Palladacycles complexes

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

Figure 1 Homogeneous palladacycle complexes commonly used in Heck couplings.

Apart from palladium(O) and palladium(II) salts, such as Pd2(dba)3, PdCl2 and Pd(OAc)2, several well-defined Pd-carbene complexes have been employed in ionic liquids, as have some palladacycle complexes. However, ligands used are in most cases neutral and have not been included to facilitate catalyst retention in the ionic liquid. 

NHC-palladacycle complexes (125) have been obtained, as depicted in equation (17), from the corresponding dimer (124) and two equivalents of NHC ligand. Conveniently, this route was found compatible with diverse functions and so aminopalladacycles and phosphapalladacycles have been prepared. 

Many types of palladacyclic complexes have been used as precursors to catalysts for a variety of coupling processes146,147. Mixtures of dimeric palladacycles containing bridging halide, acetate or trifluoroacetate ions and a phosphine or carbene ligand have been studied as catalysts for the animation of aryl halides. The isolated phosphine adducts can also be applied in catalysis. 

In 2010, Kantchev and co-workers developed a series of palladacycle complexes of the formula [Pd(NHC)(palladacycle)] and evaluated their activity in Suzuki-Miyaura coupling. Complex 84 was found to be the most active pre-catalyst, efficiently forming a number of C—C bonds, including sp —sp, sp —sp, sp —sp, and sp —sp, in good-to-excellent yield (Scheme 19). The high reactivity of complex 84 in... 

Since the first X-ray crystal structure of a palladium(IV) complex was published by Canty [29, 30], a number of groups have undertaken the study of these relatively uncommon intermediates. In an effort to elucidate the role of palladium(IV) and palladium(II) intermediates in the reaction mechanism, Catellani prepared a number of isolable palladacycle complexes in both the (II) and (IV) oxidation states using 1,10-phenanthroline as the ligand (Scheme 8) [31]. Catellani successfully isolated palladium(IV) palladacycle 22, which was subsequently characterized by... 

In the cross-coupling reaction, starting from the simple arene (with directing group), palladation by a Pd(II) salt would lead to the formation of the palladacyclic complex (Ar1Pd(II)L) (Scheme 3). After the transmetallation and reductive elimination processes, the biaryl product is obtained together with Pd(0). If the Pd(0) can be further oxidized to Pd(II) catalyst, a catalytic cycle will be formed. By accomplishing this, arenes (C-H) are used to replace the aryl halides (C-X). Similarly, arenes (C-H) can be used to replace the aryl metals (C-M). 

Also worth mentioning here are studies based around the preparation and use of silica-supported palladacyclic complexes. It was the use of these that gave valuable evidence for the decomposition of half-pincer and SCS pincer " complexes during Heck reactions, generating soluble Pd(0) species that are the true catalysts. 

Palladacycle complexes of this type readily undergo ring closure (Eq. 13), particularly in the presence of bulky ort/io substituents such as t-Bu [7,17,23]. Ring enlargement of the same complex can also be effected by reaction with internal acetylenes such as dimethyl acetylenedicarboxylate (R=C02Me, L= methyl isonicotinate) (Eq. 14) [24]. 

Several hindered ortho-di-, tri- and tetra-substituted biaryls were successfully obtained by this method in 58-98% yields [114]. Recently, the great breakthrough has been achieved since several very active palladacycle catalysts were developed [120-131]. The first report by Beller and coworkers [120] has shown very high activity of the palladacycle complex 89 (see Chapter 3) with TON S up to 74 000 in the SM reactions of aryl bromides and activated chlorides. Najera s group [121,122] has developed... 

Among A -heterocyclic carbene oxime-palladacycles, complex 341 showed very high catalytic activity with aryl iodides, bromides and chlorides giving high TON s, 2000-92000, and TOF s, 200-4300 h [131]. 

Chen, M.T., Huang, C.A. and Chen, C.T. (2006) Synthesis, characterization, and catalytic applications of palladacyclic complexes bearing C,N,S-donor ligands. Ew. J. Inorg. Chem., A6A2%. 

Bedford used a palladacycle complex [130], while Vilar investigated a variety of homobimetallic palladium complexes [131]. These species are based on Buchwald s biphenyl-type ligands and were found to be excellent precatalysts for aminations of aryl chlorides. 

Mastrorilli et al. described a combined ESI HRMS and F NMR mechanistic study of this reaction with palladacycle complex 10 (Scheme 7.5) as the precatalyst [17], from which the authors postulated the formation of the true catalytic species in a first step. It is suggested that palladacycle 10 reacts with potassium trifiuorophenylborate 8a to give Pd(0) intermediate 117 (Entry 3, Table 7.3), which starts the catalytic cycle. This species undergoes oxidative addition of the aryldiazonium salt to give the cationic aryl-palladium(II) complexes 120, which contain azobenzene 11 as a ligand (Entry 4, Table 7.3). In this study, no intermediate was detected corresponding to transmetalation species. 

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