Aryl tosylates, Suzuki coupling

The Suzuki coupling of aryl halides was also extended to tosylates recently. Benzothiazole 5-tosylate reacted with m-xylene-2-boronic acid (6.13.) to give the coupled product in 94% yield using palladium acetate and a stericly congested biphenyl based phosphine ligand as catalyst.17 Another class of less commonly utilised cross-coupling partners are methyltio derivatives. In the presence of a copper salt, which activates the carbon-sulphur bond, 2-methyltio-benzotiazol coupled readily with a series of arylboronic acids.18... 

The procedures described in Chapters 4-7 all relate to a set of broadly similar transformations that are becoming exceptionally important and well used in laboratories worldwide. In Chapter 4, eight examples of the Suzuki coupling reaction are described four accounts describe the use on activated alkene (vinyl bromide, triflate or tosylate) as the coupling partner for boronic acid derivatives. The other examples of Suzuki couplings involve aryl bromides and aryl chlorides. It is noteworthy that the methodology introduced by Nolan has been extended to include amination reactions. 

Several examples of Pd/P(t-Bu)2Me-catalyzed Suzuki couplings of primary, P-hydrogen-containing alkyl tosylates are illustrated in Table 1 (entries 9-12). Functional groups such as acetals (entry 9), amides (entry 10), ketones (entry 11), and tertiary alcohols (entry 12) are tolerated. Furthermore, both alkyl-and aryl-9-BBN compounds are suitable coupling partners. These Suzuki reactions of alkyl tosylates proceed in comparable yield at room temperature, albeit much more slowly. 

Nickel-based catalysts have also been used to activate aryl tosylates. In 1996, Kobayshi reported that the activated poro-methoxycarbonylphenyl tosylate underwent Suzuki coupling with a lithium organoborate using [NiCl2(PPh3)2] as the catalyst (Equation 2.78) [126]. 

Monteiro reported a more general method in 2001 for Suzuki couplings of aryl tosylates using a nickel catalyst derived from PCys (Equation 2.79). Mechanistic studies suggest that transmetaUation, and not reductive elimination or oxidative addition, is rate-determining [127]. 

For other examples of nickel-catalyzed Suzuki couplings of aryl tosylates, see ... 

The Suzuki reaction was described in the presence of various [Pd(7 -allyl)Cl]2 (l)/ligand systems as well. The efficiency of the tetraphosphine ligand Tedicyp was demonstrated for various Suzuki couplings, and more recently for the reaction of alkenyl-boronic acids with arylbromides. 7i The preparation of various stryrene derivatives (eq 81) has been performed in the presence of vinylboronic acid and substituted alkenyl boronic acids. The catalyst system was tolerant of electronic variation in the aryl-bromide component. Tum-over numbers of 3100-8600 could be achieved for activated substrates such as 4-bromoacetophenone. The coupling of alkyl tosylates was described either in the presence of Pd(OAc)2 or dimer (1) as precursor. 7 a fiber-supported catalyst prepared from 1 was found to be efficient for Suzuki coupling between activated aryl bromides and phenylboronic acid. 

Ligand 19 is water soluble, and has been employed in Suzuki coupling reactions conducted in aqueous media. Ligand 17 provides optimal results in Suzuki coupling reactions of aryl tosylates and aryl benzenesulfonates. Ligand 16 is thought to have the greatest substrate scope of the Buchwald biaryl-phosphine derivatives, and is widely used in... 

A recent example of the application of Suzuki reactions to introduce diversity is shown by the synthesis of 4-carbon-based substituted quinoli-nones [118]. There are limited methods for the synthesis of such derivatives and the outlined solid phase approach involved the synthesis of a resin-bound 3-methoxycarbonyl 4-tosyl quinolinone and subsequent Suzuki coupling with several aryl boronic acids and cleavage (Scheme 43). One drawback was the formation of hydrolysis byproducts, lowering the yield. 

NiCl2(PCy3)2 associated with PCyj promotes the selective cross-coupling of aryl-tosylates with arylboronic acids under relatively mild conditions, and a variety of function groups are tolerated in both arenes [70]. It is particularly noteworthy that no reducing agent such as Zn is required in this reaction. More recently, room-temperature nickel-catalyzed Suzuki coupling reactions of arenesulfonates with arylboronic acids have been disclosed [71]. 

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]. 

On the other hand, the recent studies by Percec s team of a Ni-catalyzed neopentylglycolborylation of aryl iodides, bromides, chlorides, mesylates and tosylates with freshly prepared solutions of HBnpg (Scheme 3.33) also provide, efficiently and economically, arrays of arylboronic esters that are good reagents for Suzuki-Miyaura couplings. " ... 

Alternative sulfonate leaving groups besides triflate have not been reported to be active in Pd catalyzed Suzuki-type reactions. Aryl mesylates, benzenesulfonates and tosylates are much less expensive than triflates and are usually unreactive towards palladium catalysts. However, in the first example of the use of a Ni-catalyst in the Suzuki reaction, aryl mesylates participated in cross-coupling reactions with arylboronic acids in good yields (equation 48) (745). 

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