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

Reaction of [2,3,4,5-Ph(4)(eta(5)-C(4)COH)Ru(CO)(2)H] (2) with different imines afforded ruthenium amine complexes at low temperatures. At higher temperatures in the presence of 2, the complexes decomposed to give [Ru(2)(CO)(4)(mu-H)(C(4)Ph(4)COHOCC(4)Ph(4))] (1) and free amine. Electron-rich imines gave ruthenium amine complexes with 2 at a lower temperature than did electron-deficient imines. The negligible deuterium isotope effect (k(RuHOH)/k(RuDOD) = 1.05) observed 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-determining step. In the dehydrogenation of N-phenyl-1-phenylethylamine (4) to the corresponding imine 8 by [2,3,4,5-Ph(4)(eta(4)-C(4)CO)Ru(CO)(2)] (A), the kinetic isotope effects observed support a stepwise hydrogen transfer where the isotope effect for C-H cleavage (k(CHNH)/k(CDNH) = 3.24) is equal to the combined (C-H, N-H) isotope effect (k(CHNH)/k(CDND) = 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 [2,3,4,5-Ph(4)(eta(4)-C(4)CO)]Ru(CO)(2)NH(CH(3))(CHPhCH(3)) (15), which is the complex between ruthenium and the amine newly generated from the imine. At -80 degrees C the reaction of hydride 2 with 4-BnNH-C(6)H(9)=NPh (18), with an internal amine trap, only afforded [2,3,4,5-Ph(4)(eta(4)-C(4)CO)](CO)(2)RuNH(Ph)(C(6)H(10)-4-NHBn) (19), where the ruthenium binds to the amine originating from the imine, showing that neither complex A nor the diamine is formed. Above -8 degrees C complex 19 rearranged to the thermodynamically more stable [Ph(4)(eta(4)-C(4)CO)](CO)(2)RuNH(Bn)(C(6)H(10)-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.     

Intramolecular trapping of an intermediate in the reduction of imines by a hydroxycyclopentadienyl ruthenium hydride: support for a concerted outer sphere mechanism

Reduction of imines by [2,5-Ph2-3,4-Tol(2)(eta(5)-C(4)COH)]Ru(CO)2H (1) produces kinetically stable ruthenium amine complexes. Reduction of an imine possessing an intramolecular amine was studied to distinguish between inner sphere and outer sphere mechanisms. 1,4-Bn(15)NH(c-C(6)H(10))=NBn (12) was reduced by 1 in toluene-d8 to give 85% of [2,5-Ph2-3,4-Tol(2)(eta(4)-C(4)CO)](CO)(2)RuNHBn(c-C(6)H(10))(15)NHBn (16-RuN,15N), resulting from coordination of the newly formed amine to the ruthenium center, and 15% of trapping product [2,5-Ph2-3,4-Tol(2)(eta(4)-C(4)CO)](CO)(2)Ru(15)NHBn(c-C(6)H(10))NHBn (16-Ru(15)N,N), resulting from coordination of the intramolecular trapping amine. These results provide support for an outer sphere transfer of hydrogen to the imine to generate a coordinatively unsaturated intermediate, which can be trapped by the intramolecular amine. An opposing mechanism, requiring coordination of the imine nitrogen to ruthenium prior to hydrogen transfer, cannot readily explain the observation of the trapping product 16-Ru(15)N,N.     

Isomerization and deuterium scrambling evidence for a change in the rate-limiting step during imine hydrogenation by Shvo's hydroxycyclopentadienyl ruthenium hydride

Hydroxycyclopentadienyl ruthenium hydride 5 efficiently reduces imines below room temperature. Better donor substituents on nitrogen give rise to faster rates and a shift of the rate-determining step from hydrogen transfer to amine coordination. Reduction of electron-deficient N-benzilidenepentafluoroaniline (8) at 11 degrees C resulted in free amine and kinetic isotope effects of k(OH)/k(OD) = 1.61 +/- 0.08, k(RuH)/k(RuD) = 2.05 +/- 0.08, and k(RuHOH)/k(RuDOD) = 3.32 +/- 0.14, indicative of rate-limiting concerted hydrogen transfer, a mechanism analogous to that proposed for aldehyde and ketone reduction. Reduction of electron-rich N-alkyl-substituted imine, N-isopropyl-(4-methyl)benzilidene amine (9), was accompanied by facile imine isomerization and scrambling of deuterium labels from reduction with 5-RuDOH into the N-alkyl substituent of both the amine complex and into the recovered imine. Inverse equilibrium isotope effects were observed in the reduction of N-benzilidene-tert-butylamine (11) at -48 degrees C (k(OH)/k(OD) = 0.89 +/- 0.06, k(RuH)/k(RuD) = 0.64 +/- 0.05, and k(RuHOH)/k(RuDOD) = 0.56 +/- 0.05). These results are consistent with a mechanism involving reversible hydrogen transfer followed by rate-limiting amine coordination.     

Ruthenium-catalyzed transfer hydrogenation of imines by propan-2-ol in benzene

Transfer hydrogenation of a variety of different imines to the corresponding amines by propan-2-ol in benzene catalyzed by [Ru2(CO)4(mu-H)(C4Ph4COHOCC4Ph4)] (1) has been studied. The reaction is highly efficient with turnover frequencies of over 800 per hour, and the product amines were obtained in excellent yields. A remarkable concentration dependence of propan-2-ol was observed when the reaction was run in benzene as cosolvent. An optimum was obtained at 24 equivalents of propan-2-ol to imine, and further increase of the propan-2-ol led to a dramatic decrease in rate. Also the use of polar cosolvents with 24 equivalents of propan-2-ol gave a low rate. It was found that ketimines react faster than aldimines and that electron-donating substituents on the imine increase the rate of the catalytic transfer hydrogenation. Electron-withdrawing substituents decreased the rate. An isomerization was observed with imines having an alpha-hydrogen at the N-alkyl substituent, which is in accordance with a mechanism involving a ruthenium-amine intermediate. It was demonstrated that the ruthenium-amine complex from alpha-methylbenzylamine, corresponding to the postulated intermediate, can replace 1 as catalyst in the transfer hydrogenation of imines. A primary deuterium isotope effect of kCH/CD = 2.7 +/- 0.25 was observed when 2-deuterio-propan-2-ol was used in place of propan-2-ol in the transfer hydrogenation of N-phenyl-(1-phenylethylidene)amine.     

Reduction of imines by hydroxycyclopentadienyl ruthenium hydride: intramolecular trapping evidence for hydride and proton transfer outside the coordination sphere of the metal

Reduction of imines by [2,5-Ph2-3,4-Tol2(eta(5)-C4COH)]Ru(CO)2H (2) produces kinetically stable ruthenium amine complexes. Reduction of an imine by 2 in the presence of an external amine trap gives only the complex of the newly generated amine. Reaction of 2 with H2N-p-C6H4N=CHPh (11), which contains an intramolecular amine trap, gave a 1:1 mixture of [2,5-Ph2-3,4-Tol2(eta(4)-C4CO)](CO)2RuNH(CH2Ph)(C6H4-p-NH2) (8), formed by coordination of the newly generated amine to the ruthenium center, and [2,5-Ph2-3,4-Tol2(eta(4)-C4CO)](CO)2RuNH2C6H4-p-NHCH2Ph (9), formed by coordination of the amine already present in the substrate. These results require transfer of hydrogen to the imine outside the coordination sphere of the metal to give a coordinatively unsaturated intermediate that can be trapped inside the initial solvent cage. Amine diffusion from the solvent cage must be much slower than coordination to the metal center. Mechanisms requiring prior coordination of the substrate to ruthenium would have led only to 8 and can be eliminated.     

Oxidations of hydrocarbons by manganese(III) tris(hexafluoroacetylacetonate)

Mn(hfacac)(3) is an easily prepared and reactive oxidant (hfacac = hexafluoroacetylacetonate). It forms stable solutions in benzene and methylene chloride but is rapidly reduced in acetonitrile, DMSO, acetone, and ethers. It is reduced by ferrocene to give the Mn(II) complex [Cp(2)Fe][Mn(hfacac)(3)], which has been structurally characterized. Mn(hfacac)(3) also rapidly oxidizes 1-acetylferrocene, 1,1'-diacetylferrocene, and tris(4-bromophenyl)amine. Based on an equilibrium established with tris(2,4-dibromophenyl)amine, a redox potential of 0.9 +/- 0.1 V vs Cp(2)Fe(+/0) is calculated. Mn(hfacac)(3) oxidizes 9,10-dihydroanthracene (DHA) cleanly to anthracene, with a bimolecular rate constant of 6.8 x 10(-4) M(-1) s(-1) at 25 degrees C in benzene solution. In the presence of small amounts of water, the manganese(II) product is isolated as cis-Mn(hfacac)(2)(H(2)O)(2), which has also been structurally characterized. Mn(hfacac)(3) also oxidizes xanthene to 9,9'-bixanthene, 1,4-cyclohexadiene to benzene, and 2,4-di-tert-butylphenol to the phenol dimer. Toluene and substituted toluenes are oxidized to tolylphenylmethanes. Product analyses and relative rates--for instance that p-methoxytoluene reacts much faster than toluene--indicate that the more electron rich substrates react by initial electron transfer to manganese. For the less electron rich substrates, such as 1,4-cyclohexadiene, a mechanism of initial hydrogen atom transfer to Mn(hfacac)(3) is suggested. The ability of Mn(hfacac)(3) to abstract H* is reasonable given its high redox potential and the basicity of [Mn(hfacac)(3)](-). In CH(2)Cl(2) solution, oxidation of DHA is catalyzed by chloride ion.     

Diastereospecific and diastereoselective syntheses of Ruthenium(II) complexes using N,N' bidentate ligands aryl-pyridin-2-ylmethyl-amine ArNH-CH2-2-C5H4N and their oxidation to imine ligands

Coordination of N,N' bidentate ligands aryl-pyridin-2-ylmethyl-amine ArNH-CH2-2-C5H4N 1 (Ar = 4-CH3-C6H4, 1a; 4-CH3O-C6H4, 1b; 2,6-(CH3)2-C6H3, 1c; 4-CF3-C6H4, 1d) to the moieties [Ru(bipy)2]2+, [Ru(eta5-C5H5)L]+ (L = CH3CN, CO), or [Ru(eta6-arene)Cl]2+ (arene = benzene, p-cymene) occurs under diastereoselective or diastereospecific conditions. Detailed stereochemical analysis of the new complexes is included. The coordination of these secondary amine ligands activates their oxidation to imines by molecular oxygen in a base-catalyzed reaction and hydrogen peroxide was detected as byproduct. The amine-to-imine oxidation was also observed under the experimental conditions of cyclic voltammetry measurements. Deprotonation of the coordinated amine ligands afforded isolatable amido complexes only for the ligand (1-methyl-1-pyridin-2-yl-ethyl)-p-tolyl-amine, 1e, which doesn't contain hydrogen atoms in a beta position relative to the N-H bond. The structures of [Ru(2,2'-bipyridine)2(1b)](PF6)2, 2b; [Ru(2,2'-bipyridine)(2)(1c)](PF6)2, 2c; trans-[RuCl2(COD)(1a)], 3; and [RuCl2(eta6-C6H6)(1a)]PF6, 4a, have been confirmed by X-ray diffraction studies.     

Mechanism of the copper-free palladium-catalyzed Sonagashira reactions: multiple role of amines

Amines used as bases in copper-free, palladium-catalyzed Sonogashira reactions play a multiple role. The oxidative addition of iodobenzene with [Pd(0)(PPh(3))(4)] is faster when performed in the presence of amines (piperidine>morpholine). Amines also substitute one ligand L in trans-[PdI(Ph)(L)(2)] (L=PPh(3), AsPh(3)) formed in the oxidative addition. This reversible reaction, which gives [PdI(Ph)L(R(2)NH)], is favored in the order AsPh(3)>PPh(3) and piperidine>morpholine. Two mechanisms are proposed for Sonogashira reactions, depending on the ligand and the amine. When L=PPh(3), its substitution by the amine in trans-[PdI(Ph)(PPh(3))(2)] is less favored than that of the alkyne. A mechanism involving prior coordination of the alkyne is suggested, followed by deprotonation of the ligated alkyne by the amine. When L=AsPh(3), its substitution in trans-[PdI(Ph)(AsPh(3))(2)] by the piperidine is easier than that by the alkyne, leading to a different mechanism: substitution of AsPh(3) by the amine is followed by substitution of the second AsPh(3) by the alkyne to generate [PdI(Ph)(amine)(alkyne)]. Deprotonation of the ligated alkyne by an external amine leads to the coupling product. This explains why the catalytic reactions are less efficient with AsPh(3) than with PPh(3) as ligand.     

Novel β-hydroxy-β-bis(trifluoromethyl) imines

Selected imines reacted with hexafluoroacetone non-catalyzed at ambient temperature to give β-hydroxy-β-bis(trifluoromethyl) imines in good to excellent yields. For the imines of acetone, pentan-3-one, and of cyclohexanone a 1:2 reaction was observed giving iminodiols; for N,N′-bis(propylidene)ethylene diamine an iminotetrol was formed. The diol derivative of N-isopropyl-propylidene amine could be deprotonated and O-methylated furnishing the respective ethers. Hexafluoropropylidene amine reacted with N-isopropyl-propylidene amine, unlike hexafluoroacetone, in a 1:1 manner to form an amino-imino alcohol which in its turn is able to add hexafluoroacetone. The imines of acetophenone, trifluoroacetone, 2,4-dimethyl-pentan-3-one, 2,6-dimethyl-cyclohexanone and of acetaldehyde added hexafluoroacetone to furnish β-iminoalcohols. A multifunctional β-hydroxy enaminone was obtained from 4-isopropylamino-pent-3-en-2-one. The molecular structures of the novel β-hydroxy-β-bis(trifluoromethyl) imines exhibit strong (R)N?HO hydrogen bonds.     

Investigation of a possible solvent cage effect in the reduction of 4-aminocyclohexanone by a hydroxycyclopentadienyl ruthenium hydride

No solvent cage effect could be found in the reduction of 4-aminocyclohexanone 3 by Shvo's hydroxycyclopentadienyl ruthenium hydride 2. No preference was observed in the complex formation from trapping by aminoalcohol product 4 over external trapping by aminoketone 3 or by Shvo hydride 2. A solvent cage effect has previously been proposed to support an outer-sphere mechanism in the reduction of imines by 2; this was based on the observation that there was >90% preference for complexation of the newly formed amine over an external amine. Since alcohols form stronger hydrogen bonds than amines a larger cage effect would be expected in the present study. The lack of a cage effect in the present reduction suggests that the previous results from imine reduction require an additional explanation (other than a solvent cage effect).     

Highly Enantioselective Hydrogenation of N‐Aryl Imines Derived from Acetophenones by Using Ru–Pybox Complexes under Hydrogenation or Transfer Hydrogenation Conditions in Isopropanol

The asymmetric reduction of N‐aryl imines derived from acetophenones by using Ru complexes bearing both a pybox (2,6‐bis(oxazoline)pyridine) and a monodentate phosphite ligand has been described. The catalysts show good activity with a diverse range of substrates, and deliver the amine products in very high levels of enantioselectivity (up to 99 %) under both hydrogenation and transfer hydrogenation conditions in isopropanol. From deuteration studies, a very different labeling is observed under hydrogenation and transfer hydrogenation conditions, which demonstrates the different nature of the hydrogen source in both reactions.     

Ultrafast forward and backward electron transfer dynamics of coumarin 337 in hydrogen-bonded anilines as studied with femtosecond UV-pump/IR-probe spectroscopy

Femtosecond infrared spectroscopy is used to study both forward and backward electron transfer (ET) dynamics between coumarin 337 (C337) and the aromatic amine solvents aniline (AN), N-methylaniline (MAN), and N,N-dimethylaniline (DMAN), where all the aniline solvents can donate an electron but only AN and MAN can form hydrogen bonds with C337. The formation of a hydrogen bond with AN and MAN is confirmed with steady state FT-IR spectroscopy, where the C═O stretching vibration is a direct marker mode for hydrogen bond formation. Transient IR absorption measurements in all solvents show an absorption band at 2166 cm(-1), which has been attributed to the C≡N stretching vibration of the C337 radical anion formed after ET. Forward electron transfer dynamics is found to be biexponential with time constants τ(ET)(1) = 500 fs, τ(ET)(2) = 7 ps in all solvents. Despite the presence of hydrogen bonds of C337 with the solvents AN and MAN, no effect has been found on the forward electron transfer step. Because of the absence of an H/D isotope effect on the forward electron transfer reaction of C337 in AN, hydrogen bonds are understood to play a minor role in mediating electron transfer. In contrast, direct π-orbital overlap between C337 and the aromatic amine solvents causes ultrafast forward electron transfer dynamics. Backward electron transfer dynamics, in contrast, is dependent on the solvent used. Standard Marcus theory explains the observed backward electron transfer rates.     

联二萘酚衍生手性磷酸在亚胺不对称转移氢化反应中的应用研究进展

综述了BINOL衍生手性磷酸在亚胺不对称转移氢化反应中的应用研究进展。根据亚胺不对称转移氢化反应中三类不同的氢供体--Hantzsch酯类氢源、2-取代苯并噻唑啉类氢源及其他氢源,对BINOL衍生手性磷酸催化的亚胺不对称转移氢化反应进行了重点介绍。参考文献５３篇。     

Crystal Structure Investigations on Imines derived from 3‐Ferrocenyl‐prop‐2‐enal Crystallizing in Non‐centrosymmetric Space Groups

The condensation of 3‐ferrocenyl‐prop‐2‐enal with primary amines leads to the formation of the corresponding imines in good yields. The crystal structures of imines derived from p‐dimethylamino‐aniline and furfurylamine are determined by the ability of the functional groups to act as hydrogen bond donor or acceptor sites. Although N, N‐dimethyl‐N′‐(3‐ferrocenyl‐allylidene)‐benzene‐1, 4‐diamine and furan‐2‐ylmethyl‐(3‐ferrocenyl‐allylidene)‐amine are achiral molecules they crystallize in the non‐centrosymmetric space groups P2₁ and Pca2₁, respectively. The molecular architecture of N, N‐dimethyl‐N′‐(3‐ferrocenyl‐allylidene)‐benzene‐1, 4‐diamine is realized by the incorporation of dichloromethane acting as hydrogen bond donor and acceptor with both hydrogen and both chlorine atoms. On the other hand, the molecules of furan‐2‐ylmethyl‐(3‐ferrocenyl‐allylidene)‐amine are linked by hydrogen bonds towards the centroid of one of the cyclopentadienyl ligands and towards the oxygen atom of the furan ring to produce infinite chains.     

Hydrogen transfer to carbonyls and imines from a hydroxycyclopentadienyl ruthenium hydride: evidence for concerted hydride and proton transfer.

Reaction of ([2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)CO)](2)H)Ru(2)(CO)(4)(mu-H) (6) with H(2) formed [2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)COH)Ru(CO)(2)H] (8), the active species in catalytic carbonyl reductions developed by Shvo. Kinetic studies of the reduction of PhCHO by 8 in THF at -10 degrees C showed second-order kinetics with Delta H(double dagger) = 12.0 kcal mol(-1) and Delta S(double dagger) = -28 eu. The rate of reduction was not accelerated by CF(3)CO(2)H, and was not inhibited by CO. Selective deuteration of the RuH and OH positions in 8 gave individual kinetic isotope effects k(RuH)/k(RuD) = 1.5 +/- 0.2 and k(OH)/k(OD) = 2.2 +/- 0.1 for PhCHO reduction at 0 degrees C. Simultaneous deuteration of both positions in 8 gave a combined kinetic isotope effect of k(OHRuH)/k(ODRuD) = 3.6 +/- 0.3. [2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)COSiEt(3))Ru(CO)(2)H] (12) and NEt(4)(+)[2,5-Ph(2)-3,4-Tol(2)(eta(4)-C(4)CO)Ru(CO)(2)H](-) (13) were unreactive toward PhCHO under conditions where facile PhCHO reduction by 8 occurred. PhCOMe was reduced by 8 30 times slower than PhCHO; MeN=CHPh was reduced by 8 26 times faster than PhCHO. Cyclohexene was reduced to cyclohexane by 8 at 80 degrees C only in the presence of H(2.) Concerted transfer of a proton from OH and hydride from Ru of 8 to carbonyls and imines is proposed.     

Oxidative addition of phenyl bromide to Pd(BINAP) vs Pd(BINAP)(amine). Evidence for addition to Pd(BINAP)

[STRUCTURE: SEE TEXT] The rates of oxidative addition of phenyl bromide to [Pd(BINAP)2] have been measured in the presence and absence of added amine to assess a previous hypothesis that addition to [Pd(BINAP)(amine)] is faster than addition to [Pd(BINAP)]. These data show that addition to the amine complex is not faster than addition to [Pd(BINAP)]. Instead, they are consistent with oxidative addition, even in the presence of amine, to [Pd(BINAP)] as the major pathway. These data underscore the value of studying the stoichiometric reactions of isolated complexes when assessing the mechanism of a catalytic process.     

Synthesis of beta-lactams from a N-rhenaimine: effect of the transition metal on the energetic profile of the Staudinger reaction

As depicted in the scheme, the alkylidenamido complex 1, a N-rhenaimine, reacts with ketenes to afford the beta-lactams 2-4, which possess a {Re(CO)3(bpy)} fragment as substituent at nitrogen. Clean demetalations using HOTf or MeOTf yield the free beta-lactams or N-methyl-beta-lactams along with [Re(OTf)(CO)3(bpy)]. DFT calculations help to rationalize why the reaction is faster than those of non transition metal N-substituted imines.     

Mechanistic investigations of imine hydrogenation catalyzed by dinuclear iridium complexes

Treatment of [Ir2(mu-H)(mu-Pz)2H3(NCMe)(PiPr3)2] (1) with one equivalent of HBF4 or [PhNH=CHPh]BF4 affords efficient catalysts for the homogeneous hydrogenation of N-benzylideneaniline. The reaction of 1 with HBF4 leads to the trihydride-dihydrogen complex [Ir2(mu-H)(mu-Pz)2H2(eta2-H2)(NCMe)(PiPr3)2]BF4 (2), which has been characterized by NMR spectroscopy and DFT calculations on a model complex. Complex 2 reacts with imines such as tBuN=CHPh or PhN=CHPh to afford amine complexes [Ir2(mu-H)(mu-Pz)2H2(NCMe){L}(PiPr3)2]BF4 (L = NH(tBu)CH2Ph, 3; NH(Ph)CH2Ph, 4) through a sequence of proton- and hydride-transfer steps. Dihydrogen partially displaces the amine ligand of 4 to form 2; this complements a possible catalytic cycle for the N-benzylideneaniline hydrogenation in which the amine-by-dihydrogen substitution is the turnover-determining step. The rates of ligand substitution in 4 and its analogues with labile ligands other than amine are dependent upon the nature of the leaving ligand and independent on the incoming ligand concentration, in agreement with dissociative substitutions. Water complex [Ir2(mu-H)(mu-Pz)2H2(NCMe)(OH2)(PiPr3)2]BF4 (7) hydrolyzes N-benzylideneaniline, which eventually affords the poor hydrogenation catalyst [Ir2(mu-H)(mu-Pz)2H2(NCMe)(NH2Ph)(PiPr3)2]BF4 (11). The rate law for the catalytic hydrogenation in 1,2-dichloroethane with complex [Ir2(mu-H)(mu-Pz)2H2(OSO2CF3)(NCMe)(PiPr3)2] (8) as catalyst precursor is rate = k[8]{p(H2)}; this is in agreement with the catalytic cycle deduced from the stochiometric experiments. The hydrogenation reaction takes place at a single iridium center of the dinuclear catalyst, although ligand modifications at the neighboring iridium center provoke changes in the hydrogenation rate. Even though this catalyst system is also capable of effectively hydrogenating alkenes, N-benzylideneaniline can be selectively hydrogenated in the presence of simple alkenes.     

Hydrogen elimination from a hydroxycyclopentadienyl ruthenium(II) hydride: study of hydrogen activation in a ligand-metal bifunctional hydrogenation catalyst

At high temperatures in toluene, [2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)COH)]Ru(CO)(2)H (3) undergoes hydrogen elimination in the presence of PPh(3) to produce the ruthenium phosphine complex [2,5-Ph(2)-3,4-Tol(2)-(eta(4)-C(4)CO)]Ru(PPh(3))(CO)(2) (6). In the absence of alcohols, the lack of RuH/OD exchange, a rate law first order in Ru and zero order in phosphine, and kinetic deuterium isotope effects all point to a mechanism involving irreversible formation of a transient dihydrogen ruthenium complex B, loss of H(2) to give unsaturated ruthenium complex A, and trapping by PPh(3) to give 6. DFT calculations showed that a mechanism involving direct transfer of a hydrogen from the CpOH group to form B had too high a barrier to be considered. DFT calculations also indicated that an alcohol or the CpOH group of 3 could provide a low energy pathway for formation of B. PGSE NMR measurements established that 3 is a hydrogen-bonded dimer in toluene, and the first-order kinetics indicate that two molecules of 3 are also involved in the transition state for hydrogen transfer to form B, which is the rate-limiting step. In the presence of ethanol, hydrogen loss from 3 is accelerated and RuD/OH exchange occurs 250 times faster than in its absence. Calculations indicate that the transition state for dihydrogen complex formation involves an ethanol bridge between the acidic CpOH and hydridic RuH of 3; the alcohol facilitates proton transfer and accelerates the reversible formation of dihydrogen complex B. In the presence of EtOH, the rate-limiting step shifts to the loss of hydrogen from B.     

Photoreduction of 4,4'-bipyridine by amines in acetonitrile-water mixtures: influence of H-bonding on the ion-pair structure and dynamics

The photoreduction of 4,4'-bipyridine (44BPY) by diazabicyclo[2.2.2]octane and triethylamine (TEA) is investigated by using picosecond transient absorption and time-resolved resonance Raman spectroscopy in various acetonitrile-water mixtures. The results are interpreted on the basis of a preferential solvation effect resulting from the presence of a specific interaction between 44BPY and water by hydrogen bonding. Below 10% water, the free 44BPY species is dominant and leads upon photoreduction to a contact ion pair that undergoes efficient intrapair proton transfer if TEA is the amine donor. Above 10% water, most of the 44BPY population is H-bonded and leads upon photoreduction to a hydrated ion pair in which the intrapair proton transfer is inhibited. Instead, the 44BPY(-*) species is protonated by water through the hydrogen bond with a rate constant that increases by more than 3 orders of magnitude on going from 10% to 100% water. The dependence of this rate constant on the solvent mixture composition suggests that the reaction of intracomplex proton transfer is controlled by the hydration of the residual OH(-) species by three molecules of water, leading to a trihydrated HO(-)(H(2)O)(3) species.