Asymmetric Deprotonation and Complexation Reactions Mediated by Chiral Ketals as a Route to Ortho-Disubstituted (η6-Arene)Cr(CO)3 Complexes

Jeffrey Aubé, Joseph A. Heppert, Michael L. Milligan, Mary Jane Smith, Paul Zenk

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Abstract

A series of chiral ketals derived from an aryl ketone or aldehyde and one of several C2-symmetrical diols were converted to their corresponding (η6-arene)Cr(CO)3 complexes. The resultant 1,3-dioxolanes were trans substituted at C-4 and C-5 by groups CH2X, where X = (1), OCH3 (2), or N(CH3)2 (3). Ortho deprotonation was attempted on complexes 1–3 using tert-butyllithium in THF solution to afford the corresponding lithio derivatives, which were treated with a variety of electrophiles (MeOSO2F, TMSCl, Ph2C(O), Ph2PCl). Although 1 gave a complex mixture of products, complexes 2 and 3 afforded good yields of disubstituted complexes (with the exception that the lithiated derivative of 3 did not undergo methylation when treated with MeOSO2F). The stereoselectivity of the reactions was determined by NMR spectroscopy and found to be in the range of 3:1 for 2 and >9:1 for 3. The sense of diastereoselection were identified by chemical correlations (for compounds derived from 2) and by circular dichroism spectroscopy. Poor diastereoselection was obtained when this protocol was performed on the corresponding acetal ultimately derived from benzaldehyde and N,N,N′,N′-tetramethyl-1,4-diamino-2,3-butanediol. In addition, a related series of ortho-disubstituted arenes bearing chiral ketal or acetal substituents in the benzylic position were subjected to complexation reactions with (naphthalene)Cr(CO)3 in dibutyl ether. The best diastereoselectivity observed with this methodology was 48%, obtained with the acetal derived from o-tolualdehyde and N,N,N′,N′-tetramethyltartramide.

Original languageEnglish
Pages (from-to)3563-3570
Number of pages8
JournalJournal of Organic Chemistry
Volume57
Issue number13
DOIs
StatePublished - Jun 1 1992

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