Conformational Studies by Dynamic NMR. 83. 1 Correlated Enantiomerization Pathways for the Stereolabile Propeller Antipodes of Dimesityl Substituted Ethanol and Ethers Stefano Grilli, 2 Lodovico Lunazzi,* and Andrea Mazzanti* Department of Organic Chemistry “A. Mangini”, University of Bologna, Risorgimento, 4, Bologna 40136, Italy lunazzi@ms.fci.unibo.it Received April 23, 2001 Below -100 °C, the NMR spectra of dimesityl derivatives of ethanol and of various ethers reveal how these molecules exist as M and P propeller-like stereolabile enantiomers, owing to the restricted rotation about the Ar-C bond. Single-crystal X-ray diffraction of one such derivative confirmed the existence of a two-blade propeller structure. Computer analysis of the NMR line shape allowed the barriers for the enantiomerization process to be determined. Theoretical modeling (Molecular Mechanics) of the interconversion circuit produced good agreement between the computed and experimental barrier for a correlated dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Introduction of a configurationally stable chiral center allowed two distinct NMR spectra to be detected at appropriate low temperature for two stereolabile diastereoisomers. Introduction Compounds comprising three ortho-substituted phenyl groups bonded to a configurationally stable sp 3 atom (e.g., carbon or silicon) display dynamic processes involving the interconversion of stereolabile enantiomers, according to correlated pathways (cog-wheeling circuit) described by the so-called n-ring flip mechanisms. 3-6 The correspond- ing barriers were reported 7 to cover the range 9-22 kcal mol -1 . We have recently shown that even when there are only two ortho-substituted phenyl groups bonded to a tetra- hedral center, like a sulfur atom, an analogous dynamic process does take place. Thus, in the case of dimesityl sulfoxide and sulfone a cog-wheel effect, which allows the interconversion of the two M and P propeller-like an- tipodes (conformational enantiomers) through the one- ring flip mechanism, was detected. 8 In these cases, however, the enantiomerization barriers are much lower, having ΔG q values equal to 4.5 and 5.0 kcal mol -1 , respectively, for dimesityl sulfoxide and dimesityl sulfone. On the basis of the latter results it seems conceivable to foretell that a similar effect would also occur when two mesityl groups are bonded to a sp 3 hybridized carbon atom, since stereolabile helical enantiomers are also expected to be available. 9,10 To obtain an experimental verification of this predic- tion and with the purpose of assessing the possible consequences of the related stereochemical properties, the dimesityl derivatives of ethanol (1), dimethyl ether (2), ethylmethyl ether (3), diethyl ether (4), and ethyl 2-me- thylbutyl ether (5) were investigated: Mes 2 C(Me)OH (1); Mes 2 CHOCH 3 (2); Mes 2 C(Me)OCH 3 (3); Mes 2 C(Me)OCH 2 - CH 3 (4); Mes 2 C(Me)OCH 2 CH(Me)Et (5) (Mes ) 2,4,6- trimethyl phenyl) Results and Discussion All these derivatives were found to display dynamic effects in their 1 H and 13 C NMR spectra: as an example of such temperature-dependent features, the 13 C spectra of 3 will be illustrated. At ambient temperature down to -30 °C (Figure 1) three signals due, respectively, to the methyl bonded to the quaternary carbon (24.9 ppm), to the four methyls in the ortho position (23.0 ppm) and to the two methyls in the para position (19.0 ppm) are observed in the 19- 26 ppm spectral region. Owing to the different relaxation times the intensity of these 13 C lines is not proportional * To whom correspondence should be sent. (1) Part 82. Casarini, D.; Lunazzi, L.; Mazzanti, A. Angew. Chem., Int. Ed. 2001, 40, 2536. Part 81. Grilli, S.; Lunazzi, L.; Mazzanti, A. J. Org. Chem. 2001, 66, 4444. (2) In partial fulfilment of the requirements for the Ph.D. in Chemical Sciences, University of Bologna. (3) (a) Sabacky, M. J.; Johnson, S. M.; Martin, J. C. Paul, I. C. J. Am. Chem. Soc. 1969, 91, 7542. (b) Rieker, A.; Kessler, K. Tetrahedron Lett. 1969, 1227. (c) Kessler, K.; Moosmayer, A.; Rieker, A. Tetrahedron 1969, 25, 287. (4) (a) Gust, D.; Mislow, K. J. Am. Chem. Soc. 1973, 95, 1535. (b) Finocchiaro, P.; Gust, D.; Mislow, K. J. Am. Chem. Soc. 1974, 96, 2165 and 2176. (c) Mislow, K. Chemtracts Org. Chem. 1989, 2, 151. (5) (a) Boettcher, R. J.; Gust, D.; Mislow, K. J. Am. Chem. Soc. 1973, 95, 7157. (b) Hutchings, M. G.; Maryanoff, C. A.; Mislow, K. J. Am. Chem. Soc. 1973, 95, 7158. (c) Kates, M. R.; Andose, J. P.; Finocchiaro, P.; Gust. D.; Mislow, K. J. Am. Chem. Soc. 1975, 97, 1772. (d) Mislow, K. Acc. Chem. Res. 1976, 9, 26. (6) Sedo, J.; Ventosa, N.; Molinos. Ma. A.; Pons, M.; Rovira, C.; Veciana, C. J. Org. Chem. 2001, 66, 1579. (7) Oki, M. Applications of Dynamic NMR Spectroscopy to Organic Chemistry; VCH: Deerfield Beach, 1985; Chapter 5, p 226. (8) Casarini, D.; Grilli, S.; Lunazzi, L.; Mazzanti, A. J. Org. Chem. 2001, 66, 2757. (9) The two enantiomeric forms of the highly hindered derivatives Ar2CH-COOH (Ar ) 2,4,2′,4′-tetra-tert-butyl-6,6′-dimethylphenyl and cognates) could be separated at ambient temperature having racemi- sation barriers in the range 21.5-22.9 kcal mol -1 (Akkerman, O. S.; Coops, J. Rec. Trav. Chim. 1967, 86, 755. See also: Akkerman, O. S. Rec. Trav. Chim. 1970, 89, 673.). (10) Dynamic processes were detected in the ArAr′CHMe and ArAr′CHOH derivatives (where Ar * Ar′), see: Finocchiaro, P. Gazz. Chim. Ital. 1975, 105, 149. 5853 J. Org. Chem. 2001, 66, 5853-5858 10.1021/jo010420m CCC: $20.00 © 2001 American Chemical Society Published on Web 07/26/2001