Simple and efficient: Protonation of Ru(1,2:5,6‐η‐cod)(η6‐cot) (cod=1,5‐cyclooctadiene, cot=1,3,5‐cyclooctatriene) with HBF4⋅Et2O and then reaction with chiral bisphosphane ligands ($_{\rm ...PP}^{\frown }$=Me‐DuPHOS, BINAP, Tol‐BINAP, JOSIPHOS) affords the corresponding Ru($_{\rm PP}^{\frown }$)(H)(η6‐cot)+ or Ru($_{\rm PP}^{\frown }$)(1,2,3,4,5‐η‐C8H11′)+ (C8H11′=2,4‐cyclooctadienyl; see scheme). Exposure of these cations to H2 in solvents (sol) such as acetone, methanol, and THF affords Ru($_{\rm PP}^{\frown }$)(H)(sol)3+, which are catalysts for (amongst other things) enantioselective hydrogenations of alkenes.
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 iPrOH under standard conditions (45 °C, 45 bar H2 or Ar at atmospheric pressure). HY works at a substrate catalyst ratio (s:c) of up to 106 and TRHY at s:c<104. 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, iPrOK 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 iPrOK is added. The ketone TRHY precatalyst Ru(Cl)2((S,S)‐cyP2(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=106, again as tested on 3 in iPrOH 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. iPrOK 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 iPrOK are used and diminish when more is added, while the (low) ee is again about constant in HY as more iPrOK is added (0–4500 equiv). The new Ru(H)(Cl)((S,S)‐cyP2(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 iPrOH and in benzene using (S,S)‐9 A/(S,S)‐9 B/iPrOK or (S,S)‐9 A/(S,S)‐9 B/tBuOK 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 H2 to (S,S)‐11 to give a dihydride Ru(H)2((S,S)‐cyP2(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(ethP2(NH)2) (12) has turned out to be also a powerful precatalyst for the HY of 3 in iPrOH at s:c=106 and of some other substrates. Response to the presence of iPrOK is as before, except that 12 already functions well without it at up to s:c=106.
An RuII catalyst redox pair that works in the enantioselective hydrogenation of ketones in iPrOH is likely to also work in the transfer hydrogenation with iPrOH as the reducing agent, because of the principle of microscopic reversibility, provided the pair is stable in the absence of H2. Conversely, a RuII catalyst redox pair that works in transfer hydrogenation should also work in hydrogenation if the reduced form H‐Ru‐N‐H of the redox pair can be regenerated by addition of H2 to the dehydro form RuN rather than by the backward reaction with iPrOH.
Prototypes of new families of precatalysts and catalysts, Ru((−)‐Me‐DuPHOS)(H)(η6‐1,3,5‐cyclooctatriene)(BF4) and the derived “Ru((−)‐Me‐DuPHOS)(H)(sol)(BF4)”, are presented. They are used in an ...industrial, catalytic, enantioselective hydrogenation that leads to (+)‐cis‐methyl dihydrojasmonate Eq. (1). This stereoisomer is the odorant component of an important, large volume perfumery chemical. P−P=Diphosphane ligand (for example, Me‐DuPHOS=1,2‐bis((2R,5R)‐2,5‐dimethylphospholanyl)benzene); sol=solvent.
Simple and efficient: Protonation of Ru(1,2:5,6-η-cod)(η
-cot) (cod=1,5-cyclooctadiene, cot=1,3,5-cyclooctatriene) with HBF
⋅Et
O and then reaction with chiral bisphosphane ligands ($_{\rm ...PP}^{\frown }$=Me-DuPHOS, BINAP, Tol-BINAP, JOSIPHOS) affords the corresponding Ru($_{\rm PP}^{\frown }$)(H)(η
-cot)
or Ru($_{\rm PP}^{\frown }$)(1,2,3,4,5-η-C
H
')
(C
H
'=2,4-cyclooctadienyl; see scheme). Exposure of these cations to H
in solvents (sol) such as acetone, methanol, and THF affords Ru($_{\rm PP}^{\frown }$)(H)(sol)
, which are catalysts for (amongst other things) enantioselective hydrogenations of alkenes.
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