Trimethylsilyl-Substituted Hydroxycyclopentadienyl Ruthenium Hydrides as Benchmarks To Probe Ligand and Metal Effects on the Reactivity of Shvo Type Complexes (original) (raw)
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A detailed mechanistic investigation of the previously reported ruthenium pseudo-dipeptide-catalyzed asymmetric transfer hydrogenation (ATH) of aromatic ketones was performed. It was found that the addition of alkali metals has a large influence on both the reaction rate and the selectivity, and that the rate of the reaction was substantially increased when THF was used as a co-solvent. A novel bimetallic mechanism for the ruthenium pseudodipeptide-catalyzed asymmetric reduction of prochiral ketones was proposed. There is a demand for a larger substrate scope in the ATH reaction, and heteroaromatic ketones are traditionally more challenging substrates. Normally a catalyst is developed for one benchmark substrate, and a substrate screen is carried out with the best performing catalyst. There is a high probability that for different substrates, another catalyst could outperform the one used. To circumvent this issue, a multiple screen was executed, employing a variety of ligands from different families within our group's ligand library, and different heteroaromatic ketones to fine-tune and to find the optimum catalyst depending on the substrate. The acquired information was used in the formal total syntheses of (R)-fluoxetine and (S)-duloxetine, where the key reduction step was performed with high enantioselectivities and high yield, in each case. Furthermore, a new iron-N-heterocyclic carbene (NHC)-catalyzed hydrosilylation (HS) protocol was developed. An active catalyst was formed in situ from readily available imidazolium salts together with an iron source, and the inexpensive and benign polymethylhydrosiloxane (PMHS) was used as hydride donor. A set of sterically less demanding, potentially bidentate NHC precursors was prepared. The effect proved to be remarkable, and an unprecedented activity was observed when combining them with iron. The same system was also explored in the reduction of amides to amines with satisfactory results. iv v List of publications This thesis is based on the following papers, which will be referred to by Roman numerals. Reprints were produced with the kind permission of the publisher.
Journal of Organometallic Chemistry, 2018
Organometallic ruthenium(II) complex [Ru(L 1 C ∧ N ∧ N)(PPh 3) 2 (CO)] (1) [where L 1 H 2 is (E)-N-((1H-pyrrol-2-yl)methylene)naphthalen-1-amine] [H represents dissociable proton] was synthesized via C-H bond activation using different synthetic strategies. Ruthenium hydrido carbonyl complexes [Ru(L 1 N ∧ N)(PPh 3) 2 (CO)H] (2) [where L 1 H 2 is (E)-N-((1H-pyrrol-2yl)methylene)naphthalen-1-amine] and [Ru(L 2 N ∧ N)(PPh 3) 2 (CO)H] (3) [where L 2 H 2 is (E)-N-((1H-pyrrol-2-yl)methylene)-1-phenylmethanamine] were isolated. All the complexes were characterized by UV-Vis, IR and NMR spectral studies. Molecular structures of complexes 1, 2 and 3 were authenticated using X-ray crystallography. Geometry optimization of the complexes 1-3 have been performed using Density Functional Theory (DFT) studies. Time-dependent DFT calculations were performed to better understand the electronic properties of complexes 1-3. Complex 1 was utilized as catalyst in transfer hydrogenation of ketones. On the basis of literature study, the plausible mechanisms were proposed for hydride formation and catalytic transfer hydrogenation.
Organometallics, 2009
The complexes [Rh(PhBP 3)(cod)] (1) and [{Ru(PhBP 3)(µ-Cl)} 2 ] (8), containing the tripodal phosphanoborate ligand [PhB(CH 2 PPh 2) 3 ]-, are efficient catalysts for the selective hydrogenation of cinnamaldehyde to the corresponding allyl alcohol. Complex 8 is one of the most efficient catalysts reported to date for this reaction, in terms of activity (TOF 527 h-1) and selectivity (g97%) under mild reaction conditions (6.8 atm H 2 , 75°C). The rhodium system also displays good catalytic features in the hydrogenation of cinnamaldehyde (TOF 219 h-1), particularly a high selectivity (81%) for this metal in the reduction of the CdO bond. Crotonaldehyde can also be reduced, although the selectivities are not as high as for cinnamaldehyde; 2-cyclohexenone is rapidly and specifically reduced to cyclohexanone by both catalysts. The ruthenium catalyst 8 operates via heterolytic activation of hydrogen, involving monohydride intermediates and possibly ionic hydrogen transfer, while the rhodium complex 1 involves oxidative addition of dihydrogen to form dihydride intermediates and follows a substrate route. Indeed, complex 1 reacts with hydrogen in acetonitrile to give the dihydride complex [Rh(PhBP 3)(H) 2 (NCMe)] (3), while protonation of one of the phosphane arms of the ligand occurs on treatment of complex 1 with HBF 4 to give the cationic species [Rh{PhB(PH)P 2 }(cod)]BF 4. The hydride ligands in 3 are easily removed as molecular hydrogen by reaction with CO under atmospheric pressure to give the rhodium(I) complex [Rh(PhBP 3)(CO) 2 ]. In this reaction, the replacement of acetonitrile by CO takes place previously to the elimination of hydrogen, which indicates that substrates can coordinate to the metal in 3 by replacement of the labile acetonitrile ligand. Under an atmosphere of argon, complex 3 reacts with chloroform to give an equimolecular mixture of the cis and trans isomers of [{Rh(PhBP 3)(H)(µ-Cl)} 2 ] and, eventually, complex [Rh(PhBP 3)Cl 2 ] in one day.
Journal of the American Chemical Society, 2015
A cationic ruthenium hydride complex, [(C6H6)(PCy3)(CO)RuH] + BF4 -(1), with a phenol ligand was found to exhibit high catalytic activity for the hydrogenolysis of carbonyl compounds to yield the corresponding aliphatic products. The catalytic method showed exceptionally high chemoselectivity toward the carbonyl reduction over alkene hydrogenation. Kinetic and spectroscopic studies revealed a strong electronic influence of the phenol ligand on the catalyst activity. The Hammett plot of the hydrogenolysis of 4methoxyacetophenone displayed two opposite linear slopes for the catalytic system 1/p-X-C6H4OH (ρ = −3.3 for X = OMe, t-Bu, Et, and Me; ρ = +1.5 for X = F, Cl, and CF3). A normal deuterium isotope effect was observed for the hydrogenolysis reaction catalyzed by 1/p-X-C6H4OH with an electron-releasing group (kH/kD = 1.7-2.5; X = OMe, Et), whereas an inverse isotope effect was measured for 1/p-X-C6H4OH with an electron-withdrawing group (kH/kD = 0.6-0.7; X = Cl, CF3). The empirical rate law was determined from the hydrogenolysis of 4-methoxyacetophenone: rate = kobsd [Ru][ketone][H2] −1 for the reaction catalyzed by 1/p-OMe-C6H4OH, and rate = kobsd [Ru][ketone][H2] 0 for the reaction catalyzed by 1/p-CF3-C6H4OH. Catalytically relevant dinuclear ruthenium hydride and hydroxo complexes were synthesized, and their structures were established by X-ray crystallography. Two distinct mechanistic pathways are presented for the hydrogenolysis reaction on the basis of these kinetic and spectroscopic data.
Journal of the American Chemical Society, 2008
At high temperatures in toluene, [2,5-Ph2-3,4-Tol2(η 5 -C4COH)]Ru(CO)2H (3) undergoes hydrogen elimination in the presence of PPh3 to produce the ruthenium phosphine complex [2,5-Ph2-3,4-Tol2-(η 4 -C4CO)]Ru(PPh3)(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 PPh3 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.
Catalysis Today, 1998
The effect of pH on the formation and equilibrium distribution of the water soluble ruthenium hydrides [HRuCl(TPPMS) 2 ] 2 , [HRuCl(TPPMS) 3 ] and [H 2 Ru(TPPMS) 4 ] (TPPMS(3-sulfonatophenyl)diphenylphosphine sodium salt) was studied in aqueous solution by pH-potentiometric and 1 H and 31 P NMR methods. Depending on the pH, [RuCl 2 (TPPMS) 2 ] 2 and its hydrido-derivatives hydrolyse extensively, giving rise to formation of hydroxo-ruthenium complexes. It was established that at pH 3.3 the dominant ruthenium(II) species was [HRuCl(TPPMS) 3 ], while at pH!7 it was [H 2 Ru(TPPMS) 4 ]. While [HRuCl(TPPMS) 3 ] catalyzed the slow, selective hydrogenation of the C=C bond in trans-cinnamaldehyde, [H 2 Ru(TPPMS) 4 ] was found an active and selective catalyst for C=O reduction. Consequently, the selectivity of the hydrogenation of transcinnamaldehyde could be completely inverted by minor changes in the solution pH, shifting the equilibrium between [HRuCl(TPPMS) 3 ] and [H 2 Ru(TPPMS) 4 ]. # 1998 Elsevier Science B.V. All rights reserved. Catalysis Today 42 (1998) 441±448 *Corresponding author.
New catalysts for the chemoselective reduction of a,b-unsaturated ketones: Synthesis, spectral, structural and DFT characterizations of mixed ruthenium(II) complexes containing 2-ethene-1,3bis(diphenylphosphino)propane and diamine ligands a b s t r a c t
Organometallics, 1993
The nonclassical trihydrides [(PP3)M(H)(r12-H~)lBPh4 (M = Fe, Ru, Os) are efficient catalyst precursors for the reduction of a,@-unsaturated ketones via hydrogen-transfer from secondary alcohols [PP3 = P(CH2CH2PPh2)31. a,@-Unsaturated ketones bearing bulky substituents a t the double bond (Le. benzylideneacetone) are chemoselectively reduced to allylic alcohols by using either the iron or the ruthenium catalyst. In contrast, the osmium system catalyzes the reduction of a,@-unsaturated ketones to saturated ketones uia isomerization of the initially produced allylic alcohols. A number of reducible substrates including various unsaturated and saturated ketones, aldehydes, alkenes, and alkynes have been studied in order to get information on the steric and electronic factors which may affect the interaction of the substrate with the metal center and, thus, control the selectivity of the hydrogen-transfer reductions. Evidence is provided for the formation of an +-0-benzylideneacetone complex of the formula [(PP3)0s(H){+OCMe-(CH=CHPh))]BPh4 which has been characterized by multinuclear NMR spectroscopy. The latter compound and the related complex [ (P P~) O S (H) (~~-O C M~~) I B P~~ have been used in a number of reactions. As a result, valuable information has been obtained which allows one to propose catalytic cycles for the hydrogen-transfer reduction of a,@-unsaturated ketones to unsaturated alcohols assisted by the Fe and Ru complexes, and for the isomerization of allylic alcohols to saturated ketones catalyzed by the Os complex.
2015
cleavage reactions to form new compounds as these processes are expected to provide novel ways to transformation of inexpensive hydrocarbons into more commercially valuable products such as pharmaceuticals, agrochemicals and polymers. A few examples of transition metal catalyzed cross coupling reactions involving C-N bond cleavage have been reported. A well-defined Ru catalytic system has been developed for oxidative alkylation of alcohol by deaminative coupling reactions of amines to form alkylated ketones. The catalytic method was successfully applied to the decarboxylative and deaminative coupling of amino acids with ketones. Reductive deoxygenation of aldehydes and ketones has attracted considerable attention due to its many applications in fine-chemical synthesis and biofuel production. Classical methods for the deoxygenation of carbonyl compounds are generally associated with harsh reaction conditions and the use of stoichiometric amounts of toxic reagents, and poor functional-group tolerance. A well-defined Ru-H catalyst was found to mediate the reductive deoxygenation of carbonyl compounds to produce aliphatic compounds. Two different mechanistic pathways have been investigated in detail to probe the electronic nature of the catalysts and ligands. Reductive etherification of ketones/aldehydes and alcohols have been studied intensively as cheaper and greener ways to synthesize ethers. A method for the reductive coupling of carbonyl compounds with alcohols has been developed, which involved a highly chemoselective formation of unsymmetrically substituted ether products. The catalytic etherification method employs cheaply available molecular hydrogen as the reducing agent, tolerates a number of common functional groups, and uses environmentally benign water as the solvent.
Homogeneous hydrogenations and related reductive reactions catalyzed by rhenium complexes
2013
Chapter 1 General Introduction Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes hydrogen molecule can coordinate to a metal centre in a side-on fashion (ɳ 2) primarily via donation of its two σ electrons to a vacant d orbital of the metal to form a stable dihydrogen complex. The first structurally characterized dihydrogen metal complex W(CO) 3 (Pi-Pr 3) 2 (H 2) was discovered in 1983 by Kubas and co-workers. 2 The stabilization of ɳ 2-H 2 complexes arises from the back donation of electrons from a filled metal d orbital of metal to the σ* anti bonding orbital of H 2. The back donation is analogous to that of Dewar-Chatt-Duncanson model for olefin coordination (Figure. 1.1). 3 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes 1.4. Hydrogenation of Various Functional Groups 1.4.1. Hydrogenation of Alkenes The first documented example of homogeneous hydrogenation by metal compounds was reported by Calvin in 1938, reporting that quinoline solutions of copper acetate, at 100 °C, were found to be active catalysts for the hydrogenation of quinines 7. The most significant advances in homogeneous hydrogenation catalysis have been the discovery of rhodium phosphine complex [RhH(CO)(PPh 3) 3 ] by Bath and Vaska in 1963. 8 Later in few years, the catalytic activity of this complex for hydrogenation, isomerisation and hydroformylation reactions were reported by Wilkinson and co-workers. 9 The most important rhodium catalyst, the [RhCl(PPh 3) 3 ] complex, was reported during the period 1965-1966 independently by Wilkinson, Bennett and Vaska. 10 Wilkinson and co-workers extensively studied the remarkable catalytic properties of this complex, which is usually known as Wilkinson's catalyst. This turned out to be the first practical hydrogenation system working usually at room temperature and atmospheric pressure of hydrogen. During this time [IrCl(CO)(PPh 3) 2 ] was discovered by Vaska, called Vaska's complex, 11 which was susceptible for oxidative addition-reductive elimination with dihydrogen to form [IrH 2 Cl(CO)(PPh 3) 2 ] whose activity was very weak. Also, the iridium analogue of Wilkinson catalyst, [IrCl(PPh 3) 3 ] was also weakly active. 10b The almost inactivity of these complexes towards hydrogenation were due to inability to form vacant sites by dissociation of PPh 3 ligand from [IrH 2 (PPh 3) 3 ]. For the two decades, rhodium chemistry dominated in the field of hydrogenation, due to the remarkable investigations of Wilkinson, Kagan, Osborn, and Knowles. 12 Ruthenium was slowly developing during these period starting with studies by Halpern 6,13 and Wilkinson. 6,14 In 1965, Wilkinson and co-workers found that the reaction of RuCl 2 (PPh 3) 3 with hydrogen and a base gave the hydride complex RuHCl(PPh 3) 3 , a very active catalyst for hydrogenation. 13 This monohydride complex is formed by the abstraction of proton from the Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes