Timing is critical: Effect of spin changes on the diastereoselectivity in Mn(salen)-catalyzed epoxidation

Journal of the American Chemical Society

Volumn 121, Issue 21, 1999, Pages 5083-5084

  Linde, Christian ^a   Åkermark, Björn ^a   Norrby, Per Ola ^b   Svensson, Mats ^a  

^a ROYAL INSTITUTE OF TECHNOLOGY   (Sweden)

^b UNIVERSITY OF COPENHAGEN   (Denmark)

Author keywords

[No Author keywords available]

Indexed keywords

ALKENE; MANGANESE;

ARTICLE; CATALYST; CHEMICAL BOND; DIASTEREOISOMER; ELECTRON; ENANTIOMER; EPOXIDATION; ISOMERISM; MOLECULAR INTERACTION; REACTION ANALYSIS; REACTION TIME; STEREOCHEMISTRY; STRUCTURE ANALYSIS; THERMODYNAMICS;

EID: 0033516282     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja9809915     Document Type: Article

References (36)

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5. Linker, T. Angew. Chem., Int. Ed. Engl. 1997, 36, 2060-2062.
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7. (a) Srinivasan, K.; Michaud, P.; Kochi, J. K. J. Am. Chem. Soc. 1986, 108, 2309-2320.
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9. (c) Palucki, M.; Finney, N. S.; Pospisil, P. J.; Güler, M. L.; Ishida, T.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 948-954.
10. (a) Norrby, P.-O.; Linde, C.; Akermark, B. J. Am. Chem. Soc. 1995, 117, 11035-11036.
11. (b) Hamada, T.; Fukuda, T.; Imanishi, H.; Katsuki, T. Tetrahedron 1996, 52, 515-530.
12. (c) Adam, W.; Fell, R. T.; Stegmann, V. R.; Saha-Möller, C. R. J. Am. Chem. Soc. 1998, 120, 708-714.
13. Reaction of styrenes substituted with hypersensitive radical clocks indicate that radicals are not intermediates in the formation of cis-epoxides, see: Linde, C.; Arnold, M.; Norrby, P.-O.; Akermark, B. Angew Chem., Int. Ed. Engl. 1997, 36, 1723-1725.
14. Buschmann, H.; Scharf, H.-D., Hoffmann, N.; Esser, P. Angew. Chem., Int. Ed. Engl. 1991, 30, 477-515. Eyring nonlinearities generally indicate that at least two TSs influence the reaction. Small changes in substrates or reaction conditions could easily render the breaks undetectable, and no conclusions can therefore be drawn from the absence of such breaks.
15. B3LYP: Stevens, P. J.; Devlin F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. We used a (14s,11p,6d) primitive basis from Wachters, A. J. H. J. Chem. Phys., 1970, 52, 1033. augumented by two p and one diffuse d function contracted to [6s,5p,3d]. On substrates and ligands we used the double-ζ basis set from Dunning, T. H.; Hay, P. J Modern Theoretical Chemistry; Plenum Press: New York, 1977.
16. B3LYP: Stevens, P. J.; Devlin F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. We used a (14s,11p,6d) primitive basis from Wachters, A. J. H. J. Chem. Phys., 1970, 52, 1033. augumented by two p and one diffuse d function contracted to [6s,5p,3d]. On substrates and ligands we used the double-ζ basis set from Dunning, T. H.; Hay, P. J Modern Theoretical Chemistry; Plenum Press: New York, 1977.
17. B3LYP: Stevens, P. J.; Devlin F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. We used a (14s,11p,6d) primitive basis from Wachters, A. J. H. J. Chem. Phys., 1970, 52, 1033. augumented by two p and one diffuse d function contracted to [6s,5p,3d]. On substrates and ligands we used the double-ζ basis set from Dunning, T. H.; Hay, P. J Modern Theoretical Chemistry; Plenum Press: New York, 1977.
18. Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. J. Am. Chem. Soc. 1991, 113, 7063-7064.
19. Oxomanganese(V) was early postulated as the active species, but not until recently was direct proof found for its existence: (a) Feichtinger, D.; Plattner, D. A. Angew. Chem., Int. Ed. Engl. 1997, 36, 1718-1719. (b) Groves, J. T.; Lee, J.; Maria, S. S. J. Am. Chem. Soc. 1997, 119, 6269-6273.
20. Oxomanganese(V) was early postulated as the active species, but not until recently was direct proof found for its existence: (a) Feichtinger, D.; Plattner, D. A. Angew. Chem., Int. Ed. Engl. 1997, 36, 1718-1719. (b) Groves, J. T.; Lee, J.; Maria, S. S. J. Am. Chem. Soc. 1997, 119, 6269-6273.
21. Calculations by Jorgensen et al. (Rasmussen, K. G.; Thomsen, D. S.; Jorgensen, K. A. J. Chem. Soc., Perkin. Trans. 1, 1995, 2009.), using INDO calculations, have earlier shown that the ground state for a related Mn(V)O system is the triplet, 71 kJ/mol lower than the singlet. These calculations also show that the singlet has higher MnO bond order than the triplet.
22. The reason for the small energy difference between the triplet and quintet states is the weak π-bond between the oxo group and the manganese center which in turn contributes to the high reactivity of the oxomanganese-(salen) complex.
23. (a) Bernardi, F.; Bottoni, A.; Canepa, C.; Olivucci, M.; Robb, M. A.; Tonachini, G. J. Org. Chem. 1997, 62, 2018-2025.
24. (b) Singleton, D. A.; Merrigan, S. R.; Liu, J.; Houk, K. N. J. Am. Chem. Soc. 1997, 119, 3385-3386.
25. The two C-O distances were kept equal in the calculations of the synchronous attack.
26. Structure 2 is proven to be a true alkene addition TS, whereas the perpendicular counterpart has two imaginary frequencies where the lower mode corresponds to a rotation to 2. However, steric influences from a substituted salen ligand could affect this energy difference.
27. From MM3-calculations on analogues, the barrier to rotation in the benzylic radical derived from styrene is estimated to 5-10 kJ/mol. Cis substitution lowers the barrier further.
28. + (a) Danovich, D.; Shaik, S. J. Am. Chem. Soc. 1995, 119, 1773-1786. (b) Shaik, S.; Filatov,M.; Schröder, D.; Schwarz, H. Chem.- Eur. J. 1998, 4, 193-199. (c) Shaik, S.; Danovich, D.; Fiedler, A.; Schröder, D.; Schwarz, H. Helv. Chim. Acta 1995, 78, 1393-1407. (d) Mitchell, S.; Blitz, M.; Siegbahn, P.; Svensson, M. J Chem. Phys. 1994, 100, 423-433. In our case, such calculations are very difficult due to the size of the system. The fact that the triplet and quintet surfaces in this system are nearly parallell also makes it difficult to determine the exact crossing point. However, the fact that the surfaces are parallell and close in energy also indicates a high spin transition probability according to LZ formula.
29. + (a) Danovich, D.; Shaik, S. J. Am. Chem. Soc. 1995, 119, 1773-1786. (b) Shaik, S.; Filatov,M.; Schröder, D.; Schwarz, H. Chem.-Eur. J. 1998, 4, 193-199. (c) Shaik, S.; Danovich, D.; Fiedler, A.; Schröder, D.; Schwarz, H. Helv. Chim. Acta 1995, 78, 1393-1407. (d) Mitchell, S.; Blitz, M.; Siegbahn, P.; Svensson, M. J Chem. Phys. 1994, 100, 423-433. In our case, such calculations are very difficult due to the size of the system. The fact that the triplet and quintet surfaces in this system are nearly parallell also makes it difficult to determine the exact crossing point. However, the fact that the surfaces are parallell and close in energy also indicates a high spin transition probability according to LZ formula.
30. + (a) Danovich, D.; Shaik, S. J. Am. Chem. Soc. 1995, 119, 1773-1786. (b) Shaik, S.; Filatov,M.; Schröder, D.; Schwarz, H. Chem.- Eur. J. 1998, 4, 193-199. (c) Shaik, S.; Danovich, D.; Fiedler, A.; Schröder, D.; Schwarz, H. Helv. Chim. Acta 1995, 78, 1393-1407. (d) Mitchell, S.; Blitz, M.; Siegbahn, P.; Svensson, M. J Chem. Phys. 1994, 100, 423-433. In our case, such calculations are very difficult due to the size of the system. The fact that the triplet and quintet surfaces in this system are nearly parallell also makes it difficult to determine the exact crossing point. However, the fact that the surfaces are parallell and close in energy also indicates a high spin transition probability according to LZ formula.
31. + (a) Danovich, D.; Shaik, S. J. Am. Chem. Soc. 1995, 119, 1773-1786. (b) Shaik, S.; Filatov,M.; Schröder, D.; Schwarz, H. Chem.- Eur. J. 1998, 4, 193-199. (c) Shaik, S.; Danovich, D.; Fiedler, A.; Schröder, D.; Schwarz, H. Helv. Chim. Acta 1995, 78, 1393-1407. (d) Mitchell, S.; Blitz, M.; Siegbahn, P.; Svensson, M. J Chem. Phys. 1994, 100, 423-433. In our case, such calculations are very difficult due to the size of the system. The fact that the triplet and quintet surfaces in this system are nearly parallell also makes it difficult to determine the exact crossing point. However, the fact that the surfaces are parallell and close in energy also indicates a high spin transition probability according to LZ formula.
32. Jacobsen, E. N.; Deng, L.; Furukawa, Y.; Martínez, L. E. Tetrahedron 1994, 50, 4323-4334.
33. The triplet-quintet energy differences for the substituted ethenes were calculated in the geometry of structure 2.
34. These results are in accordance with Jacobsen's findings that electron-poor alkenes give more isomerization than electron-rich alkenes, ref 18. We have also seen a similar effect: for example, cis-4,4′-dimethoxystilbene gives 84% cis-epoxide, while cis-4,4′-dinitrostilbene gives 96% trans-epoxide: Åkermark, B.; Norrby, P.-O.; Linde, C., manuscript in preparation.
35. It is interesting that while the formation of the manganaoxetane 4 is kinetically disfavored compared to the radical route (see Figure 1) it is thermodynamically accessible on the triplet surface. Formation of a manganaoxetane (Scheme 1, pathway C) would stabilize the cis orientation of the substrate substituents on the triplet surface before spin change. We have earlier suggested that the enantioselectivity in the reaction largely can be traced to an implicit twist of the salen ligand in the selectivity-determining step, see ref 5a. The suggestion has been contested on the basis of observed structures of planar Mn(III)-salen complexes, see: Pospisil, P. J.; Carsten, D. H.; Jacobsen, E. N. Chem.-Eur. J. 1996, 2, 974-980. A twist conformation is adopted upon oxidation to 1, the angle between the aromatic rings being 22° in our model system. In TS 2 the angle is smaller (5°) but is likely to increase in the real system where the backbone and the bulky C3(C3′) substituents impose an inherent twist preference. Such a twist conformation is strongly suggested also by the results of Katsuki et al.: ref 5b and Hashihayata, T.; Ito, Y.; Katsuki, T. Synlett 1996, 1079.
36. It is interesting that while the formation of the manganaoxetane 4 is kinetically disfavored compared to the radical route (see Figure 1) it is thermodynamically accessible on the triplet surface. Formation of a manganaoxetane (Scheme 1, pathway C) would stabilize the cis orientation of the substrate substituents on the triplet surface before spin change. We have earlier suggested that the enantioselectivity in the reaction largely can be traced to an implicit twist of the salen ligand in the selectivity-determining step, see ref 5a. The suggestion has been contested on the basis of observed structures of planar Mn(III)-salen complexes, see: Pospisil, P. J.; Carsten, D. H.; Jacobsen, E. N. Chem.-Eur. J. 1996, 2, 974-980. A twist conformation is adopted upon oxidation to 1, the angle between the aromatic rings being 22° in our model system. In TS 2 the angle is smaller (5°) but is likely to increase in the real system where the backbone and the bulky C3(C3′) substituents impose an inherent twist preference. Such a twist conformation is strongly suggested also by the results of Katsuki et al.: ref 5b and Hashihayata, T.; Ito, Y.; Katsuki, T. Synlett 1996, 1079.