Ligand-free reactions

A special focus of modem research in Pd-catalyzed Heck-coupling reactions is to investigate the ligand-free reaction in ionic liquids using Pd(OAc)2, PdQz or Pd/C as catalyst precursors [210]. In these examples the role of the ionic liquid is as either solvent, ligand or both. Of special interest for these studies is the regioselectivity of the Heck products when electron-rich olefins, such as acyclic enol ethers, silanes, and enol amides are applied as substrates. Possible products are the branched olefin (a. Scheme 5.3-24) and the linear olefin (P, Scheme 5.3-24). 

Buchwald disclosed arylations of a variety of ketones using biphenyl-based Ugand systems [220], and also examined phosphine ligand-free reactions, although these proved applicable to only a limited number of substrate combinations. 

Mechanism ofLP Oxo Rea.ction. The LP Oxo reaction proceeds through a number of rhodium complex equilibria analogous to those ia the Heck-Breslow mechanism described for the ligand-free cobalt process (see Fig. 1). 

Solvent-free microwave-assisted Suzuki reaction on AI2O3 as soHd support using KF as base has also been described in the hterature by Villemin [38] (Scheme 20). Ligand-free Pd(OAc)2 was used as a precatalyst. In a reaction... 

Remarkably, one year later Leadbeater described that biaryls can be synthesized via a Suzuki-type coupling under transition-metal free conditions [51, 52]. The reaction conditions were almost identical to those reported for the ligand-free process, with the difference being that a larger amoimt of Na2C03 and arylboronic acid were used. Only one successful example of a heteroaryl haUde substrate is shown namely, the coupling of 2-bromopyridine with phenylboronic acid (Scheme 32). 3-Bromothiophene did not couple under the same reaction conditions. Unfortimately, attempts to use heteroarylboronic acids such as 3-pyridinylboronic acid, 3-thienylboronic acid, and lH-indol-5-ylboronic acid on 4-bromoacetophenone completely failed. 

A discussion of ligand exchange reactions of organometallic compounds associated with oxidation-reduction processes leading to free-radical formation will be found in Volume 14 (Free-radical polymerization). 

Yamamoto and co-workers reported abase- and ligand-free palladium (II) catalysed method, in methanol at room temperature and under air [49], While the conversion of arylboronic acids bearing an electro-donating group was very efficient, the presence of an electro-withdrawing substituent led to lower conversions. To solve this problem and also prevent the fast formation of palladium black, Yamamoto and co-workers described a new approach where the reaction was catalysed by NHC-bearing complexes 21 and 22 in the presence of an oxidant (Fig. 7.4) [50]. The best results were obtained when complex 21b was used in methanol at room temperature, in the presence of a small excess of p-benzoquinone. 

Analysis of soluble Pd. To assess the amount of ligand-free soluble Pd present in the reaction mixture, the solution containing 4-bromoanisole/4-... 

Palladium-catalyzed carbon-carbon cross-coupling reactions are among the best studied reactions in recent decades since their discovery [102, 127-130], These processes involve molecular Pd complexes, and also palladium salts and ligand-free approaches, where palladium(O) species act as catalytically active species [131-135]. For example, the Heck reaction with aryl iodides or bromides is promoted by a plethora of Pd(II) and Pd(0) sources [128, 130], At least in the case of ligand-free palladium sources, the involvement of soluble Pd NPs as a reservoir for catalytically active species seems very plausible [136-138], Noteworthy, it is generally accepted that the true catalyst in the reactions catalyzed by Pd(0) NPs is probably molecular zerovalent species detached from the NP surface that enter the main catalytic cycle and subsequently agglomerate as N Ps or even as bulk metal. 

Simple bidentate ligands involving dipyridyl- or dipyrimidylamino fragment (L23) form Pd catalysts of moderate activity for the cross-coupling of terminal acetylenes (copper-free reaction) or arylboronic acids. Supported versions of such ligands were also reported (see Chapter 9.9 for more details about supported catalysts).449,450... 

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... 

TBAB = tetrabutylammonium bromide Scheme 6.12 Ligand-free Suzuki reactions using TBAB as an additive. 

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