Synthesis and characterization of in-plane and out-of-plane enone - Lewis acid complexes: Implications for Diels - Alder reactions

Journal of Organic Chemistry

Volumn 61, Issue 4, 1996, Pages 1436-1441

  Singh, D K ^a   Springer, James B ^a   Goodson, Patricia A ^a   Corcoran, Robert C ^a  

^a UNIVERSITY OF WYOMING   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000575831     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo9518112     Document Type: Article

References (50)

1. Portions of this work were presented at the 34th National Organic Symposium, June, 1995, Williamsburg, VA.
2. Two recent related studies of chiral Lewis acids which also provide leading references to the literature in this field: (a) Haase, C.; Sarko, C. R.; DiMare, M. J. Org. Chem. 1995, 60, 1777. (b) Seebach, D.; Dahinden, R.; Marti, R. E.; Beck, A. K.; Plattner, D. A.; Kühnle, F. N. M. J. Org. Chem. 1995, 60, 1788.
3. Two recent related studies of chiral Lewis acids which also provide leading references to the literature in this field: (a) Haase, C.; Sarko, C. R.; DiMare, M. J. Org. Chem. 1995, 60, 1777. (b) Seebach, D.; Dahinden, R.; Marti, R. E.; Beck, A. K.; Plattner, D. A.; Kühnle, F. N. M. J. Org. Chem. 1995, 60, 1788.
4. 4-) complexes of saturated and unsaturated carbonyl compounds with electron rich transition metals is not unusual: for leading references, see ref 8.
5. The literature is extensive. The recent work by Denmark and Almstead is particularly detailed and includes a discussion of this issue which constitutes a review for all practical purposes: Denmark, S. E.; Almstead, N. G. J. Am. Chem. Soc. 1993, 115, 3133.
6. Seeman, J. I. Chem. Rev. 1983, 83, 83.
7. Ironically, a particularly apt example of this may be found in the extensive and detailed studies of Gladysz and co-workers on π-complexes of carbonyls with chiral rhenium compounds, in which nucleophilic addition to the carbonyls was found to occur by way of a small concentration of highly reactive in-plane complex. For leading references, see (a) Wang, Y.; Gladysz, J. A. J. Org. Chem. 1995, 60, 903. (b) Klein, D. P.; Gladysz, J. A. J. Am. Chem. Soc. 1992,114, 8710.
8. Ironically, a particularly apt example of this may be found in the extensive and detailed studies of Gladysz and co-workers on π-complexes of carbonyls with chiral rhenium compounds, in which nucleophilic addition to the carbonyls was found to occur by way of a small concentration of highly reactive in-plane complex. For leading references, see (a) Wang, Y.; Gladysz, J. A. J. Org. Chem. 1995, 60, 903. (b) Klein, D. P.; Gladysz, J. A. J. Am. Chem. Soc. 1992,114, 8710.
9. Corcoran, R. C.; Ma, J. J. Am. Chem. Soc. 1991, 113, 8973.
10. Corcoran, R. C.; Ma, J. J. Am. Chem. Soc. 1992, 114, 4536.
11. Rabideau, P. W.; Marcmow, Z. Org. React. 1992, 42, 1.
12. Wilds, A. L.; Nelson, N. A. J. Am. Chem. Soc. 1953, 75, 5360.
13. Amupitan, J.; Hug, E.,; Mellor, M.; Scovell, E. G.; Sutherland, J. K. J. Chem. Soc., Perkin Trans. 1 1983, 747.
14. 4 have been unsuccessful.
15. obs = 0.0665 using 847 observed reflections with I > 3σ(I) collected at 23 °C using Mo Kα (λ = 0.71073 Å) radiation with 4° < 2θ < 45.0°. The X-ray data has been submitted to the Cambridge Crystallographic Data Centre.
16. 4 showed no enhancements, as expected.
17. 4) - δ(substrate).
18. (a) Loncharich, R. J.; Schwartz, T. R.; Houk, K. N. J. Am. Chem. Soc. 1987, 109, 14.
19. (b) Pascual-Teresa, B.; Gonzalez, J.; Asensio, A.; Houk, K. N. J. Am. Chem. Soc. 1995, 117, 4347.
20. A recently published crystal structure of a dienophile-(Ti-TADDOL) complex (Gothelf, K. V.; Hazell, R. G.; Jørgensen, K. A. J. Am. Chem. Soc. 1995, 117, 4435) differs substantially from that expected from transition state models proposed by the authors of ref 2 and by Corey and Matsumura (Corey, E. J.; Matsumura, Y. Tetrahedron Lett. 1991, 32, 6289). Only in DiMare's work (ref 2a) was a structure somewhat similar to that found in the crystal structure considered, but this was rejected due (in part) to the lack of obvious facial bias to diene approach to the assumed in-plane complex, of the dienophile. The crystal structure reveals an out-of-plane coordination geometry, and a dienophile facial bias is clearly evident.
21. A recently published crystal structure of a dienophile-(Ti-TADDOL) complex (Gothelf, K. V.; Hazell, R. G.; Jørgensen, K. A. J. Am. Chem. Soc. 1995, 117, 4435) differs substantially from that expected from transition state models proposed by the authors of ref 2 and by Corey and Matsumura (Corey, E. J.; Matsumura, Y. Tetrahedron Lett. 1991, 32, 6289). Only in DiMare's work (ref 2a) was a structure somewhat similar to that found in the crystal structure considered, but this was rejected due (in part) to the lack of obvious facial bias to diene approach to the assumed in-plane complex, of the dienophile. The crystal structure reveals an out-of-plane coordination geometry, and a dienophile facial bias is clearly evident.
22. For a recent discussion of the issue of s-cis and s-trans enone geometries in esters, see Shida, N.; Kabuto, C.; Niwa, T.; Ebata, T.; Yamamoto, Y. J. Org Chem. 1994, 59, 4068.
23. Along similar lines, Yamamoto and co-workers suggested the involvement of an out-of-plane coordinated aluminum Lewis acid to explain stereochemical results in the additions of organometallics to complexes of ketones with methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD): Maruoka, K.; Itoh, T.; Yamamoto, H. J. Am. Chem. Soc. 1985, 107, 4573. That a sterically driven out-of-plane complexation is reasonable for this Lewis acid was confirmed by a crystal structure of the methyl p-toluate-MAD complex, in which a 50° dihedral angle is seen for Al-O=C-OMe: Shreve, A. P.; Mulhaupt, R.; Fultz, W.; Calbrese, J.; Robbins, W.; Ittel, S. D. Organometallics 1988, 7, 409.
24. Along similar lines, Yamamoto and co-workers suggested the involvement of an out-of-plane coordinated aluminum Lewis acid to explain stereochemical results in the additions of organometallics to complexes of ketones with methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD): Maruoka, K.; Itoh, T.; Yamamoto, H. J. Am. Chem. Soc. 1985, 107, 4573. That a sterically driven out-of-plane complexation is reasonable for this Lewis acid was confirmed by a crystal structure of the methyl p-toluate-MAD complex, in which a 50° dihedral angle is seen for Al-O=C-OMe: Shreve, A. P.; Mulhaupt, R.; Fultz, W.; Calbrese, J.; Robbins, W.; Ittel, S. D. Organometallics 1988, 7, 409.
25. 4. Unfortunately from the standpoint of the work described here, the authors do not explictly discuss energetic differences between in-plane and out-of-plane coordination geometries. Branchadell, V.; Oliva, A. J. Am. Chem. Soc. 1992, 114, 4357.
26. Search performed on the Cambridge Structural Database, version 5.08, on Ti-O=C with the carbon constrained to be three coordinate. The dihedral angle determined in the current work was included in the calculation of the fraction of compounds having >45° dihedral angle. The references are arranged in order of decreasing Ti-O=C-Y dihedral angle, with the dihedral angle given in parentheses at the end of the citation. Only the larger of the dihedral angles is given for compounds with multiple Ti-O=C interactions. In those cases in which two angles are given, they refer to different compounds reported in the same paper, or to different crystalline forms of the same compound, (a) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1991, 417, 389 (86.8°). (b) Yuchi, A.; Shiro, M.; Wada, H.; Nakagawa, G. Bull. Chem. Soc. Jpn. 1991, 64, 760 (75.7°). (c) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem 1993, 447, 213 (59.9° and 72.2°). (d) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1990, 393, 349 (68.4°). (e) Poll, T.; Metter, J. O.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 112 (63.6°). (f) Kakkonen, H. J.; Pursianinen, J.; Pakkanen, T. A.; Ahlgren, M.; Tiskola, E. J. Organomet. Chem. 1993, 453, 175 (34.7° and 63.2°). (g) Utko, J.; Sobota, P.; Lis, T. J. Organomet. Chem. 1989, 373, 63 (60.6°). (h) Sobota, P.; Szafert, S.; Lis, T. J. Organomet. Chem. 1993, 443, 85 (57.5°). (i) Bassi, I. W.; Calcaterra, M.; Intrito, R. J. Organomet. Chem. 1977, 127, 305 (45.7°).
27. Search performed on the Cambridge Structural Database, version 5.08, on Ti-O=C with the carbon constrained to be three coordinate. The dihedral angle determined in the current work was included in the calculation of the fraction of compounds having >45° dihedral angle. The references are arranged in order of decreasing Ti-O=C-Y dihedral angle, with the dihedral angle given in parentheses at the end of the citation. Only the larger of the dihedral angles is given for compounds with multiple Ti-O=C interactions. In those cases in which two angles are given, they refer to different compounds reported in the same paper, or to different crystalline forms of the same compound, (a) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1991, 417, 389 (86.8°). (b) Yuchi, A.; Shiro, M.; Wada, H.; Nakagawa, G. Bull. Chem. Soc. Jpn. 1991, 64, 760 (75.7°). (c) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem 1993, 447, 213 (59.9° and 72.2°). (d) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1990, 393, 349 (68.4°). (e) Poll, T.; Metter, J. O.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 112 (63.6°). (f) Kakkonen, H. J.; Pursianinen, J.; Pakkanen, T. A.; Ahlgren, M.; Tiskola, E. J. Organomet. Chem. 1993, 453, 175 (34.7° and 63.2°). (g) Utko, J.; Sobota, P.; Lis, T. J. Organomet. Chem. 1989, 373, 63 (60.6°). (h) Sobota, P.; Szafert, S.; Lis, T. J. Organomet. Chem. 1993, 443, 85 (57.5°). (i) Bassi, I. W.; Calcaterra, M.; Intrito, R. J. Organomet. Chem. 1977, 127, 305 (45.7°).
28. Search performed on the Cambridge Structural Database, version 5.08, on Ti-O=C with the carbon constrained to be three coordinate. The dihedral angle determined in the current work was included in the calculation of the fraction of compounds having >45° dihedral angle. The references are arranged in order of decreasing Ti-O=C-Y dihedral angle, with the dihedral angle given in parentheses at the end of the citation. Only the larger of the dihedral angles is given for compounds with multiple Ti-O=C interactions. In those cases in which two angles are given, they refer to different compounds reported in the same paper, or to different crystalline forms of the same compound, (a) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1991, 417, 389 (86.8°). (b) Yuchi, A.; Shiro, M.; Wada, H.; Nakagawa, G. Bull. Chem. Soc. Jpn. 1991, 64, 760 (75.7°). (c) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem 1993, 447, 213 (59.9° and 72.2°). (d) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1990, 393, 349 (68.4°). (e) Poll, T.; Metter, J. O.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 112 (63.6°). (f) Kakkonen, H. J.; Pursianinen, J.; Pakkanen, T. A.; Ahlgren, M.; Tiskola, E. J. Organomet. Chem. 1993, 453, 175 (34.7° and 63.2°). (g) Utko, J.; Sobota, P.; Lis, T. J. Organomet. Chem. 1989, 373, 63 (60.6°). (h) Sobota, P.; Szafert, S.; Lis, T. J. Organomet. Chem. 1993, 443, 85 (57.5°). (i) Bassi, I. W.; Calcaterra, M.; Intrito, R. J. Organomet. Chem. 1977, 127, 305 (45.7°).
29. Search performed on the Cambridge Structural Database, version 5.08, on Ti-O=C with the carbon constrained to be three coordinate. The dihedral angle determined in the current work was included in the calculation of the fraction of compounds having >45° dihedral angle. The references are arranged in order of decreasing Ti-O=C-Y dihedral angle, with the dihedral angle given in parentheses at the end of the citation. Only the larger of the dihedral angles is given for compounds with multiple Ti-O=C interactions. In those cases in which two angles are given, they refer to different compounds reported in the same paper, or to different crystalline forms of the same compound, (a) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1991, 417, 389 (86.8°). (b) Yuchi, A.; Shiro, M.; Wada, H.; Nakagawa, G. Bull. Chem. Soc. Jpn. 1991, 64, 760 (75.7°). (c) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem 1993, 447, 213 (59.9° and 72.2°). (d) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1990, 393, 349 (68.4°). (e) Poll, T.; Metter, J. O.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 112 (63.6°). (f) Kakkonen, H. J.; Pursianinen, J.; Pakkanen, T. A.; Ahlgren, M.; Tiskola, E. J. Organomet. Chem. 1993, 453, 175 (34.7° and 63.2°). (g) Utko, J.; Sobota, P.; Lis, T. J. Organomet. Chem. 1989, 373, 63 (60.6°). (h) Sobota, P.; Szafert, S.; Lis, T. J. Organomet. Chem. 1993, 443, 85 (57.5°). (i) Bassi, I. W.; Calcaterra, M.; Intrito, R. J. Organomet. Chem. 1977, 127, 305 (45.7°).
30. Search performed on the Cambridge Structural Database, version 5.08, on Ti-O=C with the carbon constrained to be three coordinate. The dihedral angle determined in the current work was included in the calculation of the fraction of compounds having >45° dihedral angle. The references are arranged in order of decreasing Ti-O=C-Y dihedral angle, with the dihedral angle given in parentheses at the end of the citation. Only the larger of the dihedral angles is given for compounds with multiple Ti-O=C interactions. In those cases in which two angles are given, they refer to different compounds reported in the same paper, or to different crystalline forms of the same compound, (a) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1991, 417, 389 (86.8°). (b) Yuchi, A.; Shiro, M.; Wada, H.; Nakagawa, G. Bull. Chem. Soc. Jpn. 1991, 64, 760 (75.7°). (c) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem 1993, 447, 213 (59.9° and 72.2°). (d) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1990, 393, 349 (68.4°). (e) Poll, T.; Metter, J. O.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 112 (63.6°). (f) Kakkonen, H. J.; Pursianinen, J.; Pakkanen, T. A.; Ahlgren, M.; Tiskola, E. J. Organomet. Chem. 1993, 453, 175 (34.7° and 63.2°). (g) Utko, J.; Sobota, P.; Lis, T. J. Organomet. Chem. 1989, 373, 63 (60.6°). (h) Sobota, P.; Szafert, S.; Lis, T. J. Organomet. Chem. 1993, 443, 85 (57.5°). (i) Bassi, I. W.; Calcaterra, M.; Intrito, R. J. Organomet. Chem. 1977, 127, 305 (45.7°).
31. Search performed on the Cambridge Structural Database, version 5.08, on Ti-O=C with the carbon constrained to be three coordinate. The dihedral angle determined in the current work was included in the calculation of the fraction of compounds having >45° dihedral angle. The references are arranged in order of decreasing Ti-O=C-Y dihedral angle, with the dihedral angle given in parentheses at the end of the citation. Only the larger of the dihedral angles is given for compounds with multiple Ti-O=C interactions. In those cases in which two angles are given, they refer to different compounds reported in the same paper, or to different crystalline forms of the same compound, (a) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1991, 417, 389 (86.8°). (b) Yuchi, A.; Shiro, M.; Wada, H.; Nakagawa, G. Bull. Chem. Soc. Jpn. 1991, 64, 760 (75.7°). (c) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem 1993, 447, 213 (59.9° and 72.2°). (d) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1990, 393, 349 (68.4°). (e) Poll, T.; Metter, J. O.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 112 (63.6°). (f) Kakkonen, H. J.; Pursianinen, J.; Pakkanen, T. A.; Ahlgren, M.; Tiskola, E. J. Organomet. Chem. 1993, 453, 175 (34.7° and 63.2°). (g) Utko, J.; Sobota, P.; Lis, T. J. Organomet. Chem. 1989, 373, 63 (60.6°). (h) Sobota, P.; Szafert, S.; Lis, T. J. Organomet. Chem. 1993, 443, 85 (57.5°). (i) Bassi, I. W.; Calcaterra, M.; Intrito, R. J. Organomet. Chem. 1977, 127, 305 (45.7°).
32. Search performed on the Cambridge Structural Database, version 5.08, on Ti-O=C with the carbon constrained to be three coordinate. The dihedral angle determined in the current work was included in the calculation of the fraction of compounds having >45° dihedral angle. The references are arranged in order of decreasing Ti-O=C-Y dihedral angle, with the dihedral angle given in parentheses at the end of the citation. Only the larger of the dihedral angles is given for compounds with multiple Ti-O=C interactions. In those cases in which two angles are given, they refer to different compounds reported in the same paper, or to different crystalline forms of the same compound, (a) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1991, 417, 389 (86.8°). (b) Yuchi, A.; Shiro, M.; Wada, H.; Nakagawa, G. Bull. Chem. Soc. Jpn. 1991, 64, 760 (75.7°). (c) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem 1993, 447, 213 (59.9° and 72.2°). (d) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1990, 393, 349 (68.4°). (e) Poll, T.; Metter, J. O.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 112 (63.6°). (f) Kakkonen, H. J.; Pursianinen, J.; Pakkanen, T. A.; Ahlgren, M.; Tiskola, E. J. Organomet. Chem. 1993, 453, 175 (34.7° and 63.2°). (g) Utko, J.; Sobota, P.; Lis, T. J. Organomet. Chem. 1989, 373, 63 (60.6°). (h) Sobota, P.; Szafert, S.; Lis, T. J. Organomet. Chem. 1993, 443, 85 (57.5°). (i) Bassi, I. W.; Calcaterra, M.; Intrito, R. J. Organomet. Chem. 1977, 127, 305 (45.7°).
33. Search performed on the Cambridge Structural Database, version 5.08, on Ti-O=C with the carbon constrained to be three coordinate. The dihedral angle determined in the current work was included in the calculation of the fraction of compounds having >45° dihedral angle. The references are arranged in order of decreasing Ti-O=C-Y dihedral angle, with the dihedral angle given in parentheses at the end of the citation. Only the larger of the dihedral angles is given for compounds with multiple Ti-O=C interactions. In those cases in which two angles are given, they refer to different compounds reported in the same paper, or to different crystalline forms of the same compound, (a) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1991, 417, 389 (86.8°). (b) Yuchi, A.; Shiro, M.; Wada, H.; Nakagawa, G. Bull. Chem. Soc. Jpn. 1991, 64, 760 (75.7°). (c) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem 1993, 447, 213 (59.9° and 72.2°). (d) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1990, 393, 349 (68.4°). (e) Poll, T.; Metter, J. O.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 112 (63.6°). (f) Kakkonen, H. J.; Pursianinen, J.; Pakkanen, T. A.; Ahlgren, M.; Tiskola, E. J. Organomet. Chem. 1993, 453, 175 (34.7° and 63.2°). (g) Utko, J.; Sobota, P.; Lis, T. J. Organomet. Chem. 1989, 373, 63 (60.6°). (h) Sobota, P.; Szafert, S.; Lis, T. J. Organomet. Chem. 1993, 443, 85 (57.5°). (i) Bassi, I. W.; Calcaterra, M.; Intrito, R. J. Organomet. Chem. 1977, 127, 305 (45.7°).
34. Search performed on the Cambridge Structural Database, version 5.08, on Ti-O=C with the carbon constrained to be three coordinate. The dihedral angle determined in the current work was included in the calculation of the fraction of compounds having >45° dihedral angle. The references are arranged in order of decreasing Ti-O=C-Y dihedral angle, with the dihedral angle given in parentheses at the end of the citation. Only the larger of the dihedral angles is given for compounds with multiple Ti-O=C interactions. In those cases in which two angles are given, they refer to different compounds reported in the same paper, or to different crystalline forms of the same compound, (a) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1991, 417, 389 (86.8°). (b) Yuchi, A.; Shiro, M.; Wada, H.; Nakagawa, G. Bull. Chem. Soc. Jpn. 1991, 64, 760 (75.7°). (c) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem 1993, 447, 213 (59.9° and 72.2°). (d) Sobota, P.; Utko, J.; Lis, T. J. Organomet. Chem. 1990, 393, 349 (68.4°). (e) Poll, T.; Metter, J. O.; Helmchen, G. Angew. Chem., Int. Ed. Engl. 1985, 24, 112 (63.6°). (f) Kakkonen, H. J.; Pursianinen, J.; Pakkanen, T. A.; Ahlgren, M.; Tiskola, E. J. Organomet. Chem. 1993, 453, 175 (34.7° and 63.2°). (g) Utko, J.; Sobota, P.; Lis, T. J. Organomet. Chem. 1989, 373, 63 (60.6°). (h) Sobota, P.; Szafert, S.; Lis, T. J. Organomet. Chem. 1993, 443, 85 (57.5°). (i) Bassi, I. W.; Calcaterra, M.; Intrito, R. J. Organomet. Chem. 1977, 127, 305 (45.7°).
35. In addition to ref 21f, see: (a) Albanese, J. A.; Staley, D. L.; Rheingold, A. L.; Burmeister, J. L. Inorg. Chem. 1990, 29, 2209 (43.7°). (b) Bachand, B.; Belanger-Gariepy, G.; Wuest, J. D. Organometallics 1990, 9, 2860 (36.7°). (c) Oppolzer, W.; Rodriquez, I.; Blagg, J.; Bernardinelli, G. Helv. Chim. Acta 1989, 72, 123 (31.2°).
36. In addition to ref 21f, see: (a) Albanese, J. A.; Staley, D. L.; Rheingold, A. L.; Burmeister, J. L. Inorg. Chem. 1990, 29, 2209 (43.7°). (b) Bachand, B.; Belanger-Gariepy, G.; Wuest, J. D. Organometallics 1990, 9, 2860 (36.7°). (c) Oppolzer, W.; Rodriquez, I.; Blagg, J.; Bernardinelli, G. Helv. Chim. Acta 1989, 72, 123 (31.2°).
37. In addition to ref 21f, see: (a) Albanese, J. A.; Staley, D. L.; Rheingold, A. L.; Burmeister, J. L. Inorg. Chem. 1990, 29, 2209 (43.7°). (b) Bachand, B.; Belanger-Gariepy, G.; Wuest, J. D. Organometallics 1990, 9, 2860 (36.7°). (c) Oppolzer, W.; Rodriquez, I.; Blagg, J.; Bernardinelli, G. Helv. Chim. Acta 1989, 72, 123 (31.2°).
38. Instances in which the shorter Ti-O=C bond is to the carbonyl bound with a more out-of-plane coordination geometry: refs 21a-c,e-g and 22b. In ref 21h the carbonyl which has a lesser out-of-plane coordination geometry has the shorter bond (by 0.025 Å).
39. The C-C-C-C dihedral angle in eyclohexane is 56.4°, as calculated using PCModel, version 3.3, Serena Software, Bloomington, IN.
40. 2 centers and lacking heteroatom substituents in the 2-, 3-, or 4-positions; 22 structures were located. The absolute values of the larger of the two O-C(1)-C(6)-C(alkyl) dihedral angles ranged from 64.9° to 112.6°, with an average of 93.0° ± 13.0°. Absolute values of O=C-C=C dihedral angles ranged from 145.9° to 177.6° with an average of 167° ± 8°.
41. Quinkert, G.; del Grosso, M.; Bucher, A.; Bauch, M.; Doring, W.; Bats, J. W.; Dumer, G. Tetrahedron Lett. 1992, 33, 3617.
42. However, there is not always a correlation between bond lengths and bond strengths: Ernst. R. D.; Freeman, J. W.; Stahl, L.; Wilson, D. R.; Arif, A. M. J. Am. Chem. Soc. 1995, 117, 5075.
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46. Laszlo, P.; Teston, M. J. Am. Chem. Soc. 1990, 112, 8750.
47. Laszlo, P.; Teston-Henry, M. Tetrahedron Lett. 1991, 31, 3837.
48. Grinvald, A.; Rabinovitz, M. J. Chem. Soc., Perkin Trans. 2 1974, 94.
49. Mautner, H. G.; Chu, S.-H.; Lee, C. M. J. Org. Chem. 1962, 27, 3671.
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