Orbital Control of Photochemical Rearrangement of 4-Aryl-1,1-dicyano

Nov 14, 2016 - Orbital Control of Photochemical Rearrangement of 4-Aryl-1,1-dicyano-1-butenes through the Hyperconjugative Substitution on the Linker ...
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Orbital Control of Photochemical Rearrangement of 4‑Aryl-1,1dicyano-1-butenes through the Hyperconjugative Substitution on the Linker Chain Nobuo Matsuki, Yoshihisa Inoue, and Tadashi Mori* Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan S Supporting Information *

ABSTRACT: Hyperconjugative interaction was demonstrated to play a vital role in the photochemistry of 4-aryl-1,1-dicyano-1-butenes. Thus a simple substituent on the benzylic position effectively induced a new photoreactivity to afford an allylic rearrangement product that is not obtained for the parent substrate. The natural bond orbital analysis was employed to reveal the enhanced relative contributions of hyperconjugation in the excited state, which dramatically alter the photochemical outcomes not only by reducing the strength of the allylic/benzylic bond but more crucially by affecting the conformer distribution.

H

cyclization/cycloaddition) while keeping the photophysical properties essentially the same (Scheme 1). The experimental

yperconjugation is ubiquitous in many molecular systems and important to explain atypical conformational preferences. It is usually defined as the stabilization through σ−π* interaction, but secondary hyperconjugation such as σ−σ* interaction is often included.1−3 Such interaction is able to explain the preference for the staggered conformation in ethane4 and other peculiar conformer preferences (e.g., the gauche effect) and has been utilized as a tool for controlling the selectivity in various (thermal) reactions.5−7 The effects of hyperconjugation have also been discussed in some photochemical reactions. Enhanced cyclization of fluorinated ketones (Norrish Type II reaction)8 and syn-selective Paternò-Büchi reaction of 5-substituted adamantanones9 have been rationalized by the hyperconjugative stabilization in the biradical intermediate. The hyperconjugation is also operative in the excited state. For instance, the excited-state hyperconjugation has been claimed to explain unusual IR, UV−vis, fluorescence, and XAS spectral behaviors in several systems.10−14 Theoretical investigations have revealed that the rotational barriers of substituted toluenes15 and methylnaphthalenes16 significantly differ in the ground versus excited state due to the excited-state hyperconjugation. However, little is known about how such hyperconjugative interaction in the excited state can be exploited for controlling photoreactions. Here we explore whether and to what extent the orbital control is achievable in photoreaction by examining the conformer distribution in the excited state (and also in the ground state) and the photochemical outcomes, both of which are modified through the hyperconjugative effect. We employed 4-aryl-1,1-dicyano-1-butenes with a series of substituents (X) of different degrees of hyperconjugation on the benzylic position to compare the product selectivity (rearrangement versus © XXXX American Chemical Society

Scheme 1. Photoreaction of 4-Phenyl- (1a−c) and 4Naphthyl-1,1-dicyano-1-butenes (2a−c)

results and the quantum-chemical calculations demonstrated that the relative hyperconjugative contribution becomes more effective in the excited state than in the ground state. Photoreaction of 4-X-substituted 4-phenyl-1,1-dicyano-1butenes 1a17 and 1b18 affords the rearrangement (R) and ortho-cyclization (oC) products in ratios varying with substituent X (Scheme 1). We immediately realized that the R/oC ratio is always higher for methylated 1b than for parent 1a, and this trend is very enhanced upon excitation at longer wavelengths (at the charge-transfer or CT band)19 (Table 1, Received: November 10, 2016 Accepted: November 14, 2016 Published: November 14, 2016 4957

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The Journal of Physical Chemistry Letters

Encouraged by the above findings,20 we extended our study to naphthyl series 2a−c. The cyclization became much more preferred (i.e., rearrangement being discouraged) in naphthyl derivative 2a than in phenyl analog 1a.21 Indeed, the rearrangement of 2a was observed only in a small amount (∼3%) in methylcyclohexane (MCH) at 20 °C (Table 1, entry 4) and not detected at all in acetonitrile (AN) (see Table S2 in the SI). Hence, we wanted to demonstrate how much the rearrangement is enhanced in the photoreaction of the naphthyl series by making best use of the hyperconjugation effect of the substituent on the linker (2b and 2c, X = Me or OMe). Somewhat unexpectedly, the photolyses of 2b and 2c afforded the [2+2]-cycloadduct (C) as a major product, in addition to the rearrangement product (R) (Table 1, entries 5 and 6). The cyclization products (oC and pC) were only detected as minor product(s) upon prolonged irradiation (see the SI). Indeed, the irradiation of 2c at 313 nm in dichloromethane at 20 °C led to the photostationary state (PSS) with C within 10 min (2c:C = 61:39 at PSS). Essentially, no other reactions were observed in polar solvents. In a control experiment, we confirmed that substrate 2c and cycloadduct C are not converted thermally or photochemically to the cyclization products (oC or pC) even in the presence of acid, suggesting that C is formed through the concerted mechanism, presumably in the singlet manifold. Moreover, only the all-antiisomer was obtained among the possible eight stereoisomers of C (at least in the early stage of photoreaction), which was fully confirmed by using the 2D NMR techniques (see SI); other isomers, however, were also produced (in much smaller amounts) upon prolonged irradiation on a preparative scale. When the photoreaction was performed in nonpolar MCH, the rearrangement product (R) was additionally formed. Because of the reversible nature of photoreaction, the R/C ratio was time-dependent. Accordingly, the intrinsic R/C ratios used in the following discussion were obtained by extrapolating the experimentally obtained values to time zero (see Table S2

Table 1. Effect of Hyperconjugating Substituent on the Product Distribution upon Local and CT Excitation of 1a−c and 2a−c in Methylcyclohexane at 20 °Ca distribution of rearrangement versus cyclization or cycloaddition products entry

substrate

local excitation

CT excitation

1 2 3 4 5 6

1ab 1bc 1c 2ad 2b 2c

66:34 (1.9) 88:12 (7.3) >95:5 (>20) 3:97 (0.03) [7e] 59:41 (1.4) 64:36 (1.8)

23:77 (0.30) 69:31 (2.2) >95:5 (>20) 4:96 (0.04) [10e] 14:86 (0.16) 56:44 (1.3)

a Local excitation at 254 nm (for 1a−c) or 300 nm (for 2a−c); CT excitation at 280 nm (for 1a−c) or 330 nm (for 2a−c). bRef 16. cRef 17. dRef 18. eRatio of peri- and ortho-cycloadducts (pC/oC).

entries 1 and 2; see also Table S1 in the Supporting Information (SI)). Because the UV−vis spectra of both substrates are essentially the same, the selectivity change does not appear to originate from an electronic reason. The rearrangement is considered to occur from the anti conformer of the substrate, while the cyclization proceeds through the appropriate gauche conformer(s). Therefore, we hypothesized that the hyperconjugative effect of the methyl group may have dramatically affected the conformer distribution and thus the selectivity in the photoreaction. Accordingly, we first checked the photoreaction of a similar substrate with more electronegative (i.e., more strongly hyperconjugating) substituent, methoxy. As expected, the rearrangement became dominant and no cyclization was observed in the photoreaction of 1c (X = OMe), regardless of the excitation wavelength (Table 1, entry 3). Thus the relative preference for rearrangement (R/oC ratio) increased from 1.9 to >20 upon local excitation at 254 nm and from 0.3 to >20 upon CT excitation at 280 nm with increasing hyperconjugation.

Table 2. Differential Activation Parameters for Rearrangement versus Cyclization/Cycloaddition upon Local (300 nm) or C−T Band (330 nm) Excitation of 2a−ca

substrate

solvent

2a

acetonitrile (AN)

2b

acetonitrile (AN)

λex/nm 300 330 300 330 300 330 300 330

methylcyclohexane (MCH) 2c

methylcyclohexane (MCH)

ΔH‡/kJ mol−1 −16.0 −15.8 +10.0 +10.7 −3.2 −0.9 +4.0 +4.9

± ± ± ± ± ± ± ±

1.8 1.9 0.5 0.8 0.8 0.9 1.0 0.1

TΔS‡/kJ mol−1 −25.7 −26.5 +3.0 +4.1 −1.6 −4.0 +5.5 +5.5

± ± ± ± ± ± ± ±

0.8 0.8 0.6 0.2 1.0 1.0 1.2 0.1

Differential activation parameters obtained from the Eyring plots (top figures) of the ratio of the rearrangement versus cyclization/cycloaddition products (T = 293 K). a

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Table 3. Photophysical Properties of 1a−c and 2a−ca

in the SI). As shown in Table 1, the extrapolated R/C ratios were found to be dramatically enhanced in the order: 2a < 2b < 2c, varying from 0.03 to 1.8 upon local excitation and from 0.04 to 1.3 upon CT excitation. As such, the substituent on the linker (by Me or OMe) effectively controls the photochemical outcomes most likely through the change of conformer distribution in the ground and excited states. The exact reason for the reactivity switching from cyclization to [2+2]-cycloaddition and the exclusive formation of the all-anti-isomer, however, remained to be elucidated.22 The temperature-dependence study of the product selectivity in the photoreaction of 2a−c provided further insights into the nature of excited-state equilibrium among the conformers. Thus the differential Eyring plot (of the logarithm of product ratio against reciprocal temperature) for each substrate gave a single straight line, implying that the product selectivity is determined in a single step within the temperature range examined (Table 2). The substitution is an important factor influencing the activation parameters. Although both 2a and 2b afford the cyclization products in high selectivities (95−99%) upon photoirradiation in AN (Table S2 in the SI), the activation parameters reveal the strikingly contrasting origins. Thus the high preference for cyclization in 2a (X = H) is achieved entropically by strongly discouraging the rearrangement (TΔΔS‡ = −26 kJ mol−1) with modest enthalpic assistance for cyclization (ΔΔH‡ = −16 kJ mol−1), but in the case of 2b (X = Me), this is realized enthalpically by facilitating the cyclization (ΔΔH‡ = +10 kJ mol−1) with minor entropic assistance for rearrangement (TΔΔS‡ = +3 kJ mol−1). It is intriguing that the excitation mode does not significantly affect the activation parameters for all of the substrates in each solvent (excepting 2b in MCH), while much more dramatic changes are induced by altering the solvent from polar AN to nonpolar MCH, where the temperature dependence is much more pronounced in the former solvent, which are likely to be attained not by discouraging the rearrangement but by facilitating the cyclization/cycloaddition, as the overall reaction rates are enhanced in this solvent (Table S2 in the SI). This means that the transition state for cyclization/cycloaddition is more polarized and hence profoundly solvated than that for rearrangement. Similar sign inversions of ΔΔH‡ and ΔΔS‡ are found for 2b and 2c in nonpolar MCH (but to lesser extent), endorsing the crucial role of AN solvation. Table 3 summarizes the photophysical properties of 1a−c and 2a−c (see also Figures S27−S29 in the SI). It is to note that the difference in dipole moment between the ground and excited states evaluated from the Lippert−Mataga plots revealed the highly polar nature of the intramolecular exciplexes derived from 2a−c (Δμ ≈ 10 D). Although the bathochromic shift caused by changing the solvent from MCH to AN was found slightly larger for 2b and 2c (42 and 37 nm, respectively) than for 2a (35 nm), the UV−vis and fluorescence spectroscopic studies on 1a−c and 2a−c indicated that the substituent on the linker does not considerably alter the electronic nature of the substrates, confirming that the differences in photochemical outcome are not electronic in origin. Finally, we quantitatively evaluated the effects of hyperconjugation in the ground and excited states by theoretical calculations. Because the calculation of the transition-state at the excited-state hypersurfaces was not feasible, we compare the differences of orbital interaction for the ground- and excitedstate geometries. Second-order perturbation analysis of

substrate

solvent

λabs/nm

λfl/nm

τ/ns

Stokes shift/cm−1

1a 1b 1c 2ab

AN AN AN MCH Et2O CH2Cl2 AN MCH Et2O CH2Cl2 AN MCH Et2O CH2Cl2 ACN

209/234 208/235 206/238 224/286 223/283 −/284 223/284 224/284 221/281 −/282 221/281 222/287 222/285 −/287 223/284

346 407 440 481 335 397 441 477 336 397 437 473

3.1

6100 10 800 12 500 14 500 5300 10 400 12 800 14 700 5100 9800 12 000 14 000

2b

2c

3.2

2.8

a Absorption and fluorescence peak maxima (λabs and λfl), fluorescence lifetime (τ), and Stokes shift measured at 20 °C. Peak maxima were estimated by curve-fitting if they consisted of vibrational fine-structure. b Ref 21.

interactions in the natural bond orbital (NBO) framework has been frequently employed to examine the strength and nature of hyperconjugation.23−25 In 2a−c, the C−X bond plays two distinct roles for stabilization as a σ-donor and also as a σacceptor. Therefore, we defined the total second-order perturbation energy E(2)total as a sum of two components, that is, E(2)donor and E(2)acceptor, in the analysis of hyperconjugation in 2a−c (Figure 1).

Figure 1. Schematic drawings of the breakdown of the total stabilization energy E(2)total through hyperconjugation to the donor and acceptor components: E(2)donor and E(2)acceptor, depicted for the anti conformer.

All possible conformations of 2a−c were considered as follows. In 2a−c, the Cα−Cβ bond is able to take anti (A) and a pair of gauche (G+ and G−) conformations. The acceptor (dicyanoethene) unit can be located either on the peri- or ortho-side of naphthalene ring. For 2b and 2c, the diastereomeric pro-R/pro-S face selectivity should also be taken into account. Accordingly, all of the possible 12 (6 for 2a) conformations were geometrically optimized at the DFTD3(BJ)-TPDD/def2-TZVP level26 and the TD-DFT-CAMB3LYP/def2-TZVP level27 for the ground and excited states, respectively (see the SI for more detail). To simplify the discussion, an average of all anti conformers is represented by A, as they are considered to afford the rearrangement product. As representative gauche conformer G, we chose the peri-G +-pro-S isomer, which is a direct precursor to all-anti-[2+2]cycloadduct. The detailed E(2) data for the rest of conformers can be found in Table S4 in the SI, but the following discussion 4959

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The Journal of Physical Chemistry Letters Table 4. Differential E(2) Energies for Hyperconjugation and Geometrical Changes upon Photoexcitation of 2a−ca ground state

excited state

difference

substrate

conformer

ΔE(2)

rG

ϕG

rE

ϕE

Δr

Δϕ

2a

A G A G A G

−0.33 −0.67 −0.36 −0.83 −0.26 −0.64

1.561 1.566 1.569 1.571 1.549 1.557

180.3 188.7 184.2 193.6 184.8 192.5

1.575 1.579 1.581 1.588 1.554 1.570

179.1 199.9 185.4 209.5 185.0 205.4

0.014 0.013 0.012 0.017 0.005 0.013

+1.2 −11.2 −1.2 −15.9 −0.2 −12.9

2b 2c a

Difference in total E(2) between ground and excited states was calculated by the NBO analysis at the CAM-B3LYP/def2-TZVP level with geometries optimized at the DFT-D3(BJ)-TPSS/def2-TZVP level (for the ground-states) and the TD-DFT-CAM-B3LYP/def2-TZVP level (for the lowest singlet excited states). Distance between Cα and Cβ (r) and torsion angle (ϕ) for hyperconjugation around the Cα−Cβ bond are also shown. Conformation A refers the averaged values of all possible anti conformers, while G refers to the values for peri-G+-pro-S conformer, direct precursor to all-anti-[2+2]-cycloadduct.

and σ* orbitals by the alteration of torsion angle (Scheme 2).29 Despite the fact that the nonproductive G+ conformers are energetically much preferred in the excited state for all of 2a−c (Table S3 in SI), the rearrangement is dominant for 2b and 2c. This seems reasonable as the decrease in E(2) between G and A is much larger in 2b and 2c, and thus the anti conformers become more preferred in the excited state, eventually leading to the increase in the rearrangement product. In other words, although the stabilization by hyperconjugation is reduced in the excited state, the relative contribution of the anti isomer becomes much pronounced to efficiently improve the product selectivity. In summary, we demonstrated that a small substituent on the linker chain in 4-aryl-1,1-dicyano-1-butenes is capable to totally switch the photochemical outcomes. The hyperconjugation is operative not only in the ground state but also in the excited state. The second-order perturbation analysis in the NBO framework suggested that the hyperconjugation effect is decreased in general in the excited state due to the reduced orbital overlap caused by the Cα−Cβ bond elongation. However, the structural change upon excitation, in particular, the difference in torsion angle, is more conformer-dependent and generally greater in gauche conformers. As such, the relative preference for the anti isomer is substantially enhanced in the excited state than in the ground state. Accordingly, the photoreactions of methyl- and methoxy-substituted 4-aryl-1,1dicyano-1-butenes predominantly afford the corresponding rearrangement products, which are essentially not obtained from the parent substrate. Such an orbital control through the excited-state hyperconjugation is to be more recognized and utilized as a versatile tool for understanding, controlling, and fine-tuning the photochemical outcomes in various systems.

is commonly applicable to all of the other conformers without any major modifications. In the ground state, the E(2)total energy for stabilizing the A or G− conformer remains comparable for all of 2a−c. The detailed breakdown of E(2) energy revealed that the gain in E(2)acceptor is compensated by the loss in E(2)donor (Table S4 in the SI). In contrast, the E(2)acceptor gains are not as large as the E(2)donor loss in the G+ conformer. Overall, the A conformer becomes much preferred by introducing an electronegative substituent. This trend is in accord with the results observed for substituted ethanes previously reported.28 Table 4 summarizes the calculated structural parameters in the ground and excited states of 2a−c. The Cα−Cβ bond distance (r) becomes longer upon excitation, the degree of which is, however, comparable for all of the conformers and substituents examined. Interestingly, while the torsion angle (ϕ) for optimizing the hyperconjugation around the Cα−Cβ bond is not very altered in the excited state for the anti conformers, appreciable decreases are noticed for the G isomers (Scheme 2). Scheme 2. Conformation-Dependent Change of Hyperconjugation upon Excitation

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02632. Experimental and theoretical details with spectral data. (PDF)

To assess how such structural changes affect the orbital interactions, the NBO analysis was performed for the geometries optimized in the S1 state, and the differences from the ground state are listed in Table 4 (see also Table S4 in the SI). Because of the bond lengthening, the E(2)total energies for all conformers of 2a−c are decreased in the excited state. More importantly, much larger decreases were found for the G conformer due to the loss of effective overlap between the σ

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-6-6879-7923. ORCID

Tadashi Mori: 0000-0003-3918-0873 4960

DOI: 10.1021/acs.jpclett.6b02632 J. Phys. Chem. Lett. 2016, 7, 4957−4961

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The Journal of Physical Chemistry Letters Notes

butenes Controlled by Intramolecular Charge-Transfer Interaction. Effect of Medium Polarity, Temperature, Pressure, Excitation Wavelength, and Confinement. Photochem. Photobiol. Sci. 2011, 10, 1405−1414. (18) Nishiuchi, E.; Mori, T.; Inoue, Y. Control of Conformer Population and Product Selectivity and Stereoselectivity in Competitive Photocyclization/Rearrangement of Chiral Donor-Acceptor Dyad. J. Am. Chem. Soc. 2012, 134, 8082−8085. (19) Mori, T.; Inoue, Y. Charge-Transfer Excitation: Unconventional Yet Practical Means for Controlling Stereoselectivity in Asymmetric Photoreactions. Chem. Soc. Rev. 2013, 42, 8122−8133. (20) Recently, favored conformation derived from the substituent on linker has been discussed in terms of entropy effect. Navale, T. S.; Talipov, M. R.; Shukla, R.; Rathore, R. Interplay between Entropy and Enthalpy in (Intramolecular) Cyclophane-Like Folding versus (Intermolecular) Dimerization of Diarylalkane Cation Radicals. J. Phys. Chem. C 2016, 120, 19558−19565. (21) Aoki, Y.; Matsuki, N.; Mori, T.; Ikeda, H.; Inoue, Y. Exciplex Ensemble Modulated by Excitation Mode in Intramolecular ChargeTransfer Dyad: Effects of Temperature, Solvent Polarity, and Wavelength on Photochemistry and Photophysics of Tethered Naphthalene-Dicyanoethene System. Org. Lett. 2014, 16, 4888−4891. (22) For instance, relative energies of cyclization product and [2+2]adduct were found quite comparable between 2a (+27 and + 2 kcal mol−1) and 2c (+20 and −3 kcal mol−1). (23) Glendening, E. D.; Landis, C. R.; Weinhold, F. Natural Bond Orbital Methods. WIREs Comput. Mol. Sci. 2012, 2, 1−42. (24) Weinhold, F. Natural Bond Orbital Analysis: A Critical Overview of Relationships to Alternative Bonding Perspectives. J. Comput. Chem. 2012, 33, 2363−2379. (25) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Intermolecular Interactions from a Natural Bond Orbital, Donor-Acceptor Viewpoint. Chem. Rev. 1988, 88, 899−926. (26) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456−1465. (27) Yanai, T.; Tew, D.; Handy, N. A New Hybrid Exchange− Correlation Functional using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51−56. (28) Alabugin, I. V.; Zeidan, T. A. Stereoelectronic Effects and General Trends in Hyperconjugative Acceptor Ability of σ Bonds. J. Am. Chem. Soc. 2002, 124, 3175−3185. (29) Pandey, A. K.; Yap, G. P. A.; Zondlo, N. J. (2S,4R)-4Hydroxyproline(4-nitrobenzoate): Strong Induction of Stereoelectronic Effects via a Readily Synthesized Proline Derivative. Crystallographic Observation of a Correlation between Torsion Angle and Bond Length in a Hyperconjugative Interaction. J. Org. Chem. 2014, 79, 4174−4179.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by Grant-in-Aids for Scientific Research, Challenging Exploratory Research, and Innovative Area “Photosynergetics” (Grant Numbers JP15H03779, JP15K13642, JP15H01087) from JSPS and the Matching Planner Program from JST (Grant Number MP27215667549) are gratefully acknowledged.

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