Until recently, organoboronic acids have been of limited use in organic synthesis due to their low reactivity for ionic reactions. During the past two decades; however, it has become increasingly clear that they are valuable reagents capable of undergoing many carbon-carbon bond formations in organic syntheses. In 1979, catalytic carbon-carbon bond formation, via transmetalation between organoboron compounds and palladium (II) halides, was found to proceed in the presence of a base. Historical PerspectivesAlthough organoboronic acids are highly advantageous as reagents in 1 laboratories and industries because they are largely unaffected by the presence of water, tolerate a broad range of functionalities, and yield nontoxic by-products, the boron-carbon bond is highly inert to ionic reactions. Transmetalation to other metals, especially to transition metals, is one promising process that enables the carbon-carbon bond formation by using organoboronic acids [1][2][3]. The interaction between 1-hexenylboronic acid and palladium(II) acetate was first demonstrated by Heck [4]. In situ preparation of (E)-or (Z)-1-alkenylpalladium(II) acetate via transmetalation was followed by its conjugate addition to ethyl acrylate (Scheme 1). Negishi found that boron-ate complexes such as lithium 1-alkynyltributylborate can participate in Pd-catalyzed cross-coupling [5,6]. Heck-type addition-elimination mechanism was proposed because other types of lithium tetraalkylborates, such as alkyl-, alkenyl-, and aryl derivatives, were unreactive. In spite of these previous observations, it was very difficult to use organoboron compounds to the palladium-catalyzed cross-coupling reaction; thus, an alternative and two-step procedure involving transmetalation of trialkylboranes to alkylmagnesium halides was developed by Murahashi [7].<>In 1976, we attempted a stoichiometric reaction between trialkylboranes and π-allylpalladium(II) chloride [8]. However, our first attempt failed because the reaction resulted in the exclusive formation of β-hydride elimination products. It was interesting that trialkylboranes readily alkylate π-allylpalladium(II) chloride without the assistance of a base immediately precipitating palladium black; however, a major process was quarternization of the boron atom with a negatively charged base enhances the nucleophilicity of the organic group on the boron atom for alkylation of R-Pd-X (6). 4Although there is no direct evidence for analogous hydroxyboronate anions, RB(OH) 3 -(7), which exists in alkaline solution in equilibrium with a free organoboronic acid, could similarly alkylate R-Pd-X (6) (path A). The transmetalation to 1 decreasing in the order of Cl > Br > I is the reverse of the oxidative addition of organic halides to palladium(0) complexes and is highly dependent on the counter cation of the added bases.<>An alternative process is transmetalation to an alkoxo-, hydroxo-, acetoxo-, or (acetylacetoxo)palladium(II) complex (9)

Sumnary:The

No abstract

No abstract

The borylation of arenes leads to the formation of synthetically versatile products from unactivated arene reagents. We report that Ir(I) precursors in conjunction with bipyridine ligands catalyze in high yields the borylation of arenes under mild conditions. These reactions encompase arenes bearing both electron-withdrawing and electron-donating substituents. The temperatures required for the transformation are much lower than those previously reported for direct arene borylation. The combination of [Ir(COE)2Cl]2 and (4,4-di-t-butyl)bipyridine even allows for reaction at room temperature. The same catalyst system at 100 degrees C provides remarkably high turnover numbers for a hydrocarbon functionalization process. Mechanistic studies show that the reactions involve uncommon, Ir(II) tris-boryl complexes. An example of this type of complex ligated by di-t-butylbipyridine was isolated and structurally characterized. It reacted rapidly at room temperature to produce aryl boronate esters in high yields.