Aryl mesylates

In a few cases, it is possible to remove an oxygen substituent directly from the aromatic ring. Treatment of an aryl mesylate (ArOMs) with a nickel catalyst in DMF, for example, leads to the deoxygenated product, Ar—H." ... 

Aryl mesylates are used for the Ni-catalysed cross-coupling with Grignard, organozinc and organoboron reagents [100]. 

The phenolic OH group can be removed by Pd-catalysed hydrogenolysis of its triflate 522 with triethylammonium formate [261]. Naphthol can be converted to naphthalene by the hydrogenolysis of its triflate. The Ni-catalysed reduction of aryl mesylates 523 is possible using MeOH and Zn as the hydrogen donor [262]. Smooth removal of phenol groups as triflates and mesylates is not possible by any other means. 

One of the challenges in the Suzuki-type cross-coupling is to extend this reaction from electron-rich aryl iodides, bromides, and triflates to less reactive aryl sulfonates and aryl chlorides, which show poor reactivity in terms of oxidative addition in the catalytic cycle. Aryl mesylates, benzenesulfonates, and tosylates are much less expensive than triflates, and are unreactive toward palladium catalysts. The Ni(0)-catalyzed Suzuki-type cross-coupling reaction of aryl sulfonates, including mesylates, with arylboronic acids in the presence of K3P04 has been reported [123]. 

A versatile and efficient synthesis of 3-substituted-l//-indazoles 199 is based on the generation and subsequent cyclization of the intermediate hydrazones from aryl mesylates 198 and hydrazines (Scheme 119) <1999S588, CHEC-111(4.01.9.1.2)94>. 

Deoxygenation of phenols,l Aryl nonaflates, readily available by reaction of phenols with this reagent and triethylamine, undergo hydrogenolysis (H2, Pd/C) more rapidly and in higher yield (80-90%) than do aryl mesylates or tosylates. 

Very few examples of the use of nickel seem to have been described until now. Percec et al. [215] have coupled tributylphenyltin with aryl mesylates however, the main product was that derived from homocoupling rather than cross-coupling, tin reagents being much less suitable for cross-coupling than the corresponding magnesium or zinc derivatives. 

Percec, V., Bae, J. Y., Hill, D. H. Aryl Mesylates in Metal-Catalyzed Homo-Coupling and Cross- Coupling Reactions. 4. Scope and Limitations of Aryl Mesylates in Nickel-Catalyzed Cross-Coupling Reactions. J. Org. Chem. 1995, 60, 6895-6903. 

Aryl ethers. With CS2CO3 as base, aryl mesylates act as phenolate ions because desulfonylation occurs in situ. 

For performing the SM reactions of aryl mesylates, triflates, tosylates, and besylates, Percec s method involving Ni(dppf)Cl2 proved to be powerful and versatile alternative with moderate to good yields of biaryls. Thus the reaction of 4-acetylphenyl mesylate (295) with phenylboronic acid (260) resulted in 4-acetylbiphenyl (68) with a 51% yield [48], respectively. Scheme 32. 

In 2008, Buchwald and colleagues developed an efficient procedure for the carbonylation of aryl chlorides, aryl tosylates and aryl mesylates [202-204]. Under their reaction conditions, carboxylic acid derivatives were prepared in good yields (Scheme 2.23). The advantages of this procedure are (1) l,3-bis(dicyclohexyl-phosphino)propane bis(tetrafluoroborate) as the ligand used is stable and easily available and (2) the reactions were carried out in a reaction tube under 1 bar of CO, avoid the using of autoclave. 

Aryl chlorides are important starting materials in palladium-catalyzed coupling reactions [7]. Compared with the corresponding aryl iodides or aryl bromides, the advantages of aryl chlorides are obviously that they are inexpensive, easy to prepare, stable, etc. The same is true if we compare aryl tosylates or aryl mesylates with their aryl triflates analogs. Even though aryl chlorides, aryl mesylates and aryl acetates have been studied and have succeeded in cross-coupling reactions, their... 

Several studies have used palladium catalysis in the arylation of benzoxazoles. A palladium catalyst with a phosphine ligand allows their reaction with aryl mesylates without the requirement for acid or copper additives. In the reaction with arene-sulfonyl chloride, palladium is used in combination with copper. A plausible mechanism involves initial cupration of the benzoxazole followed by copper—palladium exchange and oxidative addition of the sulfonyl chloride to palladium to give (84). This intermediate may lose sulfur dioxide to give an aryl palladium species, which, on reductive elimination, yields 2-arylbenzoxazole. The arylation of benzoxazoles and benzthiazoles with aryl boronic acids is also catalysed by a combination of palladium... 

Oxidative addition is often the rate-determining step in a catalytic cycle. The relative reactivity decreases in the order of I > Br > OTf Cl. Although the palladium complexes exhibit excellent catalytic activity for various aryl iodides, bromides, and triflates, the biaryl-coupling reactions of less reactive chloroarenes and aryl mesylates are best carried out by nickel(O) complexes." " For example, the palladium-catalyzed reaction of chloroarenes is limitedly used for substituents with a > 0.45, whereas all substituents in a range of -0.83 to 0.66 accommodate the nickel-catalyzed reaction well (Table 1). ... 

One of the challenges is to extend the reaction from triflates to less reactive arylsulfonates. The palladium-catalyzed reaction of aryl mesylates results in low yields because of their slow oxidative addition to a palladium(O) complex, but they readily participate in the nickel-catalyzed cross-coupling reaction at 80°-100 °C. A nickel(O) species incorporating dppf ligand, obtained by in situ reduction of 10 mol% of NiCljCdppf) with zinc dust, is recognized as the most effective catalyst (Eq. 43). However, again the reaction needs to be optimized because a... 

Kwong and coworkers utilized amino phosphines L47-L48 for the amination of aryl mesylates [29]. The use of air-stable phenyl-bridged PJSl-ligands L49-L56 has been subsequently danon-strated for the arylation of ammonia with aiyl chlorides (Scheme 20.15) [30]. 

Use of Non-halide Electrophiles for the Direct Functionalization of Heteroarenes. A palladium-catalyzed direct arylation employing aryl tosylates for the functionalization of various heterocycles was reported (eq 182). Use of an alcoholic cosolvent ( BuOH) and X-Phos as the ligand was key to the success of the transformation. One example with more atom-economical aryl mesylates was also reported, but in a lower yield. Later, more general conditions were developed for arylation with mesylates, employing CM-Phos as the ligand for Pd (eq 183). Other intra-and intermolecular examples with mesylates have recently been reported. ... 

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