Cocktail catalysts

The complete mechanism of the autoxidation of methylaromatics mediated by the Amoco, Co/Mn/Br catalyst cocktail is depicted in Figure 18. The development of such a complex, elegant system must surely be considered a work of art. 

In 1994, Badone et al. reported that the Stille coupling of allenylstannane 77 and aryl triflates 78 resulted in formation of various aryl-substituted allenes 79 in moderate to good yield (Scheme 14.18) [39]. The choice of catalyst was certainly a crucial issue in this process for optimizing yield and rate. The best results could be obtained employing a catalyst cocktail of Pd2(dba)3-TFP-LiCl-CuI. Similar Stille coupling reactions with stannylated allenes and aromatic iodides as substrates were described by Aidhen and Braslau [40a] and Huang et al. [40b],... 

To a heavy-walled flask equipped with a nitrogen inlet side arm was added resin-bound terminal acetylene (684.4 mg resin, 0.274 mmol, 0.448 mequiv/g resin) and aryl iodide (120.6 mg, 0.3011 mmol) (Scheme 11). The flask was evacuated and back-filled with nitrogen a minimum of three times. The supernatant of a separate 0.2 M catalyst cocktail solution (previously prepared) was added via cannula (5 mL, 3.0 mmol) to the reaction flask. The flask was kept sealed at 65°C for 12 h and agitated periodically to remix polymer beads stuck on flask walls. The beads were then transferred to a fritted filter using methylene chloride and washed with methylene chloride (21 mL). Excess aryl iodide can be recovered from the first methylene chloride wash. All further washes were carried out in the ratio of 30 mL/g resin. The resin was washed sequentially with DMF, 0.05 M solution of sodium diethyl dithiocarbamate in 99 1 DMF-diisopropylethylamine,... 

In this chapter, an attempt is made to summarize the current state of understanding of the basics of the mechanism, to provide an overview over the diverse and sometimes mysterious compositions of applicable catalyst cocktails , and to review important recent developments and applications of this reaction principle. 

Thus, the regioselectivity of Heck reactions with unsymmetrical alkenes can be manipulated favorably by appropriate variations of the catalyst cocktail . T e impact of silver(I) and thallium(I) salts [97] on Pd-catalyzed reactions extends beyond just increasing... 

The influence of high pressure on the Heck reactions of selected alkenyl and aryl halides, respectively, i.e., 1-iodocyclohex-l-ene, iodobenzene, bromobenzene, with methyl acrylate has been investigated and the activation parameters of these reactions determined [142], Two different catalyst cocktails were used in this study, the classical system (Pd(OAc)2, NEtg, PPhg) and the one reported by Herrmann, Beller and others [16] (la). The temperature-dependent and the pressure-dependent rate coefficients both follow the order PhI/Pd(OAc)2 > 1-iodocyclohexene/Pd(OAc)2 > Phl/la > PhBr/la and the activation enthalpies as well as the activation entropies exhibit the trend 1-iodocyclohexene/Pd(OA)2 < Phl/Pd(OAc)2 < Phl/la < PhBr/la. The absolute values of the activation volumes, which were ascertained from the pressure-dependent rate coefficients, increase as follows l-iodocyclohexene/Pd(OAc)2 < PhI/Pd(OAc)2 Phl/la < PhBr/la. Under high pressure, the lifetime of the active palladium catalyst and thereby the turnover numbers are greatly enhanced [88]. 

Aryl-, alkenyl- and alkynylpalladium species readily undergo carbonylation reactions because carbon monoxide as a loosely bonded ligand can reversibly insert into any palladium-carbon bond [110]. Thus, 2-allyl-l-iodocyclopentene (148), under palladium catalysis, reacts with carbon monoxide in two modes, depending on the excess of carbon monoxide and the catalyst cocktail (Scheme 3-39) [110a]. With a slight excess (1.1 atm of CO) in the presence of [Pd(PPh3)4] in tetrahydrofuran, 148 cyclized with one CO insertion to yield 3-methylenebicyclo[3.3.0]oct-l(5)-en-2-one (152), and under 40 atm of CO with [Pd(PPh ,)2Cl2] in benzene/acetonitrile/methanol, methyl 2- 3 -(2 -oxobicyclo[3.3.0]oct-1 (5 )-enyl) acetate 149 after two CO insertions (Scheme 3-39). 

However, the substitution pattern on the arene is crucial for the success of the Heck reaction. When a second Heck coupling takes place in an ortho position of another alkenyl unit, cyclization of the intermediately formed o--(/3-arylalkyl)palladium complex may occur, as formation of alkylideneindanes and alkylindenes, especially under classical Heck conditions with phosphines in the catalyst cocktail, was observed (Scheme 2, Table 1). 

Other possible relays are mostly alkynes, which, after carbopalladation, undergo a surprisingly clean reductive demetallation with formic acid salts (Scheme 2). At this point it is crucial that the catalyst cocktail has the appropriate composition, as yields under different conditions can vary substantially. The high degree of regioselectivity starting from propynoic acid amides has been attributed to coordination with the amide functionality. 

The Mizoroki-Heck reaction can either be performed on immobilized aryl halides with soluble alkenes [13] or alkynes or with the latter being attached to solid phase and free aryl halides. When performed on the same type of resin and using identical catalyst cocktails, however, it generally seems preferable to use immobilized aryl halides with soluble alkenes rather than doing it the other way around [14]. Both possibilities have been reported and will be discussed below [11]. 

Thus, the regioselectivity of the Heck reactions with unsymmetrical aikenes can favorably be manipulated by appropriate variations of the catalyst cocktail [123] for example, the best conditions for the couphng of bromobenzene with t-butyl acrylate in the presence of Pd[(o-Tol)3P]2Cl2 (Tol, tolyl) were found to be with potassium carbonate in ethanol at 80 °C. This is unusual for this kind of catalyst system. The active catalyst is actually believed to be nanodispersed palladium metal generated by reduction of the catalyst precursor by ethanol under basic conditions (Table 8.5). 

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