Boron Compounds Catecholborane

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. 

Important examples of this type of compounds include pinacolborane, catecholborane, etc. These compounds are readily available by the reaction of pinacol 331 or catechol with either borane or boron halides. Transesterification also provides an attractive alternative for the synthesis of these compounds (Scheme 55). 

Catecholborane and its derivatives had a pivotal role in establishing the class of boryl complexes, since they proved to be the most versatile starting materials for the synthesis of such compounds. This was demonstrated in many works especially by Baker, Hartwig, Marder, Norman, and Smith. ia In the course of our investigations we aimed at boryl complexes with different substituents attached to boron, and we could show that boranes with B-N bonds are also very well suited for the synthesis of boryl complexes. Over the last three years a wide variety of products was obtained by salt elimination reactions between anionic transition metal complexes and haloboranes. Examples include aminoboryl- 4, 5, diborane(4)yl- 6, 7, and rj -borazine complexes 8 (Figure 4). 

Apart from aliphatic derivatives, radioiodovinyl and radioiodoaryl compounds are obtained from vinylboronic and arylboronic acid intermediates. Thus, catecholborane was used for the hydroboration of alkines. The following example published for the preparation of 11-cholesterylundec-l-en-l-yl boronic acid (Kabalka et al. 1988) may serve as a generalized hydroboration procedure. 

Hydroboration of a,(3-unsaturated carbonyl compounds have been studied [14,15], mainly due to the synthetic utility of boron enolates [ 1 ]. Phenyl-1 -alke-nylketones undergo selective 1,4-hydroboration either with 9-BBN or catechol-borane to produce the corresponding (Z)-boron enolates (Chart 20.2) [16] in high yields in solvents like chloroform, dichloromethane, THF, or benzene and in high isomeric purity. However, this selectivity has not been observed in the reactions of 9-BBN or catecholborane with alkylalkenylketones (3). 

The alkyl- or alkenyl-organoboron compound used in a Suzuki reaction is prepared by hydroboration of a terminal alkene or a terminal alkyne, respectively. Often the boron-contairting compound is catecholborane. 

Compared to the hydroboration of alkynes, the preparation of polyconjugated hydrocarbon compounds by hydroboration is still challenging both for transition metal- or non-transition metal-catalyzed processes. Moses has reported the clean conversion of enyne 230 into boronate 231, as a single diastereoisomer, when using freshly distilled catecholborane (Scheme 90) [183],... 

---