The Photochemical Reactivity of 1,6-methano[10]annulene

Article

The Photochemical Reactivity of 1,6-methano[10]annulene Laura Moroni, Marco Pagliai, Riccardo Chelli, Giangaetano Pietraperzia, Pier Remigio Salvi, and Cristina Gellini J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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The Journal of Physical Chemistry

The Photochemical Reactivity of 1,6-Methano[10]annulene Laura Moroni, Marco Pagliai, Riccardo Chelli, Giangaetano Pietraperzia, Pier Remigio Salvi and Cristina Gellini* E-mail: [email protected]

Dipartimento di Chimica “Ugo Schiff”, Università di Firenze Via della Lastruccia 3, 50019 Sesto Fiorentino (Firenze), Italy

_______________________ *Author to whom the correspondence should be addressed. 1

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Abstract 1,6-methano[10]annulene solutions into cyclohexane have been subjected to continuous and pulsed UV irradiation. Photolysis occurs in both cases, giving naphthalene as a minor and major product, respectively. The wavelength dependence of the reaction in solution indicates that the photochemical process occurs exciting 1,6-methano[10]annulene in the second and third singlet electronic excited states. The reaction kinetics has been determined under pulsed irradiation. From the time dependence of concentrations, along with the support of density functional theory calculations and early published data, two mechanisms are proposed for naphthalene production. Reaction steps such as direct migration of the bridging methylene of 1,6-methano[10]annulene to cyclohexane and 1,6-methano[10]annulene isomerization to benzotropilidene have been identified. The calculated energy diagrams relative to the ground and lowest excited states allow to relate these steps to processes such as electrocyclic closure and sigmatropic shift. The norcaradienic form of 1,6-methano[10]annulene results to be the critical species for the methylene migration and the sigmatropic [1,5] shift. The present results and those arising from photolysis in the gas phase are good examples of the photochemical reactivity of 1,6methano[10]annulene.

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1. Introduction The chemical reactivity of 1,6-methano[10]annulene (1a, Fig. 1), the first bridged monocycle higher homologue of benzene, has been the subject of a large number of studies1-8 since the pioneering synthesis9, whose results have been reported in review papers10-13 and textbooks14,15 addressing aromaticity/antiaromaticity issues. The compound 1a undergoes bromination2 and dye-sensitized oxidation3 by analogy to the aromatic behavior16,17 and, on the other hand, cycloaddition with dienophiles1,4 due to favorable steric conditions for the Diels–Alder reaction. Reactive species like nitrene and phosphinidene have been inserted between the C-C bonds of 1a5,6 resulting in the enlargement of the C10 cycle to include one nitrogen or one phosphor atom by norcaradienic – cycloheptatriene valence isomerization18,19. Furthermore, 1a behaves as a stabilizer of biradical intermediates, for instance in the rearrangement of methylenecyclopropane7. New synthetic studies to prepare 1a derivatives have been reported with substituents both on the bridge and on the annulene ring8. The thermal fragmentation of 1a in vacuum has also been attempted as a possible source of methylene19. However, heating to 500 oC, 1a isomerizes to 1,2-benzotropilidene (2a, Fig. 1) rather than dissociating to yield methylene19,20. Only the 11,11-difluoro derivative of 1a gives almost quantitatively naphthalene20 on pyrolysis at 450 oC. In contrast, the photochemical reactivity of 1a has received less attention. To the best of our knowledge, a single article21 has reported on the photolysis of 1a with generation of naphthalene and singlet methylene in an experiment where the gaseous sample was expanded in a supersonic beam and excited with laser light of ∼300 nm. Here, we report on results concerning the photochemistry of 1a in cyclohexane solution at room temperature. The excitation regimes are (a) irradiation with conventional UV light in the 230 – 350 nm spectral region and (b) irradiation with pulsed laser light at 266 nm. Photolysis occurs in both cases, with the naphthalene yield much higher with pulsed than with continuous irradiation. The wavelength dependence of the process was determined and found to follow closely the 1a absorption profile of the second and third absorption bands. In pulsed irradiation, concentration of 1a decreases, while naphthalene rises with time. In order to advance a working hypothesis about the reaction pathway, the kinetics of the process has been studied. Two possible mechanisms, both in fair agreement with experimental data, have been considered. In the first, here referred to as “two channels”, the direct migration of the methylene group from 1a to cyclohexane occurs together with a more complex path leading from reactant to products through isomerization. The second mechanism, termed “one channel”, involves only isomerization. From ab initio calculations, it 3



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turns out that both mechanisms can be described by excited state energy trajectories with critical steps allowed in the excited state. Accordingly with our experimental data, the computational analysis is consistent with an enhancement of the photolysis in the third excited state.

2. Materials and Methods 2.1. Experimental procedures The synthesis of 1,6-methano[10]annulene has been described elsewhere9. Naphthalene from Aldrich was used as received. Solutions of 1a were prepared in spectroscopic grade cyclohexane and isopentane-diethylether mixture in the concentration range 10 – 10 M and were irradiated in quartz cells at room temperature. The photochemical reaction was best observed with cyclohexane as a solvent and accordingly kinetic studies were performed with cyclohexane solutions 10 M inside a cuvette 0.2 cm thick. In a first series of experiments the solutions were subjected to continuous irradiation. A Xe lamp (Oriel, mod. 6266, 400 W) was coupled to a double monochromator (Jobin– Yvon, mod. HRD1) in order to obtain tunable continuous emission between 350 and 230 nm. The solution samples were either irradiated at fixed wavelength, 266 nm, 300 µW incident power, for 411 min and the absorption spectra periodically monitored with a Cary 5 spectrophotometer or, alternatively, irradiated for 120 min with the excitation wavelength tuned in the 350 – 230 nm range step by step every 5 nm. In this second operation, solution aliquots 10 M were prepared for three separate runs of the complete spectral interval. The absorbance decrease of the strongest band of 1a at 256 nm was determined as a function of wavelength and for normalization purposes the power of the Xe lamp at the filtered wavelength was measured by means of a power-meter (Si photodiode, Coherent mod. OP–2UV) and resulted to be in the range 10 – 20 µW in front of the solution cell. In the pulsed experiment, the excitation wavelength was the fourth harmonic of a Quanta–Ray Nd:YAG laser (266 nm, 10 Hz, pulse energy 240 µJ), the irradiation time was 320 min and the absorption spectra of the irradiated solutions were determined spectrophotometrically up to the final time. The solutions of 1a were also excited at 266 nm, 200 µJ pulse energy, for 150 min and the fluorescence spectra were subsequently acquired in the 300 – 500 nm wavelength region with an experimental apparatus assembled in our laboratory22. Briefly, the fluorescence emission was detected combining the double monochromator with a cooled photomultiplier. Fluorescence and reference from photodiode were processed by a gated integrator (Stanford, mod. SR250) to give the normalized spectrum. Fluorescence spectra of naphthalene in cyclohexane solutions were also run for comparison with those 4


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of the irradiated solutions. Standard absorption spectra of naphthalene in cyclohexane solutions at various concentrations were measured. 2.2. Computational procedures The photochemical reactivity of 1a in cyclohexane solution has been studied by DFT calculations with the Gaussian09 suite of programs23, using the B3LYP24,25 functional in combination with the 6-31G and 6-311G++(d,p) basis sets and ωB97X-D26-28 functional with the 6-311G++(d,p) basis set. The calculations refer to the reaction paths described in Section 3.2 and involve the stable molecules 1a, 2a, 3a and 3b of Fig. 1. The solvent is introduced by means of the polarizable continuum model29 using the integral equation formalism. It has been verified in each calculation that the optimized structures correspond to minima of the energy surfaces as evidenced by the absence of imaginary frequencies in the vibrational analysis. Transition states between two optimized structures were located through the Syncronous Transit-Guided Quasi-Newton method30. Transition states (saddle points) on the ground state energy surface, are recognized by the occurrence of only one normal mode with imaginary frequency. From the geometry of the transition state, the two starting structures have been reached along the intrinsic reaction coordinate31 in the forward and reverse directions. The energies of the S1, S2 and S3 excited states have been calculated through the time-dependent DFT procedure32 at the optimized ground state geometries by singly–excited promotions among valence orbitals and all virtual orbitals. The total number of singly–excited configurations employed in the excited state calculation amounts to 2190 configurations.

3. Results 3.1. Photochemical reactivity of 1,6-methano[10]annulene The UV absorption spectrum of 1a consists of three absorption regions with maxima at 361, 298 and 256 nm (corresponding to 79.1, 95.0 and 111.6 kcal/mol, respectively) with very low, medium and high oscillator strengths, respectively33-36. The spectrum of a 10 M solution in the 400 – 200 nm spectral region is reported in Fig. 2 as solid line. The first band around 361 nm, having negligible intensity due to the low solute concentration, has been magnified in the figure. Due to continuous irradiation at 266 nm, the strong 256 nm band, assigned to the S0 → S3 transition33-36, decreases smoothly with the irradiation time, reaching ∼70% of the initial absorbance after 411 min. At the end time, three weak bands are observed at 343, 285 and 221 nm, assigned to the photoreaction products (see inset of Fig. 2). 5



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The wavelength dependence of the reaction was determined between 350 and 230 nm. The 256 nm absorbances at initial time and after 120 min of irradiation have been measured tuning the excitation step by step every 5 nm and their differences normalized with respect to the power variation (see inset of Fig. 3) in the spectral range. The results are plotted as circles as a function of the excitation wavelength in Fig. 3. The diagram follows closely the absorption profile, reported for comparison, being only slightly broadened and red-shifted. The data make evident that the photoreaction is induced by the excitation of 1a in the second and third states, with a larger efficiency in the latter than in the former. The highest activity is roughly located between 270 and 250 nm. The normalized values gradually vanish toward the high wavelength limit, suggesting the absence of reactivity beyond 350 nm, in agreement with results on the gaseous sample21. When the cyclohexane solution of 1a is irradiated with pulsed 266 nm laser light for a total of 320 min, a band rises up prominently with maximum at 221 nm and a weak band structure comes up at 286, 275 and 266 nm (see Fig. 4) without a corresponding increase of the two bands observed with continuous irradiation at 343 and 285 nm. This obviously implies that at least two photoproducts are generated under continuous irradiation. The absorption spectrum at the end time, where the structured profile overlaps a high rising background absorption, is reported in the inset of Fig. 4. The excellent comparison with the spectrum of naphthalene (nph) in cyclohexane solution (inset of Fig.4) identifies the main/trace product under pulsed/continuous conditions. From the molecular extinction coefficient37 of 1a at 256 nm, , = 70000 M cm, and of nph at 221 nm, , = 117000 M cm, it is possible to extract the time dependence of the 1a and nph concentrations38, respectively, in the pulsed experiment. For instance, at = 320 min, ∼92% of the initial amount of 1a has reacted. In Fig. 5, the fractions of 1a and nph with respect to the initial concentration of 1a are reported as functions of the irradiation time. The sum of 1a and nph fractions at any time is not equal to unity. This means that additional products are formed during the pulsed irradiation, whose total amount, 1 − a a − nph, is also shown in Fig. 5. After keeping the irradiated sample in the dark for 17 h, its absorption spectrum was found unchanged. Therefore, within the experimental accuracy, no side reaction occurs to give a partial recovery of the starting material, i.e., the reaction is irreversible. The presence of nph as a product of the photoreaction was also revealed by means of fluorescence spectroscopy exciting the solution of 1a in cyclohexane at 266 nm and various concentrations, 10 –

10 M. In particular, the fluorescence spectrum of the 10 M solution after 150 min irradiation time is reported in Fig. 6. In addition to the diffuse fluorescence emission of 1a centered around 430 nm,34 a new structured emission is observed in the range 310 – 380 nm due to nph. It may be seen from the 6


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inset of Fig. 6 that there is a good correspondence between this emission and the fluorescence emission of pure nph taken in the same experimental conditions, once the reabsorption factor due to the wing of the 298 nm absorption band of 1a is taken into account. 3.2. DFT calculations The bridged system 1a has attracted much theoretical interest along the years in relation to the valence interconversion39 between the aromatic and norcaradienic forms, 1a and 1b, respectively (see Fig. 1), the aromaticity/homoaromaticity behavior in bridged 1,4; 1,5; 1,6 methano[10]annulenes40, the ground state properties of 1a41 and the fluxional molecule 11,11-dimethyl derivative42. In the latter study, the valence 1a/1b tautomerism was also investigated analyzing the molecular energy as a function of the C1C6 distance. By using several quantum-mechanical approaches, including DFT at the B3LYP/6311G(d,p) and ωB97X-D/6-311G(d,p) levels, it was found that the potential energy curve is featured by a single well and one shoulder at C1C6 distances smaller than 2 Å, and was reasonably well described by means of the ωB97X-D/6-311G(d,p) calculation42. In particular, the energy of 1b at the C1C6 distance of 1.61 Å, relative to the 1a minimum, was reported to be 13.60 and 7.22 kcal/mol for the two methods. In the present study, we have performed calculations using DFT at the B3LYP/631G, B3LYP/6-311++G(d,p) and ωB97X-D/6-311++G(d,p) levels. The energy curves as functions of the C1C6 distance are reported in Fig. S1 of the Supporting Information. Our results agree with those of Ref. 42, including the 1b energy relative to that of the 1a minimum. For instance, the B3LYP/6311++G(d,p) and ωB97X-D/6-311++G(d,p) calculations give energies of 13.7 and 7.3 kcal/mol, respectively, at C1C6 = 1.61 Å. With B3LYP/6-31G the 1b/1a energy difference increases to 17.0 kcal/mol so that the 1b stability is underestimated at most by ~10 kcal/mol with respect to the more performing ωB97X-D/6-311++G(d,p) approach. However, the dispersion of the energy values in the three types of calculation is strongly reduced when differences between ground state minima and saddle structures are considered and amounts in general to few kcal/mol, as it is evident on inspection of Tables 1 and 2. Similarly, the excitation energies of 1a, nph and 3b collected in Table S1 of the Supporting Information show only a weak dependence on the type of calculation. These considerations make us confident that ground and excited state paths determined at the B3LYP/6-31G level of theory are able to capture the essential information relevant for a discussion of the 1a photoreactivity. In order to rationalize the photochemical reactivity of 1a, two reaction mechanisms are hypothesized relying on DFT and time-dependent DFT calculations relative to the ground and lowest singlet excited state energies of 1a (and related isomers) in solution. The first reaction mechanism is proposed recalling that the addition of singlet methylene to ethylene is known since longtime43,44 to be a 7



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forbidden process in the ground state in the most symmetrical approach, i.e., C2v geometry. On the contrary, the C2v attack is allowed if methylene is in the excited singlet state, promoting one electron from the σ bonding to the p non bonding orbital44. In the 1a case, the methylene departure from the decagonal ring is a two-step process19, leading, the first, to the norcaradienic form 1b from 1a and, the second, to the methylene separation from the cyclopropane subunit of 1b, which is just the opposite of the above-mentioned addition. The a a → b b rearrangement can be viewed as the disrotatory electrocyclic closure of a polyenic chain with 6 π-electrons between C1 and C6. This process is favored in the ground state, but symmetry forbidden in the excited state.45 We have taken into account the possibility that a solvent molecule might take part to the reaction as a reactant. In fact, it is well known46,47 that singlet methylene generated from diazomethane by photolysis reacts with a large number of saturated hydrocarbons to produce their methyl derivatives by insertion into the CH bonds. It is reported, for instance, that diazomethane reacts with solvent cyclopentane upon irradiation giving methylcyclopentane46. Other reactive precursors are ketene48 and hydrocarbons methanophenantrene or phenylcyclopropane49. In our case the reaction with cyclohexane as active reactant is

1a + cyclohexane

nph + methylcyclohexane

(scheme 1)

in solvent cyclohexane and is schematically represented in Fig. 7, first line. The energies in the ground and lowest excited states of the initial, saddle and final geometries obtained from B3LYP/6-31G, B3LYP/6-311++G(d,p) and ωB97X-D/ 6-311++G(d,p) calculations are reported in Table 1. Both reactants ( + cyclohexane) and products (nph + methylcyclohexane) are weak adducts, with the products being more stable than reactants by 53.1, 49.6 and 53.1 kcal/mol in the three calculations, respectively. This is consistent with the irreversibility of the photoreaction experimentally observed (see Section 3.1). As already noted, the excited state energies do not differ appreciably in the three cases. According to expectations for a weak adduct, the S1, S2, and S3 vertical transition energies of + cyclohexane match fairly well the 1a experimental values,33-36 79.1, 95.0 and 111.6 kcal/mol, respectively. The barrier height relative to the migration process of the methylene group from 1a to cyclohexane is 60.9, 58.0 and 59.4 kcal/mol using the three methods. The S0 energy profile of the reaction in scheme 1 obtained by using the B3LYP/6-31G results, is displayed in Fig. 8, together with the energy profiles of the S1, S2, and S3 excited states obtained by vertical excitation. Analyzing the structural evolution during the reaction path in the ground state, we note that the reaction consists of three steps, (a) the norcaradienic rearrangement19 a a → b b, (b) the methylene separation from the ring 8


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and (c) the methylene insertion into the CH bond of cyclohexane. The a a → b b rearrangement has a barrier of 30 kcal/mol for S1 and 23 kcal/mol for S2, in agreement with the disrotatory nature of the 6π electrocyclic closure. At the S3 – S1 intersection (see black arrow in Fig. 8) efficient S3 − S1 internal conversion can occur. Successively, in S1 the step (b) is almost barrierless, while an energy gap of ∼15 kcal/mol is found for the step (c) giving nph and methylcyclohexane. An alternative mechanism for the photochemical dissociation of 1a in solution can be inferred from several experimental results on similar systems50-54 summarized below: 1) the thermal rearrangement a a → +a a has been reported19 to occur at high temperature, 500 oC; 2) 2a (and 3,4-benzotropilidene) rearranges photochemically in solution to benznorcaradiene 3b, which gives naphthalene upon prolonged irradiation50,51. Other experiments showed that 2a may rearrange to 2,3-benzotropilidene 3a by [1,7] sigmatropic hydrogen migration and then to 3b by valence tautomerization52 with high quantum efficiency53. Such alternative dissociation route, represented by the multistep process sketched in the second line of Fig. 7, is outlined by the DFT results of Fig. 9, where the energy profiles of the reaction path a a→

+a a → ,a a → ,b b in the S0, S1, S2 and S3 electronic states are reported. The energies of the minima and saddle structures of the ground and excited states obtained from B3LYP/6-31G, B3LYP/6311++G(d,p) and ωB97X-D/ 6-311++G(d,p) are reported in Table 2. The a a → +a a energy barrier is high, 60.6, 56.5 and 57.7 kcal/mol, respectively, thus justifying the high temperature condition of the reaction in the ground state19, while the excited state barriers are considerably lowered. Analyzing more closely the a a → +a a curve, three processes are distinguished, in the order a a → b b → +b b → +a a. The a a → b b and +b b → +a a steps are the disrotatory electrocyclic closure and opening, respectively, while

b b → +b b is the sigmatropic [1,5] shift, necessarily suprafacial in the present case. According to the Woodward – Hoffmann rules45 the suprafacial [1,5] shift is allowed/forbidden in the ground state with configuration retention/inversion at the shifting site. Here the second mechanism holds, as it has been shown by the B3LYP/6-31G calculation, and nicely justifies the height of the barrier in the ground state and conversely its reduction in the excited states until disappearance in S1. The next step is the suprafacial [1,7] H migration giving 3a from 2a, a forbidden process in the ground state but allowed in the S1 state due to 8 intervening electrons in the process45. The minimum S1 – S0 energy gap at the saddle geometry of + → ,a a, predicted by all the three types of calculation (see Table 2), is responsible

of the fast internal S1 – S0 conversion. Subsequently, due to the disrotatory ,a a → ,b b closure involving

6 π-electrons45 and then to the negligible ground state barrier, 3b is reached from 3a. It is known that 9



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3b has a strong absorption band with maximum around 270 nm55. In addition, due to the cyclopropyl group adjacent to the aromatic ring, 3b is prone to dissociate giving methylene, as reported in similar cases49. The prolonged irradiation at 266 nm then gives rise to the excitation of 3b in the final reaction step50,51 and to the dissociation into nph and excited methylene, rapidly captured by the solvent to give methylcyclohexane, as already shown for the direct mechanism.

4. Discussion In this section, the photochemistry of 1a in terms of a kinetic model able to account for the experimental results of Fig. 5 is discussed. To rationalize the data, we basically resort (a) to a fit of the curves of Fig. 5 according to possible kinetic schemes, (b) to the photochemical reactivity of 1a resulting from ab initio modeling (see Section 3.2) and (c) to early results on the photochemical reactivity of 3b56-59, which, according to DFT calculations, can be envisaged as a product of the photochemical dissociation of 1a. According to the scheme 1 of Section 3.2, the reaction rate d1a 1a/d depends on the reactant 1a

concentrations 1a 1a 1a ∙ cyclohexane (from now on, reactant and product concentrations will be intended as fractions of the initial concentration of 1a). Since cyclohexane is the solution medium, [cyclohexane] does not vary appreciably during the course of the reaction and hence, d1a 1a/d 1a

decreases linearly with 1a 1a. 1a Analogous reasoning holds for nph, whose formation rate, dnph/d, is

also linear with 1a 1a. 1a These evidences allow us to simplify the reaction scheme 1 obtained from computational modeling as

1a

nph

(1)

On the basis of the alternative mechanism for the photochemical dissociation of 1a (see Fig. 9 and related discussion), the rather complex reaction path of Fig. 7 may be represented as follows

1a

3b

nph

(2)

Such an assumption is justified from being the energy barriers leading from the S3 state of 1a to the ground state of 3b very small (see Fig. 9). This implies that the kinetic constants of the intermediate reaction steps leading to 3b (see Fig. 7) are quite large, so that no intermediate products, specifically +a a and ,a a, are expected to remain into equilibrium with reactant and products. Therefore, the formation of

3b is reasonably predicted to be quantitative (irreversible). Further photoexcitation of 3b leads to the final product, namely nph. This is indeed observed in condensed-phase photoexcitation experiments of 3b56. Further support to the above assumption is gained resorting to early experimental data. On one 10


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hand, it has been reported that 2a rearranges photochemically to 3b51 and not viceversa. On the other hand, there is a rich series of results regarding 3b and related systems56-59 which give a substantial help for a plausible assignment of the intermediate product. In particular, irradiation for 2 h of a dilute solution of 3b gives nph as major product (46%), unreacted 3b with a smaller percentage (36%)56 and other compounds, such as 1- and 2-methylnaphthalene and benzobicyclo, [3.2.0]-2,6-diene, as minor products. These observations together with our computational modeling make quite reasonable to associate the intermediate species of the reaction path (2) to 3b and all the other compounds collectively to side products P. This also suggests that a further photochemical reaction of the type

3b

P

(3)

cannot be ruled out a priori from a possible kinetic model. Here, P represents globally the products, different from nph, obtained upon photoexciting 3b. In order to complete the scenario of possible photodissociation paths of 1a, we also consider the reaction directly leading from 1a to products P, even if no experimental or computational indications are actually available:

1a

P

(4)

Possible kinetic mechanisms based on the (1), (2), (3) and (4) are discussed in the following. First, we notice that the kinetic scheme (1) alone cannot explain the experimental outcomes of Fig. 5, because it would imply that the equality 1a 1a 1a + nph = 1 holds at each irradiation time, a condition clearly in contrast with the experimental evidence. The most simple kinetic route to account for the formation of nph and other side products P, upon irradiation of 1a, can be based on (1) and (4)

1a

nph (5)

P This model can be ruled out as well, since it gives rate constants for nph and P both equal to that of the 1a photodissociation process, which is contrary to the experimental results of Fig. 5. Other possible kinetic mechanisms, such as (2) alone and that based on (1) and (2), revealed really unsatisfactory in reproducing the experimental curves of Fig. 5 (fit results are available upon request). Two kinetic routes based on the paths (1), (2) and (3) have been found to provide comparable agreement with the experimental data.

11



1

1a 1

nph 1a

1

3b

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nph

1

(6)

P 12

3b

13

14

nph (7)

P The kinetic equations for the mechanisms (6) and (7) are reported in the Appendix along with their solutions. In the former mechanism, nph is irreversibly formed either from reactant 1a [see (1)] or from the intermediate species 3b [see (2)], from which also secondary products P arise [see (3)]. The mechanism (7) is similar to (6), apart from the absence of the path leading to direct production of nph. In other words, we assume that nph can be produced through a “two channels” mechanism, (6), or through a “one channel” mechanism, (7). The three sets of data of Fig. 5, corresponding to 1a 1a, 1a nph

and 3b 3b 1a 3b + 0 (equivalent to 1 − 1a 1a − nph) in our model, have been fitted through the Eqs. (A.5)

– (A.8) by varying 1 , 1 , 1 and 1 , and through the Eqs. (A.13) – (A.16) by varying 12 , 13 and 14 .

The rate constants for the “one channel” and “two channels” kinetic mechanisms are collected in Table 3 and the best fit obtained from the “two channels” mechanism is shown in Fig. 5. The fitting curves yielded by the “one channel” mechanism are very similar and hence have not been drawn in the figure (the data are available upon request). In fact, the mean square displacements of the fitting data from the experimental ones show a small difference in the two cases, being the value obtained from the “two channels” mechanism slightly better (2.02 ∙ 10 vs 2.32 ∙ 10 ). However, this does not allow for a definite indication on the best candidate reaction path. In spite of the similar fitting outcomes, the rateconstant values suggest a sharp distinction between the two mechanisms: in the first, (6), the photochemical reaction evolves with comparable rate constant in each reaction step, while in the second, (7), nph and P are formed much more faster from 3b than 3b from 1a. An argument is provided here which, together with the moderately better fit to the experimental data, suggests a preference for the “two channels” mechanism over the “one channel” one. This is based on the transfer of the two pairs of rate constants, 1 , 1 of mechanism (6) and 13 , 14 of mechanism (7) (see Table 3), to the 3b case as 1, 1 6 , respectively:

12


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1

3b 1

nph

6

(8)

P The kinetic equations and the related solutions for the above photochemical reaction are reported in the Appendix. Assuming that these rate constants do not change significantly going from 1a to 3b photochemistry and exploiting the fact that the values of one pair are strongly different from those of the other, the comparison with experimental data56 should give a definite indication for the choice. Therefore, substituting the 1 , 1 and the 13 , 14 values in lieu of 1, 1′, it is quickly realized that the agreement with the experimental outcomes is much better with the first than with the second pair of rate constants. In fact, with the 1 , 1 values after 2 h it is found that nph is ∼40% of the initial 3b concentration and the recovered 3b is ∼30%, in fair agreement with the reported percentages56, 46% and 36%, respectively. In the other case, the calculated percentages are strikingly different, ∼78% and 8.5 ∙ 10 %, respectively.

5. Conclusions In this paper, we have reported on the photochemistry of 1,6-methano[10]annulene in cyclohexane solution at room temperature exciting in the UV with continuous and pulsed irradiation. Significant results of our study are: 1) the photochemical reactivity of 1a is promoted exciting into the second and, more actively, in the third excited state; 2) the kinetic data indicate that two photochemical processes are responsible of this activity; 3) according to B3LYP/6-31G calculations, these processes are identified as the excited state reaction of 1a with the solvent and the excited state isomerization to benzotropilidenes. These results, which are centered on the excited state dynamics of 1a, are worth of further comment. As to point (1), our data together with previous results20,21 support the view that the detachment of the methylene bridge from the decagonal ring in the most symmetrical geometry is thermally forbidden while allowed in the excited state. The methylene bridge behaves as if it were enclosed in the cyclopropane subunit of the norcaradienic form 1b with negligible interaction with the underlying polyenic ring structure. The second comment is relative to the a a → +a a isomerization and invites to a comparison with the benzene photochemistry. The photoisomerization of benzene is known to lead to the formation of Dewar benzene and benzvalene60-62 and to involve formally two double bonds 13



Page 14 of 41

undergoing a disrotatory 4π electrocyclic ring closure for the first and a [2a+2a] cycloaddition for the second product, both processes being allowed in the excited state62. In the case of the 10π annulene ring the allowed process in the excited state is the sigmatropic [1,5] shift with configuration inversion at C11 leading from 1b to 2b. As to point (3), additional calculations were made also using DFT at the B3LYP/6-311++G(d,p) ωB97X-D/6-311++G(d,p) levels of theory. The ground and excited state energies at the structural extrema do not differ significantly from those obtained with the B3LYP/631G approach, especially considering the amount of energies into play. More, the calculated profiles were in qualitative agreement with expectations based on Woodward-Hoffmann rules45.

6. Appendix According to the kinetic scheme (6), the rate equations are: : :;

= − :;

= 12 −

(A.2)

: :;

:P :;

= 11 + 13 ,>

= 14 ,>

(A.3) (A.4)

If only a a is present at = 0, i.e., a aA = 1 and ,>A = nphA = PA = 0, the analytical solutions of the four coupled equations are63 = e− = nph = F11 −

(A.5)

12 Be−C11+12D −e−C13+14D E 13 +14 −11 −12

12 13 B1−e−C13+14D E 12 13 1−e−C11 +12D G +

(A.10) (A.11)

= 1L ,>

(A.12)

Following the same treatment63 as above, if only a a is present at = 0, the solutions are = e−1J

,> = nph = P =

(A.13)

1J Be−1J −e−C1K+1LD E 1K +1L −1J

1J 1K 1−e−1J H 1K +1L −1J 1J

1J 1L 1−e−1J H 1K +1L −1J 1J

−

−

1−e−C1K+1LD I 1K +1L

1−e−C1K+1LD I 1K +1L

(A.14) (A.15) (A.16)

Finally, the kinetic equations for the mechanism (8) are :,> :;

= −

= 1′,>

(A.17) (A.18) (A.19)

whose solutions are ,> = e−C1+1′D nph =

P =

1 N1 − e−C1+1′D O 1+1′

1′ N1 − e−C1+1′D O 1+1′

(A.20) (A.21) (A.22)

Supporting Information Available: Energy curves of 1a and 1b forms as a function of the C1C6 distance calculated using DFT with the following combinations of functionals and basis sets: B3LYP/6-31G, B3LYP/6-311++G(d,p) and

15



Page 16 of 41

ωB97X-D/6-311++G(d,p). Excitation energies of 3b, 3b 1a and nph according to B3LYP/6-31G, B3LYP/6-311++G(d,p) and ωB97X-D/6-311++G(d,p) calculations.

Acknowledgments The 1,6-methano[10]annulene sample was a generous gift of the late Prof. Vogel (University of Köln, Germany). The authors wish to thank Prof. G. Cardini (Department of Chemistry, University of Firenze, Italy) for helpful suggestions and discussions.

16


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2

- Vogel, E.; Boll, W.A.; Biskup, M. Bromierung von 1.6-methano- und 1.6-oxidocyclodecapentaen Tetrahedron Lett. 1966, 7, 1569-1575.

3

- Vogel, E.; Alscher, A.; Wilms, K. Photooxidation of Annulenes Angew.Chem. Int. Ed. Engl. 1974, 3, 398-400.

4

- Ashkenazi, P.; Ginsburg, D.; Vogel, E. Propellanes—XXXVI: Reactions of Bridged [10]Annulenes with 4-Substituted-1,2,4-triazoline-3,5-diones Tetrahedron 1977, 33, 11691175.

5

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6

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7

- Creary, X.; Miller, K.M. 1,6-Methano[10]Annulene-Stabilized Radicals Organic Lett. 2002, 4, 3493-3496.

8

- Barrett, D.G.; Liang, G.-B.; Tyler McQuade, D.; Desper, J.M.; Schladetzky, K.D.; Gellman, S.H. Synthetic Studies on the 1,6-Methano[10]Annulene Skeleton: A New Route That Provides Derivatives Substituted at the Bridge and on the Annulene Ring J. Am. Chem. Soc. 1994, 116, 10525-10532.

9

- Vogel, E.; Roth, H.D. The Cyclodecapentaene System Angew. Chem. Int. Ed. Engl. 1964, 3, 228-229.

10 - Kennedy, R.D.; Lloyd, D.; McNab, H. Annulenes, 1980–2000 J. Chem. Soc. Perkin Trans. 2002, 1, 1601-1621. 11 - Gellini, C.; Salvi, P.R. Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States Symmetry 2010, 2, 1846-1924. 12 - Spitler, E.L.; Johnson II, C.A.; Haley, M.M. Renaissance of Annulene Chemistry Chem. Rev. 2006, 106, 5344-5386. 13 -

Peart, P.A.; Repka, L.M.; Tovar, J.D. Emerging Prospects for Unusual Aromaticity in

Organic Electronic Materials: The Case for Methano[10]Annulene Eur. J. Org. Chem. 2008, 17



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2008, 2193-2206. 14 - Garratt, P.J. Aromaticity, J. Wiley and Sons, New York, NY, USA, 1986. 15 - Minkin, V.I.; Glukhovtsev, M.N.; Simkin, B.Ya. Aromaticity and Antiaromaticity, J. Wiley and Sons, New York, NY, USA, 1994. 16 - Morrison, R.T.; Boyd, R.N. Organic Chemistry, Allyn and Bacon, Boston, USA, 1966. 17 - Kearns, D.R. Physical and Chemical Properties of Singlet Molecular Oxygen Chem. Rev. 1971, 71, 395-427. 18 - Vogel, E. Perspektiven der Cycloheptatrien-Norcaradien-Valenztautomerie Pure Appl.Chem. 1969, 20, 237-262. 19 – McNamara, O.A.; Maguire, A.R. The Norcaradiene-Cycloheptatriene Equilibrium Tetrahedron 2011, 67, 9-40. 20 - Rautenstrauch, V.; Scholl, H.-J.; Vogel, E. 11,11-Dihalogeno-1,6-Methano-[10]Annulenes as Dihalogenocarbene Transfer Agents Angew. Chem. Int. Ed. Engl. 1968, 7, 288-289. 21 - Park, U.-H.; Jo, S.H.; Jo, H.J.; Kim, S.G.; Choi, Y.S. Ultraviolet Photolysis of 1,6Methano[10]Annulene Generates the Singlet Methylene Bull. Korean Chem. Soc. 2001, 22, 1030-1032. 22 – Gellini, C.; Salvi, P.R.; Hafner, K. Fluorescence Emission and Conformational Changes of 1,3,5,7-tetra-tert-butyl-s-indacene (TTBI) J. Phys. Chem. 1993, 97, 8152-8157. 23 - Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. Gaussian’09 Revision C.01; Gaussian Inc.: Wallingford, CT (USA), 2010. 24 - Becke, A.D. Density‐Functional Thermochemistry. III. The Role of Exact Exchange J. Chem. Phys. 1993, 98, 5648-5652. 25 - Lee, C.; Yang, W.; Parr, R.G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density Phys. Rev. B. 1988, 37, 785-789. 26 - Chai, J.-D.; Head-Gordon, M. Systematic Optimization of Long-Range Corrected Hybrid Density Functionals J. Chem. Phys. 2008, 128, 084106. 27 - Lin, Y.-S.; Li, G.-O.; Mao, S.-P.; Chai, J.-D. Long-range Corrected Hybrid Density Functionals with Improved Dispersion Corrections J. Chem. Theory Comput. 2013, 9, 263-272. 28 - Chai, J.-D.; Head-Gordon, M. Long-range Corrected Hybrid Density Functionals with Damped Atom–Atom Dispersion Corrections Phys. Chem. Phys. Chem. Phys. 2008, 10, 66156620. 18


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29 - Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models Chem. Rev. 2005, 105, 2999-3094. 30 - Peng, C.; Schlegel, H.B. Combining Synchronous Transit and Quasi-Newton Methods to Find Transition States Isr. J. Chem. 1993, 33, 449-454. 31 - Fukui, K. The Path of Chemical Reactions - the IRC Approach Acc. Chem. Res. 1981, 14, 363-368. 32 - Furche, F.; Burke, K. Time-dependent Density Functional Theory in Quantum Chemistry Ann. Rep. Comp. Chem. 2005, 1, 19-30. 33 - Blattmann, H.-R.; Boll, W.A.; Heilbronner, E.; Hohlneicher, G.; Vogel, E.; Weber, J.-P. Die Elektronenzustände von Perimeter-π-Systemen: I. Die Elektronenspektren 1,6-überbrückter [10]-Annulene Helv. Chim. Acta 1966, 49, 2017-2038. 34 - Catani, L.; Gellini, C.; Salvi, P.R. Excited States of 1,6-Methano[10]annulene: Site Selection Fluorescence and Fluorescence Excitation Spectroscopy on S1 J. Phys. Chem. A 1998, 102, 1945-1953. 35 - Dewey, H.J.; Deger, H.; Frolich, W.; Dick, B.; Klingensmith, K.A.; Hohlneicher, G.; Vogel, E.; Michl, J. Excited States of Methano-Bridged [10]-, [14]-, and [18]Annulenes. Evidence for Strong Transannular Interaction, and Relation to Homoaromaticity J. Am. Chem. Soc. 1980, 102, 6412-6417. 36 - Catani, L.; Gellini, C.; Moroni, L.; Salvi, P.R. Two-Photon Fluorescence Excitation Spectrum of 1,6-Methano-[10]Annulene J. Phys. Chem. A 2000, 104, 6566-6572. 37 - Perkampus, H.-H. UV-VIS Atlas of Organic Compounds; VCH, Weinheim, Germany, 1992. 38 - Logan, S.R. Does a Photochemical Reaction Have a Reaction Order? J. Chem. Educ., 1997, 74, 1303. 39 - Mealli, C.; Ienco, A.; Hoyt, Jr., E.B.; Zoellner, R. W. A Comprehensive Qualitative and Quantitative Molecular Orbital Analysis of the Factors Governing the Dichotomy in the Dinorcaradiene 1,6-Methano[10]Annulene System Chem. Eur. J. 1997, 3, 958-967. 40 - Caramori, G.F.; de Oliveira, K.T.; Galembeck, S.E.; Bultinck, P.; Constantino, M.G. Aromaticity and Homoaromaticity in Methano[10]Annulenes J. Org. Chem. 2007, 72, 76-85. 41 - Gellini, C.; Salvi, P.R.; Vogel, E. Ground State of 1,6-Bridged [10]Annulenes: Infrared and Raman Spectra and Density Functional Calculations J. Phys. Chem. A 2000, 104, 3110-3116. 42 - Humason, A.; Zou, W.; Cremer, D. 11,11-Dimethyl-1,6-Methano[10]Annulene—An Annulene with an Ultralong CC Bond or a Fluxional Molecule? J. Phys. Chem. A 2015, 119, 19



Page 20 of 41

1666-1682. 43 - Hoffmann, R. Trimethylene and the Addition of Methylene to Ethylene J. Am. Chem. Soc. 1968, 90, 1475-1485. 44 - Hoffmann, R.; Hayes, D.W.; Skell, P.S. Potential Surfaces for the Addition of Methylene and Difluoromethylene to Ethylene and Isobutene J. Phys. Chem. 1972, 76, 664-669. 45 - Woodward, R.B.; Hoffmann, R. The Conservation of Orbital Symmetry Angew. Chem. Int. Ed. Engl. 1969, 8, 781-853. 46 - von E. Doering, W.; Buttery, R.G.; Laughlin, R.G.; Chaudhuri, N. Indiscriminate Reaction of Methylene with the Carbon-Hydrogen Bond J. Am. Chem. Soc. 1956, 78, 3224. 47 - Richardson, D.B.; Simmons, M.C.; Dvoretzky, The Reactivity of Methylene from Photolysis of Diazomethane J. Am. Chem. Soc. 1961, 83, 1934-1937. 48 - Frey, H.M.; Kistiakowsky, G.B.; Reactions of Methylene. I. Ethylene, Propane, Cyclopropane and n-Butane J. Am. Chem. Soc. 1957, 79, 6373-6379. 49 - Richardson, D.B.; Durrett, L.R.; Martin, Jr., J.M.; Putnam, W.E.; Slaymaker, S.C.; Dvoretzky, I. Generation of Methylene by Photolysis of Hydrocarbons J. Am. Chem. Soc. 1965, 87, 2763-2765. 50 - Pomerantz, M.; Gruber, G.W. Photochemical Reorganization of 3,4-Benzotropilidene J. Am. Chem. Soc. 1967, 89, 6798-6799. 51 - Pomerantz, M.; Gruber, G.W. Photochemical Reorganization of 1,2-Benzotropilidene J. Am. Chem. Soc. 1967, 89, 6799-6801. 52 - Pomerantz, M.; Gruber, G.W. Photochemical Reorganization Reactions of o-Divinylbenzene, 3,4-Benzotropilidene, 1,2-Benzotropilidene, and 1-Phenyl-1,3-Butadiene J. Am. Chem. Soc. 1971, 93, 6615-6622. 53 -

Swenton, J.S.; Burdett, K.A.; Madigan, D.M.; Rosso, P.D. Substituent Effects on the

Efficiency of Hydrogen Migration vs. Electrocyclic Ring Closure in 1,2-Benzotropilidenes J. Org. Chem. 1975, 40, 1280-1286. 54 - Keisall, B.J.; Andrews, L.; Trindle, C. Absorption Spectra and Photochemical Rearrangements of the 1,2-Benzotropylidene Molecule and Parent Cation in Solid Argon J. Phys. Chem. 1983, 87, 4898-4903. 55 -

von E. Doering, W.; Goldstein, M.J. An Unusual Rearrangement of an Acyl Azide

Tetrahedron 1959, 5, 53-69. 56 - Gruber, G.W.; Pomerantz, M. Photochemical Berson-Willcott Bones Rearrangement J. Am. 20


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Chem. Soc. 1969, 91, 4004-4006. 57 - Madigan, D.M.; Swenton, J.S. Deuterium Isotope Effect on Competing Hydrogen and Carbomethoxy Migrationn Benzotropilidene Photochemistry J. Am. Chem. Soc. 1971, 93, 7513-7515. 58 - Swenton, J.S.; Krubsack, A.J. Fragmentation and Rearrangement Pathways in Benzonorcaradiene Photochemistry J. Am. Chem. Soc. 1969, 91, 786-787. 59 - Madigan, D.M.; Swenton, J.S. Intermediacy of a 1,2-Benzotropilidene in the Photochemical Rearrangement of a Benzonorcaradiene to a Benzobicyclo[3.2.0]hepta-2,6-diene J. Am. Chem. Soc. 1971, 93, 6316-6318. 60 - Scott, L.T.; Jones, M. Rearrangements and Interconversions of Compounds of the Formula (CH)n Chem. Rev. 1972 , 72, 181-202. 61 - Turro, N.J. Modern Molecular Photochemistry, University Science Books, Mill Valley, California (USA) 1991. 62 - Klessinger, M.; Michl, J. Excited States and Photochemistry of Organic Molecules, VCH Publishers, New York, NY (USA) 1995. 63 - Levine, I.R. Physical Chemistry, MGraw-Hill, New York, NY, USA, 1988.

21



Page 22 of 41

Table 1 Ground and excited state energies (kcal/mol) of the initial, saddle and final structures of the reaction + cyclohexane → nph + methylcyclohexane in solvent cyclohexane. For each state, entries are related to B3LYP/6-31G (upper), B3LYP/6-311++G(d,p) (middle) and ωB97X-D/6311++G(d,p) (lower) calculations. All data are relative to the ground state energy of the adduct 1a + cyclohexane: −660.853562 a.u. for B3LYP/6-31G, −661.182301 a.u. for B3LYP/61a 311++G(d,p) and −660.958710 a.u. for ωB97X-D/6-311++G(d,p). The excited state energies

correspond to vertical transitions at the optimized ground state geometries.

S0

S1

S2

S3

1a + cyclohexane 1a

saddle

nph + methylcyclohexane

0.

60.9

−53.1

0.

59.4

−53.1

0.

58.0

85.5

105.6

88.2

106.7

84.3 94.7 91.2 97.4

115.0 109.9 114.4

103.1 127.4 128.0 146.1 144.3 136.8 148.4

−49.6 50.6 49.7 54.2 52.2 52.5 54.6 81.5 71.1 81.4

22


Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Ground and excited state energies (kcal/mol) of all the stable species and saddle (TU--W)

structures participating to the reaction a a → +a a → ,a a → ,b b in solvent cyclohexane. For each state,

entries are related to B3LYP/6-31G (upper), B3LYP/6-311++G(d,p) (middle) and ωB97X-D/6311++G(d,p) (lower) calculations. All data are relative to the ground state energy of 1a: −425.030635

a.u. for B3LYP/6-31G, −425.237227 a.u. for B3LYP/6-311++G(d,p) and −425.079050 a.u. for

ωB97X-D/6-311++G(d,p). The excited state energies are obtained through vertical transitions at the optimized ground state geometries.

S0

S1

S2

S3

a)

1a

S1a1a-2a

2a

S2a2a-3a

3a

S3a3a-3b

3b

0.

60.6

−26.8

51.5

−6.7

−5.6

−17.9

0.

57.7

−27.4

49.2

−5.9

−5.6

−23.8

67.7

51.9

0.

56.5

85.5a

103.5

88.1

104.9

91.3

120.3

84.1

94.7a 97.5

115.0a 109.9

114.7

98.2

125.3 132.7 141.7 134.5 142.5

−25.2 71.3 69.4 76.9

46.6 53.6 49.0 53.9

81.0

119.1

87.3

120.2

79.4 92.6 89.1

103.7

106.6

−6.1 68.9 74.3 86.4 84.3 92.5

3.8

85.0 82.5 88.9 91.8 89.8

−18.5 86.5 80.3 82.8 90.8 85.8 89.1

135.1

105.1

113.1

108.6

127.9

108.8

111.9

108.8

113.3

98.8

95.7

98.2

The 1a transition energy differs from that of (1a 1a + cyclohexane) in Table 1 by a few hudredth of 1a

kcal/mol. Such a negligible difference is due to the weak non-covalent interaction with cyclohexane.

23



Page 24 of 41

Table 3 Rate constants (min) resulting from the fit to the experimental data of Fig. 5 by using the functions (A.5) – (A.8) for the “two channels” mechanism (6) and (A.13) – (A.16) for the “one channel” mechanism (7) (see Appendix). The mean square displacements (m.s.d.) of the fitting data from the experimental ones are also reported.

Mechanism (6)

Mechanism (7)

1

1

1

1

m.s.d.

4.72 ∙ 10

3.17 ∙ 10

5.79 ∙ 10

4.27 ∙ 10

2.02 ∙ 10

12

13

14

7.87 ∙ 10

7.57 ∙ 10

2.16 ∙ 10

m.s.d.

2.32 ∙ 10

24


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Captions to Figures Fig. 1 – Top: molecular structures of 1,6-methano[10]annulene 1a and its norcaradienic form 1b. Middle: molecular structures of 1,2- and 2,3-benzotropilidene, 2a and 3a, respectively. Bottom: norcaradienic form of 1,2- and 2,3-benzotropilidene, 2b and 3b, respectively. The atomic numbering of carbon atoms are indicated on all structures. Fig. 2 – Absorption spectrum of a 10 M solution of 1a in cyclohexane at room temperature as a

function of the irradiation time (Yexc = 266 nm; continuous excitation). Due to its negligible intensity,

the band around 361 nm has been magnified. Considering the band at 256 nm, the curves from top to bottom correspond to 0, 70, 163, 253, 348, 411 min irradiation times. Inset: spectrum of the reaction mixture at the end of the irradiation process (411 min).

Fig. 3 – Absorbance difference, normalized to the incident power, between band maxima of 1a at 256 nm at initial time and after 120 min of continuous irradiation, as a function of the wavelength, in the spectral range 230 – 350 nm. On the ordinate axis, the quantity Z

The Photochemical Reactivity of 1,6-methano[10]annulene

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