Lithiums boron halides

One of the prominent synthetic protocols for the preparation of diazaborolines 323 involves the reaction of lithiated diamides 324 with boron halides and subsequent elimination of lithium halide salt. Another protocol involves synthesis via cyclocondensation of 1,2-diimines 325 with alkylboron halides to yield the diazaborolium salts 326 that undergo reduction with sodium amalgam yielding the diazaborolines 323 in high yield (Scheme 53) <2005EJI4715>. 

The source of the boron is conveniently a boron halide or an ester of boric acid. Initial experiments were carried out by Finholt, Bond, and Schlesinger (107), who formed diborane by reacting boron trichloride with lithium aluminum hydride in ether solution at low temperatures (at which, apparently, the ether is not appreciably cleaved by the boron trichloride) and reported a 99.4% yield. This is historically the first reaction discovered to give essentially quantitative yields of diborane... 

Transmetallation of aiyl halide (VIII) with n-butyl lithium and treating it with readily available and inexpensive 3-ethoxycyclohexen-l-one followed by acid work-up would provide enone (XI) (Scheme 8). Enone (XI) can be then aromatized to phenol (X) or any other biaryl alkyl ether. By using this protocol, 3-ethoxycyclohexen-l-one is a synthon for 3-hydroxyphenyl boronic acid. The bulk price of 3-ethoxycyclohexen-l-one is 85/kg compared to 3500 for the boronic acid, thereby significantly reducing the cost to prepare biaryl (X). 

A synthesis of comblike organoboron polymer/boron stabilized imidoanion hybrids was examined via reactions of poly(organoboron halides) with 1-hexylamine and oligo(ethylene oxide) monomethyl ether and subsequent neutralization with lithium hydride (scheme 8). The obtained polymers (10) were amorphous soft solids soluble in common organic solvents such as methanol, THF, and chloroform. In the nB-NMR spectra (Fig. 11), neutralization of the iminoborane unit with lithium hydride... 

Reagents of choice for reduction of epoxides to alcohols are hydrides and complex hydrides. A general rule of regioselectivity is that the nucleophilic complex hydrides such as lithium aluminum hydride approach the oxide from the less hindered side [511, 653], thus giving more substituted alcohols. In contrast, hydrides of electrophilic nature such as alanes (prepared in situ from lithium aluminum hydride and aluminum halides) [653, 654, 655] or boranes, especially in the presence of boron trifluoride, open the ring in the opposite direction and give predominantly less substituted alcohols [656, 657,658]. As far as stereoselectivity is concerned, lithium aluminum hydride yields trans products [511] whereas electrophilic hydrides predominantly cis products... 

Ester enolates replace bromide from a-bromo boronic esters with remarkable diastereoselcctiv-ity. (Dibromomethyl)lithium is generated by addition of lithium diisopropylamide to dibro-momethane in the presence of a boronic ester at — 78 "C to produce an a-bromo boronic ester. Reaction of the a-bromo boronic ester with lithium 1-tert-butoxy-Tpropen-l-olate yields a product that is almost exclusively the threo-isomer (d.r. = 15 1 to 60 1), as shown by conversion to the / -hydroxy carboxylic ester24. It is worth noting the facility with which a-bromo boronic esters racemize in the presence of halide ions72. 

---