Article Cite This: Organometallics XXXX, XXX, XXX−XXX
pubs.acs.org/Organometallics
Hydrosilylation of Carbonyl Compounds Catalyzed through a Lithiated Hydrazone Derivative Á lvaro Raya-Baroń ,†,§ Pascual Oña-Burgos,†,§,∥ Antonio Rodríguez-Dieǵ uez,‡ and Ignacio Fernań dez*,† †
Organometallics Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/14/18. For personal use only.
Department of Chemistry and Physics, Research Centre CIAIMBITAL, University of Almería, Ctra. Sacramento, s/n, Almería E-04120, Spain ‡ Department of Inorganic Chemistry, Faculty of Science, University of Granada, 18071 Granada, Spain S Supporting Information *
ABSTRACT: A well-defined lithiated hydrazone derivative has been synthesized and fully characterized through various analytical platforms, including multinuclear (1H, 13C, 15N, 7Li) and two-dimensional NMR, high-resolution MS spectrometry, IR, and X-ray diffraction crystallography. It behaves as a binuclear species in the solid state and as a monomeric contact ion pair in solution. It has also been tested as a catalyst in hydrosilylation reactions, being the first lithium hydrazone reported to catalyze the full conversion of carbonyls of different nature into alcohols in short reaction times, at room temperature, and with catalyst loadings equal to or below 0.5 mol %. Kinetic studies have proven fractional order dependences with respect to ketone and silane and first order dependence in the case of the catalyst. The proposed reaction mechanism is characterized by the nucleophilic addition of the lithium hydrazonide to the silicon atom of the silane to give a five-coordinate silicon species.
■
INTRODUCTION The reduction of organic substrates into value-added products has found many applications in industrial and academic settings.1 The addition of silanes to unsaturated functional groups, the so-called hydrosilylation, has attracted great interest in the last few years.2 Transition-metal catalysts, both noble metals2,3 and first-row transition metals,4 have provided remarkable activity, as well as control of the chemo-, regio-, and stereoselectivity in the reduction process. Maingroup-element compounds have also been employed since the discoveries of Volpin et al., where CsF or ammonium fluoride catalyzed the hydrosilylation of carbonyls.5 Main-groupcatalyzed reductions of unsaturated bonds have been nicely reviewed by Revunova et al. very recently.6 Electrophilic boranes have also been used as effective catalysts for the hydrosilylation of carbonyls and imine functions.7 Studies related to those described in the present article are those reported by Hosomi et al. that employed alkali-metal alkoxides to carry out the chemo-8 and stereoselective9 catalytic reduction of aldehydes, ketones, and imines. Similarly, the reduction of hydroxy esters, lactones, and tosylimines catalyzed by LiOMe has been also described.10 More recently, tertiary amides were efficiently reduced to their corresponding enamines under hydrosilylation conditions, using t-BuOK (5 mol %) and (MeO)3SiH or (EtO)3SiH as the reducing agent.11 Simple bases such as t-BuOK and KOH can catalyze polymethylhydrosiloxane (PMHS) reduction of ketones and esters to alcohols and the reduction of aldimines to amines, as described by Nikonov and co-workers.12 Most of the base© XXXX American Chemical Society
catalyzed systems described so far require typical catalyst loadings of 5−10 mol %, which in turn represent some of their main disadvantages. To the best of our knowledge, hydrosilylation catalyzed by lithium hydrazones or lithium amides have not been described. We therefore describe herein the first time where a lithium hydrazone of anthraquinonic nature catalyzes the hydrosilylation of polarized unsaturated bonds with very low catalyst loadings and at room temperature. The pivotal role of the anthraquinone moiety will be highlighted.
■
RESULTS AND DISCUSSION Ligand 1 has been synthesized from inexpensive starting materials in high yields through 1-hydrazinoanthraquinone, which was prepared following reported methods (Scheme 1).13 NMR, mass spectrometry, IR, and X-ray diffraction crystallographic data confirmed the formation of (E)-1-(2-(pyridin-2ylmethylene)hydrazinyl)anthraquinone (1) in pure form. 1 H,15N gHMBC experiments performed on a THF-d8 solution of ligand 1 allowed the identification of N1, N2, and N3 signals, which were located at δN 149.6, 326.3, and 315.9 ppm, respectively (Figure S15). Crystals of 1·HBF4 were grown by layering hexane over a dichloromethane solution of 1 in the presence of a few drops of fluoroboric acid to assist with crystallization. The X-ray diffraction structure is given in Figure S4 and confirms, on one Received: May 15, 2018
A
DOI: 10.1021/acs.organomet.8b00315 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
These dinuclear molecules are packed thanks to π-stacking interactions involving anthraquinone units (Figure S6). We have occasionally observed that the solution and solidstate structures of group 1 salts may be different and are strongly dependent on the choice of solvent.15 Consequently, we have studied the solution structure of 2 via multinuclear NMR spectroscopy together with 1H and 7Li PGSE NMR diffusion measurements. The 1H NMR spectrum of 2 at 294 K exhibited averaged signals for the two hydrazone ligands incorporated in the complex so that no distinction between the dimer and monomer could be inferred. The whole set of signals spans from δH 7.14 to 8.54 ppm, where those coming from the pyridine moiety and those from the anthraquinone were distinguished thanks to COSY correlations (Figure S22). The assignment of the 13C NMR spectrum was accomplished in combination with DEPT-135 and two-dimensional HMQC and HMBC experiments (Figures S19−S21). 1H,15N gHMQC 2D NMR allowed the observation of all three nitrogen atoms N1, N2, and N3, which were located at δN 264.3, 383.5, and 313.3 ppm, respectively. The values for N1 and N2 are clearly shifted downfield with respect to the neutral ligand (1) with coordination shifts of Δ(15Ncoord) = +115.0 and +57.6 ppm, respectively, which matches earlier observations of similar trends in δN of metal complexes and parallels the effects induced by alkylation or protonation of a nitrogen lone pair.16 Importantly, N3 did not shift significantly from the value of its neutral ligand, showing a 15N coordination shift of Δ(15Ncoord) = −2.6 ppm, which points to a solution structure where the binuclear scaffold has been broken up into mononuclear species and where the pyridine moiety is not chelating the lithium atom and, therefore, no apparent change in its nitrogen-15 chemical shift is observed. To ascertain and verify the real solution structure in comparison to the dimeric architecture found in the solid state, PGSE diffusion NMR measurements (1H and 7Li) in combination with 1H−7Li-HOESY experiments were performed. The 1H and 7Li PGSE NMR results for THF-d8 solutions of 1 and 2 are given in Table S1 (further details are given in the Supporting Information). The almost equivalent D values for both the anion and cation in 2 point to a contact ion pair in THF solution. From the X-ray diffraction structure of 2, one can estimate a radius of 6.2 Å. Therefore, the 5.2 Å rH value based on the measured D coefficients is consistent with a mononuclear contact ion pair at 294 K.17 The relative intensities of the cross-peaks in a 7Li,1H HOESY experiment are determined by the competition between the NOE buildup and the decay of signals via relaxation. For 2 these values were large enough to be adequate for their observation (Figure 2) and only one cross-peak between the 7Li nucleus and the proton H15 was detected. No other cross-peaks were observed, since all other Li···H through-space distances are greater than 4 Å (Li···H−C5 4.005 Å; see the molecular structure of 2 in Figure 1). In addition, no cross-peaks between the 7Li nuclei and the protons of THF were detected, probably because of isotopic dilution, through which the majority of the protonated THF molecules are displaced by deuterated molecules and the correlation signal may disappear under the noise. Aside from the crystallographic characterization, NMR, and FT-IR spectroscopy, MS spectrometry was also used to verify the structural nature of 2 in more detail. In particular, MS (ESI) spectrometry in THF/dichloromethane solution gave evidence for the cleavage of the binuclear scaffold in solution.
Scheme 1. Synthesis of Derivatized Anthraquinone 1 and Its Lithiated Derivative 2 Starting from 1-Chloroanthraquinone
hand, the protonation of the pyridinic nitrogen, and on the other hand, the chelating potential of the quinonic moiety, since an intramolecular hydrogen bond between NH and C O of length ca. 2.623 Å is observed. The reaction of 1 with 1 equivalent of LiHMDS in THF led to the quantitative formation of complex 2 (Scheme 1), which is stable at room temperature over several days or weeks under an inert atmosphere. This new Li hydrazone derivative has been fully characterized by NMR in THF-d8 solution and by Xray diffraction in the solid state. X-ray diffraction studies for the lithium material 2 revealed a binuclear coordination compound where both lithium ions are equivalent and contribute toward the formation of a 12-membered ring based on Li−N bonds (Figure 1). The lithium atoms are equivalent and are both
Figure 1. Molecular structure of 2 in the solid state. Hydrogen atoms are omitted for clarity. The perspective of the complex is supported by an X-diffraction study.
four-coordinated with a LiO2N2 core almost of tetrahedral geometry. The connectivity of this complex was supported by an X-ray diffraction study, but unfortunately its quality prevents its publication. This coordination environment is established from two nitrogen atoms pertaining to the pyridine ring and imine group, one oxygen atom from the anthraquinone unit (CO), and one coordinated tetrahydrofuran molecule. Similar heterometallic macrocycles have been already described by Dyson and co-workers.14 Interestingly, the dimeric scaffold is promoted by the imine-picoline fragment, which is poorly flexible and has an E geometry. The Fourier transform infrared (FT-IR) spectrum (Figure S33) corroborates this statement and shows two distinct CO stretching bands at 1655 and 1610 cm−1, indicating the unsymmetrical coordination of the two quinonic oxygens. B
DOI: 10.1021/acs.organomet.8b00315 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
higher than 99% conversion after another 1 h of reaction (entry 2). Other silanes were used (entries 3−6), being phenylsilane the only one that afforded similar conversion (entry 3). These results were compared to those obtained using other Li−N based compounds, such as LiHMDS and LDA (Table 1, entries 7 and 8), but their performance as catalysts was remarkably lower. To prove the relevance of the anthraquinone moiety in our catalyst design, we tested the performance of (E)-2-((2phenylhydrazone)methyl)pyridine (3) by using it under catalytic conditions in the presence of the same amount of n-BuLi. This ligand is analogous to 1 but lacks the anthraquinone scaffold and bears only a phenyl substituent instead. The obtained conversion was notably lower (60%; Table 1, entry 9) than that from 2 (92%; entry 1), indicating that the presence of the anthraquinone substituent in the catalyst’s structure must have a significant influence on the hydrosilylation process. It is important to note that no additive other than our complex 2, in catalytic amount, was needed for the reactions to take place. This strategy was extended to more sterically demanding substrates, aliphatic ketones and aromatic aldehydes and ketones. The alkane functionality of ketonic substrates such as 3,4-dihydronaphthalen-1(2H)-one (entry 2, Table 2) and cyclohexanone (entry 3, Table 2) produced different outcomes. The former needed 24 h to reach 84% conversion, while the latter was complete in 2 h. When aryl aldehydes were employed, quantitative yields in less than 1 h were obtained in all the cases (Table 2, entries 4− 9). When electron-donating substituents such as methoxy (entry 6)- and 4-dimethylaminobenzaldehyde (entry 7) were used, extended reaction times of 37 and 45 min, respectively, were necessary to reach full conversion. When electronwithdrawing groups were attached at the para position with respect to the aldehyde functional group, as in 4fluorobenzaldehyde (entry 5) and 4-nitrobenzaldehyde (entry 9), shorter reaction times of 15 and 10 min, respectively, were required. In the case of 2-pyridinecarboxaldehyde, the metal coordination abilities of the pyridine could explain its retardation down to 40 min. The highest TOF value observed was for 4-nitrobenzaldehyde, at 19.8 min−1 (1188 h−1). Remarkably, the nitro group, which is often not tolerated by iron-based catalysts, provides in this case excellent conversion in a very short reaction time. Hydrosilylation of esters and amides was attempted, but no reaction product was observed similar conditions under to those used for ketones and aldehydes. Thus, these functional groups are not expected to interfere with the reduction of aldehydes or ketones if they are present in the same molecule. Overall, the major part of the catalysis assayed occurs on the order of minutes, and this therefore represents a straightforward method for the fast reduction of a great variety of carbonyls. To test the recyclability of the catalyst, we performed six consecutive catalytic runs on a J. Young NMR tube (entry 2, Table 1) with only one loading of catalyst (0.5 mol %). No significant loss of catalytic activity was observed after six reaction cycles, where conversions of 97, 97, 95, 97, 95, and 95% for all of the successive cycles were determined by 1H NMR. The reactivities of some of these substrates were monitored by 1H NMR spectroscopy in J. Young NMR tubes. These reaction profiles (Figure 3) allowed us to distinguish initial
Figure 2. (a) Section of the 7Li,1H-HOESY NMR (194 MHz) spectrum of compound 2 in THF-d8 at 294 K. The 1D projections correspond to 7Li and 1H NMR spectra. (b) Section of the 1H,15N gHMBC NMR (500 MHz) spectrum acquired with a 500 MHz spectrometer of compound 2 in THF-d8 at 294 K. Chemical shifts are referenced to NH3.
The positive ion mode revealed parent ions corresponding to mononuclear lithium derivatives with two or three bonded THF molecules ([C 2 0 H 1 2 N 3 O 2 Li(THF) 2 (OH)] + or [C20H13N3O2Li(THF)3]+, respectively), with m/z 494.26821 and 550.23066, respectively. As expected, the observed isotope pattern matches well with those theoretically calculated for the exact masses (Figures S36 and S37). Interestingly, a fragment located at m/z 661.21946 was detected and assigned to [C40H27N6O4Li], which in fact represents the substitution of the THF molecules by another molecule of ligand 1, corroborating the coordination abilities of the ligand itself (Figure S38). Hydrosilylation of Carbonyl Compounds. With the new lithiated hydrazone characterized in solution and the solid state, we first tested its efficiency in the hydrosilylation of acetophenone (Table 1). Stirring the reaction mixture of acetophenone and (EtO)2MeSiH in the presence of 0.5 mol % of 2 at room temperature for 1 h afforded the corresponding alcohol (after acidic workup) in 92% conversion (entry 1) and Table 1. Optimization of the Reaction Conditions for the Hydrosilylation of Acetophenonea entry
silane
time (h)
conversn (%)b
1 2 3 4 5 6 7 8 9
(EtO)2MeSiH (EtO)2MeSiH PhSiH3 Ph2SiH2 Et3SiH PMHS (EtO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH
1 2 1 24 24 24 1 1 1
92 >99 [80]c 86 29 0
