REFORMATSKY-TYPE REACTION WITH NEW CALCIUM REAGENTS BY USING CALCIUM ATOM - THF CO-CONDENSATES
Main Group Metal Chemistry
Calcium vapor (atom) when co-condensed with THF reacts very smoothly with α-bromo-and achloroesters at -20 ~ -50 °C to afford enolates, which react with aldehydes to give good yields of ßhydroxyesters in high stereoselectivity after hydrolysis. Although the Reformatsky reaction is a generally applicable procedure for converting ahaloesters to ß-hydroxyesters, there are inherent problems in this method  . The initiation of the reaction and its control for extended times in solvents heated to
... solvents heated to reflux often lead to the desired products in low yields and with poor stereoselectivity. Numerous modifications of the reaction using various metal or metal complexes have been proposed in order to resolve such problems            . During our continuous investigation on the reaction of calcium atoms in the condensed phase [13-15], we have found that calcium atom -THF co-condensates, prepared by a metal vapor method, very smoothly reacts with α-bromoesters or even a-chloroesters at -20 --50 °C to give calcium enolates. The enolates react with aldehydes to give ß-hydroxyesters in good yields after hydrolysis. Herein, we now wish to report the first example of a Reformatsky-type reaction with new calcium reagents by using calcium atom -THF co-condensates. The stereoselectivities are disclosed. At first, the effect of reaction conditions (stoichiometric ratio, temperature, and time) on the yield of the present Reformatsky-type reaction was investigated. As a model reaction using the calcium atom -THF co-condensate, the synthesis of 3-acetoxy-2-methyl-3-phenylpropionate from methyl 2bromopropionate and benzaldehyde after acetoxylation was examined as shown in Scheme 1. The representative results of the reaction conditions are summarized in Table 1 . Adjusting the ratio of calcium atom -THF co-condensate/methyl 2-bromopropionate ratio to ca. 6:1 gave a modest yield of the coupling product. A rather high yield was observed under conditions of -50 °C and for 15-30 min.