Article pubs.acs.org/Organometallics
Mechanistic Understanding of a Silver Pyridylpyrrolide as a Catalyst for 3 + 2 Cyclization of a Nitrile with Diazo Ester Jaime A. Flores, Kuntal Pal, Maria E. Carroll,† Maren Pink, Jonathan A. Karty, Daniel J. Mindiola,*,† and Kenneth G. Caulton* Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *
ABSTRACT: The trinuclear argentate complex Ag3(μ2-3,5(CF3)2PyrPy)3 (PyrPy = 2,2′-pyridylpyrrolide) catalyzes the 3 + 2 cycloaddition of several NCR (R = Me, Ph, tBu) and N2CHCO2Et to disubstituted oxazoles, even in the presence of light and air. Structural and theoretical studies imply a three-coordinate silver carbene complex, (3,5-(CF3)2PyrPy)Ag(CHCO2Et), to be responsible in a stepwise nitrile addition and cyclization step to form the heterocycle.
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The 2,2′-pyridylpyrrolide ligand 3,5-(CF3)2PyrPy12 employed here has an electron-rich pyrrolide moiety but carries two electron-withdrawing CF3 substituents in the pyrrolide ring. This ligand scaffold can also enforce a bent geometry for coinage metals that generally coordinate in a linear fashion. We have shown that attachment of a cisoid bidentate pyridylpyrrolide ligand to monovalent copper or silver, in the absence of an additional ligand/Lewis base source, yields an unprecedented structure in which the pyridylpyrrolide twists internally to bridge two metals, therefore yielding a trimer, M3(μ2-3,5-(CF3)2PyrPy)3 (M = Cu, Ag)13−16 Despite the fact that M3(μ2-3,5-(CF3)2PyrPy)3 forms a trinuclear cluster, we view it as a possible precursor to the unsaturated and reactive “bent” M(I) fragment M(3,5-(CF3)2PyrPy),14 when confronted by some substrate. We report here that a silver analogue to Cu3(μ2-3,5-(CF3)2PyrPy)3,13 Ag3(μ2-3,5-(CF3)2PyrPy)3,15,16 is indeed a source of the highly unsaturated argentate fragment [Ag(3,5-(CF3)2PyrPy)] fragment, together with its catalytic utility for a practical 3 + 2 cyclization of a nitrile with a diazo ester, affording some examples of disubstituted oxazoles (eq 1).
ive-membered-ring heterocycles such as pyrroles, oxazoles, and oxadiazoles are quintessential building blocks in both natural products and organic synthesis. Some forms of oxazoles are biomolecules that result from cyclization and oxidation reactions of serine and threonine nonribosomal peptides; therefore, synthetic entry to these molecules has gathered enormous interest, given their role in many biological processes as well as the importance of this motif in the construction of various natural products.1 For this reason, an effective synthetic approach to these skeletons, especially via a catalytic process, would constitute an important advance in the field. In addition, if an air- and water-stable catalyst could be obtained readily from easy-to-prepare and -handle starting materials, such a reaction would appeal from a practical standpoint. There have been reports of some coinage-metal-based catalysts (Cu(I)) that can promote the coupling of acyl azides and terminal alkynes to oxazoles,2 while other copper catalysts have been used in tandem with oxidative cyclization reactions.3 Au(I) catalysts have been reported for the coupling of alkynes and nitriles under oxidative conditions (using various N-oxides),4 among other situations.5 Despite these efforts, the only example of a metal-catalyzed 3 + 2 addition of nitrile and a carbene source (derived from a diazo ester precursor), has been reported using lanternlike Rh2II,II catalysts.6 Other metals such as iron,7 palladium,8 and some lanthanides9 have been also applied toward the synthesis of substituted oxazoles, but none derive from a carbene source. Lastly, non-metal-catalyzed and stoichiometric reactions have been reported in the assembly of oxazoles,10 but only in the presence of powerful Lewis acids such as BF3 can these reactions result in the addition of carbenes to nitriles to form oxazoles.11 To our knowledge, the use of group 11 metals as catalysts, in particular Ag, in polar 3 + 2 addition reactions to form oxazoles has not been documented. © 2014 American Chemical Society
Since preliminary studies showed higher catalytic performance by the silver complex, we report only those here. We present Received: July 30, 2013 Published: March 24, 2014 1544
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14.7 (NCCH3), 14.0 (CH3). EI-MS: found 127.06 [M]+, C6H9NO2, calcd 127.06.18 Isolated yield: 70%. 5-Ethoxy-2-phenyl-1,3-oxazole. 1H NMR (25 °C, CD2Cl2): δ 8.07 (d, JHH = 7.5 Hz, 2H, o-Ar), 7.09 (t, JHH = 7.5 Hz, 2H, m-Ar), 7.02 (t, JHH = 7.0 Hz, 1H, p-Ar), 6.09 (s, 1H, HCC), 3.47 (q, JHH = 7.0 Hz, 2H, OCH2), 0.93 (t, JHH = 6.8 Hz, 3H, CH3). 13C NMR (25 °C, CD2Cl2): δ 160.7 (CCO), 153.1 (NCO), 129.8 (C-Ar), 129.3 (CAr), 128.9 (C-Ar), 126.0 (C-Ar), 101.6 (NCHC), 68.0 (OCH2), 14.7 (CH3). EI-MS: found 189.08 [M]+, C11H11NO2, calcd 189.08.19 Isolated yield: 40%. A second round of catalysis, in an air atmosphere, was performed by adding more EDA to the solution of Ag trimer in neat NCPh and confirmed complete conversion to the corresponding oxazole in another 3 days; a third run was performed, and a similar outcome was obtained. 5-Ethoxy-2-tert-butyl-1,3-oxazole. 1H NMR (25 °C, CD2Cl2): δ 5.88 (s, 1H, HCC), 3.54 (q, JHH = 7.0 Hz, 2H, OCH2), 1.25 (s, 9H, C(CH3)3), 0.98 (t, JHH = 7.00, CH3). 13C NMR (25 °C, CD2Cl2): δ 161.8 (NCO), 160.3 (CCO), 99.8 (NCHC), 67.8 (OCH2), 34.1 (C(CH3)3), 28.8 (C(CH3)3), 14.7 (CH3); EI-MS: found 169.11 [M]+, C9H15NO2, calcd 169.11. Isolated yield: 40%. The reaction with NCtBu was found to be complete after 3 days at room temperature. A second round of catalysis, in an air atmosphere, was performed by adding more EDA to the solution of Ag trimer in neat NCtBu and confirmed complete conversion to the corresponding oxazole in another 3 days; a third run was performed, and a similar outcome was obtained.20 Note that C−Cl activation of CH2Cl2 was observed as a major byproduct when this reaction was performed either in this solvent or in the presence of light. Ethyl 2,3-Dichloropropionate. 1H NMR (25 °C, CDCl3): δ 4.48 (dd, JHH = 8.8 Hz, 5.4 Hz, 1H), 4.37 (q, JHH = 7.1 Hz, 2H), 4.03 (dd, JHH = 11.2 Hz, 8.8 Hz, 1H), 3.86 (dd, JHH = 11.2 Hz, 5.2 Hz, 1H), 1.40 (t, JHH = 7.2 Hz, 3H).21 NMR Spectroscopic Study of the Reaction of 1 with Diazo Compounds. In a J. Young NMR tube [Ag(3,5-(CF3)2PyrPy)]3 (0.01 g, 0.026 mmol Ag) was dissolved in 0.5 mL of CD2Cl2. To this solution was added the diazo compound (0.026 mmol), and the mixture was agitated for 10 min. 19F NMR spectra of the resulting solutions were then obtained immediately at different temperatures.22 Computational Details. All calculations were carried out using DFT as implemented in the Jaguar 7.723 suite of ab initio quantum chemistry programs. Geometry optimizations were performed with the B3LYP24−27 functional and the 6-31G** basis set. Ag was represented using the Los Alamos LACVP basis,28,29 which includes relativistic effective core potentials. The energies of the optimized structures were reevaluated by additional single-point calculations on each optimized geometry using Dunning’s correlation consistent triple-ζ basis set30 ccpVTZ(-f) that includes a double set of polarization functions. For Ag, we used a modified version of LACVP, designated as LACV3P, in which the exponents were deconstructed to match the effective core potential with triple-ζ quality. Solvation energies were evaluated by a self-consistent reaction field31−33 (SCRF) approach based on accurate numerical solutions of the Poisson−Boltzmann equation. In the results reported, solvation calculations were carried out with the 6-31G**/ LACVP basis at the optimized gas-phase geometry employing the dielectric constant of ε = 9.08 and solvent dichloromethane. Analytical vibrational frequencies within the harmonic approximation were computed with the 6-31G**/LACVP basis to confirm proper convergence to well-defined minima or saddle points on the potential energy surface. The energy components have been computed with the following protocol. The free energy in solution phase G(sol) has been calculated as
computational studies to assess the role of the Ag+ ion in the polar cyclization of nitrile and carbene fragment to form the disubstituted oxazole. Previously we have found that the compound Ag3(μ2-3,5-(CF3)2PyrPy)3 can efficiently catalyze carbene insertion into the C−H bonds of alkanes (including volatile examples such as ethane, propane, and butane).15 Likewise, we have discovered the compound Ag3(μ2-3,5-(CF3)2PyrPy)3 to be a versatile catalyst, promoting carbene addition across aromatic CC bonds,16 a process known as the Büchner ring expansion reaction.17 Here we show how this catalyst system can be extended to a few examples of cyclization reactions of polar substrates, but the ultimate goal of this study is to provide some understanding of the organometallic role of the silver precatalyst 1 in this unique transformation.
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EXPERIMENTAL DETAILS
General Considerations. All manipulations were carried out in air (unless otherwise stated). NMR spectra were recorded in CD2Cl2, C6D6, and CD3CN at 25 °C (1H, 400.11 MHz; 13C, 100.61 MHz; 19F, 376.48 MHz). Proton and carbon chemical shifts are reported in ppm versus Me4Si, and 19F NMR chemical shifts are referenced relative to external CF3CO2H. LDI samples were applied to the target in a volatile solvent that was allowed to evaporate in air, and samples were irradiated at 355 nm. Ethyl diazoacetate (85% in CH2Cl2), Ag2O, acetonitrile, benzonitrile, trimethylacetonitrile, diphenyldiazomethane, trimethylsilyldiazomethane, and dimethyl diazomalonate were purchased from commercial sources and used as received. H(3,5-(CF3)2PyrPy)12 and compound 115 were synthesized following published procedures. See the Supporting Information for further details. Synthesis of Complex 2. Complex 1 (0.01g, 0.026 mmol Ag) was dissolved in acetonitrile (1 mL), and ethyl diazoacetate (3 μL, 0.026 mmol) was added at room temperature. After 6 h of stirring a white solid precipitated; total conversion was observed within 12 h. Crystals as transparent rectangles were obtained from slow evaporation in a 3/1 benzene/dichloromethane solution at room temperature. The title compound is air and light stable as a solid and in solution for more than 1 month. Isolated yield: 95%. Mp: 172−178 °C dec. 1H NMR (25 °C, CD2Cl2): δ 8.26 (apparent doublet, J = 4 Hz, 1H, H-pyridyl), 7.84 (m, 2H, H-pyridyl), 7.26 (t, J = 7 Hz, 1H, H-pyridyl), 6.90 (s, 1H, H-pyrrolide), 6.04 (s, 1H, H-4-oxazole), 4.13 (q, J = 7 Hz, 2H, OCH2), 2.44 (s, 3H, CH3-2-oxazole), 1.42 (t, J = 7 Hz, 3H, OCH2CH3) . 19F NMR (25 °C, CD2Cl2): δ −53.84 (s, 3F), −60.40 (s, 3F) . 13C NMR (25 °C, CD2Cl2): δ 160.42, 155.07, 153.50, 150.44, 138.89, 123.90, 122.59, 111.19, 100.23 (CO2), 69.35 (OCH2), 15.13 (CH3), 14.84 (CH3). MS (LDI, positive): found 512.97 [L107Ag(oxazole)]+, C17H14107AgF6N3O2, calcd 513.00; found 514.97 [L109Ag(oxazole)]+, C17H14109AgF6N3O2, calcd 515.00. Catalytic Reactions with Nitriles and EDA To Form Disubstituted 1,3-Oxazoles. A 10 mg amount of complex 1 (5% of catalyst based on monomer Ag(μ2-3,5-(CF3)2PyrPy), 0.026 mmol) was dissolved in 3 mL of neat nitrile, then EDA (64 μL, 0.520 mmol) was added with a syringe at once, and the reaction was monitored over time by GC-MS or 1H NMR spectroscopy. The products were isolated by evaporation of the solvent. For 5-ethoxy-2-methyl-1,3-oxazole, pure product was obtained upon removal of solvents. For 5-ethoxy-2phenyl-1,3-oxazole and 5-ethoxy-2-tert-butyl-1,3-oxazole the products were purified by silica gel column chromatography, with a 1/1 mixture of petroleum ether and diethyl ether as eluent. Yields were calculated on the basis of integration of 1H NMR spectra containing hexamethylbenzene as an internal standard. The procedure does not require protection from light, except in the case of trimethylacetonitrile; in the presence of light, that reaction mixture yields catalyst decomposition with precipitation of a silver mirror on the walls of the reaction vessel. 5-Ethoxy-2-methyl-1,3-oxazole. 1H NMR (25 °C, CD2Cl2): δ 5.91 (s, 1H, HCC), 3.47 (q, JHH = 7.0 Hz, 2H, OCH2), 1.95 (s, 3H, CCH3), 0.92 (t, JHH = 7.3 Hz, 3H, CH3). 13C NMR (25 °C, CD2Cl2): δ 160.5 (CCO), 152.2 (NCO), 99.9 (NCHC), 67.9 (OCH2), 1545
G(sol) = G(gas) + G(solv)
(2)
G(gas) = H(gas) − TS(gas)
(3)
H(gas) = E(SCF) + ZPE
(4)
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Organometallics ΔE(SCF) =
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∑ E(SCF for products) − ∑ E(SCF for reactants)
Scheme 2. Proposed Mechanism of Formation of 2,5Disubstituted Oxazoles via a Lewis Acid Promoted 3 + 2 Cyclization
(5) ΔG(sol) =
∑ G(sol for products) − ∑ G(sol for reactants)
(6) where G(gas) is the free energy in the gas phase, G(solv) is the free energy of solvation as computed using the continuum solvation model, H(gas) is the enthalpy in the gas phase, T is the temperature (in K), S(gas) is the entropy in the gas phase, E(SCF) is the self-consistent field energy (i.e. “raw” electronic energy as computed from the SCF procedure), and ZPE is the zero-point energy. Note that by entropy here we refer specifically to the vibrational/rotational/translational entropy of the solute(s); the entropy of the solvent is incorporated implicitly in the continuum solvation model. To locate transition states, the potential energy surface was first explored approximately using the linear synchronous transit (LST)34 method, followed by a quadratic synchronous transit (QST)35 search using the LST geometry as an initial guess. IRC (intrinsic reaction coordinate) scans were performed to verify the convergence of the transition states to the appropriate reactants and products.
compounds such as TiF4 have been used in similar cyclization reactions,36 but in each case the product retention on the metal in the reaction prevents catalytic turnover. Mechanistic experiments were in best agreement with electrophilic activation of the nitrile, followed by nucleophilic attack of an intact diazo ester carbonyl oxygen on the nitrile carbon, followed by N2 loss and cyclization (C/N bond formation), as suggested in Scheme 2.36 Notably, no free or coordinated carbene transient was invoked in this type of transformation.36 For the silver pyridylpyrrolide system it was found that trimer 1 reacts readily with MeCN, either 3/1 MeCN/Ag3(μ23,5-(CF3)2PyrPy)3 or neat MeCN (or CD3CN), to form a new species consistent with a monomeric adduct, (3,5-(CF3)2PyrPy)Ag(NCMe) (A); this shows simpler 1H and 19F NMR spectra than does the trimer in CD2Cl2, indicating that nitrile does break up the trimer into a discrete monomer (Scheme 3).
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RESULTS AND DISCUSSION Synthesis and Characterization of the Silver Trimer Ag3(μ2-3,5-(CF3)2PyrPy)3 (1). Previously we reported the synthesis of the silver complex Ag3(μ2-3,5-(CF3)2PyrPy)3 (1), by reaction of Ag2O with H(3,5-(CF3)2PyrPy)3 in MeCN (Scheme 1).15 Scheme 1. Synthesis of Complex 1 from Ag2O and H(3,5(CF3)2PyrPy)3 in MeCN
Scheme 3. Generation of the MeCN Adduct (3,5(CF3)2PyrPy)Ag(NCMe) (A) and Subsequent 3 + 2 Cyclization To Yield the 2,5-Disubstituted Oxazole Silver(I) Adduct 2
Trimer 1 is quite Lewis acidic and it is necessary to dissolve the crude product in toluene and then evaporate all volatiles to completely remove any water or adventitious Lewis base from coordination to silver. As a solid complex 1 is both air and light stable at room temperature for several weeks; dichloromethane solutions are stable for at least 2 days.15 The molecular structure of 1 has been established by a single-crystal X-ray diffraction study and showed a non-C3symmetric trimer analogous to (but not crystallographically isomorphous with) that of the copper analogue.13 A detailed description of the molecular structure of 1 has been published recently.15 A larger expansion of the metal/metal distances in the silver complex compared with copper renders the silver centers more accessible to external reagents. The ease in preparation of 1 also makes it an ideal catalyst to study insertion reactions. 3 + 2 Cyclization Reactions Catalyzed by Complex Ag3(μ2-3,5-(CF3)2PyrPy)3 (1) To Form Oxazoles. Previous work11 has shown Lewis acid acceleration of the cyclization between a nitrile and a diazo ester, via the stoichiometric use BF3 (Lewis acid = LA). Under such a scenario the reaction is complete within minutes at 25 °C (Scheme 2). Attempts at catalytic application of BF3 failed due to competing fluorination of the diazo reagent. Likewise, highly Lewis acidic transition-metal
Intermediate A is stable in MeCN against deposition of silver metal over 24 h at 25 °C even without protection from light. If 1 equiv of MeCN per Ag(I) is added to a solution of 1 in C6D6, the MeCN chemical shift is shifted by over 1 ppm from the value of free MeCN in benzene, clearly confirming adduct formation. Addition of EDA (ethyl diazoacetate, Scheme 3) to an MeCN solution of this monomeric adduct shows no significant change in 1H or 19F NMR spectra of either EDA or that of the adduct A, but within 5 min new resonances for the CH(CO2Et) moiety of the coordinated oxazole grow in. The formation of “carbene dimers”, (olefins) such as the ethyl esters of maleate and fumarate, are not observed, a factor promising for catalysis. Reaction (1/1) of 1 with EDA in MeCN at 25 °C slowly (12 h) produces a white precipitate which exhibits all 1546
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NMR spectroscopic signals of a coordinate pyridylpyrrolide ligand, together with OEt and CH resonances derived from EDA, in a 1/1 mole ratio. Single crystals grown from benzene/ dichloromethane identified this as an oxazole Ag(I) adduct, (3,5-(CF3)2PyrPy)Ag(κ1N-5-ethoxy-2-methyl-1,3-oxazole) (2), resulting from N2 loss with the EDA carbene fragment cyclized to the nitrile triple bond in a 3 + 2 addition mode (Figure 1).
Figure 1. Structural drawing (50% probability thermal ellipsoids) of the non-hydrogen atoms of complex 2, showing selected atom labeling. Selected structural parameters (bond distances in Å and angles in deg): Ag1−N3, 2.117(2); Ag1−N2, 2.157(2); Ag1−N1, 2.419(2); N3−Ag1−N2, 161.42(8); N3−Ag1−N1, 125.78(8); N2− Ag1−N1, 72.80(8); C12−N3−Ag1, 125.94(18); C14−N3−Ag1, 127.37(16). Figure 2. (top) Structural drawing of 2 showing intermolecular H··F bonds (50% thermal ellipsoid probability). (bottom) Argentophilic interactions observed in the packing of 2.
The structure of 2 reveals that the three-coordinate silver center has a T shape, since one of the N−Ag−N angles deviates from an ideal 120° (N3−Ag1−N2, 161.42(8)°; N3−Ag1−N1, 125.78(8)°).37 The resulting oxazole heterocycle is coordinated to the Ag(I) ion via the nitrogen atom, is rigorously coplanar with the pyridylpyrrolide ligand, and is formed without any substituent migration. There are some intermolecular CH···FC contacts to the ortho pyridyl hydrogens (in the distance range currently discussed as weak hydrogen bonds; Figure 2, top),38 as well as an Ag/Ag distance between the stacked planar molecules of 3.44 Å (in the range of those currently described as “argentophilic interactions”,39 but they are much longer than that in trimer 1) seem to contribute to the good tendency of this compound to crystallize from MeCN, with efficient planar stacking (Figure 2, bottom). As shown in Figure 1, the methyl group (from MeCN) is syn to the CF3 group of the pyridylpyrrolide. The clean formation of complex 2 suggested that a catalytic reaction could be performed with an excess of EDA, given the labile nature of both the oxazole and the nitrile adducts. Accordingly, 2,5-disubstituted oxazole formation at 25 °C can be run catalytically, in MeCN and using 5 mol % of precatalyst 1, with the reaction being highly selective for oxazole in 70% yield (Table 1, entry 1). In the reaction there is complete conversion of EDA and negligible formation of “carbene dimer” products fumarate and maleate esters or the corresponding azine, RHCNNCHR.40 Since the oxazole complex can be isolated in neat MeCN, a slow step in catalysis must be replacement of oxazole by MeCN. This preference is apparently the reason BF3 promotion of oxazole formation is stoichiometric, and not catalytic in boron. Other Lewis acids have been shown to catalyze this reaction, even boron or aluminum Lewis acids, where carbene complexes are unlikely to be involved.36
Table 1. Catalytic 3 + 2 Cyclization Reactions of Nitriles and Diazoalkyl Estera
a
The reaction was performed using neat nitrile and 85% EDA (solution in CH2Cl2).
However, slower rates and side products from halogenations by other catalysts or from solvent render complex 1 overall a superior catalyst. In MeCN, the catalysis is complete in 12 h at 60 °C, and no carbene dimers are produced under standard conditions, giving a turnover number (TON) of 14. At the end of a catalytic run, only one set of resonances is detected, indicating that all of the catalyst has converted to one species in the 1547
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also found to be facilitated by the ability of the PyrPy ligand to easily twist from planarity around the inter-ring C−C bond (∼4 kcal/mol).13,14 The 3 + 2 cyclization reactions can be narrowly expanded to other groups on the nitrile. For example, reactions under the above standard conditions in PhCN as solvent are faster than those in MeCN (90% of EDA is consumed in 12 h at room temperature) to give 5-ethoxy-2-phenyl-1,3-oxazole (Table 1, entry 2).43 However, further heating for several days provides full conversion to product. Analysis of a stoichiometric reaction by 1H and 19F NMR spectroscopy (Ag/EDA ratio of 1/1) showed complete conversion in less than 1 h, to give the coordinated oxazole (3,5-(CF3)2PyrPy)Ag(κ1N-5-ethoxy-2phenyl-1,3-oxazole), therefore implying that the rate-determining step (RDS) in the catalytic cycle is most likely product release: nitrile or Lewis base coordination displacing oxazole to close the cycle. The faster reaction observed with PhCN is therefore most likely due to faster product release, which we attribute to steric conflict between the oxazole phenyl substituent and the Ag(3,5-(CF3)2PyrPy) scaffold. Addition of a second charge of EDA to this solution again gave complete conversion of added EDA, as did a third charge of EDA to the resulting solution. Therefore, our case is likely an example of a living catalyst. We have also found that repetitive batch studies conducted in an air atmosphere do not affect our yields; therefore, catalyst 1 does not appear to be affected by normal atmospheric constituents. Reaction under standard conditions (in the dark) in neat t BuCN shows catalytic production of the 2-tBu-substituted oxazole (Table 1, entry 3), which was again faster in comparison to the catalytic reaction of 1 using MeCN. Likewise, no dimer coupling products were observed in the reaction mixture. The oxazole was identified by both GC/MS in combination with 1H and 13C NMR spectroscopy. The same catalytic test, but performed with ordinary room light, not only produced a lower yield of oxazole, with precipitation of Ag by formation of a mirror on the walls of the reaction vessel, but also showed formation of the fumarate and maleate esters, a broad array of less volatile products, and unconsumed EDA; the latter led to some C−Cl insertion product,41 proving that the catalyst was still active when CH2Cl2 diluent was added prior to GC/MS analysis. Proposed Mechanism and Computational Studies. Scheme 4 depicts our proposed catalytic 3 + 2 addition reaction involving precatalyst 1, nitrile RCN, and EDA. On the basis of the fact that nitrile MeCN readily forms a new species with 1, namely A, in addition to oxazole forming monomer 2, we propose that nitrile or Lewis base (LB) first breaks trimer 1 to form the adduct A, (3,5-(CF3)2PyrPy)Ag(LB). Breaking of the trimer could also be triggered by other potential Lewis bases such as EDA, and not nitrile alone. This process mostly involves a pre-equilibrium scenario. Displacement of the nitrile or other Lewis bases by EDA forms intermediate B, which then undergoes N2 extrusion to form carbene C, a previously discussed species relevant to C−H insertion chemistry.15 As suggested in Scheme 4, the role of nitrile could be 3-fold. Not only could it participate in activation of precatalyst 1 to form a monomeric species but it could also be a substrate in the addition step to C to yield the oxazole adduct 2 and also could displace the oxazole to close the catalytic cycle by re-forming the adduct (3,5-(CF3)2PyrPy)Ag(NCR). As in the formation of B, the latter step might also involve a pre-equilibrium process, and displacement of oxazole might also be promoted by excess
catalytic cycle. Our catalytic studies were intentionally limited to 25 °C in order to extend catalyst life and also to optimize selectivity for a single product, but under these conditions this transformation is slow to reach completion. In general, and even at high temperatures, only negligible formation of products from carbene insertion into the C−Cl bond was observed.41 It should be noted that an attempt to conduct the same transformation using simply AgOTf as a catalyst resulted in ∼10% conversion to product (60 °C, 12 h) accompanied by formation of silver mirror even when the reaction was performed in darkness. In an attempt to detect the intermediate (e.g., a silver carbene complex such as (3,5-(CF3)2PyrPy)Ag(CHCO2Et) (C)) formed simply from 1 and EDA (i.e., in the absence of MeCN), these were combined (1/3 mole ratio) in CD2Cl2. The mixture showed an immediate color change from yellow to red and only one small broad additional resonance by 19F NMR spectroscopy at 25 °C. If 19F NMR spectra of this solution are recorded with decreasing temperature, one can observe not only decoalescence of the six resonances for the trimer 1 but also the sharpening of two resonances due to one new compound, which we assign to be an adduct, (3,5(CF3)2PyrPy)Ag(EDA) (B), presumably monomeric in metal. The ratio of reagent to product in this ∼0.05 M solution at −50 °C is 6/1 by intensity, hence a 2/1 mole ratio. Variabletemperature 1H NMR spectra of this same solution shows analogous equilibrium (partial) conversion of 1 to adduct, including the prominent appearance of signals due to free EDA. The unique CH resonance of EDA is broad at room temperature and decoalesces below ∼0 °C into two resonances in a 60/40 ratio apparently due to two conformers of the ester group. Attempts with other diazoalkane reagents such as N2CPh2 give only slightly higher equilibrium conversions to adduct, even at −50 °C (established by 19F NMR spectroscopy). The fact that partial conversion to adduct is detected with N2CPh2 suggests that the carbonyl group is not essential to binding and is thus not necessarily a critical donor to silver. Using another diazoalkane derivative such as N2CH(SiMe3) evinced greater (but not 100%) conversion to an adduct, but the new complex contained six 19F chemical shifts (all in a 1/1 ratio intensity), indicative of a product where the trimer structure remains intact in the adduct. We therefore conclude that EDA can effect at most partial conversion of catalyst precursor onto the catalytic cycle. Although EDA could bind via its terminal nitrogen, the C-bound adduct is thermodynamically more stable and competent along the reaction coordinate.16,42 We questioned whether these adducts contained an intact diazoalkane or if nitrogen had already been expelled. Accordingly, we attempted to use the equilibrium conversion established above to answer this question. First, complex 1 was contacted with EDA (1/3 mole ratio) for 30 min, to form the equilibrium mixture of adduct and reactants. After this time volatiles were then removed at 25 °C, followed by addition of fresh CD2Cl2 (19F and 1H NMR). The NMR spectra revealed a complete reformation to 1, which rules out a nitrogen-loss carbene as the equilibrium participant (adduct identity) without any evidence for carbene dimers being formed (the olefins). Overall, our data indicate that the adduct contains intact EDA, in accord with intermediate B, and that EDA is bound weakly enough to be removed merely by vacuum. Indeed, the regeneration of 1 by evaporation of the solvent from a solution containing a three-coordinated Ag(PyrPy) complex is not only feasible, as is evident from the synthesis of 1 (vide supra), but is 1548
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EDA. (Note: we favor the nitrile as the Lewis base since it is used as the solvent medium; however, coordination of EDA is also plausible.) Our simplistic proposed cycle shown in Scheme 4 excludes critical steps such as the binding mode of EDA to the fragment (3,5-(CF3)2PyrPy)Ag or the role of carbene C in the addition of nitrile to form an oxazole-based ligand. For these reasons, we examined the above transformation using computational studies. Specifically, theoretical studies for the above reaction coordinate could allow us to predict whether N2 extrusion or exchange steps are rate-determining and what intrinsic details could be important in the C−O and N−C bond forming processes in C → 2. As expected, the reaction of EDA with MeCN to form N2 and oxazole is an exergonic reaction with ΔG(solv) = −24.7 kcal/mol. Figure 3 depicts our computed reaction profile for a silver-catalyzed 3 + 2 cycloaddition of a :CHCO2Et fragment with MeCN. Binding of MeCN to 1 breaks the trimer to give the three-coordinate silver monomer A, which is nearly isothermal with its predecessor. The computed structure of A suggests this species to possess a highly distorted T-shaped structure whereby the nitrile coordinates trans to the pyrrolide nitrogen (a similar structure was obtained by X-ray crystallography for the Cu(I) analogue).14 The predicted geometry of A closely resembles that of oxazole adduct 2. Displacement of MeCN by EDA results in formation of B, which is 6.5 kcal/mol higher in energy than A. Binding of EDA to the (3,5(CF3)2PyrPy)Ag fragment does not occur via the diazo nitrogen
Scheme 4. Proposed Catalytic Formation of Disubstituted Oxazoles Promoted by Complex 1 and the Reagents RCN and EDAa
a
For simplicity, we exclude equilibrium arrows in the catalytic cycle. It is also quite possible that EDA reacts with 1 to form B directly.
Figure 3. Computed reaction profile for the catalytic cycloaddition of a carbene (:CHCO2Et) and a nitrile (MeCN) by precatalyst 1 to form an oxazole. The 2,2′-pyridylpyrrolide has been abbreviated as the monoanionic chelate ligand N1N2. *The transition states for initial formation of A, B, and D cannot be located on the electronic surface, as these are dominated by the translational entropy change. 1549
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but instead by the γ carbon, presumably given the canonical form NN+C−HCO2Et. The binding mode of EDA to silver(I) and N2 extrusion to form the carbene C has been discussed elsewhere, and we will focus our attention on subsequent steps.15 Given the electrophilic nature of the carbene carbon in C, MeCN binds to this site to form a remarkably stable Lewis base adduct of the silver carbene species, the intermediate (3,5-(CF3)2PyrPy)Ag{CH(NCMe)CO2Et} (D). This conversion is analogous to that shown by some electrophilic “Fischer-type” carbene complexes.44 Binding of the nitrile to carbene carbon in C greatly stabilizes this species by 19.0 kcal/mol. As expected for a σ donor, Figure 4 shows how binding of NCMe to the electrophilic carbene results in negligible change in the nitrile NC bond (1.156 Å) from that of free NCMe (1.159 Å).45 Note (Figure 4) that the C1/N5/C3
electrophile that activates the nitrile carbon to nucleophilic attack is the carbene carbon, not silver, but silver (and CO2R) is an EWG which enhances the carbene electrophilicity. We have demonstrated by the use of a silver complex, which is stable to water and light, that there are significant advantages of this methodology from previous studies, since the products arising from carbene dimers are essentially absent. This feature eliminates the usage of a controlled slow addition of EDA via a syringe pump as well as the dilution of the reagents using copious amount of solvent. Although our silver catalyst is sometimes living, such a system also reduces the necessity to rigorously exclude water or light from the reactor, a factor which is ideal for larger scale synthesis. In summary, it appears that the electrophilic character of the putative complex [Ag(3,5-(CF3)2PyrPy)] is responsible for the catalytic capability of the system we report here, and sensitivity to light is not excessive; therefore, the catalyst is practical as well as robust to atmospheric O2 and adventitious water. In addition to carbene insertion into C−H bonds and CC bonds, complex 1 is a versatile reagent and can also promote 3 + 2 cycloaddition chemistry presumably via a transient, three-coordinate silver carbene reagent.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Tables, figures, and a CIF file giving computational details such as computed structures and coordinates, spectral data, and information on the X-ray crystal structure analysis of compound 2. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 4. Computed structures for transition state D-TS (left) and intermediate E (right). Distances are reported in Å and angles in deg. For D-TS: Ag−C1, 2.252; C1−C2, 1.471; C2−O1, 1.242; C1−N5, 1.403; N5−C3, 1.192; O1−C3, 2.205; N1−Ag−C1, 149.3; N2−Ag− C1, 139.5. For E: Ag−C1, 2.359; C1−C2, 1.389; C2−O1, 1.354; C1− N5, 1.412; N5−C3, 1.288; O1−C3, 1.392; N1−Ag−C1, 153.1; N2− Ag−C1, 134.7.
Corresponding Authors
*E-mail for D.J.M.:
[email protected]. *E-mail for K.G.C.:
[email protected].
unit in D-TS is already nonlinear, hence developing electrophilicity at C3, which is needed for cyclization. The silver ion facilitates electron flow in the 3 + 2 addition step involving D-TS, and its computed structure is shown in Figure 4. In DTS, there is little C−O bond formation (O1−C3 van der Waals distance of 3.22 Å), therefore implying a relatively early transition state with respect to formation of E. There is no change in the connectivity of silver with substrate in these steps, but silver serves as an electron-withdrawing substituent on the carbenoid carbon; that is its catalytic role. Carbon−oxygen bond formation between the nitrile carbon and ester CO group results in an overall concerted ring-closing step to form the C-bound oxazole (3,5-(CF3)2PyrPy)Ag{C(H)NC(Et)OC(OEt)} (E), traversing a small barrier of 13.4 kcal/mol. The computed structure of E is best described as a C-bound oxazole bound to Ag(I) whereby a single C−O bond bond has formed (1.39 Å). From E, a nearly barrierless 1,2-silver migration results in the more thermodynamically favorable N-bound oxazole isomer 2.
Present Address †
University of Pennsylvania, Department of Chemistry, 231 S. 34 Street, Philadelphia, PA 19104, United States. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Indiana University Bloomington for financial support of this research through a Faculty Research Support Program (FRSP). Financial support of this research was also provided by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Science, Office of Science, U.S. Department of Energy (DE-FG02-07ER15893 to D.J.M.).
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CONCLUSIONS In the sense that it is the nucleophilic substituent pendant on the carbene that initiates these reactions, the transformations catalyzed by the trimer precursor 1 have some analogy to nitrile hydration (forming RC(O)NH2), where higher valent cobalt complexes activate the nitrile carbon for subsequent nucleophilic attack by water.46 Cyclization then is a subsequent mechanistic event to ultimately form the oxazole. Here the 1550
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