Alkylboron halides

One of the prominent synthetic protocols for the preparation of diazaborolines 323 involves the reaction of lithiated diamides 324 with boron halides and subsequent elimination of lithium halide salt. Another protocol involves synthesis via cyclocondensation of 1,2-diimines 325 with alkylboron halides to yield the diazaborolium salts 326 that undergo reduction with sodium amalgam yielding the diazaborolines 323 in high yield (Scheme 53) <2005EJI4715>. 

Alkylboron halides of the types RBX2 and R2BX can be obtained by reaction of boron trihalides with dialkylzincs212 or tetraalkyltins 213... 

The alkylboron halide, 9-haIo-9-borabicyclo[3.3.I]nonane, readily undergo haloboration reactions in a high stereo-, regio-, and chemoselective manner (Scheme 23.31). Terminal carbon-carbon triple bonds are selectively haloborated in the presence of internal carbon-carbon triple bonds and terminal double bonds. Functional groups such as aldehydes, ketone, esters, and halogens are not affected by B-X-9-BBN. 

Prior to 2000, alkylboron halide reagents had not been used to alkylate carbonyl compounds. In 2000, it was discovered that alkylation could be achieved by simply adding a base to a mixture of aldehyde and diorganoboron halide (Scheme 23.57). " " The reaction occurs under mild reaction conditions and tolerates a variety of functional groups. This new alkylation reaction provides a useful alternative to traditional Grignard and organolithium reactions. The survival of base-sensitive functional groups under these reaction conditions is attractive. 

Although the reaction of alkylboronic acids is significantly slower than that of trialkylboranes, methylboroxine (MeBO)3 or methylboronic acid exceptionally alkylates bromo- and chloroarenes [125, 126]. Methylation of electron-poor bromoarenes was efficient, whereas prolonged reaction time was required for more electron-rich substrates. The reaction times can be shortened by use of the polar solvent DMF at 115 °C. B-Methyl-9-BBN and its oxy-derivative (48) [127] undergo methylation of aryl and vinyl halides under more mild conditions (Scheme 22). 

Alkylboronic acids and arylboronic acids, RB(OH)2, and ArB(OH)2, respectively, are increasingly important in organic chemistry. The palladium catalyzed coupling reaction of aryl halides and aryl triflates with arylboronic acids (the Suzuki-Miyaura... 

Alkylboronic acids are coupled to aryl halides using a palladium catalyst, " analogous to the Suzuki reaction in 13-12. Conversely, arylboronic acids can be coupled to aliphatic halides Arylboronic acids can be coupled to allylic alcohols as well. " Arylboronic acids (12-28) were shown to react directly with benzene in the presence of Mn(OAc)3. " Arylboronic acids also couple with alkyl halides in... 

The hydroboration-coupling approach for the construction of carbon skeletons affords several advantages [139]. The high stereoselectivity of the hydroboration reaction provides a stereodefined alkyl center on boron. For instance, in the reaction shown in Scheme 2-49, the hydroboration occurs chemoselectively at the less hindered C(19)-C(20) double bond. In addition, the alkylboron group thus constructed can be readily cross-coupled with alkenyl or aryl halides under mild conditions. 

Molander, G. A., Yun, C.-S. Cross-coupling reactions of primary alkylboronic acids with aryl triflates and aryl halides. Tetrahedron 2002, 58, 1465-1470. 

Although alkyl halides are prone to P-hydride elimination, alkylzinc or alkylboron... 

Suzuki-Miyaura cross-coupling reactions of alkylboronic acid derivatives or alkyltrifluoroborates widi aryl, alkenyl or alkyl halides and triflates ,... 

Similar reactions of Tl(III) halides with alkylboronic acids give only a small amount of dialkylthallium halide. The main product -is TlCl. 

The alkyl- and aryl-borons can be made from the halides by lithium or Grignard reagents. However, as noted above (page 240), the alkylborons, which have great importance as intermediates in organic synthesis, are often most conveniently made by hydroboration. The lower alkyls are reactive substances and inflame in air, but the aryls are stable. 

Except for trimethylboron, which is a gas at room temperature, the trialkylborons are colorless liquids, but the triarylborons are crystalline. Unlike the alkyl derivatives of other elements of the Third Group, alkylborons are monomeric they are unaffected by water but react with halogens and hydrogen halides, yielding organic boron halides. 

The combination of NiBr2 diglyme and 7 (R = r Pr) catalyses the crosscoupling of unactivated primary, secondary and tertiary alkyl halides with diboron reagent, pinB-Bpin, affording alkylboronates in good yields (Scheme 14.12). Compared with related nickel-catalysed carbon-carbon bond-forming reactions, this reaction exhibits an inverse reactivity order of all l halides. Thus, tertiary all l halides are more reaetive than seeondaty or primary all l halides in this C-B bond formation. ... 

Suzuki-Miyaura Cross-Coupling Reactions of Alkylboronic Acid Derivatives or Alkyltrifluoroborates with Aryl, Alkenyl or Alkyl Halides and Triflates Doucet, H. Eur. J. Org. Chem. 2008, 2013. 

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