A chiral spiroborate anion from diphenyl- l -tartramide [B{ l -Tar(NHPh) 2 } 2 ] − applied to some challenging resolutions

• Published in: CrystEngComm 2018-01, Vol.20 (33), p.4831-4848
• Main Authors: Wong, Lawrence W.-Y., Tam, Gemma S.-S., Chen, Xiaoyan, So, Frederick T.-K., Soecipto, Aristyo, Sheong, Fu Kit, Sung, Herman H.-Y., Lin, Zhenyang, Williams, Ian D.
• Format: Article
• Language: English
• Subjects: Amines; Anions; Binding sites; Cation exchanging; Crystal structure; Enantiomers; Hydrogen bonds; Single crystals
• ISSN: 1466-8033; 1466-8033
• DOI: 10.1039/C8CE00855H

The chiral spiroborate anion [B{ l -Tar(NHPh) 2 } 2 ] − has been prepared as simple salts K, NH 4 , Na in high yield and purity. Its structural features have been examined by single crystal XRD and DFT calculations and indicate that conformational arrangements are dominated by intramolecular NH---OC inter-amide hydrogen bonds. These confer predictable shape, as well as a clear binding site for ion-pair formation. The potential of this anion for resolution by diastereomeric salt formation was then tested using five challenging racemic amines of type NH 2 CHR 1 R 2 with high shape similarity between their enantiomers. Organo-ammonium salts from these were made directly from a simple 1-pot reaction from racemic amine, boric acid and 2 eq. N , N ′-diphenyl- l -tartramide in MeOH. The products are single phase crystalline solids with moderate to excellent enantioexcesses up to 95% ee. They show conserved NH 3 R + binding and layered packing arrangements, all with short 5.5 Å axes. Based on chiral HPLC the initial [B{ l -Tar(NHPh) 2 } 2 ] salt from rac -phenylglycinol has [S-NH 3 CH(CH 2 OH)Ph] + with 95% ee and the salt from rac -1-phenylpropylamine is also well resolved (>91% ee) in a single step. Three disorder modes that limit resolution in the other salts were identified at the cation site – H/R 1 site exchange, R 1 /R 2 site exchange or C–H re-pyramidalization. Extension to a family of such aryltartramide anions may allow crystal engineering of the cation binding pockets to overcome the disorder inherent to such resolutions.