Transition State Modeling of Asymmetric Epoxidation Catalysts

Chapter 3

Transition State Modeling of Asymmetric Epoxidation Catalysts K. N. Houk, Jian Liu, and Thomas Strassner

Downloaded by UNIV OF ARIZONA on August 7, 2012 | http://pubs.acs.org Publication Date: April 8, 1999 | doi: 10.1021/bk-1999-0721.ch003

Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569

The catalytic epoxidation of alkenes produces optically active epoxides from prochiral precursors. Reaction of peracids, dioxiranes, oxaziridines, and oxaziridinium cations with alkenes have been explored with density functional theory. Examples of catalytic reactions involving chiral ketones, and iminium cations, using Oxone as oxidant, have been modeled by quantum mechanical and force field methods. The geometries and multiplicities of the manganese-oxo intermediates involved in the Jacobsen asymmetric catalytic epoxidation using manganese-salen complexes have been explored.

A large number of catalytic and stoichiometric asymmetric epoxidations have been developed (1). What are the mechanisms of these reactions, and what is the origin of their high selectivities? It is the purpose of our theoretical studies to understand how these reactions work, to develop computational models to be used for the prediction of reactions of new substrates, and to design new catalysts. This paper describes the current status of our work. Figure 1 shows examples of some stereoselective epoxidations using a variety of optically active epoxidizing agents, such as oxaziridines (2), peroxyacids (3-6), and dioxiranes (7-11). Peracids and hydroperoxides give low enantioselectivities. There are now many examples of dioxirane epoxidations producing optically active products, and several groups are working actively on catalytic versions of these reactions. Davis and coworkers have prepared oxaziridines which epoxidize alkenes with moderate stereospecificity (2). A few examples of chiral oxaziridinium compounds have been reported (12-17).

© 1999 American Chemical Society

In Transition State Modeling for Catalysis; Truhlar, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

33


34

Hydroperoxides

Oxaziridines

Figure 1. Examples of stereoselective epoxidations. Dioxirane and oxaziridinium reactions have been made catalytic. Figure 2 outlines the general strategy for catalytic asymmetric epoxidations. For this cycle to be effective, the oxidation of the ketone, imine or iminium should be efficient, and in the case of imines and iminium, the oxidation should be stereoselective, or else diastereoisomeric oxaziridine derivatives will be formed.

NaHS0

NaHS0

4

5

(Oxone) or other Figure 2. Catalytic cycle for epoxidation by dioxiranes (X=0), oxaziridine (X=NR), or oxaziridinium (X=NR2 ). +


35 Our initial work established the utility of B 3 L Y P calculations for the prediction of transition states. In a collaboration with Dan Singleton at Texas A & M , we compared computed isotope effects for several different transition structures to experimental (18).

(

" ,A -° * If — A * R

0

0


O

° * If — J ' * ocf

Figure 3. Prototype epoxidations. Pioneering calculations by Bach and others (19-24) have provided insights into the nature of epoxidation transition states. Over the years, as increasingly accurate computational methods became available, models of epoxidation transition states have evolved. For the extensively discussed oxaziridine epoxidations, we have found that the spiro transition structures are always favored, but either synchronous or asynchronous transition structures can be formed (25). Figure 3 shows the reactions we have studied in detail. Computational Methodology Calculations reported here were conducted using the Gaussian94 program (26). Full geometry optimizations were carried out using density functional theory with the Becke3LYP functional and the 6-31G* basis set. Both restricted and unrestricted Becke3LYP/6-31G* were used for the transition structure searches for the parent systems with oxygen transfer from unsubstituted performic acid, dioxirane, peroxynitrous acid and oxaziridine to ethylene. Frequency calculations have been carried out for all the transition structures to ensure the presence of only one imaginary frequency corresponding to C - 0 bond forming and 0 - X (X= O, N) bond breaking. The optimized reactants were also checked by frequency calculations in order to confirm that they are minima. Charges cited in the discussions are Mulliken charges. Transition Structures for the Epoxidation of the Parent Systems. Our previous results for the parent systems are summarized in Figure 4, which shows the transition states for epoxidations of ethylene by performic acid, dioxirane, peroxynitrous acid (27), and oxaziridine calculated by Becke3LYP/6-31G* method. The transition structures for the epoxidations by performic acid and by dioxirane are very similar: the breaking 0 - 0 bond lengths are 1.86 A and 1.87 A, respectively, and the two forming C - 0 bond lengths are 2.03 A and 2.01 A, respectively.


36 O

A E = 14.1 kcal/mol


a

AE = 13.2 kcal/mol a

A E = 12.9 kcal/mol a

A E = 33.0 kcal/mol a

Figure 4. Transition structures (B3LYP/6-31G*) for epoxidations of ethylene by performic acid, dioxirane, peroxynitrous acid, and oxaziridine. The activation energies for the epoxidations by performic acid and dioxirane by both Becke3LYP/6-31G* and MP2/6-31G* method are in reasonable agreement with experimental results in solution, which are 15-18 kcal/mol for performic acid (28) and 14.1 kcal/mol for dioxirane (29). The epoxidation of ethylene by oxaziridine has been studied by different methods (19-25). Previous calculations using MP2/6-31G* yielded a synchronous transition structure (19-24). Peroxynitrous acid is a very short-lived species which has not yet been observed to epoxidize alkenes (27). The activation energy for the oxygen transfer from peroxynitrous acid to ethylene is only 13.2 kcal/mol by Becke3LYP/6-31G* theory. It has an asynchronous transition structure, in between the synchronous peracid and the asynchronous oxaziridine in character. The difference in energy between the spiro and planar geometries was assessed computationally using Becke3LYP/6-31G*. The preference for spiro increases as the length of the most fully formed CO bond decreases, from 5.1 kcal/mol with performic acid to 11.4 kcal/mol with oxaziridine. In all cases, there is a very large spiro preference. There is obvious diradical character in the asynchronous transition structures of epoxidation by oxaziridine. The partial radical centers are on the N terminus of the oxaziridine and one C-terminus of ethylene. Because of this, we also explored these reactions with unrestricted DFT theory (UBecke3LYP/6-31G*) for the parent system. We have demonstrated the suitability of U B 3 L Y P calculations for reactions involving diradicals and have discussed elsewhere the


37


potential problems in radical calculations (30). In the case of the oxaziridine transition state, the unrestricted solution is more stable than the restricted by 4.7 kcal/mol, and the spin densities are -0.68 and 0.53 at the N and C termini, respectively. Transition structures for epoxidations of ethylene substituted by four different groups - methoxy, methyl, vinyl and cyano, ranging from electrondonating to electron-withdrawing - were located. We concentrate here on the reactions of dioxirane, which are characteristic of all the oxidants, and are most relevant to the observed catalytic processes. For the oxygen transfer from dioxirane, all the transition structures are asynchronous, with the substituted groups on the carbon with the longer forming C - 0 bonds (Figure 5).

AEa= 10.2 kcal/mol

AEa= 15.2 kcal/mol

Figure 5. Transition structures (B3LYP/6-31G*) for epoxidations of substituted ethylenes by dioxirane and ethylene by methyldioxirane. Unsymmetrical substitution such as that of methyl on the dioxirane causes the transition structure to become somewhat asynchronous. The methoxy, methyl and vinyl group all decrease the activation energies of the epoxidation reaction, while the electron-withdrawing cyano group increases the activation energy. The transition structure for oxygen transfer from oxaziridine to ethylene is already very asynchronous for the parent system, and the four different substituents have little influence on the timing of the two forming C - 0 bonds (Figure 6). A l l four substituents decrease the activation energies as compared to the parent system, with the methoxy and vinyl groups stabilizing the transition structures the most. Instead of destabilizing the transition structures as in the performic acid and dioxirane system, the cyano group decreases the activation energy of epoxidation


38 by oxaziridine. This reflects the significant diradical character of the transition structure for the oxaziridine system.


-0.23

AE = 27.4 kcal/mol

AE = 31.4 kcal/mol

a

a

-0.14

AE = 27.4 kcal/mol a

AE = 29.8 kcal/mol

Figure 6. Transition structures (B3LYP/6-31G*) for epoxidations of substituted ethylenes by oxaziridine. The partial charges are given next to the heavy atoms in the Figures. A l l the transition structures have no more than 0.35 charge transfer from alkene to the epoxidizing reagent, indicating diradical, rather than zwitterionic, character for the transition structures. The more asynchronous the transition structures are, the less the charge transfer, and the more diradical character. There is very little CO bondbreaking in the dioxirane and oxaziridine reactions. Figure 7 summarizes the principal frontier orbital interactions occurring in the transition states. Relatively early transition states involve SN2-like interactions have been described in detail by Bach (22). The alkene H O M O interacts in a stabilizing fashion with the OX a* orbital, and a synchronous geometry is favored. When the leaving group is less able to stabilize a negative charge, the transition state becomes more advanced and SH2 in character, which favors an asynchronous geometry. This involves interaction of the O lone pair with the alkene 7t* orbital,


39


which is enhanced in an asynchronous geometry. Those two F M O interactions give a diradicaloid transition state when they are comparable in magnitude.

Figure 7. F M O interactions in the transition states of epoxidations: (a) alkene HOMO-oxidant L U M O and (b) oxidant HOMO-alkene L U M O .

The trajectory predicted by this model follows that trajectory which is wellknown for carbene cycloadditions. The spiro geometry is favored significantly, because the oxygen lone-pair H O M O interaction with the alkene L U M O is maximized in the spiro geometry but is eliminated in the planar transition state. The importance of this increases as the transition state becomes more advanced. This transition state model provides a good qualitative explanation for stereoselectivities which have been observed. Figure 8 shows the transition states of epoxidation of frans-stilbene by one of the Davis chiral oxaziridine reagents (2) and the Yang chiral binaphthyl dioxirane (5). The transition states were built by constraining the five atoms involved in bonding changes to the transition state optimal geometry determined by DFT calculation and minimizing all the other parts by M M 2 * using MacroModel 5.0 (31). The energy difference predicted between the transition states corresponding to experimentally favored and disfavored products are 4.2 kcal/mol for oxaziridine and 2.0 kcal/mol for dioxirane (Figure 8). This energy difference corresponds to a higher selectivity than that observed by experiment, but the models are nevertheless useful for the prediction of the sense of the observed selectivity.


40

(R.R) 4.2 kcal/mol


(S.S) 0.0 kcal/mol

Oil

(S.S) 0.0 kcal/mol

(R,R) 2.0 kcal/mol

Figure 8. Force field models of asymmetric epoxidations with a Davis oxaziridine and a Yang chiral dioxirane. The factors which control the stereoselectivity are shown in Figure 9. In the oxaziridine, experimental results can be rationalized by the model shown: on the left carbon of the alkene, the larger substituent prefers to be back, away from the protruding H of the oxaziridine. At the other terminus,fl-stackingcauses aromatic substituents to prefer the forward position. Alkyl or bulky groups will prefer to be back. Radical stabilization causes the substituents to be favored at one terminus in an unsymmetrical case; rr-stacking as shown in the sketch and steric hindrance, involving groups on the reagent and on the alkene, dictate the position of groups in the transition state. The qualitative model shown for dioxirane epoxidation also rationalizes experimental data obtained by Yang (27). Here the interaction of the larger dioxirane substituent with the alkene substituent dominates, and fran,?-alkenes place groups at S1 and L in spite of the fact that the large substituent on the left carbon would prefer to be back.


41

Z'OgS % re-stacking attractive * interaction favors Aromatic substituent.


Steric repulsion with oxaziridine hydrogen requires a smaller substituent in this position on the alkene

Steric interaction between R l and SI, requiring the smaller alkene substituent/^N in the S1 position. p j '

Larger substituent on the dioxirane (R2) causes a tilt to reduce repulsion between R2 and the alkene. The smallest alkene substituent prefers the S2 position.

Figure 9. Qualitative models to rationalize stereoselective epoxidations by oxaziridines and dioxiranes.

Catalytic epoxidations involving oxaziridinium intermediates are most promising. Iminium salts can be oxidized to oxaziridinium salts very easily in situ, and the oxaziridinium salts can epoxidize alkenes (12-17). Iminium salts are organic salts which can have good solubility in aqueous and organic solvents. Hence, iminium salts can act as catalysts for epoxidation reactions using oxidants such as Oxone. The pioneering work by Hanquet and Lusinchi demonstrates a catalytic reaction involving an iminium salt. The iminium salt can be converted by Oxone to the corresponding oxaziridinium which effectively epoxidize alkenes (Figure 10) (12-15). In the investigation of the asymmetric epoxidation catalyzed by chiral iminium salts, two chiral iminium salts, 1 and 2, have been reported (12-16). The asymmetric epoxidations of stilbene catalyzed by both chiral iminium salts give promising results of about 30% ee, and the epoxidation of 1-phenylcyclohexene catalyzed by 2 gives 71% ee. In a recent paper by Armstrong and co-workers (77), a series of pyrrolidine-derived iminium salts 3 of different substituted benzaldehyde have been prepared and used in the catalytic epoxidation. The electron-releasing substituent MeO- on phenyl completely deactivates the catalyst, while the electronwithdrawing group such as 0 - C F 3 or 0-Cl increases the catalytic activity of the iminium salt.



42

1

2

3

X= H, p-MeO, p-CI, o-CI, 0-CF3,2,6-dichloro

Figure 10. Iminium ion catalysts for epoxidations involving oxaziridinium ions.

B3LYP/6-31G* calculations predicts a symmetric spiro transition structure (TS1 in Figure 11) for the epoxidation of ethylene by oxaziridinium, similar to that obtained for performic acid or dioxirane. The activation energy for oxygen transfer from oxaziridinium to ethylene is only 4.9 kcal/mol, and the transition state is a very early one. The two forming epoxide C - 0 bonds are 2.6 A, much longer than those of about 2.0 A in the epoxidation by performic acid or dioxirane. ITie bond length of O-N bond has elongated from 1.44 A to 1.57 A in the transition state. The influence of the methyl substituent on the transition structure has been investigated. Figure 11 also shows the transition states for the epoxidations of ethylene by N,N-dimethyl oxaziridinium (TS2), N-methyl oxaziridinium (TS3) and C-methyl oxaziridinium (TS4). The methyl substituents on oxaziridinium make the transition structure substantially later than TS1, but it is still quite early. The geometries of transition states still are spiro synchronous, even though TS3 and TS4 have unsymmetrical substituents. Since all these epoxidations by oxaziridiniums have early transition states, the methyl substituents do not have as big an influence on the synchronicity of the transition structure as those for dioxirane, performic acid and oxaziridine. The transition state TS3 and TS4 have negative activation energies, indicating that a complex between the cation and ethylene forms before the transition states. Compared to the epoxidation by performic acid, dioxirane and oxaziridine, calculations predicts much lower activation energy for the epoxidation by oxaziridinium. Experimentally, epoxidations by oxaziridinium generated from 1 are faster than m-CPBA epoxidations, since a catalytic amount of iminium 1 can accelerate the reaction, similar to trifluoroacetone. The calculation most probably



43

1.35

AE = -1.0 kcal/mol a

TS3

AE = -1.4 kcal/mol a

TS4

Figure 11. Transition structures for the epoxidations of ethylene by oxaziridinium (TS1), N,N-dimethyl oxaziridinium (TS2), N-methyl oxaziridinium (TS3), Cmethyl oxaziridinium (TS4).


44


underestimated the activation energy, since the transition state was located in gas phase, and there is likely to be a large solvent effect for the cationic mechanism. TS2 has been used to build transition state models for the study of epoxidations by chiral oxaziridiniums in the same way we have modeled epoxidations by oxaziridines and dioxiranes. We have tried to use force field modeling to elucidate the selectivity for the epoxidation of frans-stilbene catalyzed by iminium salt 1. Since it was reported that oxaziridinium ring can only form on the side of the methyl substituent of iminium salt while the other side is blocked by the phenyl substituent, we used the oxaziridinium formed from iminium salt 1 for modeling. Figure 12 shows the modeled transition structures for the epoxidations of mzns--stilbene by which lead to (R,R) and (S,S) epoxides.

The modeling predicts that the transition structure which leads to (R,R) is 1.3 kcal/mol lower in energy than stilbene-l-(S,S). The steric hindrance of the two phenyl rings in stilbene-l-(S,S) cause the higher energy of this model. The calculation is consistent with the experiment which gave 33% ee (R,R) selectivity. The top face of the iminium double bond is blocked by substituent groups. The bottom face of the iminium double bond is open to the formation of oxaziridinium.

Jacobsen Oxidations. We have followed avidly the development of manganese salen catalyzed epoxidation reactions and have developed models to understand the stereoselectivity (32). The reaction is summarized in Figure 13.


45 o R NaCI


NaOC!

(L)

Figure 13. The Jacobsen asymmetric epoxidation. There have been several suggestions about the mechanism of this reaction, including a concerted reaction, a stepwise process involving diradical intermediates, and a stepwise process involving a manganooxetane intermediate. A recent article by Jacobsen has summarized electronic effects on stereoselectivity and has used a stepwise/diradical model to interpret the results (34). Here, we concentrate on characterization of geometries and multiplicities of the metal-oxo intermediates, and consider the possibilities that two states of different multiplicity are involved in these reactions, analogous to the model recently proposed for cytochrome p-450 reactions (35). Calculations were performed with B3LYP using the split valence doublezeta (DZ basis set, 3-21G, or a D Z plus polarization basis set, 6-31G*, for C, H , N , and O, together with double-zeta (DZ) (36) or triple zeta (TZ) (37) valence basis sets for manganese. The model systems used in these calculations are shown in Figure 14.

Figure 14. The models for Mn(V)-oxo complexes.

Calculations were performed on the structures in the oxidation states +111 to compare to existing X-ray data and +V to make predictions about the structure of the actual catalyst. The manganese (III) compounds have d and the manganese (V) compounds d electronic configurations. For the d electron configuration on an octahedral field, each electron will occupy one of the ^-orbitals with parallel spins. 4

2

2



46 It is well known that the Mn (III) favors an octahedral coordination sphere, while litde is known about manganese (V) compounds. The geometries of the Mn (HI) complexes compare favorably with ample X ray crystallographic data available in the literature. These results will be reported elsewhere (38). For the manganese (V) systems, calculations on oxo compounds lacking the CI ligand give the geometries shown in Figure 15. Both calculations predict low- and high-spin to be close in energy. The best calculations predict the low-spin system to be lower in energy by 3.5 kcal/mol. By contrast, with the CI ligand, the high-spin state is 10.5 kcal/mol more stable. Thus, with relatively strong ligands, the triplet is the ground state, and reactions are expected to be the characteristic of a diradical (35): the Jacobsen mechanism (34) is fully consistent with this. By removal of the ligand, or perhaps by reducing to a less nucleophilic ligand, there may be a change of the spin state from a triplet species to a singlet spin state. This may be related to the fact that the experimental results show that for different ligands (39) or ligand substitutents (34), there might be different reaction pathways and enantioselectivities (39). The nature of the ligand might play a role for the stereoselectivity of the reaction.

Figure 15. The low-spin (singlet) and high-spin (triplet) forms of the M n (V) model system.

Figure 16. Manganese (V) model compounds calculated by the manganese triplezeta basis/Becke3LYP/6-31G(d). The multiplicity of the oxo-complex is important for the course of the reaction. A high-spin complex is more likely to react via a stepwise mechanism which involves a radical intermediate than a low-spin complex which would more easily react via a concerted pathway. On the other hand, the multiplicity of the Mn (III) catalyst is quintet, and so the correlation may be much more complex than indicated in this simple picture. These findings make a recently proposed (2+2) intermediate unlikely, but would very well agree with a radical intermediate in the transition state. We are currently exploring possible transition state structures.


47


Acknowledgments. We thank Kevin R. Condroski and Nicholas C. DeMello for initial results and helpful discussions. We are grateful to the 'Deutsche Forschungsgemeinschaft' for a fellowship (T.S.) and to the National Institute of General Medical Sciences, National Institutes of Health for financial support of this research. Literature Cited. (1) Jacobsen, E. N. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York, 1993; Chapter 4.2. (2) Davis, F. A.; Sheppard, A. C. Tetrahedron 1989, 45, 5703-5742, and references therein. (3) Ewins, R. C.; Henbest, H. B.; McKervey, M. A. Chem. Commun. 1967, 1085. (4) Monlanari, F.; Moretti, J. ; Torre, G. Gazz. Chim. Ital. 1974, 104, 7. (5) Pirkle, W. H.; Rinaldi, P. L. J. Org. Chem. 1977, 42, 2080-2082. (6) For a claim of planar transition state, see Rebek, Jr., J.; Marshall, L.; McMairy, J.; Wolak, R. J. Org. Chem. 1986, 51, 1649. (7) Curci, R.; D'Accolti, L.; Fiorentino, M.; Rosa, A. Tetrahedron Lett. 1995, 36, 5831. (8) Murray, R. W.; Singh, M.; Williams, B. L.; Moncrieff, H. M. Tetrahedron Lett. 1996, 36, 2437. (9) Yang, D.; Yip, Y.-C.; Tang, M.-W.; Wong, M.-K; Zheng, J.-H.; Cheung, K.K. J. Am. Chem. Soc. 1996, 118, 491-492. (10) Tu, Y.; Wang, Z.-X.; Shi, Y. J. Am. Chem. Soc. 1996, 118, 9806. (11) Adam, W.; Smerz. A. K. J. Org. Chem. 1996, 61, 3506. (12) Hanquet, G.; Lusinchi, X.; Milliet, P. Tetrahedron Lett. 1988, 29, 3941. (13) Hanquet, G.; Lusinchi, X. Tetrahedron Lett. 1993, 24, 5299. (14) Bohe, L.; Hanquet, G.; Lusinchi, M.; Lusinchi, X. Tetrahedron Lett. 1993, 34, 7271. (15) Lusinchi, X.; Hanquet, G. Tetrahedron 1997, 53, 13727. (16) Aggarwal, V. K.; Wang, M. F. Chem. Commun. 1996, 191. (17) Armstrong, A.; Ahmed, G.; Garnett, I.; Goacolou, K. SYNLETT 1997, 1075. (18) Singleton, D. A.; Merrigan, S. R.; Liu, J.; Houk, K. N. J. Am. Chem. Soc. 1997, 119, 3385-3386 (19) Bach, R. D.; Coddens, B. A.; McDouall, J. J. W.; Schlegel, H. B.; Davis, F. A. J. Org. Chem. 1990, 55, 3325-3330. (20) Bach, R. D.; Andres, J. L.; Davis, F. A. J. Org. Chem. 1992, 57, 613-618. (21) Bach, R. D.; Winter, J. E.; McDouall, J. J. W. J. Am. Chem. Soc. 1995, 117, 8586-8593, and references therein. (22) Bach, R. D.; Glukhovtsev, M. N. Gonzalez, C.; Marquez, M.; Estevez, C. M.; Baboul, A. G.; Schlegel, H. B. J. Phy. Chem. 1997, 101, 6092. (23) Yamabe, S.; Kondou, C.; Minato, T. J. Org. Chem. 1996, 61, 616-620. (24) Bach, R. D.; Canepa, C.; Winter, J. E.; Blanchette, P. E. J. Org. Chem. 1997, 62, 5191. (25) Houk, K. N.; Liu, J.; DeMello, N. C.; Condroski, K. R. J. Am. Chem. Soc. 1997, 119, 10153. (26) Gaussian 94 (Revision A.1), Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Peterson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Oriz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;



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Wong, M . W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian, Inc., Pittsburgh PA, 1995. (27) Houk, K. N.; Condroski, K. R.; Pryor, W. A., J. Am. Chem. Soc. 1996, 118, 13002. (28) Dryuk, V. G. Tetrahedron 1976, 32, 2855. (29) Murray, R. W. Chem. Rev. 1989, 89, 1187. (30) Goldstein, E.; Beno, B.; Houk, K. N. J. Am. Chem. Soc. 1996, 118, 6036. (31) Macromodel V5.0: Mohamadi, F.; Richards, N . G. J.; Guida, W. C.; Liskamp. R.; Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still W. C. J. Comput. Chem. 1990, 11, 440. MM2* is the MacroModel implementation of MM2. (32) Houk, K. N.; DeMello, N.; Condroski, K.; Fennen, J.; Kasuga, T. Electronic Conference on Heterocyclic Chemistry (www.choic.ac.uk/ectoc/echet96/ echet_ma.html), 1996. (33) Katsuki, T. J. Mol. Catalysis A: Chemical 1996, 113, 87-107. (34) Palucki, M . ; Finney, N. S.; Pospisil, P. J.; Güler, M . L.; Ishida, T.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 948-954, and references therein. (35) Shaik, Sason, Filatov, M.; Schroder, D.; Schwarz, H. Chem. Eur. J. 1998, 4, 193-199. (36) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Phys. Chem. 1992, 97, 2571. (37) Schäfer, A.; Huber, C.; Ahlrichs, R. J. Phys. Chem. 1994, 100, 5829. (38) Strassner, T.; Houk, K. N. In preparation. (39) Irie, R.; Noda, K.; Ito, Y.; Matsumoto, N.; Katsuki, T. Asymmetry 1991, 2, 481.


Transition State Modeling of Asymmetric Epoxidation Catalysts

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