Well defined, stable, one-point binding ruthenium complexes and selectively bind and activate alpha,beta-unsaturated carbonyl compounds for cycloaddition reactions. These mild Lewis acids catalyze asymmetric 1,4-addition reactions of aryl thiols to enones with product selectivities up to 87% ee. (31)P NMR experiments provide an insight into the intricate equilibria governing the reaction mechanism. The absolute configuration of the major products indicates enones to react in the syn-s-trans orientation. Models based on X-ray structures of the Ru complexes can be used to rationalize selectivity.
The highly tuned, one-point binding cationic cyclopentadienyl-iron and -ruthenium complexes 1 and 2 that incorporate chiral bidentate pentafluoroaryl-phosphinite ligands selectively coordinate and activate alpha,beta-unsaturated carbonyl compounds towards asymmetric catalytic cycloaddition reactions with diaryl nitrones. The reaction gives isoxazolidine products in good yields, with complete endo selectivity and high enantioselectivity. The products are obtained as a mixture of regioisomers in ratios varying from 96:4 to 15:85. The regioselectivity correlates directly with the electronic properties of the nitrone. This is shown by the experimental and computational data.
1,3-Dipolar cycloadditions afforded fast access to isoxazolidines bearing N-alkyl or N-benzyl substituents. The electronic properties of the substituents in the nitrones define the activity of the dipoles and modulate diastereoselectivity in the non-catalyzed reactions. Using a chiral one-point binding ruthenium Lewis acid catalyst, products were obtained in good yields and with excellent regio-, diastereo-, and enantioselectivity.
Highly tuned, one-point binding chiral iron and ruthenium complexes selectively coordinate and activate α,β-unsaturated aldehydes and ketones toward asymmetric catalytic Diels-Alder cycloaddition reactions. Here we focus on the application of these transitionmetal Lewis acids to asymmetric catalytic 1,3-dipolar cycloaddition reaction between enals and cyclic and acyclic nitrones as well as aryl nitrile oxides to give isoxazolidines and isoxazolines, respectively.
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