Organoboron compounds Organoboronic acids

Much attention has recently been focused on organoboronic acids and their esters because of their practical usefulness for synthetic organic reactions including asymmetric synthesis, combinatorial synthesis, and polymer synthesis [1, 3, 7-9], molecular recognition such as host-guest compounds [10], and neutron capture therapy in treatment of malignant melanoma and brain tumor ]11]. New synthetic procedures reviewed in this article wiU serve to find further appHcations of organoboron compounds. 

A major advantage of this MCR is that organoboronic acids are readily available in a large variety of structural configurations and they can be formed in isomeri-cally pure forms. As a result of their widespread utility in Suzuki-Miyaura coupling [27, 28] and other reactions [29, 30], a variety of aryl and heteroaryl [31] boronic adds are now commercially available and can be employed in this MCR process. Most of these compounds are also air and water stable as well as non-toxic and environmentally friendly. They also tolerate many functional groups, thereby... 

Scheme 1 Synthesis of organoboronic acids, esters, and amides via transmetallation with Li or Mg compounds.

The metal-catalyzed addition reaction of organoboron compounds have not yet been well developed, but die reaction of NaBPh4 or arylboronic acids with enones in the presence of Pd(OAc)2 and NaOAc or SbCL, was recently reported by Uemura and his coworkers.2 The reaction was proposed to proceed through the oxidative addition of the C-B bond to the Pd(0) species however, another probable process, the transmetalation to transition metals, may allow a similar catalytic transformation by the use of organoboronic acids. We report here the 1,4-addition reaction of organoboronic acids to oc, J-unsaturated ketones or esters and 1,2-addition to aldehydes or imines catalyzed by a (acac)Rh(CHj=CH2)2/phosphine complex, which may involve the B-Rh transmetalation as the key step. 

The main limitations of using organoboronic acids in cross-coupling chemistry are threefold. These compounds are quite reactive towards many nucleophiles and oxidants, which prohibits installation of the boronic acid functional group early in a synthetic sequence. Moreover, purification of boronic acids is often problematic. Finally, since the C—B bond is relatively nonpolar, transmetalation of an organoboron reagent to Pd(ll) will not occur without coordination of an anionic base or a fluoride ion to the boron atom to afford a four-coordinate ate -complex (e.g. 20 shown below). This requirement poses some limitations with respect to transformations of base- or fluoride-sensitive substrates, although these... 

Practically all the organoboron compounds of interest in the current context are boronic acids or closely related compounds. Boronic acids are relatively weak acids that ionise by association, not dissociation - stable salts of tetrahedral aryl trihydroxyboronates can be isolated as stable solids and used in coupling... 

The most commonly employed aryl sources are arylboronic acids. Due to their widespread application in the Suzuki-Miyaura reaction, organoboronic acids have found wide acceptance, and currently more than 450 different arylboronic acids are available commercially [47]. These compounds unite several beneficial properties such as high air and moisture stability and great functional group... 

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