Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.
J Am Chem Soc. 2011 Jun 22;133(24):9572-9. doi: 10.1021/ja202676a. Epub 2011 May 27.
1,3-Dipolar cycloaddition reactions of nitrones with α,β-unsaturated aldehydes catalyzed by a cationic chiral dirhodium(II,III) carboxamidate with (R)-menthyl (S)-2-oxopyrrolidine-5-carboxylate ligands in toluene increase reaction rates, give optimum regioselectivities, and enhance stereoselectivities compared to the same reactions performed in traditionally used halocarbon solvents. Rate and enantioselectivity enhancements were also obtained in hetero-Diels-Alder and carbonyl-ene reactions performed in toluene over those obtained in dichloromethane using the diastereomeric chiral cationic dirhodium(II,III) carboxamidate with (S)-menthyl (S)-2-oxopyrrolidine-5-carboxylate ligands. These enhancements are attributed to diminished or absent association of toluene with the catalyst which lessens the relative importance of the uncatalyzed background reaction, and they may also be a consequence of different coordination angles for aldehyde association with rhodium in the different solvent environments. Overall, the enhancement of reaction rates and selectivities with cationic chiral dirhodium(II,III) carboxamidates in toluene suggests broad applications for them in Lewis acid catalyzed reactions.
硝酮与α，β-不饱和醛的 1,3-偶极环加成反应在阳离子手性二钌（II，III）羧酸酯催化剂的催化下进行，该催化剂具有（R）-薄荷基（S）-2-氧代吡咯烷-5-羧酸酯配体，在甲苯中增加反应速率，提供最佳的区域选择性，并提高立体选择性，与在传统卤代烃溶剂中进行的相同反应相比。在手性阳离子二钌（II，III）羧酸酯催化剂的存在下，在甲苯中进行的杂 Diels-Alder 和羰基-烯反应也获得了速率和对映选择性的提高，与在二氯甲烷中进行的反应相比，使用（S）-薄荷基（S）-2-氧代吡咯烷-5-羧酸酯配体的非对映异构体手性阳离子二钌（II，III）羧酸酯催化剂。这些增强归因于甲苯与催化剂的缔合减少或不存在，从而降低了未催化背景反应的相对重要性，并且它们也可能是由于不同溶剂环境中醛与钌的配位角度不同所致。总的来说，在甲苯中使用阳离子手性二钌（II，III）羧酸酯提高反应速率和选择性表明它们在手性 Lewis 酸催化反应中有广泛的应用。
