Preparation, Properties, and Uses of Borate Esters

Abstract

The esters of orthoboric acid, although known for more than 100 years (58), have only recently become commercially available. This paper presents process development data; the synthesis and properties of a new series of hydrolytically stable borates; the variation in boiling point, density, index of refraction, and viscosity with temperature; a summary of reactions and uses; and physical evidence that tetracoordinate boron is involved in the reactions of borate esters. A number of methods for preparing borate esters have been described. They in clude: reaction of an alcohol (or phenol) with boron trichloride (48, 134) (Equation 1); boron acetate (8, 146) (Equation 2); boric acid (47, lp) (Equation 3); boron oxide (188) (Equation 4) ; or sodium borohydride (36) (Equation 5) ; and transesterification with a lower boiling borate ester (170, 204) (Equation 6). When water is a product of the esterification, it has been removed by azeotropic distillation with an excess of the alcohol (14, 92, 164) o r a n mer t medium (149) ; or by dehydration with sulfuric acid (45,197), copper sulfate (56, 93), or boron oxide (120,126,172,197) (Equation 7). 3ROH + BC13 -> (RO)3B + 3HC1(1) 6ROH + (CH3COO)4B20 -> 2(RO)3B + 4CH3COOH + H20(2) 3ROH + H3B03 -> (RO)3B + 3H20 (3) 6ROH + B203 -> 2(RO)3B + 3H20 (4) 3ROH + NaBH4 + H+ -> (RO)3B + 4H2 + Na +3ROH + (RO)3B -> 3(RO)3B + 3ROH (6) 3ROH + B203 -> (RO)3B + H3B03 (7) Stanley (180) has claimed the preparation of borate esters as an intermediate step in the conversion of alkylenes into alcohols.The alkylene is converted to the alkyl sulfate, alkyl phosphate, or alkaryl sulfonate by absorption into acid or acid ester; the borate ester formed on subsequent reaction with boric acid is then distilled and hydrolyzed to the alcohol: 3S02(OEt)2 + H3B03 -> 3S02(OEt)OH 4-(EtO)8B 1 H3B°3 > 3H2S04 + (EtO)8B (8) 129 Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 8, 2014 |

Densities and viscosities of trialkyl borates

“…The over-all Table I average deviations for densities and absolute viscosities are ±0.0180 and ±0.225%, respectively. Although extensive and systematic physical data for this class of compounds are somewhat lacking, some of the observed p's may be compared with a few previous findings (4,5,6). A search of the literature disclosed one observation of the kinematic viscosities of propyl and butyl borates at 28°C.…”

“…A search of the literature disclosed one observation of the kinematic viscosities of propyl and butyl borates at 28°C. (3), and another (6) which reports the kinematic viscosities of methyl, ethyl, propyl, isopropyl, and butyl borates at three different temperatures. No temperature correlations were attempted.…”

Efficient and Selective Iron-Complex-Catalyzed Hydroboration of Aldehydes

“…Catalytic hydroboration of carbonyl compounds using pinacolborane (HBpin) represents an attractive strategy in organic synthesis not only for synthesizing widely used borate ester intermediates , but also for conversion into the corresponding alcohols . Because the Bpin group can serve as a versatile directing and protecting group, this catalytic process enjoys further advancement in synthetic chemistry. − Intrigued by recent reports from Gunanathan and co-workers of selective aldehyde (vs ketone) hydroboration catalyzed by a simple Ru precursor, we disclose herein an efficient and strictly selective iron-based catalyst containing an imine-coupled, redox-active N 2 S 2 ligand.…”

“…C atalytic hydroboration of carbonyl compounds using pinacolborane (HBpin) represents an attractive strategy in organic synthesis 1 not only for synthesizing widely used borate ester intermediates 2,3 but also for conversion into the corresponding alcohols. 4 Because the Bpin group can serve as a versatile directing and protecting group, this catalytic process enjoys further advancement in synthetic chemistry.…”

2-Nitrobenzyl Borate Based Photolabile Linker for Breakable Polymer Vesicles

“…The general procedure for the preparation of borate esters involves an azeotropic distillation of boric acid and alcohols in toluene medium or transesterifi cation with a lower boiling borate ester. [ 6 ] However, borate esters are very susceptible to hydrolysis in aqueous systems. Fortunately, the hydrolytic stability of borate esters can be much improved by introducing the intramolecular-coordination of nitrogen to boron or forming chelate esters with molecules bearing cis -diol groups or polyalcohols, such as mannitol, glycerol, sugar, and poly(vinyl alcohol).…”