A-haloboronates

Figure 13 Ring expansion of a haloboron(III) subphthalocyanine (49) to give an unsymmetrically substituted phthalocyanine, here the zinc(II) benzonaphthoporphyrazine (51).252,253 As usual, the product is...

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],... 

The reactive nature of compound 22 is illustrated by the series of transformations shown in Scheme 7.12, in which its Zr—C bond reacts selectively with electrophilic reagents to produce a-haloboronates 36—38. Compound 22 also catalyzes the polymerization of styrene. The polymers thus obtained had weight-average molecular masses in the range 75000—100000 with polydispersities of 1.8—2.1. An X-ray analysis of 22 confirmed it to be a four-coordinate Zr complex with two cyclopentadienyl rings, chlorine, and the aliphatic C-l carbon atom as the ligands (Fig. 7.4). 

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). 

By reduction of the aromatic rings, the enantiomers of 1,2-dicyclohexyl-l, 2-ethanediol 14 are obtained, which have also found application for the alkylation of carbanions via a-haloboronic acids (Section D.1.1.2.1.). 

Some boron heterocycles have already been mentioned in the section on terpenes, in particular some derivatives of the pinenes with the 9-borabicyclo[3.3.1] moiety, e.g., Alpine-borane and NB-Enantrane (Section 3.1.2.). In this section, five-membered heterocycles will be discussed. Review articles on such compounds and their applications as allylboronates15, a-haloboronic acids20, and catalysts21 are available. 

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). 

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). 

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