­(COD) 2 is relatively expensive, thermally unstable, and air-sensitive. ,, As opposed to nickel-based systems in SMC, various well-defined palladium catalysts have been reported. ,− Consequently, studies have been conducted for the development of more practicable well-defined nickel-based catalysts. ,,− Nonetheless, the greater expansion of nickel-based catalytic systems in SMC is facing inherent challenges from the activation of precatalysts ,, and air and moisture sensitivity. ,,,, Moreover, high catalyst loading (2.5–10 mol %), ,,,,, elevated temperature (100–130 °C), ,,− and prolonged reaction time (12–48 h) ,,,,,, significantly affect their prospects in SMC compared to the Pd counterparts. Also, there is a need for a large excess of boronic acid (2.5–5 equiv) ,,,, for generating the desired yield because of competitive protodeboronation …”

Synthesis of 7-hydroxydibenzopyran-6-ones via benzannulation of coumarins

“…benzyl, 2-nitrobenzyl, p -tolunesulfonyl, and trifluoromethylsulfonyl; Scheme 5). Accordingly, we prepared sulfones 17l–o by converting OH into OBn/O-2-Nb 23 /OTs 24 /OTf 25 and their reactions with ethyl cyanoacetate 21e under optimized conditions provided cyclic products 19l–o and open chain products 22p–s as per the details given in the Scheme 5. While the reaction of coumarins 17l–n with ethyl cyanoacetate provided the cyclized products 19l–n to a varying degree (OBn: 15%; O-2-Nb: 30% and OTs: 56%) at the cost of the open chain products 22p–r , the reaction of the coumarin with C7-OTf 17o furnished the cyclic product 19o exclusively in 75% yield; there was no trace of the non-cyclized product 22s .…”