Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Transfer Hydrogenation of Aldehydes, Allylic Alcohols, Ketones, and Imines Using Molybdenum Cyclopentadienone Complexes Weiwei Wu, Tomohiro Seki, Katherine L. Walker, and Robert M. Waymouth* Department of Chemistry, Stanford University, Stanford, California 94306, United States S Supporting Information *
ABSTRACT: The molybdenum tetraphenylcyclopentadienone complex (C5Ph4O)Mo(CO)3(CH3CN) 1a is an effective precatalyst for the transfer hydrogenation of aldehydes, allylic alcohols, ketones, and imines under mild conditions with either 2-propanol or formic acid as reducing reagent. Mechanistic studies suggest that these molybdenum cyclopentadienone complexes can be reduced to the corresponding hydroxycyclopentadienyl Mo hydrides. These complexes, by virtue of the hydroxyl group on the cyclopentadienyl ligand, are more reactive and chemoselective than the analogous cyclopentadienyl molybdenum complexes for the reduction of ketones, aldehydes, and imines.
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INTRODUCTION Transfer hydrogenation is a powerful and convenient method for the hydrogenation of a wide variety of substrates.1−5 Dihydrogen donors such as 2-propanol and formic acid are inexpensive and available in large scale from renewable energy sources. Among various classes of transition metal transfer hydrogenation catalysts, those that adopt metal−ligand bifunctional mechanisms6,7 are particularly attractive due to their exceptional reactivity, chemo- and enantioselectivity in transfer hydrogenation, and asymmetric transfer hydrogenation of polar bonds. Ruthenium tetraphenylcyclopentadienone (tetracyclone) complexes, first developed by Shvo,4,8,9 represent a class of metal−ligand bifunctional catalysts, where the transfer of separate hydrogen atoms occurs from the metal center and the hydroxyl group on the cyclopentadienyl ligand. Subsequently, several other transition metal analogues have been reported (Fe, Os, Co, Rh, Re, Ir, Ni);4,10−14 the Fe13 and Re14 analogues were shown to be active transfer hydrogenation catalysts. Herein, we report that the molybdenum tetraphenylcyclopentadienone complex 1a15,16 functions as an efficient catalyst precursor for the transfer hydrogenation of aldehydes, ketones, and imines (Scheme 1). Mechanistic studies imply that these molybdenum cyclopentadienone complexes can be reduced to the corresponding hydroxycyclopentadienyl Mo hydrides. These complexes, by virtue of the hydroxyl group on the cyclopentadienyl ligand, are more reactive and chemoselective than the analogous cyclopentadienyl molybdenum complexes.17−19
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Scheme 1. Catalytic Transfer Hydrogenation with Mo Tetracyclone Complex 1a
excess 2-propanol. Cinnamaldehyde was fully reduced to the saturated alcohol, whereas, for a nonconjugated enal, the aldehyde was reduced selectively (entries 2, 3). Aldehydes were hydrogenated much more rapidly than ketones (entries 1, 6, 10). In an internal competition experiment with benzaldehyde and acetophenone, 1a demonstrated 47:1 selectivity for benzaldehyde, which is comparable to the original dimeric ruthenium Shvo catalyst.20 Aldimines were reduced in higher yield than the corresponding ketimines (entries 8, 9). The substituted α,β-unsaturated alcohol was hydrogenated slightly less efficiently than the unsubstituted allyl alcohol (entries 4, 5). Molybdenum complex 1a also showed high chemoselectivity: esters, amide, primary imines, and isolated alkenes and alkynes are not hydrogenated. 2-Acetylpyridine was not hydrogenated possibly due to the inhibition of the pyridine moiety binding to the molybdenum center. Formic acid can also serve as reducing agent for aldehydes, ketones, and allylic alcohols, but not imines. The reaction rate is comparable with 2-propanol as reducing reagent, and was
RESULT AND DISCUSSION
The synthesis of molybdenum complex 1a was carried out as previously reported by Morris et al.15 Molybdenum complex 1a was found to be an efficient and selective catalyst for hydrogenation of polar multiple bonds of aldehydes, allylic alcohols, ketones, and imines (Table 1). Benzaldehyde was hydrogenated to benzyl alcohol (>99%) in 12 h at 65 °C in the presence of © XXXX American Chemical Society
Received: February 8, 2018
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DOI: 10.1021/acs.organomet.8b00086 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Table 1. Transfer Hydrogenation of Aldehydes, Allylic Alcohols, Ketones, and Imines Catalyzed by Molybdenum Complex 1ac
a c
For entries 3−5, formic acid (2.0 M) was used as reducing reagent for the ease of product isolation. bNMR yields (isolated yields in parentheses). Conditions: substrate (0.33 M), molybdenum precatalyst 1a (10.0 mol %), d6-benzene (0.4 mL), 2-propanol (2.0 M) at 65 °C.
to the hydroxyl group on the cyclopentadienyl ligand could not be identified in the 1H NMR spectrum of 1b, presumably as a consequence of hydrogen bonding and exchange. After 12 h at 25 °C, the reaction of 1a with 2-propanol reaches equilibrium; integration of the 1H NMR spectra yields an equilibrium constant Keq = 0.98, as determined from a series of experiments at different initial 2-propanol concentrations. Due to the reversible nature of the reaction, attempts to isolate and fully characterize 1b were unsuccessful. Nevertheless, the identity of 1b could be established by analysis of the reaction mixture of 1a and 2-propanol by 1H NMR and 13C NMR spectroscopy of and by in operando high resolution mass spectrometry21,22 which revealed an ion at m/z 591.0492, consistent with the sodium adduct of 1b [(C5Ph4OH)Mo(H)(CO)3(Na)]+. Treatment of molybdenum complex 1a with excess formic acid in benzene-d6 at 25 °C afforded a mixture of species characterized as the protonated cationic molybdenum complex 2a and the molybdenum formate complex 3 (eq 2). The molybdenum formate complex 3 was identified by 1H NMR and by high resolution mass spectroscopy in negative mode as the deprotonated anion (measured: m/z 611.0386; calcd: m/z 611.0389). The identity of 2a was corroborated by an independent synthesis of the analogous air- and moisture-stable cationic complex 2b (93% yield) from 1a and excess HBF4 in benzene (eq 3; see the Supporting Information).
adopted for entries 3, 4, and 5 for the ease of isolation of volatile products. Molybdenum complex 1a is stable in the presence of water, and the transfer hydrogenation reaction can be run in air, but 1a has limited air stability in its solid state. The kinetics was monitored by 1H NMR spectroscopy with benzaldehyde as dihydrogen acceptor and 2-propanol as the reducing agent. When the catalytic reaction of benzaldehyde and 2-propanol was monitored at 65 °C by 1H NMR, both 1a and a molybdenum hydride, formulated as (C5Ph4OH)Mo(H)(CO)3 1b (δ −3.92), could be identified in the reaction mixtures. The rate of transfer hydrogenation was determined to be firstorder in 1a, 2-propanol, and benzaldehyde for [2-propanol]0 < 1.6 M. At higher concentrations of 2-propanol ([2-propanol]0 > 1.6 M), the rate is insensitive to 2-propanol concentrations, indicative of saturation behavior (Figure S5, Supporting Information). These data suggest that, at modest 2-propanol concentrations, the turnover-limiting step is the formation of a molybdenum hydride complex, formulated as 1b; whereas, in the saturation regime, dihydrogen transfer from molybdenum hydride 1b to benzaldehyde becomes turnover-limiting. The pseudo-second-order rate constant for the catalytic process at 65 °C was determined to be 3.4 × 10−3 M−1 s−1 when [2-propanol]0 > 1.6 M.
A series of stoichiometric experiments were carried out in an effort to identify plausible reaction intermediates. Upon treatment of Mo complex 1a with a stoichiometric amount of 2-propanol in benzene-d6 at 25 °C, analysis by 1H NMR spectroscopy revealed the slow formation of acetone (δ 1.55 ppm) concomitant with that of a new hydride resonance at δ −3.88 ppm. These data are consistent with the oxidation of 2-propanol to acetone and the conversion of the Mo complex 1a to the molybdenum hydride complex 1b (eq 1). Resonances corresponding
The formate complex 3 and the cationic acetonitrile complex 2a are in dynamic exchange in benzene-d6 at 25 °C. Upon mild heating of the reaction mixture (65 °C), partial decarboxylation B
DOI: 10.1021/acs.organomet.8b00086 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 2. Proposed Mechanism for Catalytic Transfer Hydrogenation with Mo Complex 1a
diphenylacetone to diphenylmethanol with a selectivity of 29% in competition with decarboxylation and disproportionation.24 In contrast, complex 1a catalyzes the reduction of benzaldehyde with excess formic acid with high selectivity (90% isolated yield of benzyl alcohol; see the Supporting Information). We propose that the higher selectivity observed with Mo complex 1a is due to the cooperative effect of the hydroxyl substituent on the cyclopentadienyl ligand, as supported by the stoichiometric investigations. While 1a is more active than cyclopentadienyl Mo complexes, it is less active than the corresponding Ru Shvo complexes.4,20,25 Nevertheless, as observed for the Shvo-type complexes,4,25 the Mo complex 1a catalyzes the reduction of carbonyl complexes with formic acid as the reducing agent.26−40 Formic acid is a convenient and inexpensive reducing agent; release of CO2 as byproduct renders the reaction effectively irreversible.41−46 In addition, H2 gas can also be used, but it is less effective than either 2-propanol or formic acid for the reduction of benzaldehyde (see the Supporting Information, p S9).
was observed to generate resonances consistent with the molybdenum hydride complex 1b (eq 2). Similar behavior was observed for the analogous Ru formate complex.23 Treatment of the molybdenum complex 1a with a stoichiometric amount of triethylsilane at room temperature generated the molybdenum hydride species 4 (eq 4). 1H NMR spectroscopy showed upfield shifted resonances for the ethyl groups at δ 0.70 and δ 0.23, and a molybdenum hydride resonance at δ −4.20 ppm. This complex was isolated in 84% yield and characterized by NMR and high-resolution mass spectroscopy as the deprotonated anion of 4 in negative mode (measured: m/z 681.1358; calcd: m/z 681.1368).
The silylated Mo-H 4 was inactive for the catalytic transfer hydrogenation of benzaldehyde with 2-propanol as the dihydrogen source; no reaction was observed with complex 4 over 2 days. When transfer hydrogenation of benzaldehyde was attempted with formic acid as the reducing agent, complex 4 decomposed to unknown products and no reduction was observed for benzaldehyde. Similarly, the cationic complex 2b did not exhibit catalytic activity for the transfer hydrogenation of benzaldehyde with either 2-propanol or formic acid. The lack of activity for either 2b or 4 suggests that both the free hydroxyl group and hydride play an important role for the catalytic transfer hydrogenation reactivity. These data are consistent with a mechanism analogous to that proposed for the Shvotype transfer hydrogenation catalysts4 where both the O-H of the hydroxycyclopentadienyl ligand and the Ru-H are critical for mediating the reduction of carbonyl compounds (Scheme 2). The catalytic reactivity of Mo complex 1a can be contrasted with that of related cyclopentadienyl complexes. The Mo precatalyst 1a exhibits higher catalytic activity and chemoselectivity for carbonyl reduction compared to those reported for cyclopentadienyl molybdenum catalysts that do not contain a hydroxycyclopentadienyl ligand.17−19 For cyclopentadienyl molybdenum complexes (Cp′Mo(CO)3H, Cp′ = cyclopentadienyl or substituted cyclopentadienyl ligand), hydrogenation of carbonyl compounds was proposed to proceed by an ionic mechanism involving oxidative addition of H2 to a cationic Mo center to generate a cationic dihydride.17−19 Parkin24 demonstrated that the related CpMo(PMe3)3−x(CO)xH complexes catalyze dehydrogenation, disproportionation, and transfer hydrogenation reactions of formic acid. In the presence of formic acid, the CpMo(CO)3H complexes catalytically reduce
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CONCLUSION In summary, the molybdenum cyclopentadienone complex 1a15,16 is an efficient catalyst precursor for the transfer hydrogenation of aldehydes, ketones, and imines. Mechanistic studies reveal that these Mo complexes catalyze transfer hydrogenation by a mechanism analogous to that of the Shvo-type complexes of Ru, Re, and Fe.13,14 These studies reveal that the cooperative behavior of redox- and proton-active tetracyclone ligands with metal hydrides, as pioneered by Shvo,4,8,9 is a general strategy for the design of bifunctional transfer hydrogenation catalysts. Efforts to improve the stability and reactivity by modification of the cyclopentadienone ligand are currently in progress.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00086. Experimental procedures, characterization data, and spectra (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Weiwei Wu: 0000-0002-7951-3487 Katherine L. Walker: 0000-0002-7924-8185 Robert M. Waymouth: 0000-0001-9862-9509 C
DOI: 10.1021/acs.organomet.8b00086 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge support from the National Science Foundation (CHE-1565947). K.L.W. acknowledges the Center for Molecular Analysis and Design (CMAD, Stanford) for a fellowship and the National Science Foundation Graduate Research Fellowship Program for support (GRFP).
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DOI: 10.1021/acs.organomet.8b00086 Organometallics XXXX, XXX, XXX−XXX
