Electron-rich hgands

N-Heterocyclic carbenes are relatively stable, electron-rich hgands and can be made sterically demanding. They are donor ligands with httle 7r-acid properties, hke trialkylphosphines. These new ligands are readily available by deprotonation of azolium precursors, and the deprotonation can be carried out in the presence of palladium salts to generate the complexes directly (Scheme 12). 

In most of the mechanistic schemes described below, the ligands on palladium have been omitted for the sake of clarity and simplicity. However, the nature of the ligands is often crucial for the reactivity and selectivity of palladium catalysts. For example, in many instances Pd-catalyzed amination reactions of aryl halides provide low yields with PPh,-ligated palladium complexes but proceed in excellent yields when catalysts bearing bulky electron-rich hgands are employed (see Section 1.7.1 below). Thus, the choice of the appropriate catalysVhgand is often crucial for success in these reactions. 

Such dearomatization and aromatization of these electron-rich hgand systems in cooperation with metal centers present new opportunities for homogeneous catalysis. 

A similar positive effect is also achieved with bulky electron-rich hgands. Palladium complexes such as 237 derived from air-stable secondary phosphine chloride ligands catalyze the coupling of a range of aUcyl Grignard reagents with aryl bromides in high yields [65] (Scheme 5.57). 

Verkade etal. usedproazaphosphatranes (Scheme 6.13) for the Stille coupHngs of aryl chlorides under mild reaction conditions. These unusually electron-rich Hgands are also suitable for reactions of sterically congested substrates, and crossaryl bromides can be achieved at room temperature [31b,c]. 

Significant advance in the field of asymmetric catalysis was also achieved with the preparation of l,2-bis(phospholano)benzene (DuPHOS 4) and its confor-mationally flexible derivative (l,2-bis(phospholano)ethane, known as BPE) by Burk et al. [59]. Two main distinctive features embodied by these Hgands, as compared to other known chiral diphosphine ligands, are the electron-rich character of the phosphorus atoms on the one hand and the pseudo-chirality at phosphorus atoms, on the other. These properties are responsible for both the high activity of the corresponding metal complex and an enantioselection indepen-... 

The region of a metal ion complex where hgands make direct binding interactions with the central metal ion. When the ligands do not completely neutralize the positive ionic charge of the central ion, other ions or electron-rich substances will become loosely associated with the complex through so-called outer coordination sphere interactions. 

Hartwig reported that the ferrocene-derived phosphines 13,20, and 21 are all effective as supporting hgands in the Pd-catalyzed reaction of aniline derivatives and aryl chlorides, Eq. (107) [72]. These bulky, electron-rich ligands allow for the desired C-N formation to be performed with as httle as 1 mol% Pd. 

Reductive elimination can often be facilitated by the use of catalysts bearing bulky, monodentate phosphine hgands, and is beheved to be most rapid when the two coupling partners have opposite electronic properties (i.e. one electron-rich and one electron-poor) [14]. The rates of all three steps in the catalytic cycle are believed to be maximized by employing conditions that favor the formation of intermediates bearing a single phosphine hgand [15]. 

Subsequently, the enantioselective route was replaced by a resolution process (33—>rac-34—>rac-36—>rac-31—>(S,S,i )-31—>32) [26]. This synthesis proved superior due to the reduced number of steps, although the undesired stereoisomer R,R,S)-3l could not be recycled. Ecological considerations led to the investigation of another enantioselective approach which is based on the hydrogenation of a-pyrone 35 to dihydropyrone (i )-34 [27]. Ee-values up to 96% were achieved with a cationic Ru catalyst derived from the electron-rich and sterically bulky f-Bu-MeOBIPHEP Hgand. The relatively low TON of 1,000 calls for further catalyst improvement in this hitherto unprecedented type of reduction. 

Recent work has led to the use of huUc electron-rich phosphine hgands to facihtate palladium-catalyzed processes with problematic oxidative addition. For examples, see (a) Littke, A.F. Fu, G.C. A convenient and general method for Pd-catalyzed Suzuki cross-couplings of aryl chlorides and arylboronic acids. Angew. Chem., Int. Ed. Engl. 1998, 37, 3387-3388. (b) Shen, W. Palladium catalyzed coupling of aryl chlorides with arylboronic acids. Tetrahedron Eett. 1997, 38, 5575-5578. (c) Old, D.W. Wolfe,... 

In conclusion, for C-C bond forming reactions on aryl bromides and iodides there is no need for the use of complexes, palladacycles or pincers as simple ligand-free Pd(OAc)2 will perform as well. For C—C bond forming reactions on aryl chlorides some palladacycles may be the catalyst of choice. For problematic cases, the palladium complexes based on bulky electron-rich phosphines or catalysts based on carbene Hgands are likely to give the best results. 

Pincer ligands, that is, tridentate Hgands that enforce meridional geometry upon complexation to transition metals, result in pincer complexes which possess a unique balance of stability versus reactivity [3]. Transition-metal complexes of bulky, electron-rich pincer ligands have found important appHcations in synthesis, bond activation, and catalysis [4, 5]. Among these, pincer complexes of Pr-PNP (2,6-bis-(di-iso-propylphosphinomethyl)pyridine), Bu-PNP (2,6-bis-(di-terPbutyl-phosphinomethyl)pyridine), and PNN ((2-(di-tert-butylphosphinomethyl)-6-diethyl-aminomethyl)pyridine), PNN-BPy (6-di-tert-butylphosphinomethyl-2,2 -bipyridine) ligands exhibit diverse reactivity [6-8]. These bulky, electron-rich pincer ligands can stabilize coordinatively unsaturated complexes and participate in unusual bond activation and catalytic processes. 

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