Borates, boron promoters

High polymers are generally obtained on treatment with Lewis acids at low temperatures in an inert solvent. Boron trifluoride and boron trifluoride etherate are the most widely used catalysts, but a small amount of water must be present, which is termed a promoter triethylaluminum and triisobutylaluminum are also useful initiators and are generally used with addition of water. Alkylating agents, such as ethyl triflate, triethyloxonium tetrafluoro-borate and hexafluorophosphite, and 2-methyl-l,3-dioxolenium perchlorate, are also effective initiators (76MI51301,72MI51304). 

All the reactions of the hydrocyanation process are catalyzed by zero-valent nickel phosphine or phosphite complexes. These are used in combination with Lewis acid promoters such as zinc chloride, trialkyl boron compounds, or trialkyl borate ester. The ability of the precatalyst to undergo ligand dissociation... 

Aqueous solutions of boric acid (H3BO3) and borax (Na2B207 x IOH2O) at a mass ratio of 5 to 6 were successfully used for fire-retardance of cellulosic textiles in the twenties. Sodium, potassium, and lithium borates are readily water-soluble. They therefore, continue to be used in the textile, wood, and paper industry. The increasing demand for and the rising price of antimony trioxide is another factor which has promoted research into the use of boron compounds for the flame-retardancy of plastics. 

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]. 

Zinc chloride is the usual Lewis acid promoter of the rearrangement of borate 2 into (a-haloalkyl)boronic ester 4. However, lithium cation functions in the same capacity if zinc chloride is not added, but usually does not provide optimum stereoselection. One factor in favor of the zinc chloride is that it sequesters chloride as ZnCl3" or ZnCl4 , thus slowing epimerization of the product 4 by free chloride ion [14]. 

Zinc borate works predominantly in the condensed phase and is a good char promoter. Analysis of residues show that around 80% of the boron and perhaps 63% of the zinc can remain bound up in the char [5]. 

A chiral boron reagent, derived from equimolar amounts of (R)-or (5)-binaphthol and triphenyl borate, promotes the condensation of chiral imines with t-butyl acetate silyl ketene acetal in high diastereomeric excess (eq 12). ... 

---