Cobalt-Catalysed Mizoroki-Heck-Type Reactions

5 Cobalt-Catalysed Mizoroki-Heck-Type Reactions 

A heterogeneous cobalt catalyst was employed for arylations of styrene (2) and two acrylates with aryl iodides. Generally, isolated yields were significantly lower than those observed for heterogeneous nickel catalysts [24]. Further, a silica-supported poly-y-aminopropylsilane cobalt(II) complex was reported as a highly active and stereoselective catalyst for Mizoroki-Heck-type reactions of styrene (2) and acrylic acid (16) using aryl iodides [23,25]. 

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An early example for cobalt-catalysed Mizoroki-Heck-type reactions with aliphatic halides by Branchaud and Detlefsen showed that an intermolecular substitution of styrene (2) could be achieved with [Co(dmgH)2py] (70) (dmgH = dimethylglyoxime monoanion) as catalyst in the presence of visible light. This radical reaction led selechvely to the substitution products when using stoichiometric amounts of Zn (27) and pyridine (31) as additives (Scheme 10.24) [52]. 

A more efficient and more generahy applicable cobalt-catalysed Mizoroki-Heck-type reaction with aliphatic halides was elegantly developed by Oshima and coworkers. A catalytic system comprising C0CI2 (62), l,6-bis(diphenylphosphino)hexane (dpph 73)) and Mc3 SiCH2MgCl (74) allowed for intermolecular subshtution reactions of alkenes with primary, secondary and tertiary alkyl hahdes (Scheme 10.25) [51, 53]. The protocol was subsequently applied to a cobalt-catalysed synthesis of homocinnamyl alcohols starting from epoxides and styrene (2) [54]. 

The catalytic system proved not only applicable to alkyl hahdes, but also allowed for the intramolecular conversion of aryl halides. Interestingly, the corresponding Mizoroki-Heck-type cyclization products were formed selectively, without traces of reduced side-products (Scheme 10.27) [55]. Therefore, a radical reaction via a single electron-transfer process was generally disregarded for cobalt-catalysed Mizoroki-Heck-type reactions of aromatic hahdes. Instead, a mechanism based on oxidative addition to yield an aryl-cobalt complex was suggested [51]. 

Scheme 10.22 Electrochemical cobalt-catalysed Mizoroki-Heck-type reaction.

Cobalt-catalysed electrochemical arylation reactions of acrylates were achieved by Gosmini and coworkers. The presence of 2,2 -bipyridine (Bpy, 63) was found crucial to reduce the formation of conjugate addition products in this transformation. Notably, this Mizoroki-Heck-type reaction proved applicable to aryl iodides and bromides and to an alkenyl chloride (Scheme 10.22) [48]. 

Scheme 10.25 Cobalt-catalysed intermolecular Mizoroki-Heck-type reaction with alkyl halides.

As part of comparative studies, Iyer [47] reported the use of Vaska s complex [IrCl(CO)(PPh3)2l (92) in intermolecular Mizoroki-Heck-type reactions of methyl acrylate (1) and styrene (2). Aryl iodides could be used as electrophiles, while bromobenzene, chlorobenzene and aliphatic halides gave no desired product. The catalytic activity was found to be lower than that observed when using Wilkinson s complex [RhCl(PPh3)3] (84). Thus, a higher reaction temperature of 150 °C was mostly required. In contrast to the corresponding cobalt-catalysed reaction, however, Vaska s complex (92) proved applicable to orf/io-substituted aryl iodides (Scheme 10.33). 

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