Asymmetric Cyclopropanation of Olefins with an in situ Generated Phenyliodonium Ylide

Abstract

A procedure for the in situ generation of the phenyliodonium ylide (2) derived from Meldrum’s acid (1) and its Rh(II)-catalyze­d decomposition to afford an intermediate metallocarbene is described. In the presence of olefins, cyclopropanes are formed with yields and enantioselectivity comparable to that resulting from cyclopropanation with the isolated ylide 2.

asymmetric catalysis - alkenes - carbenes - rhodium - ylides

References

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<A NAME="RG16503ST-14">14</A>

For characterization of the cyclopropanes 5a-f, see ref. [7]

<A NAME="RG16503ST-15">15</A>

Representative procedure (Method A): CH₂Cl₂ (10 mL) was added through syringe into a 50 mL round bottom flask containing a mixture of Meldrum’s acid (1, 10 mmol, 1 equiv), PhI(OAc)₂ (1.4 equiv), [Rh₂(OAc)₄] or [Rh₂{(S)-nttl}₄] (5 mol%), Al₂O₃ (2.3 equiv) and molecular sieves 4 Å (250 mg), followed by the addition of the olefin (10 equiv). The reaction mixture was thermostated in an oil bath to 30 °C and stirred under Ar. 100 µL Samples were taken after several time intervals. The samples were filtered using a syringe filter holder (0.2 µm pore size) and the organic layer was diluted with 100 µL of CH₂Cl₂ and analyzed by GC. The reaction progress was monitored qualitatively by TLC using pentane-EtOAc (2:1) as eluent. An aliquot of the supernatant was used for GC. When maximum conversion was reached, the reaction was terminated by filtration through celite. The residue on the celite was washed twice with CH₂Cl₂. Evaporation of the combined filtrates under reduced pressure followed by chromatography on silica gel column with pentane-EtOAc (2:1) as eluent afforded the desired cyclopropane derivatives.