C₁‐Symmetric Rh/Phebox‐Catalyzed Asymmetric Alkynylation of α‐Ketoesters

Rh‐Catalyzed Direct Enantioselective Alkynylation of α‐Ketiminoesters

“…Next, we examined the effects of substituents on the Phebox ligand (Table 1, entries 4–7), and found that a catalyst with an indanyl substituent, 1 e , gave the highest enantioselectivity (Table 1, entry 7). We also tested ligands of different symmetry ( C 2 and C 1 ) because reactivity and enantioselectivity are significantly affected by ligand symmetry 13. The results indicated that C 2 ‐symmetric catalyst 1 e afforded better enantioselectivity than C 1 ‐symmetric catalysts 1 g and 1 h (Table 1, entries 8 and 9), results that are in sharp contrast to those previously obtained with α‐ketoesters.…”

“…We previously reported Rh–Phebox‐catalyzed direct enantioselective alkynylation of α‐ketoesters 13. The reactions proceed at room temperature to give chiral tertiary propargyl alcohols in high yield with high enantioselectivity.…”

“…Catalyst loading and reaction time were successfully reduced to 2.5 mol % and 12 h, respectively, without a loss of either reactivity or selectivity (Table 1, entry 10). Finally, the use of catalyst 1 f , which has a nitro group at the para position on Phebox, afforded the best result (Table 1, entry 11) 13. Notably, the use of Cu I ‐based catalysts, which are frequently used in direct alkynylation of imines,4–7 only gave the product, 4 a , in low yield and enantioselectivity, thus suggesting that the Rh–Phebox catalyst system is superior for this type of substrate 16.…”

Copper/Guanidine‐Catalyzed Asymmetric Alkynylation of Isatins

“…Initiated by the pioneering work of Carreira and co-workers on the alkynylation of aldehydes, [8a,b] Jiang and co-workers utilized the Zn II /N-methylephedrine derivative/Et 3 Ns ystem in the asymmetric alkynylation of a-ketoesters at 70 8 8C. [11] Ohshima, Mashima, and co-workers [12] reported an efficient asymmetric alkynylation of ethyl trifluoropyruvate catalyzed by a C 1 -symmetric Rh/phebox complex. Both Zn II and Rh III weaken the terminal C(sp)ÀHsothat weakly basic amines or acetate counterions can effect deprotonation with concomitant generation of the corresponding metal alkynilides.T he groups of Chan [10d,i] and Wang [10h] realized the Cu(OTf) 2 /chiral camphorsulfonamide promoted alkynylation of ketones in the presence of as toichiometric amount of alkynylzinc.T he complexation of terminal alkynes with copper(I) in the presence of amines is well known, [13] and both Cu I /diphosphine and Cu I /pybox complexes in the catalytic alkynylation of trifluoromethyl ketones gave only moderate enantioselec-tivity (up to 52 % ee), [14a] and good results in the reaction of isochroman ketals.…”

Copper/Guanidine‐Catalyzed Asymmetric Alkynylation of Isatins

“…[6] Moreover,incomparison with the asymmetric alkynylation of aldehydes, [7][8][9] the reaction of ketones [10] to form at etrasubstituted carbon center has been very limited. [11] Ohshima, Mashima, and co-workers [12] reported an efficient asymmetric alkynylation of ethyl trifluoropyruvate catalyzed by a C 1 -symmetric Rh/phebox complex. [11] Ohshima, Mashima, and co-workers [12] reported an efficient asymmetric alkynylation of ethyl trifluoropyruvate catalyzed by a C 1 -symmetric Rh/phebox complex.…”