Transmetallation of organoboronic acids

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]. 

Scheme 5.5 Proposed mechanism for the transmetallation of organoboronic acids. [Rh] = Rh(l)/L [9b, 14].

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

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... 

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. 

Mechanism of the palladium-catalyzed ketone synthesis, comprising oxidative addition of carboxylic anhydride, transmetallation with organoboronic acid, and reductive ehmination, is shown in Scheme 1.12. 

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. 

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... 

The B-C bonds of tetraorganohorate, BRJ, and organoboronic acid, RB(OH)2, are more reactive toward transmetalation than that of triorganoborane, BR3. Phenyl group transfer from BPhJ to transition metals takes place under mild conditions. The reaction of PdCl2 with NaBPh4 in the presence of norbornadiene produces a Pd complex with a phenylnorbornenyl ligand (Eq. 5.16) [53]. 

In his Nobel lecture, Suzuki has summarized work on the use of organoboranes in the formation of carbon-carbon bonds.A wide-ranging review has been published covering the transmetalation reactions involving the use of organoboron compounds in the formation of metal-carbon bonds.There has also been a review of the use of copper-promoted reactions of arylboronic acids in the formation of carbon-heteroatom bonds. 

The transmetallation with organoboron reagent precedes the carbometallative addition of the arylrhodium intermediates 394 to the unsaturated substrate, and the intermediate alkenylrhodium species 395 then undergo hydrolysis to close the catalytic cycle with regeneration of the catalytically active species 393 and formation of the observed enamide products. An intermolecular carborhodation event was proposed to occur in a three-component coupling of arylboronic acids with disubstituted alkynes in the presence of methyl acrylate (Scheme 10.135) [113]. 

The preparation of functionalized aryl boronic reagents can be achieved by directed metallation followed by a transmetallation of aryllithiums with organoboron compounds. Thus, Caron and Hawkins have described a directed ortho-metalla-tion of aryl neopentyl esters such as 1 for the synthesis of substituted ortho-horo-nyl neopentyl benzoates using lithium diisopropylamide (LDA) as the base and B(OiPr)3 as an in situ trap [3]. The crude boronic acids obtained by acidic hydrolysis were subsequently treated with ethanolamine and converted to stable diethanolamine complexes such as 2. This methodology allows the preparation of a new class of boronic acids with ortho-carbonyl substituents and other functionalities... 

The role of the base was unraveled through DFT-B3LYP studies by Maseras, Ujaque and co-workers [70] concluding that the role of the base was to react with the organoboronic acid to generate the anionic species [RB(OH)3] this is the reactive species taking part in the transmetalation process by substituting the halide and the subsequent transmetalation step. 

---