Borinic acid

Isopinocampheyl(l-isopinocampheyl-2-octenyl)borinic acid (1), available with a diastereomer-ic purity of approximately 80 85% de, reacts smoothly with aldehydes at —15 °C in tetrahy-drofuran to provide a homoallylic alcohol with 79-85% ee after oxidative workup (30% hydrogen peroxide, 40 °C)6. 

Improved methods for the preparation of reagents such as isopinocampheyl(l-isopinocam-pheyl-2-alkenyl)borinic acids will certainly lead to a more enantioselective synthesis of anti-homoallylic alcohols, since the enantiomeric purity of the reagent is the only significant limitation to the synthetic utility of this reagent system. 

In addition to trialkylboranes, various alkoxyboron compounds have prominent roles in synthesis. Some of these, such as catecholboranes (see. p. 340) can be made by hydroboration. Others are made by organometallic or related substitution reactions. Alkoxyboron compounds are usually named as esters. Compounds with one alkoxy group are esters of borinic acids and are called borinates. Compounds with two alkoxy groups are called boronates. Trialkoxyboron compounds are borates. 

Replacement of pyridine I-oxide by borinic acid in the above reaction leads to formation of vinyloxyborane intermediate 7 which further reacts with aldehydes to give the aldol products 8 and 9. 2-Deoxy-D-ribose is prepared efficiently by application of this reaction O). 

Alkoxyboron compounds are usually named as esters. Compounds with one alkoxy group are esters of borinic acids and are called borinates. Compounds with two alkoxy groups are... 

Tetrafluoroborin acid can be prepared at starting with hydrogen fluoride and borin acid.55-57... 

The present procedure is also applicable to the synthesis of substituted benzeneboronic acids.4 Benzeneboronic acid and its anhydride are of use as starting materials for the synthesis of phenylboron dichloride 12 and of various substituted boronic and borinic acids and esters.6 13... 

Boron heterocycles bearing a hydroxy group on the boron atom are acidic. Boric, boronic and borinic acids are Lewis acids towards hydroxide ion. It was suggested that six-membered 5-hydroxy compounds behaved as Brdnsted acids. The experimental evidence for this was the similarity between the UV spectra of the hydroxy compounds in neutral and basic solution. The rationale for this fact was the maintained aromaticity which was supposed to favour the formation of, for example, ion (163) over ion (164). This suggestion was at first supported by nB NMR spectroscopy, which differentiates between anions of Lewis-acidic... 

The oxidation of cholesteryl esters and low-density lipoproteins by free radicals has been reviewed.228 The use of bis(pentafluorophenyl)borinic acid as a strong Lewis acid allows efficient Oppenauer oxidation of allylic and benzylic alcohols using Bu CHO as oxidant.229 Saturated alcohols were only slowly oxidized and this allowed selective conversion of allylic alcohols in the presence of saturated alcohols. 

Chiral ketones.3 Asymmetric hydroboration of a prochiral alkene with monoisocampheylborane followed by a second hydroboration of a nonprochiral alkene provides a chiral mixed trialkylborane. This product reacts with acetaldehyde with elimination of a-pinene to give a chiral borinic acid ester in 73-100% ee. Treatment of this intermediate with a,a-dichloromethyl methyl ether (2,120 5, 200-203) and lithium triethylcarboxide followed by oxidation results in an optically active ketone in 60-90% ee. 

Asymmetric hydroboration 2171 of prochiral alkenes with monoisopinocampheyl-borane in the molar ratio of 1 1, followed by a second hydroboration of non-prochiral alkenes with the intermediate dialkylboranes, provides the chiral mixed trialkylbo-ranes. Treatment of these trialkylboranes with acetaldehyde results in the selective, facile elimination of the 3-pinanyl group, providing the corresponding chiral borinic acid esters with high enantiomeric purities. The reaction of these intermediates with base and dichloromethyl methyl ether provides the chiral ketones (Eq. 130)2l8>. A simple synthesis of secondary homoallylic alcohols with excellent enantiomeric purities via B-allyldiisopinocampheylborane has been also reported 219),... 

Nitrogen-donor macrocycles have also been prepared with other heteroatoms in the ring but not coordinated to the metal. Schrauzer (124) observed that chelate complexes (LVI) could be obtained when certain metal salts were present in the reaction of borinic acid ester... 

A brief summary is included of carbohdyrate borinates—acyclic esters derived from borinic acids (R2BOH)—which have received less attention than the related boronates. 

Significant features of these active borinic acid catalysts are that they are strong Lewis acids and have a hydroxy group on the boron atom. Dehydration is strongly favored in THF. The reaction usually proceeds smoothly, and a,/3-enones are obtained in high yields as ( ) isomers. In reactions of a-substituted-/3-hydroxy carbonyl compounds, a,/3-enones are preferentially obtained from anti aldols, whereas most syn aldols are recovered. This dehydration thus represents a useful and convenient method for isolating pure syn aldols from syn anti isomeric mixtures (Eq. 106). 

The mechanism we have proposed to explain borinic acid-catalyzed dehydration is also depicted in Eq. (106). Reaction of the 8-hydroxy function with the diarylborinic acid leads to a cyclic intermediate, which should be susceptible to dehydration. Subsequent transformation to a,/3-enones occurs via an enolate intermediate resulting from selective abstraction of a pseudo-ayaaX a-proton perpendicular to the carbonyl face. A cyclic intermediate formed from a syn aldol and a diarylborinic acid would be thermodynamically less stable than the cyclic intermediate. Thus, dehydration to ( )-a,/3-enones occurs selectively for anti aldols. 

Chemistry of boron and silicon subhalides Chemistry of boronic and borinic acids Polypyrazolylborates, a ncnv class of ligands... 

Ishihara, K., Kurihara, H., Yamamoto, H. Bis(pentafluorophenyl)borinic Acid as a Highly Effective Oppenauer Oxidation Catalyst for Allylic... 

---