Aryl cobalt-catalyzed

A cobalt-catalyzed method for arylation of heteroarenes including thiazole and benzothiazole was reported in 2003 <030L3607>. According to this report, the direct C-5 arylation of thiazole with iodobenzene was carried out in the presence of cobalt catalyst [Co(OAc)2/IMes] and cesium carbonate, and a complete reversal of arylation from C-5 to C-2 was observed with the bimetallic Co/Cu/IMes system. This report has been retracted as the laboratory of the senior author has not been able to reproduce the key results disclosed in the communication <06OL2899>. 

Aryl-4,5-dihydrooxazoles 556 underwent cobalt-catalyzed carbonylation to give 4,5-dihydro-l,3-oxazin-6-ones 557, usually in good yields, but an exception was the ring enlargement of 556 containing a 5-methyl (R = Me) or a sterically bulky 2-(o-tolyl) substituent (Equation 67) <20030L1575>. 

Since Wakamatsu serendipitously discovered amidocarbonylation while performing the cobalt-catalyzed hydroformyla-tion of olefins in 1971, this unique carbonylation reaction, affording a-amino acids directly from aldehydes, has been extensively studied.More recently, palladium-catalyzed processes have been developed to expand the scope of this reaction.The Pd-catalyzed amidocarbonylation has been applied to aldehydes,aryl halides, and imines. As a related reaction, lactamization " of aryl halides catalyzed by a rhodium complex has also been developed. 

Fig. 58 Cyclizations of aryl iodides catalyzed by low-valent cobalt complexes...

Fig. 70 Electrochemically mediated cobalt-catalyzed aryl radical 5-exo cyclizations...

Aryl-4,5-dihydrooxazoles 380 undergo cobalt-catalyzed carbonylation to give 4,5-dihydro-l,3-oxazin-6-ones 381, generally in good yields (Scheme 177) <20030L1575>. 

Kom, T. J., Cahiez, G., Knochel, P. New cobalt-catalyzed cross-coupling reactions of heterocyclic chlorides with aryl and heteroaryl magnesium halides. Syn/etf 2003,1892-1894. 

Cobalt-catalyzed Aryl Radical Cyclizations with Grignard Reagent... 

Other C-C bond-forming reactions have been successfully developed using SCCO2 and liquid CO2 as reaction media. Examples include the synthesis of cyclopentenones via cobalt-catalyzed cocyclizations of alkynes with alkenes and carbon monoxide (Pauson-Khand reaction) (Scheme 30) , enantioselective nickel-catalyzed hydrovinylation of styrenes (Scheme 31) , and the palladium-catalyzed hydroarylation of acyclic jS-substimted-o , j8-enones with aryl iodides (formal conjugate addition) (Scheme 32). ... 

Facile insertion of CO takes place, and the reaction offers a synthetic method of orf/to-substituted benzoic acid derivatives. The 2-aryl-3-indazolone 421 was obtained in high yield by carbonylation of 4-methylazobenzene complex 420 in alcohol or water [176]. Cobalt-catalyzed carbonylation of 421 and subsequent hydrolysis afforded aniline and 5-methylanthranilic acid (422). The results show that the cr-bond formation is an electrophilic substitution of PdCl2 on the benzene ring [176,177]. 

Copper- and Cobalt-Catalyzed Arylations of Alkyl (Pseudo]Halides... 

Although cobalt catalysis is somewhat less developed compared to other transition metal catalysis for cross-coupUngs, but has shown rather unconventional, novel reactivity profiles [50]. For example, Oshima reported a cobalt-catalyzed tandem radical cyclization/cross-coupling reaction between an aryl Grignard reagent and an alkyl halide bearing an (o-alkenyl group (Equation 5.44) [51]. 

Highly stereoselective arylation reactions were reported, as shown in Equation 5.46. With an optically active diamine ligand, a modest asymmetric induction was observed (Equation 5.47). An asymmetric synthesis of a synthetic prostaglandin AH 13 205 was accomplished using the diastereoselective cobalt-catalyzed cycliza-tion/arylation sequence as key step [55]. 

Table 5.5 Cobalt-catalyzed cross-coupling (and tandem cyclization/arylation) reactions between unactIvated alkyl halides and aryl metal reagents.

In 1990 Alper and Lee reported on a cobalt-catalyzed carbonylation of aryl iodides and alkyl iodides [293, 294]. In the presence of cobalt chloride or acetate, potassium cyanide, base, and PEG-400, carboxylic acids were produced from the corresponding iodo compounds. Lewis acids, such as boron trifluoride etherate... 

Chiral l-aryl-5,6,7,8-tetrahydroquinolines (136) were synthesized by photo induced chiral cobalt-catalyzed [2-1-2-1-2] cycloaddition reaction of l-aryl-l,7-octadiynes (134) and nitriles (135). ... 

Cobalt catalysis has also received increased attention [351, 352]. Cobalt-catalyzed heterobiaryl coupling reactions between aryl chlorides and arylmagnesium halides take place with low loadings of Co(acac)j as the precatalyst under mild conditions [353]. Kinetic studies indicate that the active catalyst is an arylcobaltate(I) species. 

Wang, Shi, and coworkers [70] reported the cobalt-catalyzed directed C-H alkylation of benzo[h]quinoHne with alkyl Grignard s reagents (Scheme 19.44). The reaction occurred at room temperature but proved less efficient and general than the corresponding C-H arylation. Interestingly, when isopropylmagnesium bromide was employed, a mixture of Hnear and branched products was obtained. The... 

L3444).The cobalt-catalyzed electrophilic coupling of an arylzinc reagent with an N-chloro-l,4-diazepane derivative resulted in the corresponding arylated 1,4-diazepane (13CEJ6225). 

An interesting ligand effect was observed in the cobalt-catalyzed regioselective Diels-Alder reaction of 2-aryl-1,3-butadiene with an alkynylborane. The use of dppe as a ligand predominately afforded the ]oara-substituted product. On the contrary, the use of a pyridine-imine ligand predominately afforded the mefa-substituted product (Scheme 21.37) [43]. 

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