@article{6556c46242c24d319d8690ce3e1ce05e,
title = "What is the origin of highly asymmetric induction by a chiral (salen)manganese(III) complex? Design of a conformationally fixed complex and its application to asymmetric epoxidation of 2,2-dimethylchromenes",
abstract = "The optically active (salen)manganese(III) complex I having a carboxylate group on the ethylenediamine moiety was found to be an efficient catalyst for the asymmetric epoxidation of several 2,2-dimethylchromene derivatives (up to 99% ee). A high level of turn-over number (up to 9,200) was also achieved in this epoxidation. The pseudo-axially oriented carboxylate in I was postulated to coordinate to the manganese ion and to fix the conformation of the salen ligand in the form enantiomeric to that of normal chiral (salen)manganese,(III) complexes. This was supported by the absolute configurations of the produced epoxides.",
author = "Ito, {Yoshio N.} and Tsutomu Katsuki",
note = "Funding Information: Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan Received 9 March 1998; revised 6 April 1998; accepted 8 April 1998 Abs tract: The optically active (salen)manganese(III) complex 1 having a carboxylate group on the ethylenediamine moiety was found to be an efficient catalyst for the asymmetric epoxidation of several 2,2-dimethylchromene derivatives (up to 99% ee). A high level of turn-over number (up to 9,200) was also achieved in this epoxidation. The pseudo-axially oriented carboxylate in 1 was postulated to coordinate to the manganese ion and to fun the conformation of the salen ligand in the form enantiomeric to that of normal chiral (salen)manganese(III) complexes. This was supported by the absolute configurations of the produced epoxides. {\textcopyright} 1998 Elsevier Science Ltd. All rights reserved. Funding Information: rotation and retention time in the HPLC analysis using a chiral column. Several solvents were employed and, among them, acetonitrile was found to be the most effective for the present reaction. With 2 mol% of 1, the reaction proceeded at 0 °C quantitatively and 98% ee was observed (entry 4). Contrary to the previously reported (salen)manganese(III) catalysts having uncoordinating substituents on their ethylenediamine moiety, high asymmeuic induction was observed without addition of an axial ligand such as pyridine N-oxide or an imidazole derivative. Accordingly, the present procedure is ver b' simple. Furthermore, it was found that 1 was highly stable in the epoxidation conditions. The catalyst could be recovered by column chromatography 'after the reaction and recycled for another run. Moreover, reduction of the catalyst amount of 1 from 2 mol% to 0.01 mol% maintained the high yield and high ee (>98% ee) though prolonged reaction time was required at 0 ~C (entry 9). The turn-over number of the catalyst could be estimated 9,200 times. For the other substrates 5b-e, similar high asymmetric inductions of more than 90% ee were observed (entries 13-20). The observed enantioface selectivity is consistent with that expected from our working hypothesis (vide supra). The equilibration in Figure 2 should shift exclusively to conformer B where the carboxylate group coordinates with the manganese ion. Differing from a Mn-salen complex, external axial ligand, and iodosylbenzene system in which excess external ligand might doubly coordinate with the manganese ion and retard the formation of the oxo-manganese species, the present system which does not use any additional axial ligand allows the new complex 1 to leave one axial coordination site unoccupied. This may partly explain the high catalytic activity of 1. As described here, we have succeeded in developing a novel type of catalyst. The present study provides a conceptually new catalyst for asymmetric reaction. Further application of 1 to other asymmetric reactions, further elucidation of the mechanism, and the optimization of the catalyst are under investigation in our laboratory.l 3 Acknowledgment: Financial support from a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan and from Asahi Glass Foundation is gratefully acknowledged. The authors also thank Nissan Chemical Industries Co. Ltd. for generous supply of some chemicals.",
year = "1998",
month = jun,
day = "11",
doi = "10.1016/S0040-4039(98)00763-1",
language = "English",
volume = "39",
pages = "4325--4328",
journal = "Tetrahedron Letters",
issn = "0040-4039",
publisher = "Elsevier Limited",
number = "24",
}