1.3.2- Benzodioxaborole hydroboration

Hydroboration, the addition of a boron-hydrogen bond across an unsaturated moiety, was first discovered by H. C. Brown in 1956. Usually, the reaction does not require a catalyst, and the borane reagent, most commonly diborane (B2H6) or a borane adduct (BH3-THF), reacts rapidly at room temperature to afford, after oxidation, the /AMarkovnikov alkene hydration product. However, when the boron of the hydroborating agent is bonded to heteroatoms which lower the electron deficiency, as is the case in catecholborane (1,3,2-benzodioxaborole) 1 (Scheme 1), elevated temperatures are needed for hydroboration to occur.4 5... 

In the hydroboration/oxidation reaction of 2-cyclohexen-l-ol derivatives the regio- as well as the stereoselectivities can be steered in either of two directions by use of different reagents and reaction conditions. Dialkylboranes, e.g., 9-BBN, give mainly tn rs-l,2-diols, whereas with 1,3,2-benzodioxaborole ( catecholborane") in the presence of the rhodium hydrogenation catalyst Rh(PPh3)jCl the trans-1,3-diols become predominant. The cis- 1,2-diols are usually only formed in trace amounts, but the cis-l,3-diols are always produced as byproducts in 10 — 20% yield (D.A. Evans, 1988). 

Stereospecific synthesis of (2Z,4E,6E)-3,7,11-trimethyl-2,4,6,10-dodecatetraene, trans (Cio)-allofamesene 49), isolated from Perilla fmtscem Makino, was realized by the ptdladium-catalyzed cross-coupling of 4,8-dimethyl-l,3,7-nonatrienyl-l,3,2-benzodioxaborole 48) with (Z)-2-bromo-2-butene. The benzodioxaborole derivative 48) was prepared by hydroboration of 4,8-dimethylnona-3,7-dien-l-yne 47), obtained via two steps from geranial, with 1,3,2-benzodioxaborole (Eq- 112) Bombykol and its geometrical isomers were also synthesized selectively... 

Table 3. Selected Chiral Alcohols via Catalytic Asymmetric Hydroboration/Oxidation of Alkenes with 1,3,2-Benzodioxaborole...

Catalytic asymmetric hydroboration has been most extensively studied with styrene (4) as the substrate which produces 1-phenylethanol (6) after treatment of the hydroboration product, l-phenyl-l,3,2-benzodioxaborole (5), with alkaline hydrogen peroxide (Scheme 2). The regioselectivity favoring the branched isomer 5 over the linear isomer 5 is usually high when the reaction is carried out with rhodium complexes coordinated with chelating ligands such as bisphos-... 

The attachment of one alkenyl group to boron can be achieved by the hydroboration of alkynes with dihalogenoboranes or 1,3,2-benzodioxaborole. Dichloroborane reacts sluggishly with alkynes in EtjO or THF and the reaction is accompanied by disproportionation. However, in pentane the addition of BCI3 generates the nascent HBCI2 and the reaction proceeds to completion in 1 h at RT. Alkenyldichloroboranes are isolated by distillation in high yields ... 

Yields of the catechol derivative of 3-hexyne and 1-hexyne are 84 and 44%, respectively, indicating substantial dihydroboration of the terminal triple bond. Quantitative yields of 2-alkenyl-1,3,2-benzodioxaboroles are obtained by hydroboration of alkynes with 1,3,2-benzodioxaborole . ... 

Complexes of rhodium compounds with enantiomerically pure phosphines facilitate enantiose-lective hydroboration of prostereogenic olefins with 1,3,2-benzodioxaborole (catecholboranc), an achiral hydroborating agent. The reactions are carried out using a 1 1.1 -1.2 molar ratio of the olefin and catecholboranc in the presence of 1 mol % of the catalyst which is generated in situ111"113. The following catalysts are used ... 

Catecholborane (1,3,2-Benzodioxaborole), 4,25, 69-70 5, 100-101 6, 33-34, 98. Hydroboration, This process has been reviewed (66 references). Deoxygenation (6, 98). Tosylhydrazones of a, -unsaturated carbonyl compounds are reduced by 1 equiv. of this hydride (1) to methylene derivatives, with migration of the double bond. The reaction is believed to proceed through a diazene intermediate. ... 

Sato, M. Miyaura, N. Suzuki, A. Rhodium(i)-catalyzed Asymmetric Hydroboration of Alkenes with 1,3,2-benzodioxaborole. Tetrahedron Lett. 1990,31,231-234. 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the acetylene. Catechol borane (l,3>2-benzodioxaborole), which is prepared from equimolar amounts of catechol (1,2-dihydroxybenzene) and borane, is a particularly useful reagent for hydroboration of acetylenes.Protonolysis of the adduct with acetic acid results in reduction of the original alkyne to the corresponding c/5-alkene. Oxidative workup with hydrogen peroxide gives ketones via an enol intermediate. Treatment of the vinyl borane with bromine and base leads to the vinyl bromide. The net anh-addition has been rationalized on the basis of anh-addition of bromine followed by a second z/tr/-elimination of bromide and boron but there are exceptions to this generalization. 

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