Singh R P, Matteson D S
Department of Chemistry, Washington State University, Pullman 99164-4630, USA.
J Org Chem. 2000 Oct 6;65(20):6650-3. doi: 10.1021/jo005522b.
In the asymmetric homologation of boronic esters with a (dihalomethyl)lithium, substituents that can bind metal cations tend to interfere. Accordingly, we undertook the introduction of weakly basic oxygen and nitrogen substituents into boronic esters in order to maximize the efficiency of multistep syntheses utilizing this chemistry. Silyloxy boronic esters cannot be made efficiently by direct substitution, but a (hydroxymethyl)boronic ester has been silylated in the usual manner. Conversion of alpha-halo boronic esters to alpha-azido boronic esters has been carried out with sodium azide and a tetrabutylammonium salt as phase-transfer catalyst in a two-phase system with water and either nitromethane or ethyl acetate. These are safer solvents than the previously used dichloromethane, which can form an explosive byproduct with azide ion. Boronic esters containing silyloxy or alkoxy and azido substituents have been shown to react efficiently with (dihalomethyl)lithiums, resulting in efficient asymmetric insertion of the halomethyl group into the carbon-boron bond.
在硼酸酯与（二卤甲基）锂的不对称同系化反应中，能够结合金属阳离子的取代基往往会产生干扰。因此，我们将弱碱性的氧和氮取代基引入硼酸酯中，以最大限度地提高利用该化学方法进行多步合成的效率。甲硅烷氧基硼酸酯不能通过直接取代有效地制备，但（羟甲基）硼酸酯已按常规方法进行了甲硅烷基化。在水与硝基甲烷或乙酸乙酯组成的两相体系中，以叠氮化钠和四丁基铵盐作为相转移催化剂，已将α-卤硼酸酯转化为α-叠氮硼酸酯。这些溶剂比先前使用的二氯甲烷更安全，二氯甲烷会与叠氮离子形成爆炸性副产物。含有甲硅烷氧基或烷氧基以及叠氮取代基的硼酸酯已被证明能与（二卤甲基）锂高效反应，从而使卤甲基有效地不对称插入碳-硼键中。
