Organoboron compounds oxidation

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... 

Atmospheric boron is in the form of particulates or aerosols of borides, boron oxide, borates, boranes, organoboron compounds, trihalide boron compounds, or borazines. The half-time persistence of airborne boron particles is short, usually on the order of days (USPHS 1991). 

Organoboron compound, geminal, 219 Organodiboron derivatives, 193 Organodielement halides, 79-83 Organoelement halides, 99 Os-Cl exchange, 186 Osmium boryl complexes, 179 Oxidation, ionic liquids, 278-279... 

Among organoboron compounds, the greatest practical interest is presented nowadays by trialkylborates. They are used as additives to hydrocarbon oils, to fuels and lubricants, as well as a protective medium which prevents hot metal in the process of molding from oxidation (trialkylborate vapours are sent over the surface of the metal). 

Fig. 16.18. Representative mechanism of the Pd-catalyzed C,C coupling of an organoboron compound. The elementary steps, discussed in the text, are (1) complexation, (2) oxidative addition, (3) transmetalation of the alkenylboron compound to afford an alkenylpalladium compound, (4) reductive elimination, and (5) dissociation of the coupled product from the metal. - Note Regarding the arrangement of the ligands around the metal center of the individual intermediates and the details of the transmetalation the present mechanistic analysis is less complete than the mechanistic analysis of other Pd-catalyzed C,C couplings, namely the Stille coupling (Figure 16.27) or Heck reaction (Figure 16.35, part II), which have been investigated in great detail.

Fig. 13.14. Representative mechanism of the Pd-catalyzed C,C coupling of an organoboron compound. The elementary steps, discussed in the text, are (1) complexation, (2) oxidative addition, (3) transmetallation of the alkenylboron compound to afford an alkenylpalladium compound, (4) reductive elimination, and (5) dissociation of the coupled product from the metal.

Organic Synthesis Using Organoboron Compounds 9.05.3.1 Protodeboronation, Oxidation, Halodeboronation, Amination... 

Trialkylboranes add to propellane to give a zwitterion that can rearrange to give 60 or react with another molecule of la to give 61. Both organoboron compounds were oxidized with hydrogen peroxide and isolated as the corresponding alcohols in 65% and 21% yield, respectively (equation 23). 

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. 

Oxidative addition [1, 38] of 1-alkenyl, i-alkynyl, allyl, benzyl, and aiyl halides to a palladium(O) complex affords a stable rra .s-<7-palladium(II) complex (11). The reaction proceeds with complete retention of configuration for alkenyl halides and with inversion for allylic and benzylic halides. Alkyl halides having /3-hydrogens are rarely useful because the oxidative addition step is very slow and may compete with /3-hydride elimination from the a-organopalladium(II) species. However, it has been recently shown that iodoalkanes undergo the cross-coupling reaction with organoboron compounds (Section 2.4.5). 

An extensive review has been written which covers in detail the oxidation of the carbon-silicon bond under both Tamao and Fleming oxidation conditions.6 One virtue of the use of these procedures as compared to the related oxidation of organoboron compounds is the fact that the silicon groups can often be carried through several synthetic steps while masking the hydroxyl functionality. 

Meanwhile, Lei and his colleagues discovered a novel protocol that makes use of air as an oxidant at low temperatures [111], Using a balloon pressure of CO/air mixture, arylboronates were converted into the corresponding esters in good yields (Scheme 8.29). This was the first example that could apply simply air in the oxidative carbonylation of organoboron compounds with alcohols. 

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