Catalysis using pincer complexes

Introduction - Pros and Cons of Using Pincer Complexes in Catalysis... 

Arylation ofAlkenes Using Pincer Complex Catalysis... 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

To study the effect of the chiral backbone on the behavior of the NCN-pincer complex 4 in catalysis, the Michael addition of methyl vinyl ketone to (R/S)-ethyl a-isocyanopropionate was used as a model reaction (Scheme 3). The results of the experiment show 80% conversion after 24 h which is higher than the 38% conversion obtained in the same Michael addition without catalyst. After full conversion, the product and loaded nanocapsules could be recovered separately in almost quantitative yields (> 96%) by dialysis. Product analysis revealed that no enantiomeric excess was found and racemic mixtures were obtained (ee = 0%) (Scheme 3). 

C-H borylation is a widely used methodology for the synthesis of organoboronates [63-65]. Most of the applications have been presented for the synthesis of aryl-boronates. However, functionalization of alkenes has also attracted much interest [66, 67]. In most applications, iridium catalysis was used. However, in case of alkenes, borohydride forms as a side product of the C-H borylation, which undergoes hydroboration with alkenes. This side reaction can be avoided using palladium catalysis under oxidative conditions. In a practically useful implementation of this reaction, pincer-complex catalysis (Ig) was appHed (Figure 4.17) [51]. The reaction can be carried out under mild reaction conditions at room temperature using the neat aUcene 34 as solvent. In this reaction, hypervalent iodine 36, the TFA analog of 29, was employed. In the absence of 36, borylation reaction did not occur. 

A nickel-catalyzed hydroamination of acrylonitrile using cationic pincer complexes has been described (Scheme 3-109). Mechanistic studies suggested that a simple Lewis acidic role of nickel may be responsible for the catalysis in this group... 

In 2003 The Uses of Pincer Complexes in Organic Synthesis, by Singleton [30] In 2004, PaUadacycles in Catalysis—a Critical Survey, by Beletskaya and Cheprakov [105]... 

Vicente et al. were first to report cross-coupling catalysis with an acyclic car-bene complex at very low catalyst loadings (as low as 0.011 mol%) using ADC-containing Pd -pincer complex 17 (Figure 16.5) [38]. Coupling of phenylboronic acid with /i-bromoanisole occurred readily at 100-110 °C in toluene or 95 5 toluene H2O, and the coupling of an activated aryl chloride was even accomplished... 

Pincer-ligated metal complexes have displayed extraordinarily rich chemistry and have found widespread use in catalysis. Pincer complexes of numerous transition metals have been synthesized, but the most well-studied probably involve Ru, Rh, Ir, and Pd [1-7], Our group has largely focused on pincer-iridium complexes, which have shown a strong tendency toward the activation of C-H bonds. These complexes have been found to effect the oxidative addition of a variety of C-H bonds including those with sp - and sp-hybridized carbon [8-10], Most notable, however, has been the activation of C(sp )-H bonds, leading to alkane dehydrogenation [6, 7],... 

Pincer (mer, tridentate) phosphines have proved resistant to degradation and useful in alkane dehydrogenation catalysis. Recently, Xu et al. [118] found that a dihydrido Ir complex containing a tridentate monoaifionic aryl bis(phosphino) (PCP) pincer, 31 (R = terf-butyl), is highly active catalyst for dehydrogenation... 

C—H borylation of indoles has been reported by a number of groups. Chirik and coworkers utilized pincer-ligated cobalt complexes with N-methyhndole and recorded the C-2 borylated indole (133) as the major product (2014JAC4133). The catalysts employed demonstrated high catalysis turnover and low catalytic loading and also demonstrated efficacy with other electron-rich heteroarenes (furan, thiophene, benzofuran) as well as electron-deficient pyridines. More recendy, platinum-NHC complexes have been used in the selective C—H borylation of indoles (2015JAC12211). The authors reported higher isolated yields with this... 

It was shown that palladacydes 1 [3c, 24] prepared from palladium] I) acetate and tris(o-tolyl)- or trimesitylphosphine are excellent catalysts for the Heck coupHng of triflates and halides including certain aryl chlorides. In some of these cases, a possible involvement of oxidation states +II and +IV in the catalytic cycle has been considered [25]. Similarly, other palladacydes such as 3 [26e,h] or 6 [27] have been used in the Heck reactions (Figure 8.1) [24, 26, 28]. It has been proposed that, at least for NC palladacydes, the reaction proceeds through the classical phosphine-free Pd(0)/Pd(II) catalytic cycle and that the active catalysts are actually slowly formed palladium clusters [29]. Besides classical palladacydes, complexes with pincer-type ligands such as 2 [30] have become very popular in palladium catalysis [31]. 

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