Chelation-Assisted Oxidative Addition

Other Group 10 metal complexes, such as Pd2(dba)3 and Pt(cod)2, were tested for chelate-assisted oxidative addition of 55. Use of Pd2(dba)3 in the reaction of 55... 

An alternative synthetic route to platinum(II) thiolates is by the oxidative addition of the S—S bond to platinum(O). When the sulfur atom has phenyl or electron-withdrawing substituents such as CF3, this reaction is a useful one to synthesize the thiolato platinum(II) complexes (equation 503).1703-1705 Simple alkyl disulfides such as Me2S2 and Et2S2 do not form stable dithiolato complexes of platinum(II) by S—S addition to Pt(PPh3)3, but if chelation can occur, chelate-assisted oxidative addition can induce S—S cleavage (equation 504).30 An unusual cyclic thiolato complex is obtained by the decarbonylative cleavage of a C—S bond (equation 505).1707... 

Spectrophotometrically, we could detect no reaction between 1 and AsCHO. This result demonstrates that the identity of the neutral donor component of the bifunctional substrate is crucial for the success of our methodology. This observation substantiates the mechanism outlined in Scheme 2 for the chelate assisted oxidative addition reaction. 

In addition, the reaction of [ Rh(cod)Cl 2] (cod= 1,5-cyclooctadiene) with Ph2P(o-C6H4CHO) 93 in the presence of l,2-phenylenediamine(daphen) 95 led to the formation of the chelate-assisted oxidative addition product [Rh(Cl)(H)[PPh2((7-C6H4CO)](daphen)] 96 with displacement of 1,5-cyclooctadiene as shown in Equation (30) <2004ICA2818>. 

While alkyl halides do not generally oxidatively add to neutral, 18-electron d systems, aromatic C-X bonds (X = F, Cl, Br, I) will undergo chelate-assisted oxidative addition to W(CO)3(EtCN)3 and ( 7 -toluene)Mo(CO)3 . The cleavage of C-F bonds is notable. The W(II) products have been investigated for further functionalization of the C-X bonds . 

Deuterium-labeling experiments suggested that the catalytic cycle involves chelation-assisted oxidative addition of the C-H bond to the cobalt center, followed by insertion of styrene into the Co-H bond to afford either branched or... 

Based on studies directed towards chelation-assisted oxidative homocouplings of 2-arylpyridines, a protocol was developed for intermolecular coupling reactions [132]. Thus, benzo h quinoline (186) was efficiently arylated with a variety of arenes using silver salts and benzoquinone as additives (Scheme 9.53). 

The photo-assisted formation of a chelating diphos ligand from PPh3 and a eyelopalladated [P(G6H4)(G6H5)2] ligand was reported.New P,G-chelated Pd(ii) complexes were formed via phosphine-assisted oxidative addition of an aliphatic G-Gl bond. ... 

Several reaction pathways for reaction 1 are possible. A clear reaction mechanism has not been elucidated. Although it is premature to discuss the details of the reaction pathway for this silylation reaction, one possible pathway for the chelation-assisted silylation of C-H bonds is shown in Scheme 2. The catalytic reaction is initiated by oxidative addition of hydrosilane to A. Intermediate B reacts with an olefin to give C. Then, addition of a C-H bond to C leads to intermediate D. Dissociation of alkane from D provides Ru(silyl)(aryl) intermediate E. Reductive elimination making a C-Si bond gives the silylation product and the active catalyst species A is regenerated. Another pathway, addition of a C-H bond to A before addition of hydrosilane to A is also possible. At present, these two pathways cannot be distinguished. 

An important factor in the success of these reactions involves chelation-assistance by a heteroatom. Thus, the coordination of the heteroatom to the metal, brings the metal closer to the C-H bond and stabilizes the thermally unstable C-M-H species formed by the oxidative addition of a C-H bond to a low-valent transition metal complex. In addition, the use of the chelation-assistance leads to a high regioselectivity, which is an essential factor in organic synthesis. For reactions, a number of transition metal complexes - including ruthenium, rhodium, and iridium - are used as a catalyst, and ruthenium-catalyzed reactions will be described in this chapter [5]. 

This potential catalyzing effect of polar solvents is supported by the discovery a few years ago of neighboring group participation, or chelate assistance, in aiding a variety of oxidative additions 195). Reactions of HCl, MeCl, MeBr, Mel, CCU, CI2, and PhCOCl with phosphine complexes of Rh(I), Ir(I), or Pt(II) are all enhanced when the phosphine is PMe2(o-MeOC6H4), which contains a nucleophilic MeO group, com-... 

Keywords Disilanes / Oxidative Addition / Phosphinoalkylsilanes / Chelate Assistance / Hydrido-Silyl Complexes... 

Our group recently showed that addition of I2 to chelated Pt(II) diaryl complexes resulted in the formation of free iodoarene and Pt(II) aryl iodo complexes, with the exception of the small dmpe ligand system, where stable oxidative addition Pt(IV) complex fra y-(dmpe)Pt(Ar)2l2 was isolated (Scheme 26) [70]. Heating this complex in polar solvents gave the mixture of products the thermodynamically stable cis-isomer and Ar-I reductive elimination products (Scheme 27a) [71]. The isomerization reaction was light-assisted with the light triggering the diphosphine chelate... 

When the iridium hydride is reacted with a hydrogen acceptor, simple oxidative addition adducts can be seen for aromatic and vinylic C-H containing substrates. With nitrobenzene, although a thermodynamic preference is seen for an orthometallated chelate product, the kinetic preference is for meta- and para-Gr-W activation, which is then followed by rearrangement to the o/n4n-activated product, which in turn coordinates the nitro group. Hence, chelate assistance is found to have no kinetic benefit for C-H activation in this complex (Equation (22)). 

Chen et al. [20], for example, reported on chelation-assisted reactions in an article entitled Chelation-Assisted Carbon-Halogen Bond Activation by a Rhodium(I) Complex in 2009. These reactions proceed by C-Br bond activation via an oxidative addition mechanism. They take place in reactions of [Rh(PPh3)2(acetone)2] PFg" with 2-(2-bromophenyl)pyridine at room temperature to give the cyclometa-lated rhodium bromide shown in Eq. (6.4). 

More recently, the first direct observation of C-O oxidative addition of an aromatic elher to a transition metal complex has been reported by Kakiuchi (Equation 4.76). This reaction occurs by initial chelate-assisted C-H bond cleavage, followed by isomerization to form the final Ru(II) aryloxide complex. 

In an effort to move away from precious metal catalysts, various reports in recent years have focused on the use of first-row metal catalysts for direct arylations [57-60]. As a representative example of these new developments, we illustrate in Scheme 23.15 the chelate-assisted ortho-C-H arylation of arenes with Fe catalysts [61]. With iron being cheap, nontoxic, and ubiquitous, this protocol is highly attractive for pharmaceutical syntheses. Using the catalyst precursor Fe(acac)j in conjunction with bidentate pyridine ligands, Zn-aryl reagents as aryl transfer reagents and 1,2-dichloroisobutane as the oxidant, excellent yields of the arylated product were obtained. An interesting feature of this reaction is the hydrolysis of the imine moiety after work-up. The reaction conditions tolerate additional functionalities such as cyanides, chlorides, triflates, tosylates, and thiophenes. 

Despite these challenges, several groups have developed chelate-assisted fluorinations of C-H bonds. The earliest example from 2006 uses 25 as F+ source and oxidant in combination with microwave irradiation (Scheme 23.47) [149]. Using a pyridine directing group, difluorination is observed when the arene has no additional substituents in ortho or meta position. 

In 2004 Kakiuchi disclosed that atkoxyarenes bearing an ortho carbonyl group can be coupled with arylboronic esters in the presence of RuH2(CO)(PPh3)3 [Eq. (16)] [65]. The reaction represents the first example of a Suzuki-Miyaura-type reaction of aryl ethers. Chelation assistance facilitates the oxidative addition of C-O bonds, which allows for a regioselective C-O bond cleavage of diaryl ethers. 

Despite the intrinsic difficulties mentioned above, a number of strategies have been devised to realize oxidative addition of C-C a-bonds. For example, release of ring strain of a substrate molecule affords both kinetic and thermodynamic drive for oxidative addition. A chelating effect also assists both kinetically and... 

Copper(I) complexed with 1,10-phenanthroline has been used in the arylation of imidazo[l,2-a]pyridines by aryl bromides, iodides, and triflates to give products, (139), substituted at the 3-position. The mechanism is likely to involve initial deprotonation at the 3-position followed by cupration and oxidative addition of the aryl halide. The copper-mediated cross-coupling of indoles with 1,3-azoles is assisted by chelation of a, readily removed, 2-pyrimidyl group and leads to products such as (140). In this reaction, two carbon-hydrogen substitutions are required and it is likely that initial cupration of the 1,3-azole is followed by chelation-assisted formation of a bis(heteroaryl) copper species before oxygen-promoted reductive elimination... 

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