A-haloboronic esters

The use of N-bromosuccinimide resulted in very complex mixtures and not the expected a-haloboranes. Apparently, the succinimide moiety may have acted as a base, causing various side reactions [81—85]. However, a-haloboronic esters are much more stable than a-halotrialkylboranes. Halogenation of boriozirconocene bimetallics 18 based on boronic esters proceeded very smoothly, affording the expected a-bromoboronic esters 35 (Scheme 7.11) [52],... 

By replacing the methyl groups of 2,3-butanediol by isopropyl groups, a more sterically hindered analog 12 is obtained. It is an important intermediate for chiral a-haloboronic esters (Section D.1.1.2.1.). Multistep synthesis of 11 uses tartaric acid as the starting material which is converted to dimethyl (2/ ,3/ )-2.3-0-isopropylidenetartrate (see following section). 

Boro zincioalkane and 1,1-boro zincioalkene can be prepared by a different route, that is, the reaction of a-haloboronic esters with zinc dust in N,N-dimethylacetamide (DMA). The zinc insertion (see Lnser-tion) into a-haloalkenylboronic esters is not stereospecific, so a pure Z reagent will be converted to an EjZ mixture of the corresponding alkenylzinc iodide (equation 53). 

Boronic esters (8) react with dichloromethyllithium to form intermediate borate complexes which then undergo rearrangement to form 2-haloboronic esters (9). When treated with a Grignard reagent, the 2-haloboronic esters (9) rearrange to secondary alkylboronic esters (10 Scheme 3). 

In principle, any difunctional chiral compound, such as a diol, diamine, or amino alcohol, can form a cyclic boron derivative by reaction with borane, haloboronic acid or its esters, alkylboronic acids, trihaloboranes or similar compounds. Several syntheses of this type are described in chapters D.1.3.3.3. (together with applications for allylic additions to carbonyl compounds) and D. 1.1.2.1. (with applications of a-haloboronic acids). 

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