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Cite this: Org. Biomol. Chem., 2014, 12, 1717 Received 2nd January 2014, Accepted 23rd January 2014
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Dissecting non-covalent interactions in oxazaborolidinium catalyzed cycloadditions of maleimides† Robert S. Paton
DOI: 10.1039/c4ob00009a www.rsc.org/obc
Substrate association and asymmetric induction in oxazaborolidinium-catalyzed cycloadditions of maleimides are shown to depend strongly on the catalyst’s aromatic substituents. Favourable dispersive forces bias complexation to the catalyst’s convex (exo) face exposing a single diastereoface of the substrate preferentially.
Protic or Lewis acid activation of oxazaborolidines forms cationic oxazaborolidinium catalysts,1 which have emerged as effective promoters of cycloadditions involving a broad range of dienes and dienophiles with high levels of enantioselectivity.2 Originally developed by Corey and co-workers, the activation of α,β-unsaturated aldehydes, esters, carboxylic acids, ketones and quinones as dienophiles has been achieved using oxazaborolidinium catalysis, and most recently maleimides and anhydrides were found to undergo enantioselective cycloadditions with a range of dienes (Scheme 1).3,4 The interaction of neutral and cationic oxazaborolidines with carbonyl compounds has been the subject of much interest, from both experimental and theoretical viewpoints.5 Based on X-ray structures of aldehyde–BF3 complexes6 Corey proposed that a stabilizing formyl CH⋯O interaction
Scheme 1 Oxazaborolidinium catalyzed cycloaddition of maleimides and anhydrides.
Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, UK. E-mail: [email protected], paton.chem.ox.ac.uk † Electronic supplementary information (ESI) available: Details of all calculations performed, comparison of NCI and QTAIM analyses, absolute energies and Cartesian coordinates. See DOI: 10.1039/c4ob00009a
This journal is © The Royal Society of Chemistry 2014
plays an important rigidifying role in aldehyde binding of oxazaborolidines, and was responsible for highly enantioselective reduction and cycloadditions.7 Calculations by Goodman8,9 suggested a balance of formyl hydrogen bonding and nO→σ*B–F hyperconjugation dictate conformational preferences in these complexes.‡ Non-classical CH⋯O interactions10 involving other types of C–H bonds (e.g. vinylic) have since been invoked to account for transition state preorganization in oxazaborolidinium catalyzed cycloadditions.2 Calculated transition structures (TSs) for cycloadditions of cyclopentadiene with methacrolein11 and quinone with butadiene12 have identified CH⋯O interactions, while cycloadditions of butadiene with five dienophiles were found to closely resemble Corey’s working transition state model.13 Nevertheless, recent studies indicate alternative coordination modes, not involving such interactions, are also energetically feasible.14,15 Given the intense interest in understanding the role of non-bonding interactions in asymmetric catalysis16 we sought to understand maleimide binding in oxazaborolidinium-catalyzed cycloadditions. Unlike previous studies, optimizations are performed with M06-2X and B97-D density functionals, which provide an effective treatment of non-covalent interactions.17 A computational assessment of non-classical CH⋯X interactions in carbonyl boron Lewis acid complexes is presented in Fig. 1a. The existence of weak interactions are perhaps best viewed as a continuum of strengths rather than in absolute terms: from shorter, more pronounced CH⋯F interactions in carbonyl–BF3 complexes to the longer and weaker CH⋯O interactions in carbonyl–oxazaborolidinium complexes. This interpretation of hydrogen bond strength is supported by values of nX→σ*C–H delocalization energies calculated through second order perturbation theory.18 The non-covalent interactions (NCI) index19 computed from the electron density (and supported by QTAIM calculations, ESI†) in Fig. 1b shows an attractive CH⋯O region in the maleimide–catalyst complex, but also emphasises the existence of favourable interactions between the substrate and a phenyl group on the underside of the catalyst. On the evidence of optimized distances and nO→σ*C–H interactions, evidently oxazaborolidinium CH⋯O
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Fig. 3 Most stable complex conformers of maleimide with azaborolidinium catalysts (kcal mol−1); complexation of maleimide with BH3.
Fig. 1 (a) M06-2X/6-31G(d) optimized carbonyl–boron Lewis acid complexes; (b) attractive non-covalent interactions in the maleimide complex; (c) a significant formyl CH⋯O interaction in the aldol reaction.
interaction does not confer appreciable stabilization to binding. In sharp contrast, a dramatic change results from the replacement of the catalyst’s gem-diphenyl group with H atoms. This illustrates the role that (one of ) these rings play(s) in stabilizing convex face coordination. In the absence of these phenyl groups, conformers 1a/1b are destabilized by around 4 kcal mol−1 while 1c/1d are unaffected. Evidently, attractive non-covalent interactions between substrate and the aromatic substituent on the catalyst’s convex face play a critical role in substrate binding and recognition. How then do we account for the preference for 1a over 1b? Near identical structures obtained with oxazaborolidinium and the oxygen-deleted analogue in Fig. 3 show that a CH⋯O interaction is not necessary to favour this conformation, while binding of maleimide with BH3 displays the same preference for Lewis acid coordination trans-to the N-phenyl substituent. TSs were located for the oxazaborolidinium catalyzed Diels– Alder reaction of N-phenylmaleimide with cyclopentadiene (Fig. 4). As in experiment the endo pathway is favoured to a significant extent over exo, by 3.3 kcal mol−1. The M06-2X computed activation barrier relative to the most stable catalyst– substrate complex, ΔGact, is 15.4 kcal mol−1 at −78 °C (the
Fig. 2 SMD-M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) complexation free energies at −78 °C of substrate and azaborolidinium catalysts (kcal mol−1).
interactions are noticeably weaker than formyl CH⋯O bonds found in boron aldol reactions of β-alkoxy ketones (Fig. 1c), where they play an import role in transition state preorganization.20 Rather, relatively weak CH⋯O interactions are an unlikely determinant of conformation in catalyst–substrate complexes,21 and so we further explored maleimide binding with an oxazaborolidinium catalyst, along with analogues chosen to isolate non-covalent interactions and therefore probe the origins of conformational preference (Fig. 2). Binding of the maleimide to the Lewis acidic boron may occur syn or anti to the N atom and to either catalyst convex (exo) or concave (endo) enantioface, giving rise to four possible conformations (Fig. 2, 1a–d). The most favourable is 1a, which has been proposed previously to benefit from a stabilizing CH⋯O interaction.2,22 However, replacement of oxygen by methylene CH2, gives the same ordering of conformer stabilities for the optimized complexes while leading to a more negative free energy of association, which suggest a CH⋯O
1718 | Org. Biomol. Chem., 2014, 12, 1717–1720
Fig. 4 SMD-M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) Diels–Alder endo-transition structures with cyclopentadiene (kcal mol−1). Boronoxygen Wiberg bond order (BO) also shown.
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Fig. 5 SMD-M06-2X/6-311++G(d,p)//M06-2X/6-31G(d) complexation free energies of 2-methyl maleimide and azaborolidinium catalysts (kcal mol−1).
CBS-QB3 value differs by only 1.9 kcal mol−1) for the uncatalyzed reaction, while the two most stable TSs, formed from complexes 1a and 1c have significantly lower free energy barriers of 6.4 and 8.0 kcal mol−1. The TS formed from conformer 1b lies a further 2.8 kcal mol−1 above TS-3, such that the preference for convex over concave face coordination dictates which diastereoface of the maleimide is exposed to the diene. The magnitude of the CH⋯O interaction is relatively small in each TS (0.5–0.7 kcal mol−1). Attractive interactions involving the lower phenyl ring appear to be significant, however. Replacing the gem-diphenyl group with H atoms leads to a significant reduction in the energy gap favouring TS-2 over TS-3 by 2.2 kcal mol−1 (gas phase) or 1.1 kcal mol−1 (in dichloromethane). Therefore TS-2 is favoured due to attractive noncovalent interactions to the catalyst’s phenyl substituent on the convex face, coupled with the maleimide preference to coordinate trans-to the N-phenyl group. Conformational preferences in the binding of 2-methyl-Nphenylmaleimide to the oxazaborolidinium catalyst was then explored (Fig. 5). The two carbonyl groups are no longer equivalent, giving rise to more possible binding modes. However, as above, relative stabilities are in line with those computed for binding to BH3. Along with the miniscule computed nO→σ*C–H energy, these data suggest that this conformational preference is largely driven by coordination to the more electron rich, sterically more accessible carbonyl oxygen trans to the N-phenyl group, rather than decisive CH⋯O interactions. What then are the decisive interactions controlling enantioselectivity in the cycloaddition? The lowest energy TSs leading to the observed major (TS-5) and minor (TS-6) enantiomer differ not by the presence or absence of CH⋯O interactions, but instead by whether the substrate is coordinated to the convex or concave catalyst face (Fig. 6). Attractive dispersive forces between substrate and oxazaborolidinium catalyst phenyl substituent thus dictate both conformation and enantioselectivity. To further illustrate the key role of this non-covalent interaction, we computed the selectivity for a catalyst without the phenyl groups, and with 3,5-dimethylphenyl groups.
This journal is © The Royal Society of Chemistry 2014
Fig. 6 SMD-M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) TSs for the catalyzed cycloaddition of 2-methyl-N-phenylmaleimide with cyclopentadiene (kcal mol−1).
Accordingly, enantioselectivity (which agrees well with the observed level of 93% ee) is predicted to drop to 72% in the absence of phenyl groups and increase to 94.8% with a disubstituted, electron rich aromatic group. Thus in this, and potentially in other oxazaborolidinium-catalyzed cycloadditions, enantioselectivity may be enhanced by increasing the strength of this dispersive interaction on the catalyst convex face. Quantum chemical calculations have shown that in the oxazaborolidinium catalyzed cycloadditions of maleimides, nonattractive non-covalent interactions dictate substrate binding and enantioselectivity. Analysis of the electron density suggest that non-classical CH⋯O interactions are indeed present, but computed substituent effects show their importance has been overstated in oxazaborolidinium catalysis, and instead that dispersive forces between the substrate an aromatic substituent on the catalyst’s convex face are critical for controlling the observed selectivity. This interaction may be modulated to enhance (or to erode) levels of stereoinduction.
Notes and references ‡ Quantum chemical calculations were performed with Gaussian09 and NBO6, while topological analysis of the electron density was carried out with NCIplot and AIMALL. Images were prepared with CYLview, Pymol and VMD. Optimizations were performed with M06-2X and B97-D functionals; relative trends in conformer energies are identical, however, M06-2X gave closer agreement with CBS-QB3 for cycloaddition barrier heights and reaction energies. A full list of citations is provided in the ESI.†
1 E. J. Corey, T. Shibata and T. W. Lee, J. Am. Chem. Soc., 2002, 124, 3808–3809. 2 E. J. Corey, Angew. Chem., Int. Ed., 2009, 48, 2100–2117. 3 S. Mukherjee and E. J. Corey, Org. Lett., 2010, 12, 632–635.
Org. Biomol. Chem., 2014, 12, 1717–1720 | 1719
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4 S. Mukherjee and E. J. Corey, Org. Lett., 2010, 12, 1024–1027. 5 H. Zhu, D. J. O’Leary and M. P. Meyer, Angew. Chem., Int. Ed., 2012, 51, 11890–11893. 6 E. J. Corey, J. J. Rohde, A. Fischer and M. D. Azimioara, Tetrahedron Lett., 1997, 38, 33–36. 7 E. J. Corey and T. W. Lee, Chem. Commun., 2001, 1321– 1329. 8 J. M. Goodman, Tetrahedron Lett., 1992, 33, 7219–7222. 9 M. D. Mackey and J. M. Goodman, Chem. Commun., 1997, 2383–2384. 10 R. C. Johnston and P. H.-Y. Cheong, Org. Biomol. Chem., 2013, 11, 5057. 11 Z. Pi and S. Li, J. Phys. Chem. A, 2006, 110, 9225–9230. 12 N. Y. M. Omar, N. A. Rahman and S. M. Zain, Bull. Chem. Soc. Jpn., 2011, 84, 196–204. 13 M. N. Paddon-Row, C. D. Anderson and K. N. Houk, J. Org. Chem., 2009, 74, 861–868.
1720 | Org. Biomol. Chem., 2014, 12, 1717–1720
Organic & Biomolecular Chemistry
14 K. Sakata and H. Fujimoto, J. Org. Chem., 2013, 78, 3095–3103. 15 M. N. Paddon-Row, L. C. H. Kwan, A. C. Willis and M. S. Sherburn, Angew. Chem., Int. Ed., 2008, 47, 7013–7017. 16 R. R. Knowles and E. N. Jacobsen, Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 20678–20685. 17 T. Lu, R. Zhu, Y. An and S. E. Wheeler, J. Am. Chem. Soc., 2012, 134, 3095–3102. 18 U. Adhikari and S. Scheiner, J. Phys. Chem. A, 2013, 117, 10551–10562. 19 E. R. Johnson, S. Keinan, P. Mori-Sánchez, J. ContrerasGarcía, A. J. Cohen and W. Yang, J. Am. Chem. Soc., 2010, 132, 6498–6506. 20 (a) R. S. Paton and J. M. Goodman, Org. Lett., 2006, 8, 4299–4302; (b) R. S. Paton and J. M. Goodman, J. Org. Chem., 2008, 73, 1253–1263. 21 C. R. Jones, P. K. Baruah, A. L. Thompson, S. Scheiner and M. D. Smith, J. Am. Chem. Soc., 2012, 134, 12064–12071. 22 E. J. Corey, Angew. Chem., Int. Ed., 2002, 41, 1650–1667.
This journal is © The Royal Society of Chemistry 2014
Published on 11 February 2014. Downloaded by Purdue University on 02/10/2014 16:58:04.
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Cite this: Org. Biomol. Chem., 2014, 12, 1717 Received 2nd January 2014, Accepted 23rd January 2014
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Dissecting non-covalent interactions in oxazaborolidinium catalyzed cycloadditions of maleimides† Robert S. Paton
DOI: 10.1039/c4ob00009a www.rsc.org/obc
Substrate association and asymmetric induction in oxazaborolidinium-catalyzed cycloadditions of maleimides are shown to depend strongly on the catalyst’s aromatic substituents. Favourable dispersive forces bias complexation to the catalyst’s convex (exo) face exposing a single diastereoface of the substrate preferentially.
Protic or Lewis acid activation of oxazaborolidines forms cationic oxazaborolidinium catalysts,1 which have emerged as effective promoters of cycloadditions involving a broad range of dienes and dienophiles with high levels of enantioselectivity.2 Originally developed by Corey and co-workers, the activation of α,β-unsaturated aldehydes, esters, carboxylic acids, ketones and quinones as dienophiles has been achieved using oxazaborolidinium catalysis, and most recently maleimides and anhydrides were found to undergo enantioselective cycloadditions with a range of dienes (Scheme 1).3,4 The interaction of neutral and cationic oxazaborolidines with carbonyl compounds has been the subject of much interest, from both experimental and theoretical viewpoints.5 Based on X-ray structures of aldehyde–BF3 complexes6 Corey proposed that a stabilizing formyl CH⋯O interaction
Scheme 1 Oxazaborolidinium catalyzed cycloaddition of maleimides and anhydrides.
Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, UK. E-mail: [email protected], paton.chem.ox.ac.uk † Electronic supplementary information (ESI) available: Details of all calculations performed, comparison of NCI and QTAIM analyses, absolute energies and Cartesian coordinates. See DOI: 10.1039/c4ob00009a
This journal is © The Royal Society of Chemistry 2014
plays an important rigidifying role in aldehyde binding of oxazaborolidines, and was responsible for highly enantioselective reduction and cycloadditions.7 Calculations by Goodman8,9 suggested a balance of formyl hydrogen bonding and nO→σ*B–F hyperconjugation dictate conformational preferences in these complexes.‡ Non-classical CH⋯O interactions10 involving other types of C–H bonds (e.g. vinylic) have since been invoked to account for transition state preorganization in oxazaborolidinium catalyzed cycloadditions.2 Calculated transition structures (TSs) for cycloadditions of cyclopentadiene with methacrolein11 and quinone with butadiene12 have identified CH⋯O interactions, while cycloadditions of butadiene with five dienophiles were found to closely resemble Corey’s working transition state model.13 Nevertheless, recent studies indicate alternative coordination modes, not involving such interactions, are also energetically feasible.14,15 Given the intense interest in understanding the role of non-bonding interactions in asymmetric catalysis16 we sought to understand maleimide binding in oxazaborolidinium-catalyzed cycloadditions. Unlike previous studies, optimizations are performed with M06-2X and B97-D density functionals, which provide an effective treatment of non-covalent interactions.17 A computational assessment of non-classical CH⋯X interactions in carbonyl boron Lewis acid complexes is presented in Fig. 1a. The existence of weak interactions are perhaps best viewed as a continuum of strengths rather than in absolute terms: from shorter, more pronounced CH⋯F interactions in carbonyl–BF3 complexes to the longer and weaker CH⋯O interactions in carbonyl–oxazaborolidinium complexes. This interpretation of hydrogen bond strength is supported by values of nX→σ*C–H delocalization energies calculated through second order perturbation theory.18 The non-covalent interactions (NCI) index19 computed from the electron density (and supported by QTAIM calculations, ESI†) in Fig. 1b shows an attractive CH⋯O region in the maleimide–catalyst complex, but also emphasises the existence of favourable interactions between the substrate and a phenyl group on the underside of the catalyst. On the evidence of optimized distances and nO→σ*C–H interactions, evidently oxazaborolidinium CH⋯O
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Fig. 3 Most stable complex conformers of maleimide with azaborolidinium catalysts (kcal mol−1); complexation of maleimide with BH3.
Fig. 1 (a) M06-2X/6-31G(d) optimized carbonyl–boron Lewis acid complexes; (b) attractive non-covalent interactions in the maleimide complex; (c) a significant formyl CH⋯O interaction in the aldol reaction.
interaction does not confer appreciable stabilization to binding. In sharp contrast, a dramatic change results from the replacement of the catalyst’s gem-diphenyl group with H atoms. This illustrates the role that (one of ) these rings play(s) in stabilizing convex face coordination. In the absence of these phenyl groups, conformers 1a/1b are destabilized by around 4 kcal mol−1 while 1c/1d are unaffected. Evidently, attractive non-covalent interactions between substrate and the aromatic substituent on the catalyst’s convex face play a critical role in substrate binding and recognition. How then do we account for the preference for 1a over 1b? Near identical structures obtained with oxazaborolidinium and the oxygen-deleted analogue in Fig. 3 show that a CH⋯O interaction is not necessary to favour this conformation, while binding of maleimide with BH3 displays the same preference for Lewis acid coordination trans-to the N-phenyl substituent. TSs were located for the oxazaborolidinium catalyzed Diels– Alder reaction of N-phenylmaleimide with cyclopentadiene (Fig. 4). As in experiment the endo pathway is favoured to a significant extent over exo, by 3.3 kcal mol−1. The M06-2X computed activation barrier relative to the most stable catalyst– substrate complex, ΔGact, is 15.4 kcal mol−1 at −78 °C (the
Fig. 2 SMD-M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) complexation free energies at −78 °C of substrate and azaborolidinium catalysts (kcal mol−1).
interactions are noticeably weaker than formyl CH⋯O bonds found in boron aldol reactions of β-alkoxy ketones (Fig. 1c), where they play an import role in transition state preorganization.20 Rather, relatively weak CH⋯O interactions are an unlikely determinant of conformation in catalyst–substrate complexes,21 and so we further explored maleimide binding with an oxazaborolidinium catalyst, along with analogues chosen to isolate non-covalent interactions and therefore probe the origins of conformational preference (Fig. 2). Binding of the maleimide to the Lewis acidic boron may occur syn or anti to the N atom and to either catalyst convex (exo) or concave (endo) enantioface, giving rise to four possible conformations (Fig. 2, 1a–d). The most favourable is 1a, which has been proposed previously to benefit from a stabilizing CH⋯O interaction.2,22 However, replacement of oxygen by methylene CH2, gives the same ordering of conformer stabilities for the optimized complexes while leading to a more negative free energy of association, which suggest a CH⋯O
1718 | Org. Biomol. Chem., 2014, 12, 1717–1720
Fig. 4 SMD-M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) Diels–Alder endo-transition structures with cyclopentadiene (kcal mol−1). Boronoxygen Wiberg bond order (BO) also shown.
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Fig. 5 SMD-M06-2X/6-311++G(d,p)//M06-2X/6-31G(d) complexation free energies of 2-methyl maleimide and azaborolidinium catalysts (kcal mol−1).
CBS-QB3 value differs by only 1.9 kcal mol−1) for the uncatalyzed reaction, while the two most stable TSs, formed from complexes 1a and 1c have significantly lower free energy barriers of 6.4 and 8.0 kcal mol−1. The TS formed from conformer 1b lies a further 2.8 kcal mol−1 above TS-3, such that the preference for convex over concave face coordination dictates which diastereoface of the maleimide is exposed to the diene. The magnitude of the CH⋯O interaction is relatively small in each TS (0.5–0.7 kcal mol−1). Attractive interactions involving the lower phenyl ring appear to be significant, however. Replacing the gem-diphenyl group with H atoms leads to a significant reduction in the energy gap favouring TS-2 over TS-3 by 2.2 kcal mol−1 (gas phase) or 1.1 kcal mol−1 (in dichloromethane). Therefore TS-2 is favoured due to attractive noncovalent interactions to the catalyst’s phenyl substituent on the convex face, coupled with the maleimide preference to coordinate trans-to the N-phenyl group. Conformational preferences in the binding of 2-methyl-Nphenylmaleimide to the oxazaborolidinium catalyst was then explored (Fig. 5). The two carbonyl groups are no longer equivalent, giving rise to more possible binding modes. However, as above, relative stabilities are in line with those computed for binding to BH3. Along with the miniscule computed nO→σ*C–H energy, these data suggest that this conformational preference is largely driven by coordination to the more electron rich, sterically more accessible carbonyl oxygen trans to the N-phenyl group, rather than decisive CH⋯O interactions. What then are the decisive interactions controlling enantioselectivity in the cycloaddition? The lowest energy TSs leading to the observed major (TS-5) and minor (TS-6) enantiomer differ not by the presence or absence of CH⋯O interactions, but instead by whether the substrate is coordinated to the convex or concave catalyst face (Fig. 6). Attractive dispersive forces between substrate and oxazaborolidinium catalyst phenyl substituent thus dictate both conformation and enantioselectivity. To further illustrate the key role of this non-covalent interaction, we computed the selectivity for a catalyst without the phenyl groups, and with 3,5-dimethylphenyl groups.
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Fig. 6 SMD-M06-2X/6-311+G(d,p)//M06-2X/6-31G(d) TSs for the catalyzed cycloaddition of 2-methyl-N-phenylmaleimide with cyclopentadiene (kcal mol−1).
Accordingly, enantioselectivity (which agrees well with the observed level of 93% ee) is predicted to drop to 72% in the absence of phenyl groups and increase to 94.8% with a disubstituted, electron rich aromatic group. Thus in this, and potentially in other oxazaborolidinium-catalyzed cycloadditions, enantioselectivity may be enhanced by increasing the strength of this dispersive interaction on the catalyst convex face. Quantum chemical calculations have shown that in the oxazaborolidinium catalyzed cycloadditions of maleimides, nonattractive non-covalent interactions dictate substrate binding and enantioselectivity. Analysis of the electron density suggest that non-classical CH⋯O interactions are indeed present, but computed substituent effects show their importance has been overstated in oxazaborolidinium catalysis, and instead that dispersive forces between the substrate an aromatic substituent on the catalyst’s convex face are critical for controlling the observed selectivity. This interaction may be modulated to enhance (or to erode) levels of stereoinduction.
Notes and references ‡ Quantum chemical calculations were performed with Gaussian09 and NBO6, while topological analysis of the electron density was carried out with NCIplot and AIMALL. Images were prepared with CYLview, Pymol and VMD. Optimizations were performed with M06-2X and B97-D functionals; relative trends in conformer energies are identical, however, M06-2X gave closer agreement with CBS-QB3 for cycloaddition barrier heights and reaction energies. A full list of citations is provided in the ESI.†
1 E. J. Corey, T. Shibata and T. W. Lee, J. Am. Chem. Soc., 2002, 124, 3808–3809. 2 E. J. Corey, Angew. Chem., Int. Ed., 2009, 48, 2100–2117. 3 S. Mukherjee and E. J. Corey, Org. Lett., 2010, 12, 632–635.
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4 S. Mukherjee and E. J. Corey, Org. Lett., 2010, 12, 1024–1027. 5 H. Zhu, D. J. O’Leary and M. P. Meyer, Angew. Chem., Int. Ed., 2012, 51, 11890–11893. 6 E. J. Corey, J. J. Rohde, A. Fischer and M. D. Azimioara, Tetrahedron Lett., 1997, 38, 33–36. 7 E. J. Corey and T. W. Lee, Chem. Commun., 2001, 1321– 1329. 8 J. M. Goodman, Tetrahedron Lett., 1992, 33, 7219–7222. 9 M. D. Mackey and J. M. Goodman, Chem. Commun., 1997, 2383–2384. 10 R. C. Johnston and P. H.-Y. Cheong, Org. Biomol. Chem., 2013, 11, 5057. 11 Z. Pi and S. Li, J. Phys. Chem. A, 2006, 110, 9225–9230. 12 N. Y. M. Omar, N. A. Rahman and S. M. Zain, Bull. Chem. Soc. Jpn., 2011, 84, 196–204. 13 M. N. Paddon-Row, C. D. Anderson and K. N. Houk, J. Org. Chem., 2009, 74, 861–868.
1720 | Org. Biomol. Chem., 2014, 12, 1717–1720
Organic & Biomolecular Chemistry
14 K. Sakata and H. Fujimoto, J. Org. Chem., 2013, 78, 3095–3103. 15 M. N. Paddon-Row, L. C. H. Kwan, A. C. Willis and M. S. Sherburn, Angew. Chem., Int. Ed., 2008, 47, 7013–7017. 16 R. R. Knowles and E. N. Jacobsen, Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 20678–20685. 17 T. Lu, R. Zhu, Y. An and S. E. Wheeler, J. Am. Chem. Soc., 2012, 134, 3095–3102. 18 U. Adhikari and S. Scheiner, J. Phys. Chem. A, 2013, 117, 10551–10562. 19 E. R. Johnson, S. Keinan, P. Mori-Sánchez, J. ContrerasGarcía, A. J. Cohen and W. Yang, J. Am. Chem. Soc., 2010, 132, 6498–6506. 20 (a) R. S. Paton and J. M. Goodman, Org. Lett., 2006, 8, 4299–4302; (b) R. S. Paton and J. M. Goodman, J. Org. Chem., 2008, 73, 1253–1263. 21 C. R. Jones, P. K. Baruah, A. L. Thompson, S. Scheiner and M. D. Smith, J. Am. Chem. Soc., 2012, 134, 12064–12071. 22 E. J. Corey, Angew. Chem., Int. Ed., 2002, 41, 1650–1667.
This journal is © The Royal Society of Chemistry 2014
