Article pubs.acs.org/Organometallics
Büchner Reactions Catalyzed by a Silver(I) Pyridylpyrrolide: Understanding Arene CC Insertion Selectivity Nobuyuki Komine, Jaime A. Flores, Kuntal Pal, Kenneth G. Caulton,* and Daniel J. Mindiola* Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *
ABSTRACT: The complex Ag3(μ2-3,5-(CF3)2PyrPy)3 (3,5-(CF3)2PyrPy = 2,2′pyridylpyrrolide(1−) ligand) catalytically promotes the insertion of the carbene of ethyl diazoacetate (EDA), at room temperature, into the CC bond of a series of arenes to ultimately ring-open them and form the corresponding cycloheptatrienes. In one case, the norcaradiene intermediate can be isolated, while regioselective CC insertion can be promoted with certain arene substrates. The mechanism of CC insertion, preference over C−H insertion, and origin of CC regioselectivity has been probed by a combination of experimental and theoretical studies.
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Ag(I), still showed some C−H insertion and “slow decomposition,” attributed to reduction to silver metal presumably by the B−H functionality in the Tp ligand. However, inserting the carbene fragment regioselectively into the CC bond of a substituted arene, to form a ring-expanded product, is an issue that has not been clearly addressed, in part because of the competing C−H insertion pathway or challenges associated with minimizing carbene dimer formation, which wastes the EDA carbene source. To date, no study has examined the origin and regioselectivity of CC insertion versus C−H insertion. We have reported separately14 that the product of reaction of Ag2O with HL, where L is the bidentate 3,5-(CF3)2PyrPy− (2,2′-pyridylpyrrolide) ligand, gives not a simple twocoordinate, mononuclear LAg species but instead the trimer Ag3(μ2-3,5-(CF3)2PyrPy)3 (1), where each ligand bridges adjacent metals (Scheme 1). The reaction of this molecule with ethyl diazoacetate occurs quickly (minutes) at 25 °C to produce the monomeric adduct AgL(EDA) containing intact EDA: in spite of the idea that N2 loss might be facile, the observed adduct is formed with intact EDA, and adduct formation can even be reversed simply by vacuum at 25 °C. This adduct has a structure where the carbenoid carbon, carbanionic in one Lewis structure, binds to silver. The trimer, interesting as it might be, is simply illustrative of how silver, in the ligand-deficient environment of its synthesis, increases its number of donor neighbors; when offered the substrate EDA, it coordinates to form a three-coordinate monomer. This adduct was shown to have a significant barrier to lose N2 to form a transient carbene, where the carbene carbon is established to be
INTRODUCTION Selectively adding a carbene unit, intermolecularly, across aromatic CC bonds as opposed to the more common C−H insertion pathway (eq 1) is an exceedingly rare occurrence,
given the strength of that CC bond and also the loss of aromaticity of the molecule. Frequently, the primary norcaradiene product of addition of carbene to one arene C/C bond (“cyclopropanation”) readily isomerizes to the cycloheptatriene. Examples of catalytic systems capable of promoting some selectivity in intermolecular cyclopropanation and subsequent ring opening of aromatic CC bonds (the Büchner reaction, discovered in 1885, or alternatively called the Büchner−Curtius−Schlotterbeck reaction)1−4 over C−H and C−X (X = halogen) insertion have been reported for Rh2(II,II) lantern type systems,5 Au(I),6−8 Ag(I),9 and Cu(I)6,10 and, more recently, in a noncatalytic Ru(II) system.11 Interestingly, an intramolecular Büchner type reaction has been implicated in the decomposition pathway of second-generation Grubbs olefin metathesis catalysts.11−13 In general, the carbene precursor is typically a diazoacetate ester (e.g., EDA, N2CHCO2Et). In the context of silver catalysis, Lovely and Dias reported (THF)AgTpCF3, where TpCF3 = [HB(3,5(CF3)2Pz)3]−, the only Ag(I) catalyst that can preferentially insert carbene into aromatic CC bonds rather than C−H bonds.9 This report, with a highly fluorinated Tp ligand on © XXXX American Chemical Society
Received: December 20, 2012
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Scheme 1. Catalytic Reactions of Compound 1 with Benzene To Prepare an Ester-Substituted Cycloheptatriene
highly electrophilic, not only because of the poorly π donating silver but also due to the ester substituent. Consistent with the facile reversibility of monomer EDA adduct returning to compound 1 by loss of intact EDA under vacuum, the loss of N2 and formation of the reactive carbene complex is rate determining in the catalytic conversion of H−C(sp3) bonds to esters by insertion of the carbene into aliphatic C−H bonds, including volatile alkanes such as ethane, propane, and butane.14 On the basis of theoretical work, only for methane does a hypothetical C−H bond cleavage become ratedetermining. In the mechanism established with alkanes, the arriving alkane never forms any bond to silver. Since the EDA-derived carbene is electrophilic enough to attack alkane C−H bonds, we next investigated other substrates that might be transformed by reaction with the EDA-derived carbene. However, an arene substrate might prefer carbene insertion into arene C−H bonds, and indeed there are precedents to that reactivity, with gold carbenoids.6−8 In addition, the previously reported15,16 Tp catalyst precursors have received speculative comment on possible conversion of the tridentate ligand to a bidentate form, in order to explain some Lewis base poisoning studies. We were therefore interested in how our two-coordinate AgL fragment might react with arenes, what selectivity might be preferred, and also the mechanism of any such reaction, with special reference to how the lower coordination number of our L vs Tp affected catalytic performance. In particular, we wondered whether the extra open coordination site furnished by use of pyridylpyrrolide leads to a mechanism where an arene donor substituent ever forms a bond to silver or whether the mechanism involves purely attack by arene as a nucleophile on the electrophilic carbene carbon. In addition, a lower silver coordination number should leave the carbene carbon increasingly electrophilic, benefitting reactivity. Finally, we wondered how the unsymmetrical electronic character (π acid pyridine and π donor pyrrolide) of this bidentate ligand will change the mechanism vs C3-symmetric tris(pyrazolyl)borates. In this work we demonstrate the ability of 1, available in 95% yield,14 to selectively insert the carbene fragment of EDA into CC bonds of various arenes. Complex 1 is a source of a highly electrophilic silver center and is a catalyst for the intermolecular Büchner reaction. We report an evaluation of our catalyst robustness (air and light resistant) and its selectivity (regioselective CC insertion and preference for aromatic CC bonds over C−H bonds, without parasitic activation of a C−Cl bond in the presence of CH2Cl2), using various substituted arenes. We also present competition experiments including KIE measurements for this reaction type. Our studies establish the response of the mechanism to substrate electron-donating and -withdrawing substituent
effects on the regioselective CC insertion of the arene, which is also examined with computational methods.
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RESULTS As noted before,14 the solid-state structure of complex 1 is a trinuclear system in which each 3,5-(CF3)2PyrPy ligand bridges pairs of Ag(I) ions through the nitrogen atoms (Scheme 1). The trinuclear cluster does not have 3-fold symmetry, and each silver has a different formal charge of 1−, 0, and 1+, as implied by the asymmetry in the bridging of the 2,2′-pyridylpyrrolide ligand (Scheme 1).14 When 5 mol % of complex 1 is dissolved in a mixture of benzene substrate and CH2Cl2 (for catalyst dissolution) and excess EDA is added, quantitative formation of the ester-substituted cycloheptatriene occurs after 2 h at room temperature (Scheme 1).17 In neat benzene the yield of the product decreases to 87%, which we attribute to the poor solubility of the catalyst. Insertion into the C−H bond of benzene or the formation of fumarate and maleate esters is not observed in the reaction mixture, as suggested by 1H NMR spectroscopy and GC-MS. Within 2 h at 25 °C, there is complete consumption of EDA and syringe pump addition of EDA is not necessary (in contrast with the 4 h slow addition reported for (THF)AgTpCF3 systems).9 At the end of the reaction, catalyst 1 is recovered unchanged, as established by 19 F NMR spectroscopy. In fact, complex 1 is a living catalyst, since the addition of more EDA (after its consumption is complete) results in renewed formation of the ring-expanded product.17 Complex 1 persists for six such cycles without noticeable decomposition into colloidal silver (solution remains clear) or significant drop in the yield (after six cycles the yield is 54%), even in the presence of light.17 The reaction occurs also in the presence of a trace amount of water (0.13−1.3 mmol) without noticeable decomposition of 1 or drop in the yield of ring-expanded product. The selectivity in CC versus C−H insertion is rather surprising, since the thermodynamics for insertion of :CH(CO2Et) into the benzene C−H bond is much more favorable (−52.5 kcal/mol) than insertion into a benzene ring CC bond to form the norcaradiene intermediate or the cycloheptatriene product (−19.5 kcal/mol).17 This thermodynamic difference is presumably a reflection of the aromaticity only present in the C−H insertion product,18 and the energy difference between these isomeric products is close to the resonance energy of benzene. Product thermodynamics might have been expected to be reflected in transition state energies, which would favor C−H insertion; breaking the aromaticity should raise that barrier. Since the observed catalytic selectivity favors the cycloheptatriene, this is clearly an example of a catalytic mechanism overruling the thermodynamic preference: kinetic control of product selectivity. B
dx.doi.org/10.1021/om301240d | Organometallics XXXX, XXX, XXX−XXX
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Table 1 depicts monosubstituted arenes explored in catalytic carbene insertion into CC bonds and subsequent ring-
Scheme 2. Catalytic Reactions of Compound 1 with Highly Substituted Arenes and Naphthalene
Table 1. Catalytic Büchner Reaction Promoted by Complex 1a
ratio solvent yield of CC insertion/% entry 1 2 3 4 5 6 7 8 9 10 11
X NMe2 MeO Me3C Me F Cl CO2Me CF3 NO2
solvent
yield/%
A
B
C
CD2Cl2 CD2Cl2 C6F6 CD2Cl2 CD2Cl2 CD2Cl2 C6F6 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2
0 52 52 67 100 76 56 56 6 12 0
0 21 25 0 25 18 17 8 0 0 0
0 0 0 36 35 18 25 36 50 42 0
0 79 75 64 49 67 63 59 50 58 0
observed.20,21 Combining 1H and 13C NMR spectroscopy and comparison to the literature,20,21 we are confident that cyclopropanation at that site is the only observed product, without evidence for ring expansion. Likewise, the lower coupling of the CH to the two vicinal CH groups (triplet, 3JHH = 4.4 Hz) is in accord with the exo isomer being produced.20,22 In addition, DFT calculations confirm this product to be more stable than the ring-opened product. If more EDA is then added in the naphthalene case, the catalyst again is still active (living), and additional product is formed.17 For both the arene and naphthalene catalytic reactions, 19F NMR spectroscopic analysis of the reaction solutions at the end of the catalysis shows the presence of intact 1, indicating that this is indeed a living catalyst. We propose that catalyst decomposition is somehow a direct result of the trinuclear core attacking the electron-poor arenes, leading to the formation of colloidal silver. Since our standard reaction time for the assay, 24 h, resulted in complete consumption of EDA, relative rates could not be easily determined. Consequently, we carried out internal competition studies to learn the substituent influence on the rate in the catalysis. Treating complex 1 with an equimolar amount of cyclohexane and benzene revealed CC insertion to be 4 times more favorable than alkane C−H insertion, thus unambiguously establishing that the selectivity must be kinetic in nature rather than thermodynamic.17 Reaction of 1, at 25 °C in CH2Cl2, with equimolar amounts of benzene and fluoro- or chlorobenzene (20:1 vs EDA) gave mostly the product of insertion into benzene (∼3:1), at both 2 and 20 h reaction times.17 In contrast, anisole, toluene, and tBuC6H5 show rates similar to that of benzene. The corresponding competition reactions of benzene and p-xylene showed a similar insertion ratio (1.0:1.2 product ratio) and revealed very little rate difference between the more sterically encumbered and more electron rich xylene and benzene.17 Of special interest, the two insertion products at the two inequivalent CC bonds of pxylene are produced in essentially equal amounts, thus indicating no large steric inhibition of the reaction. Markedly too, the relative rates of CC insertion are similar but are slightly faster for the hindered arene, even when benzene was
a
Reaction conditions: Ag(μ2-3,5-(CF3)2PyrPy)3 (0.013 mmol), EDA (0.26 mmol), aromatic compound (1 mL), solvent (0.5 mL), 25 °C, 1 day, in the dark.
expansion reactions. For comparison, the isolated yield from benzene under these standard conditions is 82%. Reaction with toluene, under conditions comparable to those for benzene, shows the three isomeric products, with insertion in the distal position being the major product (insertion C).18 Although the yields are slightly lower, fluoro- and chlorobenzene also show three isomers (entries 6−8), with insertion C also being the major product.19 The substrates tBuC6H5 and CF3C6H5 form only isomers B and C; insertion proximal to the tBu or CF3 group (isomer A) is not observed (entries 4 and 10). In the case of CF3C6H5 the electron-withdrawing CF3 reduces the yield, as found with the TpAg catalyst.9 A very low yield of ringexpansion product is observed for the substrate PhCO2Me, albeit with similar regioselectivity (entry 9). In contrast, anisole shows two isomers being formed, and insertion B is not observed (entries 2 and 3). Nitrobenzene resulted in decomposition of 1 (colloidal silver), while DMAP or pyridine poisoned the catalyst by forming the adduct (3,5(CF3)2PyrPy)Ag(L) (L = p-(dimethylamino)pyridine or pyridine), as evidenced by 1H and 19F NMR spectroscopy. Highly substituted arenes can also be catalytically ringexpanded by 1 (Scheme 2a), whereas the para-disubstituted arenes X2C6H4 (X = tBu, OMe, CF3) yielded very little or no detectable insertion product, and p-difluorobenzene gives the CC insertion product in only 4% yield.17 Surprisingly, hindered arenes such as mesitylene (Scheme 2b) and hexamethylbenzene (Scheme 2c) yielded the ring-expanded product (100 and 69%, respectively), with only a trace of the carbene dimers being observed in the latter reaction. These last two results suggest insertion into the more hindered CC bonds is not kinetically prohibited. Insertion into only the C1/ C2 bond of naphthalene is also observed, to form the estersubstituted norcaradiene (Scheme 2d), with complete consumption of EDA. No other cyclopropanation isomers are C
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Figure 1. Potential energy surface (PES) for the catalytic insertion of carbene into an arene C−H bond (broken line) versus a CC bond (solid line) by the monomer form of 1, namely (3,5-(CF3)2PyrPy)Ag. Asterisks denote transition states which cannot be located because of the flat electronic surface. N′ designates the pyrrolide fragment.
subsequent insertion of carbene into benzene. The energy to break trimer 1 to produce the nonlinear and monomeric fragment (3,5-(CF3)2PyrPy)Ag is 8.9 kcal/mol, and calculations show that this step is compensated by the energy of binding EDA to monomeric silver to form (3,5-(CF3)2PyrPy)Ag(EDA) (A). The most stable isomeric form of binding intact EDA to silver is via the methylene carbon, which is consistent with the fact that the C−N distance in f ree EDA is 1.31 Å, hence not involving much multiple-bonding character. Binding of EDA to silver via its terminal N or its carbonyl O is less stable by >10 kcal/mol. In all three cases, the EDA is weakly coordinated to the Ag(I) center to form the T-shaped intermediate A with the pyridine motif at the base.14 Loss of N2 from A involves a negligible energy change of −1.8 kcal/mol and leads to a carbene species, (3,5-(CF3)2PyrPy)Ag(CHCO2Et) (B), having a Y-shaped coordination geometry where the pyridyl coordinates more strongly than in its predecessor. The loss of N2 from A involves a transition state (TS-A) which lies at 22.1 kcal/mol. This species has the carbene plane perpendicular to the AgN2 plane in (3,5-(CF3)2PyrPy)Ag, and the ester plane is perpendicular to the carbene plane, meaning the last two functionalities are not conjugated. The Ag−C distance, 2.01 Å, is very similar to distances observed in silver complexes supported by N-heterocyclic carbene ligands.14,25−28 This orthogonality of ester and carbene planes leaves the carbene moiety in B highly electrophilic, and the LUMO (Figure 2) reveals that this orbital is indeed dominated by the carbon pπ orbital, with only minor participation by the corresponding silver dπ orbital. The makeup of this empty orbital π*AgC shows that the bonding counterpart πAgC is primarily metal-localized; hence, there is no significant metal− carbene π bond. The structure of the carbene complex has the
compared to hexamethylbenzene (1.0:1.3 product ratio). Electronic effects are certainly important, however, since attempted catalysis of the reaction of EDA with m-bis(trifluoromethyl)benzene gave no products of CC or of C−H insertion; after 24 h, only carbene dimers were formed.17 Finally, naphthalene was cyclopropanated twice as quickly as benzene, consistent with the former having a more reactive diene motif.17 Using a similar competition experiment, we determined the KIE of the reaction by examining equimolar ratios of C6H6 and C6D6 with or without CD2Cl2 to solubilize complex 1. Duplicate experiments revealed an average KIE of 0.68(2) in the absence of CD2Cl2 versus a KIE of 0.70(2) in CD2Cl2, thus consistent with an inverse secondary isotope effect.23 Our inverse kH/kD is consistent with two arene carbon centers undergoing rehybridization from sp2 to sp3 at the transition state of the cycloaddition step. Likewise, our low KIE value also suggests the transition state involving the insertion of carbene into the CC bond to be early, since the rehybridization of the carbon from sp2 to sp3 has been estimated to have a kH/kD value of 1.43 in Cope type rearrangements.24 As a result, insertion of the carbene to form the substituted norcaradiene intermediate in the silver-catalyzed product-determining step involves significant rehybridization of the CC group undergoing carbene insertion prior to the ring-expansion step. DFT calculations were carried out to understand the origin of CC vs C−H selectivity and of arene substituent effects on product selectivity. Specifically, we calculated the reaction of EDA with 1, the loss of N2 to form a silver carbene, and finally the attack of benzene on the resulting silver carbene complex. Shown in Figure 1 is the computed reaction energy profile (solvent corrected free energy) involving 1, EDA, and D
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the arene π system (ring face) on the carbene carbon without any arene−Ag interaction gave only collapse to structure C. This indicates that the empty silver orbital here significantly differentiates the mechanism from that in the presence of the η3-Tp ancillary ligand. After C is produced, the next energy minimum, (3,5(CF3)2PyrPy)Ag(η2-C6H6CHCO2Et) (D), has formed a second bond from the carbene carbon to a benzene ring carbon (i.e., reductive coupling), yielding a norcaradiene coordinated to silver via an η2-CC bond.17 Species D, at −9.9 kcal/mol, lies much lower than the metallacyclobutane C; this indicates that four-coordinate AgIII in C is relatively unfavorable, which is consistent with the rarity of trivalent silver. Another possible mechanistic scenario, ring expansion in C to form an eight-membered metallacycle, lies at very high energy (48.2 kcal/mol) and is thus unrealistic to propose, given our reaction conditions.17 The TS for electrocyclic opening of the strained three-membered ring of this coordinated norcaradiene D to form (3,5-(CF 3 ) 2 PyrPy)Ag(η 2 C7H7CO2Et) (E) lies nearby at −4.5 kcal/mol, and E would liberate the substituted cycloheptatriene, regenerating the silver monomer (3,5-(CF3)2PyrPy)Ag. Alternatively, the ΔG° value for norcaradiene complex E to react with EDA to form free norcaradiene and A is only −4.9 kcal/mol. To address substituent directing effects, we analyzed the frontier orbitals of three electronically different substrates.17 Any ring substituent lifts the degeneracy of HOMO and HOMO-1, and the resulting energies can be used to understand the regioselectivity; donating and withdrawing substituents in general raise the energy of the formerly degenerate orbitals. Shown in Figure 2 are the most important orbitals for anisole, trifluoromethylbenzene, and naphthalene. In each case the HOMO has a node at the C−C bond which does not undergo insertion of the electrophilic carbene. For trifluoromethylbenzene, the bond which would form product A has low electron density for donating to the carbene carbon; this originates from a reversal of HOMO and HOMO-1 energies for electronwithdrawing vs -donating groups. In both cases, the location of the most energetically accessible orbital (HOMO) alone is sufficient to explain the regioselectivity in these substrates, showing how kinetic control dominates regioselectivity. The more negative HOMO energy of the CF3 derivative (vs OCH3) correlates with a lower catalytic yield due to decreased substrate nucleophilicity with an electron-withdrawing ring substituent. Since the transient complex B is electrophilic at the carbene carbon, we argue for regioselectivity among ring C−C bonds on the basis of the electron density pattern of the HOMO.4,37 We have further investigated the regioselectivity by DFT calculations of two anisole isomers of species C (Figure 1). As shown in Figure 3, the energy of the metallacycle from 3,4addition to the Ag−carbene bond is much more stable than that from addition to the 1,2-arene bond, consistent with the observed 79:21 selectivity. Thus, reaction energies of intermediates fully support the simpler reasoning based on frontier molecular orbitals that the more electron rich C−C bond remote from the methoxy group avoids higher energy intermediates. Note also that the energy of the favorable anisole intermediate lies lower than that of the benzene analog (Figure 1), accounting for faster rates with this electron-donating substituent. Thus, in spite of the fact that the RDS in the energy profile of Figure 1 is loss of N2 from coordinated EDA, productdetermining selectivity in competition experiments can probe
Figure 2. Frontier orbitals for the silver carbene B, anisole, CF3C6H5, and naphthalene.
carbene plane perpendicular to the AgN2 plane, which creates two inequivalent “sides” of the carbene for approach of an arene substrate; this is a distinct difference from the case for the Tp ancillary ligand. In the CC insertion step, one energy minimum is found as the metallacyclobutane species (3,5-(CF3)2PyrPy)Ag(CH[CO2Et]C6H6) (C), with two nearly equal Ag−C bond lengths. In addition, the ring C−C bond length of the bound benzene has lengthened to 1.50 Å and hence is near to a single bond; a benzene C−C bond has added across the Ag−C bond. Species C has all four coordinating atoms, N2C2, in a planar array around silver, consistent with what would be expected for a d8 configuration: hence Ag(III). The energy of species C is 10.9 kcal/mol,29 which is clearly lower than that calculated for C−H insertion (27.0 kcal/mol; Figure 1), despite the latter leading to the more thermodynamically favorable product (3,5(CF3)2PyrPy)Ag(EtCO2CH2C6H5) (F) (dashed line in Figure 1). This step contrasts with theoretical studies involving metalcatalyzed cyclopropanation of olefins, where the metal does not play a direct role in the CC insertion step (or [1:2] cycloaddition step of an olefin).30−36 As mentioned above, there are two sides of the carbene plane to which the benzene could approach. Examination of the approach of benzene cis to pyridyl instead of cis to pyrrolide, illustrated in Figure 1, showed no stationary state after several searches from varied starting geometries. We traced the origin of this selectivity to several stabilizing hydrogen bond attractions between two ortho benzene hydrogens and two fluorines of one CF3 group of the pyrrolide ring.17 Efforts to find a TS for direct attack of E
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CONCLUSIONS In conclusion, we report a new silver catalyst for selective intermolecular C−C carbene insertion in preference to the more common C−H insertion pathway. The use of several substituted arenes reveals the regioselectivity to be governed mostly by frontier orbitals of the arene. Likewise, modest steric bulk does not appear to preclude C−C insertion reactions and insertion of the carbene involves an early [2 + 2] cycloaddition transition state where significant rehybridization occurs at two arene carbons, giving rise to a significant inverse secondary isotope effect. Our computed reaction coordinate suggests the origin of C−C selectivity over C−H to be more kinetic than thermodynamic; the C−H bond is thus not mechanistically competitive with the more nucleophilic arene π system, leading to the higher TS energy for adding that bond to the silver carbene carbon. In comparison to precedent, the catalyst presented here offers both structural and electronic advantages. First, the ancillary ligand is only bidentate (and preventing a monomeric Ag(I) from adopting a linear bonding mode), so that the metal and thus the carbene unit are left especially electrophilic. Second, C−C insertion is more sterically demanding than C−H insertion at the catalytic site, and a low-coordinate metal can allow for a πCC approach (even for heavily alkylated arenes), so that its inherently lower barrier can control product selectivity, even without ever forming an η2benzene complex with the empty silver orbital. This feature differentiates our silver catalyst from the widely studied Rh2(O2CR)4 catalysis for olefin cyclopropanation, where the rhodium is apparently saturated, without capacity to form a metallacycle.38 Finally, the electronically unsymmetrical character of pyridylpyrrolide translates into arene substrate arriving preferentially trans to the weaker (pyridyl) ligand, but influenced in part by interactions with ligand fluorine substituents.
Figure 3. Thermodynamic effect on the selectivity of product formation with a methoxy substituent used as the substrate (carbene attack at two different anisole ring carbons, Cα and Cδ, yielding isomeric products).
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later steps in the reaction profile, as do deuterium kinetic isotope effects.
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ASSOCIATED CONTENT
S Supporting Information *
DISCUSSION Selectivity for CC over C−H is attributed to the bidentate character of our ligand leaving one metal site orbital open. In the C−H case the unsaturation at (3,5-(CF3)2PyrPy)Ag was found to not be an active mechanistic participant (the alkane substrate never forms any bond to silver in the lowest energy mechanism), while Figure 1 shows that, in Büchner catalysis, one arene carbon forms a bond to silver and the resulting metallacycle provides a low-energy path that is responsible for the observed selectivity favoring CC insertion. Clearly the π donor ability of our pyrrolide ligand assists in forming this metallacycle by [2 + 2] cycloaddition of CC to the Ag− carbene bond, which demands formally a Ag(III) ion. We report here a new catalyst which is not by all means “perfect”, but the markedly different ancillary ligand offers new mechanistic alternatives and shows different selectivity, which assists in establishing the mechanistic origin of such selectivity. The empty metal orbital of this unsaturated complex (a consequence of the bidentate pyridylpyrrolide ligand) leads to a metallacycle intermediate, which is uniquely planar and suggests trivalent silver (absent for the C−H insertion PES). A frontier orbital approach helps to understand arene substituent effects on regioselectivity. Note also (Figure 2) that the HOMO energy of naphthalene is high, explaining its observed ability to outcompete benzene.
Text, tables, and figures giving experimental procedures, catalytic reactions and ratios, spectral data, and computational information. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (K.G.C.);
[email protected] (D.J.M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Indiana University Bloomington (FRSP) for financial support of this research. We acknowledge Prof. Theodore S. Widlanski and Dr. Jonathan A. Karty. N.K. acknowledges support from the JSPS Institutional Program for Young Researcher Overseas Visits.
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REFERENCES
(1) Büchner, E.; Curtius, T. Chem. Ber. 1885, 18, 2371. (2) Carey, F. A.; Sundberg, R. J. In Advanced Organic Chemistry, 3rd ed.; Plenum Press: New York, 1990; Part B, pp 522−528. (3) Wu, Y.-J. Buchner reaction. In Name Reactions for Carbocyclic Ring Formations; Wiley: New York, 2010; pp 424−450.
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Organometallics
Article
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dx.doi.org/10.1021/om301240d | Organometallics XXXX, XXX, XXX−XXX
