ABSTRACT
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 [H2Spa][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 [NBu4][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 [H2Spa][B R (Sal)2]2 is limited to 70% ee, however an enantiopure analogue [H2Spa][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.
ABSTRACT
Bis(mandelato)borate [B(Man)2](-) (R- or S-) anions are simply prepared and appear widely effective for resolution of racemic cations. Three examples demonstrate their scope; the alkaloid tetrahydropalmatine (THP), 1,2-diaminopropane (1,2-dap) and the metal-organic complex [Co(phen)3](3+) are readily resolved, either by a facile one-pot procedure, or via counter-ion metathesis.
