The performance of recently reported highly cis-diastereoselective Rh(I) cyclopropanation catalysts has been significantly improved by a systematic study of different reaction parameters (catalyst activation, solvent, temperature, stoichiometry). The catalyst efficiency and diastereoselectivity were enhanced by changing the activating agent from AgOTf to NaBArf. With this new system, the Rh(I) catalyst was shown to be a highly efficient and cis-diastereoselective cyclopropanation catalyst in reactions between α-diazoacetates and a range of different alkenes and substituted derivatives. Particularly noteworthy is the remarkable reactivity and cis-diastereoselectivity displayed in the reactions between ethyl diazoacetate and cyclopentene, 2,5-dihydrofuran, and benzofuran, with yields up to 99% and cis-selectivities greater than 99%.
Four bis(tetrahydroisoquinoline) N,N′-dioxides were used as catalysts for the epoxide ring opening with tetrachlorosilane under various conditions. A strong solvent effect on asymmetric induction was observed for each of the used catalysts. The highest achieved asymmetric induction for the opening of meso-stilbene oxide was 69% ee. Regarding the cycloalkene oxides, 56% ee was obtained in the reaction with cyclooctene oxide.
Enantioselective allylation of aromatic and α,β‐unsaturated aldehydes with allyltrichlorosilane catalyzed by two diastereoisomeric (R,Rax,R)‐ and (R,Sax,R)‐bis‐1,1′‐[5,6,7,8‐tetrahydro‐3‐(tetrahydrofuran‐2‐yl)isoquinoline] N,N′‐dioxideswas studied. The course of the reaction was profoundly influenced by the chosen solvent. The (R,Sax,R) catalyst efficiently promoted the reaction in THF with enantioselectivity up to 96 %. On the other hand, the allylation of aromatic aldehydes in the presence of the (R,Rax,R) catalyst proceeded only in MeCN (up to 67 % ee), and the level of asymmetric induction was strongly influenced by the presence of electron‐donating and ‐accepting groups in the aldehyde. The allylation of α,β‐unsaturated aldehydes proceeded only in dichloromethane (enantioselectivity up to 68 %).
Enantioselective Epoxide Ring Opening Catalyzed by Bis(tetrahydroisoquinoline) N,N'-Dioxides. -Two earlier synthesized (compounds Ia) and two novel (compounds Ib) axially chiral N-oxide catalysts are tested in the ring opening of epoxides. A strong solvent influence on the catalytic activity and the induced enantioselectivity is observed in all cases. -(KADLCIKOVA, A.; VLASANA, K.; KOTORA*, M.; Collect. Czech. Chem. Commun. 76 (2011) 5, 415-422, http://dx.
Enantioselective Allylation of Aldehydes Catalyzed by Diastereoisomeric Bis(tetrahydroisoquinoline) N,N'-Dioxides. -Asymmetric allylation of aromatic (I) and α,β-unsaturated aldehydes (IV) with allyltrichlorosilane (II) proceeds smoothly in THF in the presence of (Sax)-bis(tetrahydroisoquinoline N-oxide) to give the products (III) and (V) in good yields and moderate to high enantioselectivity. Interestingly, the corresponding (R ax )-ligand does not catalyze this reaction in THF but needs MeCN or CH2Cl2 as solvents to give the products with the same sense of stereoinduction but lower enantioselectivity. -(VLASANA, K.; HRDINA, R.; VALTEROVA, I.; KOTORA*, M.; Eur.
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