A Brief History of Boronic Ester Chemistry

simple achiral boronic acids and esters are readily available from various sources (Chapter 1). The first synthesis from an organozinc reagent preceded general acceptance of Avogadro s hypothesis [1], and was superseded nearly a century ago by the now standard preparation from Grignard reagents [2, 3]. Hydroboration has 

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The first synthesis of an (a-haloalkyl)boronic ester [8], a free radical addition of a tetrahalomethane, was followed by mechanistic studies that indicated the potential for stereospecific alkylation with Grignard reagents via borate intermediates [9], if only there had been a way to obtain asymmetric examples. The discovery of the efficient reaction of (dichloromethyl)lithium with boronic esters to form (a-chloroalkyl)boron-ic esters by insertion of a CHCl group into the B-C bond opened a new opportunity [10]. Boronic esters of pinanediol, prepared from (+)-a-pinene by osmium tetroxide catalyzed oxidation, were soon found to undergo the insertion reaction with a strong asymmetric bias, with diastereomeric selectivities frequently in the 90-95% range [llj. It was subsequently found that anhydrous zinc chloride promotes the reaction and increases diastereoselectivity to as high as 99.5% in some cases [12]. 

The (dichloromethyl)lithium insertion reaction or its bromo analog provides most of the (a-haloalkyl)boronic esters of interest in this chapter, and is described in detail in the next section. (Halomethyl)boronic and (2-bromo-2-propyl)boronic esters require different approaches and are described under Section 8.3.2. 

2 General Description of ((X-HaloalkYl)boronic Ester Chemistry j 307 UCHCI2 

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