Journal
CHEMISTRYSELECT
Volume 4, Issue 29, Pages 8526-8533Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/slct.201902408
Keywords
aryl boronic acids; bioxazoline fused imidazole rings; 1; 4-conjugate addition; N-heterocyclic carbene; rhodium; C-2-symmetry alpha; beta-unsaturated ketone
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Funding
- BASF India, New Delhi, India
- Department of Science and Technology, New Delhi, India [EMR/2014/000254]
- Council of Scientific & Industrial Research (CSIR), New Delhi [01(2880)/17/EMR-II]
- CSIR, New Delhi, India
- BASF India
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Rhodium(I) complexes (1-3) of chiral C-2-symmetric tricyclic bioxazoline fused imidazole derived N-heterocyclic carbene ligands were efficient precatalysts for the 1,4-conjugate addition of aryl boronic acids to cyclohexenone yielding 3-aryl cyclohexanones in moderate to excellent yields but without any chirality transfer. Quite significantly, the intermediates of the proposed catalytic cycle namely, a rhodium hydroxo species (A), a phenyl bound rhodium species (B) and a 3-phenyl cyclohexenolate bound rhodium species (D) have been detected by mass spectrometry. Lastly, the rhodium(I) precatalysts of the type, {(3R,7R)-3,7-di-R-2,3,7,8-tetrahydrodioxazolo[3,2-c:3 ',2 '-e]imidazol-5-ylidene}Rh(COD)Cl, [R=s-butyl (1), i-butyl (2), i-propyl (3), COD=1, 5-cyclooctadiene] were prepared by the direct metalation of tricyclic oxazoline fused imidazole derived N-heterocyclic carbene precursors namely, (3R,7R)-3,7-di-R-2,3,7,8-tetrahydrodioxazolo[3,2-c:3 ',2 '-e]imidazol-4-ium trifluoromethanesulfonate, [R=s-butyl, i-butyl, i-propyl] with {(COD)RhCl}(2) in presence of t-BuOK at room temperature.
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