Synthetic reactions mediated by a Ti(O-i-Pr)4 / 2 i-PrMgX reagent

Synlett

Volumn , Issue 6, 2000, Pages 753-775

  Sato, Fumie ^a   Urabe, Hirokazu ^a   Okamoto, Sentaro ^a  

^a TOKYO INSTITUTE OF TECHNOLOGY   (Japan)

Author keywords

( 2 alkyne)titanium complex; Allytitanium; Cyclometallation; Intramolecular nucleophilic acyl substitution; Low valent titanium

Indexed keywords

ACETYLENE; TITANIUM DERIVATIVE;

ARTICLE; CHEMICAL STRUCTURE; COMPLEX FORMATION; MOLECULAR INTERACTION; REACTION ANALYSIS; SYNTHESIS;

EID: 0034047332     PISSN: 09365214     EISSN: None     Source Type: Journal    
DOI: None     Document Type: Article

References (132)

1. Sato, F. Janssen Chim. Acta 1990, 8, 3.
2. Sato, F. J. Organomet. Chem. 1985, 285, 53. Sato, F.; Urabe, H. In Handbook of Grignard Reagents, G. S. Silverman, Rakita, P. E. Eds.; Marcel Dekker, New York, 1996, p. 23. Sato, F.; Urabe, H. In Grignard Reagents - New Developments; Richey, H. G., Ed.; Wiley: New York 1999, p65.
3. Sato, F. J. Organomet. Chem. 1985, 285, 53. Sato, F.; Urabe, H. In Handbook of Grignard Reagents, G. S. Silverman, Rakita, P. E. Eds.; Marcel Dekker, New York, 1996, p. 23. Sato, F.; Urabe, H. In Grignard Reagents - New Developments; Richey, H. G., Ed.; Wiley: New York 1999, p65.
4. Sato, F. J. Organomet. Chem. 1985, 285, 53. Sato, F.; Urabe, H. In Handbook of Grignard Reagents, G. S. Silverman, Rakita, P. E. Eds.; Marcel Dekker, New York, 1996, p. 23. Sato, F.; Urabe, H. In Grignard Reagents - New Developments; Richey, H. G., Ed.; Wiley: New York 1999, p65.
5. Sato, F. J. Synth. Org. Chem. Jpn. 1982, 40, 744. Urabe, H.; Sato, F. In Handbook of Grignard Reagents, G. S. Silverman, Rakita, P. E. Eds.; Marcel Dekker, New York, 1996. p. 577.
6. Sato, F. J. Synth. Org. Chem. Jpn. 1982, 40, 744. Urabe, H.; Sato, F. In Handbook of Grignard Reagents, G. S. Silverman, Rakita, P. E. Eds.; Marcel Dekker, New York, 1996. p. 577.
7. Gao, Y.; Urabe, H.; Sato, F. J. Org. Chem. 1994, 59, 5521. Gao, Y.; Harada, K.; Hata, T.; Urabe, H.; Sato, F. J. Org. Chem. 1995, 60, 290.
8. Gao, Y.; Urabe, H.; Sato, F. J. Org. Chem. 1994, 59, 5521. Gao, Y.; Harada, K.; Hata, T.; Urabe, H.; Sato, F. J. Org. Chem. 1995, 60, 290.
9. Gao, Y.; Sato, F. J. Chem. Soc., Chem. Commun. 1995, 659.
10. Martin, H. A.; Jellinek, F. J. Organomet. Chem. 1968, 8, 115; 1968, 12, 149.
11. Martin, H. A.; Jellinek, F. J. Organomet. Chem. 1968, 8, 115; 1968, 12, 149.
12. (a) Sato, F.; Iijima, S.; Sato, M. Tetrahedron Lett. 1981, 22, 243.
13. (b) Sato, F.; Iijima, S.; Sato, M. J. Chem. Soc., Chem. Commun. 1981, 180.
14. (c) Urabe, H.; Yoshikawa, K.; Sato, F. Tetrahedron Lett. 1995, 36, 5595.
15. (d) Gao, Y.; Iijima, S.; Urabe, H.; Sato, F. Inorg. Chim. Acta 1994, 222, 145 and references cited therein. See also, Klei, E; Teuben, J. H.; de L. Meijer, H. J.; J. Chem. Soc., Chem. Commun. 1981, 342. Klei, E.; Teuben, J. H.; de L. Meijer, H. J; Kwak, E. J.; Bruins, A. P. J. Organomet. Chem. 1982, 224, 327.
16. (d) Gao, Y.; Iijima, S.; Urabe, H.; Sato, F. Inorg. Chim. Acta 1994, 222, 145 and references cited therein. See also, Klei, E; Teuben, J. H.; de L. Meijer, H. J.; J. Chem. Soc., Chem. Commun. 1981, 342. Klei, E.; Teuben, J. H.; de L. Meijer, H. J; Kwak, E. J.; Bruins, A. P. J. Organomet. Chem. 1982, 224, 327.
17. (d) Gao, Y.; Iijima, S.; Urabe, H.; Sato, F. Inorg. Chim. Acta 1994, 222, 145 and references cited therein. See also, Klei, E; Teuben, J. H.; de L. Meijer, H. J.; J. Chem. Soc., Chem. Commun. 1981, 342. Klei, E.; Teuben, J. H.; de L. Meijer, H. J; Kwak, E. J.; Bruins, A. P. J. Organomet. Chem. 1982, 224, 327.
18. (a) Reetz, M. T. Top. Curr. Chem. 1982, 106, 1.
19. (b) Seebach, D.; Weidmann, B.; Widler, L. In Modern Synthetic Methods. Transition Metal in Organic Synthesis; Seheffald, R., Ed.; Otto Salle Verlag: Frankfurt and Main, 1983: p. 217.
20. (c) Reetz, M. T. Organotitanium Reagents in Organic Synthesis; Springer-Verlag: Berlin/Heidelberg, 1986.
21. (d) Roush, W. R. Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon: Oxford, 1991; Vol. 2, p 1.
22. (e) Yamamoto, H. Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon: Oxford, 1991; Vol. 2. p 81.
23. Harada, K.; Urabe, H.; Sato, F. Tetrahedron Lett. 1995, 36, 3203.
24. Kulinkovich, O. G.; Sviridov, S. V.; Vasilevskii, D. A.; Pritytskaya, T. S. Zh. Org. Khim. 1989, 25, 2244. Kulinkovich, O. G.; Sviridov, S. V.; Vasilevski, D. A. Synthesis 1991, 234.
25. Kulinkovich, O. G.; Sviridov, S. V.; Vasilevskii, D. A.; Pritytskaya, T. S. Zh. Org. Khim. 1989, 25, 2244. Kulinkovich, O. G.; Sviridov, S. V.; Vasilevski, D. A. Synthesis 1991, 234.
26. This synthetically unique and useful reaction has attracted considerable interest and the synthetic utility of the reaction as well as the extension of this type of reaction have been reported by several research groups, see: (a) Lee, J.; Kim, H.; Cha, J. K. J. Am. Chem. Soc. 1995, 117, 9919.
27. (b)Corey, E. J.; Achyutha Rao, S.; Noe, M. C. J. Am. Chem. Soc. 1994, 116, 9345.
28. Kulinkovich, O. G.; Savchenko, A. I.; Sviridov, S. V.; Vasilevskii, D. A. Mendeleev Commun. 1993, 192.
29. 3,and an alkyl Grignard reagent and its synthetic utility as a vicinal alkyl dianion equivalent has been a subject of continuous study, see: (a) Lee, J.; Kang, H.; Kim, H.; Cha, J. K. J. Am. Chem. Soc. 1996, 118, 291.
30. (b) Lee, J.; Kim, Y. G.; Bae, J. G.; Cha, J. K. J. Org. Chem. 1996, 61, 4878.
31. (c) Lee, J.; Cha, J. K. Tetrahedron Lett. 1996, 37, 3663.
32. (d) Lee, J.; Kim, H.; Cha, J. K. J. Am. Chem. Soc. 1996, 118, 4198.
33. (e) Lee, J.; Cha, J. K. J. Org. Chem. 1997, 62, 1584.
34. (f) Lee, J.; Ha, J. D.; Cha, J. K. J. Am. Chem. Soc. 1997, 119, 8127.
35. (g) Cho, S. Y.; Lee, J.; Lammi, R. K.; Cha, J. K. J. Org. Chem. 1997, 62, 8235.
36. (h) U, J. S.; Lee, J.; Cha, J. K. Tetrahedron Lett. 1997, 38, 5233.
37. (i) Lee, K. L.; Kim, S.-I.; Cha, J. K. J. Org. Chem. 1998, 63, 9135.
38. (j) Mizojiri, R.; Urabe, H.; Sato, F. Tetrahedron Lett. 1999, 40, 2557.
39. (a) Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124.
40. (b) Hanzawa, Y.; Ito, H.; Taguchi, T. Synlett 1995, 299.
41. Takayanagi, Y.; Yamashita, K.; Yoshida, Y.; Sato, F. Chem. Commun. 1996, 1725.
42. 2 at low temperature but it might be unstable due to the steric bulkiness of the ligated iso-butene.
43. It is now revealed that some kinds of the reaction proceed well also by using an alkyl Grignard reagent other than i-PrMgX; however, even in these cases, i-PrMgX affords better results with respect to the yield. See ref. 13a-f,h,i and 27.
44. Hewlett, D. F.; Whitby, R. J. J. Chem. Soc., Chem. Commun. 1990, 1684. Takahashi, T.; Swanson, D. R.; Negishi, E. Tetrahedron Lett. 1987, 623. Buchwald, S. L.; Watson, B. T.; Huffman, J. C. J. Am. Chem. Soc. 1987, 109, 2544. Kataoka, Y.; Miyai, J.; Oshima, K.; Takai, K.; Utimoto, K. J. Org. Chem. 1992, 57, 1973. Hartung, J. B.; Pedersen, S. F. J. Am. Chem. Soc: 1989, 111, 5468.
45. Hewlett, D. F.; Whitby, R. J. J. Chem. Soc., Chem. Commun. 1990, 1684. Takahashi, T.; Swanson, D. R.; Negishi, E. Tetrahedron Lett. 1987, 623. Buchwald, S. L.; Watson, B. T.; Huffman, J. C. J. Am. Chem. Soc. 1987, 109, 2544. Kataoka, Y.; Miyai, J.; Oshima, K.; Takai, K.; Utimoto, K. J. Org. Chem. 1992, 57, 1973. Hartung, J. B.; Pedersen, S. F. J. Am. Chem. Soc: 1989, 111, 5468.
46. Hewlett, D. F.; Whitby, R. J. J. Chem. Soc., Chem. Commun. 1990, 1684. Takahashi, T.; Swanson, D. R.; Negishi, E. Tetrahedron Lett. 1987, 623. Buchwald, S. L.; Watson, B. T.; Huffman, J. C. J. Am. Chem. Soc. 1987, 109, 2544. Kataoka, Y.; Miyai, J.; Oshima, K.; Takai, K.; Utimoto, K. J. Org. Chem. 1992, 57, 1973. Hartung, J. B.; Pedersen, S. F. J. Am. Chem. Soc: 1989, 111, 5468.
47. Hewlett, D. F.; Whitby, R. J. J. Chem. Soc., Chem. Commun. 1990, 1684. Takahashi, T.; Swanson, D. R.; Negishi, E. Tetrahedron Lett. 1987, 623. Buchwald, S. L.; Watson, B. T.; Huffman, J. C. J. Am. Chem. Soc. 1987, 109, 2544. Kataoka, Y.; Miyai, J.; Oshima, K.; Takai, K.; Utimoto, K. J. Org. Chem. 1992, 57, 1973. Hartung, J. B.; Pedersen, S. F. J. Am. Chem. Soc: 1989, 111, 5468.
48. Hewlett, D. F.; Whitby, R. J. J. Chem. Soc., Chem. Commun. 1990, 1684. Takahashi, T.; Swanson, D. R.; Negishi, E. Tetrahedron Lett. 1987, 623. Buchwald, S. L.; Watson, B. T.; Huffman, J. C. J. Am. Chem. Soc. 1987, 109, 2544. Kataoka, Y.; Miyai, J.; Oshima, K.; Takai, K.; Utimoto, K. J. Org. Chem. 1992, 57, 1973. Hartung, J. B.; Pedersen, S. F. J. Am. Chem. Soc: 1989, 111, 5468.
49. (a) Hungerford, N. L.; Kitching, W. Chem. Commun. 1996, 1697.
50. (b) Launay, V.; Beaudet, I.; Quintard, J.-P. Synlett 1997, 821.
51. (c) Hungerford, N. L.; Kitching, W. J. Chem. Soc., Perkin Trans. I 1998, 1839.
52. Mézailles, N.; Avarvari, N.; Bourissou, D.; Mathey, F.; Floch, P. L. Organometallics 1998, 17, 2677.
53. Yamashita, K.; Sato, F. Tetrahedron Lett. 1996, 37, 7275.
54. Yamashita, K.; Urabe, H.; Sato, F. Tetrahedron Lett. 1997, 38, 4619.
55. Gao, Y.; Harada, K.; Sato, F. Tetrahedron Lett. 1995, 36, 5913. Gao, Y.; Yoshida, Y.; Sato, F. Synlett 1997, 1353
56. Gao, Y.; Harada, K.; Sato, F. Tetrahedron Lett. 1995, 36, 5913. Gao, Y.; Yoshida, Y.; Sato, F. Synlett 1997, 1353
57. Gao, Y.; Shirai, M.; Sato, F. Tetrahedron Lett. 1996, 37, 7787. Gao, Y.; Shirai, M.; Sato, F. Tetrahedron Lett. 1997, 38, 6849.
58. Gao, Y.; Shirai, M.; Sato, F. Tetrahedron Lett. 1996, 37, 7787. Gao, Y.; Shirai, M.; Sato, F. Tetrahedron Lett. 1997, 38, 6849.
59. Gao, Y.; Harada, K.; Sato, F. Chem. Commun. 1996, 533.
60. Urabe, H.; Hamada, T.; Sato, F. J. Am. Chem. Soc. 1999, 121, 2931.
61. Kasatkin, A.; Nakagawa, T.; Okamoto, S.; Sato, F. J. Am. Chem. Soc. 1995, 117, 3881.
62. 2 with allyl ethers; Ito, H.; Taguchi, T.; Hanzawa, Y. Tetrahedron Lett. 1992, 33, 1295 and see also ref. 14b.
63. Hikichi, S.; Gao, Y.; Sato, F. Tetrahedron Lett. 1997, 38, 2867.
64. Teng, X.; Kasatkin, A.; Kawanaka, Y.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1997, 38, 8977.
65. Teng, X.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1998, 139, 6927.
66. Kasatkin, A.; Sato, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 2848.
67. Gao, Y.; Sato, F. J. Org. Chem. 1995, 60, 8136.
68. Lorthiois, E.; Marek, I.; Normant, J. J. Org. Chem. 1998, 63, 2442.
69. Matsuda, S.; An, D. K.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1998, 39, 7513.
70. Nakagawa, T.; Kasatkin, A.; Sato, F. Tetrahedron Lett. 1995, 36, 3207.
71. 4 and thus, the variety of available compounds is limited; see ref. 8e.
72. Yoshida, Y.; Nakagawa, T.; Sato, F. Synlett 1996, 437.
73. An, D. K.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1998, 39, 4861.
74. An, D. K.; Hirakawa, K.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1999, 40, 3737.
75. Okamoto, S.; An, D. K.; Sato, F. Tetrahedron Lett. 1998, 39, 4551.
76. An, D. K.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1998, 39, 4555.
77. Unpublished results of this laboratory.
78. Okamoto, S.; Sato, H.; Sato, F. Tetrahedron Lett. 1996, 37, 8865.
79. Yamazaki, T.; Kasatkin, A.; Kawanaka, Y.; Sato, F. J. Org. Chem. 1996, 61, 2266.
80. Kasatkin, A.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1995, 36, 6075. Okamoto, S.; Kasatkin, A.; Zubaidha, P. K.; Sato, F. J. Am. Chem. Soc. 1996, 118, 2208. Zubaidha, P. K.; Kasatkin, A.; Sato, F. J. Chem. Soc., Chem. Commun. 1996, 197.
81. Kasatkin, A.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1995, 36, 6075. Okamoto, S.; Kasatkin, A.; Zubaidha, P. K.; Sato, F. J. Am. Chem. Soc. 1996, 118, 2208. Zubaidha, P. K.; Kasatkin, A.; Sato, F. J. Chem. Soc., Chem. Commun. 1996, 197.
82. Kasatkin, A.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1995, 36, 6075. Okamoto, S.; Kasatkin, A.; Zubaidha, P. K.; Sato, F. J. Am. Chem. Soc. 1996, 118, 2208. Zubaidha, P. K.; Kasatkin, A.; Sato, F. J. Chem. Soc., Chem. Commun. 1996, 197.
83. 2, and oxygen.
84. (a) Kasatkin, A.; Sato, F. Tetrahedron Lett. 1995, 36, 6079.
85. (b) Kasatkin, A.; Kobayashi, K.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1996, 37, 1849.
86. Mizojiri, R.; Urabe, H.; Sato, F. Angew. Chem., Int. Ed. Engl. 1998, 37, 2666.
87. Chaplinski, V.; de Meijere, A. Angew. Chem., Int. Ed. Engl. 1996, 35, 413. Chaplinski, V.; Winsel, H.; Kordes, M.; de Meijere, A. Synlett 1997, 111.
88. Chaplinski, V.; de Meijere, A. Angew. Chem., Int. Ed. Engl. 1996, 35, 413. Chaplinski, V.; Winsel, H.; Kordes, M.; de Meijere, A. Synlett 1997, 111.
89. Yoshida, Y.; Okamoto, S.; Sato, F. J. Org. Chem. 1996, 61, 7826.
90. Kasatkin, A.; Yamazaki, T.; Sato, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1966. Yamazaki, T.; Urabe, H.; Sato, F. Bull. Chem. Soc. Jpn. 1998, 71, 1673.
91. Kasatkin, A.; Yamazaki, T.; Sato, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1966. Yamazaki, T.; Urabe, H.; Sato, F. Bull. Chem. Soc. Jpn. 1998, 71, 1673.
92. Okamoto, S.; Iwakubo, M.; Kobayashi, K.; Sato, F. J. Am. Chem. Soc. 1997, 119, 6984.
93. Shirai, M.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1999, 40, 5331.
94. Ito, Y.; Fujji, S.; Nakatsuka, M.; Kawamoto, F.; Saegusa, T. Org. Synth. Coll. Vol. 6, p327 (1988).
95. Davies, S. G.; Hedgecock, C. J. R.; McKenna, J. M. Tetrahedron; Asymmetry 1995, 6, 827.
96. (a) Hikichi, S.; Hareau, G. P.-J.; Sato, F. Tetrahedron Lett. 1997, 38, 8299.
97. (b) Hareau, G. P-J.; Koiwa, M.; Hikichi, S.; Sato, F. J. Am. Chem. Soc. 1999, 121, 3640.
98. (c) Koiwa, M.; Hareau, G. P-J.; Morizono, D.; Sato, F. Tetrahedron Lett. 1999, 40, 4199.
99. Hareau, G. P-J.; Hikichi, S.; Sato, F. Angew. Chem., Int. Ed. Engl. 1998, 37, 2099.
100. Mincheva, Z. P.; Gao, Y.; Sato, F. Tetrahedron Lett. 1998, 39, 7947.
101. (a) Buchwald, S. L.; Nielsen, R. B. Chem. Rev. 1988, 88, 1047.
102. (b) Negishi, E. In Comprehensive Organic Synthesis; Trost, B. M.; Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 5, p 1163.
103. (c) Broene, R. D.; Buchwald, S. L. Science 1993, 261, 1696. Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124.
104. (c) Broene, R. D.; Buchwald, S. L. Science 1993, 261, 1696. Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124.
105. (d) Maier, M. In Organic Synthesis Highlights II; Waldmann H., Ed.; VCH; Weinheim, 1995; p 99.
106. (e)Ohff, A.; Pulst, S.; Lefeber, C.; Peulecke, N.; Arndt, P.; Burkalov, V. V.; Rosenthal, U. Synlett 1996, 111.
107. (f) Sato, F.; Urabe, H. In Handbook of Grignard Reagents; Silverman, G. S.; Rakita, P. E., Eds.; Marcel Dekker: New York, 1996; p 23.
108. (g) Negishi, E.; Takahashi, T. Bull. Chem. Soc. Jpn. 1998, 71, 755.
109. Urabe, H.; Hata, T.; Sato, F. Tetrahedron Lett. 1995, 36, 4261.
110. Probert, G.D.; Harding, R.; Whitby, R.J.; Coote, S.J. Synlett 1997, 1371.
111. Urabe, H.; Sato, F. J. Org. Chem. 1996, 61, 6756.
112. Copêret, C.; Negishi, E.; Xi, Z.; Takahashi, T. Tetrahedron Lett. 1994, 35, 695.
113. Urabe, H.; Sato, F. J. Am. Chem. Soc. 1999, 121, 1245.
114. For an example, see: Arai, M.; Lipshutz, B. H.; Nakamura, E. Tetrahedron 1992, 48, 5709.
115. Urabe, H.; Sato, F. Tetrahedron Lett. 1998, 39, 7329.
116. Urabe, H.; Takeda, T.; Sato, F. Tetrahedron Lett. 1996, 37, 1253.
117. (a) Yamaguchi, S.; Jin, R.-Z.; Tamao, K.; Sato, F. J. Org. Chem. 1998, 63, 10060.
118. (b) Block, E.; Birringer, M.; He, C. Angew. Chem., Int. Ed. Engl. 1999, 38, 1604. See also ref. 19b.
119. Hamada, T.; Suzuki, D.; Urabe, H.; Sato, F. J. Am. Chem. Soc. 1999, 121, 7342.
120. Cf. Hill, J. E.; Balaich, G.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1993, 12, 2911.
121. Hideura, D.; Urabe, H.; Sato, F. Chem. Commun. 1998, 271.
122. Urabe, H.; Takeda, T.; Hideura, D.; Sato, F. J. Am. Chem. Soc 1997, 119, 11295.
123. Duthaler, R. O.; Hafner, A. Chem. Rev. 1992, 92, 807; Hoppe. D.; Krämer, T. Angew. Chem. Int. Ed. Engl. 1986, 25, 160. See also ref. 35 and 36.
124. Duthaler, R. O.; Hafner, A. Chem. Rev. 1992, 92, 807; Hoppe. D.; Krämer, T. Angew. Chem. Int. Ed. Engl. 1986, 25, 160. See also ref. 35 and 36.
125. (a) Takayama, Y.; Gao, Y.; Sato, F. Angew. Chem. Int. Ed. Engl. 1997, 36, 851.
126. (b) Takayama, Y.; Okamoto, S.; Sato, F. Tetrahedron Lett. 1997, 38, 8351.
127. Campbell, A. D.; Raynham, T. M.; Taylor, R. J. K. Chem. Commun. 1999, 245.
128. Yamazaki, T.; Urabe, H.; Sato, F. Tetrahedron Lett. 1998, 39, 7333.
129. Takayama, Y.; Okamoto, S.; Sato, F. J. Am. Chem. Soc. 1999, 121, 3559.
130. (a) Suzuki, K.; Urabe, H.; Sato, F. J. Am. Chem. Soc. 1996, 118, 8729.
131. (b) Urabe, H.; Suzuki, K.; Sato, F. J. Am. Chem. Soc. 1997, 119, 10014.
132. Although the structure of the titanacycle (and the relevant species hereafter) is tentatively drawn in the form of α-titanaester for simplicity, its exact nature is unknown and, accordingly, it may be the corresponding titanium enolate.