Oxidative addition of aryl triflates

The initial step of the catalytic cycle is oxidative addition of aryl triflate to a BINAP-coordinated Pd(0) species. Since, in the actual catalytic system, Pd(OAc)2 and (/ )-BINAP are used as the precursors of the Pd(0) species, reduction of Pd(OAc)2 into the BINAP-coordinated Pd(0) species should be operative prior to the catalytic reaction. Although Pd(OAc)2 is the most commonly used precursor of a Pd(0) species in many palladium-catalyzed organic reactions, no direct information has been reported so far on its reduction process. In this study, we confirmed for the first time that the reduction proceeds according to the process involving a combination of tertiary phosphine (BINAP) and water as the reducing reagent (Scheme 8) (Ozawa, F. Kubo, A. Hayashi, T., submitted for publication). 

Cationic tra i-ArPd(PPh3)25 complexes are quantitatively produced in oxidative addition of aryl triflates to free-chloride palladium(0) complexes (Scheme 8). " However, Stille reactions from aryl triflates are more efficient when performed in the presence of added chloride Neutral trans-ArPdCU complexes are then formed... 

Jutand, A. and Mosleh, A. (1995) Rate and mechanism of oxidative addition of aryl triflates to zerovalent palladium complexes. Evidence for the formation of cationic (aryl)palladium complsx s. Organometallics, 14, 1810-7. 

Their first study was based on the rate of oxidative addition of aryl triflates to Pd (PPh3)4 in a coordinating solvent such as DMF [19]. In one case, the authors isolateda... 

B.ii.c. Cationic trsms-ArPdL2S Complexes (S = Solvent, L = PPhj). Oxidative addition of aryl triflates to a free-chloride complex such as Pd°(PPh3>4 gives cationic trans-ArPd(PPh3)25 (S = THE, DMF) characterized by conductivity measurements (Eq. 1 of Scheme Neutral trfl 5 -ArPdCl(PPh3)2 complexes are formed as soon as chloride ions are added to Pd°(PPh3)4 (Eq. 2 of Scheme 8). The faster oxidative addition observed in the presence of chloride ions ( app° > app) confirms the involvement of three-coordinate anionic Pd°(PPh3)2Cl complexes. ... 

In the competition experiment, electron-deficient 4-trifluoromethyl aryl triflate reacted 21 times fasterthan electron-rich 4-methyl substrates. Intermolecular KIE kulko = 1.5 suggested that oxidative addition of aryl triflate to Pd(0) is most likely the turnover-limiting step of the catalytic cycle. A plausible mechanism was also proposed in Scheme 4.36. 

Aryl triflates have proved to possess approximately the same reactivity as aryl halides in various Pd-catalyzed cross-coupling reactions [100, 101]. In 2009, Alami s group employed aryl triflates for Pd-catalyzed coupling reaction of polyoxygenated aryl A-tosyUiydrazones (Fig. 22) [102]. The catalytic system Pd(OAc)2/Xphos with LiO Bu as base in dioxane was found to be suitable for the reaction. The reaction is initiated by the oxidative addition of aryl triflate to Pd(0) species. Then similar migratory insertion and p-hydride elimination subsequently take place to afford a series of 1,1-diarylethylenes which are of biological interest. 

C-0 bond cleavage of aryl triflates or tosylates is also studied in relation to Mizoroki-Heck type reactions [101], Oxidative addition of PhOTf to Pd(PPh3)4 is 10 times slower than that of Phi. Since similar trend is observed for the catalytic Mizoroki-Heck reaction, the oxidative addition of aryl compound is considered to be the rate-determining step in the overall catalytic process. This feature suggests that the C-0 bond cleavage of aryl triflate proceeds by the concerted SNAr mechanism. However, since the triflate normally acts as a non-coordinating anion, thermally unstable cationic arylpalladium(II) complexes are formed in this reaction (Scheme 3.54). 

The oxidative addition of aryl or alkenyl halides to Pd(0), followed by transmetalation of organohn to Pd(II) complex, causes achvation of the Sn-C bond and the formation of a new Pd-C bond. For aryl or alkenyl triflates the reaction needs often to be promoted by the addihon of Cl , which was thought to... 

A two-step one-pot synthesis of 2,3,5-trisubstituted furans from epoxyalkynyl esters was reported, in which a facile Sml -mediated reduction was used for the generation of the 2,3,4-trien-l-ols, and the reduction was followed by a Pd(ll)-catalyzed cycloisomerization <01JOC564>. An attractive variant of this reaction was extended to the preparation of tetrasubstituted furans. Thus, when electrophilic Pd(ll) complexes were generated in situ by an oxidative addition of aryl halides or triflates to Pd(0), the oxypalladation process was followed by a reductive elimination and tetrasubstituted furans were formed <01TL3839>. 

Oxidative addition of aryl haUdes to an electron-rich paUadium(O) species depends substantially on the nature of the C(sp )—X bond. Relatively weak bonds, as in aryl iodides or triflates, lead to a faster oxidative addition, but the activations of stronger bonds, as in aryl bromides-and especially chlorides-require very good donor ligands. This dependence of oxidative addition rates on the nature of the electrophile in multistep reactions may lead to different rate-deterriiiiittig steps. Apart from the nature of the aromatic substrate, the possible interaction of the terminal alkyne starting material and the internal alkyne product with the metal center of the catalyst can alter the kinetics of oxidative addition. The kinetics of addition of... 

It has been shown that the use of ionic liquids may be beneficial in aromatic fluorinations in protic solvents." Aryl fluorides may also be obtained using a copper-catalysed halide exchange reaction. The evidence suggests a redox Cu(I)/Cu(III) catalytic cycle involving oxidative addition of aryl halide at the copper(I) centre followed by halide exchange and reductive elimination." A mechanistic investigation of the palladium-catalysed conversion of aryl triflates to fluorides has shown that C-F reductive elimination from the palladium—arene complex does not occur when the aryl group is electron rich and requires in situ modification of the catalyst." ... 

The oxidative addition of aryl and vinyl halides and triflates was presented in Chapter 7. A few features of this mechanism that are important for understanding the scope and limitations of the cross-coupling processes are presented here. 

One of the most common examples of this approach involves the coupling of aryl or vinyl halides with palladium-catalyzed cydization. This chemistry typically employs palladium(O) catalysts, which are postulated to undergo an initial oxidative addition of aryl or vinyl halides to form a palladium(II) complex to mediate cydization. This is followed by reductive elimination of the heterocyde-aryl or heterocyde-vinyl bond. This approach has been employed to construct a range of polysubstituted indoles. For example, Cacchi has shown that the Pd(PPh3)4 catalyzed coupling of trifluoroace-tanilides with aryl halides or vinyl halides/triflates proceeds to substituted indoles in good yield (Scheme 6.14) [18]. 

This reaction is not a bona fide Heck reaction per se for two reasons (a) the starting material underwent a Hg Pd transmetallation first rather than the oxidative addition of an aryl halide or triflate to palladium(O) (b) instead of undergoing a elimination step to give an enone, transformation 134 136... 

Intramolecular arylation of G-H bonds gives cyclic aromatic compounds. In this intramolecular arylation, the carbon-palladium cr-bond is first formed by the oxidative addition of Pd(0) species and then the resulting electrophilic Pd(n) species undergoes the intramolecular G-H bond activation leading to the formation of the palladacycle, which finally affords the cyclic aromatic compounds via reductive elimination.87 For example, the fluoroanthene derivative is formed by the palladium-catalyzed reaction of the binaphthyl triflate, as shown in Scheme 8.88 This type of intramolecular arylation is applied to the construction of five- and six-membered carbocyclic and heterocyclic systems.89 89 89 ... 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

Fig. 16.1. Presumed elementary steps of a C,C coupling between a Gilman cuprate and an alkenyl or aryl triflate (X = 03S—CF3), bromide (X = Br), or iodide (X = I). The four elementary steps of the reaction, discussed in the text, are (1) complexation, (2) oxidative addition of the substrate to the metal, (3) reductive elimination, and (4) dissociation of the w-bound ligand.

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