Alkyne Hydroamination Catalyzed by Silica-Supported Isolated Zn(II

Communication Cite This: Organometallics XXXX, XXX, XXX−XXX

Alkyne Hydroamination Catalyzed by Silica-Supported Isolated Zn(II) Sites Amanda K. Cook and Christophe Copéret* Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1-5, CH-8093 Zürich, Switzerland S Supporting Information *

ABSTRACT: Hydroamination is an atom-economical reaction to form C−N bonds, which are ubiquitous in organic compounds. Hydroamination has seen prolific advancements and has mostly focused on the development of homogeneous catalysts based on lanthanides or transition metals. Here, we have developed silica-supported, site-isolated Zn(II) sites through a combined surface organometallic chemistry (SOMC) and thermolytic molecular precursor (TMP) approach and show that they catalyze the intramolecular hydroamination of alkynes. This material is applicable to a broad range of substrates. On the basis of kinetics and in situ IR spectroscopic studies, we propose that the mechanism involves coordination of the aminoalkyne onto Zn(II) followed by the heterolytic activation of the N−H bond and subsequent cyclization and proton transfer. −N bond formation is a ubiquitous reaction in chemistry, used in the synthesis of polymers, ligands, and drugs, among others.1 A highly atom economical method of C−N bond formation is the hydroamination reaction, which is the addition of an N−H bond across a carbon−carbon multiple bond.2 This reaction, while typically thermodynamically feasible,3 shows high kinetic barriers when it is uncatalyzed, due to the requirement of two electron-rich components to react with each other.4 Progress has been made with the employment of d- and f-block metal catalysts that can be divided into two categories (Figure 1a):

C

1980s, BASF and Air Products developed acidic zeolite catalysts for hydroamination, and BASF patented a process for the hydroamination of isobutene with ammonia.8 These materials catalyze the hydroamination of few substrates in low conversions.9 Subsequently, metal-exchanged solids,10 especially with zinc,10,11 have shown improved activity, albeit restricted to a narrow class of substrate due to the small pore size of zeolites (ca. 1 nm).12 In addition, due to the difficulty in understanding the nature of the active site of these materials, it is difficult to obtain a molecular level mechanistic understanding, making their development mostly empirical. We reasoned that the synthesis of a nonporous material with structurally controlled metal sites, here Zn(II), would be helpful toward the development of supported catalysts applicable to a broader range of substrates (no diffusion limitation due to microporosity in zeolites). We therefore synthesized isolated Zn(II) sites supported on SiO2 as hydroamination catalysts by combining surface organometallic chemistry (SOMC), a method that allows for the generation of site-isolated metal centers,13 and the thermolytic precursor approach (TMP),14 which allows for the generation of ligand-free active sites with controlled nuclearity and oxidation state.15 This approach has enabled the formation of a broad range of well-defined metal sites, e.g. Cr(II),16 Cr(III),17 Co(II),18 Ga(III),19 and Ln(III), with high reactivities and noteworthy catalytic properties, e.g. hydrogenation/dehydrogenation through the heterolytic activation of C−H and H−H bonds (Figure 1b).16,17,18a,19,20 Because N−H bonds are generally more acidic than C−H and H−H bonds, isolated M/SiO2 could potentially cleave N−H bonds, which is thought to be a key step in hydroamination.18a Here, we have developed the synthesis of silica-supported, isolated Zn(II) sites.14a,b,21 This material displays good activity for the intramolecular hydroamination of

Figure 1. (a) Classification of catalysts for the hydroamination reaction, and (b) heterolytic X−H bond activation across M−O bonds of M/SiO2 catalysts.

group 4 and lanthanide catalysts (type I),5 which are highly active but typically lack functional group tolerance, and the group 9−12 precious-metal-containing catalysts (type II),4b,c,6 which show the opposite properties. Therefore, there is still a need to develop catalysts that combine the advantages of both types of catalysts. Despite the atom-economical efficiency of hydroamination, its use in industry is scarce,7 due to the current performance and cost of molecular catalysts. Ideally, the hydroamination catalysts should be based on a nonprecious metal and show both good catalytic performance typical of homogeneous catalysts and high recyclability associated with heterogeneous catalysts. In the © XXXX American Chemical Society

Received: April 7, 2018

A

DOI: 10.1021/acs.organomet.8b00202 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Scheme 1. Synthesis of Zn(II)/SiO2‑700 from SiO2‑700 and 1 via Grafting (Step a) and Thermal Treatment under Vacuum (Step b)

o-alkynylanilines, in relation to its Lewis acidic properties revealed by IR and NMR spectroscopy. The reaction of [Zn(OSi(O-t-Bu)3)2] (1)22,23 (1.2 equiv of [Zn]/SiOH) with SiO2‑700 (0.30 mmol [OH] g−1) cleanly releases 0.87 equiv of HOSi(O-t-Bu)3 upon grafting (Scheme 1, step a).21 Elemental analysis of the material 1/SiO2‑700 shows a Zn:C:H molar ratio of 1:11:28, which corresponds well to the proposed structure (SiO)Zn(OSi(O-t-Bu)3) in 1/SiO2‑700 (calculated ratio of 1:12:27). Furthermore, the IR spectrum of the grafted material (Figure S1b) shows disappearance of the isolated silanol peak at 3745 cm−1 (Figure S1a) and appearance of C−H stretching (3015−2830 cm−1) and bending bands (1500−1350 cm−1). Treatment of 1/SiO2‑700 under vacuum (10−5 mbar) at high temperature (300 °C for 1 h and then at 400 °C for 3 h) yields the material Zn(II)/SiO2‑700, with all organic moieties removed from the surface according to IR spectroscopy, while OH groups are regenerated (Scheme 1, step b, and Figure S1c). Analysis of the volatiles formed during the thermal treatment revealed the formation of 2.3 equiv of isobutene and 0.46 equiv of t-butyl alcohol (2.8 equiv of total C4 units), which agrees with the expected release of 3 equiv of C4 units per Zn. Elemental analysis shows a Zn loading of 1.94 wt %, which corresponds to 0.30 mmol of Zn g−1. These results support the formation of Zn(II) surface sites in Zn(II)/SiO2‑700. In fact, contacting a self-supporting pellet of Zn(II)/SiO2‑700 with CO shows the appearance of a peak with low intensity at 2208 cm−1 (Figure S2), blue-shifted relative to free CO and consistent with the presence of Lewis acidic sites.24 Pyridine was also used as a probe molecule. Contacting Zn(II)/SiO2‑700 with pyridine and monitoring the disappearance of bands by a thermal desorption study (see the Supporting Information for details) indicate the presence of Lewis acidic sites along with nonacidic surface silanols.25 Using dynamic nuclear polarization solid-state NMR spectroscopy (DNP NMR),26 the 15N spectrum of 15N-pyridine adsorbed onto Zn(II)/SiO2‑700 shows peaks at 259 and 290 ppm (Figure S9).27 While the peak at 290 ppm is characteristic of pyridine hydrogen bonding to surface silanols, the peak at 259 ppm corresponds to pyridine bound to a Lewis acidic site,28 consistent with the IR data. Pyridine coordination to Lewis acidic sites shows that the Lewis acidic Zn(II) sites in Zn/SiO2‑700 are accessible for coordination and activation of basic molecules, which is one of the requirements for the targeted hydroamination reaction. Having shown that Zn(II)/SiO2‑700 contains Lewis acid sites to bind basic probe molecules, we next evaluated its catalytic activity in the intramolecular hydroamination of alkynes.4a,6c,29 Using 6 mol % [Zn] of Zn/SiO2‑700, 2-(phenylethynyl)aniline (2a) was cleanly converted at 90 °C to 2-phenylindole (3a) (98% GC yield, Table 1, entry 1; see the Supporting Information for evaluation of reaction conditions and control reactions). Hardly any conversion was observed with pure silica, supporting the requirement of Zn(II) as active sites. Other Lewis acidic M/ SiO2‑700 materialsM = Co(II), Cr(II), Cr(III)show lower yields of the desired product (see the Supporting Information). Moreover, 2b with more acidic protons (Ts, toluenesulfonyl) is converted by only 15% (Table 1, entry 2), indicating that the N−

Table 1. Substrate Scope in Alkyne Hydroamination

entry

substrate

R1

R2

R3

yield (%)a

1 2 3 4 5 6 7 8 9

2a 2b 2c 2d 2e 2f 2g 2h 2i

H H H CH3 F Cl H H H

Ph Ph n-Bu Ph Ph Ph 4-CH3Ph 4-ClPh 4-CF3Ph

H Tsb H H H H H H H

98 15c 87 89 81 98 98 25 11

a

Yields were determined by GC, using a calibration curve based on 2phenylindole (3a) with 1,3,5-(OCH3)3Ph as an internal standard. bTs = toluenesulfonyl, −SO2C7H7. cConversion, determined using 1H NMR spectroscopy with 1,3,5-(OCH3)3Ph as an internal standard.

H acidity is less important than nucleophilicity with this catalyst.30 Replacing the alkyne by an alkene moiety also leads to hydroamination, albeit in low conversions (6%), suggesting that the alkyne is required for high reactivity (see the Supporting Information). However, replacing the phenyl substituent of the alkyne with a butyl group provides a high yield (87% yield) of the hydroamination product 3c (Table 1, entry 3), showing that the aromatic group is not essential. We also examined the effect of various substituents on the aromatic rings: electron-rich and -poor substituents para to the amino group (R1) are tolerated well, with yields ranging from 81% to 98% (entries 4−6). In contrast, while an electron-donating methyl substituent para to the alkyne gave a high yield (98%; entry 7), electron-deficient substituents para to the alkyne show diminished yields (entries 8 and 9). To further understand this catalytic reaction, the reaction of substrate 2a was monitored over time using GC. The reaction is complete after approximately 8 h, reaching 90% yield (Figure 2, red circles). Recycling studies shows that Zn sites remain active for catalysis for multiple cycles, albeit with a continuous decrease in activity (Figure S11). A hot filtration test was also performed

Figure 2. Yield of 3a versus time under standard reaction conditions (red circles) or on filtration of the reaction mixture while hot after 1 h (blue diamonds). B

DOI: 10.1021/acs.organomet.8b00202 Organometallics XXXX, XXX, XXX−XXX

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Organometallics to probe the nature of the catalyst: homogeneous or heterogeneous.31 After monitoring for 1 h (Figure 2, blue diamonds), the yield has reached ca. 20% as previously observed and the solid was filtered under inert conditions while the reaction mixture was still hot. The filtrate was further heated at 90 °C, and no further conversion was observed over 24 h, confirming that the catalyst is Zn(II)/SiO2‑700. To further support the solid-state nature of the catalyst, a self-supporting pellet with 2a (2.6 mol equiv per Zn) adsorbed onto Zn(II)/ SiO2‑700 was heated to 90 °C, and IR spectra were recorded (Figure S8). Using the distinct IR bands of the starting material 2a and product 3a, the reaction was determined to be complete after 20 h. Therefore, catalysis is shown to occur in the solid phase; these data further support the heterogeneous nature of the active sites in Zn(II)/SiO2‑700. To quantify the number of active sites of the material, poisoning studies were performed using 4dimethylaminopyridine (DMAP) as a poison. Increasing the amount of DMAP with respect to Zn shows a linear loss of activity up to ca. 0.6 equiv of DMAP per Zn, at which activity ceases (Figure S10). This result indicates that a large proportion of Zn(II) sites are active in Zn(II)/SiO2‑700. With substrate 2a, there are two probable moieties that can bind to Zn, the alkyne or the amine. We initially hypothesized that the amine would preferentially bind, since Zn(II) generally prefers to bind hard rather than soft Lewis bases. Thus, 2a was adsorbed onto Zn(II)/SiO2‑700, and an IR spectrum was recorded. The spectrum, seen in Figure S6b, shows three broad resonances in the O−H and N−H stretching region (3200−3800 cm−1). When this spectrum was compared to that of 2a (KBr pellet; Figure S6a), the N−H bands have broadened and shifted from 3459 and 3366 cm−1 to 3319 and 3269 cm−1. 4Chloroaniline was also adsorbed onto Zn(II)/SiO2‑700, and similarly to 2a, broadening and shifting of N−H stretches is also seen (Figure S5). Therefore, even at room temperature, there is a strong interaction of the amine moiety with the acidic sites and this interaction is not limited to substrate 2a but also occurs with simpler aniline derivatives. The alkyne stretching band of 2a can be seen at 2205 cm−1, and there is minimal change in the wavenumber (2215 cm−1) or peak shape of this alkyne stretch upon adsorbing 2a on Zn/SiO2‑700. Taken together, these IR data suggest that the substrate 2a is indeed coordinating to Zn/ SiO2‑700 via the amine functional group. The same experimental procedure is used to adsorb the product 3a on Zn(II)/SiO2‑700 (Figure S7b). Two broad bands are observed in the O−H and N−H stretching region; the band at 3435 cm−1 is similar in peak shape and wavenumber to the N−H band of product 3a (3442 cm−1; Figure S7a), suggesting that 3a does not bind strongly to Zn/SiO2‑700. This poor binding is consistent with the decreased basicity of the lone pair of 3a in comparison to that of 2a. Additionally, the poor binding of 3a to Zn would facilitate the product/substrate exchange step of the mechanism, encouraging catalytic turnover. To further probe the mechanism, the reaction order in 2a and [Zn] was determined by measuring the initial rate of the reaction at varying initial concentrations of 2a (15−46 mM, Figures S14 and S15) and [Zn] (2.4−12 mM, Figures S17 and S18). The rate of the reaction is inversely proportional to the initial concentration of 2a and is proportional to the concentration of Zn. Therefore, the reaction is inverse first order in 2a and first order in Zn (Figures S16 and S19). A proposed mechanism32 of hydroamination is shown in Figure 3, where Zn(II) active sites coordinate the substrate prior to hydroamination, and the negative order in substrate indicates that one of the 2a ligands

Figure 3. Proposed mechanism of the Zn(II)/SiO2‑700-catalyzed, intramolecular hydroamination of alkynes.

must dissociate from A for catalysis to occur; the remaining bound substrate 2a is then activated, probably via a heterolytic N−H bond activation step, which parallels what has been previously observed for C−H and H−H activation on silicasupported Lewis acid sites.16,17,18a,19,20c−g Following the 1,2addition of the N−H bond across a Zn−O bond, cyclization affords D, which via proton transfer and ligand exchange concludes the catalytic cycle and releases the product. In conclusion, a tailored silica-supported Zn(II) material, Zn(II)/SiO2‑700, has been developed using the combined SOMC/TMP approach; it is a Lewis acid catalyst for the hydroamination of alkynes. For instance, 2-(phenylethynyl)aniline (2a) is cleanly converted to 2-phenylindole (3a) via a heterogeneously catalyzed process according to hot filtration tests and catalysis in the solid state. IR spectroscopic studies suggest that 2a coordinates to Zn(II)/SiO2‑700 via the amino group, not the alkyne. While the substrate 2a shows a strong interaction with Zn(II)/SiO2‑700, the product 3a shows a weaker interaction, which is likely due to its decreased basicity. A mechanism of the catalytic cycle was proposed and supported by kinetic experiments, which showed inverse first order in substrate and first order in Zn. We are currently exploring this approach for the development of supported Lewis acid catalysts for a broader range of reactions.

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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.8b00202. General experimental details, IR spectra, and kinetic measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for C.C.: [email protected]. ORCID

Amanda K. Cook: 0000-0003-3501-8502 Christophe Copéret: 0000-0001-9660-3890 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Deven Estes is thanked for donation of Co(II)/SiO2 material. Wei-Chih Liao is thanked for measurement of DNP NMR spectra. A.K.C. is supported by an ETH Postdoctoral Fellowship C

DOI: 10.1021/acs.organomet.8b00202 Organometallics XXXX, XXX, XXX−XXX

Communication

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(jointly funded by ETH Zürich and the European Union’s Seventh Framework Program, grant agreement 608881).

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DOI: 10.1021/acs.organomet.8b00202 Organometallics XXXX, XXX, XXX−XXX