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.
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