Carbon has unfaired advantages in material properties to be used as electrodes. It offers a low cost, light weight cathode that minimizes the loss in specific energy of lithium-oxygen batteries as well. To date, however, carbon dioxide evolution has been an unavoidable event during the operation of non-aqueous lithium-oxygen batteries with carbon electrodes, due to the reactivity of carbon against self-decomposition and catalytic decomposition of electrolyte. Here we report a simple but potent approach to eliminate carbon dioxide evolution by using an ionic solvate of dimethoxyethane and lithium nitrate. We show that the solvate leads to deactivation of the carbon electrode against parasitic reactions by electrochemical doping of nitrogen into carbon. This work demonstrates that one could take full advantage of carbon by mitigating the undesired activity.
In the presence of a catalytic amount of BF 3 .Et 2 O, ethyl diazoacetate reacts with CF 3 -imines to afford smoothly the corresponding cis CF 3 -aziridine-2-carboxylates in high yields and with high diastereoselectivity. These CF 3 -aziridines are good precursors of b-amino acids.Aziridine-2-carboxylates are potentially very useful synthetic precursors of non proteogenic a-and b-amino acids, 1 which are important residues of enzyme inhibitors. In the last few years a great deal of research has been devoted to the synthesis of fluorinated analogues of amino acids as highly biologically active molecules with a wide range of potential applications. 2,3 Surprisingly, ring opening of fluoroalkyl aziridine-2-carboxylates has not received much attention and only one example of the preparation of CF 3 -isoserinates was reported by this approach. 4 Furthermore, the described preparation of the starting CF 3 -substituted aziridine carboxylates, involving the ring closure of b-bromo trifluoromethyl amines, gives rise only to the trans isomer. Considering the synthetic potentiality of such CF 3 -subtituted synthons, we became interested in an access to cis 3-CF 3 -2-carboxyl aziridines. A number of methods are available for the synthesis of aziridines. Besides classical ring closure of pre-built b-functionalised amines, 5,6 more direct methods involve C-C bond formation onto imines through the addition of either an a-haloester enolate, 7 or of a carbene moiety generated in the presence of transition metal complexes, 8 or directly of an a-diazoacetate, using a Lewis acid as catalyst. 9 This latter approach presents the great interest to provide preferentially the cis aziridines.We have thus investigated the reaction of ethyl diazoacetate with CF 3 -imines under Lewis acid catalysis. In this letter, we report our preliminary results on the preparation and the ring-opening of cis-CF 3 -aziridines.The imines 1 and 2 were treated with 1.5 equivalents of ethyl diazoacetate using 10% of Lewis acid, in various solvents and at different temperatures, and afforded the corresponding aziridines 3 and 4 (Table 1). a) Recovering of starting imine. b) Determinated by GC. Table 1 Addition of Ethyl Diazoacetate to CF 3 -Imines 1 or 2 in the Presence of Lewis Acids Downloaded by: Chinese University of Hong Kong. Copyrighted material.
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