Improving phase-transfer catalysis by enhancing non-covalent interactions

Iñigo Iribarren and Cristina Trujillo *
School of Chemistry, Trinity Biomedical Sciences Institute, Trinity College Dublin, 152-160 Pearse Street, Dublin 2, Ireland. E-mail: trujillc@tcd.ie

Received 14th April 2020 , Accepted 8th July 2020

First published on 8th July 2020


A wide variety of asymmetric transformations catalysed by chiral catalysts have been developed for the synthesis of valuable organic compounds in the past several decades. Within the asymmetric catalysis field, phase-transfer catalysis has been recognized as a powerful method for establishing useful procedures for organic synthesis. In the present study intermolecular interactions between a well-known alkaloid quinine-derived phase transfer catalyst and four different anions were characterised, analysing the competition between the pure ion-pair interaction and the intermolecular hydrogen bond established upon complexation. Finally, a theoretical study of the free-energy profile corresponding to the enantioselective conjugate cyanation of an α,β-unsaturated ketone in the presence of two different catalysts was performed.


image file: d0cp02012e-p1.tif

Iñigo Iribarren

Iñigo carried out his chemistry studies at “Universidad Autónoma de Madrid” as well as his master studies in theoretical and computational chemistry. He spent three months at “Sorbonne Université” in Paris under the supervision of Dr Contreras developing a module for the Non-covalent interaction index software. He carried out his master thesis at “Instituto de Química Médica (CSIC, Madrid)” under the supervision of Prof. Alkorta focused on studying charged dimers linked by hydrogen bonds. Currently, Iñigo is at Trinity College Dublin doing his PhD under the supervision of Dr Cristina Trujillo focused on weak interactions on catalysis.

image file: d0cp02012e-p2.tif

Cristina Trujillo

Cristina obtained her PhD (Cum Laude) in Theoretical and Computational Chemistry in 2008 at Universidad Autónoma de Madrid (UAM). During 2008–2016, she has held several postdoctoral positions in Spain, Prague and Ireland. She joined Trinity College Dublin in 2019 being awarded with a Science Foundation of Ireland-Starting Investigator Research Grant. She has received the 2019 L’Oréal-UNESCO for Women in Science UK and an Ireland Highly Commended certificate. She has expertise in fundamental topics within Computational Organic Chemistry. Cristina's research group investigates the application of computational techniques in the design of organocatalysts, as well as the prediction and control of catalytic processes.


Introduction

During the past few decades, there has been great interest in organocatalytic processes; the recovery and reuse of catalysts is highly desirable from both economic and environmental standpoints. Due to the absence of transition metals, organocatalytic methods are attractive for the preparation of pharmaceutical compounds where levels of certain metal-ion contamination are tightly controlled.

Phase-transfer catalysts (PTCs), since Starks first introduced the term in 1971,1 have been widely applied in organic reactions in different immiscible phases. Asymmetric PTCs based on the use of a variety of structurally well-defined chiral motifs have also attracted the attention of chemistsand have since been recognised as a versatile strategy for preparing chiral functional molecules. As a result, many novel organic transformations have been achieved with high enantioselectivity.2–6

Several catalyst classes have been developed based on the Cinchona alkaloid, and chiral binaphthyls have emerged among the most successful examples, including quaternary-onium salts,2,7–12 and have been applied successfully to highly enantioselective transformations.13 In 1997, Corey et al.14 stated a stereochemical rational in which the nitrogen of a Cinchona alkaloid quaternary salt is the centre of a tetrahedron and three of the four faces are sterically hindered by the alkaloid quinuclidine structure. In consequence, the PTC should be structured to preclude the approach of the anion to three of the faces of this tetrahedron, while the fourth face should be adequately open to assist the ion-pairing interaction (Fig. 1, left).


image file: d0cp02012e-f1.tif
Fig. 1 Traditional concept on asymmetric ammonium ion-pairing catalysis.

As described in the extensive synthetic literature,5 it was hypothesised that the anion of PTCs is usually situated proximally to the positively-charged nitrogen or phosphorous atom, establishing a strict (/tight) ion pairing interaction (Fig. 1, right),15,16 and therefore in the proposed general mechanism of asymmetric PTCs (Fig. 1, right) the nucleophilic addition of an achiral anion to prochiral electrophiles occurs, and a new stereocentre is created.

Catalytic asymmetric cyanation of prochiral unsaturated compounds has been extensively studied in recent years as their reaction products are considered highly desirable building blocks for pharmaceutical compounds.17 The conjugate cyanation of α,β-unsaturated carbonyl compounds is included in this category.17–19 The first major discovery was reported by Jacobsen20,21 using chiral Al-Salen and bimetallic cooperative catalyst systems for the enantioselective conjugate additions of trimethylsilyl cyanide (TMSCN) to α,β unsaturated imides (Scheme 1, A). Shibasaki22,23 and co-workers reported two chiral bifunctional catalysts derived from gadolinium, strontium, and different carbohydrate-based chiral ligands for a highly enantioselective 1,4-addition of cyanide to α,β-unsaturated N-acyl pyrroles and enones (Scheme 1, B), using trimethylsilyl cyanide (TMSCN) as a cyanide source. In 2008, the first organocatalysed PTC conjugate addition of a cyanide ion to β,β′-disubstituted nitroolefins that leads to the efficient formation of an all-carbon quaternary stereocentre (Scheme 1, C) was studied by Fochi.24 Very recently, Deng et al. presented the development of highly enantioselective catalytic 1,4-additions of cyanide with readily accessible and easy-to-handle cyanation reagents (Scheme 1, D).25 In this work, two different modes of activation for bifunctional PTCs were proposed: the first one in which a pure ion-pairing interaction is established and, alternatively, one in which simultaneous interactions with both the cyanation species and the enone occur. Therefore, by introducing secondary interactions in bifunctional PTCs, as proposed by Deng, ion pairing processes could compete with the establishment of an intermolecular hydrogen bond between the PTC counteranion and potential hydrogen bond donors (Fig. 2). There is a diversity of non-covalent interactions, but the introduction of strong, directional hydrogen bonds (HBs)26–28 in particular is known to be crucial in the success of organocatalysed reactions.


image file: d0cp02012e-s1.tif
Scheme 1 Catalytic asymmetric cyanation studied in the synthetic literature.

image file: d0cp02012e-f2.tif
Fig. 2 The two different interactions analysed in the present study.

While the field of asymmetric organocatalysis is currently growing exponentially, an understanding of the mechanistic details involved in most of these reactions has often lagged far behind the pace of catalyst development, which in turn slows down the rational design of catalysts. Therefore, continuous efforts should be made toward the design and development of new catalyst classes, as well as understanding existing relationships between the structure of the catalyst and its ability to transfer stereochemical information.16,29–34

Herein, with the objective of providing a definitive binding mode for bifunctional PTCs, we present a theoretical study of different interactions that can be established between a well-known alkaloid quinine-derived PTC and four different anions of interest in organocatalysis: Cl, Br, MeCO2 and CN.

Finally, the mechanistic picture of the enantioselective conjugate cyanation of α,β-unsaturated ketones catalysed by a Cinchona alkaloid quaternary salt was studied. An improved PTC derivative to the existing system was proposed in order to increase the enantioselectivity of a model reaction and therefore a theoretical study of the free-energy profile was performed.

Computational details

The structures of the complexes were optimized at the M062X/aug-cc-pVDZ35,36 computational level. Harmonic vibrational frequencies were computed at the same level used for geometry optimization in order to confirm that the stationary points are local minima. Calculations were performed using the Gaussian16 software.37 Single point energies for the lowest energy small basis set calculations were computed using the M062X/aug-cc-pVTZ computational level. Interaction energies (ΔGi) were calculated as the difference between the energy of the optimised complex and the energy of each monomer in its optimised geometry. The free energies reported in the document were obtained by adding the free energy correction from the small basis set calculations to the potential energy obtained from the high-level single-point energy calculations. Solvent effects (toluene) were included in the optimization by means of a continuum method, the Solvation Model based on Density (SMD) approach38 and the refined SMD1839 version for Br atoms implemented in Gaussian16. The molecular electrostatic potential (MEP) of the isolated monomers was calculated on the electron density isosurface of 0.001 a.u. This isosurface was shown to resemble the van der Waals surface.40 These calculations were carried out using Gaussian-16 software and the numerical results were analysed using the Multiwfn program41 and plotted using Jmol.42 The atoms in molecules (AIM) methodology43,44 was used to analyse the electron density of the systems with the AIMAll program.45 The Natural Bond Orbital (NBO) method46 was used to evaluate atomic charges using the NBO-6 program and to analyse charge-transfer interactions between occupied and unoccupied orbitals.

On the basis of the Curtin–Hammett principle and according to the calculated Gibbs free energies (evaluated within the standard harmonic-oscillator rigid-rotor model) of the transition states, the theoretical ee (enantiomeric excess) value was determined. In these calculations, the ee is defined as ([R] − [S])/([R] + [S]) and [R]/[S] is obtained as the ratio of the rate constants at T = 243 K; image file: d0cp02012e-t1.tif.47

Results and discussion

Conformational analysis of the catalyst

We began by exploring the low-energy chemical space of the catalyst by rotating three different parts of the catalyst plotted in Fig. 3. The relative energies are summarised in Table S1 (ESI).
image file: d0cp02012e-f3.tif
Fig. 3 Left: Different rotations taken into account for the conformational analysis. Right: Lowest-energy optimised conformer for the catalyst under study, conf_1.

From the conformational analysis 12 different conformers were obtained (Fig. S1, ESI). The relative free energies range from 0.1 to 36.0 kJ mol−1. Following the different conformations, it is clear that compared to catalyst conf_1 (no rotations made, Fig. 3, right) when only φ2 or φ3 is rotated (conf_5 or conf_6, respectively, Fig. S1, ESI) the energy difference is ca. 14 kJ mol−1, and therefore the value of ca. 30 kJ mol−1 is obtained when two dihedral angles are changed (conf_11 and conf_12, Fig. S1, ESI). Rotations of the dihedral angle φ1 do not provoke significant changes regarding the energy outcome. Conf_1 is the most stable one among all the 12 conformations studied, in complete agreement with studies reported in the literature by means of computational methods48 and X-ray techniques.6 The Boltzmann population of all conformers was calculated (ESI, Table S2) showing that conf_1 and conf_2 are the majority conformers, 47% and 46%, respectively. The difference between both conformers is the rotation of the vinyl group, φ1, having no relevance since it is not involved at the binding side.

Catalyst–anion interactions

With the optimal conformations in hand, we began by calculating the molecular electrostatic potential (MEP) surface for the isolated catalyst in order to analyse the areas susceptible to anion interactions. The maxima values of the MEP on the 0.001 a.u. electron density isosurface are plotted in Fig. 4 and summarised in Table 1.
Table 1 Interaction energies (ΔG, kJ mol−1) and X⋯H–O and X⋯N+ intermolecular distances (Å) calculated at the M062X/aug-cc-pVTZ//aug-cc-pVDZ computational level
Complex ΔGint Dist. X⋯H Dist. X⋯N+
Complex_Cl_1 −150.7 2.054 4.020
Complex_Cl_2 −118.1 3.713
Complex_Cl_3 −101.4 4.024
Complex_Cl_4
Complex_Cl_5 −95.9 4.575
Complex_Br_1 −139.6 2.259 4.217
Complex_Br_2 −105.5 3.934
Complex_Br_3 −92.4 4.255
Complex_Br_4 −96.8 4.771
Complex_Br_5 −86.9 4.758
Complex_MeCO2_1 −172.3 1.491 4.729
Complex_MeCO2_2 −133.6 3.410
Complex_MeCO2_3 −107.0 3.591
Complex_MeCO2_4
Complex_CN_1 −142.8 1.684 3.810
Complex_NC_1 −139.6 1.776 3.931
Complex_CN_2 −107.5 3.461
Complex_CN_3 −74.5 3.708


Four MEP maxima values (black dots) were found within the catalyst (Vmax), one corresponding to the OH group and three localised around the quinuclidine ring, where the positive charge is placed (Fig. 4). It is noteworthy that the minimum 2 corresponds to the face of the tetrahedron of the quinuclidine structure (Fig. 1, left). All the values are very close, but the most positive one corresponds to the OH group. Considering the previous results, it is clear that the hydrogen bond becomes a significant competitive interaction for the anion.


image file: d0cp02012e-f4.tif
Fig. 4 Molecular electrostatic potential on the 0.001 a.u electron density isosurface and the maxima (Vmax) values of the molecular electrostatic potential (in a.u.) for the catalyst under study at the M062X/aug-cc-pVDZ computational level. Colour scheme range: red (+0.015 a.u.) to blue (+0.15 a.u.).

In order to provide an insight into the different interactions that can be established with the catalyst a thorough computational study upon complexation was performed (Fig. 5). The nomenclature chosen for the formed complexes is complex_X_N, where X refers to the different anions involved (X = Cl, Br, MeCO2 and CN) and N corresponds to the different maxima values found in the MEP of the catalyst (N = 1–4). For both halogen anions (X = Cl, Br) an extra complex N = 5 was found. The optimised structures are shown in Table S6 (ESI). The interaction free energy values (ΔGint) are presented in Table 2 and they range from −74.5 to −172.3 kJ mol−1.


image file: d0cp02012e-f5.tif
Fig. 5 Structure of the different interaction systems analysed in the present study.
Table 2 Interaction energies (ΔG, kJ mol−1) calculated at the M062X/6-311+g(d,p)//6-31g(d) computational level (toluene and 243 K)
ΔGint
Complex Cat1 Cat2
R
Pre-TS assembly −137.9 −144.0
TS −58.2 −93.7
Product −120.9 −172.9
S
Pre-TS assembly −141.6 −172.1
TS −58.8 −81.6
Product −139.3 −167.0


Regarding the different anions, the acetate anion complexes present the strongest interactions while the bromide ion complexes show the weakest interaction energy among of all them. It is clear that, within the same anion–complex interaction, the one establishing an intermolecular hydrogen bond exhibits the strongest interaction. If a comparison is made between the HB-bound complex (1) and the one exhibiting a strict ion-pair interaction, in which both charged atoms are closest (2), the former is in general more than 30 kJ mol−1 more stable compared to the latter, even though the X⋯N+ distance is shorter.

The quantum theory of atoms in molecules (QTAIM) analysis was carried out. The main electron density properties at the bond critical points (BCPs) of the different HBs found in these complexes within the QTAIM analysis are summarized in Table S4 (ESI). Molecular graphs for all the complexes are summarised in Table S6 (ESI). The values of the electron density at the BCP, ρ(BCP), were found to range from 0.079 to 0.004 a.u. The large ρ(BCP) values indicate strong hydrogen bonds, while the positive values of ∇2ρ(BCP) are characteristic of closed shell systems. From the molecular graphs and the analysis of the density at the BCPs, it is clear that the different anions can establish either a strong intermolecular HB with the hydroxyl group of the catalyst (OH⋯X) or mild hydrogen bonds with weaker hydrogen bond donors such as CH groups from the quinuclidine ring of the catalyst. The values of both ρ(BCP) and ∇2ρ(BCP) increase with the intermolecular hydrogen bond established between the anion and the OH group of the catalyst as shown in Table S4 (ESI). Also, the strength of the most relevant intermolecular hydrogen bond (OH⋯X) is also dependent on the nature of the anion. It becomes weaker in the order of MeCO2 > CN > Cl > Br, evidenced by the interaction energy, binding distance and electron density. That pattern is in complete agreement with the electronegative trend; the greater electronegativity of the hydrogen bond acceptor will lead to an increase in hydrogen-bond strength. Exponential relationships between the intermolecular distance with the anion and ρ(BCP) in each BCP were found (Fig. 6), showing the decay of the electron density with distance.49–52


image file: d0cp02012e-f6.tif
Fig. 6 Exponential relationships of the intermolecular distance between the catalyst and the anion, in Å, with ρ(BCP).

In order to provide a visual description of the electron density overlap established in the intramolecular interactions, non-covalent interaction index (NCI) plots for the most representative examples (complex_X_1) were obtained and are presented (Fig. 7). Besides, the 2D-NCI plots are depicted in Fig. S4 (ESI).


image file: d0cp02012e-f7.tif
Fig. 7 3D-NCI plots of the interactions found for the most stable complex with Cl (a), Br (b), MeCO2 (c) and CN (d). RDG isosurfaces provide a 3D representation of interaction regions. The sign of the second eigenvalue of the ρ Hessian matrix is used to differentiate repulsive (λ2 > 0) from attractive (λ2 < 0) interactions.

In all the cases a green area corresponding to the values of λ2 ≈ 0 (weakly attractive) appears between the anions and the catalyst indicating the interactions taking place. In the case of MeCO2 and CN, two blue areas λ2 < 0 (strongly attractive) are shown to coincide with the position of the established hydrogen bond interaction. Those are consistent with the ΔGint values shown for both anion complexes. Also, the Cl complex presents small blue areas, which are weaker compared with the other two anions, but stronger than those in the bromide case. The obtained 2D-NCI plots show an intense peak corresponding to both anions, MeCO2 and CN, at more negative values of λ2.

Finally, NBO analysis was used to identify and characterise the intermolecular charge transfer between the occupied molecular orbitals and the empty ones upon complexation. In Table S5 (ESI) the second order perturbation energies E(2) present in the found complexes are reported. Among all the molecular interactions observed the largest contributions correspond to the donations from the X electron lone pairs into the σ* antibonding orbital of the hydrogen bond donor (σ*O–H). In all the anion-hydrogen bonding established in the series the orbital interaction decreases following the interaction energy trend (MeCO2 > CN > Cl > Br). The smallest contributions of the series come from the ion pair interaction corresponding to those complexes in which both ions present the larger interatomic distance (complex_X_3 and complex_X_5).

Conjugate cyanation

Having obtained a clearer understanding of the different interactions that can be established between the catalyst and the different anions, an application was studied in order to improve the phase transfer catalysis of cat1.

We began by studying the pathway of the enantioselective conjugate cyanation of the α,β-unsaturated ketone, (E)-Chalcone (1), in the presence of the catalyst studied previously, cat1, and then a theoretical modification of the catalyst was made, cat2 (Scheme 2). In order to improve the enantioselectivity of this reaction, an extra anchorage point was added (C6′-OH). Even though the reaction was not performed experimentally, the reactants as well as the conditions were proved to be widely used in previous synthetic studies (Scheme 1) but not under the promotion of this particular catalyst, cat1.


image file: d0cp02012e-s2.tif
Scheme 2 Asymmetric conjugate cyanation of (E)-Chalcone.

Only the first step, the addition of CN, was studied, since it is responsible for the stereochemistry of the global process. Therefore the global product of the free-energy profile is the cat-bound enolate (2) shown in Scheme 2. Besides, from the methodology perspective, due to convergence issues, a 6-31g(d)53,54 basis set was used to perform the energy profile, instead of the previously used aug-cc-pVDZ basis set, since this specific Pople basis set was widely used in the frame of theoretical asymmetric catalysis,33,34 A benchmark between both basis sets for the conformational study as well as for the catalyst–anion interaction study was performed (Table S3 and Fig. S2, S3, ESI), obtaining, in general, the same trend.

The free-energy profile for the formation of both enantiomers of cat1-bound enolate product (2) (Fig. 8, left) indicates that the barrier corresponding to the C–C bond formation is very similar for both pathways, around 80 kJ mol−1 (ΔΔG = 0.6 kJ mol−1). It is clear from the free-energy profile that no steric issues are presented by the two TSs (R and S) and therefore both are energetically accessible by the cyanide.


image file: d0cp02012e-f8.tif
Fig. 8 Left: Free-energy profiles for the reaction in the presence of cat1. Right: Free-energy profiles for the reaction in the presence of cat2.

Regarding the cat-enolate complexes (2), they present similar energies to the pre-TS assembly, with the corresponding enantiomer S being slightly more stable than the R one.

All the interaction free energies (in kcal mol−1) were obtained at the M062X/6-311+g(d,p) level (toluene and 243 K).

When the OMe group is replaced by a hydrogen bond donor group, OH (cat2), a second anchoring point is added, forcing the intermolecular hydrogen bond between the two moieties, the new hydroxyl group of cat2 and the oxygen of (E)-Chalcone. The free-energy profile study was performed (Fig. 8, right), showing that the energetic barrier between the two enantiomers had increased up to 12 kJ mol−1. Regarding the four energy pathways studied, in three of them there is a possibility of reversibility since the products are slightly less stable than the reactants. However, it is noteworthy to remark that cat-bound enolate is not the final product of the reaction, since a protonation has to take place and very likely will stabilise the final product.

To probe the origins of the observed asymmetric induction, we examined the transition states associated with the key stereocentre forming the addition step for both major enantiomers (Fig. 9). The binding mode is analogous for both TSs. The inspection of the calculated transition state leading to the major antipode provides an understanding of why the simple modification, 6′-hydroxylation, is superior in this process and also sheds light on the catalyst–substrate interactions. Cat2 forms a discernible non-covalent network identified by QTAIM calculations (see the ESI) in the transition state. This, together with the binding of the O–H proton to one of the carbonyl groups, aligns the electrophile.


image file: d0cp02012e-f9.tif
Fig. 9 Optimised geometries for both TSs in the presence of cat1 (left) and cat2 (right) for the major enantiomer in each reaction (S and R, respectively).

The calculated ee is 13% when the reaction proceeds in the presence of cat1, while for cat2 it increases up to 99% (Fig. S6, ESI).

Conclusions

A theoretical study of different interactions that can be established between a well-known alkaloid quinine-derived PTC and four different anions of interest in organocatalysis: Cl, Br, MeCO2 and CN was performed. The competition between the ion pairing processes with the establishment of an intermolecular hydrogen bond between the PTC counteranion and potential hydrogen bond donors was studied.

It was found that the acetate anion complexes exhibit the strongest interactions while the bromide ion complexes show the weakest interaction energy among of all them; therefore, the one establishing an intermolecular hydrogen bond presents the strongest interaction. The complex exhibiting a strict ion-pair interaction, in which both charged atoms are closest (2) (the shortest X⋯N+ distance), is 30 kJ mol−1 less stable than the HB-bound complex (1).

A theoretical study of the free-energy profile corresponding to the enantioselective conjugate cyanation of the α,β-unsaturated ketone, (E)-Chalcone, in the presence of the catalyst cat1 was performed. The barrier for both enantiomers of the cat1-bound enolate product indicates that the C–C bond formation is very similar for both pathways (ΔΔG = 0.6 kJ mol−1). From the free-energy profile both enantiomers (R and S) are energetically accessible by the cyanide, obtaining a theoretical ee value of 13%.

In order to improve the enantioselectivity of this reaction, an extra anchorage point was added, cat2. The corresponding free-energy profile was studied. The calculated ee value when the reaction proceeds in the presence of cat2 increases up to 99%.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This publication has emanated from the research supported by the Science Foundation Ireland (SFI 18/SIRG/5517). We are grateful to the Irish Centre for High-End Computing (ICHEC) for the provision of computational facilities. We are indebted to Dr Goar Sánchez from the ICHEC for the AIM graphical outputs and Prof. Ibon Alkorta for the NBO-6 software. We would like to thank Prof. Stephen Connon for all useful discussions, ideas and his continuous support.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cp02012e

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