This work is focused on the development of spiroborate anions for crystallization and chiral resolution. In particular we seek to answer questions about the enantio- and compositional stability of spiroborate anions and also develop new classes of spiroborate resolving agent involving either labile B- based, or stable C-based chirality. In Chapter 2 we describe the synthesis, structure and stability of salicylate-based spiroborate salts based on the anions [B(Sal*)
2], derived from substituted salicylic acids. Structures of eight new salts are described. These salts are then the focus of studies to determine the conditions for possible ligand exchange between two spiroborates [B(Sal-1)
2] and [B(Sal-2)
2] to form an equilibrium mixture with hetero-spiroborate [B(Sal-1)(Sal-2)] anion. This was attempted by mass spectrometry but finally achieved using
1H NMR. The result is that in protic solvents like MeOH equilibrium is achieved after approximately 2 h at room temperature. However in aprotic media the [B(Sal*)
2] anions are compositionally stable. In Chapter 3 the resolution of salicyl spiroborates by Sparteinium cations is examined. The successful resolution was achieved for about ten different salicylspiroborates [B(Sal*)
2] anions. A total of 13 new crystalline phases are reported which belong to one of two families of salts, the thermodynamically preferred 1:1 salts [SPA-H][B(Sal*)
2] which tend to have B
S conformation and the kinetic 1:2 salts [SPA-H
2][B(Sal*)
2]
2, which are either B
R or racemic. The supramolecular factors involved in these preferences are explored, but appear to arise from a major change of shape for the mono- and di-protonated Sparteinium cations. Although the [B(Sal*)
2] anions are enantiolabile in protic media their stability in aprotic solvents, as well as their ease of preparation and low cost may yet allow them to be successfully employed as resolving anions. In Chapter 4 the synthesis and characterization of 10 new crystalline phases are reported based on two chiral tartramide spiroborate anions in various salts. In these L-tartaric acid has been derivatized by either phenylamine or dimethyl amine to afford the diol ligands R,R-[CHOHCONR
1R
2]
2 with NHPh and NMe
2 groups respectively. The chiral anion bis(L-N,N’-diphenyltartramido)borate [B(L-DPTA)
2] has in principle two symmetric chelating ligands, but intramolecular NH—O hydrogen bonding leads to asymmetry and different possible boron configurations, and thus shape. In general the anion is found in various structures as a rather flattened, but chirally puckered shape. Chiral resolutions were attempted using this anion for alpha-methylbenylamine and sec-butylamine with some degree of success and up to 80% ee on a single crystallization. The bis(L-N,N’-tetramethyltartramido)borate [B(L-TMTA)
2] lacks the intramolecular hydrogen bonding. However all four ligand arms are independent can have different torsional twists for the NMe
2 groups. This type of anion may have better prospects for chiral ionic liquid formation than chiral resolution. In conclusion several new chiral spiroborate systems have been explored and offer excellent prospects for resolution. Furthermore the ligand exchange rate for spiroborates has been established under different media conditions. The boron center is highly labile in protic, but quite stable in aprotic solvents. This offers prospect of controlled synthesis of hetero-spiroborates in labile conditions and their subsequent use under stable conditions.
| Date of Award | 2014 |
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| Original language | English |
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| Awarding Institution | - The Hong Kong University of Science and Technology
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