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Enantioselective Hydrogenations of Arylalkenes Mediated by [Ir(cod)(JM‐Phos)] + Complexes
Author(s) -
Hou DuenRen,
Reibenspies Joseph,
Colacot Thomas J.,
Burgess Kevin
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20011217)7:24<5391::aid-chem5391>3.0.co;2-1
Subject(s) - oxazoline , chemistry , enantioselective synthesis , phosphine , asymmetric hydrogenation , catalysis , substituent , nitrobenzene , stereochemistry , denticity , medicinal chemistry , metal , organic chemistry
Phosphine oxazoline ligands 1 a – j were converted to the corresponding [Ir(cod)(phosphine oxazoline)] + complexes 2 a – j . X‐ray diffraction analyses of complexes 2 b, 2 h, 2 i , and 2 j were performed. The tert ‐butyl‐, 1,1‐diphenylethyl‐, and phenyl‐oxazoline complexes ( 2 b, 2 h , and 2 i , respectively) had typical square planar metal environments with chair‐like metallocyclic rings. However, the 3,5‐di‐ tert ‐butylphenyl oxazoline complex 2 j was distorted toward a tetrahedral metal geometry. This library of complexes was tested in asymmetric hydrogenations of several arylalkenes. High enantioselectivities and conversions were observed for some substrates. A possible special role for the HPh 2 C‐oxazoline substituent in asymmetric hydrogenations was identified and is discussed. In attempts to rationalize why high enantioselectivities were not observed for some alkenes, a series of deuterium labeling experiments were performed to probe for competing reactions that occurred prior to the hydrogenation step. Double bond migrations were inferred for several substrates, and this is a significant complication in asymmetric hydrogenations of arylalkenes that had not been discussed prior to this study. A mechanistic rationale is proposed involving competing double bond migration for some but not all substrates. Appreciation of this complication will be valuable in further studies aimed at optimization of enantioselection in asymmetric hydrogenations of unfunctionalized alkenes.