N-Halocarbamate Salts Lead to More Efficient Catalytic Asymmetric Aminohydroxylation†‡
Dr. Guigen Li
Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA), Fax: Int. code +(619)784-7562, e-mail: [email protected]
Search for more papers by this authorDr. Hubert H. Angert
Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA), Fax: Int. code +(619)784-7562, e-mail: [email protected]
Search for more papers by this authorCorresponding Author
Prof. K. Barry Sharpless
Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA), Fax: Int. code +(619)784-7562, e-mail: [email protected]
Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA), Fax: Int. code +(619)784-7562, e-mail: [email protected]Search for more papers by this authorDr. Guigen Li
Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA), Fax: Int. code +(619)784-7562, e-mail: [email protected]
Search for more papers by this authorDr. Hubert H. Angert
Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA), Fax: Int. code +(619)784-7562, e-mail: [email protected]
Search for more papers by this authorCorresponding Author
Prof. K. Barry Sharpless
Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA), Fax: Int. code +(619)784-7562, e-mail: [email protected]
Department of Chemistry, The Scripps Research Institute, 10550 N. Torrey Pines Road, La Jolla, CA 92037 (USA), Fax: Int. code +(619)784-7562, e-mail: [email protected]Search for more papers by this authorThis research was supported by the National Institutes of Health (GM 28384), the National Science Foundation (CHE 9531152), and the W. M. Keck Foundation. We thank Dr. Pui Tong Ho for many helpful discussions. H. H. A. is grateful to the Deutsche Forschungsgemeinschaft (DFG) for providing a post-doctoral fellowship.
Dedicated to Professor Koji Nakanishi
Graphical Abstract
Better enantioselectivities and convenient deprotection make the sodium salts of N-chlorocarbamates the oxidant/nitrogen source of choice for the osmium/cinchona alkaloid catalyzed asymmetric aminohydroxylation (AA) of olefins [e.g. Eq. (a)]. This methodology simplifies the synthesis of a vast array of compounds such as unnatural amino acids and other pharmacologically important compounds. DHQH = dihydroquinine, PHAL = 1,3-phthalazinediyl, Z = benzyloxycarbonyl.
References
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- 8 See Scheme 2 in ref. [2]. In the present case × = COOR. This assumption is supported by our finding (not in tables) that methyl trans-cinnamate gives the BOC-protected product in only 78% ee when tert-BuOCONClNa is used as the oxidant (cf. 99% ee and 94% ee in Tables 1 and 2 for the reaction of the same substrate with the corresponding benzyl and ethyl carbamates (R = Bn and Et), respectively).
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- 14 In reexamining the results of the old experiments [9, 11] one sees that only trace amounts of water were used (2–4.5 equiv based on olefin (1% of total solvent)), probably due to concern that the O3OsNCOOR species might hydrolyze to OsO4 faster than it reacted with the olefin. In any case, it is frustrating to realize how very close, and on how many different occasions, one can come to the “right” conditions over years of experimenting, and still miss the target zone or zones within which the reaction conditions support useful, or at least detectable, catalytic turnover.–Metal-catalyzed redox processes are inherently multistep reactions, and the principal challenge in developing them is to find reaction conditions under which each of the steps can function at an acceptable rate. The search is guided above all by the knowledge that compromises are inevitable. A given set of reaction conditions can never be optimal for all the component reactions of the cycle. The slowest step constitutes the turnover-limiting step and defines the throughput (turnover rate) of the entire system [15]. In terms of the utility of a catalyst, the turnover rate (activity) is “not everything, it is the only thing” (paraphrased from a quote about the importance of winning, attributed to Vince Lombardi).–The complexity and interdependence of catalytic cycles means that “apparently” minor variations in the reaction conditions sometimes have major effects on the catalytic activity. This has certainly been the case for osmium-catalyzed oxidations of olefins, for which the maximum turnover rates (vmax) are virtually always identical with the rates for hydrolytic release of the chelated diol or amino alcohol product from the osmium coordination sphere. This is probably the least “interesting” step in the entire sequence, but in catalysis every step in the cycle is by definition of equal importance.
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