Modification of binap: implications of 3,3'-disubstitution and the 3,5-dialkyl meta effect in asymmetric catalysis
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2010
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Abstract
Changes in the structure of the BINAP framework were shown to provide improved enantioselectivities in asymmetric reactions. This dissertation describes two novel modifications of the BINAP scaffold and their implications in asymmetric reactions. Two BINAP derivatives containing 3,3'-substituents bound to the binaphthalene core via a C-C bond have been prepared. The syntheses of (S)-3,3 '-bis(phenethyl)BINAP and (S)-3,3 '-bis(phenoxymethyl)-BINAP hinged on the successful ortholithiation and iodine incorporation at the I-position of 2-(diphenylphosphinoyl)-3-phenyethyny 1 naphthalene. Electronically similar (S)-BINAP and (S)-3,3 '-bis(phenethyl)-BINAP were compared in Rh- and Pd-catalyzed asymmetric reactions. It was concluded that levels of asymmetric induction and product enantiosense reversals can be attributed solely to a change in the steric properties of the BINAP ligand used. Isosteric yet electronically different (S)-3,3 '-bis(phenethyl)-, (S)-3,3 '-bis(phenoxymethyl)- and (S)-3,3 '-(OBnhBINAP in the same reactions afforded mixed results. This indicated that the nature of the 3,3 '-substituent, not necessarily the cr-basicity of the ligand, might dictate asymmetric induction and product enantiosense. A series of 3,3 '-disubstituted xy!BINAP derivatives were prepared by combining strategies employed in the syntheses of xy!BINAP and 3,3 '-substituted BIN AP ligands. The use of a naproxen ether-based auxiliary allowed for biaryl axis resolution and access to diastereomeric 3,3 '-disubstituted xy!BINAP derivatives. Auxiliary cleavage and further manipulation of a bisnaphthol intermediate afforded 3,3 '-(OiPr)2-xylBINAP and 3,3 '-(OPiv)2-xylBINAP. 3,3 '-disubstituted xylBINAP derivatives were used in Rh- and Pd-catalyzed asymmetric reactions. The effect of 3 ,3 '-disubstitution and the 3 ,5-dialkyl meta effect are cooperative in the Pd-catalyzed arylation of 2,3-dihydrofuran and the Rh-catalyzed hydrogenation of MAC. The naproxen ether-substituted xylBINAP derivatives outperformed all other 3,3 '-substituted BINAP and xylBINAP ligands, with the naproxen and P-xylyl substituents being necessary for high levels of asymmetric induction. The reversal of product enantiosense observed in Rh-catalyzed hydrogenations upon 3,3'-substitution of the BINAP scaffold was further investigated. The energies of the catalyst-substrate complexes involved in the Rh-catalyzed hydrogenation of MAC obtained from OFT calculations were consistent with an anti-lock-and-key mode of enantioselection. Temperature and pressure studies, as well as quadrant diagram analyses indicate that a change in the kinetics of olefin could be responsible for the enantiosense reversals and high levels of asymmetric induction observed with 3,3 '-disubsituted BINAP and xylBINAP ligands.
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Bibliography: p. 316-326
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Citation
Rankic, D. A. (2010). Modification of binap: implications of 3,3'-disubstitution and the 3,5-dialkyl meta effect in asymmetric catalysis (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/3832