Theoretical Elucidation on the Regio-, Diastereo-, and Enantio

May 29, 2012 - ABSTRACT: The asymmetric direct aldol reactions of aliphatic ketones. (acetone, butanone, and cyclohexanone) with 4-nitrobenzaldehyde...
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Theoretical Elucidation on the Regio-, Diastereo-, and EnantioSelectivities of Chiral Primary−Tertiary Diamine Catalyst for Asymmetric Direct Aldol Reactions of Aliphatic Ketones Xiangting Sun, Rongxiu Zhu, Jun Gao, Dongju Zhang,* and Dacheng Feng Key Lab of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong University, Jinan, 250100, People's Republic of China S Supporting Information *

ABSTRACT: The asymmetric direct aldol reactions of aliphatic ketones (acetone, butanone, and cyclohexanone) with 4-nitrobenzaldehyde catalyzed by a chiral primary−tertiary diamine catalyst (trans-N,Ndimethyl diaminocyclohexane) have been investigated by performing density functional theory calculations to rationalize the experimentally observed stereoselectivities. Focused on the crucial C−C bond-forming steps, we located several low-lying transition states and predicted their relative stabilities. The calculated results demonstrate that the catalytic direct aldol reactions of acetone favors the (S)-enantiomer and that butanone prefers the branched syn-selective product, while cyclohexanone yields predominantly the opposite anti-selective product. The theoretical results are in good agreement with the experimental findings and provide a reasonable explanation for the high enantioselectivity and diastereoselectivity, as well as regioselectivity, of the aldol reactions under consideration. syn versus anti diastereoselectivities, although Luo et al.14 have proposed plausible transition state modes to rationalize the observed syn and anti products of aldol reactions of acyclic and cyclic ketones (Scheme 1). Herein, we present a detailed computational study on the several optimal catalytic systems screened by Luo et al.14 to better understand the mechanism and stereoselectivities of the chiral primary−tertiary diamine catalyst. In recent years, density functional theory (DFT)15 has been widely used to study the thermodynamic and kinetic properties of various chemical reactions,16,17 so our calculations were conducted in the framework of DFT.

1. INTRODUCTION Among the numerous asymmetric C−C bond-forming reactions, direct aldol reaction plays a particularly important role because it represents one of the most economical ways to introduce chirality into an aldol product through asymmetric amine catalysis.1−5 During the past decades, people have witnessed the extraordinary success of chiral amines as efficient amine-based asymmetric direct aldol catalysts.1,6,7 However, highly stereoselective amine-catalyzed effects have been limited to anti-selectivity.8,9 Therefore, looking for new routes of synselective direct aldol reactions has received much research interests. In recent years, Barbas10 and Gong’s11−13 groups have independently synthesized a number of chiral primary amine catalysts that enable syn-aldol reactions of ketones. However, the reported catalysts generally offered low efficiency and only worked for few specific substrates. Thus, designing new primary amine catalysts with high efficiency and stereoselectivity, as well as broad substrate scope, becomes highly desired. Recently, Luo’s group14 reported a chiral primary−tertiary diamine catalyst for asymmetric direct aldol reactions, which allows for high enantioselectivity, diastereoselectivity, and regioselectivity for a broad range of substrates including the once challenging small aliphatic ketone donors under mild conditions. Moreover, the primary−tertiary diamine catalyst was shown to provide unprecedented syn-stereoselectivity toward acyclic ketones but excellent anti-stereoseletivity toward cyclohexanone. To the best of our knowledge, there are no theoretical illuminations concerning the origin of the opposite © 2012 American Chemical Society

2. REACTION SYSTEMS AND COMPUTATIONAL METHODS Three reaction systems to be studied in this work are summarized in Scheme 2. Reaction I denotes the aldol reaction of acetone, the smallest linear aliphatic ketone, which was found to afford high yield (84%) and excellent enantiomeric excesses (up to 94% ee). The aldol donor in reaction II is a small linear aliphatic ketone, 2-butanone, which favors the syn-aldol product (syn/anti = 10:1) with good regioselectivity (b/l = 9:1) and even higher enantiomeric excess (>95% ee). In contrast, cyclohexanone in reaction III renders the anti-selectivity Received: December 26, 2011 Revised: May 25, 2012 Published: May 29, 2012 7082

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frequency) or first-order saddle points (one imaginary frequency). The intrinsic reaction coordinate (IRC) pathways were traced to verify the energy profiles connecting transition structures to two desired minima. To obtain more reliable energies, single point energy calculations of all species involved in this work were further carried out by using the more flexible basis set 6-311+G(d, p). All calculations were carried out using the Gaussian03 program.22

Scheme 1. Sketch Map Illustrating Luo’s Transition States and Corresponding syn and anti Products of Aldol Reactions of Acyclic and Cyclic Ketones

3. RESULTS AND DISCUSSION It is well established that the asymmetrical direct aldol reactions catalyzed by primary and secondary amines proceed via enamine intermediates for the crucial C−C bond-forming step (nucleophilic addition of the enamine intermediate to an electrophilic aldehyde), which controls the stereoselectivity of products.23,24 As shown by models A and B in Scheme 1, Luo et al. assumed the (Z)- and (E)-enamine transition states to explain the syn- and anti-diastereoselectivities, and believed that the protonated tertiary amine served as a directing group through the hydrogen bonding interaction with the aldehyde.14 Thus, our study focuses on the crucial C−C bond-forming process involving the transition states of nucleophilic addition of enamine to the aldehyde, while the mechanism forming enamine intermediates is only schematically given in Scheme 3 and the relevant details are not considered here because it has been well-established in the literature.25−36 To understand the observed stereoseletivities, we have considered several stereochemical pathways for the C−C bond-forming steps of all the three reactions shown in Scheme 2. Considering the syn and anti orientations of the enamine double bond relative to the cyclohexane ring of the chiral diaminocyclohexane and its (Z)- and (E)-configurations, the enamine intermediate could have four stereoisomers, as shown in Scheme 3. Furthermore, when the enamine attacks aldehyde, the C−C bond can be formed via the different approaching modes of the re and si faces of the enamine and the carbonyl group of the aldehyde. In addition, in view of the importance of the H-bond for stabilizing the transition states, only those involving H-bond interactions between the enamine intermediates and the aldehyde are considered. For convenience in discussion, following the line of the study of Fu et al.,25,26 we have defined several dihedral angles of enamine intermediates and transition states, as shown in Scheme 4. 3.1. Enamine Intermediates. Figure 1 shows the optimized geometries and calculated relative energies of the

Scheme 2. Asymmetric Direct Aldol Reactions Studied in This Work

product (syn/anti = 1:9) with an ee value of 98%. In our calculations, the chiral primary−tertiary diamine catalyst transN,N-dimethyl diaminocyclohexane was selected for all these three reactions because it was found to show the optimal stereoselectivity for the direct aldol reactions of acetone with small aliphatic ketone donors.14 All calculations presented in this work were carried out using the popular B3LYP functional18−21 with the standard 6-31G (d) basis set for all atoms. Geometries of minima and transition states were completely optimized by minimizing the total energy with the use of analytic gradient techniques. Harmonic vibrational frequency calculations have also been conducted to verify all stationary points as minima (zero imaginary

Scheme 3. Sketch Map Illustrating the Formation of Enamine Intermediates and the Definition of Enamine Stereoisomers

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2.0 kJ/mol. For the enamines involved in reaction II, the anti(Z)-conformer is found to be the energetically most favorable geometry in the four conformers. Based on these enamine intermediates, we have performed calculations on the crucial C−C bond-forming steps controlling to the stereochemistry of reactions I, II, and III. The relevant transition state structures are illustrated in Figures 2−4 and Figures S1−3 in the Supporting Information (SI), respectively, where we listed the several key geometrical parameters, including the lengths of the forming C−C bonds, hydrogen bond distances, the dihedral angles ωI−IV (which measure the deviation of the developing iminium bond from the previous enamine planarity, ideally are 0 and ±180°, see Scheme 4), the dihedral angles ωV−VIII (which represent the different arrangements of aldehyde and enamines along the forming carbon− carbon bond, ideally are ±60° and ±180° for staggering conformation, see Scheme 4), and the relative energy (Erel) with respect to the energetically favorable transition state. The calculated barriers (ΔE, the energy demand to reach the transition state) are listed in Tables S1 and S2 in the SI. In the following sections, we will discuss these three reactions in details one by one to better understand the observed stereoselectivities. 3.2. Reaction I. For reaction I, the enamine intermediates, where the CC double bond is in the syn or anti arrangement

Scheme 4. Definition of Dihedral Angles of the Enamine Intermediates and the Transition States

stereoisomers of the enamine intermediates between the catalyst and three ketone donors. In reaction I, acetone reacts with the catalyst to form a syn-enamine and an anti-enamine, and cyclohexanone involved in reaction III gives the syn- and anti-enamines with (E)-configuration, while in reaction II, 2butanone can render (Z)- and (E)-configurations for both synand anti-enamines as approaching the catalyst. For the two enamine intemediates involved in reaction I, the syn-conformer is more favorable in energy by 6.1 kJ/mol than the anti-conformer; while in the (E)-enamines involved in reaction III, the syn-conformer lies below the anti-conformer by

Figure 1. Optimized geometries and the calculated relative energies (in kJ/mol) of the enamine intermediates formed between the catalyst and the ketone donors in the three reactions studied. 7084

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Figure 2. Selected four lower-energy transition state structures and predicted their relative stabilities for reaction I by different arrangements of aldehyde and enamine to form the corresponding stereoisomers of the product. Erel is the relative energy of the transition state respect to the energetically most favorable transition state.

relative to the cyclohexane ring of the catalyst due to the rotation of the C−N bond, can attack the re or si face of 4nitrobenzaldehyde via two sides of the double bond of the enamine. Thus, eight potential transition state (TS) structures (denoted as 1a, 1b, ..., 1h, respectively) were located for the C− C bond-formation of the aldol reaction. Of which four lowerenergy TSs 1a−1d are shown in Figure 2. The other four are shown in Figure S1 in the SI. We find that in all these transition structures, the protonated ammonium forms an effective hydrogen bond with the carbonyl oxygen atom of aldehyde, which is in similar with Luo’s transition state models shown in Scheme 1. From Figure 2, it is found that the new forming C−C bond lengths are between 1.80 and 2.14 Å. Among these transition states, 1a is predicted to be the energetically most favorable, which involves the attack of anti-enamine to the si face of the aldehyde, leading to the (S)-enantiomer, the major product

observed experimentally. The second more favorable structure in energy is 1b, which involves the attack of syn-enamine to the si face of the aldehyde, also leading to the same (S)-enantiomer, is calculated to be less favorable by 6.5 kJ/mol in energy. The other two TSs 1c and 1d, leading to the (R)-enantiomers, are predicted to be 15.2 kJ/mol (for 1c) and 17.5 kJ/mol (for 1d) higher in energy than 1a. These results are in good agreement with the observed high excellent enantiomeric excesses (up to 94% ee). The reaction preferentially proceeds via transition state 1a can be attributed to the favorable configuration of the enamine moiety and the suitable arrangement of the chemical groups adjacent to the forming C−C bond in 1a. The previous studies on catalyzed aldol processes10,25,26 have predicted that two important factors that contribute to the stereoselectivity are the deviation of the developing iminium double bond from planarity and the different arrangements of aldehyde and 7085

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Figure 3. Selected lower-energy transition state structures and predicted their relative stabilities for reaction II by different arrangements of aldehyde and enamine to form the corresponding diastereoisomers of the product. Erel is the relative energy of the transition state respect to the energetically most favorable transition state.

enamines along the forming C−C bond, and that the transition state will be more favorable in energy if the chemical groups adjacent to the developing iminium bond are closer to be planar and those around new-forming C−C bond are in the low-energy staggering orientation.10,25,26 In 1a, we find that (i) the chemical groups adjacent to the developing iminium bond are almost in a plane (ωII = 6°); (ii) the chemical groups around new-forming C−C bond tend to adopt a staggering conformation (ωVI = −178°), and (iii) the cyclohexane ring of the catalyst is far from the methyl group of the enamine moiety. All these three factors are responsible for the preferred formation of of 1a. Furthermore, we also listed the calculated barriers (ΔE) that must be overcome to reach the transition state along each active channel, as shown in Table S1 in the SI. We find that the channel involving 1a has the lowest barrier (48.1 kJ/mol), indicating this is also kinetically more favorable than other three channels. 3.3. Reaction II. It was reported that the direct aldol reaction of butanone with 4-nitrobenzaldehyde favors the synaldol product (syn/anti = 10:1) with high enantiomeric excess (>95% ee) and high regioisomer ratio (the ratio of branched and linear products b/l = 9:1). To explain the observed high enantioselectivity, diastereoselectivity, and regioselectivity, the transition state structures corresponding to the C−C bondformation step have been considered. Different from the reactions of acetone (not involving (E)- or (Z)-configuration) and cyclohexanone (only involving (E)-enamines) with 4nitrobenzaldehyde, in the aldol reaction of butanone, both (E)and (Z)-enamines are possible. A total of 24 energetically lowlying transition state structures were located at the present theoretical level. Eight lower-energy structures 2a−2h (2a−2d lead to the branched products, and 2e−2h lead to the linear

Figure 4. Selected four transition state structures and predicted their relative stabilities for reaction III by different arrangements of aldehyde and enamine to form the four diastereoisomers of the product. Erel is the relative energy of the transition state respect to the energetically most favorable transition state.

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calculated barrier values along mach each channel are also given in Table S1 in the SI. We find that the barrier that must be overcome to reach 2a is lowest (48.3 kJ/mol) among the eight transition states discussed above. 3.4. Reaction III. For reaction III, the reaction of cyclohexanone with 4-nitrobenzaldehyde, the high antidiastereoselectivity was observed. In this reaction, the enamine intermediates, where the CC double bond is in the syn or anti arrangement relative to the tertiary amino group of the catalyst due to the rotation of the C−N bond can attack the re or si face of 4-nitrobenzaldehyde from its re or si face. Here, cyclohexanone acting as the aldol donor only forms (E)enamine intermediates. Thus, we have located eight potential transition state structures for the crucial C−C bond-forming step, of which four lower-energy structures 3a−3d are shown in Figure 4. The remainder is shown in Figure S3 in the SI. It can be found that their geometrical characteristics are similar to those located in reactions I and II, that is, the formation of the C−C bond is accompanied by the formation of the effective hydrogen bond between the N−H group of enamine and the carbonyl oxygen atom of aldehyde. The four lower-energy transition states can be divided into two pairs of enantiomers, 3a versus 3c and 3b versus 3d, of which 3a is calculated to be the energetically most favorable one. It involves the attack of the si face of anti-(E)-enamine to the si face of the aldehyde, leading to the (3R,4S)-enantiomer. Structure 3c corresponds to the (3S,4R)-enantiomer, however, it lies above 3a by 18.8 kJ/mol. Thus, we believe that the experimentally observed anti-product should mainly origin from 3a. Similarly, structures 3b and 3d, which are less favorable in energy than 3a by 15.7 and 21.8 kJ/mol, result in two syn-enantiomers with configurations (3S,4S) and (3R,4R), respectively. From a purely energy point of view, we consider that the experimentally observed syn-product is formed primarily via 3b, where the re face of syn-(E)-enamine attacks the si face of aldehyde. Considering the fact that 3b is less favorable by 15.7 kJ/mol than 3a, we can easily understand the high anti diastereoselectivity observed in the experiment (syn/ anti = 1:9, 98% ee). Obviously, the channel involving 3a is energetically the most favorable, which involves a lowest barrier of 49.2 kJ/mol (Table S1 in the SI). One hand, this can be attributed to a small deviation of the developing iminium double bond from planarity in 3a, where the iminium plane almost still remain (ωII = 6°). On the other hand, in 3a, the chemical groups around new-forming C−C bond tend to adopt an even more staggering conformation (such as ωV = −55°), which leads to the small steric repulsion between the enamine moiety and 4nitrobenzaldehyde.

products) are shown in Figure 3, where the notation used, for example, “syn-(Z)” in “syn-(Z)-si−re” describes the conformers of enamine, and “si” and “re” mean the si face of enamine and the re face of aldehyde, respectively. And the remainder is shown in Figure S2 in the SI. All these transition states can be divided into two catalogues, which lead in the branched and linear products, respectively. In transition states 2a−2d, the forming carbon−carbon double bond is located between C2 and C3 atoms of butanone, leading to branched products; while in transition states 2e−2h, it is located between C1 and C2 atoms, giving the linear products. First we discuss the observed regioselectivity. As seen in Figure 3, 2a is the energetically most favorable among 2a−2d, so it should be mainly responsible for the observed branched product. This is because of the small steric repulsion between the aldehyde and the enamine intermediate. Similarly, 2e is expected to be the decisive contributor for the linear aldol product, as seen in Figure 3. We find that 2e lies above 2a by 9.8 kJ/mol. This fact rationalizes the observed high yield of the branched aldol product well. Then we focus our attention on the diastereoselectivity. Transition states 2a−2d correspond to the four addition products: two syn-aldol adducts (3S,4S) and (3R,4R), and two anti-aldol adducts (3S,4R) and (3R,4S). As mentioned above, 2a is the energetically most favorable transition state, which involves the re face attack of the anti-(Z)-enamine to the si face of aldehyede leading to the (3S,4S)-enantiomer, a syn-aldol product. Transition state 2c also results in the syn-aldol product, however, it is less favorable in energy by 17.3 kJ/mol than 2a. Thus we believe that the observed syn-aldol adduct should be formed mainly from 2a. The second favorable structure in energy is 2b, which involves the si attack of the anti-(E)-enamine to si face of aldehyede leads to the anti-aldol adduct, the (3R,4S)-enantiomer. Although 2d also results in the anti-aldol products (3S,4R), it can not compete with 2b because it lies above 2b by 12.5 kJ/mol. So the obverted anti-aldol product should mainly come from 2b. It should be noted that 2b is less favorable in energy by 7.0 kJ/mol than 2a. This clearly indicates that the syn-aldol product should be dominant, which is in good agreement with the experimentally observed high diastereoselectivity (syn/anti = 10:1). Our concern turns to the enantioselectivity. As shown in Figure 3, the energetically most favorable transition state that results in the (3R,4R)-enantiomer (syn-aldol adduct) is 2c, while 2a is considered as the main contributor to the (3S,4S)enantiomer (syn-aldol adduct). We find that 2c is less favorable in energy by 17.3 kJ/mol than 2a, which agrees with the observed syn-aldol product with high enantioselectivity (96% ee).14 From the calculated relative energies of transition states shown in Figure 3, we have rationalized the high regioselectivity, diastereoselectivity, and enantioselectivity of reaction II. Our calculations indicate that these high selectivities closely relates to the transition state geometries involved. In structure 2a, the chemical groups around the new-forming C− C bond adopt a nearly staggering arrangement, which results in the smaller steric repulsion between the phenyl of the 4nitrobenzaldehyde and the methyl in the enamine intermediate as compared with those in 2b and 2d. For example, ωV = −53° and 13° in 2b and 2d, while it is −65° in 2a, as shown in Figure 3. So we can conclude that the most important factor that controls the stereoselectivity of the reaction is steric repulsions between 4-nitrobenzaldehyde and the enamine moiety. The

4. CONCLUSION The details of the crucial C−C bond-formation step for the reaction of acetone, butanone, and cyclohexanone with 4nitrobenzaldehyde catalyzed by a chiral primary−tertiary diamine catalyst (trans-N,N-dimethyl diaminocyclohexane) have been shown by performing DFT calculations at the B3LYP/6-31G(d) level. The favorable (S)-product for the reaction of acetone generates from anti-enamine, and the C−C bond formation involves the attack of the anti-emamine to the si face of aldehyde. The observed branched syn-product for the reaction of butanone originates from the anti-(Z)-enamine intermediate, and the C−C bond formation relates to the attack of the re surface of the anti-(Z)-enamine to the si face of the 7087

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aldehyde. The opposite anti product for the reaction of cyclohexanone starts from anti-(E)-enamine intermediate, and the C−C bond formation corresponds to the attack of the si face of anti-(E)-enamine intermediate to the si face of the aldehyde. Our calculated results confirm that the aldol reactions catalyzed by chiral trans-N,N-dialkylated diaminocyclohexanes present good enantioselectivity and opposite syn−anti diastereoselectivities for different ketones. The theoretical results are in good agreement with the experimental findings and provide a reasonable explanation of the high enantioselectivity, diastereoselectivity, and regioselectivity for the aldol reactions under consideration.

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ASSOCIATED CONTENT

S Supporting Information *

Energetically less favorable transition state structures involved in reactions I−III with the calculated relative energies, Cartesian coordinates for all optimized structures, and the complete author list for ref 19. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-531-88365833. Fax: +86-531-88564464. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 20873076). REFERENCES

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