Mechanistic Insights from the Gas-Phase Reactivity of Phosphorus

Jun 10, 2010 - The daughter channels were identified as cyclopropanation, metathesis, and regeneration of the starting benzylidene carbene via olefin ...
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Organometallics 2010, 29, 2994–3000 DOI: 10.1021/om100224h

Mechanistic Insights from the Gas-Phase Reactivity of Phosphorus-Ylid-Supported Benzylidene Gold Complexes Alexey Fedorov and Peter Chen* Laboratorium f€ ur Organische Chemie, ETH Z€ urich, Wolfgang-Pauli-Strasse 10, CH-8093 Z€ urich, Switzerland Received March 23, 2010

The chemoselectivity as a function of ligand electronic properties was studied for the adducts of phosphorus-ylid-supported gold benzylidenes with methyl vinyl ether by gas-phase collision-induced dissociation experiments. The daughter channels were identified as cyclopropanation, metathesis, and regeneration of the starting benzylidene carbene via olefin loss. The observed product ratios provided linear Hammett plots with a negative slope for the cyclopropanation channel and similar positive F values for metathesis and olefin loss. These results are consistent with the presence of a gold-coordinated cyclopropane intermediate as a global minimum energy structure in the gas phase, from which the pathways to the different products proceed. Additionally, we discuss the anomalous case of the para-dimethylamino gold benzylidene species, which appears to be too stabilized to engage in the reactivity under investigation.

Introduction The past decade has witnessed a tremendous growth of transformations that were accomplished via homogeneous electrophilic gold catalysis.1 Many of the mechanistic rationales that were put forward to explain the observed reactivity invoked gold carbene/carbenoid intermediates.1 Spectroscopic proof of these transient species, however, has been very scarce. Indeed, until very recently the literature on unstabilized reactive gold carbenes contained only examples of mass spectrometric characterization of species with no close analogues in the condensed phase.2 Insightful studies for clearing the identity of solution-phase intermediates came from the F€ urstner laboratories, suggesting that the proposed Au(I) carbene/carbenoid intermediates in cycloisomerizations of enynes are more consistently described as *Corresponding author. Tel: þ41-44-632-2898. Fax: þ41-44-6321280. E-mail: [email protected]. (1) (a) Zhang, L.; Sun, J.; Kozmin, S. A. Adv. Synth. Catal. 2006, 348, 2271–2296. (b) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896–7936. (c) F€urstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007, 46, 3410–3449. (d) Gorin, D. J.; Toste, F. D. Nature 2007, 446, 395– 403. (e) Jimenez-N u~ nez, E.; Echavarren, A. M. Chem. Commun. 2007, 333– 346. (f) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180–3211. (g) Yamamoto, Y. J. Org. Chem. 2007, 72, 7817–7831. (h) Muzart, J. Tetrahedron 2008, 64, 5815–5849. (i) Kirsch, S. F. Synthesis 2008, 20, 3183–3204. (j) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239–3265. (k) Arcadi, A. Chem. Rev. 2008, 108, 3266–3325. (l) Jimenez-Nu~nez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326–3350. (m) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351–3378. (n) Patil, N. T.; Yamamoto, Y. Chem. Rev. 2008, 108, 3395–3442. (o) Michelet, V.; Toullec, P. Y.; Gen^et, J.-P. Angew. Chem., Int. Ed. 2008, 47, 4268–4315. (p) Soriano, E.; Marco-Contelles, J. Acc. Chem. Res. 2009, 42, 1026–1036. (2) (a) Chowdhury, A. K.; Wilkins, C. L. J. Am. Chem. Soc. 1987, 109, 5336–5343. (b) Aguirre, F.; Husband, J.; Thompson, C. J.; Metz, R. B. Chem. Phys. Lett. 2000, 318, 466–470. (c) Schwarz, H. Angew. Chem., Int. Ed. 2003, 42, 4442–4454. (d) Li, F.-X.; Armentrout, P. B. J. Chem. Phys. 2006, 125, 133114. For reviews on stabilized gold carbene adducts, see: (e) Lin, I. J. B.; Vasam, C. S. Can. J. Chem. 2005, 83, 812–825. (f) Raubenheimer, H. G.; Cronje, S. Chem. Soc. Rev. 2008, 37, 1998–2011. pubs.acs.org/Organometallics

Published on Web 06/10/2010

gold-stabilized carbocations.3 Shortly afterward the same group reported the NMR characterization of a resonancestabilized gold alkylidene complex.4a Currently, the understanding of the properties of gold carbene species and bonding situations in these transient intermediates is rapidly developing.4 We have contributed to this area by reporting a facile gasphase synthesis of a series of cationic benzylidene gold complexes of the general type IMesAuCHArþ (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene).5 Their ion-molecule reactions with electron-rich olefins gave adducts, collision-induced dissociation (CID) of which provided an apparent metathesis channel,6 which had been (3) (a) F€ urstner, A.; Morency, L. Angew. Chem., Int. Ed. 2008, 47, 5030–5033. (b) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2008, 47, 6754– 6756. (c) Echavarren, A. M. Nat. Chem. 2009, 1, 431–433. (4) (a) Seidel, G.; Mynott, R.; F€ urstner, A. Angew. Chem., Int. Ed. 2009, 48, 2510–2513. (b) Benitez, D.; Shapiro, N. D.; Tkachouk, E.; Wang, Y.; Goddard, W. A., III; Toste, F. D. Nat. Chem. 2009, 1, 482–486. (c) Correa, A.; Marion, N.; Fensterbank, L.; Malacria, M.; Nolan, S. P.; Cavallo, L. Angew. Chem., Int. Ed. 2008, 47, 718–721. (d) F€urstner, A. Chem. Soc. Rev. 2009, 38, 3208–3221. (e) Shapiro, N. D.; Toste, F. D. Synlett 2010, 5, 675–691. For spectroscopic characterization of a gold Fischer alkenyl carbene, see: Fa~nanas-Mastral, M.; Aznar, F. Organometallics 2009, 28, 666–668. (5) (a) Fedorov, A.; Moret, M.-E.; Chen, P. J. Am. Chem. Soc. 2008, 130, 8880–8881. (b) Fedorov, A.; Chen, P. Organometallics 2009, 28, 1278– 1281. (6) For related gas-phase work on metathesis reactivity from our group, see: (a) Hinderling, C.; Adlhart, C.; Chen, P. Angew. Chem., Int. Ed. 1998, 37, 2685–2689. (b) Adlhart, C.; Volland, M. A. O.; Hofmann, P.; Chen, P. Helv. Chim. Acta 2000, 83, 3306–3311. (c) Adlhart, C.; Chen, P. Helv. Chim. Acta 2000, 83, 2192–2196. (d) Adlhart, C.; Hinderling, C.; Baumann, H.; Chen, P. J. Am. Chem. Soc. 2000, 122, 8204–8214. (e) Volland, M. A. O.; Adlhart, C.; Kiener, C. A.; Chen, P.; Hofmann, P. Chem.;Eur. J. 2001, 7, 4621–4632. (f) Chen, X.; Zhang, X.; Chen, P. Angew. Chem., Int. Ed. 2003, 42, 3798–3801. (g) Zhang, X.; Chen, X.; Chen, P. Organometallics 2004, 23, 3437–3447. (h) Frech, C. M.; Blacque, O.; Schmalle, H. W.; Berke, H.; Adlhart, C.; Chen, P. Chem.;Eur. J. 2006, 12, 3325–3338. (i) Torker, S.; Merki, D.; Chen, P. J. Am. Chem. Soc. 2008, 130, 4808–4814. r 2010 American Chemical Society

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unprecedented for gold, along with cyclopropanation7 and regeneration of the starting carbene. Intrigued by this unexpected result, we set about to gain further insights into these processes via electrospray ionization tandem mass spectrometry (ESI-MS/MS) high-throughput screening. This approach constitutes a very powerful tool for the refinement of mechanistic hypotheses and allows for equally efficient use of pooled libraries, that is, mixtures of compounds rather than pure substances, that more rapidly scan structure or parameter space, provided that no isobaric species complicate analysis of the results.8 We report herein mechanistic studies unraveling the influence of supporting ligand electronic properties on the rates of cyclopropanation, olefin loss, and metathesis channels (vide supra) in the gas phase. Not only do our experiments shed light on the intermediates and rate-limiting transition states involved in these transformations, but they also lead to guidance for the rational design of new solution-phase reactions yet to be made.

Experimental Section Unless stated otherwise, all manipulations were carried out under an argon atmosphere on a vacuum line using standard Schlenk techniques. THF and dichloromethane were dried prior to use by distillation from Na/K alloy and CaH2, respectively, and were transferred under N2. The mass spectrometric experiments are described in the Supporting Information. Synthesis of the Bis-ylid Adducts 1 and 2. Method 1 was applied for the synthesis of bis-ylid adducts with the following groups in the para-position of a benzyl substituent in the phosphonium salt: SO2Me, CN, CF3, CO2Me, NMe2. Potassium hexamethyldisilazane (20 mg, 0.1 mmol, 1 equiv) was added to a stirred suspension of the corresponding phosphonium salt(s) (equimolar, 0.05 mmol) in THF (4 mL) under Ar at ice bath temperature. The reaction mixture was stirred for 10 min, whereupon (Me2S)AuCl (29.5 mg, 0.1 mmol, 1 equiv) was added and the ice bath was removed. After an additional 1 h of stirring, the reaction mixture was passed through a plug with Celite and the filtrate was concentrated to 0.5 mL. Addition of pentane (10 mL) to the filtrate caused precipitation of the product mixture of homo- and heteroleptic ylid adducts 1 and 2 as an air- and moisture-stable solid. (Decomposition in air was observed only for the dimethylamino-substituted homoleptic bis-ylid adduct 1.) Samples for the gas-phase experiments were prepared in dry CH2Cl2 with a concentration of ∼1  10-5 mol/L and were filtered through a short plug of Celite before spectrometric analysis. They decompose with time, depositing metallic gold. Method 2 was applied for the synthesis of bis-ylid adducts with the following groups in the para-position of a benzyl substituent in the phosphonium salt: Me, Ph, F, H, Cl, Br. The homoleptic bis-ylid adducts were prepared according to method 1 with the corresponding phosphonium salt. (4-Methoxybenzylidene)triphenylphosphorane (2 mg, 0.005 mmol) was added to the solution of the homoleptic adduct 1 (0.005 mmol, 1 equiv) in CH2Cl2 (2 mL), and the reaction mixture was stirred (7) Numerous gold-catalyzed ring-closures furnishing cyclopropanes are covered in the reviews.1 For application of ylid complexes in stoichiometric cyclopropanation, see: (a) Brookhart, M.; Studabaker, W. B. Chem. Rev. 1987, 87, 411–432. (b) Cohen, T. Pure Appl. Chem. 1996, 68, 913–918. (c) M€ uller, P. Acc. Chem. Res. 2004, 37, 243–251. (8) (a) Chen, P. Angew. Chem., Int. Ed. 2003, 42, 2832–2847. (b) M€ uller, C. A.; Markert, C.; Teichert, A. M.; Pfaltz, A. Chem. Commun. 2009, 1607–1618. (c) Markert, C.; Pfaltz, A. Angew. Chem., Int. Ed. 2004, 43, 2498–2500. (d) M€ uller, C. A.; Pfaltz, A. Angew. Chem., Int. Ed. 2008, 47, 3363–3366. (e) Teichert, A.; Pfaltz, A. Angew. Chem., Int. Ed. 2008, 47, 3360–3362.

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for 20 min prior to dilution of a sample to ∼1  10-5 mol/L concentration for electrospray mass spectrometric analysis.

Results To benefit from the ESI-MS/MS screening approach, we considered ylid complexes of the general type [(Ylid)2Au]þX-.9 These monomeric homoleptic adducts were intensively studied and always feature ylid ligands serving as twoelectron monohapto donors when forming Au-C σ-bonds. We reasoned that ancillary-bonded ylids could be a suitable replacement for the N-heterocyclic carbene ligand,10 which after all, can be described in one of its principal resonance structures as an ylid as well. Thus, an array of para-substituted phosphonium ylids that we utilized before5b can be employed to assess the branching ratio between the daughter channels (vide infra) as a function of ligand donating properties. For this purpose a series of heteroleptic bis-ylid adducts was prepared in situ (Scheme 1). In particular, we have targeted adducts of type 2 where one of the aryl substituents is always para-methoxyphenyl (ArOMe) and the second ArR is varied (Scheme 1). When ArR contained strongly withdrawing groups (R=CO2Me, CF3, CN, SO2Me), simple addition of the gold precursor to the equimolar mixture of ylids in THF at 0 °C led to the rapid disappearance of the intensive ylid color upon formation of a mixture of homo- and heteroleptic bis-ylid adducts (a typical ESI-MS spectrum of such a reaction mixture is given in the Supporting Information, Figure 1). An alternative way proved better for the remaining members of the series; signals from the heteroleptic bis-ylid adducts (R = Me, Ph, F, H, Cl, Br) were more intense when the corresponding homoleptic adduct 1-(YR)(YR) was initially prepared and then treated with (4-methoxybenzylidene)triphenylphosphorane 3 in CH2Cl2 (Scheme 1, method 2). To our delight, in analogy with the IMes-supported gold ylid complexes,5 electrospray of the reaction mixtures containing 1 and 2 under higher tube lens voltage led to the smooth detachment of the PPh3 group in the bis-ylid adduct series to produce reactive intermediates of types 4 and 5 (Scheme 2). A representative example of the ESI-MS spectra obtained from such a reaction mixture is given in Figure 1. It shows the clean formation of the desired adducts (top), which undergo ion-molecule reactions with the thermalization gas upon increasing the tube lens voltage (bottom). Namely, the three parent bis-ylid adducts 1-(YOMe)(YOMe), 1-(YPh)(YPh), and 2-(YOMe)(YPh) produced the corresponding η2-coordinated complexes 7 and 8 (Figure 1, top) with methyl vinyl ether via triphenylphosphine elimination and coordination f cyclopropanation events (vide infra). Ion beams of thus obtained heteroadducts 7-YR (8-ArR) were directed into the collision chamber for CID with xenon. In analogy with our previous linear free energy relationship (LFER) study,5b several experimental conditions were used to confirm the consistency of the results.11,12 At a collision (9) (a) Grohmann, A.; Schmidbaur, H. In Comprehensive Organometallic Chemistry II; Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Elsevier: Oxford, 1995; Vol. 3, pp 1-56. (b) Schmidbaur, H.; Schier, A. In Comprehensive Organometallic Chemistry III; Crabtree, R. H.; Mingos, D. M. P., Eds.; Elsevier: Oxford, 2007; Vol. 2 pp 272-279. (c) Schmidbaur, H. Angew. Chem., Int. Ed. Engl. 1983, 22, 907–927. (d) Schmidbaur, H.; Franke, R. Angew. Chem., Int. Ed. Engl. 1973, 12, 416–417. (10) Alcarazo, M.; Lehmann, C. W.; Anoop, A.; Thiel, W.; F€ urstner, A. Nat. Chem. 2009, 1, 295–301. (11) For details, see Supporting Information.

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Scheme 1. Synthesis of Mixtures of Homo- and Heteroleptic Ylid Adducts 1 and 2a

a Compounds are abbreviated as follows: YR specifies the entire ylid ligand with R-substituted aryl group ArR, where R is one of the 11 substituents listed above.

Scheme 2. Gas-Phase Synthesis of P-Ylid-Supported Gold Benzylidenesa

a Only one resonance extreme is shown for intermediates 4 and 5. Note that in 4-YR the substitution of the remaining ylid ligand is being varied, versus that of the benzylidene fragment in 5-CHArR.

gas pressure of 60 μTorr and collision energy ranging from 5 to 7 eV in the center-of-mass energy frame the expected fragmentation occurs, as shown in Figure 2.13 The integrated intensities of products and undissociated adduct were summed to obtain the overall ion current. Every reaction channel was then normalized to this total ion current, from which we derived the Hammett plots,14 shown in Figure 3. Satisfactory linear correlations were obtained with σ Hammett constants.14 Remarkably, two channels had the same slopes, namely, that of olefin dissociation to regenerate the starting gold benzylidene and that of metathesis. Values of 0.23 ( 0.06 and 0.23 ( 0.04 were obtained for F, which suggests that the ratelimiting transition state for these processes is identical. In contrast, the cyclopropanation manifold is disfavored by electron-withdrawing substituents with F = -0.28 ( 0.05. We also wish to report that para-dimethylamino-substituted analogues of 1 and 2 demonstrate anomalous behavior with respect to the other members of the series studied. When the homoleptic bis-ylid adduct 1-(YNMe2)(YNMe2) was electrosprayed, already at moderately high tube lens voltage the main signal in the parent mode corresponded to a species that had undergone detachment of two PPh3 mole(12) The conditions for the CID measurements were similar in both series: 60 μTorr of xenon pressure in the collision cell and 5 eV (center-ofmass) herein or 40 eV (Lab frame) offset in ref 5b. We point out that use of the center-of-mass energy frame should be preferred for comparison of CIDs for species with different mass. (13) We have estimated the probability of collisions under a given xenon pressure as described by Armentrout, P. B. The Encyclopedia of Mass Spectrometry; Elsevier: Amsterdam, 2003; Vol. 1, pp 451-455. As obtained using relative intensities for 7-OMe, approximately 30% of parent molecules are expected to have had single collision; 10%, double collision; and 3%, triple collision. (14) (a) Hammett, L. P. Chem. Rev. 1935, 17, 125–136. (b) Jaffe, H. H. Chem. Rev. 1953, 53, 191–261. (c) Chapman, N. B.; Shorter, J. Correlation Analysis in Chemistry; Plenium Press: New York, 1978. (d) Applicability of σ-constants for the gas-phase experiments was discussed: Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165–195.

Figure 1. ESI-MS spectra obtained from the in situ prepared mixture of homo- and heteroleptic bis-ylid complexes 1 and 2. The tube lens offset was set to -80 V (top) and -150 V (bottom) with 1.5  10-2 Torr of methyl vinyl ether (6) in the thermalization chamber. Signals with m/z 757 and 849 are analogues of 7 and 8 produced from the homoleptic adducts 1-(YOMe)(YOMe) and 1-(YPh)(YPh), respectively.

cules (Figure 4, i). Under various pressures of 6 and tube lens voltages, Me2N-substituted species 12 do not form an adduct

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Figure 2. Representative fragment-ion spectra11 produced by mass selection of adducts 7-YR (8-ArR) in the first quadrupole and subsequent dissociation with 60 μTorr of Xe in the octopole collision cell at 5 eV collision energy (center-of-mass). Spectra cover a 450 mass units range and are shown at 30% of the normalized relative intensity with the parent signals aligned to the right. CP stands for cyclopropanation, M for metathesis, and OL for olefin loss channels. A decrease of the electron density on the supporting P-ylid ligand suppresses the cyclopropanation channel in favor of metathesis and olefin dissociation.

with methyl vinyl ether. In fact, spectrum (i) was recorded with the standard, 1.5  10-2 Torr pressure of 6 in the thermalization chamber, and it features only signals originating from the parent bis-ylid adduct 1 with m/z 987 and the starting phosphonium salt. The unexpected species 13 that we assign to the gold bis-benzylidene (m/z 463) does not react with 6 either. The identity of 13 is confirmed by the following CID experiments. When monobenzylidene 12 is mass-selected and collided with xenon under near-single-collision conditions, clean formation of 13 occurs along with a minor signal from an ylid radical cation (m/z 395) (ii). In turn, CID of 13 furnishes 14 via detachment of one Me2N-benzylidene fragment, thus ruling out an isomeric stilbene gold adduct structure for 13 (iii). Lastly, the same species 14 are produced upon mass-selection and CID of the hetero species 15 (Figure 4, iv). The strongly electron-withdrawing methanesulfonyl substituent on the ylid aryl group destabilizes the corresponding species 15 so that C-Au ylid binding energy is significantly lowered, and therefore P-ylid dissociation occurs almost exclusively.

Discussion As pointed out above, IMes-supported gold benzylidenes were prepared in the gas phase earlier, and their identity was confirmed via an energy-resolved CID measurement of the triphenylphosphine detachment that agreed well with the

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Figure 3. Plots of relative rates of the olefin dissociation, metathesis, and cyclopropanation versus the Hammett parameters. The data presented were obtained at 60 μTorr of xenon pressure in the collision chamber and collision energy of 5 eV (center-ofmass). The point with Br for the olefin loss manifold deviates most from the others and was omitted for the calculation of the F-value.

barrier predicted by DFT calculations.5a Computations to establish the potential energy surface (PES) of the metathesis pathway identified η2-aryl-bound adduct15 18-ArCF3 as the lowest energy structure on the transformation from 17-CHArCF3 þ cis-dimethoxyethylene to the products.16 The first piece of experimental credence for intermediacy of 18 came when an adduct of IMesAuþ and 1-ethoxy-2-methoxycyclopropane was subjected to CID experiment. At mild collision offset two daughter signals were obtained, which we ascribed to 19 and its nearest homologue.17 Computational rationale for the latter process demonstrated that this transformation is energetically feasible and thus justified the assignment. The results from a LFER study indicated that three different rate-determining transition states govern the ratio of these competing processes.5b Hammett plots demonstrated that metathesis is hardly affected by electronic effects; the cyclopropanation channel is favored by electron-withdrawing substituents on the benzylidene moiety, whereas electron-donating groups facilitate the regeneration of starting carbene 17.5b Having in mind this precedent, we now turn to the interpretation of the presented results. A priori, dissociation of (15) Schmidbaur, H.; Schier, A. Organometallics 2010, 29, 2–23. (16) Fedorov, A; Batiste, L.; Bach, A.; Birney, D. M.; Chen, P. Unpublished results. (17) Batiste, L.; Fedorov, A.; Chen, P. Chem. Commun. 2010, 46, 3899-3901.

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Figure 4. ESI-MS spectra obtained from the in situ prepared homoleptic, para-dimethylamino-substituted adduct 1-(YNMe2)(YNMe2) at -110 V tube lens potential with 1.5  10-2 Torr of methyl vinyl ether in the thermalization 24-pole (i) and fragment-ion spectra of the corresponding mass-selected parent signals after fragmentation with 60 μTorr of xenon at 10 (ii) and 20 (iii) eV collision energy in the center-of-mass frame. The parent ion in part (iv) was prepared from heteroleptic adduct 2-(YSO2Me)(YNMe2) via PPh3 cleavage, and its fragmentation at 10 eV is shown. Scheme 3. Synthetic Routes to the Gaseous IMesAu-Benzylidene Complexes and Their Reactivitya

a

Only MS-active CID products of 18 are shown.

PPh3 from 2-(YOMe)(YR) could occur from either of the ylid ligands. In this regard, the ratio between the [Au-YOMe]+ and [Au-YR]+ daughter channel intensities is instructive. The electronic desymmetrization of the aryl fragments in 2(YOMe)(YR) suppresses triphenylphosphine detachment from the ArR side, which, as evident from the absence of any appreciable intensity of [Au-YOMe]+ already for parachlorophenyl (Figure 2), renders the CID results suitable for quantitative Hammett analysis. Our LFER data treatment relies on the following assumptions: (i) We assumed that linear free energy relationships may be constructed with microcanonical rates, which, strictly speaking, can be considered theoretically question(18) (a) Cooks, R. G.; Wong, P. S. H. Acc. Chem. Res. 1998, 31, 379-386, and references therein. (b) Cooks, R. G.; Koskinen, J. T.; Thomas, P. D. J. Mass Spectrom. 1999, 34, 85–92. (c) Bursey, M. M.; Harvan, D. J.; Hass, J. R.; Becker, E. I.; Arison, B. H. Org. Mass Spectrom. 1984, 19, 160–164. (d) Majumdar, T. K.; Clairet, F.; Tabet, J.-C.; Cooks, R. G. J. Am. Chem. Soc. 1992, 114, 2897–2903. (e) Chen, G.; Wong, P.; Cooks, R. G. Anal. Chem. 1997, 69, 3641–3645.

Table 1. Comparison of the Two LFER Studies slopes (F) and uncertainties proposed lowenergy species R

18-Ar (ref 7-YR

5b

)

cyclopropanation

metathesis

olefin loss

σ: 0.53 ( 0.12 σ+: 0.37 ( 0.09 σ: -0.28 ( 0.05

-0.04 ( 0.11 -0.02 ( 0.08 0.23 ( 0.04

-1.27 ( 0.14 -0.89 ( 0.13 0.23 ( 0.06

able. However, based on our past experience,5b as well as the successful application of Cooks’ kinetic method in gasphase ion-molecule chemistry,18 we believe the assumption is sufficiently justified. While not normally treated as such, Cooks’ kinetic method is closely related to the linear free energy relationships used commonly in the condensed phase. (ii) Competing PPh3 dissociation from the phenyl side in 2(YOMe)(YH) was neglected based on the low intensity of the [Au-YOMe]+ signal (Figure 2). When the [Au-YOMe]+ signal was more intense, that is, for R = Me, Ph, F, quantification of the CID channels originating from 2-(YOMe)(YR) and 6

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Scheme 4. Simplified Energetic Surface for the Reaction of Gold Benzylidenes with Electron-Rich Olefinsa

a

L could be IMes or P-ylid ligand, as taken from the literature.17

was not performed. (iii) Ion transmission was assumed not to be strongly mass-dependent at our experimental CID range. (iv) The absence of the [Au-YOMe]+ signal was taken as evidence for the identity of 7-YR and 4-YR. The last point requires clarification. The cyclopropanation process necessitates the intermediacy of the corresponding gold benzylidene, for which carbene transfer to 6 is the preferred pathway under the conditions of this study. When the formation of [AuYOMe]+ was not detected, we assumed that the corresponding benzylidene did not form and therefore that the signal of 4-YR contains no admixture of the isobaric 5-CHArR. The presented F-values are in remarkable contrast to those reported previously (Table 1).5b We explain this trend here from the point of view of the intermediacy of the related η2-aryl-bound adducts 7 and 18. It is expected that the dissociation of the cationic gold fragment from the metalbound cyclopropanated species (Scheme 4) occurs via a loose transition state, that is, without an extra barrier beyond that due to the reaction endothermicity and centrifugal effects. We propose that the height of this barrier is mainly responsible for the observed F-values for the cyclopropanation channel. At elevated collision energies this manifold clearly predominates over metathesis and olefin loss, which are entropically disfavored.11 This behavior was also observed earlier5 and is consistent with the high density of states for the simple ligand dissociation. Further, the analysis relies on the assumption that a change of the supporting ligand on gold from IMes to P-ylid, while changing the relative energies of some species, does not significantly modify the basic topology of the potential energy surface. The simplified PES in Scheme 4 allows for the rationalization of our two complementary LFER studies. As we have recently reported, formation of the gold Fischer carbenes via ring-opening of the electron-rich cyclopropanes is a stepwise process operating via open-chain intermediates.17 Since the latter reaction is the microscopic reverse for the cyclopropanation of 6 with 19, an adaptation of this computational rationale for the species presented in this study requires the existence of related resonance-stabilized cationic intermediates A and F (Scheme 4). Note that we assumed a similar stabilizing effect from the para-methoxyphenyl group in F as was originally found for methoxy analogues.16,17 Two possible pathways for the interconnection of A and F were provided. At the PW91/cc-pVDZ(-PP) level of theory, analogues of A and F could be connected through either the metallacyclobutane pathway or direct auration

of the cyclopropane ring via nearly isoenergetic transition states.17 In turn, one-step rupture of the Lewis-acidic gold coordination to the cyclopropane fragment in 7-YR or 18-ArR would result in the MS-observable [Au-YR]+ or 20. Consider the dissociation of the gold center from the proposed global minimum structure 7-YR first. Using the ylid approach we vary the electronics of the supporting P-ylid ligand only; the methoxy substituent on the arylcyclopropane fragment is kept constant. The barrier for such dissociation will be determined by the stability of the species [Au-YR]+. Namely, the more electron donating the substituent R is on the ylid aryl group, the more stabilized the cationic [Au-YR]+ species will be, and hence the easier the coordination to the aryl moiety in 7-YR will be broken. Electron-withdrawing substituents will have the opposite effect on the relative stability of [Au-YR]+ species, increasing their Lewis acidity and therefore the binding energy to the cyclopropane. Indeed, we observe a nearly 2 times more intense signal for [Au-YOMe]+ as compared to [Au-YSO2Me]+ upon CID of 7-YOMe and 7-YSO2Me at identical conditions (Figure 2). This behavior is reflected in the negative F-values for the cyclopropanation channel (Table 1). In contrast to 7-YR, adducts 18-ArR contain the varying substituent on the aryl-cyclopropane fragment; IMes-ligand on the gold center stays unchanged. The same argumentation can be applied to rationalize the positive F-values, which we observed earlier for the alteration of the electronic properties on the benzylidene fragment.5b Cyclopropane ligand detachment is expected to be faster for the least electron-rich adducts 18-ArR, as is experimentally observed. We conclude the discussion of this channel by stating that it appears that no change in the rate-determining step for cyclopropanation occurs as one replaces the supporting ligand IMes with the P-ylid. The donor strength of the aryl-cyclopropane ligand seems not to lead to gross modifications on the energy surface, only altering the gold fragment binding energy. It follows from Scheme 4 that the metathesis pathway and regeneration of the starting benzylidene mechanistically are closely related. We speculate that part of the PES that connects the metal-bound cyclopropane 7-YR or 18-ArR with the products of the metathesis pathway should be similar to that reported in the literature.17 It is perhaps most straightforward to account for the same slopes for the formation of [Au-YR]dCHOMe+ and regeneration of [Au-YR] dCHArOMe+, which are obtained in the present study. Given that the methoxymethylidene and para-methoxybenzylidene

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Fedorov and Chen

fragments in the series 4-YR and 19 remain unchanged, it is only the supporting properties of the P-ylid ligand that will alter the relative stability of these Fischer carbenes. This change for the metathesis and olefin loss channels must occur in a very similar fashion that reflects the electronic properties of the ArR group. We further speculate that the relative energy of the transition states connecting intermediates A and F (Scheme 4) would change in a similar manner as well. It was pointed out that although the olefin dissociation from the species similar to F to form methoxymethylidene gold carbene 19 is expected to be the most endothermic, the transition states that precede formation of F are not significantly lower in energy.17 In a case like this the overall reaction kinetics will contain contributions from all the corresponding maxima on the given PES. It is therefore difficult to reliably identify the rate-limiting transition state for the formation of product Fischer carbenes 10-YR and 19 at this point. The fact that olefin loss is suppressed by depleting electron density from the aryl group in the cyclopropane adduct 18-ArR is easily explained by the energetic destabilization of the corresponding electrophilic benzylidenes 17-CHArR. Since the negative slope for this process originates from the increasing relative energy of IMesAudCHArR+ as the substituent R becomes more electron-withdrawing, it is legitimate to propose that carbene dissociation from intermediate A to form 17-CHArR (Scheme 4) will be then rendered rate-determining. A few solution-phase Hammett studies on the metalcatalyzed cyclopropanation reaction have been reported in the literature.19,20,7a Perhaps the most related to our system is the Cu(I)-catalyzed carbene transfer to para-substituted styrenes, which shows a preference for electron-rich substrates with F-values ranging from -0.51 (bis-oxazoline complex + ethyl diazoacetate (EDA)),19a -0.85 ([hydrotris (3,5-dimethyl-1-pyrazolyl)borate]CuC2H4 + EDA),19b -1.06 (stoichiometric reactions of isolated aryl- and diarylcarbene complexes),19c down to -1.19 ([dihydrodobis(pyrazolyl)borate] Cu catalyst + EDA).19d Iron-based cyclopropanation features negative F-values as well.7a,20 These results are in contrast, but not in contradiction, with the electronic substituent effects that we have observed in the gas phase (vide supra).5b One can envision that the potential energy surfaces for catalyzed carbene

transfer reactions21 with different metals and ligand environments and in different phases need not be similar.22 Lastly, we explain the chemical inertness of 12 and 13 (Figure 4) by their strong resonance stabilization that is offered by the donor substituent on the aryl. On the basis of the observed reactivity, one should point out the similarity of these alkyl-aminobenzylidene carbenes to N-heterocyclic carbenes, which are widely used in catalysis as supporting ligands.23 In fact, given the good transmission of the electronic effects through the phenyl ring, species 13 could be considered as analogues of regular aminocarbenes with the substantial ylid character contribution.24 Recently, we have also observed an analogous effect of the dimethylamino substituent on the aziridination pathway that metal nitrenes usually feature. Cationic gold aminonitrene species were inactive in the gas-phase reaction with an olefin, thus demonstrating their suppressed nitrene-type reactivity.25

(19) (a) Rasmussen, T.; Jensen, J. F.; Østergaard, N.; Tanner, D.; Ziegler, T.; Norrby, P.-O. Chem.;Eur. J. 2002, 8, 177–184. (b) DíazRequejo, M. M.; Perez, P. J.; Brookhart, M.; Templeton, J. L. Organometallics 1997, 16, 4399–4402. (c) Shishkov, I. V.; Rominger, F.; Hofmann, P. Organometallics 2009, 28, 1049–1059. (d) Díaz-Requejo, M. M.; Nicasio, M. C.; Perez, P. J. Organometallics 1998, 17, 3051–3057. (e) Dai, X.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 10085–10094. (20) (a) Wolf, J. R.; Hamaker, C. C.; Djukic, J.-P.; Kodadek, T.; Woo, L. K. J. Am. Chem. Soc. 1995, 117, 9194–9199. (b) Li, Y.; Huang, J.-S.; Zhou, Z.-Y.; Che, C.-M.; You, X.-Z. J. Am. Chem. Soc. 2002, 124, 13185–13193. (21) For gold-based carbene transfer from diazo compounds, see: (a) Fructos, M. R.; Belderrain, T. R.; de Fremont, P.; Scott, N. M.; Nolan, S. P.; Dı´ az-Requejo, M. M; Perez, P. J. Angew. Chem., Int. Ed. 2005, 44, 5284–5288. (b) Li, Z.; Ding, X.; He, C. J. Org. Chem. 2006, 71, 5876–5880. (c) Ricard, L.; Gagosz, F. Organometallics 2007, 26, 4704– 4707. (d) Prieto, A.; Fructos, M. R.; Díaz-Requejo, M. M.; Perez, P. J.; PerezGalan, P.; Delpont, N.; Echavarren, A. M. Tetrahedron 2009, 65, 1790– 1793. (e) Zhou, Y.; Trewyn, B. G.; Angelici, R. J.; Woo, L. K. J. Am. Chem. Soc. 2009, 131, 11734–11743. (22) A concerted mechanism for carbene transfer from propargyl pivolate with gold catalysis was proposed: Johansson, M. J.; Gorin, D. J.; Staben, S. T.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 18002– 18003. See also: Nieto-Oberhuber, C.; Lopez, S.; Mu~noz, M. P.; JimenezN u~ nez, E.; Bu~ nuel, E.; Cardenas, D. J.; Echavarren, A. M. Chem.;Eur. J. 2006, 12, 1694–1702.

Supporting Information Available: Reference mass spectra and data for the Hammett correlations. This material is available free of charge via the Internet at http://pubs.acs.org.

Conclusions ESI-MS provides a facile, direct access to the rare gold benzylidene reactive species. As an extension of our earlier work,5,17 the reported data are in agreement with the cyclopropanated complex as a lowest energy intermediate from which formation of gold carbenes evolves. The literature on related solution-phase transformations contains insightful reports by Gassman, where a tungsten metathesis catalyst was shown to abstract a carbene moiety from a cyclopropane and transfer it to another olefin.26 While it is well known that the same metal carbene species could mediate cyclopropanation and metathesis reactions,27 no practical transcyclopropanation or cyclopropane metathesis processes have been developed to date. On the other hand, the silylene transfer from silacyclopropane to alkenes is established and relies on silver catalysis.28 The results presented herein suggest that d10 coinage metal complexes and suitably substituted cyclopropanes are good candidates for delivering such transformations in the condensed phase. Our efforts in this direction will be reported in due course.

Acknowledgment. Support from the ETH Z€ urich and the Swiss Nationalfonds is gratefully acknowledged. A.F. appreciates helpful discussions with Dr. Marc-Etienne Moret and Dr. Erik P. A. Couzijn.

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