Do the Anti-Selectivities of 2,3-endo,endo-Dimethylnorbornan-7-one and the Corresponding Diethyl Analog Obey the Cieplak Model? An ab Initio MO Investigation and Application of the Cation Complexation Model Veejendra K. Yadav* Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India vijendra@iitk.ac.in Received October 9, 2000 The anti-selectivity of 2,3-endo,endo-dimethylbicyclo- [2.2.1]heptan-7-one, 1 (Figure 1), is appreciably inferior to that of the corresponding diethyl derivative, 2. For instance, in reaction with LiA1H 4 , the anti/syn selectivity is 55:45 for the dimethyl derivative and 79:21 for the diethyl material. 1 However, if the hyperconjugation ef- fects were indeed the control elements as perceived by the Cieplak model, 2 both the materials will be predicted for syn-selectivity. The Cieplak model considers a C-H bond more electron-donating than a C-C bond, and thus, the C1-C6/C4-C5 bonds must be more electron rich than the C1-C2/C3-C4 bonds. In this paper, we present the results of an application of the cation complexation model 3 and explain (a) why these materials are anti- selective, to begin with, and (b) why the ethyl derivative is better at its selectivity than the corresponding methyl species. In application of the cation complexation model to bicy- clo[2.2.1]heptan-7-ones, we have calculated 4-7 the torsion angles Dl ) O-C7-C1-C2, D2 ) O-C7-C1-C6, D3 ) O-C7-C4-C3, and D4 ) O-C7-C4-C5, both before and after complexation, to assess the direction of carbonyl pyramidalization. 3,8 We call the pyramidalization “anti” when Dl and D3 are smaller than D2 and D4 and “syn” when Dl and D3 are larger than D2 and D4, respectively. The anti-pyramidalization leads to anti- addition and the syn-pyramidalization leads to syn- addition of a nucleophile. The directional changes in the torsion angles D5 ) H-Cl-C7-O, D6 ) H-C4-C7-O, and D7 ) C1-C7-O-C4 also provide information about the direction of pyramidalization. 9 The torsion angles Dl-D7 are collected in Table 1. We have considered two conformations, 2a and 2b, for the diethyl derivative. 10 In 2a, the carbon-carbon bonds of the ethyl groups are antiperiplanar to the C2-C3 bond (C-C-C2-C3 ) C-C-C3-C2 ) 178.28°). In 2b, one such bond is near antiperiplanar to the C1-C2 bond (C-C-C2-C1 ) 159.90°) and the other near synperipla- nar to the C3-H bond (C-C-C3-H )-18.57°). The conformer 2a is 3.40 kcal mol -1 more stable than the conformer 2b. The torsion angles D1 and D3 are smaller than the torsion angles D2 and D4, respectively, in both * To whom correspondence should be addressed. Fax: Int. Code- 91-512-597436. (1) Mehta. G.; Khan, F. A. J. Am. Chem. Soc. 1990, 112, 6140. Mehta, G.; Khan, F. A. J. Chem. Soc., Chem. Commun. 1991, 18. Li, H.; Mehta, G.; Padma, S.; le Noble, W. J. J. Org. Chem. 1991, 56, 2006. Paddon-Row: M. N.; Wu, Y.-D.; Houk, K. N. J. Am. Chem. Soc. 1992, 114, 10638. Ganguly, B.; Chandrasekhar J.; Khan, F. A.; Mehta, G. J. Org. Chem. 1993, 58, 1734. (2) Cieptak, A. S. J. Am. Chem. Soc. 1981, 103, 4540. Cieplak, A. S.; Taft, B. D.; Johnson, C. R. J. Am. Chem. Soc. 1989, 111, 8447. (3) (a) Jeyaraj, D. A.; Yadav, A.; Yadav, V. K. Tetrahedron Lett. 1997, 38, 4483. (b) Jeyaraj, D. A.; Yadav, V. K. Tetrahedron Lett. 1997, 38, 6095. (c) Yadav, V. K.; Jeyaraj, D. A. J. Org. Chem. 1998, 63, 3474. (d) Yadav, V. K.; Senthil, G.; Jeyaraj, D. A. Tetrahedron 1999, 55, 14211. (e) Yadav, V. K.; Jeyaraj. D. A.; Balamurugan, R. Tetrahedron 2000, 56, 7581. (f) Yadav, V. K.; Balamurugan, R. J. Chem. Soc., Perkin Trans 2 2001, 1. (4) All the geometry optimizations and the calculations of NBO charges were performed at Becke3LYP/6-31G* level using the program Gaussian 94, Revision C.2: Frish, M. J.; Trucks, G. W.; Schlegel, H. B.; Jones, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al- Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresnan, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; 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. (5) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (6) For the 6-31G* basis set, see: Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (7) NBO 3.1 program: Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. QCPE Bull. 1990, 10, 58. For detailed information, see: Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 8899. Weinhold, F. Natural Bond Orbital Methods. In Encydopedia of Computational Chemistry; Schleyer, P. v. R., Allinger, N. L., Clark, T., Gasteiger, J., Kollman, P. A., Schaefer, H. F., III, Schreiner, P. R., Eds.; Wiley: Chichester, UK, 1998; Vol. 3, pp 1792-1811. (8) Laube, T. J. Org. Chem. 1999, 64, 8177 and references therein. (9) I am indebted to one reviewer who has kindly suggested the torsion angle D7 ) C1-C7-O-C4 as a better proof for the direction of carbonyl pyramidalization than the torsion angles D1-D6. (10) The conformer 2a was selected on the basis of intuitive chemical knowledge aimed at minimizing the possible steric interactions between the two ethyl groups. The conformer 2b was a product of geometry optimization of an initial guess wherein the two ethyl carbon-carbon bonds were set synperiplanar to the C-H bonds on C2 and C3. Figure 1. Structures of the molecules studied: 2,3-endo,endo- dimethylnorbornan-7-one (1) and conformers of 2,3-endo,endo- diethynorbornan-7-one (2). Table 1. Selected Becke3LYP/6-31G* Geometrical Parameters on 2,3-endo,endo-Disubstituted Bicyclo[2.2.1]heptan-7-ones a substrate D1 D2 D3 D4 D5 D6 D7 b 1 121.63 -125.06 -121.63 125.06 -1.14 1.14 -178.80 1-H + 115.33 -132.28 -115.69 131.97 -6.46 6.02 -173.70 1-Li + 119.46 -127.70 -119.47 127.70 -3.03 3.02 -176.90 2a 121.22 -125.59 -121.22 125.59 -1.73 1.73 -178.50 2a-H + 112.57 -135.20 -113.18 134.60 -8.78 8.11 -171.90 2a-Li + 118.72 -128.61 -118.91 128.45 -3.81 3.74 -176.50 2b 122.46 -124.50 -120.75 126.40 -0.75 1.63 -178.50 2b-H + 114.21 -133.68 -112.34 135.86 -7.86 8.18 -171.80 2b-Li + 120.29 -127.15 -118.22 129.38 -2.77 3.77 -176.50 a Dl ) O-C7-C1-C2; D2 ) O-C7-C1-C6; D3 ) O-C7-C4- C3; D4 ) O-C7-C4-C5; D5 ) H-C1-C7-O; D6 ) H-C4-C7- O; D7 ) C1-C7-O-C4. b The D7 torsion angles were calculated using the program ORTEP-3.2. 2501 J. Org. Chem. 2001, 66, 2501-2502 10.1021/jo001454h CCC: $20.00 © 2001 American Chemical Society Published on Web 03/13/2001