COMMUNICATIONS
(2: I), which indicates that during reaction two ligands are bound to the copper ion. The remaining sites in the tetrahedral coordination sphere of the copper ion are likely to be occupied by n-complexation of the enone's double bond,"" and an ethyl fragment transfered from zinc."'] 2) The pyridyl-substituted chalcone 5c gave the corresponding 1,4-product in only 29 % ee, presumably due to competitive binding of the copper catalyst to the enone's pyridine moiety.
Preliminary experiments showed that dioctylzinc, prepared directly from I-octene by boron-zinc exchange,['91 can be used as well; it furnishes the 1,4-products with comparable ee values. This protocol can be extended to functionalized diorganozinc reagents. For instance, with the chiral catalyst derived from 12 and Cu(OTf),, 4-penten-1-y1 acetate was added to 3a with 56% ee. [Eq. (d)]. 0 1) HBEt2 CU(OTf)2 / 12 3a 0 0 51 %, 56% ee
In conclusion a new highly efficient catalyst for asymmetric conjugate addition of diorganozinc reagents to enones has been developed. Remarkable features are the excellent chemoselectivity to give nearly pure 1,4-products, the effective ligand acceleration by new phosphorus amidite ligands, the relatively high ee values for both cyclic and acyclic enones, the efficiency of a monodentate ligand in this asymmetric catalysis, and the fact that alkenes can be used as starting material.
Experimental Procedure
General procedure for ligands 8-16 (argon atmosphere): A warm solution (60Β°C) of (S)-2,Zbinaphthol (2) (860 mg, 3 mmol) in toluene (25 mL) was added in 5 minutes to a cooled solution (-60Β°C) of PCI, (270 pL, 3 mmol), Et,N (860 pL. 6 mmol), and toluene (5 mL). The reaction mixture was stirred for 2 h, warmed to room temperature, and filtered. The filtrate was treated with Et,N (410pL, 2.9 mmol) and 2.9 mmol of the appropriate secondary amine at -40 "C. After 16 h at room temperature, the reaction mixture was filtered, concentrated, and purified by chromatography (SO,, hexane:CH,CI, 2.1) to give the pure amidite (yield 30-80%). (S)-12: [el, = + 591 ( c = 0.68 in CHCI,); 'H NMR (200 MHz, CD-