Mechanistic Study of Nickel-Catalyzed Reductive Coupling of Ynoates

Article pubs.acs.org/joc

Mechanistic Study of Nickel-Catalyzed Reductive Coupling of Ynoates and Aldehydes Sanjeewa K. Rodrigo and Hairong Guan* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, United States S Supporting Information *

ABSTRACT: In this work, (1,5-hexadiene)Ni(SIPr) (SIPr = 1,3-bis(2,6diisopropylphenyl)imidazolidin-2-ylidene) is used in place of Ni(COD)2/ SIPr·HBF4/KOtBu (COD = 1,5-cyclooctadiene) as a more robust catalyst for regioselective reductive coupling of ynoates and aldehydes with triethylsilane. The catalytic reaction of ethyl 3-(trimethylsilyl)propiolate and methyl 4-formylbenzoate shows first-order dependence on aldehyde and catalyst concentrations, inverse first-order dependence on [ynoate], and no dependence on [silane]. The kinetics data, coupled with deuteriumlabeling experiments, support a mechanism involving dissociation of the ynoate from a catalytically dormant nickelacyclopentadiene intermediate prior to turnover-limiting formation of a catalytically active nickeladihydrofuran.

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DFT calculations,36−40 and the nickeladihydrofurans have been characterized spectroscopically and crystallographically.41,42 The complexity of these catalytic systems and the low stability of the nickel species, however, present daunting challenges for kinetic analysis of the reactions. Nevertheless, Baxter and Montgomery investigated the kinetics of intramolecular reductive coupling of an ynal with Et3SiH catalyzed by Ni(COD)2-PCy3 (COD = 1,5-cyclooctadiene; Cy = cyclohexyl).43 Consistent with the hypothesis that the nickeladihydrofuran formation is turnover-limiting, the overall reaction is first order in both ynal and catalyst concentrations but independent of the concentration of Et3SiH. Studying intramolecular coupling of alkyne and aldehyde built within one molecule, however, does not provide information on the sequence of activating π-bonds at the nickel center. In addition, kinetics of Ni(COD)2-NHC catalyzed reductive coupling of alkynes and aldehydes have not been studied in detail, so it is unclear if the kinetic features observed in the phosphine case are reproduced in the NHC systems. We are interested in using reductive coupling of ynoates and aldehydes to make molecules with 1,4-difunctionalities (eq 1).44 The selectivity favors the ester group (CO2R1) being placed distal to the newly formed C−C bond as long as R1 is not sufficiently bulky (e.g., R1 = Me, Et, and 2-naphthyl). To gain a deeper understanding of the mechanism, we conducted a kinetics study of the reaction. As illustrated in this paper, for the NHC system, nickeladihydrofuran formation is also the turnover-limiting step. Our results provide additional mechanistic insight, particularly the involvement of an offcycle species.

INTRODUCTION Reductive coupling of alkynes and aldehydes has proven to be an efficient and versatile method for the synthesis of allylic alcohols and their derivatives.1−10 Nickel-based catalytic systems in particular are notable for achieving high regioselectivity in both intermolecular11−28 and intramolecular29−31 reductive couplings, including selective macrocyclization of ynals.23,32,33 Asymmetric variants of these reactions have also been successfully developed using chiral phosphine or N-heterocyclic carbene (NHC) ligands.12−14,23,25 High tunability of the supporting ligand combined with a wide variety of choices for the reductant allows a complete reversal of alkyne orientation through altering steric interactions, thus offering opportunities for regiodivergent synthesis.34 The general consensus is that these reactions proceed via turnover-limiting formation of nickeladihydrofuran intermediates (Scheme 1), although using a bulky NHC ligand and a bulky reductant (e.g., tBu2MeSiH) can change the turnover-limiting step to the subsequent σ-bond metathesis with the reductant.35 The mechanistic details and nuances have been fully elucidated by Scheme 1. Proposed Mechanism for Nickel-Catalyzed Reductive Coupling of Alkynes and Aldehydes

Received: February 28, 2017 Published: May 1, 2017 © 2017 American Chemical Society

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DOI: 10.1021/acs.joc.7b00483 J. Org. Chem. 2017, 82, 5230−5235

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The Journal of Organic Chemistry

However, reducing the catalyst loading further or having a more dilute solution results in significant erosion in yield. In any case, the observed regioselectivity (3:4 = 90:10) is comparable to the one obtained from the reaction catalyzed by Ni(COD)2/ SIPr·HBF4/KOtBu (3:4 = 85:15).44 The presynthesized nickel complex (1,5-hexadiene)Ni(SIPr) was then tested for catalytic reductive coupling of 1 and 2, which were the substrates chosen for the kinetics study (eq 2).

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RESULTS AND DISCUSSION For the reductive coupling of ynoates and aldehydes (eq 1), the ynoate substrate is typically added slowly via a syringe pump to the mixture containing the rest of the reagents to prevent nickel-catalyzed trimerization of alkynes.45 This inevitably complicates the kinetics study. Fortunately, ethyl 3-(trimethylsilyl)propiolate (1) was identified as a substrate that reacts with aldehydes at rates much faster than those in the trimerization process so that syringe-pump addition is not needed. The progress of the reductive coupling reaction can thus be conveniently monitored by in situ IR spectroscopy. To avoid band overlap between the starting materials and the products, methyl 4-formylbenzoate (2) was selected as the coupling aldehyde.46 The initial kinetics study was carried out using a catalytic mixture comprised of Ni(COD)2, SIPr·HBF4 and KOtBu (10 mol % each).47 After several runs, it became obvious that the data collected using the in situ generated catalyst were irreproducible. This could be attributed to variable degree of catalyst decomposition. Ni(COD)2-NHC catalyzed reductive coupling reactions usually require a catalyst loading of 10 mol % or even higher,32,33 indicating high sensitivity or low stability of the catalyst. Because the catalyst is made from three components, we suspect that weighing errors might also contribute to the irreproducibility. In searching for a more robust nickel precatalyst, Hazari’s (1,5-hexadiene)Ni(SIPr) caught our attention as it was shown to be stable in refluxing toluene48 and a superior substitute for Ni(COD)2 in nickelcatalyzed reactions.49 To ensure this particular nickel complex catalyzes the reductive coupling process, the reaction of methyl propiolate and benzaldehyde with triethylsilane was examined first (Table 1). Compared to the in situ generated catalyst, (1,5-hexadiene)Ni(SIPr) appears to live longer, requiring a lower amount of catalyst (5 mol %) to achieve high yield.

The reaction is highly regioselective, producing 5 exclusively. In this specific case, the catalyst concentration could be kept relatively low ([Ni] = 0.0050 M) without compromising the product yield. These results suggested that using (1,5-hexadiene)Ni(SIPr) would address the stability issues associated with the in situ generated catalyst. To measure the kinetics for the reaction shown in eq 2, the disappearance of 1 was monitored by in situ IR spectroscopy, and the absorbance values were converted to concentrations using a calibration plot (see Supporting Information). Initial rates were calculated based on data points acquired up to 30% conversion. The reaction order for each reagent was determined by varying their initial concentrations (Figure 1). Interestingly, the overall reaction is inversely proportional to the concentration of the ynoate, suggesting the presence of an off-cycle species from which 1 can reversibly dissociate. The reductive coupling process shows first-order dependence on both aldehyde and catalyst concentrations but zeroth-order dependence on [Et3SiH], implying that the interaction with Et3SiH occurs after the turnover-limiting step. It should be mentioned that the initial rates show a slight nonlinear dependence on catalyst loading at a low catalyst concentration (0.0050 M, Figure 1, lower left plot). This is likely due to irreversible catalyst deactivation in a dilute solution (Table 1). To discern whether or not the reaction features oxidative addition of Et3SiH, the reductive coupling reaction was performed in the presence of a 1:1 mixture of Et3SiH and Et3SiD (eq 3). Analyzing the protio content of the product reveals a

Table 1. Reductive Coupling of Methyl Propiolate and Benzaldehyde Catalyzed by (1,5-hexadiene)Ni(SIPr)a

entry 1 2 3 4

catalyst loading (mol %) 10 5 5 2.5

[Ni catalyst] (M) 0.020 0.025 0.0050 0.0025

1:1 mixture of 5 and 5-D. Similar results have been previously observed for nickel-catalyzed intramolecular reductive coupling of an ynal43 and rhodium-catalyzed hydrosilylation of acetophenone with PhMe2SiH.50,51 This type of intermolecular competition experiment can be used to differentiate a mechanism involving direct E−H bond cleavage from the one involving substrate binding followed by E−H bond cleavage.52 The lack of kinetic isotope effect observed here supports the latter scenario. A mechanism consistent with the kinetics results is outlined in Scheme 2. We propose that nickelacyclopentadiene A is in

yield (%)b 98 93 40 12

a

Methyl propiolate was added to the reaction mixture over a period of 8 h using a syringe pump, after which the reaction was stirred for another 1 h. bCombined GC yield for 3 and 4 (90:10). 5231


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Figure 1. Initial rates as functions of [1], [2], [Ni catalyst], and [Et3SiH]. Each data point is the average of three runs, and the error bar represents the standard deviation of the three runs.

an equilibrium53 with complex B and 1, while the subsequent insertion of aldehyde 2 or the formation of nickeladihydrofuran C is the turnover-limiting step. The rate law for the overall catalytic process is thus given in eq 4 with the assumption that the equilibrium is fully established and favors A (or [1] ≫ Keq). Under this mechanistic scenario, increasing the initial concentration of 1 would shift the equilibrium more toward the dormant species A, resulting in a slower catalytic reaction.

The silane participates in catalyst regeneration after the turnover-limiting step; therefore, changing its concentration does not impact the rate for the overall reaction. To account for the lack of kinetic isotope effect from silane, we propose intermediate D in which the Si−H bond is not yet broken. Related intermediates have been suggested in DFT calculations on the cleavage of Ni−O bonds by boranes.54 Vcat =

Scheme 2. Proposed Catalytic Cycle for Nickel-Catalyzed Reductive Coupling of Ynoates and Aldehydes

k 2Keq[Ni][2] [1] + Keq

≈

k 2Keq[Ni][2] [1] (when[1] ≫ K eq)

(4)

Unfortunately, our attempts to synthesize A independently were unsuccessful (see Supporting Information). Nickelacyclopentadienes have been previously proposed as intermediates in nickel-catalyzed cycloaddition reactions,55 although well-characterized ones often involve coordination of the nickelacyclopentadiene ring to another metal.56−59 Reductive coupling of ynoates or their derivatives to form metallocyclopentadienes is, however, precedented with palladium60−63 and iridium.64 Envisioning severe steric repulsions between the Me3Si groups and the iPr groups of SIPr, we tentatively place the Me3Si groups at the β-positions of the nickelacyclopentadiene. This regioisomer is also electronically favored by placing the electron-withdrawing ester groups at the α-positions.65 We cannot however rule out the possibility that A exists as a bis(alkyne) complex.66 Our results are consistent with DFT calculations of related reactions showing that the alkyne substrate is activated first at the nickel center.37,40 Whittaker and Dong recently proposed oxidative addition of an aldehyde C−H bond to an NHC-ligated Ni(0) species.67 According to DFT calculations,68 such a process is promoted by the binding of another substrate with π-accepting ability, which in this case could be the ynoate. If this mechanism were 5232


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in other catalytic processes,71−73 deconvolution of productive and nonproductive pathways can lead to the improvement of the catalytic efficiency. Our ongoing efforts focus on modification of the reaction protocol and identification of ynoates that are unlikely to form the off-cycle species.

Scheme 3. Alternative Mechanism Featuring C−H Bond Activation

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

General Experimental Methods. Unless otherwise mentioned, all air-sensitive compounds were handled under an inert atmosphere using standard Schlenk line and inert-atmosphere box techniques. THF was deoxygenated and dried in a solvent purification system by passing through an activated alumina column and an oxygen-scavenging column under argon. Substrates used in this study were purchased from commercial sources (1 from Acros Organics; 2 and Et3SiH from Aldrich) and used without further purification. (1,5-Hexadiene)Ni(SIPr) was synthesized according to literature procedures.48 In situ FT-IR data were acquired using a Remspec ReactionView Mid IR Fiber-optic System equipped with an ATR probe. HRMS data were acquired using a Micromass Q-TOF-2 (hybrid quadrupole time-of-flight) mass spectrometer. Reductive Coupling of 1 and 2 Catalyzed by (1,5Hexadiene)Ni(SIPr). In a flame-dried Schlenk flask, (1,5-hexadiene)Ni(SIPr) (13.3 mg, 0.025 mmol) was dissolved in 4 mL of THF, resulting in an orange solution. In a separate flame-dried Schlenk flask, 1 (95 μL, 0.50 mmol), 2 (82 mg, 0.50 mmol), and Et3SiH (80 μL, 0.50 mmol) were mixed with 1 mL of THF. This mixture was then added to the orange catalyst solution in one portion using a syringe. The resulting mixture was stirred at room temperature (23 °C) until the starting materials were fully consumed (∼5 h as monitored by TLC). The volatiles were removed under vacuum, and the crude product was purified by column chromatography (eluted by diethyl ether/hexanes) to yield 5 (218 mg, 97% yield) as a colorless oil. 1 H NMR (400 MHz, CDCl3, δ) 7.98 (d, J = 8.0 Hz, 2H), 7.34 (d, J = 8.0 Hz, 2H), 6.83 (s, 1H), 5.42 (s, 1H), 4.21 (q, J = 6.8 Hz, 2H), 3.89 (s, 3H), 1.32 (t, J = 6.8 Hz, 3H), 0.90 (t, J = 8.0 Hz, 9H), 0.59 (q, J = 8.0 Hz, 6H), 0.00 (s, 9H); 13C{1H} NMR (101 MHz, CDCl3, δ) 167.2, 167.0, 164.1, 147.1, 130.4, 129.6, 129.4, 127.6, 78.4, 60.5, 52.2, 14.4, 6.9, 5.0, 0.1; IR (neat, cm−1) 2953, 2877, 1720, 1610, 1458, 1435, 1411, 1369, 1305, 1276, 1244, 1186, 1083, 1037, 1005, 971, 905, 838, 811, 728, 707, 617, 579, 536, 495; HRMS (ESI) m/z: [M + Na]+ Calcd for C23H38O5Si2Na 473.2155; Found 473.2154. General Procedures for Determining the Initial Rates Using in Situ IR Spectroscopy. At room temperature (23 °C) under an argon atmosphere, (1,5-hexadiene)Ni(SIPr) (13.3 mg, 0.025 mmol) was dissolved in 4 mL of THF in a 10 mL scintillation vial capped by a Teflon septum. The IR probe of ReactionView was inserted into a 25 mL three-necked flask through its center neck under a positive argon flow. The flask with the probe was immersed in a constant temperature bath set to 25 °C. The catalyst mixture was transferred into the flask via a cannula and allowed to equilibrate for at least 15 min. Solvent subtracted spectra were recorded at 60 s intervals until the baseline of the IR spectrum and the temperature of the solution had stabilized (∼15 min). A mixture of 1 (95 μL, 0.50 mmol), 2 (82 mg, 0.50 mmol), and Et3SiH (80 μL, 0.50 mmol) in 1 mL of THF (premixed in another 10 mL scintillation vial capped by a Teflon septum) was added to the catalyst solution all at once via a thin cannula. The progress of the reaction was monitored by observing the absorbance of the ynoate C−O stretch (1233 cm−1) as a function of time. Absolute absorbance values were obtained by comparing the band at 1233 cm−1 to the baseline at 1345 cm−1. The absorbance values were converted to concentration values using a calibration plot. The reaction was monitored up to 30% conversion to reflect the catalytic rates for several turnovers.

operative (Scheme 3), insertion of ynoate into the resulting Ni−H bond and then reductive elimination of 6 followed by rapid nickel-catalyzed hydrosilylation of 6 would also agree with our kinetics results. However, this mechanism contradicts our deuterium-labeling experiments (Scheme 4). Deuterium Scheme 4. Deuterium-Labeling Experiments

originated from the silane is located at the α-carbon to the ester group, whereas deuterium originated from the aldehyde remains attached to its original carbon. In addition, our control experiments show that at room temperature Ni(COD)2-SIPr does not catalyze the hydrosilylation of ynoate or aldehydes.69,70 On the other hand, the deuterium-labeling experiments are consistent with the nickeladihydrofuran mechanism shown in Scheme 2.

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CONCLUSION In summary, we identified a relatively stable Ni(0)-NHC complex for catalytic reductive coupling of ynoates and aldehydes, which gives more reliable kinetics data than using the catalyst generated in situ from Ni(COD)2, an imidazolinium, and KOtBu. Our mechanistic study suggests that the ynoate reacts with nickel to produce a catalytically dormant nickelacyclopentadiene outside of the catalytic cycle. Its dissociation of ynoate to form a nickelacyclopropene or an η2-ynoate complex allows the aldehyde molecule to interact with nickel to generate a nickeladihydrofuran. The turnover-limiting and regioselectivity-determining step of the reductive coupling process is the formation of the nickeladihydrofuran. The discovery of the off-cycle species, a nickelacyclopentadiene, is important for future development of nickel-catalyzed reductive coupling reactions. As demonstrated

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b00483. Experimental details, characterization data for 5, and kinetics data (PDF) 5233


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hairong Guan: 0000-0002-4858-3159 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (Grants CHE0952083 and CHE-1464734) for support of this research.

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Mechanistic Study of Nickel-Catalyzed Reductive Coupling of Ynoates

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