Organoboronic acids formation

Suzuki reaction A palladium catalyzed reaction that results in the formation of new carbon to carbon bonds by reacting an organohalide or organotriflate with an organoboronic acid. 

Addition reactions of organoboronic acids to electron-deficient alkenes were found to be catalyzed by rhodium(i)931 or dicationic palladium(n) complexes.932,933 The reaction proceeding through the transmetallation to a transition metal has been proved to be a general technique for a wide range of selective carbon-carbon bond formation via 1,4-addition to a,/ -unsaturated ketones, aldehydes, esters, and amides, and the 1,2-addition to aldehydes and imines (Equation (217)).934... 

Although there is no direct evidence that the boronate anions, such as RB(OH)3, are capable of effecting the transmetafl tiQQjit is quite reasonable to assume a similar effect of the base for the transmetallation of organoboronic acids. The cross-coupling reaction of arylboronic acids with aryl halides at pH 7-8.5 is retarded relative to the reaction at pH 9.5-11 [51]. The of phenylboronic acid is 8.8, thus suggesting the formation of the hydroxyboronate anion [RB((OH)3 at pH > pA and its transmetallation to the palladium(II) halides. The formation of ArB(OH)3 at pH 11-12 has been reported [52]. 

Lautens recently reported a novel strategy forbenzothiophene synthesis, employing intramolecular Pd-catalyzed vinylation of thiols. Concomitant C-S and C-C bond formation can be achieved in the presence of organoboronic acids (10) [116]. 

Scheme 1.12. Catalytic cycle for the formation of ketones from carboxylic anhydrides and organoboronic acids.

Interestingly, and in contrast to the vast majority of rhodium-based catalysts, addition of organoboronic acids to conjugated enals using complex 15 as catalyst resulted exclusively in the formation of 1,2-addition products. With the exception of this particular system, NHC-based catalysts generally produce the 1,4-addition products. 

The overall mechanism is closely related to that of the other cross-coupling methods. The aryl halide or triflate reacts with the Pd(0) catalyst by oxidative addition. The organoboron compound serves as the source of the second organic group by transmetala-tion. The disubstituted Pd(II) intermediate then undergoes reductive elimination. It appears that either the oxidative addition or the transmetalation can be rate-determining, depending on reaction conditions.134 With boronic acids as reactants, base catalysis is normally required and is believed to involve the formation of the more reactive boronate anion.135... 

Aliphatic amines can be readily oxidized by Pd(II) to imines or iminium salts and hydrido complexes. The latter can transfer hydrogen to alkenes, leading to the formation of alkanes as byproducts of the Heck reaction (last example, Scheme 8.18). Such reactions can be avoided by using alkali carbonates as base instead of aliphatic amines [148]. Treatment of stannanes or organoboron derivatives with electron-deficient alkenes under acidic reaction conditions can also lead to formal products of Michael addition instead of the products of a Heck-type reaction [149, 150] (Scheme8.19). 

The covalent B-G bond of organoboron compounds shows little reactivity toward representative electrophiles, but the presence of an empty low-lying orbital facilitates transmetallation to other metals. The transmetallation to zinc709 and other metal reagents has been extensively studied for carbon-carbon bond formation via addition and coupling reactions of trialkylboranes and alkylboronic acids. 

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