Journal
EUROPEAN JOURNAL OF INORGANIC CHEMISTRY
Volume -, Issue 20-21, Pages 2285-2299Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/ejic.201701344
Keywords
Ruthenium; Ligand effects; Reaction mechanisms; Transfer hydrogenation; Homogeneous catalysis
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Funding
- Centre National de la Recherche Scientifique (CNRS)
- Institut Universitaire de France (IUF)
- Russian Science Foundation [14-13-00801]
- French Ministry of Foreign Affairs through the French Embassy in Moscow
- Russian Science Foundation [17-13-00102] Funding Source: Russian Science Foundation
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Complex [((6)-Cym)RuCl(NPN)] {Cym = p-cymene; NPN = (pTolN)(2)PPh2} (1) yields a thermally sensitive hydride derivative [((6)-Cym)RuH(NPN)] (2) by reaction with iPrOH in the presence of a strong base, via an observable isopropoxide intermediate [((6)-Cym)Ru(OiPr)(NPN)] (3), or with NaBHEt3 in toluene. Partial conversion also occurs in iPrOH in the absence of base. 2 is stabilized by dihydrogen bonding with isopropyl alcohol, but attempts to isolate it induce isomerization by hydride migration to a ring CH position to yield a 16-electron cyclohexadienyl derivative [{(5)-p-C6H5(Me)(iPr)}Ru(NPN)], which has been crystallographically characterized as a disordered mixture of two regioisomers (4/4). Complex 2 is able to release H-2 upon treatment with medium strength proton donors (fluorinated alcohols), but also slowly with iPrOH. 2 is an active catalyst for the transfer hydrogenation of acetophenone to phenylethanol in isopropyl alcohol. The catalytic transformation is first order in acetophenone and first order in catalyst, with k = 117 +/- 10 m(-1)h(-1) at 40 degrees C. The temperature dependence of the rate constant (25-80 degrees C) gave the activation parameters H-double dagger = 9.6 +/- 1.3 kcalmol(-1) and S-double dagger = -31 +/- 4 calmol(-1)K(-1). DFT calculations have validated the slow isomerization of 2 to 4/4 (high energy TS), the preference of the cyclohexadienyl system for 4/4 relative to the other isomers 4(Me) and 4(iPr), where the hydride has migrated to the CMe or CiPr position, and suggest that the hydrogen transfer mechanism involves outer sphere hydride transfer to the ketone substrate with H-bonding assistance of isopropyl alcohol to yield a sigma complex intermediate [((6)-Cym)Ru+(NPN){H-C(Me)(Ph)O-}].
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