Solvation of Dichlorocarbene: Complexation with Aryl Ethers - The

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Solvation of Dichlorocarbene: Complexation with Aryl Ethers Robert A. Moss,* Lei Wang, Christina M. Odorisio, Min Zhang, and Karsten Krogh-Jespersen* Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of New Jersey, New Brunswick, New Jersey 08903 ReceiVed: August 5, 2009; ReVised Manuscript ReceiVed: September 24, 2009

Dichlorocarbene (CCl2), generated by laser flash photolysis of dichlorodiazirine, formed π- and O-ylidic complexes with aromatic ethers such as anisole, 1,3-dimethoxybenzene, 1,3,5-trimethoxybenzene, dibenzofuran, and dibenzo-18-crown-6 and with the aromatic ester phenyl acetate. These complexes were visualized by UV-vis spectroscopy, and they retarded the addition of CCl2 to tetramethylethylene by factors of 18-152. Computational studies based on density functional theory provided structures and energetics for the transient species and rationalized their absorption spectra. Complexes were not observed between CCl2 and simple, nonaromatic ethers such as THF, dioxane, or 18-crown-6, nor did these ethers much affect the addition rate of CCl2 to tetramethylethylene. Computations also suggested that π-complexes of CCl2 and, e.g., mesitylene and durene, were energetically reasonable transients. Although these species were not detected spectroscopically, the aromatic compounds did slow the addition of CCl2 to tetramethylethylene by factors of 15 and 31, respectively. 1. Introduction Evidence for the solvation of carbenes and the concomitant modulation of their reactivity has gradually accumulated. Singlet carbenes feature a formally vacant p orbital so that electron donation to that orbital by a solvent provides an attractive possibility for stabilizing the carbene, decreasing its reactivity, and enhancing its selectivity. For example, Tomioka et al. noted that dioxane altered the selectivity of phenylcarbene toward alcohols, and also the carbene’s discrimination between O-H insertion and CdC addition.1 Similarly, Ruck and Jones observed that the intramolecular partition between C-H insertion and methyl migration of tertbutylcarbene was sensitive to solvent identity; electron-donating solvents like THF, benzene, anisole, and pyridine induced significant changes in the product distribution.2 Our laboratories found that a related solvent effect on RCCl (R ) benzyl, propyl, or cyclopropyl) by benzene or anisole altered intramolecular/ intermolecular carbene product distributions,3 a result that was rationalized by computational studies.4 Negative or inconclusive effects of solvents on carbenes have also been reported. Platz and co-workers found little evidence that THF or aromatic solvents modified the rates of addition to tetramethylethylene (TME) of phenylchlorocarbene, phenylbromocarbene, or p-nitrophenylchlorocarbene (PNPCC).5 Timeresolved IR spectroscopy revealed little effect of benzene or THF on key absorptions of phenylchlorocarbene or phenylfluorocarbene, and their rates of addition to TME were only marginally affected by, e.g., benzene, THF, or MeCN.6 Analogous IR experiments gave no evidence for the complexation of PNPCC by benzene.7 However, evidence was obtained for the specific solvation of two amidohalocarbenes by THF and dioxane.8 Of course, the strongest evidence for specific carbene solvation is the direct detection of the carbene-solvent complex. An early example was the use of photoacoustic spectroscopy * Authors to whom correspondence should be addressed. E-mail: [email protected]; [email protected].

to monitor heat deposition ascribed to the formation of a π-complex between methylene and benzene.9 More recently, we reported that alkylchlorocarbenes such as methylchlorocarbene and benzylchlorocarbene reacted with anisole and 1,3dimethoxybenzene to give weak π- and O-ylidic complexes that displayed signature UV-vis bands in laser flash photolysis (LFP) experiments.10-12 Analogous experiments revealed that PNPCC formed similar complexes with the same solvents.13 Even benzene afforded UV spectroscopic evidence for weak complexation with PNPCC,13 despite the lack of positive results in the earlier IR study.7 What about the iconic divalent carbon species dichlorocarbene (CCl2)? Computational studies suggest that CCl2 should complex weakly with π donors like benzene,4 although a search for CCl2-ether interactions found that neither dioxane nor THF much affected the rates of CCl2 additions to TME.14 Here we describe UV-vis spectroscopic evidence and computational results supporting the formation of π- and O-ylidic complexes of CCl2 with aryl ethers such as anisole, 1,3-dimethoxybenzene, 1,3,5-trimethoxybenzene, and dibenzofuran. These solvents also significantly alter the rates of CCl2 additions to TME. Complexation is not observed by UV-vis spectroscopy for ethereal solvents (THF, dioxane) or for aromatics like mesitylene or durene, although the latter do significantly modulate the CCl2-TME addition rates. 2. Experimental Details and Computational Methods 2.1. Experimental Details. The preparation of dichlorodiazirine (1), a photochemical precursor of CCl2, has been

described in detail.15 Solvents and other reagents were commercial materials, mostly used as received: pentane (99%,

10.1021/jp9075542  2010 American Chemical Society Published on Web 10/30/2009

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spectrophotometric grade), anisole (99.7%), 1,3-dimethoxybenzene (98%), 1,3,5-trimethoxybenzene (99%), dibenzofuran (99%), dibenzo-18-crown-6 (98%), 18-crown-6 (99%), 1,3,5trimethylbenzene (98%), 1,2,4,5-tetramethylbenzene (98%), and phenyl acetate (99%). Dioxane (99%) was dried over sodium, then refluxed, and distilled from sodium. THF (99.9%) was dried over sodium, then refluxed, and distilled from sodium and benzophenone. All LFP experiments were carried out under nitrogen at 25 °C on 2 mL solutions of diazirine 1 in quartz cells. The diazirine absorbance was A ∼ 0.4 at λmax ) 359 nm. Carbene signals were monitored from 60 to 150 ns after the laser pulse. A description of our LFP system appears in the Supporting Information of ref 10. 2.2. Computational Methods. Electronic structure calculations based on density functional theory (DFT) made use of the PBE16a and B3LYP16b,c exchange-correlation functionals and 6-311+G(d) basis sets.17 Geometry optimizations of carbene and solvent monomers, O-ylidic complexes, dimeric carbenesolvent π-complexes,12 transition states, and reaction products were carried out using the PBE functionals (PBE/6-311+G(d)). Stationary points were characterized further by normal-mode analysis, and the (unscaled) vibrational frequencies formed the basis for the calculation of vibrational zero-point energy corrections. Standard statistical mechanical expressions (based on harmonic oscillator/rigid rotor approximations and ideal gas behavior) were then used to evaluate the thermodynamic corrections necessary to convert from purely electronic energies to standard enthalpies (H; T ) 298.15 K) and Gibbs free energies (G; T ) 298.15 K, P ) 1 atm).18 The differential free energies presented in the tables have been corrected to a reference state of 1 M concentration for each species participating in the reaction by subtracting 1.89 kcal/mol from the Gibbs free energies at P ) 1 atm (T ) 298.15 K). Electronic population analysis employed the NBO scheme of Weinhold and coworkers.19 Calculations of electronically excited state properties (transition wavelengths (λ) and oscillator strengths (f)) were performed at the optimized ground-state geometries using the timedependent DFT formalism,20 B3LYP functionals, and 6-311+G(d) basis sets (TD-B3LYP/6-311+G(d)//PBE/6-311+G(d)). The character of a particular electronic transition was assigned by consideration of the largest transition amplitude(s) for the excitation and by visualization of the contributing MOs. The polarizable conductor self-consistent reaction field model (CPCM) was used to incorporate general polar solvent effects into the excited-state calculations.21 Default parameters were applied, except that we chose to use Pauling atomic radii with explicit hydrogens for the solvent molecules. The pentane, anisole, and methylene chloride solvents used experimentally were simulated with heptane, THF, and methylene chloride parameters, respectively. The computational scheme outlined above has provided useful information in our previous investigations of carbene-solvent interactions (see, e.g., refs 10, 13, and 15). All electronic structure calculations were carried out with the Gaussian03 suite of programs.22 3. Results and Discussion 3.1. Spectroscopy. Matrix-isolated CCl2 absorbs at 460-530 nm,23 in reasonable agreement with the computed transition of lowest energy for CCl2 in simulated heptane at 492 nm (σ f p, f ∼ 0.004).15 LFP of diazirine 1 in pentane at 25 °C gives a transient absorbing at 465 nm with a lifetime of ∼0.5 µs.15,24

Moss et al.

Figure 1. LFP spectrum of CCl2 in a 1:1 anisole-pentane solution 150 ns after the laser pulse; O-ylidic complex at 460 nm, π-complex(es) at 500 nm.

We initially assigned this absorbance to CCl2,24 but this proved incorrect; the signal disappeared when the LFP experiment was performed under nitrogen on nitrogen-degassed diazirine solutions.15 A more reasonable carrier of the 465 nm absorption is carbonyl oxide 2, formed by the reaction of CCl2 with oxygen.15 The LFP experiments to be described here were all performed under nitrogen on degassed solutions of diazirine 1 (A ∼ 0.4 at λmax ) 359 nm). Under these conditions, the CCl2 generated in the 60-70 mJ laser flash is insufficiently concentrated to afford a detectable UV absorbance. LFP of diazirine 1 in a 1:1 solution of anisole (3) in pentane

affords two absorptions observed at 460 and 500 nm, 150 ns after the laser pulse; cf., Figure 1 and Table 1. CCl2 is a highly electrophilic carbene and the methoxy group is a strong π-donor, ortho-para directing as a phenyl substituent. We thus anticipate potential CCl2/anisole complex formation to be most probable in the proximity of atoms carrying excess electron density, i.e., the oxygen atom and the para (C4) and ortho (C2, C6) carbon atoms (computed net atomic charges for 3 are shown in Scheme 1). Indeed, electronic structure calculations readily located CCl2/anisole complexes in which the dominant carbene interaction occurs with the oxygen atom (CCl2/3a), the para carbon (CCl2/3c), or one of the ortho carbons (CCl2/3b, CCl2/3d). Computed interaction energies (PBE/6311+G(d)) are listed in Table 2, and views of these complexes are presented in Figure 2. The complexes bear considerable resemblance to the recently described O-ylidic and π-complexes formed between MeCCl10 or PNPCC13 and anisole. The

Solvation of Dichlorocarbene

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TABLE 1: Dichlorocarbene Complexes donor anisole 1,3-dimethoxybenzene 1,3,5-trimethoxybenzene dibenzofuran dibenzo-18-crown-6 phenyl acetate

λmax (nm) 460 500 500 548 580 660 484 580 468 564 460 516

kf (s-1)a

kd (s-1)b

1.62 0.645 1.85 1.63 5.79 6.06 1.84 3.07 1.96 2.26 2.34 1.77

0.313 1.50 1.29 2.98 4.83 6.20 0.88 1.36 1.21 1.47 1.93 4.24

a Observed rate of formation of transient; rate constants are in the 107 regime. b Observed rate of transient decay; rate constants are in the 106 regime.

computed complexation enthalpies are low (-2.5 to -3.5 kcal/ mol), and the (standard) free energies are well into the positive realm. In the O-ylidic CCl2/3a complex the C(carbene)-O distance is long (∼2.6 Å) and the O atom approaches tetrahedral coordination (a lone pair occupies the fourth site). The smallest C(carbene)-C(phenyl) distances in the π-complexes (CCl2/ 3b-3d) are of the order 2.7-2.8 Å; C(carbene) is always located essentially right above a C(phenyl) atom and one Cl atom points toward the center of the phenyl ring. The O-ylidic species CCl2/3a is computed as the thermodynamically most stable CCl2/anisole complex (∆G ) 3.7 kcal/mol), while the relative stabilities of the three π-complexes appear about equal (∆G ∼ 4.4-5.0 kcal/mol). The concentration of any individual complex will thus be modest but statistically there are many anisole sites available to interact with CCl2; furthermore, small variations in complex geometry do not induce major destabilization so that, overall, the concentration of CCl2/3 complexes may be sufficiently large to be detected (viz. Figure 1). The electronic transitions of lowest energy computed for each complex (TD-B3LYP/6-311+G(d)//PBE/6-311+G(d)) are also listed in Table 2. The experimental absorption around 460 nm is assigned to the O-ylidic complex CCl2/3a for which a strong absorption band is predicted at 486 nm (f ) oscillator strength ) 0.11). The transition has strong charge-transfer character with a lone pair electron on O being excited into the formally empty carbene p orbital (n f p type transition). The lower energy absorption peak observed at 500 nm is assigned to the π-complexes collectively; individual absorptions (π f p transitions) are predicted at 513 nm (f ) 0.11) for CCl2/3b, also at 513 nm (f ) 0.15) for CCl2/3c, and at 534 nm (f ) 0.08) for CCl2/3d. The peak is probably dominated by CCl2/3b,3c absorption. As can be seen from the observed rate constants for transient formation and decay (Table 1), the 500 nm absorption assigned to the π-complexes (CCl2/3b-3d) decays ∼5 times faster than the 460 nm absorption assigned to the O-ylidic complex, consistent with the slightly greater thermodynamic stability calculated for the ylidic complex (Table 2). For some complexes, higher energy transitions with significant oscillator strengths are computed but not observed; strong background absorption by the diazirine precursor is probably responsible. Other aryl ethers also afford complexes with CCl2. Thus, 1,3dimethoxybenzene (4), 1,3,5-trimethoxybenzene (5), dibenzofuran (6), and dibenzo-18-crown-6 (7) each give LFP UV-vis spectra of transient complexes; λmax values and the rate constants for their formation and decay are summarized in Table 1. Complexation was also observed between CCl2 and an aryl ester, phenyl acetate (8).

The absorptions of the CCl2/1,3-dimethoxybenzene and CCl2/ 1,3,5-trimethoxybenzene complexes appear more intense than the CCl2/anisole absorptions (compare the ordinate scale of Figure 1 with Figures 3 and 4). The absorption peaks undergo a red shift with the increase in π-electron richness of the phenyl ring, but the relative intensities of the two peaks present in each spectrum change. Furthermore, the magnitudes of the red shifts going from 1,3-dimethoxybenzene to 1,3,5-trimethoxybenzene are approximately twice those going from anisole to 1,3dimethoxybenzene (Table 1). In the CCl2-anisole spectrum (Figure 1), the higher wavelength absorption is more intense, whereas the peak absorptions show about equal intensity in the CCl2-1,3-dimethoxybenzene spectrum (Figure 3) and the higher wavelength band has broadened. In the CCl2-1,3,5-trimethoxybenzene spectrum (Figure 4), the low wavelength absorption is distinctly stronger than the high wavelength absorption, a reversal from the CCl2-anisole spectrum (Figure 1). Thus, assignments of the spectra illustrated in Figures 3 and 4 are not expected to be identical to those of Figure 1. Three 1,3-dimethoxybenzene conformational minima may be generated by simple rotations around the C(phenyl)-O(OMe) bonds (4A-4C). These conformers are very close in energy (∆∆G ∼ 0.5 kcal/mol, Scheme 2) and thus coexist as a Boltzmann distribution in a solution of 4 (the computed barrier to Me rotation around the C(phenyl)-O bond is about 4 kcal/ mol). We have computationally investigated complexes between CCl2 and only the most stable 1,3-dimethoxybenzene rotamer, 4A. Likely binding sites based on net atomic charges (Scheme 1) are C2, C4, C6, and of course the O atoms (essentially the same sites as in anisole). CCl2/4 complexes have been located at all these sites and, overall, they look very similar to those presented above in Figure 2 (see Figure S-1 in Supporting Information). The computed complexation energies are shown in Table 3. Relative to CCl2/anisole complexes, the stabilities of the π-complexes (CCl2/4b-4d) are preferentially increased by the presence of the second methoxy donor substituent, and the interaction distances are concomitantly slightly reduced; the O-ylidic complex CCl2/4a is very similar to CCl2/3a in both interaction energy and distance. We actually located two minima for CCl2 interacting with C2; they differ in the relative orientation of a Cl atom with respect to the methoxy group on C3 (CCl2/4b and CCl2/4b′ in Figure S-1). The free-energy difference between the two minima is 1.8 kcal/mol, but identification of these two very similar complexes illustrates that many shallow structural minima undoubtedly exist for a CCl2 species embedded in the π-cloud of the methoxybenzenes. Interestingly, when searching for a complex as a minimum near π-electron deficient C5, only a cyclopropane product could be located. The electronic absorption spectrum shows two maxima near 500 and 548 nm (broad) with about equal peak intensities (Figure 3). The calculations assign comparable stability to the complexes CCl2/4a-4d and predict transitions with significant oscillator strengths around 500 nm for all, making detailed spectral assignments nearly impossible. Thus, tentative assignments for the absorption observed at 500 nm are π-complexes of type CCl2/4b and CCl2/4c, which have strong computed absorptions at 482 nm (f ) 0.10) and 496 nm (f ) 0.16), respectively. The higher wavelength absorption would then be attributed to CCl2/4d and CCl2/4a complexes, which possess intense computed absorptions at 507 nm (f ) 0.15) and 519 nm (f ) 0.07), respectively. However, the experimental mixture of complexes actually sampled in the LFP experiment must also include CCl2 complexes with other 1,3-dimethoxybenzene

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SCHEME 1: PBE/6-311+G(d) Net Atomic Charges (NPA Analysis) and Atom-Numbering Scheme for Anisole (3), 1,3-Dimethoxybenzene (4), 1,3,5-Trimethoxybenzene (5), Dibenzofuran (6), and Dibenzo-18-crown-6 (7)

TABLE 2: Complexation Energies (PBE/6-311+G(d)) and Electronic Transition Wavelengths and Intensities (B3LYP/ 6-311+G(d) with CPCM Solvent Correction) for CCl2/Anisole O-Ylidic and π-Complexes (CCl2/3a-3d) species

binding site

∆Ha,b

∆Ga,c

C-Xd

λ1 e

f1 f

λ2 e

f2 f

λ3 e

f3 f

CCl2/3a CCl2/3b CCl2/3c CCl2/3d

O C2 C4 C6

-2.7 -2.4 -2.5 -3.5

3.7 5.0 4.6 4.4

2.6 2.7 2.8 2.7

486 513 513 534

0.11 0.11 0.15 0.08

433 457 408 461

0.02 0.01 0.09 0.01

371 354 394 357

0.00 0.14 0.00 0.14

a ∆H and ∆G in kcal/mol, relative to the energies of separated reactants. b T ) 298.15 K. c The free-energy differences were computed using a reference state of 1 M concentration for each species participating in the reaction, and T ) 298.15 K. d Computed C(carbene)-O or C(carbene)-C(phenyl) bond lengths in Å. e Wavelengths in nm. f Oscillator strengths (dimensionless).

rotamers (4B, 4C) and their spectral effects (band broadening and energy shifts) are unknown. The trimethoxybenzene complexes are particularly interesting. Only two 1,3,5-trimethoxybenzene rotamers are possible (5A, 5B; Scheme 2); they are separated by ∆∆G ) 1.0 kcal/mol and must thus coexist in solution. We have computationally investigated complexes with both isomers and discuss here only complexes formed with the most stable rotamer, 5A; data for CCl2/5B complexes are available in the Supporting Information (Table S-1). On the basis of the computed net atomic charges, there are only two prospective binding sites in 5A (Scheme 1): the unsubstituted C atoms (all fully equivalent) and the O atoms (also equivalent). The computed complexation energies for CCl2 with 5A are presented in Table 3. Not surprisingly, we find that the O-ylidic complex CCl2/5a (Figure S-2 in the Supporting Information) is very similar to CCl2/3a and CCl2/4a in interaction enthalpy (∆H ) -3.0 kcal/mol) and C(carbene)-O distance (2.6 Å). However, we have located two π-complexes (CCl2/ 5b,5c) in which the carbene center interacts with the highly charged C2 site (Figure 5) and substantial interaction energies result (∆H ∼ -9 kcal/mol, Table 3). Analogous to CCl2/4b,4b′ described above, the complexes CCl2/5b,5c could be considered as a pair of rotamers: CCl2/5b has a Cl atom pointing toward an adjacent Me group, whereas CCl2/5c has a Cl atom pointing in the opposite direction toward an O atom (Figure 5). The interaction distances are very short in CCl2/5b,5c (C(phenyl)C(carbene) ∼ 1.65 Å) and distinct pyramidalization occurs at C2 in both species (Figure 5). The absorption observed at 580 nm in Figure 4 is assigned to the π-complex of lowest energy, CCl2/5b, with a computed intense transition at 607 nm (f ) 0.12). The absorption observed peaking at 660 nm (Figure 4) is assigned to the less stable

species CCl2/5c for which a transition is calculated at 683 nm (less intense with f ) 0.07). Since the computed binding energies of CCl2/5b,5c are substantially larger (by ∼6 kcal/mol) than that of the alternative O-ylidic complex CCl2/5a (absorption maxima calculated at 509 and 493 nm, f ) 0.01 and 0.09), the latter is presumably not observed as a separate spectroscopic feature. We note that the complexes of 1,3,5-trimethoxybenzene form and decay more rapidly than those of 1,3-dimethoxybenzene or anisole (Table 1). This is likely due to the high reactivity of the very electron-rich trimethoxybenzene toward the electrophilic CCl2. We have probed the potential energy surface for CCl2 and rotamer 5A in more detail and investigated the formation and some possible decay pathways for the CCl2/5 complexes, assuming an idealized gas-phase environment (data available in Table S-2 in the Supporting Information). Starting from the separated reactants, there are no potential energy barriers to the formation of the CCl2/5 (O-ylidic or π-type) complexes. The free-energy barrier for the most stable complex, CCl2/5b, toward cyclopropanation to form 1,3,5-trimethoxy-7,7-dichloronorcaradiene is only 2.0 kcal/mol, and the reaction is exergonic by 21.0 kcal/mol. Complex CCl2/5c encounters a slightly higher barrier of 5.0 kcal/mol when forming the norcaradiene product directly. However, the free-energy barrier for the formation of CCl2/5b from CCl2/5c, a process essentially occurring via rotation of the CCl2 fragment, is only 2.9 kcal/mol and thus offers a lower energy path to decay by norcaradiene formation. In contrast, formation of a true O-ylide (C-O ) 1.43 Å) from the less favorable O-ylidic complex CCl2/5a (C-O ) 2.6 Å) is predicted to be endergonic by 1.0 kcal/mol. We were unable to locate an appropriate TS for this process. There is a small



Figure 4. LFP spectrum of CCl2 in a 0.3 M 1,3,5trimethoxybenzene-pentane solution 60 ns after the laser pulse; π-complex absorptions at 580 and 660 nm.

SCHEME 2: Relative Free Energies (PBE/6-311+G(d), kcal/mol) of 1,3-Dimethoxybenzene (4A-4C) and 1,3,5-Trimethoxybenzene (5A, 5B) Rotamers

Figure 2. PBE/6-311+G(d) computed structures of representative CCl2/anisole complexes.

Figure 3. LFP spectrum of CCl2 in a 1:1dimethoxybenzene-pentane solution 150 ns after the laser pulse; transient absorptions at 500 and 548 nm.

enthalpic barrier of 1.3 kcal/mol for the decay of complex CCl2/ 5a to form complex CCl2/5b, but the free-energy barrier for this process is almost nonexistent (0.1 kcal/mol relative to CCl2/ 5a). The O-ylidic complex is thus both thermodynamically and kinetically unfavorable relative to CCl2/5b,5c. The π-complexes CCl2/5b,5c owe their existence to the substantial electron density present at the C2 site in 5 and the presence of energy barriers for cyclopropane (norcaradiene) formation; these small but finite

barriers are presumably caused by the loss of aromaticity that occurs with cyclopropanation. Spectra of transient complexes are also observed with dibenzofuran (6) and dibenzo-18-crown-6 (7); their absorptions appear at 484 and 580 nm and 468 and 564 nm, respectively; see Figures S-3 and S-4 in the Supporting Information. The net atomic charges for dibenzofuran suggest low selectivity among the ring sites C2, C3, and C4; detailed calculations reveal small and approximately equal stabilization energies for the various CCl2/6 π-complexes and the O-ylidic species (∆H ∼ -1.4 to -2.4 kcal/mol; Table S-3). The computed transition intensity is particularly large for the π-complex formed at C2 (CCl2/6d, Figure 6) for which an absorption wavelength of 540 nm (f ) 0.15) is predicted (Table 4). A π-complex formed at C1 (CCl2/ 6c, see Table 4) shows significant intensity in a transition computed at 509 nm (f ) 0.09). The O-ylidic complex CCl2/ 6a, in which the carbenic carbon is situated in the dibenzofuran plane, enjoys the largest stability among the located complexes (Table 4 and Figure 6). Unfortunately, this geometry renders the lowest energy transition in the complex very weak. We tentatively assign the absorptions observed for CCl2-dibenzofuran to π-complexes CCl2/6c (484 nm band) and CCl2/6d (580 nm band), although additional complexes most likely contribute to the full spectrum (see Table S-3).

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TABLE 3: Complexation Energies (PBE/6-311+G(d)) and Electronic Transition Wavelengths and Intensities (B3LYP/ 6-311+G(d) with CPCM Solvent Correction) for CCl2/1,3-Dimethoxybenzene (CCl2/4a-4d) and CCl2/1,3,5-Trimethoxybenzene (CCl2/5a-5c) O-Ylidic and π-Complexes species

binding site

∆Ha,b

∆Ga,c

C-Xd

λ1 e

f1 f

λ2 e

f2 f

λ3 e

f3 f

CCl2/4a CCl2/4b CCl2/4b′ CCl2/4c CCl2/4d CCl2/5a CCl2/5b CCl2/5c

O1 C2 C2 C4 C6 O C2 C2

-2.9 -4.7 -3.3 -3.8 -5.2 -3.0 -9.4 -8.8

4.1 3.9 5.7 4.4 3.2 4.0 1.7 1.9

2.6 2.5 3.1 2.8 2.5 2.6 1.6 1.7

519 530 579 496 507 509 607 683

0.07 0.01 0.05 0.16 0.15 0.01 0.12 0.07

445 482 495 429 421 493 368 373

0.01 0.10 0.03 0.01 0.01 0.09 0.08 0.10

426 433 437 375 359 434 334 332

0.04 0.01 0.03 0.14 0.20 0.00 0.10 0.09


formed with the central oxygen atom (O1, CCl2/7a) with predicted absorption at 485 nm (f ) 0.01). The higher wavelength absorption around 564 nm is assigned to π-complexes predominantly formed at C3 (CCl2/7d) with predicted absorption at 551 nm (f ) 0.11), Table 4. In contrast to PNPCC, where simple ethers such as diethyl ether,13 THF,5 and 18-crown-613 afford spectra of PNPCC-Oylides, no O-ylides or O-ylidic complexes were observed in reactions of CCl2 with nonaromatic ethers. Thus, LFP of diazirine 1 in 1:1 pentane solutions of THF (9), dioxane (10), or 0.15 M 18-crown-6 (11) did not give observable transients in UV or visible spectral regions. The contrasting behavior of

Figure 5. PBE/6-311+G(d) computed structures of highly stabilized CCl2/1,3,5-trimethoxybenzene complexes: CCl2/5b with computed absorption at 607 nm (f ) 0.12) and CCl2/5c with computed absorption at 683 nm (f ) 0.07).

Figure 6. PBE/6-311+G(d) computed structures of the most stable CCl2/dibenzofuran and CCl2/dibenzo-18-crown-6 complexes.

Several dibenzo-18-crown-6 conformers could be competitive in solution.25 We have investigated only complexes in which the crown ether adopts a boat conformation; according to our calculations, this is the conformer of lowest energy. In this conformer, CCl2 complex formation with the O2 site may become sterically demanding and, furthermore, there should not be much discrimination among the C binding sites of the benzene units (Scheme 1). We have characterized four CCl2/7 complexes (Table S-3) and believe the spectroscopic observations can be rationalized by consideration of two complexation sites, O1 and C3 (Table 4). The low wavelength absorption observed around 468 nm is assigned to the O-ylidic species

CCl2 with dibenzofuran versus THF and of dibenzo-18-crown-6 versus 18-crown-6 is dramatic evidence that an aromatic donor, and an oxygen donor, are needed for the formation of observable CCl2 complexes. Computational studies (PBE/6-311+G(d)) indicate that both the enthalpies and free energies of formation of CCl2/THF and CCl2/dioxane ylidic complexes (Table 5) are considerably less favorable than those of the CCl2/methoxybenzene complexes (Tables 2 and 3), so our failure to observe the ylidic complexes is understandable. Also, we did not observe transient spectra due to complexation of CCl2 with mesitylene (12) or durene (13), although computational studies suggest that π-complexation with CCl2 is as favorable with 12 and 13 as it is for anisole (Table 5). Low-energy transitions (∼400-500 nm) for CCl2 complexes

with substrates 9-13 are uniformly weak (Table 5); more intense transitions at higher energy (∼300-350 nm) are predicted but never detected. As noted above, the sensitivity of our LFP detection system may be reduced in this spectral region due to strong background absorption by the diazirine precursors.



TABLE 4: Complexation Energies (PBE/6-311+G(d)) and Electronic Transition Wavelengths and Intensities (B3LYP/ 6-311+G(d) with CPCM Solvent Correction) for Some CCl2/Dibenzofuran (CCl2/6a,6d) and CCl2/Dibenzo-18-crown-6 (CCl2/ 7a,7d) O-Ylidic and π-Complexes species

binding site

∆Ha,b

∆Ga,c

C-Xd

λ1 e

f1 f

λ2 e

f2 f

λ3 e

f3 f

CCl2/6a CCl2/6c CCl2/6d CCl2/7a CCl2/7d

O1 C1 C2 O1 C3

-1.4 -1.8 -1.8 -3.9 -2.8

4.0 4.9 4.8 2.8 3.9

3.2 2.9 2.9 3.2 2.9

566 536 540 577 551

0.00 0.01 0.15 0.00 0.11

539 509 500 547 499

0.00 0.09 0.00 0.00 0.00

484 448 443 485 472

0.01 0.02 0.02 0.01 0.01


TABLE 5: Complexation Energies (PBE/6-311+G(d)) and Electronic Transition Wavelengths and Intensities (B3LYP/ 6-311+G(d) with CPCM Solvent Correction) for CCl2/THF (CCl2/9), CCl2/Dioxane (CCl2/10), CCl2/Mesitylene (CCl2/12), and CCl2/Durene (CCl2/13) Complexes species

binding site

∆Ha,b

∆Ga,c

C-Xd

λ1 e

f1 f

λ2 e

f2

CCl2/9 CCl2/10 CCl2/12 CCl2/13

O O C2 C3

-1.9 1.0 -4.4 -3.8

7.5 9.1 4.9 3.5

2.2 2.2 2.6 2.7

392 398 476 499

0.03 0.04 0.06 0.01

277 301 419 477

0.12 0.12 0.00 0.04

f

λ3 e

f3 f

262 265 342 358

0.01 0.00 0.17 0.13


Moreover, the retarding effects of mesitylene and durene on the rate of CCl2 addition to TME (see section 3.2) are similar to those observed with anisole and dibenzofuran, suggesting at least weak complexation of the carbene. It is possible that π-complexation of CCl2 by durene or mesitylene is not observed in our experiments because the complexes are not formed in sufficient concentration and their absorption strength is too low or strong background absorption by the diazirine precursor interferes. The results currently at hand indicate that aromatic ethers afford obserVable complexes with CCl2, but not ethers or aromatics alone. Given this situation, we examined phenyl acetate (8), where the electron-donating group attached to the aromatic residue has been rendered less potent than the alkoxy units of substrates 3-7.26 Despite the reduced π-electron donating power of the acetoxy group of 8, complexation with CCl2 still occurred; absorptions of a transient appeared at 460 and 516 nm, similar to those of the CCl2-anisole complexes (cf., Table 1 and Figure S-5 in the Supporting Information). Calculations find the enthalpy change for CCl2/8 complex formation at the carbonyl oxygen of phenyl acetate (∆H ) -1.1 kcal/mol) to be considerably less favorable than at its ether oxygen (∆H ) -2.6 kcal/mol). Hence, we assign the 460 nm band primarily to absorption by O-ylidic ether complexes (analogous to CCl2/ 3a). The absorption is computed at 487 nm (f ) 0.004). We assign the second absorption band at 516 nm to CCl2/8 π-complexes. Table 1 contains the observed rate constants (kf and kd) for the formation and decay of the CCl2 complexes formed with substrates 3-8. The absorptions of the complexes arise with kf ) (0.6-6.1) × 107 s-1 and decay with kd ) (0.3-6.2) × 106 s-1. For a given substrate, kf and kd are generally similar for the two observed bands, so that one band does not persist long at the expense of the second band. In the case of trimethoxybenzene 5, the kf of each observed transient was studied as a function of the concentration of 5. The resulting linear correlations appear in Figures S-6 and S-7 of the Supporting Information, affording kf ) 1.3 × 108 M-1 s-1 at 580 nm and kf ) 1.2 × 108 M-1 s-1 at 660 nm for the two π-complex rotamers (see

Figure 7. Activation energy for complex formation between CCl2 and 5 in pentane, monitored at 580 nm: Ea ) -1.8 kcal/mol, log A ) 6.81 M-1 s-1, r ) 0.997.

Figure 5, above). These CCl2 complexes are formed rapidly, but at rates that are substantially below diffusion control. Given that our computational studies indicate that there should be no enthalpic barrier to complex formation (see above), we undertook an activation energy study of the complexation of CCl2 by 5. Thus, kf values for the formation of the CCl2/5 complex absorbing at 580 nm (see Figure 4), presumably the computed complex CCl2/5b, were obtained by LFP at five temperatures between 283 and 308 K. Precise temperatures ((0.1 K) were ascertained at the instant of LFP via a thermocouple immersed in the target solution. The individual kinetic results at each temperature resemble those in Figure S-6 and appear in the Supporting Information as Figures S-15-S-19. An Arrhenius correlation of the rate constants (r ) 0.997) appears here in Figure 7 and provides the following activation parameters: Ea ) -1.8 kcal/mol, log A ) 6.81 M-1 s-1, ∆H‡ ) -2.4 kcal/ mol, ∆S‡ ) -29 e.u., and ∆G‡ ) 6.4 kcal/mol.

216


Moss et al.

TABLE 6: Rate Constants for Additions of CCl2 to TMEa solvent or additive

kadd (M-1 s-1)

kpentane/kadditive

pentane anisole trimethoxybenzeneb dibenzofuranc,d mesitylene durenee THF dioxane phenyl acetate

4.7 × 10 2.6 × 108 3.1 × 107 3.3 × 108 3.2 × 108 1.5 × 108 1.0 × 109 8.5 × 108 3.3 × 108

1.0 18 152 14 15 31 4.7 5.5 14

9

a At 25 °C; [pyridine] ) 0.12 mM. Solvent:pentane ) 1:1 unless otherwise noted. b [trimethoxybenzene] ) 0.30 M. c In 1:1 pentane/CH2Cl2. d [dibenzofuran] ) 1.0 M. e [durene] ) 0.58 M.

The negatiVe activation energy is precedented in the negative activation energy (-1.2 kcal/mol) observed for the addition of CCl2 to the highly reactive alkene, tetramethylethylene.27 Houk and Rondan showed that negative activation energies, and the associated entropic control of reactivity, can “arise in fast reactions having no inherent potential energy barrier.”28 This situation is now seen in the reactions of CCl2 with both trimethoxybenzene and tetramethylethylene. In both cases, there is no activation energy barrier and no enthalpic barrier. There is, however, a free-energy barrier, created by the very negative entropy of activation. The latter arises from a combination of unfavorable translational, vibrational, and rotational effects as the two reactants combine to afford a single-product species, and also the effects of solvent reorganization. The entropy-based free-energy barrier causes the rate of CCl2/5 complexation to be less than the diffusion controlled rate, despite the absence of an enthalpic barrier to complexation. The TS is so early on the free-energy surface that the favorable enthalpy release upon complexation is insufficient to overcome the unfavorable entropic component. The parallel kinetic behavior observed in the reactions of CCl2 with either trimethoxybenzene or tetramethylethylene is very satisfying. 3.2. Kinetics of CCl2 Additions. Does the complexation of CCl2 by aryl ethers or other agents affect the carbene’s reactivity? Platz et al. reported that THF and dioxane minimally affected the rate of addition of CCl2 to TME: retardation factors relative to kadd in Freon-113 were only 2.6 and 3.4 with THF and dioxane, respectively.14 Are retardations induced by the molecules that we have studied? We used the pyridine ylide method14,29 to determine rate constants for the additions of CCl2 to TME in pentane and in the presence of various additives; cf., Table 6. The apparent formation rate of the CCl2-pyridine ylide 14,15 measured at 404 nm in pentane or pentane-additive solution, is increased by the addition of TME at a constant concentration of pyridine. A correlation of the observed rate constants for the formation

of 14 versus [TME] is linear, and its slope gives kadd for the addition of CCl2 to TME.29 In this manner, we determined that CCl2 generated by LFP of diazirine 1 adds to TME in pentane with kadd ) 4.7 × 109 M-1 s-1.30 Repetition of this experiment in a 1:1 anisole-pentane solvent mixture, containing 0.12 mM pyridine, gives kadd ) 2.57 × 108 M-1 s-1; cf., Figure 8. This corresponds to an 18-fold retardation of kadd induced by the anisole, relative to kadd in pentane alone. Similarly, slightly

Figure 8. Rate constant for addition of CCl2 to TME in 1:1 anisole-pentane: kadd ) 2.6 × 108 M-1 s-1, r ) 0.999.

smaller retardations are obtained with dibenzofuran and phenyl acetate; cf., Table 6 and Figures S-8 and S-9 in the Supporting Information. A much larger retardation of 152 occurs with trimethoxybenzene (Figure S-10), which is calculated to complex CCl2 more strongly than anisole. Interestingly, mesitylene and durene, which do not give spectroscopic evidence for CCl2 complexation, retard CCl2 additions to TME as effectively as anisole and dibenzofuran (see Table 6 and Figures S-11 and S-12 in the Supporting Information). We take these kinetics results as experimental evidence for at least weak π-complexation of the carbene by these aromatic hydrocarbons, as supported by the calculations described above (Table 5). Platz reported that THF and dioxane gave only minor retardations of kadd for CCl2 additions to TME.14 Our measured retardations of 4.7 and 5.5, respectively, in pentane (cf., Table 6 and Figures S-13 and S-14) are slightly larger than Platz’s values of 2.6 and 3.4 in Freon-113, but our conclusion is similar: ylidic complexation between CCl2 and THF or dioxane is neither observable spectroscopically nor very significant kinetically. These negative results are in agreement with computational studies that suggest minimal complex stability in these cases (Table 5). On one hand, the behavior of CCl2 resembles that of MeCCl, which also presents both spectroscopic and kinetics evidence for complexation with anisole and 1,3-dimethoxybenzene, but not with THF and dioxane.10 PNPCC, on the other hand, readily forms ylides with simple ethers, and these are both spectroscopically visible and kinetically effective in retarding the rate of PNPCC additions to TME.13 Anisole and 1,3-dimethoxybenzene also afford ylidic and π-complexes with PNPCC.13 The precise mechanism of retardation by complexation is unclear. The deceleration of addition could be due to the reversible formation of CCl2-additive complexes, or it could reflect the kinetic advantage of unencumbered CCl2 in pentane relative to a solvated or complexed carbene. Related effects are now known for MeCCl,10 PNPCC,13 and amidohalocarbenes.8 4. Conclusions Dichlorocarbene (CCl2), generated by laser flash photolysis of dichlorodiazirine, forms π- and O-ylidic complexes with aromatic ethers such as anisole, 1,3-dimethoxybenzene, 1,3,5trimethoxybenzene, dibenzofuran, and dibenzo-18-crown-6, and

Solvation of Dichlorocarbene also the aromatic ester, phenyl acetate. These complexes can be visualized by UV-vis spectroscopy and are substantiated by DFT computations that provide structures and energetics for the species and rationalize their absorption spectra. Moreover, complexation of CCl2 by the aromatic ethers retards the addition of the carbene to tetramethylethylene by factors of 18-152. Complexes are not observed between CCl2 and simple, nonaromatic ethers such as THF, dioxane, or 18-crown-6, nor do these ethers much affect the addition rate of CCl2 to tetramethylethylene. Computational results indicate that the corresponding O-ylides should be relatively unstable in these cases. Computations also suggest that π-complexes of CCl2 and (e.g.) mesitylene and durene are energetically reasonable transients. Although these species could not be detected spectroscopically, the aromatic compounds do slow the addition of CCl2 to tetramethylethylene by factors of 15 and 31, respectively. Acknowledgment. We thank the National Science Foundation and the Petroleum Research Fund (administered by the American Chemical Society) for financial support. Supporting Information Available: Figures S-1-S-19, Tables S-1-S-3, and computational details (complete reference,22 optimized ground-state geometries, absolute energies, and excited-state data for all species relevant to Tables 2-5). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Tomioka, H.; Ozaki, Y.; Izawa, Y. Tetrahedron 1985, 41, 4987. (2) Ruck, R. T.; Jones, M., Jr. Tetrahedron Lett. 1998, 39, 2277. (3) Moss, R. A.; Yan, S.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1998, 120, 1088. (4) Krogh-Jespersen, K.; Yan, S.; Moss, R. A. J. Am. Chem. Soc. 1999, 121, 6269. (5) Celebi, S.; Tsao, M.-L.; Platz, M. S. J. Phys. Chem. A 2001, 105, 1158. (6) Sun, Y.; Tippmann, E. M.; Platz, M. S. Org. Lett. 2003, 5, 1305. (7) Tsao, M.-L.; Zhu, Z.; Platz, M. S. J. Phys. Chem. A 2001, 105, 8413. (8) Tippmann, E. M.; Platz, M. S.; Svir, I. B.; Klymenko, O. L. J. Am. Chem. Soc. 2004, 126, 5750. (9) Kahn, M. I.; Goodman, J. L. J. Am. Chem. Soc. 1995, 117, 6635. (10) Moss, R. A.; Tian, J.; Sauers, R. R.; Krogh-Jespersen, K. J. Am. Chem. Soc. 2007, 129, 10019.

J. Phys. Chem. A, Vol. 114, No. 1, 2010 217 (11) Moss, R. A.; Tian, J.; Chu, G.; Sauers, R. R.; Krogh-Jespersen, K. Pure Appl. Chem. 2007, 79, 993. (12) We use the term “π-complex” to refer to weakly bound chargetransfer complexes formed between aromatic π donors and carbene receptors. We use “O-ylidic complex” to indicate ether-carbene transients in which the O-C separation significantly exceeds typical O-C bond lengths.13 (13) Moss, R. A.; Wang, L.; Weintraub, E.; Krogh-Jespersen, K. J. Phys. Chem. A 2008, 112, 4651. (14) Presolski, S. I.; Zorba, A.; Thamattoor, D. M.; Tippmann, E. M.; Platz, M. S. Tetrahedron Lett. 2004, 45, 485. (15) Moss, R. A.; Tian, J.; Sauers, R. R.; Ess, D. H.; Houk, K. N.; KroghJespersen, K. J. Am. Chem. Soc. 2007, 129, 5167. (16) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5468. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (17) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 721. (b) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209. (c) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (d) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (e) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (18) McQuarrie, D. A. Statistical Thermodynamics; Harper and Row: New York, 1973. (19) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899. (20) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108, 4439. (21) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995. (22) Frisch, M. J. Gaussian03, ReVision E.02; Gaussian, Inc.: Wallingford, CT, 2004; see the Supporting Information for the complete reference to Gaussian03. (23) (a) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1967, 47, 703. (b) Jacox, M. E.; Milligan, D. E. J. Chem. Phys. 1970, 53, 2688. (24) Chu, G.; Moss, R. A.; Sauers, R. R. J. Am. Chem. Soc. 2005, 127, 14206. (25) Kusaka, R.; Inokuchi, Y.; Ebata, T. Phys. Chem. Chem. Phys. 2007, 9, 4452. (26) For example, σR of MeO is-0.58, while σR of AcO is-0.23: Charton, M. Prog. Phys. Org. Chem. 1981, 13, 119 ff; see, especially, p 193. (27) Moss, R. A.; Wang, L.; Zhang, M.; Skalit, C.; Krogh-Jespersen, K. J. Am. Chem. Soc. 2008, 130, 5634. (28) (a) Houk, K. N.; Rondan, N. G. J. Am. Chem. Soc. 1984, 106, 4293. (b) Houk, K. N.; Rondan, N. G.; Mareda, J. Tetrahedron 1985, 41, 1555. (29) Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M. T. H. J. Am. Chem. Soc. 1988, 110, 5595. (30) Moss, R. A.; Tian, J.; Sauers, R. R.; Skalit, C.; Krogh-Jespersen, K. Org. Lett. 2007, 9, 4053.

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Solvation of Dichlorocarbene: Complexation with Aryl Ethers - The

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