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DOI: 10.1055/s-2007-965926
Copper(I)-Catalyzed Enantioselective 1,3-Dipolar Cycloaddition of Azomethine Ylides with Vinyl Sulfones
Publikationsverlauf
Publikationsdatum:
08. Februar 2007 (online)
Abstract
The combination of copper(I)-Taniaphos (5 mol%) is an efficient Lewis acid catalyst for the promotion of the asymmetric 1,3-dipolar cycloaddition of azomethine ylides to aryl vinyl sulfones, providing 3-sulfonylpyrrolidines in good yields and with nearly complete exo-selectivity and good enantiocontrol (typically 65-85% ee). The transformation of the cycloadducts into cis-2,5-disubstituted pyrrolidines of high enantiopurity (>99% ee) has been accomplished after simple recrystallization followed by N-methylation and subsequent reductive desulfonylation.
Key words
copper - azomethine ylides - 1,3-dipolar cycloaddition - ferrocene ligand - asymmetric catalysis - vinyl sulfones - pyrrolidines
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References
The silver(I) acetate catalyzed reaction between 1a and 2a was also studied, providing generally poorer reactivities and enantioselectivities. Interestingly, Taniaphos proved to be also the best ligand in the silver-promoted reaction.
15A number of well-established commercially available P,P-ligands such as Binap, Chiraphos, Norphos, and Phanephos led to low to moderate enantioselectivities (19-42% ee). Very poor results (<10% ee) were obtained with BOX, PyBOX and phosphoramidite ligands. Ferrocene-based ligands gave superior results in this reaction, the Fesulphos family and the Solvias’ ligands Walphos and Taniaphos providing enantioselectivities in the range of 50-83% ee (see ref. 13).
16Sulfones 2b-d were readily available by reaction of the corresponding aryl disulfide with vinylmagnesium bromide and subsequent oxidation of the resulting sulfide to sulfone with Na2WO4·2 H2O/H2O2. On the other hand, 2e was prepared from 2-(dimethylamino)phenyl methyl sulfone and formaldehyde. See the Supporting Information in reference 13 for details.
21Experimental details and analytical data regarding the preparation of imines 1a-j are found within the Supporting Information of reference 13.