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Article

Direct Measurement of the Rate of Interconversion of Zirconaaziridine Enantiomers

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Sarah A. Cummings, Jon A. Tunge, and Jack R. Norton*

Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027

J. Am. Chem. Soc., 2008, 130 (14), pp 4669–4679

DOI: 10.1021/ja0757935

Publication Date (Web): March 14, 2008

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, jrn11@columbia.edu

Abstract

The insertion of enantiopure C₂-symmetric diphenylethylene carbonate into the Zr−C bonds of zirconaaziridines leads to the asymmetric synthesis of amino acid methyl esters. Because the zirconaaziridine enantiomers interconvert, the reaction is a dynamic kinetic resolution (DKR). The efficiency of the DKR (the ratio of the two diastereomeric products) is determined by the balance between the rate of enantiomer interconversion and the rate of insertion; slow addition of the inserting enantiopure carbonate is often required to maximize the stereoselectivity. For a case when enantiomer interconversion is fast, its rate constant k_inv has been determined by NMR line broadening; for a case when interconversion is slow, k_inv has been determined by computer simulation of the formation of the diastereomeric products as a function of time; for several intermediate cases, k_inv has been determined by making the zirconaaziridine enantioenriched and monitoring its racemization by CD spectroscopy. The observed k_inv is independent of [THF], implying that interconversion occurs with THF coordinated. Interconversion presumably occurs via an achiral intermediate, either a rapidly inverting (via an η¹-N structure) η³-azaallyl hydride or an η¹-imine. As addition of THF slows insertion without affecting enantiomer interconversion, it produces a more efficient DKR without slow addition of the enantiopure carbonate.

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History

Published In Issue April 09, 2008
Received August 2, 2007

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Article

Direct Measurement of the Rate of Interconversion of Zirconaaziridine Enantiomers