Transient Innermolecular Carbene–Hemicarcerand Complex of

Nov 3, 2011 - Edward J. Dale , Nicolaas A. Vermeulen , Andy A. Thomas , Jonathan C. Barnes ... Udo H. Brinker , Jean-Luc Mieusset , Murray G. Rosenber...
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Transient Innermolecular Carbene Hemicarcerand Complex of Fluorophenylcarbene Zhifeng Lu, Robert A. Moss,* Ralf Warmuth,* and Karsten Krogh-Jespersen* Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States

bS Supporting Information ABSTRACT: Laser flash photolysis of fluorophenyldiazirine incarcerated in hemicarcerand 2 affords incarcerated fluorophenylcarbene [2.3], which forms a metastable, innermolecular π-complex with aryl moieties of 2. This carbene complex can be observed spectroscopically. Extensive computational studies provide insights into the structure, spectroscopy, energetics, and kinetics of the 2.3 carbene complex.

1. INTRODUCTION Extrinsic stabilization of normally unstable carbenes1,2 can be achieved by cryogenic matrix isolation3 and by inclusion in cyclodextrins,4 cavitands,5 zeolites,6 or hemicarcerands.7 We reported that the otherwise evanescent fluorophenoxycarbene (1)8 is persistent for days at ambient temperature when generated inside hemicarcerand 2.9

The substantial intrinsic stabilization afforded carbene 1 by its phenoxy and fluoro substituents inhibits attack on the C H bonds or aromatic rings of 2, while incarceration prevents the dimerization of carbene 1. In contrast, the less stabilized fluorophenylcarbene (3) is not persistent when incarcerated in 2. It reacts with an aromatic component of 2, ultimately affording the tropone derivative 4 (eq 1).10 NMR studies at 80 °C or above indicate that the reaction between 3 and 2 occurs rapidly on the macroscopic time scale.10

Nevertheless, we suspected that PhCF could form a transient innermolecular carbene complex with an aromatic moiety of 2 before reaction and that this complex might be detected by UV vis spectroscopy. Related carbene complexes have recently been identified in solution for several chlorocarbenes with electron-rich aromatic partners.11 For example, chlorophenylcarbene (PhCCl) and 1,3,5-trimethoxybenzene reversibly form π-complexes characterized by an equilibrium constant K ∼ 1250 M 1 and ΔHo = 7.1 kcal/mol at 294 K; a signature electronic absorption band for the complexes has λmax at 484 nm.12 Platz and co-workers have also reported the formation of carbene complexes in solution.13 We are pleased to report here that laser flash photolytic (LFP) generation of PhCF, incarcerated in 2, affords short-lived innermolecular 2.3 π-complexes that absorb over a broad spectral range with a maximum at 468 nm and persist for at least 4 μs. Extensive computational studies provide insight into the nature of the carbene hemicarcerand complexes.

2. EXPERIMENTAL DETAILS AND COMPUTATIONAL METHODS 2.1. Experimental Details. The incarceration of 3-fluoro3-phenyldiazirine (5)14 was carried out as previously described10 and afforded the hemicarceplex 2.5. For LFP experiments, we employed a XeF2 excimer laser emitting 14 ns light pulses at 351 nm with 55 65 mJ power.15

Received: September 12, 2011 Revised: October 21, 2011 Published: November 03, 2011 r 2011 American Chemical Society

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The Journal of Physical Chemistry A 2.2. Computational Details. Electronic structure calculations were carried out using methodologies implemented in the Gaussian09 suite of programs.16 Density functional theory provided the methodologies for the calculation of ground, transition, and excited state structures and energetics.17 Ground state geometry optimizations of hemicarcerand 2 (R = H), fluorophenylcarbene (3), and inner- and outermolecular complexes formed between 2 and 3 were carried out using the dispersioncorrected B97D18 exchange-correlation functionals with 6-31G19a and 6-31+G19b basis sets (B97D/6-31G etc.); close to 200 atoms and up to 1600 basis functions (6-31+G) are included in calculations on the complexes. The potential energy surfaces for the complexes are very soft with respect to relative carbene hemicarcerand motion, and the located stationary points possess low-frequency vibrational modes. Hence, geometry optimizations (minima or transition states) and normal mode calculations were conducted with enhanced integration grid sizes (integral(grid = ultrafine)). Normal mode analyses were performed at the B97D/6-31G level, and the (unscaled) vibrational frequencies formed the basis for the calculation of vibrational zero-point energy (ZPE) corrections. Standard thermodynamic corrections (based on harmonic oscillator/rigid rotor approximations and ideal gas behavior) were then applied to convert from potential energies (E) to (standard) enthalpies (H; T = 298.15 K) and free energies (G; T = 298.15 K, P = 1 atm).20 Counterpoise corrections were applied to the potential energies of the carbene hemicarcerand complexes computed at the B97D/6-31+G level to approximately correct for basis set superposition errors (BSSE).21 A set of “best” enthalpies and Gibbs free energies was subsequently obtained by combining the BSSE-corrected potential energy differences obtained at the B97D/6-31+G level with the potential energy enthalpy and enthalpy-free energy corrections obtained at the B97D/6-31G level. These “best” enthalpies and free energies are used in the text below; details with intermediate data are available in the Supporting Information (Table S-1). Excited state calculations (transition wavelengths (λ) and oscillator strengths (f)) at optimized B97D/6-31+G ground state geometries utilized the time-dependent DFT formalism22 with the B3LYP functionals23 and 6-31+G basis sets (TDB3LYP/6-31+G//B97D/6-31+G). The higher wavelength features of the experimental spectrum (above ∼450 nm) could be represented reasonably well by consideration of just the lowest 10 15 excited states. To cover the lower wavelength features around 300 nm, however, it was necessary to consider 40 50 states (roots). Satisfactory convergence in these resourcedemanding calculations was achieved by combining incremental expansion of the diagonalization space with a gradual tightening of the energy and wave function convergence criteria.

3. RESULTS AND DISCUSSION LFP15 of 2.5 in 1,2-dichloroethane (DCE) afforded the calibrated24 UV vis spectrum shown in Figure 1. The spectrum shows distinct peaks at 316 and 468 nm. On the basis of computational analysis (see below), these absorptions are attributed to the formation of π-complexes between PhCF and aromatic moieties of 2. The broad absorption above 375 nm with the peak at 468 nm signifies, in particular, the presence of a new species. It is important to note that the incarcerated PhCF is too large to escape from its hemicarcerand “prison” on the time scale of the

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Figure 1. Calibrated UV vis spectrum acquired 200 ns after LFP of 2.5 in DCE solution. The absorbances are attributed to an innermolecular fluorophenylcarbene hemicarcerand complex.

LFP experiment. For typical monosubstituted benzene guests in hemicarcerand 2, the barrier to egress is ∼25 30 kcal/mol.7e,f LFP studies at 468 nm, where product formation is minimal, reveal that the π-complexes decay about 10% over 4 μs (see Figure S-1 in the Supporting Information). On the macroscopic time scale, 2.3 affords the hemicarcerand tropone derivative 4, formed in 55% isolated yield.10 Previous reports from our laboratories establish that judiciously calibrated DFT calculations nicely rationalize the structures, interaction energies, and electronic transitions of carbene complexes with aromatic substrates.11,12 More than 20 configurations of 2.3, differing in the relative orientations of 3 inside 2, were subjected to geometry optimization (potential energy minimization) with conformers featuring carbene 3 in polar (long axis of the carbene parallel to the polar axis of 2) or equatorial (long axis of the carbene perpendicular to the polar axis of 2) orientations of particular interest. Even though conformations with equatorial orientations are not populated in hemicarceplexes with monosubstituted benzene guests and are models for the transition state of guest tumbling,7f we included these orientations as reference structures in which π-complexation between 3 and the aromatic moieties of 2 is geometrically impossible. A distinct, deep minimum was located starting from several initial conformers (see Figure 2A, B). Here, the carbene is in a polar orientation wedged almost symmetrically between a pair of neighboring aryl rings. The carbenic carbon of 3 is 3.17, 3.26, and 3.49 Å away from a triad of bonded C atoms of one aryl ring, and 3.25, 3.39, and 3.48 Å away from an analogous triad of bonded C atoms in the adjacent aryl ring (Figure 2B; B97D/6-31+G). Although the alignment of the carbene’s LUMO (p) orbital with the aryl HOMO (π) orbitals is not optimal, significant stabilization of the “double-decker” arylcarbene aryl-π-complex nevertheless appears to occur. We find a stabilization enthalpy of 22.0 kcal/mol for the complex configuration, relative to separated 2 and 3 (with corrections for BSSE included). A more appropriate comparison may be made with the least stable minimum located for 2.3, a configuration in which the carbene lies in the equatorial plane (Figure 2C, D), for which a stabilization enthalpy of 10.2 kcal/mol is computed (relative to separated 2 and 3). Thus, the computed enthalpy of complex formation is almost 12 kcal/mol. The endohedral 13800

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Figure 2. Optimized structures of 2.3 (R = H; B97D/6-31+G) with carbene 3 in polar (A, B) or equatorial orientations (C, D). The carbene is shown in space-filling format with the carbenic carbon atom colored purple and the fluorine, yellow. The two cavitand aryl units that are involved in the π-complex are shown in either space-filling (A) or balland-stick (B) format. The three shortest arylcarbene C C contacts for both aryl units are marked with green lines in (B); see text for optimized distances.

complex (Figure 2A, B) is also computed as thermodynamically stable with ΔGo = 7.0 kcal/mol, relative to the separated reactants, whereas equatorial orientations are not stable (ΔG o = +2.8 kcal/mol or more). Equatorial orientations of the carbene produce very shallow minima and are most likely present only in silico. Favorable carbene interactions with the electron-rich alkoxy-substituted aryl moieties provide a substantial driving force toward polar orientations of the carbene. In contrast, as mentioned above, near-equatorial orientations effectively resemble transition states for tumbling of the carbene inside the hemicarcerand cavity. The computed Gibbs energy difference between optimal polar (i.e., the π-complex) and equatorial orientations is 9.8 kcal/mol, slightly higher than the expected tumbling barrier, which is estimated to be 470 nm). This suggests that the diazo compound may have weak absorption in this spectral region, but the experimental UV spectrum was not described.25 The matrix-isolated UV vis absorption spectrum of PhCF reveals a dominant peak centered around 270 nm.26 The LFP spectrum of PhCF in DCE shows two absorptions: an intense feature at 316 nm and a weaker absorption at 548 nm.27 Our calculations find the weaker carbene-center-localized σ f p transition at 592 nm (f = 0.0043). An intense π(phenyl) f p charge-transfer transition is computed at 260 nm (f = 0.29), too high in energy, although in fair agreement with the maximum observed in the low-temperature matrix spectrum.28,29 A remainder of this latter PhCF transition is computed at 266 nm (f = 0.080) in the 2.3 π-complex (see the Supporting Information), and we assign this transition to the spectroscopic feature observed at 316 nm (Figure 1). The 266 nm transition terminates in computed excited state number 42 (!) and is composed of several elementary excitations; the dominant contributor by far (∼55%) is, however, the PhCF π(phenyl) f p charge-transfer excitation. In contrast, the computed spectrum for 2.3 with PhCF in the equatorial orientation shows the carbene σ f p absorption at 579 nm (f = 0.0087), and a remainder (∼67%) of the π(phenyl) f p transition at 277 nm (f = 0.065; excited state number 29), with no distinguishing features between 400 and 550 nm. The calculated UV vis spectra for 2.3 in the polar (π-complex) and equatorial configurations appear in Figure 3. The calculated spectrum for the polar configuration (Figure 3, left panel) compares favorably with the experimental spectrum in Figure 1. Presumably, the first step in the decay of the 2.3 π-complex is cyclopropanation of an aryl ring on the way to product 4; see eq 1. We have located the transition state for cyclopropanation (R(Ccarbene Caryl) = 2.12 Å) and find an activation enthalpy of ΔH‡ = 9.7 kcal/mol and an activation free energy of ΔG‡ = 11.6 kcal/mol relative to the π-complex (resting state). At 298 K, this barrier height translates to a lifetime for the carbene complex of 13801

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Figure 3. Computed (TD-B3LYP/6-31+G//B97D/6-31+G) UV vis spectra of 2.3 in the polar (π-complex, left panel) and equatorial (right panel) orientations. Each electronic transition was represented by a Gaussian of height proportional to its computed oscillator strength and a half-width at halfheight of 2700 cm 1. Note the difference in scale for the absorption intensities (y axes) in the two panels. See the Supporting Information for the actual transition energy and oscillator strength values.

∼50 μs, consistent with our observation of ∼10% of carbene complex decay over 4 μs (see above). We also examined the LFP of diazirine 5 in the presence of 0.025 M empty hemicarcerand 2. This generated PhCF that afforded outermolecular complex(es) presumably with the electron-rich alkoxy-substituted aromatic units of 2. The experimental UV spectrum appears in the Supporting Information as Figure S-2, where we note broad absorption extending from ∼375 to 700 nm with a peak at 500 nm along with a weaker absorption peak at 316 nm. Computational studies afford several outermolecular π-complexes and an O(acetal)-ylide complex of PhCF and 2; the π-complexes are more stable than the ylide. The π-complexes generally feature an intense peak at 275 nm, a second peak around 410 nm of medium intensity, and broad absorption at longer wavelengths with a third, weak peak near 600 nm. A sample computed spectrum is provided in the Supporting Information as Figure S-3. There is not a particularly good match between the observed and computed spectra so that more precise assignments of the outermolecular complexes are precluded at this time.

4. CONCLUSIONS LFP of diazirine 5 incarcerated in hemicarcerand 2 affords incarcerated PhCF [2.3], which forms a transient, innermolecular π-complex with aryl moieties of 2. This carbene complex can be detected spectroscopically. Extensive computational studies provide insights into the structure, spectroscopy, energetics, and kinetics of the 2.3 carbene complex. ’ ASSOCIATED CONTENT

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Supporting Information. Figures S-1 S-3; complete reference to Gaussian 09; Table S-1; B97D/6-31G and B97D/ 6-31+G optimized geometries and absolute energies of 2, 3, and 2.3 (π-complex, equatorial minimum, and TS for cyclopropane formation); TD-B3LYP/6-31+G//B97D/6-31+G electronic excitation energies and oscillator strengths for 2, 3, and 2.3 (π-complex and equatorial minimum). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (R.A.M.); warmuth@ rutgers.edu (R.W.);[email protected] (K.K.-J.).

’ ACKNOWLEDGMENT We thank Dr. Lei Wang for the determination of the spectrum of PhCF with empty hemicarcerand 2. We are grateful to the National Science Foundation for financial support. ’ REFERENCES (1) For a brief overview, see: Kirmse, W. Angew. Chem., Int. Ed. 2005, 44, 2476. (2) For thorough coverage, see: Brinker, U. H.; Mieusset, J.-L., Ed.; Molecular Encapsulation; Wiley: Chichester, U.K., 2010. (3) Reviews:(a) Sander, W. In Carbene Chemistry; Bertrand, G., Ed.; Dekker: New York, 2002, pp 1 f. (b) Maier, G.; Reisenauer, H. P. In Advances in Carbene Chemistry; Brinker, U. H., Ed.; Elsevier: Amsterdam, 2001, Vol. 3, pp 115 f. (4) (a) Mieusset, J.-L.; Brinker, U. H. In Brinker, U. H.; Mieusset, J.-L., Ed.; Molecular Encapsulation; Wiley: Chichester, U.K., 2010, pp 269 f. (b) Rosenberg, M. G.; Brinker, U. H. Eur. J. Org. Chem. 2006, 5423. (c) Rosenberg, M. G.; Brinker, U. H. Adv. Phys. Org. Chem. 2005, 40, 1. (d) Rosenberg, M. G.; Brinker, U. H. J. Org. Chem. 2003, 68, 4819. (e) Krois, D.; Bobek, M. M.; Werner, A.; Kahlig, H.; Brinker, U. H. Org. Lett. 2000, 2, 315. (5) Wagner, G.; Knoll, W.; Bobek, M. M.; Brecker, L.; van Herwijnen, H. W. G.; Brinker, U. H. Org. Lett. 2010, 12, 332. (6) (a) Moya-Barrios, R.; Cozens, F. L. J. Am. Chem. Soc. 2006, 128, 14386. (b) Moya-Barrios, R.; Cozens, F. L. Org. Lett. 2004, 6, 881. (7) (a) Warmuth, R. In Brinker, U. H.; Mieusset, J.-L., Ed.; Molecular Encapsulation; Wiley: Chichester, U.K., 2010, pp 227 f. (b) Kerdelhue, J.-L.; Langenwalter, K. J.; Warmuth, R. J. Am. Chem. Soc. 2003, 125, 973. (c) Warmuth, R.; Marvel, M. A. Chem.—Eur. J. 2001, 7, 1209. (d) Warmuth, R. J. Am. Chem. Soc. 2001, 123, 6955. (e) Cram, D. J.; Blanda, M. T.; Paek, K.; Knobler, C. B. J. Am. Chem. Soc. 1992, 114, 7765. (f) Liu, Y.; Warmuth, R. Angew. Chem., Int. Ed. 2005, 44, 7107. (g) Warmuth, R.; Maverick, E. F.; Knobler, C. B.; Cram, D. J. J. Org. Chem. 2003, 68, 2077. (h) Kemmis, C.; Warmuth, R. J. Supramol. Chem. 2003, 1, 253. (8) Moss, R. A.; Kmiecik-Lawrynowicz, G.; Krogh-Jespersen, K. J. Org. Chem. 1986, 51, 2168. (9) Lu, X.; Chu, G.; Moss, R. A.; Sauers, R. R.; Warmuth, R. Angew. Chem., Int. Ed. 2005, 44, 1994. (10) Lu, Z.; Moss, R. A.; Sauers, R. R.; Warmuth, R. Org. Lett. 2009, 11, 3866. (11) (a) Moss, R. A.; Tian, J.; Sauers, R. R.; Krogh-Jespersen, K. J. Am. Chem. Soc. 2007, 129, 10019. (b) Moss, R. A.; Wang, L.; Weintraub, E.; Krogh-Jespersen, K. J. Phys. Chem. A 2008, 112, 4651. (c) Moss, R. A.; Wang, L.; Odorisio, C. M.; Krogh-Jespersen, K. J. Phys. Chem. A 2010, 114, 209. (12) Moss, R. A.; Wang, L.; Odorisio, C. M.; Krogh-Jespersen, K. J. Am. Chem. Soc. 2010, 132, 10677. 13802

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