Synthesis 2005(3): 429-436  
DOI: 10.1055/s-2005-861800
PAPER
© Georg Thieme Verlag Stuttgart · New York

Planar Chiral Imidazolium Salts in the Asymmetric Rhodium-Catalyzed 1,2-Addition of Arylboronic Acids to Aldehydes

Thilo Focken, Jens Rudolph, Carsten Bolm*
Institut für Organische Chemie der RWTH Aachen, Professor-Pirlet-Str. 1, 52056 Aachen, Germany
Fax: +49(241)8092391; e-Mail: Carsten.Bolm@oc.rwth-aachen.de;
Further Information

Publication History

Received 15 October 2004
Publication Date:
26 January 2005 (online)

Abstract

Planar chiral imidazolium salts 8 and 13 were used as precursors for N-heterocyclic carbene ligands, which were applied in rhodium-catalyzed additions of arylboronic acids to aromatic aldehydes. With pseudo-ortho-disubstituted [2.2]paracyclophane (S,R p)-8b bearing a phosphinoyl substituent at the paracyclophane backbone, 38% ee has been achieved in this rather unexplored reaction.

22

Without addition of TMEDA, the yields were up to 10% lower.

24

Preliminary attempts to use rhodium catalysts bearing analogous planar chiral ligands obtained from oxazoline- or phosphine-containing imidazolium salts (cf. Ref. [17a] or Ref., [17b] respectively) revealed a low catalytic activity of those complexes.

25

With the same catalyst R-configured diarylmethanols can also be obtained. For example, starting from 1-naphthylboronic acid and benzaldehyde, use of (S,R p)-7b afforded the (R)-3g with the same ee as stated in Table [2] .