Phosphine-free catalytic systems

The palladium(n)-catalyzed direct carbonylation proceeds with remarkable site selectivity to afford a variety of five- or six-membered benzolactams from A-alkyl-cc-arylalkylamines in a phosphine-free catalytic system using Pd(OAc)2 and Cu(OAc)2 in an atmosphere of CO gas containing air (Equation (90)).118... 

Scenario 4. Nanoparticles are formed during the reaction initiated by soluble palladium precatalysts. Our experience in phosphine-free (mostly aqueous) systems led us at an early stage to hypothesize that these protocols go hand in hand with formation of palladium sols [4, 83]. Since 2000, the observation of palladium sols in Mizoroki-Heck reactions has become ubiquitous. So, if one were to decide to compile a list of references that mention the formation of sols, that list would practically coincide with a list of references which deal with the development of new phosphine-free catalytic systems. In fact, the idea that it is the dispersed palladium metal which catalyses (or, more correctly, serves as precatalyst) the Mizoroki-Heck reaction should be traced back to the seminal pubUcations by Heck in 1972 [2], who clearly attiibuted catalytic activity in a phosphine-free catalytic system to palladium metal formed by reduction of Pd(OAc)2 by the alkene or the amine. This idea fell into oblivion until its rediscovery in the course of a general interest in nanoscale systems in the late 1990s. 

Direct arylation polycondensation using the phosphine-free catalytic system can be adapted to the synthesis of bithiazole-based alternating copolymers (P34). In comparison with conventional polycondensation via other cross-coupling reactions, the polycondensation proceeded with a reduced amount of Pd catalyst (2 mol%) in a short reaction time (10 min to 3 h). Owing to the differenee in reactivity of the C-H bond, controlling the reaction time was effective for suppressing the side reaetion at the unexpeeted C-H bond (Scheme 1.35). ... 

While this protocol relied on the in situ generation of the relevant phosphite for catalytic hydroarylation reactions, Murai and coworkers developed effective methodologies for the direct use of Lewis-basic substrates, such as acetophenone 20 (Scheme 9) [18, 59], Thereby, regioselective ruthenium-catalyzed anti-Markovnivkov alkylations and alkenylations were accomplished using alkenes or alkynes [60] as substrates, respectively. Recently, an extension of this protocol to terminal alkynes was reported, which involved a phosphine ligand-free catalytic system (see below), along with stoichiometric amounts of a peroxide [61]. 

High-temperature and pressure IR studies with the original phosphine-free cobalt system under conditions for catalytic hydroformylation of cyclohexene (150 , 250 atm) normally show Co2(CO)s and HCo(CO)4 as the only spectroscopically detectable species, with the hydride as the major species. With 1-octene or ethylene, the hydride is not observed and the only detectable species are Co2(CO)g and RCOCo(CO)4. With... 

Typically, type 2 catalytic systems are exploited at high temperatures starting approximately at 120 °C, but usually exceeding 140 °C lower temperatures are sometimes possible after optimization. Thus, Giirtler and Buchwald [25] showed that, using their phosphine-free catalyst system, not only aryl iodides, but also aryl bromides 44, including electron-rich substrates, afford high yields in the arylation of disubstituted alkenes 45 at temperatures as low as 85-100 °C (44 46, Scheme 2.9). Both base and additive in this nice protocol are... 

Simple Pd salts and complexes which contain neither phosphines nor any other deliberately added ligands are well known to provide catalytic activity in cross-coupling reactions. Such catalytic systems (often referred to as ligand-free catalysts ) often require the use of water as a component of the reaction medium.17 In the majority of cases such systems are applicable to electrophiles easily undergoing the oxidative addition (aryl iodides and activated bromides), although there are examples of effective reactions with unactivated substrates (electron-rich aiyl bromides, and some aryl chlorides).18,470... 

Tert-butyl carbonates 229 can be used instead of free acids to obtain cyclic carbonates 230. The influence of the eledronic and steric properties of phosphine ligand in the catalytic system was studied and optimal behavior was observed for eledron-deficient phosphines such as P(C6F5)3 [48]. 

The above catalytic systems for the asymmetric hydrogenation of quinolines are mainly iridium catalysts. In 2008, Fan and coworkers developed recyclable phos phine free chiral cationic ruthenium catalyzed asymmetric hydrogenation of qui nolines [23]. They found that the phosphine free cationic Ru/TsDPEN catalyst exhibited unprecedented reactivity and high enantioselectivity in the hydrogenation of quinolines in neat ionic liquid. The results were very excellent and enantioselec... 

The carbonylation of aryl halides with alcohols and amines catalysed by palladium complexes with triphenylphosphine ligand is the convergent and direct route to the synthesis of aromatic esters as well as aromatic amides. Even though these palladium complexes are widely employed as the best catalytic system, those catalysts are difficult to separate and reuse for the reaction without further processing. The major drawbacks are oxidation of triphenylphosphine to phosphine oxide, reduction of palladium complex to metal and termination of the catalytic cycle. The phosphine-free, thermally stable and air resistant catalyst (1) containing a carbon-palladium covalent bond (Figure 12.3) has been found to be a highly selective and efficient catalyst for the carbonylation of aryl iodides.[1]... 

Littke and Fu reported an interesting and important suggestion that commercially available Pd[P(f-Bu)3]2 is a resting state of the system in the HR, and not a catalytically active species. Change of the ratio of Pd to phosphine from 1 1 to 1 2 leads to a marked decrease in the rate of reactions, and the Pd monophosphine adduct PdP(f-Bu)3 is the active catalyst. The combination of Pd[P(t-Bu)3]2 and a phosphine-free Pd complex generates in situ the active 1 1 adduct [7]. 

The Mizoroki-Heck reaction involves an immense variety of ancillary ligands, since it has been a matter of common agreement that phosphines play a distinct role different from any other ancillary ligand. Heck himself introduced two major types of catalytic system (1) phosphine-free for aryl iodides [2] (2) using PhsP [6, 7] or (2-tolyl)3P [8] for aryl bromides. This dichotomy has laid the basis for further division of all new protocols into those requiring phosphine ligands (phosphine assisted) and those that are phosphine free (quite often erroneously referred to as ligand free). 

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