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Hydrogenation versus Transfer Hydrogenation of Ketones: Two Established Ruthenium Systems Catalyze Both
Author(s) -
Rautenstrauch Valentin,
HoangCong Xuân,
Churlaud Raphaël,
AbdurRashid Kamaluddin,
Morris Robert H.
Publication year - 2003
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/chem.200304884
Subject(s) - catalysis , ketone , chemistry , acetophenone , ruthenium , substrate (aquarium) , organic chemistry , medicinal chemistry , oceanography , geology
The established standard ketone hydrogenation (abbreviated HY herein) precatalyst [Ru(Cl) 2 (( S )‐tolbinap){( S , S )‐dpen}] (( S ),( S , S )‐ 1 ) has turned out also to be a precatalyst for ketone transfer hydrogenation (abbreviated TRHY herein) as tested on the substrate acetophenone ( 3 ) in i PrOH under standard conditions (45 °C, 45 bar H 2 or Ar at atmospheric pressure). HY works at a substrate catalyst ratio (s:c) of up to 10 6 and TRHY at s:c<10 4 . Both produce ( R )‐1‐phenylethan‐1‐ol (( R )‐ 4 ), but the ee in HY are much higher (78–83 %) than in TRHY (4–62 %). In both modes, i PrOK is needed to generate the active catalysts, and the more there is (1–4500 equiv), the faster the catalytic reactions. The ee is about constant in HY and diminishes in TRHY as more i PrOK is added. The ketone TRHY precatalyst [Ru(Cl) 2 (( S , S )‐cyP 2 (NH) 2 )] (( S , S )‐ 2 ), established at s:c=200, has also turned out to be a ketone HY precatalyst at up to s:c=10 6 , again as tested on 3 in i PrOH under standard conditions. The enantioselectivity is opposite in the two modes and only high in TRHY: with ( S , S )‐ 2 , one obtains ( R )‐ 4 in up to 98 % ee in TRHY as reported and ( S )‐ 4 in 20–25 % ee in HY. i PrOK is again required to generate the active catalysts in both modes, and again, the more there is, the faster the catalytic reactions. The ee in TRHY are only high when 0.5–1 equivalents i PrOK are used and diminish when more is added, while the (low) ee is again about constant in HY as more i PrOK is added (0–4500 equiv). The new [Ru(H)(Cl)(( S , S )‐cyP 2 (NH) 2 )] isomers ( S , S )‐ 9 A and ( S , S )‐ 9 B (mixture, exact structures unknown) are also precatalysts for the TRHY and HY of 3 under the same conditions, and ( R )‐ 4 is again produced in TRHY and ( S )‐ 4 in HY, but the lower ee shows that in TRHY ( S , S )‐ 9 A /( S , S )‐ 9 B do not lead to the same catalysts as ( S , S )‐ 2 . In contrast, the ee are in accord with ( S , S )‐ 9 A /( S , S )‐ 9 B leading to the same catalysts as ( S , S )‐ 2 in HY. The kinetic rate law for the HY of 3 in i PrOH and in benzene using ( S , S )‐ 9 A /( S , S )‐ 9 B / i PrOK or ( S , S )‐ 9 A /( S , S )‐ 9 B / t BuOK is consistent with a fast, reversible addition of 3 to a five‐coordinate amidohydride ( S , S )‐ 11 to give an ( S , S )‐ 11 ‐substrate complex, in competition with the rate‐determining addition of H 2 to ( S , S )‐ 11 to give a dihydride [Ru(H) 2 (( S , S )‐cyP 2 (NH) 2 )] ( S , S )‐ 10 , which in turn reacts rapidly with 3 to generate ( S )‐ 4 and ( S , S )‐ 11 . The established achiral ketone TRHY precatalyst [Ru(Cl) 2 (ethP 2 (NH) 2 )] ( 12 ) has turned out to be also a powerful precatalyst for the HY of 3 in i PrOH at s:c=10 6 and of some other substrates. Response to the presence of i PrOK is as before, except that 12 already functions well without it at up to s:c=10 6 .