Copper complexes of chiral, C2‐symmetric semicorrin ligands were found to be efficient catalysts for the cyclopropane formation from olefins with diazo compounds. In the presence of 1 mode‐% of catalyst, alkyl diazoacetates reacted smoothly with terminal olefins such as styrene, butadiene, and 1‐heptene to give the corresponding optically active cyclopropanecarboxylic‐acid derivatives (Table 1 Scheme 2). With one of the catalysts, enantioselectivities up to 97% ee were obtained (Table 2). Usually, the reactions were carried out using bis(semicorrinato)copper(II) complexes as precatalysts. In order to produce active catalysts, these complexes had to be activated first by heating in the presence of diazoacetate or by treatment with phenylhydrazine. Experiments with (semicorrinato)copper(I) complexes, prepared in situ from copper(I) tert‐butoxide (Scheme 4), suggest that the actual catalyst is a [mono(semicorrinato)]copper(I).

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An efficient synthesis of chiral semicorrin ligands is described (see 6-9, Schemes 2 and 3 ) . Both enantiomers are readily obtained in enantiomerically pure form starting either from D-or L-pyroglutamic acid (1). Semicorrins of this type possess several features that make them attractive ligands for enantioselective control of metal-catalyzed reactions. Their structure is characterized by C, symmetry, a conformationally rigid ligand system, and two stereogenic centers adjacent to the coordination sphere. In a metal complex, the two substituents at the stereogenic centers shield the metal atom from two opposite directions and, therefore, are expected to have a pronounced effect on the stereochemical course of a reaction occurring in the coordination sphere. The structure of these two substituents can be easily modified in a variety of ways. A series of (semicorrinato)copper(II) complexes (see 1 W 4 , Scheme 4 ) has been prepared, and in one case (14), the three-dimensional structure has been determined by X-ray analysis (Fig. I).Esterification of L-pyroglutamic acid ((-)-1) in MeOH, catalyzed by acidic ion-exchange resin, gave the corresponding methyl ester in essentially quantitative yield without notable racemisation. Methyl pyroglutamate Scheme I CN HOOC

Optical yields of up to 97% ee characterize the cyclopropanation of olefins with diazoacetates in the presence of the semicorrin complex CuL2 (R = CMe2OH). The chiral, C2‐symmetrical semicorrin ligands LH are readily accessible in optically pure form from pyroglutamic acid as starting material.

Chiral C2-symmetric chelate ligands having the structure type of semicorrins 1-3 are readily accessible in enantiomerically pure form starting from pyroglutamic acid.l1I The substituents at the two chirality centers can be varied quite easily and are held firmly in position by the rigid ligand skeleton in immediate proximity to the coordination center of the ligand. Semicorrins of this type therefore appeared to offer ideal prerequisites for use in enantioselective catalysis with chiral metal complexes. As a first example of a metal-catalyzed process that proceeds enantioselectively under the influence of chiral semicorrin ligands, we have investigated the cyclopropanation of olefins with diazo compounds. With copper complexes of the ligand 3 as catalysts we obtained enantioselectivities of up to 97% ee.'"We now report on the enantioselective reduction of a,bunsaturated carboxylates with sodium borohydride in the presence of cobalt-semicorrin complexes as catalysts.Our studies in this direction were stimulated by the work of Fischli et al., ['] who had shown that a,b-unsaturated carbonyl compounds can be reduced selectively at the C C double bond with optical yields between 0 and 33% with zinc/acetic acid and cobalamine as ~atalyst.'~] In our case, sodium borohydride in ethanol/dimethylformamide (DMF) proved to be the better choice as reducing agent.["] The ester 4a employed as test substrate''] is virtually inert to this reagent at room temperature in the absence of a catalyst (< 5% conversion after 48 h). In the presence of catalytic amounts (I mol-%) of the semicorrin complex formed in situ from CoCl2 and the ligand 1, on the other hand, a smooth, remarkably uniform reduction to the optically active ester (+)-5a is observed, which proceeds to completion at room temperature within 1-2d (crude product: > 99.8% 5a according to GC).I5' The enantiomeric purity of the product was reproducible at 94% ee in a series of experiments.['] The other substrates investigated so far, with the exception of the phenyl-substituted derivatives 4d and 6d, react with simiIarly high enantioselectivity (cf. Table 1). The (Q-and (3-isomers each afford products of opposite configuration. Ethyl geranate 4b and the (3-isomer 6b are selectively reduced at the conjugated CC double bond, whereas the isolated double bond remains untouched. Upon work-up, the ligand 1 forms a (catalytically inactive) cobalt-bis(semicorrinate) complex, and can be recovered in good yield by decomplexation with acetic acid.The method described here for the enantioselective reduction of a,b-unsaturated carboxylic acid esters can be carried out very easily on a laboratory scale. It has numerous possible applications in organic synthesis ; e.g. isoprenoid chains, as occur inter alia, in the vitamins E and K1, can be synthesized stereoselectively in this ~a y~ ' ,~] (cf. the reactions 4b-5b and 6b-7b). The enantioselectivities lie in the same range as in the catalytic hydrogenation of structurally related a$-unsaturated carboxylic acids and allylic alcohols with t...