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Mechanistic Study of Hydrogen Transfer to Imines from a Hydroxycyclopentadienyl Ruthenium Hydride. Experimental Support for a Mechanism Involving Coordination of Imine to Ruthenium Prior to Hydrogen Transfer.
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2006 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 128, no 44, 14293-14305 p.Article in journal (Refereed) Published
Abstract [en]

Ruthenium hydroxycyclopentadienyl complex exhibits activity in hydrogenation of imines; the mechanism of the reaction was explored by isolation of the intermediates, evaluation of substituent effects and kinetic isotope effect detn. Reaction of [(Cp'OH)Ru(CO)2H] (2; Cp'OH = η5-2,3,4,5-Ph4C4COH) with different imines afforded ruthenium amine cyclopentadienone [(η4-Ph4C4O)Ru(NHR1R2)(CO)2] complexes at low temps. At higher temps. in the presence of 2, the complexes decompd. to give mixed hydroxycyclopentadiene-cyclopentadienone dimer [Ru2(CO)4(Ό-H)(Ό-C4Ph4COHOCC4Ph4)] (1) and free amine. Electron-rich imines gave ruthenium amine complexes with 2 at a lower temp. than did electron-deficient imines. The negligible deuterium isotope effect (kRuHOH/kRuDOD = 1.05) obsd. in the reaction of 2 with N-phenyl[1-(4-methoxyphenyl)ethylidene]amine (12) shows that neither hydride (RuH) nor proton (OH) is transferred to the imine in the rate-detg. step. In the dehydrogenation of N-phenyl-1-phenylethylamine (4) to the corresponding imine (8) by cyclopentadienone complex [(η4-Ph4C4CO)Ru(CO)2] (A), the kinetic isotope effects obsd. support a stepwise hydrogen transfer where the isotope effect for C-H cleavage (kCHNH/kCDNH = 3.24) is equal to the combined (C-H, N-H) isotope effect (kCHNH/kCDND = 3.26). Hydrogenation of N-methyl(1-phenylethylidene)amine (14) by 2 in the presence of the external amine trap N-methyl-1-(4-methoxyphenyl)ethylamine (16) afforded 90-100% of complex [(η4-Ph4C4CO)Ru(CO)2[NHMe(CHPhMe)]] (15), which is the complex between ruthenium and the amine newly generated from the imine. At -80° the reaction of hydride 2 with cyclohexylideneamine 4-PhCH2NH-C6H9:NPh (18), with an internal amine trap, only afforded [(η4-Ph4C4CO)(CO)2Ru[NH(Ph)(C6H10-4-NHCH2Ph)]] (19), where the ruthenium binds to the amine originating from the imine, showing that neither complex A nor the diamine is formed. Above -8° complex 19 rearranged to the thermodynamically more stable [(η4-Ph4C4CO)(CO)2Ru[NH(PhCH2)(C6H10-4-NHPh)]] (20). These results are consistent with an inner sphere mechanism in which the substrate coordinates to ruthenium prior to hydrogen transfer and are difficult to explain with the outer sphere pathway previously proposed. [on SciFinder(R)]

Place, publisher, year, edition, pages
2006. Vol. 128, no 44, 14293-14305 p.
Keyword [en]
ruthenium half sandwich hydroxycyclopentadienyl hydride imine redn mechanism, cyclopentadienone ruthenium amine complex formation imine redn hydroxycyclopentadienyl hydride, crystal structure ruthenium cyclopentadienone amine dicarbonyl half sandwich, mol structure ruthenium cyclopentadienone amine dicarbonyl half sandwich
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-174517DOI: 10.1021/ja061494oOAI: oai:DiVA.org:uu-174517DiVA: diva2:527602
CAPLUS AN 2006:1079279(Journal)Available from: 2012-05-21 Created: 2012-05-21 Last updated: 2012-05-21

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Samec, Joseph S. M.
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