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Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins with Chiral Rhodium Catalysts in the Presence of Anions
Author(s) -
Buriak Jillian M.,
Klein Jason C.,
Herrington Deborah G.,
Osborn John A.
Publication year - 2000
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/(sici)1521-3765(20000103)6:1<139::aid-chem139>3.0.co;2-u
Subject(s) - chemistry , olefin fiber , rhodium , catalysis , enantioselective synthesis , isomerization , halide , medicinal chemistry , sulfonate , methanol , pentane , organic chemistry , photochemistry , sodium
The strong influence of various anions upon the hydrogenation of 2‐phenyl‐1‐butene, catalyzed by chiral rhodium catalysts was investigated. Both sulfonates and halides exert large increases in the enantioselectivity when [Rh{(−)‐bdpp}(NBD)]ClO 4 (bdpp=2,4‐bis(diphenylphosphino)pentane, NBD=2,5‐norbornadiene) is used as the catalyst precursor at high pressures (70 atm) of dihydrogen in nonpolar solvents. A dihydride mechanism similar to that for Wilkinson's catalyst [RhCl(PPh 3 ) 3 ] was shown to be operating at both high‐ and low‐pressure conditions through a combination of catalytic studies, 31 P, 1 H and parahydrogen‐induced polarization (PHIP) NMR experiments. With sulfonate and in neat methanol, however, a mechanistic switch takes place from a dihydride route (dihydrogen addition before olefin binding) at high pressure to an unsaturate route (olefin binding before dihydrogen addition) at low pressures (<30 atm). Olefin isomerization is inhibited by halide addition, but occurs with sulfonate and in neat methanol through what is most likely a π‐allyl mechanism. A detailed understanding of the effects of addition of these anions is crucial for development of new classes of catalysts capable of efficient enantioselective reduction of prochiral olefins lacking a secondary polar binding group.