Isolation and enantiostability of the B-chiral bis(salicylato)borate anions [B(R)(Sal)(2)] and [B(S)(Sal)(2)]

The chiral spiroborate anions [B(S)(Sal)(2)] and [B(R)(Sal)(2)], (R and S subscripts indicate boron stereochemistry) have been isolated as 1 : 1 quininium and 1 : 2 sparteinium salts, [HQuin][B(S)(Sal)(2)] and [H(2)Spa][B(R)(Sal)(2)](2) respectively, by either cation metathesis or a simple one-pot synthesis involving reaction of boric and salicylic acids with the alkaloid base. Circular dichroism (CD) spectroscopy shows that the B-based chirality is stable in polar aprotic media, such as DMF or DMSO, though labile in protic solutions. Enantiopure salts with achiral counter-cations such as [NBu(4)][B(R)(Sal)(2)] may then be prepared by exchange, so these B-chiral anions may have use in metathesis-based resolutions. Due to a site disorder the anion in [H(2)Spa][B(R)(Sal)(2)](2) is limited to 70% ee, however an enantiopure analogue [H(2)Spa][B(R)(5-Cl-Sal)(2)](2) is readily formed using 5-chlorosalicylic acid. This also indicates a wide family of stable enantiopure B-chiral anions may be isolated by this approach.