Borane, dialkyl synthesis

The synthesis of enantiomerically pure compounds is the challenging problem for organic chemists. The synthesis becomes obsolete if the intermediates produce racemic mixtures. The problem is particularly acute when the asymmetric centers do not reside in a rigid cyclic or polycyclic framework. To be able to carry out efficient syntheses of complex molecules, chemists have to control the sense of chirality at each chiral center as it is introduced in the course of synthesis. Monoalkyl- or dialkyl-boranes exhibit a remarkable chemo-, stereo-, and regioselectivity for the hydroboration of unsaturated compounds. This property, coupled with the capability for asymmetric creation of chiral centers with chiral hydroboration agents, makes the reaction most valuable for asymmetric organic synthesis. In some of the cases, however, this has been achieved by diborane itself as shown in the synthesis of monensin by Kishi et al. A stereospecific synthesis of its seven carbon. component has been accomplished by two hydroboration reactions (Eq. 129) 209. ... 

Dialkyl metallocenes and other dialkyl Group 4 transition metal complexes are useful as precatalysts in combination with co-catalysts such as tris(perfluoro-aryl)boranes or tetrakis(periluoroaryl)borate salts [18]. Recently, an expedient procedure for the production of dimethyl metallocenes and Cp-amido dimethyl metal complexes in high yields and purity has been reported. The direct synthesis of Group 4 dimethylmetallocenes [19] consists of the one-pot reaction between the r-ligand, a 2-fold excess of MeLi, and MtCU. This simple method produces the dimethylated complexes in higher overall yield, and saves on reaction time and solvents, compared to the classic two-step route, which consists in the synthesis of the metallocene dichloride followed by its methylation with 2 equiv. MeLi. 

The general approach for the synthesis of dibenzoboroles is ring closure of appropriate biphenyls with boron compounds. The earliest practical syntheses of 5-akyl or aryl dibenzoboroles (100) required reaction of o-lithiumbiphenyl (98) with a dialkyl or diarylchloroborane to give 2-biphenyl-yl(disubstituted)boranes (99). The latter were then thermally cyclized to the 5-substituted dibenzoboroles (100) (Scheme 16) <63AG1079,67LA(702)197>. 

General Synthesis.—Terminal acetylenes can be transformed into carboxylic acids by conversion into the trimethylsilyl derivatives followed by addition of a dialkyl-borane and oxidative hydrolysis (Scheme 1). Conjugated enynes can undergo a similar conversion, and yields throughout are high. 

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