A demonstration of the primary stereoelectronic effect in the Baeyer-Villiger oxidation of α-fluorocyclohexanones

Angewandte Chemie - International Edition

Volumn 39, Issue 16, 2000, Pages 2851-2855

  Crudden, Cathleen M ^a   Chen, Austin C ^a   Calhoun, Larry A ^a  

^a UNIVERSITY OF NEW BRUNSWICK   (Canada)

Author keywords

Dipoles; Fluorine; Oxidations; Stereoelectronic effects; Substituent effects

Indexed keywords

CYCLOHEXANONE DERIVATIVE; FLUORINE; LACTONE;

ARTICLE; CHEMICAL BOND; CHEMICAL REACTION; CONFORMATION; DIPOLE; OXIDATION; STEREOCHEMISTRY; STEREOSPECIFICITY;

EID: 0000533051     PISSN: 14337851     EISSN: None     Source Type: Journal    
DOI: 10.1002/1521-3773(20000818)39:16<2851::AID-ANIE2851>3.0.CO;2-Y     Document Type: Article

References (43)

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15. The interpretation of reactions in which mass balance was not considered has caused difficulties in the past for the study of the Baeyer-Villiger oxidation. The case of camphor is most compelling. For an excellent discussion of this debate, see Ret. [2b].
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23. J. Meinwald, E. Frauenglass, J. Am. Chem. Soc. 1960, 82, 5235. Relief of steric strain in the breakdown of the Criegee intermediate has also been postulated to be responsible for relative migratory aptitudes: P. A. Jacobi, D. G. Walker, J. Am. Chem. Soc. 1981, 103, 4610.
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28. [13] This work has also been described in a recent review article: S. E. Denmark, Z. Wu, Synlett 1999, 847.
29. 19F NMR, a calibration plot was prepared to account for the imperfect pulse power across the large chemical shift range. The transmitter offset was set to δ = - 7159.8 (or δ = - 124) before each acquisition.
30. The importance of the secondary stereoelectronic effect must also be considered. There is ample evidence that the hydrogen atom in systems such as 1-hydroxy-2-fluoro-4-tert-butyl-cyclohexanone is oriented toward the fluorine, although there is considerable controversy over whether or not a true hydrogen bond is involved: a) J. M. Bakke, L. H. Bjerkeseth, T. E. C. L. Ronnow, K. Steinsvoll, J. Mol. Struc. 1994, 321, 205;
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32. cq, only the fluorinated carbon is aligned for migration in the preferred conformation I. Thus, the secondary stereoclectronic effect could he considered to be the sole determinant of regiochemistry in this case. Experiments are currently underway to determine the relative importance of the secondary electronic effect in this system.
33. Although 2-fluorocyclohexanone would seem to be a more appropriate model, it has conformational biases, existing as a mixture of equatorial and axial isomers interconverting by a ring flip, the proportion of which is dependent on the polarity of the solvent. The equatorial isomer, which has a higher dipole moment, is preferred in more polar solvents, and the axial in less polar solvents. As this equilibrium will unnecessarily complicate our results, acyclic ketone 9 was chosen as the model compound, a) Y. Pan, J. B. Stothers, Can. J. Chem. 1967, 45, 2943;
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37. For a recent example where dipole minimization is predicted to be responsible for the stereochemistry of the reaction, sec: J. M. Concellón, P. L. Bernad, J. A. Pérez-Andrés, Angew. Chem. 1999, 111, 2528; Angew. Chem. Int. Ed. 1999, 38, 2384.
38. For a recent example where dipole minimization is predicted to be responsible for the stereochemistry of the reaction, sec: J. M. Concellón, P. L. Bernad, J. A. Pérez-Andrés, Angew. Chem. 1999, 111, 2528; Angew. Chem. Int. Ed. 1999, 38, 2384.
39. a) C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, Weinheim, 1988;
40. I (an empirical parameter used to describe solvent polarity relative to tetramethylsilane (0) and water (1)) is related to the transition energy for the longest-wavelength solvatochromic absorption hand of a pyridinium-N-phenoxide betaine dye: K. Dimroth, C. Reichardt, Liebigs Ann. Chem. 1963, 661, 1. See also: Ref. [20a], pp. 363-371.
41. [23]
42. 19F, HMQC, HMBC, COSY, and TOCSY experiments. NOE difference experiments carried out on both 13 and 14 demonstrated that the major compound had a cis relationship between the fluorine of the oxidized substituent and the tert-butyl group, indicating that the migration had occurred, as expected, from the carbon with the equatorially disposed fluorine atom. Compound 13 was more sensitive to hydrolysis than its isomer 14 and we were thus unable to obtain spectra of 13 unencumbered by the side product of this reaction. As compound 13 was still present in substantial amounts, and as the resonances of interest are clearly separated, we were able to assign the structure from these spectra.
43. cq, 10, 11, 13, and 14 are available as supporting information.