Bulky ligands Heck reaction

An interesting parallel was found while the microwave-enhanced Heck reaction was explored on the C-3 position of the pyrazinone system [29]. The additional problem here was caused by the capability of the alkene to undergo Diels-Alder reaction with the 2-azadiene system of the pyrazinone. An interesting competition between the Heck reaction and the Diels-Alder reaction has been noticed, while the outcome solely depended on the substrates and the catalyst system. Microwave irradiation of a mixture of pyrazinone (Re = H), ethyl acrylate (Y = COOEt) and Pd(dppf)Cl2 resulted in the formation of a mixture of the starting material together with the cycloaddition product in a 3 1 ratio (Scheme 15). On the contrary, when Pd(OAc)2 was used in combination with the bulky phosphine ligand 2-(di-t-butylphosphino)biphenyl [41-44], the Heck reaction product was obtained as the sole product. When a mixture of the pyrazinone (Re = Ar) with ethyl acrylate or styrene and Pd(dppf)Cl2 was irradiated at 150 °C for 15 min, both catalytic systems favored the Heck reaction product with no trace of Diels-Alder adduct. 

The Pd-catalysed Heck reaction performed with thiourea as the Ugand exhibit good activities for some catalysts. As for carbene ligands [104], steric hindrance improves catalytic results. Thus, thioureas wearing bulky substituents afford the formation of air- and moisture-stable Pd complexes [105]. For example, the catalyst obtained with 2mol% Pd(dba)2 and Ar,M -dimesitylene-ethylene thiourea (Scheme 24) was still active even after 2 months in an air atmosphere. 

Phosphine ligands based on the ferrocene backbone are very efficient in many palladium-catalyzed reactions, e.g., cross-coupling reactions,248 Heck reaction,249 amination reaction,250 and enantioselective synthesis.251 A particularly interesting example of an unusual coordination mode of the l,l -bis(diphenylphosphino)ferrocene (dppf) ligand has been reported. Dicationic palladium(II) complexes, such as [(dppf)Pd(PPh3)]2+[BF4 ]2, were shown to contain a palladium-iron bond.252,253 Palladium-iron bonds occur also in monocationic methyl and acylpalladium(II) complexes.254 A palladium-iron interaction is favored by bulky alkyl substituents on phosphorus and a lower electron density at palladium. 

Figure 13.17. Mechanism of the Heck reaction for bulky ligand systems [15]...

Bulky ligands as above have also proved to be effective in other palladium-catalyzed reactions of aryl halides, e.g., amination [16-19], Suzuki-Miyaura reaction [20-22], Mizoroki-Heck reaction [23, 24], Migita-Kosugi-Stille reaction [25], and aryloxylation and alkoxylation [26-28] as well as the reaction with various carbon nucleophiles as described below. The ligands are considered to enhance both the initial oxidative addition of aryl halides and the reductive elimination of products [29, 30]. The effectiveness of the commercially available simple ligand, P(f-Bu)3, was first described for the amination by Nishiyama et al. [16]. 

These and other enantioselective Heck reactions were performed with a variety of chiral ligands (Table 3-8 and Figure 3-1). However, high enantioselectivities could so far only be achieved with BINAP (accessible in both enantiomeric forms) or oxazoline ligands (preferably the bulky 208). The latter is easily obtained as the (S )-enantiomer. 

Bulky tri(o-tolyl)phosphine was used first by Heck [11]. A palladacycle obtained from it is known as the Herrmann complex (XVIII-1) and is used extensively in HR [12]. Also, palladacycles XVIII-7 [13] and XVIII-2 [14] are high performance catalysts. Turnover numbers as high as 630-8900 were achieved by tetraphosphine Tedicyp (X-1) [15]. Recently, the remarkable effect of electron-rich and bulky phosphines, typically P(t-Bu)3 and other phosphines shown in Tables 1.4, 1.5 and 1.6, have been vmveiled. Smooth reactions of aryl chlorides using these ligands are treated later. Electron-rich ligands accelerate oxidative addition of aryl chlorides, and reductive elimination is accelerated by bulky ligands. HR can be carried out in an aqueous solution by use of a water-soluble sulfonated phosphine (TPPMS, II-2) [16]. 

Scheme 1.49 Ionic mechanism for Mizoroki-Heck reactions catalysed by a Pd(0) coordinated to one or two C—C saturated or C=C unsaturated fi-heterocyclic monocarbenes (only one way for the coordination of the alkene is presented). The reactive species is PdP(Cb) for a bulky carbene and Prf(Cb)2 for a nonbulky carbene. The aryl-palladium complex formed in the oxidative addition is always ligated by two Cb ligands delivered by the Pd(0) or PdfII) precursor even if Pcf(Cb) is the reactive species.

Mechanism of the Mizoroki-Heck Reaction when the Ligand is a Bulky and Electron-Rich Monophosphine... 

The efficiency of bulky and electron-rich phosphines in Mizoroki-Heck reactions seems to be due to their ability to generate monophosphine-Pd(O) or -Pd(II) complexes in each step of the catalytic cycle (Scheme 1.55). Steric factors are probably more important than electronic factors. One sees from Fu s studies that the last step of the catalytic cycle in which the Pd(0) complex is regenerated in the presence of a base may be rate determining. The role of this last step has been underestimated for a long time. Provided this step is favoured (e.g. with P-r-Bus as ligand and Cy2NMe as base), the oxidative addition of aryl chlorides would appear to be rate determining. However, Mizoroki-Heck reactions performed from the same aryl chloride with the same Pd(0) catalyst and same base but... 

The Mizoroki-Heck reaction is a subtle and complex reaction which involves a great variety of intermediate palladium complexes. The four main steps proposed by Heck (oxidative addition, alkene insertion, )3-hydride elimination and reductive elimination) have been confirmed. However, they involved a considerable number of different Pd(0) and Pd(Il) intermediates whose structure and reactivity depend on the experimental conditions, namely the catalytic precursor (Pd(0) complexes, Pd(OAc)2, palladacycles), the Ugand (mono- or bis-phosphines, carbenes, bulky monophosphines), the additives (hahdes, acetates), the aryl derivatives (ArX, ArOTf), the alkenes (electron-rich versus electron-deficient ones), which may also be ligands for Pd(0) complexes, and at least the base, which can play a... 

The mechanism of action of these base-free Mizoroki-Heck reactions is an intriguing question. These reactions are performed under very mild conditions, so any uncontrolled thermal decomposition pathways might be ruled out. Evidently, palladium(II) hydride intermediates sometimes regenerate palladium(0) spontaneously, either by reductive elimination or because of enhanced acidity. While there is no well-documented evidence of reductive elimination, Milstein and coworkers [11] presented a system with enhanced acidity that is rendered by an electron-rich bulky phosphine 14 in the coordination sphere. These authors described base-free Mizoroki-Heck reactions in the presence of ligand 14 (12 15, Scheme 2.3), which was shown to increase the acidity of palladium(II) hydride... 

Even hybrid carbene-phosphine precatalysts such as 136-139 showed weak and uneven dependence on the phosphine ligand (Table 2.4, entries 14-17) [204], The complexes with bulky electron-rich phosphines 138 and 139 were less reactive than those with conventional triarylphosphines 136 and 137. This can hardly be regarded as surprising, as the deligation of the carbene is likely to be less probable than the deligation of the phosphine at the applied temperatures activation of the precatalyst should inevitably occur by dissociation of the phosphine first. In this respect, the design of hybrid carbene-phosphine complexes for Mizoroki-Heck reactions is not promising. 

The Matt group has also investigated cavitands as catalysts. For example, cavitand 42 is a catalyst for Heck reactions [120]. Conversion percentages were highest when the cavitand was present in a 1 1 ratio with the Pd ligand. Semeril and Matt have also examined mono- and complexes with ruthenium [121], but the properties of these bulky ligands have to our knowledge not yet been reported. 

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