Synthesis, photochemistry, and photophysics of butadiene derivatives: Influence of the methyl group on the molecular structure and photoinduced behavior

ARTICLE

pubs.acs.org/joc

Synthesis, Photochemistry, and Photophysics of Butadiene

Derivatives: Influence of the Methyl Group on the Molecular Structure

and Photoinduced Behavior^†

,‡

‡

§

||

^

ꢀ

^§ꢀ

Irena Skoriꢁc,* Ilijana Kikaꢀs, Margit Kovꢁacs, Lajos Fodor, Zeljko Mariniꢁc, Kreꢀsimir Molꢀcanov,

Biserka Kojiꢁc-Prodiꢁc,^{^}and Ottꢁo Horvꢁath*^,§

^‡Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, 10000 Zagreb, Croatia

^§Department of General and Inorganic Chemistry, Institute of Chemistry, Faculty of Engineering, University of Pannonia, POB 158,

Veszprꢁem H-8201, Hungary

NMR Center and ^{^}Laboratory for Chemical and Biological Crystallography, Department of Physical Chemistry,

Rudjer Boꢀskoviꢁc Institute, Bijeniꢀcka cesta 54, 10000 Zagreb, Croatia

S Supporting Information

b

ABSTRACT: Novel butadiene derivatives display diverse

photochemistry and photophysics. Excitation of 2-methyl-

1-(o-vinylphenyl)-4-phenylbutadiene leads to the dihydronaph-

thalene derivative, whereas photolysis of the corresponding

model o-methyl analogue results in the formation of the

naphthalene-like derivative, deviating from the nonmethylated analogue of the prior starting compound and producing benzobi-

and -tricyclic compounds. The eﬀect of the methyl substituents is even more dramatic in the case of the dibutadienes. The parent

unsubstituted compound undergoes photoinduced intramolecular cycloaddition giving benzobicyclo[3.2.1]octadiene, whereas the

photochemical reaction of the corresponding dimethylated derivative shows only geometrical isomerization due to the steric eﬀect

of the substituents. Methyl groups on the butadiene backbones reduce the extent of conjugation, causing a blue-shift of the

characteristic absorption band. The ﬂuorescence eﬃciency is dramatically decreased, as a consequence of nonplanarity and reduced

rigidity of the molecules due to the crowding by the methyl and phenyl groups together. Four molecules of very similar structures

show dramatically diﬀerent photoinduced behavior, revealing how changes of the nature and position of the substituents are valuable

in understanding the photophysics and photochemistry of these types of compounds.

’ INTRODUCTION

leading to tricyclic structure (endo-3), or it can be used as a suitable

substrate for other transformations on the isolated double bond,

easily derivatized to new compounds with various functionalities.

Moreover, the bicyclo[3.2.1]octane skeleton is found in numerous

biologically important active natural products.^23ꢀ26

The photochemistry of 1,3-butadiene¹and 1,3,5-hexatriene²

derivatives has been studied in detail. The photochemical reac-

tions of 1,4-diphenyl-1,3-butadienes, their photoisomerization

reactions,^3ꢀ11and some electrocyclization reactions^12,13have

already been studied. However, the photochemistry of 1-(o-

vinylphenyl)-4-phenylbutadiene derivative 1 (Figure 1) was

described for the ﬁrst time by our group.¹⁴The synthesis and

photochemistry of 1, a molecule that combines properties of

both butadiene and hexatriene systems, were examined. Previous

publications^15ꢀ22on the photochemistry of diﬀerent heteroaryl-

substituted hexatrienes showed interesting intramolecular cy-

cloaddition reactions and formation of bicyclo[3.2.1]octadiene

structures. By insertion of an additional double bond into the

stilbene-like moiety, we obtained the prolonged conjugated system

of 1 that might allow the formation of a new polycyclic structure

with the double bond functionality for further transformations.

The compound 1 undergoes intramolecular photocycloaddition

reaction to benzobicyclo[3.2.1]octadiene derivative 2 in very

good yield, giving only the endo-phenyl-benzobicyclo[3.2.1]-

octadiene isomer (endo-2) due to the stereoselectivity of the

photoreaction. The phenyl-benzobicyclo[3.2.1]octadiene deri-

vative (endo-2) undergoes further di-π-methane rearrangement

As a part of our increasing interest in the photochemistry of

conjugated butadiene systems, we extended our research to new

ω,ω⁰-diarylbutadiene derivatives 4ꢀ7 (Figure 1) in order to

prepare new polycyclic structures by photochemical methodol-

ogy as well as to get a deeper understanding of the photoinduced

behavior of these conjugated systems. In the continuation of our

study of 1-(o-vinylphenyl)-4-phenylbutadienes 1,¹⁴the synthesis

of four novel butadiene derivatives 4ꢀ7 (unsubstituted or having

one or two methyl groups as substituents on the double bond),

their spectral characterization, and the detailed investigation of

the photochemical and, to a less extent, photophysical behavior

are presented. In these new systems with prolonged conjugation,

the inﬂuence of the introduced methyl group at the double

bond is examined in order to see the possibility of the intramolec-

ular [2 + 2] photocycloaddition followed by formation of the

Received: April 6, 2011

Published: September 19, 2011

r

2011 American Chemical Society

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Figure 1. Structures of the known (1) and new butadiene derivatives (4ꢀ7) and photoproducts of 1 (2 and 3).

Scheme 1. Possible Conformations of trans-Isomers of

Compounds 1 and 4ꢀ6

Scheme 2. Syntheses of New Butadiene Derivatives 4ꢀ6 by

Wittig Reaction

benzobicyclo[3.2.1]octadienes and/or photoelectrocyclization to

the photoproducts possessing six-membered rings. The steric

hindrance of the methyl groups in 4 and 6 should cause deviations

from planarity²⁷and should shift the conformer equilibrium toward

s-cis-butadiene conformers, (e.g., B and D, Scheme 1), aﬀecting the

reaction pathways and yields. As only some of the conformers can

react in the formation of intramolecular photoproducts, it is not

expected that the photoreactions take place always from the most

populated conformer (e.g., D for compound 4 and 6, Scheme 1)

regardless of the conformational distribution of the starting mole-

cule. In the case of 1,¹⁴the same conformer B was responsible for the

formation of 2 at diﬀerent wavelengths; even various conformers

exist mainly at diﬀerent reaction conditions. Although the confor-

mers of each isomer studied in this work may display deviating

photoinduced behavior, similarly to some 2-vinylbiphenyls,²⁸1,2-

dinaphthylethenes,²⁹and 1-naphthyl-2-phenylethenes,³⁰investiga-

tion of the eﬀects of the conformational diversity within the steric

inﬂuence of the starting geometric isomers on the reaction pathway

was not our aim.

Scheme 3. Synthesis of New Butadiene Derivative 7 by

Wittig Reaction

geometric isomers (compounds 5 and 6) according to the

procedures described in previous papers for heteroarylstilbene

derivatives^31,32and butadiene analogues.¹⁴Because the syntheses

were performed always from the corresponding (E) geometric

isomer of the cinnamaldehydes, the second double bond (looking

from the ortho-substituted central benzene ring) in the starting

compounds 4ꢀ7 retain the (E) conﬁguration. Hence, the number

of the geometric isomers was reduced; the reaction mixtures

contain cis- and trans-isomers of 4 and 7 and cis,cis-, cis,trans-,

and trans,trans-isomers of 5 and 6 (only the ﬁrst double bond

connected to the ortho-substituted central benzene ring is desig-

nated).

’ RESULTS AND DISCUSSION

The starting materials, unsubstituted (5) and methyl-substi-

tuted butadiene derivatives (4, 6, and 7), were prepared by the

Wittig reaction from the corresponding mono- or diphosphonium

salts and cinnamaldehydes (Schemes 2 and 3) in very good yields

(65ꢀ91%) as mixtures of two (compounds 4 and 7) or three

To conﬁrm that the vinyl group in 4 is involved in the

photoreaction and to resolve geometric isomers of starting

compound 4, the o-methyl-substituted derivative 7 was irradiated

as the model of an o-vinyl-substituted conjugated system 4

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Figure 4. Absorption spectra of the isomers of compound 6.

Figure 2. Absorption and emission spectra of the isomers of compound 4.

Figure 5. Absorption and emission spectra of the isomers of compound 7.

Figure 3. Absorption and emission spectra of the isomers of compound 5.

be attributed to the increased molecular planarity and the

possibility of better delocalization of π electrons. Accordingly,

in the case of compounds 4, the main band of the trans-isomer

(315 nm) is about 36 nm red-shifted compared to that of the

cis-isomer (279 nm). Notably, the main bands of the isomers of

this methyl-substituted butadiene derivative are signiﬁcantly (about

25ꢀ30 nm) blue-shifted in comparison to those of the corre-

sponding isomers of the unsubstituted analogue 1.¹⁴The same

relation is valid to the corresponding isomers of the 1,4-diphenyl-

1,3-butadiene (DPB).³³This indicates that the methyl group

decreases the conjugation in the bond structure, signiﬁcantly

changing the geometry of the molecule^27,34and inﬂuencing

its conformational equilibrium and photoinduced behavior.

The absorption spectrum of the trans-isomer of compound 7 is

quite similar to that of the corresponding derivative 4. Both the

positions and the molar absorbances of the main band agree well,

indicating that the change of the substituent (methyl vs vinyl)

does not inﬂuence the S₀ꢀS₁transition assigned to this band.

The ﬁrst excited state of both compounds is delocalized over

both the butadiene backbone and the phenyl groups.²⁷Since

neither the methyl nor the vinyl group on the aromatic ring

aﬀects the absorption spectrum of these butadiene derivatives,

apparently these substituents do not inﬂuence the conjugation.

without a possibility of intramolecular photoreaction and poly-

merization due to the lack of the vinyl group.

The geometric isomers of 4ꢀ7 can be easily identiﬁed accord-

ing to their ¹H NMR spectra by the characteristic vicinal coupling

constants of the cis- and trans-ethylenic protons. According to ¹H

NMR spectra of the crude reaction mixtures, the ratio of cis- and

trans-isomers of 4 and 7 was very similar (∼1:3), whereas the ratio

of cis,cis-, cis,trans-, and trans,trans-isomers of 5 and 6 was ∼1:5:4).

The isomers of 4ꢀ7 were separated by combining column and

thin-layer chromatography on silica gel, completely analyzed, and

identiﬁed spectroscopically.

UV Absorption Spectra. The UV spectra (Figures 2, 3, 4, and 5)

of separated isomers of 4ꢀ7 clearly show the substituent and con-

figurational influence on absorption characteristics. Table 1 sum-

marizes the characteristic data (maxima wavelengths and vibronic

progression of the absorption and emission spectra, the fluorescence

quantum yields and lifetimes) of the compounds studied in n-hexane.

The positions and intensities of the absorption bands indicate

that the increasing participation of the trans conﬁguration in

the double bond causes a bathochromic shift and generally an

increase in the molar absorption coeﬃcients compared to the

bands of the corresponding cis-isomers. This phenomenon can

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Table 1. Absorption and Emission Parameters of Butadiene Derivatives in n-Hexane at Room Temperature

compound

λ^m_ab^a_s^x, nm^a

Δν~_abs, cm^ꢀ1

λ^m_F^ax, nm

Δν~_F, cm^ꢀ1

Φ_F^b(λ_exc, nm)

τ_F, ns ^c

cis-4

271, 279 (4.13)

315 (4.47)

335

0.021 (280)

∼0.003 (300)

0.246 (280)

0.135 (280)

0.017 (330)

<0.001

5.97

2.54

0.52

0.49

0.21

∼0.1

nm^d

1.09

trans-4

399

cis,cis-5

313 (3.83)

360, 381, 400

360, 380, 399

406

1530

1460

cis,trans-5

trans,trans-5

cis,cis-6

318 (4.48), 331, 353^sh

303 (4.37), 312, 349^sh

299 (4.49)

291 (4.09), 326^sh

298 (4.43), 331^sh

289 (4.34)

3120

4350

cis,trans-6

trans,trans-6

cis-7

3690

3350

<0.001

trans-7

312 (4.47)

349, 368, 384

1480

0.021 (297)

^aThe main maximum is underlined. ^bStandard deviation: (5%. ^c(4%. ^dNot measurable.

However, the second, higher-energy (240ꢀ260 nm) bands of the

trans-isomers of 4 and 7 are signiﬁcantly diﬀerent. Since the

corresponding second excited state is localized on just the phenyl

moieties,³⁵these substituents signiﬁcantly aﬀect its energy and

the intensity of the S₀ꢀS₂transition (Figures 2 and 5).

ISC. A similar phenomenon was observed in the case of DPB,^11,36

suggesting that the methyl substituent on the phenyl group

barely affects the fluorescence properties of the diphenyl-buta-

diene derivatives. This conclusion is confirmed by the fact that

the emission band of trans-7 displays a vibronic fine structure,

which agrees well with that of the trans,trans-DPB.^7,27,37The

strong deviation of the emission spectral properties of derivatives

4 (and also 5 as seen later) from those of 7 and DPB,^7,11i.e.,

the \reversed tendency regarding the features of the trans- and

cis-isomers, is rather surprising and may be attributed to the role of

conformers and fast photoisomerization as one of our reviewers

suggested. Thus, a reasonable explanation of such observations is

that at the excitation wavelengths employed weakly fluorescent

conformers of trans-4 (and also trans,trans-5) were excited, but

excitation of the cis-isomers led to adiabatic formation of strongly

fluorescent conformers of trans-4 (and trans,trans-5). Relevant

precedents of such phenomena were observed for cis-1-(2-

naphthyl)-2-phenylethene³⁸and cis-1-(2-anthryl)-2-phenylethene³⁹

and systematically analyzed for various diarylalkenes.⁴⁰

From the viewpoint of emission, the most interesting phenom-

enon can be observed in the case of the dibutadiene derivatives 5

and 6. From isomers of 5, which do not contain any substituent

on either the butadiene backbones or the phenyl rings, the cis,

cis-isomer eﬃciently ﬂuoresces (Φ_F= 0.246), and its emission

spectrum is characterized by a well resolved vibronic ﬁne structure

(Figure 3) (Table 1). The ﬂuorescence of the cis,trans-isomer is

very similar, but its quantum yield is 45% lower (Φ_F= 0.135).

However, trans,trans-5 displays an order of magnitude weaker

(Φ_F= 0.017) and rather structureless emission. This tendency

suggests that the all-trans conﬁguration is not favorable for emission

from the ﬁrst excited state, and the main routes of the relaxation of

the excited molecule are photochemical and internal conversion

processes. Accordingly, the measured emission lifetime is the

shortest one in this case (0.2 ns). Considering the values of the

quantum yield and the vibronic ﬁne structure of the ﬂuorescence

of cis,cis-5 and cis,trans-5, it is clearly seen that the intensity

(eﬃciency) of the characteristic ﬂuorescence of the ﬁne-structured

spectrum is in direct proportion with the degree of cis conﬁgura-

tion in these molecules. The tendency regarding the emission

spectral properties of the derivatives 5, similarly to those of the

isomers 4, is just the reverse for that observed for 7 and DPB.^7,11A

reasonable explanation for this phenomenon is previously given in

the discussion of the ﬂuorescence features of the derivatives 4.

Nevertheless, the double butadiene structure of 5 may also play a

considerable role in this photophysical behavior strongly deviating

from that of the parent compound (DPB).

The bathochromic eﬀect of the trans conﬁgurations is more

signiﬁcantly manifested in the case of the dibutadiene derivatives

5 and 6 (Figures 3 and 4). While the cis,cis-isomers display one

(merged) band assigned to the S₀ꢀS₁transition, in the spectra of

the cis,trans- and especially of the trans,trans-isomers the low

energy absorption splits into two bands. This phenomenon

conﬁrms that trans conﬁguration increases the extent of con-

jugation and planarity of the molecule. The blue-shifts of the

corresponding bands in the spectra of the derivatives 6 compared

to those of the isomers of 5 are also in accordance with the eﬀects

of the methyl group on the butadiene backbones, reducing the

conjugation and the planarity of the former compounds. Since,

apart from the butadienes, no further substituents are on the

phenyl groups, the position of the second, higher-energy

(230ꢀ241 nm) band is almost constant for these compounds,

and only its intensity is changed by the isomerization.

Fluorescence Emission Spectra and Photophysics. In n-

hexane at room temperature these butadiene derivatives display

diverse emission features strongly depending on their structures.

The fluorescence spectrum of cis-4 is characterized with a strong

band at the shortest wavelengths (335 nm) among the emission

bands of these compounds. At the same time, the fluorescence

lifetime of this derivative is the longest one (Table 1). The low

quantum yield indicates that fluorescence is not the main decay

route of the S₁excited state of this compound. In the case of the

trans-isomer of 4 the efficiency of fluorescence is even weaker

(about 1 order of magnitude). In the case of derivatives 7,

deviating from the compounds 4, the trans-isomer displays a

characteristic fluorescence with a low efficiency (0.02) and a

relatively short lifetime (1.1 ns), whereas no emission could be

detected for the cis-isomer under these circumstances. This

phenomenon suggests that the substituent on the phenyl group

(vinyl for 4 and methyl for 7) strongly influences the emission

properties, whereas it just slightly affects the absorption features

related to the first excited state. In the case of derivatives 4, the

vinyl substituent decreases the fluorescence efficiency in the trans

configuration. From derivatives 7 (with methyl substituent on

the phenyl group) the trans-isomer displays appreciable fluores-

cence, whereas the emission of the cis-isomer is hardly detectable.

This observation indicates that the excited singlet state of the

latter species decays via nonradiative processes, such as IC and

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Figure 6. Spectral change during the irradiation of cis-4 in n-hexane (a) after 0, 10, 20, 40, 80, 160, 320 s; and trans-4 in n-hexane (b) after 0, 10, 20, 40,

80, 160 s (λ_ir= 313 nm, l = 1 cm).

Figure 7. Transient absorption spectra of trans-4 recorded at 100 (b) and 3000 ns (2) after the excitation and the estimated spectrum of the short-lived

intermediate (dash-dot line) along with the spectrum of the starting material (solid line) and that of the ﬁnal product (dashed line) (a). The decay curves

at 315 nm in shorter (b) and longer (c) time scale are also displayed.

Methyl substituents on the butadiene backbones of dibuta-

diene derivative 6 result in dramatic change in the emission.

Although due to the high sensitivity of our equipment extre-

mely weak (Φ < 0.001) structureless ﬂuorescences were de-

tected for these compounds at longer wavelengths (not shown

in Figure 4) with rather short lifetimes (about 0.1 ns), these

emissions can be considered negligible compared to those of

derivatives 5. This may be attributed to the strong steric

crowding as a mutual eﬀect of the phenyl and methyl groups,

besides the reduction of the extent of conjugation. This steric

strain may cause the lack of planarity even in the case of trans,

trans-6 and the twisting of the two terminal phenyl groups

around the butadiene backbones. A similar phenomenon was

observed for the sterically hindered tetraphenylmethylbuta-

diene (TPMB).²⁷

Photochemistry. Irradiation experiments with each isomer of

4ꢀ7 were carried out in n-hexane, and the reaction course was

followed by taking the UV spectra at indicated times. These

experiments were performed to study the effect of the config-

uration and the methyl substituents on the route of the photo-

lysis. The systems were argon-saturated to avoid the photoindu-

ced oxidation of the excited molecules. The concentration of the

starting compounds was on the order of 10^ꢀ4M. The excitation

wavelengths of 313 and 350 nm were applied for each photolysis

experiment. The preparative irradiation experiments of 4ꢀ6

were performed in petroleum ether solutions under anaerobic

conditions at 313 and 350 nm and at low concentrations in the

order of 10^ꢀ3M. Depending on the starting material, different

photochemical routes and, as a consequence, different types of

photoproducts were identifed and confirmed. The formation of

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the photoproducts was generally accompanied by the formation

of some polymeric products, which were not further investigated.

Regardless of the derivative, upon irradiation of each isomer

the longest-wavelength absorption band gradually disappeared

(see Figures 6, 9, 12, and 16 for 313 nm photolysis; the spectral

changes at 350 nm irradiation (not shown) are very similar to the

corresponding ones at λ_ir= 313 nm). These spectral changes

indicate that the long-range conjugation between the phenyl

groups via the butadiene link has been ceased by the photolysis,

and the electronic system of the aromatic rings became isolated.

In numerous cases, as the initial changes of the absorption spectra

indicate (Figures 6a, 9b, 12ab, 16a), photoisomerization is one of

the primary process, which is either followed or accompanied by

a competitive photochemical reaction.^41,42Hence, in these cases,

the quantum yield for the disappearance has been determined

from the spectra recorded after the quasi-photostationary state of

the isomers was reached. This took place very fast, within 20 s of

irradiation. The initial slope (at t = 0 s) of the polynom ﬁtted to

the absorbance vs time plot at the most characteristic wavelength

has been taken for the calculation of the quantum yield. Also the

molar absorbance of the ﬁnal product at this wavelength was

taken into account. The quantum yields of the photoinduced

disappearance determined in this way are summarized in Table 2.

As Figure 6a shows, in the initial period of irradiation of cis-4

an eﬃcient photoisomerization to trans-4 takes place (in accor-

dance with the interpretation of the surprising ﬂuorescence

properties of isomers 4), which is followed by a gradual disap-

pearance of the longer-wavelength band, while a moderate

increase of the shorter-wavelength band (below 250 nm) can

be observed (Figure 6a). The latter suggests the increase of the

absorption assigned to the phenyl ring independently of the

butadiene backbone. In the case of the trans-4 as starting material,

no isomerization can be observed on the basis of the spectral

change, only the gradual disappearance of the longer-wavelength

band accompanied by a slight decrease of the absorption band at

higher energies. The quantum yield of the disappearance at

313 nm irradiation is quite high (0.75) for cis-4, whereas it is

signiﬁcantly lower, although still appreciable, for trans-4 (0.31).

The corresponding quantum yields at λ_ir= 350 nm are similar but

somewhat lower (0.29 and 0.22). Since only the cis-isomer

displays inclination to thermal electrocyclization as quite recently

observed,^43,44such a reaction may signiﬁcantly contribute to

the higher quantum yield for cis-4. Also polymerization can occur

in the case of 4, due to the vinyl substituent. Since the ﬂuorescence

quantum yields of these derivatives are about 2 orders of mag-

nitude lower than those of the photochemical disappearance,

emission from the S₁excited state does not eﬃciently compete

with the latter process in these cases. The sum of the quantum

yields regarding the ﬂuorescence and the photochemical disap-

pearance is signiﬁcantly lower than unity, especially in the case of

Figure 8. Structure of photoproduct 8 with proton assignments (AꢀF).

Figure 9. Spectral change during the irradiation of (a) cis,cis-5 in n-hexane after 0, 10, 20, 40, 80, 160, 320, 640, 1280, 2560, 5120 s; (b) cis,trans-5 in

n-hexane after 0, 10, 20, 40, 80, 160, 320, 640, 1280, 2560 s; and (c) trans,trans-5 in n-hexane after 0, 10, 20, 40, 80, 160, 320, 640 s (λ_ir= 313 nm, l = 1 cm).

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Figure 10. Transient absorption spectrum of trans,trans-5 recorded at 150 (b) and 3000 ns (2) after the excitation and the estimated spectrum of the

intermediate (dash-dot line) along with the spectrum of the starting material (solid line) and that of the ﬁnal product (dashed line) (a). The decay curves

at 305 nm (b) and 400 nm (c) are also displayed.

the trans-isomer, indicating that other processes, such as internal

conversion (IC), may play important role as decay channels of

the excited-state molecule. A similar phenomenon was recently

observed for the trans,trans-isomer of DPB.⁴⁵

Time-resolved absorption measurements gave some hints

regarding the photochemical decay routes of the excited states.

Although all derivatives in this study were investigated by laser

Figure 11. Structure of photoproduct exo,endo-13 with proton assign-

ﬂash photolysis, only the all-trans-isomers gave appreciable signals,

ments (AꢀF).

similarly to the observations on 1,4-diphenylbutadienes.¹¹Flash

photolysisexperiments withtrans-4 resulted insigniﬁcanttransient

absorption changes, mostly bleaching in the wavelength range of

the starting compound (Figure 7). Because of the strong absorp-

tion of the initial derivative, the estimation of the transient

absorption spectrum (see in the Experimental Section) is very

uncertain. It shows a band at 320ꢀ350 nm (Figure 7a), and its

lifetime is 270 ( 20 ns (Figure 7b). This transient might be

attributed to the formation and decay of the triplet excited state of

trans-4; however, according to the previous studies on the parent

compound (DPB), the intersystem crossing in that case is very

ineﬃcient.^4,11

The triplet state of DBP in solution displayed a wide band (in

the range of 360ꢀ420 nm) centered at about 390 nm,¹¹and its

lifetime was determined to be 1.6 μs.^4,11On the basis of the

absorption band, the transient in our work (Figure 7a) may be

assigned to the triplet state, but its lifetime is much shorter than

that observed for the triplet state of its parent compound. The

ﬂuorescence eﬃciencies of the isomers of 4 are rather low (0.021

and 0.003 for the cis- and trans-isomers, respectively), whereas

the quantum yields of their photochemical disappearance are

appreciably high (0.75 and 0.31 at λ_ir= 313 nm, 0.29 and 0.22 at

λ_ir= 350 nm), indicating that the singlet excited state much more

eﬃciently decays via chemical conversion than through radiative

pathways. Since on the basis of the observations regarding DPB

the triplet formation without appropriate sensitizer is not favorable

and it has a relatively long lifetime (1.6 μs),^4,11the short-lived

transient observed for trans-4 may be assigned to a radical

intermediate. This short-lived species decays to form a long-lived

one with a lifetime of 290 ( 30ms. Its molar absorbanceat310 nm

is lower than that of the former intermediate, as seen in Figure 7c.

Also this long-lived species may be a biradical, as shown later in

Scheme 3.

On preparative irradiation of 2-methyl-1-(o-vinylphenyl)-

4-phenylbutadiene derivative (4) at 350 nm, cyclohexadiene

derivative 8 was found and isolated as the main photoproduct

as a result of an electrocyclic ring-closure process characteristic

for 1,3,5-hexatriene (Scheme 3).^3,46The proposed reaction

mechanism for transformation of the o-vinyl derivative 4 involves

a six-membered ring closure followed by a sigmatropic 1,5-H

shift to give 8. A primary photochemical process observed after

very short irradiation time is cisꢀtrans isomerization giving

preferably the trans-4 isomer (existing in diﬀerent conforma-

tions; Scheme 1 and 4) followed by competitive processes from

two diﬀerent conformations of trans-4 (Scheme 4).

After the complete conversion from the favored conformation,

the main photoproduct 8 was obtained with the yield of 52% at

350 nm and less selectively with 34% yield at 313 nm and

separated by column chromatography on silica gel, and its

structure was deduced unequivocally from spectral studies. In

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Figure 12. Spectral change during the irradiation of (a) cis,cis-6 in n-hexane after 0, 10, 20, 40, 80, 160, 320, 640, 1280 s; (b) cis,trans-6 in n-hexane after 0,

10, 20, 40, 80, 160, 320, 640, 1280 s; and (c) trans,trans-6 in n-hexane after 0, 10, 20, 40, 80, 160, 320, 640, 1280, 2560 s (λ_ir= 313 nm, l = 1 cm).

the last fractions minor product 9, 7-methyl-6-phenyl-6,9-

dihydro-5H-5,9-methano-benzocycloheptene, was isolated in

traces at 350 nm but in 11% yield at 313 nm. Irradiation at

313 nm excites also the conformer that leads to 8, but less

intensively than the 350 nm irradiation. It is obvious that the

350 nm excitation excites more intensively the other conformer

than the shorter wavelength favoring the formation of minor

product 9.

was identiﬁed according to the very well resolved four-proton

pattern in the H NMR spectrum and by the characteristic

1

couplings for this type of rigid bicyclo[3.2.1]octadienes as

published by us earlier in the photochemical reactions of β-

heteroaryl-substituted o-divinylbenzenes^15ꢀ18and unsubstituted

butadiene derivatives.¹⁴It might be assumed that even if the 1,4-

ring closure⁴¹to benzobicyclo[2.1.1]hexene derivative 11 is also

operating from 10, the formed derivative 11 could thermaly

reverse to 10 and subsequently give the more stable product 9.

Possible photoproduct 12 (Scheme 4) having benzobicyclo-

[3.1.0]hexene structure has not formed.

Regarding the mechanistic considerations, the formation of

the six-membered ring in 8 is obviously preferable and more

plausible due to the steric hindrance of the methyl group²⁷in

comparison with the photochemistry of unsubstituted derivative

1 giving benzobicyclic structure 2 as the only photoproduct

(Figure 1). The initial intramolecular photocycloaddition leading

to the bicyclic structure 9 is a less favorable process, while the

initial 6π electrocyclization leading to 8 is the dominant one. We

assume that due to the dramatic changes induced by the steric

eﬀect of the methyl group in the excited state of trans-4 the

resonance through the nonplanar part of the molecule, which

contains the methyl group, is generally not favorable (especially

at 350 nm) even if it is formed from the more preferred con-

former of trans-4.

1

From the H and ¹³C NMR spectra of 8, using diﬀerent

techniques (COSY, NOESY, HSQC, and HMBC), all protons

and carbons were completely assigned. As it can be seen from the

structure (Figure 8) the photoproduct 8 contains one pair of

geminal protons (A and B), allyl proton C, and vinyl proton D,

which occur in the ¹H NMR spectrum between 3.5 and 3.7 and

4.4ꢀ4.9 ppm, respectively. Its ¹³C NMR spectrum shows four

singlets, 11 aromatic and 1 aliphatic doublet, 1 triplet, and 1

quartet. Additionally, the MS spectra were very informative. The

main photoproduct 8 had the same molecular ion m/z 246 as the

starting compound 4. The byproduct 9 in the photoreaction of 4

had also molecular ion m/z 246 and was formed as a result of

intramolecular cycloaddition process from the other conforma-

tion of trans-4 (Scheme 4).

Minor quantities of byproduct 9, having benzobicyclo[3.2.1]

octadiene structure, were formed by initial intramolecular cy-

cloaddition and formation of resonance-stabilized intermediate

10, followed by 1,6-ring closure and rearomatization (Scheme 4).

Product 9 was seen in enriched chromatographic fractions and

In an eﬀort to study the inﬂuence of prolonged conjugation in

our systems on the course of the photoreaction, we introduced in

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Figure 13. Transient absorption spectra of trans,trans-6 recorded at 80 (b), 150 (9), and 6000 ns (2) after the excitation and the estimated spectrum of

the short-lived intermediate (dash-dot line) along with the spectrum of the starting material (solid line) and that of the ﬁnal product (dashed line) (a).

The decay curves at 255 nm (b) and 310 nm (c) are also displayed.

Figure 14. Two-stage (biexponential) transient absorption signal

(decay curve) of trans,trans-6 at 300 nm on a longer time scale.

Figure 15. Molecular structure of (a) trans,trans-6 and (b) cis,cis-5.

the compound 1 a second styryl substituent at the β-position of

the vinyl group, gaining new dibutadiene derivative 5. Photolyses

of these isomers display diﬀerent characteristics at the initial

period of irradiation, depending on the type of the starting

isomer. Whereas in the case of cis,trans-5 an eﬃcient photo-

isomerization to cis,cis-5 can be observed (Figure 9b), no spectral

change indicates the geometrical isomerization of either the cis,

cis-5 (Figure 9a) or the trans,trans-5 (Figure 9c) derivative. This

phenomenon suggests that the 313 and also 350 nm excitation

does not promote eﬃcient isomerization of trans,trans-5 through

cis,trans-5 to cis,cis-5, probably due to the too high activa-

tion barrier between the trans,trans and cis,trans geometries.

While the central o-dimethine moiety is less strained in the cis,cis

conﬁguration, due to the presence of methyl groups in 6, the overall

steric strain is smaller for trans,trans-isomer.

Surprisingly, however, the ﬂuorescence spectra in Figure 3

indicate eﬃcient adiabatic photoisomerization of cis,cis-5 to

trans,trans-5. The spectral change at longer irradiation times

indicates disruption of the long-range conjugation and the

increased part of the structural elements absorbing at shorter

wavelengths. The quantum yields of the photoinduced disapper-

ance at λ_ir= 313 nm are relatively low for the cis,cis- and cis,trans-

isomers, Φ = 0.14 and 0.11, respectively. These similar values of

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Figure 16. Spectral change during the irradiation of (a) cis-7 in n-hexane after 0, 10, 20, 40, 80, 160, 320, 640, 1280, 2560, 5120 s; and (b) trans-7 in

n-hexane after 0, 10, 20, 40, 80, 160, 320, 640, 1280, 2560, 5120 s (λ_ir= 313 nm, l = 1 cm).

Table 2. Quantum Yields for the Photolysis of Butadiene

isomers. In the case of 5, intramolecular cycloaddition reaction is

the favorable photochemical path giving, besides exo,endo-13, the

other stereoisomer endo,endo-13 in traces. The formation of 13 is

explained by the same mechanism (Scheme 5) as described for 9

(Scheme 4), from the preferred conformation of trans,trans-5 via

a 1,4-biradical 14 followed by the preferred cyclohexene ring

closure to 15 and photochemically allowed suprafacial 1,3-H

shift. The preferred ring closure with endo-orientation of the

phenyl group can be ascribed to the stabilization of the transition

state in an endo-orientation by the strong attractive intramole-

cular πꢀπ interactions of the benzo-phenyl groups, in both

photoproducts.^47ꢀ49The stereoselective formation of exactly the

exo,endo-13 with the exo-orientation of the unreacted styryl

group can be explained (Scheme 5) by preferred trans ring

closure to the indane intermediate 14, presumably because of

steric reasons, which then closes to yield exo,endo-13 via 15.

The formation of the endo,endo-isomer, found only in traces,

was explained by cis ring closure to the indane intermediate 14⁰,

which via 15⁰gave endo,endo-13. The proposed mechanism

(Scheme 5) suggests the formation of the intermediate 14 from

the trans,trans-5, which is in accordance with the photophysics

showing that the all-trans conﬁguration of 5 contributes the most

to the relaxation via photochemical and internal conversion

processes. Upon irradiation with higher energy more cis,exo,

endo-13 was formed conﬁrming that the further transꢀcis

photoisomerization on the styryl group took place, after primary

formation of trans,exo,endo-13.

Derivatives in Argon-Saturated n-Hexane (λ_ir= 313 nm)^a

compound^b

c-4

t-4 c,c-5 c,t-5 t,t-5 c,c-6 c,t-6 t,t-6

c-7

t-7

Φ

0.75 0.31 0.14 0.11 0.29 0.13 0.16 0.13 0.013 0.040

^aStandard deviation: ( 7%. ^bAbbreviations: c for cis, t for trans.

the quantum yield are in accordance with the fast isomerization of

cis,trans-5 to cis,cis-5, followed by the disappearance. For trans,

trans-5 the photochemical quantum yield is much higher (Φ =

0.29) than those for the other two isomers, indicating higher

photoactivity of this derivative. Similar eﬃciency values, although

a bit lower, were obtained for the 350 nm photolyses of these

isomers (e.g., 0.11 for trans,trans-5). Connecting to this fact, the

absorption spectra at the end of the photolyses (Figure 9)

suggest that the composition of the ﬁnal products from trans,

trans-5 are diﬀerent from those of the other two isomers. These

observations are in accordance with the results of product

analysis indicating that the conformers of the trans,trans-isomer

are favored for the formation of the ﬁnal products (see later in

Scheme 5).

Upon excitation of trans,trans-5 with a 5-ns laser pulse at

355 nm, a transient absorption of 450-ns lifetime was observed

(Figure 10). On the basis of its relatively long lifetime and

estimated absorption spectrum compared to those of the starting

compound and the ﬁnal product, this transient might be assigned

to the triplet excited state of the initial compound. However,

similarly to the case of trans,trans-4, the low eﬃciency of the ISC

for DPB suggests the formation of another type of intermediate,

probably a radical (see later in Scheme 5). The very short lifetime

of the singlet excited state of trans,trans-5 (τ_F= 0.2 ns) suggests

that this reaction can take place during the laser pulse. The single-

exponential decay of this intermediate indicates monomolecular

reactions producing further intermediates or ﬁnal products.

During the preparative photochemical experiment, introduc-

tion of the second styryl moiety results in the formation of

bicyclo[3.2.1]octadiene structure 13, the trans,exo,endo-isomer

being predominant in 49% yield at 350 nm besides 10% of

the corresponding cis-isomer (Scheme 5). Upon irradiation at

313 nm 20% of trans- and 30% of cis,exo,endo-13 was detected

according to NMR analyses followed by traces of endo,endo-13

After the complete conversion, the exact main photoproduct

trans,exo,endo-13 was separated and isolated by column chroma-

tography on silica gel, and its structure was deduced unequi-

vocally from spectral studies. From the ¹H and ¹³C NMR spectra

of exo,endo-13, using diﬀerent techniques (COSY, NOESY, and

HSQC), all protons and carbons were completely assigned. It can

1

be seen the very well resolved four-proton pattern in the H

NMR spectrum between 3.2 and 4.1 ppm unmistakably points to

the bicyclo[3.2.1]octadiene structure. The exo-orientation of the

styryl at the methano bridge carbon is deﬁned on the basis of

COSY interaction between protons E and F (Figure 11). In

previously obtained bicyclo[3.2.1]octadiene derivatives,^16,21

with no substituent on the methano bridge carbon, the proton

E couples only with the exo-oriented proton F. As the endo-

oriented proton does not couple with the proton E, the exo-

structure was assigned. The trans-conﬁguration on the styryl

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Scheme 4. Proposed Mechanism for the Photochemistry of o-Vinylbutadiene 4

substituent on the methano bridge is conﬁrmed by the existence

of two coupling constants with 16.1 Hz for protons 1 and 2

(Figure 11). The signals in the lower ﬁeld between 5.4 and

6.4 ppm are assigned to the A and B protons on double bond.

One of the aromatic protons, H_arof the benzo moiety is shifted to

the higher ﬁeld at 6.2 due to the anisotropic eﬀect of phenyl endo-

substituent. The cis-conﬁguration on the styryl substituent of the

other exo,endo-stereoisomer (Figure 11) is conﬁrmed by the

existence of two coupling constants with 11.9 Hz for protons 1

and 2. The small quantitiy of the diastereomer endo,endo-13 is

also well recognizable in ¹H NMR spectrum.

By introducing two methyl groups, both on the same double

bond of the butadiene moiety to get the new starting material 6,

the inﬂuence of the substituent could be studied under the same

conditions of intramolecular photochemical reaction as in the

case of the starting compound 5. In the initial period of

irradiation, the behavior of the methyl-substituted derivatives 6

displays just the opposite tendency as that shown by the isomers

of 5. As seen in Figure 12, both cis,cis-6 and cis,trans-6 are

eﬃciently transformed to trans,trans-6. This dramatic diﬀerence

is caused by the methyl groups on the butadiene backbones,

signiﬁcantly reducing the extent of conjugation, leading to a

nonplanar structure, also because of crowding. Thus, these

molecular stuctures are less stable and more ﬂexible than those

of the derivatives 5, making the photoinduced geometrical

isomerization of both cis,cis-6 and cis,trans-6 possible. In this

case the trans,trans-isomer is favored in the photostationary

equilibrium. Accordingly, the quantum yields for the photoche-

mical disappearance, from the quasi-photosationary state, are very

similar at λ_ir= 313 nm (Table 2). The quantum yields at λ_ir= 350 nm

are also similar to each other, with an average value of 0.10.

Deviating from the case of derivatives 5, the methyl groups in the

isomers of 6 hamper the decay of the excited state via ﬂuores-

cence (Φ_F<0.001), promoting vibrational energy dissipation

due to the less rigid structure.

In ﬂash photolysis experiments with trans,trans-6, several

transient absorptions of signiﬁcantly diﬀerent lifetimes were

observed. Similarly to the corresponding unsubstituted deriva-

tive (trans,trans-5), on the nanosecond time-scale a short-lived

transient of 150-ns lifetime was recorded, which might be

attributed to the formation and decay of triplet excited state

(Figure 13). However, similarly to the cases of trans,trans-4 and -

5, this transient may be assigned to another type of intermediate,

e.g., a radical species. The negligible ﬂuorescence of trans,trans-6

and the moderate quantum yields of its photochemical disap-

pearance (Φ = 0.13 at λ_ir= 313 nm and 0.09 at λ_ir= 350 nm)

suggest an appreciably eﬃcient formation of this short-lived

intermediate.

On a longer time scale, however, two other transients were

observed with 50-μs and 15-ms lifetimes, respectively (Figure14).

On the basis of the lifetime and the change of the absorption, the

decay of the latter intermediate probably leads to the formation of

the ﬁnal product in the photochemical reaction of the starting

compound.

Contrary to the unsubstituted dibutadiene derivative 5, intro-

duction of the two methyl substituents resulted in diﬀerent

photoreaction of the starting molecule 6, showing only photo-

isomerization upon irradiation (Scheme 6). On irradiation of the

mixture of isomers of 6, under the same conditions as for the

starting material 5, a diﬀerent ratio of the geometric isomers of 6

was obtained. During the column chromatography in the ﬁrst

fractions trans,Z-6 was isolated followed by trans,trans-6 in the

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Scheme 5. Proposed Mechanism for the Formation of the

Bicyclo[3.2.1]octadiene Structures 13

Scheme 6. Mechanism Proposed for the Photochemistry of 6

o-dimethine moiety is nearly planar (α-methine carbons deviate

less than 0.01 Å from the ring plane), the hydrogen atoms bound

to them are twisted signiﬁcantly out of plane. The distances

between α-methine hydrogen atoms are 2.931 Å in trans,trans-6;

in two symmetry-independent molecules of cis,cis-5 they are

2.463 and 2.521 Å. Torsion angles between the two α-methinic

CꢀH bonds are larger for cis,cis-5 (86.0ꢀ and 84.5ꢀ, respectively)

than for trans,trans-6 (64.8ꢀ). Since methyl groups in trans,

trans-6 create very little steric strain, there is no reason to believe

that geometry of trans,trans-5 would diﬀer signiﬁcantly, especially

in the central part of the molecule. However, the methyl groups

in 6 complicate the situation. In a ﬁctive model obtained by

attaching methyl groups on the cis,cis-5 methyl carbons would

come as close as 2.6 Å to carbon atoms of the central aromatic

ring, making the cis,cis conﬁguration prohibitively strained.

Therefore, trans,trans is energetically the most favorable conﬁg-

uration of 6. An additional steric eﬀect of the methyl groups can

be noticed: the forcing of the terminal phenyl group out of the

plane of the butadiene moiety. In trans,trans-6 it is rotated relative

to butadiene by 24.6ꢀ (Figure 15a), whereas in cis,cis-5 the

rotation is less than 5ꢀ.

If the experiment is performed at 350 nm from the starting

compound 6 (as a mixture of isomers) but in the presence of

iodine, 2,7-dimethyl-1,8-diphenylphenanthrene (17) was formed

as a result of complete electrocyclization process from cis,cis-6. The

structure of this symmetrical condensed aromatic system was

obviousfromthepresence of thecharacteristic aromatic protonsof

phenanthrene moiety. As it is seen in the ¹H NMR spectrum, one

doublet and one doublet of doublets appeared between 8.8 and

9.0 ppm with one signal for the methyl group in the lower

magnetic ﬁeld at 2.67 ppm. In the MS spectrum of 17⁰, molecular

ion m/z 360, 2 mass units lower than that of the starting

compound 6, indicated that an electrocyclization process and loss

of a hydrogen molecule occurred.

last fractions, and the high-molecular-weight products remained

on the column. Although only geometric isomerization was

indicated by the product analysis, photoinduced disappearance

of 6 from the quasi-photostationary state was observed in the 313

and the 313 nm photolyses with moderate quantum yields

(average values of 0.14 and 0.10, respectively). This phenome-

non may be attributed to photoinduced polymerization, which

may easily occur between such dibutadiene units.

Regarding the mechanistic considerations,⁵⁰the cis,cis-isomer

of 6 being less preferred in the photostationary equilibrium

during irradiation cyclizes to give intermediate 16, which could

be detected by laser ﬂash photolysis and has a lifetime of several

milliseconds (see Figure 14). Rearomatization of this intermedi-

ate to the starting compound 6 gives back the mixture of

geometric isomers of 6, in our case, trans,Z-6 and trans,trans-6.

This unique behavior of the methyl-substituted dibutadiene 6

could be ascribed to nonplanarity and diﬀerent geometry of the

starting material in comparison with 5, also resulting in the

absence of cycloaddition and/or electrocyclization reaction of 6.

The steric inﬂuence of methyl groups is illustrated by compari-

son of the two most stable conﬁgurations (according to the

observations at the beginning of the photolyses, Figures 9 and

12), cis,cis-5 and trans,trans-6, whose structures were conﬁrmed

by X-ray structure analysis (Figure 15). Both compounds reveal

molecular symmetry C₂(in cis,cis-5 the symmetry is approximate,

whereas trans,trans-6 possesses a crystallographic 2-fold rotation

symmetry; details given in Supporting Information), and the

conformations are twisted due to steric crowding of two α-

methine groups bound to the central aromatic ring. While the

Photolyses of derivatives 7 (Figure 16) display a similar

tendency as was observed for compounds 4. At the initial period

of irradiation cis-7 is transformed into trans-7, clearly indicated by

the shift of the main band, whereas in the case of trans-7 as

starting material apparently no geometrical isomerization takes

place. The main band gradually disappears during the irradiation

as in all photolyses of the compounds in this study, indicating the

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Scheme 7. Photochemical Reactions of Compound 7

phenylnaphthalenes 18 and 19, with the former being predominant.

As shown in the Scheme 6, photocyclization can occur to the

substituted phenyl ring from trans,Z-7 to give 18 and to the

unsubstituted phenyl ring from cis-7 to give 19. The photoreaction

of 7 gave the phenylnaphthalene 18 in 52% of isolated yield

besides traces of 19, along with the formation of some dimeric and

polymeric products, which were not further investigated. The

formation of both phenylnaphthalene 18 and 19 can be explained

by electrocyclization process from diﬀerent conformations of cis

conﬁguration (Scheme 7) followed by an aromatization step with

concomitant loss of hydrogen.^51,52

Figure 17. Transient absorption spectra of trans-7 recorded at 100 ns

(b) and 3000 ns (2) after the excitation and the estimated spectrum of

the short-lived intermediate (dash-dot line) along with the spectrum of

the starting material (solid line) and that of the ﬁnal product (dashed

line). The decay curve at 335 nm (insert) is also displayed.

The photoproduct 18 was isolated by column chromatogra-

phy and characterized spectroscopically. The structure was

obvious from the presence of two characteristic singlets of

1

break of the long-range conjugation. The increase of absorbance at

shorter wavelengths indicates the formation of new compounds

containingless aromatic and conjugated parts. The quantum yields

of the photoinduced disappearance of the starting derivatives 7 are

rather low (0.013 for the cis- and 0.040 for the trans-isomer at

λ_ir= 313 nm, 0.42, and 0.030 at λ_ir= 350 nm), similarly to the

ﬂuorescence eﬃciencies.

aromatic protons of the naphthalene moiety in the H NMR

spectrum characteristic only for 18. In the ¹H NMR spectrum of

some enriched chromatographic fractions traces of the recogniz-

able naphthalene pattern of 19 have been seen but in too small

quantities to be better identiﬁed. Additionally, MS spectra were

very informative. Molecular ions m/z 244 for both cyclization

products, 2 mass units lower than for starting compounds 7,

indicated that an electrocyclization process and loss of hydrogen

molecule happened. These data clearly revealed that photocycli-

zation occur to the substituted phenyl ring from trans,Z-7.

It should be noted that in the absence of I₂only geometric

isomerization takes place upon irradiation of 7. This observation

is in accordance with the very low quantum yields (in the order of

hundredths) for the photoinduced disappearance of these iso-

mers (from the quasi-photostationary equilibrium), which may be

attributed to polymerization, similarly to the case of 6. However,

the chance for polymerization of 7 is much lower because of the

presence of one instead of two butadiene units. This is manifested

in the 1 order of magnitude lower quantum yields.

This phenomenon is in accordance with the nonplanar, more

ﬂexible structure with smaller extent of conjugation due to the

eﬀect of the methyl group on the butadiene moiety, promoting

the nonradiative decay (energy dissipation) of the excited state.

Similarly to the other derivatives in this study, ﬂash photolysis

experiments with trans-4 resulted in signiﬁcant transient absorp-

tion changes in the nanosecond time scale (Figure 17a). Also in

this case, on the basis of its lifetime (280 ( 30 ns, Figure 17b) and

estimated absorption spectrum compared to those of the starting

compound and the ﬁnal product (Figure 17a), this transient

might be assigned to the triplet excited state of the initial

compound. Similarly to the previous cases, however, on the basis

of the ineﬃcient ISC, this transient may be attributed to a

chemically formed species, e.g., a radical or an isomer intermedi-

ate (see in Scheme 7). The rather short lifetime of the singlet

excited state of trans-7 (τ_F= 1.09 ns) indicates that the formation

of this species can take place within the 5-ns duration of the laser

pulse. The low ﬂuorescence quantum yield (0.02) of this isomer

suggests an eﬃcient formation of this intermediate.

’ CONCLUSIONS

The versatile photoinduced behavior of butadiene derivatives

has been revealed by the diverse photochemistry and photo-

physics of four compounds of rather similar structures. The

results reveal that the introduction of a substituent in the parent

compound 1 changes the course of the photoreaction in several

instances. The introduction of the second styryl substituent at

the β-position of the vinyl group of 1 does not inﬂuence the

cycloaddition reaction qualitatively; however, the compound 5

gives the main benzobicyclo photoproduct in a strongly lowered

yield in comparison with 1. Introduction of a methyl group at the

The preparative photochemical reaction of the model com-

pound 7, methyl analogue of 2-methyl-1-(o-vinylphenyl)-4-phe-

nylbutadiene derivative 4, gave substituted arylnaphthalenes 18

and 19 as is expected under aerobic conditions (Scheme 7).^12,13

Irradiation of 3 ꢁ 10^ꢀ3M solution of 7 (as mixture of isomers) in

dry toluene containing iodine gave, after 15 h, the mixture of

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second double bond of 1 leads to the formation of electrocycliza-

tion product 8, whereas two methyl groups introduced to 5,

causing nonplanarity and steric crowding, totally hinders the

photocycloaddition in 6, resulting in geometric isomerization as

the only photochemical reaction. These dramatic eﬀects of the

methyl group on the photochemical features are in accordance

with those on the photophysical properties due to the reduction

of the conjugation and planarity of the molecule. Since methyl

substituent(s) had no direct electronic inﬂuence on the reaction,

the diﬀerences between the reactions of the individual com-

pounds had to be ascribed to steric factors. Furthermore, it can be

considered that variations in the ground-state conformations of

the compounds investigated are responsible for their diverse

photobehavior. In the case of the studied butadiene systems, the

excitation of conformers of each derivative at the two excitation

wavelengths is probably selective but without considerable

photochemical and photophysical consequences. Understanding

of the governing eﬀects of the substituents on the photochemical

reactions of these new butadiene derivatives and elucidation of

their reaction mechanisms make the synthesis of these systems

suitable for directing the photoreaction to diﬀerent interesting

structures that can be further transformed and functionalized.

wavelength of its absorption maximum. Luminescence spectra were

corrected for detector sensitivity.

Transient absorption measurements were carried out with a laser ﬂash

photolysis system, which involved a Nd:YAG laser yielding 355 nm

pulses of about 5 ns duration and a digital oscilloscope system. The

molar absorption spectra of the short-lived intermediates detected by

ﬂash photolysis was estimated by compensation of the bleaching. Since

the absorption spectra of the intermediates signiﬁcantly overlap with

those of the corresponding ground-state starting compounds and thus

the concentration of the intermediate formed could not be reliably

determined, a very rough estimation was made for it. In the case of each

compound studied by ﬂash photolysis, at the wavelength at which the

transient absorbance decrease after the light pulse divided by the molar

absorbance of the starting compound gave the highest value, the molar

absorbance of the short-lived intermediate was taken to be zero. Hence,

the absorbance decrease at this wavelength was exclusively estimated by

the bleaching of the starting compound. Although the molar absor-

bances estimated in this way for the intermediate are the minimum

possible values, the positions of the bands have been reliably determined.

Chromatographic separations were perfomed on silica gel columns

(0.063ꢀ0.2 mm) and thin layer plates (silica gel 0.2 mm). All solvents

were distilled prior the use. The starting compounds trans-cinnamalde-

hyde, trans-α-methyl-cinnamaldehyde, and paraformaldehyde were

purchased from a commercial source, and known procedures were

used to prepare o-methylbenzyltriphenylphosphonium bromide from

the corresponding bromide and diphosphonium salt of α,α⁰-o-xylylene-

dibromide.

’ EXPERIMENTAL SECTION

1

The H and ¹³C NMR spectra were recorded in CDCl₃solutions

containing tetramethylsilane as internal standard at 300 or 600 and at 75

or 150 MHz, respectively. The mass spectra were recorded on a

GCꢀMS instrument (GC temperature program: delay 3 min, injector

temperature 350 ꢀC, heating from 110 to 300 ꢀC within 6 min, then

300 ꢀC isothermal for 7 min). High-resolution mass spectra (HRMS)

were obtained on a matrix-assisted laser desorption/ionization time-of-

ﬂight MALDI-TOF/TOF mass spectrometer equipped with Nd:YAG

laser operating at 355 nm with ﬁring rate 200 Hz in the positive ion

reﬂector mode; 1600 shots per spectrum were taken with mass range

100ꢀ1000 Da, focus mass 500 Da and delay time 100 ns. Nicotinamide

and azithromycin were used for external mass calibration in positive ion

mode. Each spectrum was internally calibrated, providing measured

mass accuracy within 5 ppm of theoretical mass. Melting points are

uncorrected. All preparative irradiation experiments were carried out

with about 10^ꢀ3molar solutions in a quartz tube and in a photochemical

reactor equipped with UV lamps. For quantum yield measurements a

photolysis equipment, containing a 200 W XeHg lamp and a mono-

chromator was used.⁵³Irradiation wavelengths were 313 and 350 nm.

Incident light intensity was determined with a thermopile calibrated by

ferrioxalate actinometry.^54,55Typically, the irradiations were carried out

with 3.5-cm³solutions of about 10^ꢀ4M concentration in 1-cm cells at

room temperature. HPLC grade n-hexane were used as solvents in these

experiments. During the photolyses the reaction mixtures were con-

tinuously homogenized by magnetic stirring. Following the change of

the absorption spectrum of the solution photolyzed, the quantum yield

for the photochemical transformation of the starting material was

determined from the initial rate calculated from the absorbance versus

time plot at a characteristic wavelength where the products did not

absorb.

1-(3-Methyl-4-phenylbuta-1,3-dienyl)-2-vinylbenzene (4).

To a stirred solution of α,α⁰-o-xylyl(ditriphenylphosphonium) bromide

(7.380 g, 9.36 mmol) and trans-α-methyl-cinnamaldehyde (1.53 g,

10.30 mmol) in absolute ethanol (100 mL) was added a solution of

sodium etoxide (0.23 g, 10.0 mmol in 20 mL ethanol) dropwise. Stirring

was continued under a stream of nitrogen for 1 h at rt. Under the stream of

nitrogen gaseous formaldehyde (obtained by the decompostion of an

excess of paraformaldehyde, 1.41 g, 46.7 mmol) was introduced, and an

additional quantity of sodium etoxide (0.23 g, 10.0 mmol in 20 mL

ethanol) was added. After removal of the solvent, the residue was worked

up with water and benzene. The benzene extract was dried and concen-

trated. The crude reaction mixture was purified, and the mixture of

isomers was isolated in 65% yield (24% cis- and 76% trans-isomer

according to ¹H NMR). The mixture of isomers was separated by column

chromatography on silica gel using petroleum ether as eluent.

cis-4: colorless oil; UV (n-hexane) λ_max/nm (ε/dm³mol^ꢀ1cm^ꢀ1

)

279 (13456); ¹H NMR (CDCl₃, 600 MHz, ppm) δ 7.52 (d, 1H, J = 7.7

Hz, H_ar), 7.33ꢀ7.27 (m, 3H, H_ar), 7.21ꢀ7.16 (m, 3H, H_ar), 7.13ꢀ7.11

(m, 1H, H_ar), 7.03ꢀ7.00 (m, 1H, H_ar), 6.95 (dd, 1H, J₁₃= 17.6 Hz, J₂₃

=

10.9 Hz, H₃), 6.54 (d, 1H, J = 12.1 Hz, H-A/B), 6.52 (broad s, 1H,

HꢀC), 6.42 (d, 1H, J = 12.1 Hz, H-A/B), 5.70 (dd, 1H, J₁₃= 17.6 Hz, J₁₂

= 1.1 Hz, H₂), 1.68 (d, 3H, J = 1.2 Hz, CH₃); ¹³C NMR (CDCl₃, 150

MHz, ppm) δ 136.4 (d), 135.3 (d), 132.7 (d), 129.1 (2d), 128.1 (d),

128.0 (2d), 127.9 (d), 127.6 (d), 127.1 (d), 126.5 (d), 125.0 (d), 115.0

(t), 17.8 (q), singlets are not seen due to small quantities; MS m/z (EI)

246 (M⁺, 100%), 231 (20), 117 (30).

trans-4: colorless oil; UV (n-hexane) λ_max/nm (ε/dm³mol^ꢀ1cm^ꢀ1

)

315 (29537); ¹H NMR (CDCl₃, 600 MHz, ppm) δ 7.52 (d, 1H, J = 7.6

Hz, H_ar), 7.46 (d, 1H, J = 7.6 Hz, H_ar), 7.38ꢀ7.32 (m, 5H, H_ar),

7.28ꢀ7.24 (m, 2H, H_ar), 7.07 (dd, 2H, J₁₃= 17.5 Hz, J₂₃=11.1 Hz, H₃),

6.94 (d, 1H, J = 16.2 Hz, H-A/B), 6.85 (d, 1H, J = 16.2 Hz, H-A/B), 6.66

(broad s, 1H, HꢀC), 5.64 (dd, J₁₃= 17.5 Hz, J₁₂= 1.3 Hz, H₁), 5.35 (dd,

J₂₃= 11.1 Hz, J₁₂= 1.3 Hz, H₂), 2.14 (d, 3H, J = 1.0 Hz, CH₃); ¹³C NMR

(CDCl₃, 150 MHz, ppm) δ 137.3 (s), 136.0 (d), 135.8 (s), 135.6 (s),

135.4 (s), 134.6 (d), 131.9 (d), 128.8 (2d), 127.7 (2d), 127.3 (d), 126.9

(d), 126.2 (d), 126.1 (d), 125.6 (d), 125.1 (d), 115.8 (t), 13.5 (q); MS

m/z (EI) 246 (M⁺, 100%), 231 (25), 117 (20); HRMS (TOF ES⁺) m/z

Absorption spectra were recorded by using a double-beam and a

diode array spectrophotometer. For the measurement of ﬂuorescence

spectra a high-sensitivity spectroﬂuorimeter was applied. This equip-

ment supplemented with a time-correlated single-photon counting

accessory was applied for determination of ﬂuorescence lifetimes too.

Ru(bpy)₃Cl₂,⁵⁶quinine bisulphate (in 0.5 M H₂SO₄), and 9,10-diphenyl-

anthracene⁵⁷were utilized as references for determination of the

ﬂuorescence quantum yields. Each compound studied was excited at the

8654

dx.doi.org/10.1021/jo200691x |J. Org. Chem. 2011, 76, 8641–8657

The Journal of Organic Chemistry

ARTICLE

calculated (for the mixture of geometrical isomers of 4) for C₁₉H₁₈

246.0910, found 246.0914.

126,8 (d), 126,2 (d), 126,1 (d), 126,0 (d), 125,9 (d), 124,4 (d), 17,4 (q),

13.4 (q); MS m/z (EI) 362 (M⁺, 100%).

trans,trans-6: white crystals; mp 93ꢀ95 ꢀC; UV (n-hexane) λ_max/nm

(ε/dm³mol^ꢀ1cm^ꢀ1) 298 (27188); ¹H NMR (CDCl₃, 600 MHz, ppm)

δ 7.52 (m, 2H, H_ar), 7.38ꢀ7.32 (m, 8H, H_ar), 7.24ꢀ7.22 (m, 4H, H_ar),

6.99 (d, 2H, J = 15.9 Hz, H-A/B), 6.87 (d, 2H, J = 15.9 Hz, H-A/B), 6.67

(ꢀsiroki s, 1H, HꢀC), 2.16 (d, 6H, J = 0.9 Hz, CH₃); ¹³C NMR (CDCl₃,

150 MHz, ppm) δ 137.4 (s), 136.1 (d), 135.7 (s), 135.6 (s), 132.0 (d),

128.8 (2d), 127.7 (2d), 126.9 (d), 126.2 (d), 126.0 (d), 125.4 (d), 13.6

(q); MS m/z (EI) 362 (M⁺, 100%); HRMS (TOF ES⁺) m/z calculated

(for the mixture of geometrical isomers of 6) for C₂₈H₂₆362.1414,

found 362.1401.

1,2-Bis(4-phenylbuta-1,3-dienyl)benzene (5) and 1,2-Bis-

(3-methyl-4-phenylbuta-1,3-dienyl)benzene (6). To a stirred

solution of α,α⁰-o-xylyl(ditriphenylphosphonium) bromide (4.472 g,

5.50 mmol) and corresponding aldehydes (trans-cinnamaldehyde and

trans-α-methyl-cinnamaldehyde, 1.5 g, 11.0 mmol) in absolute ethanol

(100 mL) was added a solution of sodium etoxide (0.261 g, 11.35 mmol

in 20 mL ethanol) dropwise. Stirring was continued overnight at rt. After

removal of the solvent on a rotary evaporator, the residue was worked up

with water (20 mL) and benzene (3 ꢁ 30 mL). The benzene extract were

dried over MgSO₄and concentrated. The crude reaction mixture was

purified, and products were isolated in 91% yield (7% cis,cis-, 45% cis,trans-,

and 48% trans,trans-isomer according to ¹H NMR) for 5 (1.488 g) and 81%

yield (10% cis,cis, 50% cis,trans, and 40% trans,trans according to ¹H NMR)

for 6 (1.812 g). The mixture of isomers was separated by TLC or repeated

column chromatography on silica gel using petroleum ether as eluent.

cis,cis-5: white crystals; mp 65ꢀ67 ꢀC; UV (n-hexane) λ_max/nm

(ε/dm³mol^ꢀ1cm^ꢀ1) 285 (6816); ¹H NMR (CDCl₃, 600 MHz, ppm) δ

7.35 (d, 4H, J = 7.7 Hz, H_ar), 7.28 (t, 4H, J = 7.7 Hz, H_ar), 7.23ꢀ7.19 (m,

6H, H_ar), 7.09 (dd, 2H, J = 15.6 Hz, J = 10.8 Hz, H_C), 6.69 (d, 2H, J = 15.6

Hz, H_D), 6.57 (d, 2H, J = 11.2 Hz, H_A), 6.47 (dd, 2H, J = 11.2, J = 10.7 Hz,

H_B); ¹³C NMR (CDCl₃, 75 MHz, ppm) δ 137.8 (s), 136.6 (s), 134.2 (d),

130.4 (d), 130.0 (d), 129.8 (d), 129.7 (d), 128.6 (2d), 127.6 (d), 127.3

(d), 126.5 (d), 125.5 (d); MS m/z (EI) 334 (M⁺, 100%), 206 (15).

cis,trans-5: white crystals; mp 72ꢀ74 ꢀC; UV (n-hexane) λ_max/nm

(ε/dm³mol^ꢀ1cm^ꢀ1) 318 (30120); ¹H NMR (CDCl₃, 600 MHz, ppm)

δ 7.55 (d, 2H, J = 7.6 Hz, H_ar), 7.44 (d, 4H, J = 7.6 Hz, H_ar), 7.33 (t, 4H,

J = 7.6 Hz, H_ar), 7.23 (t, 2H, J = 7.6 Hz, H_ar), 7.21ꢀ7.14 (m, 2H, H_ar),

7.99 (dd, 2H, J = 15.8 Hz, J = 9.0 Hz, H_B, H_C), 6.89 (d, 1H, J = 16.0 Hz,

1-Methyl-2-(3-methyl-4-phenylbuta-1,3-dienyl)benzene (7).

To a stirred solution of o-tolyl-(benzyltriphenylphosphonium)bromide

(7.38 g, 9.26 mmol) and trans-α-methyl-cinnamaldehyde (1.53 g, 10.30

mmol) in absolute ethanol (100 mL) was added a solution of sodium

ethoxide (0.23 g, 10.0 mmol in 20 mL ethanol) dropwise, and the mixture

was stirred for 3 h at rt. After removal of solvent an extraction with benzene

was carried out. The extract was dried and concentrated. The crude reaction

mixture was purified, and the mixture of cis- and trans-isomers of the

compound 7 was separated by column chromatography on silica gel using

petroleum ether as eluent. The product 7 was isolated in 77% yield (27% cis-

and 73% trans-isomer according to ¹H NMR).

cis-7: colorless oil; UV (n-hexane) λ_max/nm (ε/dm³mol^ꢀ1cm^ꢀ1

)

289 (21775); ¹H NMR (CDCl₃, 600 MHz, ppm) δ 7.30 (t, 2H, J = 7.6

Hz, H_ar), 7.24ꢀ7.10 (m, 7H, H_ar), 6.52 (broad s, 1H, HꢀC), 6.49 (d,

1H, J = 12.2 Hz, H-A/B), 6.38 (d, 1H, J = 12.2 Hz, H-A/B), 2.30 (s, 3H,

CH₃), 1.69 (d, 3H, J = 1.2 Hz, CH₃); ¹³C NMR (CDCl₃, 150 MHz,

0

ppm) δ 136.6 (s), 135.0 (d), 132.9 (s), 131.8 (d), 131.7 (s), 129.0 (d),

128.9 (d), 128.6 (2d), 128.0 (d), 127.9 (s), 127.5 (d), 126.5 (d), 125.9

(2d), 124.7 (d), 19.5 (q), 17.3 (q); MS m/z (EI) 234 (M+, 100%), 219

(25), 143 (15), 115 (15).

0

H_A/D/D), 6.88 (d, 1H, J = 12.0 Hz, H_A), 6.87ꢀ6.84 (m, 2H, H_B, H_C),

6.68 (d, 2H, J = 15.8 Hz, H_A/D/D); ¹³C NMR (CDCl₃, 75 MHz, ppm) δ

0

133.4 (d), 132.9 (d), 132.6 (d), 131.4 (d), 130.9 (d), 130.1 (d), 129.7

(d), 129.4 (d), 129.1 (d), 128.7 (d), 128.5 (2d), 128.0 (d), 127.8 (d),

127.4 (d), 127.1 (d), 126.7 (2d), 126.6 (d), 125.9 (d) (singlets are not

seen due to small quantities); MS m/z (EI) 334 (M⁺, 100%), 206 (10).

trans,trans-5: yellow-green crystals; mp 169ꢀ170 ꢀC; UV (n-hexane)

trans-7: white crystals; mp 63ꢀ64 ꢀC; UV (n-hexane) λ_max/nm

(ε/dm³mol^ꢀ1cm^ꢀ1) 312 (29350); ¹H NMR (CDCl₃, 600 MHz, ppm)

δ 7.56 (d, 1H, J = 7.1 Hz, H_ar), 7.40ꢀ7.31 (m, 4H, H_ar), 7.24ꢀ7.12 (m,

4H, H_ar), 6.88 (ABq, 2H, J = 16.2 Hz, H-A, HꢀB), 6.67 (broad s, 1H,

HꢀC), 2.40 (s, 3H, CH₃), 2.15 (d, 3H, J = 1.1 Hz, CH₃); ¹³C NMR

0

1

λ_max/nm (ε/dm³mol^ꢀ1cm^ꢀ1) 303 (23369); H NMR (CDCl₃, 600

(CDCl₃, 75 MHz, ppm) δ 137.9 (s), 136.6 (s), 136.1 (s), 135.6 (s),

135.5 (d), 132.2 (d), 130.4 (d), 129.3 (2d), 128.2 (2d), 127.2 (d), 126.6

(d), 126.2 (d), 125.6 (d), 125.2 (d), 19.9 (q), 14.0 (q); MS m/z (EI) 234

(M+, 100%), 219 (25), 143 (10), 115 (10); HRMS (TOF ES⁺) m/z

calculated (for the mixture of geometrical isomers of 7) for C₁₈H₁₈

234.1050;, found 234.1045.

MHz, ppm) δ 7.59ꢀ7.49 (m, 4H, H_ar), 7.49ꢀ7.42 (m, 4H, H_ar), 7.34 (t,

4H, J = 7.4 Hz, H_ar), 7.25ꢀ7.21 (m, 2H, H_ar), 7.03 (dd, 2H, J = 15.6 Hz, J =

10.3 Hz, H_C), 7.02 (d, 2H, J = 15.1 Hz, H_A), 6.85 (dd, 2H, J = 15.1 Hz, J =

10.3 Hz, H_B), 6.70 (d, 2H, J = 15.6 Hz, H_D); ¹³C NMR (CDCl₃, 75 MHz,

ppm) δ 137.3 (s), 135.6 (s), 133.1 (d), 131.6 (d), 130.3 (d), 129.5 (d),

128.7 (2d), 128.6 (d), 127.6 (d), 126.4 (2d), 126.3 (d); MS m/z (EI) 334

(M⁺, 100%); HRMS (TOF ES⁺) m/z calculated (for the mixture of

geometrical isomers of 5) for C₂₆H₂₂334.1207; found 334.1199.

Photochemistry of Compound 4. A mixture of isomers of 4 in

petroleum ether (6.0 ꢁ 10^ꢀ3M) was purged with argon for 20 min and

irradiated in Rayonet reactor at 313 or 350 nm in a quartz tube for 20 h.

Solvent was removed in vacuum, and the residue was chromatographed

on silica gel column using petroleum ether as eluent. After repeated

column chromatography photoproduct 8 was isolated in the first

fractions in 34% at 313 nm and 52% at 350 nm yield, while the minor

photoproduct 9 was obtained in the last fractions in 11% yield at 313 nm

and in traces at 350 nm according to NMR analyses.

(E)-2-(1-phenylprop-1-en-2-yl)-1,2-dihydronaphthalene (8).

Colorless oil; UV (n-hexane) λ_max/nm (ε/dm³mol^ꢀ1cm^ꢀ1) 256 (5311);

¹H NMR (CDCl₃, 600 MHz, ppm) δ 7.40ꢀ7.08 (m, 10H), 6.77 (d.d., 1H,

J = 5.4; 1.7 Hz, H_F), 6.39 (dd, 1H, J = 5.4; 2.0 Hz, H_E), 4.88 (dd, 1H, J =

9.8; 1.3 Hz, H_D), 4.46 (dt, 1H, J = 9.8; 2.0 Hz, H_C), 3.73 (d, 1H, J =

14.6 H_A/H_B), 3.63 (d, 1H, J = 14.6 H_A/H_B), 1.70 (d, 1H, J = 1.3 Hz, CH₃);

¹³CNMR(CDCl₃, 150 MHz, ppm) δ144.3 (s), 144.1 (s), 139.9 (s), 138.8

(d), 137.1 (s), 131.4 (d), 128.9 (2d), 128.5 (d), 126.7 (2d), 126.1 (d),

125.0 (d), 123.7 (d), 123.3 (d), 121.0 (d), 50.1 (d), 38.4 (t), 23.6 (q); MS

m/z (EI) 246 (M⁺, 100%), 102 (45); HRMS (TOF ES⁺) m/z calculated

for C₁₉H₁₈246.0894, found 246.0899.

cis,cis-6: yellow oil; UV (n-hexane) λ_max/nm (ε/dm³mol^ꢀ1cm^ꢀ1

)

299 (30752); ¹H NMR (CDCl₃, 600 MHz, ppm) δ 7.32ꢀ7.28 (m, 4H,

H_ar), 7.24ꢀ7.21 (m, 6H, H_ar), 7.19ꢀ7.14 (m, 4H, H_ar), 6.53 (broad s,

2H, HꢀC), 6.50 (d, 2H, J = 12,1 Hz, H-A/B), 6.37 (d, 2H, J = 12.1 Hz,

H-A/B), 1.79 (d, 6H, J = 1.2 Hz, CH₃); ¹³C NMR (CDCl₃, 150 MHz,

ppm) δ 137.3 (s), 136.7 (s), 135.1 (d), 135.0 (s), 131.7 (d), 129.0 (d),

128.6 (2d), 127.7 (d), 127.6 (2d), 126.1 (d), 126.0 (d), 17.7 (q); MS m/

z (EI) 362 (M⁺, 100%).

cis,trans-6: yellow oil; UV (n-hexane) λ_max/nm (ε/dm³mol^ꢀ1cm^ꢀ1

)

291 (12229); ¹H NMR (CDCl₃, 600 MHz, ppm) δ 7.60 (d, 1H, H_ar),

7.36ꢀ7.32 (m, 2H, H_ar), 7.31ꢀ7.28 (m, 3H, H_ar), 7.25ꢀ7.21 (m, 6H,

H_ar), 7.20ꢀ7.16 (m, 2H, H_ar), 6.94 (d, 1H, J = 15.9 Hz, H-A⁰/B⁰), 6.90

(d, 1H, J = 15.9 Hz, H-A⁰/B⁰), 6.66 (broad s, 1H, HꢀC⁰), 6.58 (d, 1H, J =

12.2 Hz, H-A/B), 6.56 (broad s, 1H, HꢀC), 6.47 (d, 1H, J = 12.2 Hz,

0

H-A/B), 2.09 (d, 3H, J = 1.0 Hz, CH₃), 1.70 (d, 3H, J = 1.1 Hz, CH₃);

¹³C NMR (CDCl₃, 150 MHz, ppm) δ 137.4 (s), 137,2 (s), 136,9 (s),

136,1 (d), 135,8 (s), 135,4 (s), 135,3 (s), 134,8 (d), 132,3 (d), 131,8 (d),

129,5 (2d), 128,7 (2d), 128,6 (2d), 127,6 (d), 127,5 (2d), 127,4 (d),

8655

dx.doi.org/10.1021/jo200691x |J. Org. Chem. 2011, 76, 8641–8657

The Journal of Organic Chemistry

ARTICLE

Photochemistry of Compound 5. A mixture of isomers of 5 in

petroleum ether (2 ꢁ 10^ꢀ3M) was purged with argon for 20 min and

irradiated at 313 or 350 nm in a Rayonet reactor in a quartz tube for 15 h.

After irradiation the solvent was removed in vacuo, and the oily residue

was chromatographed on silica gel using petroleum ether/diethyl ether

(3%). The major isolated product at 350 nm was bicyclic derivative trans,

exo,endo-13 in 49% yield in addition to 10% of the corresponding

cis-isomer. Upon irradiation at 313 nm, in the first fractions 20% of trans-

and 30% of cis,exo,endo-13 was isolated according to NMR analyses,

followed with traces of endo,endo-13 isomers. High-molecular-weight

products remained on the column.

ppm) δ 7.80 (d, 1H, J = 8.3 Hz), 7.62 (s, 1H), 7.42 (dt, 1H, J = 7.4;

1.2 Hz), 7.39 (d, 1H, J = 7.4 Hz), 7.30ꢀ7.26 (m, 4H), 7.20 (t, 1H, J =

7.4 Hz), 7.17 (d, 1H, J = 1.5 Hz), 2.54 (s, 3H, CH₃), 2.02 (s, 3H, CH₃);

MS m/z (EI) 232 (M⁺, 100%); HRMS (TOF ES⁺) m/z calculated for

C₁₈H₁₆232.1007, found 232.1012.

1-Methyl-2-((1E,3Z)-3-methyl-4-phenylbuta-1,3-dienyl)-

benzene (trans,Z-7). Colorless oil; UV (n-hexane) λ_max/nm (ε/dm³

mol^ꢀ1cm^ꢀ1) 293 (23237); ¹H NMR (CDCl₃, 300 MHz, ppm) δ 7.55

(d, 1H, J = 7.1 Hz), 7.36ꢀ7.31 (m, 4H), 7.20ꢀ7.13 (m, 4H), 6.87 (AB_q,

2H, J = 15.3 Hz), 6.66 (s, 1H), 2.40 (s, 3H, CH₃), 2.15 (s, 3H, CH₃); ¹³

C

NMR (CDCl₃, 150 MHz, ppm) δ 137.4 (s), 136.2 (s), 135.6 (s), 135.1

(s), 134.9 (d), 131.7 (d), 129.9 (d), 128.8 (2d), 127.7 (2d), 126.7 (d),

126.1 (d), 125.7 (d), 125.2 (d), 124.7 (d), 20.7 (q), 19.4 (q); MS m/z (EI)

MS m/z (EI) 234 (M⁺, 100%), 219 (35), 143 (10), 115 (15); HRMS

(TOF ES⁺) m/z calculated for C₁₈H₁₈234.0040, found 234.0049.

6-endo-Phenyl-10-exo-styryl-6,9-dihydro-5H-5,9-metha-

no-benzocycloheptene (trans,exo,endo-13). Colorless oil; UV

1

(n-hexane) λ_max/nm (ε/dm³mol^ꢀ1cm^ꢀ1) 254 (15655); H NMR

(CDCl₃, 600 MHz, ppm) δ 7.24ꢀ7.11 (m, 7H H_ar), 7.06 (dt, 2H, J = 7.4

Hz, J = 1.0 Hz, H_ar), 6.83 (dt, 1H, J = 7.4 Hz, J = 1.0 Hz, H_ar), 6.76ꢀ6.72

(m, 3H, H_ar), 6.50 (d, 1H, J = 16.1 Hz, H₁), 6.44 (ddd, 1H, J = 9.5 Hz, J =

’ ASSOCIATED CONTENT

0

6.7 Hz, 2.5 Hz, H_A), 6.18 (d, 1H, J = 7.3 Hz, H_ar), 6.02 (dd, 1H, J = 16.1

1

Supporting Information. H NMR and ¹³C NMR spectra

Hz, J = 8.7 Hz, H₂), 5.38 (dt, 1H, J = 9.5 Hz, J = 2.5 Hz, H_B), 4.07 (ddd,

1H, J = 4.6 Hz, J = 2.5 Hz, J = 2.5 Hz, H_C), 3.79 (d, 1H, J = 9.7 Hz, H_E),

3.30 (d,1H, J = 4.6 Hz, H_D), 3.23ꢀ3.21 (m, 1H, H_F); ¹³C NMR (CDCl₃,

150 MHz, ppm) δ 150.1 (s), 141.3 (s), 139.5 (s), 139.0 (s), 134.4 (d),

131.9 (d), 129.9 (d), 128.0 (2d), 127.9 (d), 127.3 (2d), 126.6 (d), 126.5

(d), 126.2 (d), 126.0 (d), 125.8 (2d), 125.6 (2d), 124.9 (d), 120.6 (d), 58.5

(d), 54.0 (d), 46.8 (d), 46.1 (d); MS m/z (EI) 334 (M⁺, 100%), 206 (45);

HRMS (TOF ES⁺) m/z calculated for C₂₆H₂₂334.1406, found 334.1412.

Photochemistry of Compound 6. In a quartz tube, the mixture

of isomers of 6 was dissolved in petroleum ether (3.4 ꢁ 10^ꢀ3M), purged

with argon for 20 min, and irradiated at 313 or 350 nm for 15 or 22 h,

respectively. The residue after evaporation of the solvent was chroma-

tographed on a column filled with silica gel using petroleum ether/

diethyl ether (0ꢀ2%) as eluent. After irradiation at 350 nm, in the first

fraction the new geometric isomer trans,Z-6 was isolated (0.032 g, 41%),

followed by the starting trans,trans-isomer of 6 (0.039 g, 50%). At

313 nm the yields of trans,Z-6 and trans,trans-6 were 32% and 45%,

respectively. In both cases, high-molecular-weight products remained on

the column.

S

b

for compounds 4ꢀ8, 13, and 18 and crystallographic data for cis,

cis-5 and trans,trans-6 in CIF format. This material is available

free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION

Corresponding Author

*E-mail: iskoric@fkit.hr; otto@vegic.uni-pannon.hu.

Notes

†

ꢀ

Dedicated to Professor Marija Sindler-Kulyk on the occasion of

her 70th birthday.

’ ACKNOWLEDGMENT

This research was funded by grants from the Ministry of

Science, Education and Sports of the Republic of Croatia (125-

0982933-2926, 098-0982929-2917, and 098-1191344-2943) and

also in the frame of the Hungarian-Croatian Intergovernmental

S&T Cooperation Program for 2009-2011 jointly ﬁnanced by the

Hungarian National Oﬃce of Research and Technology (OMFB-

01247/2009). We would also like to thank Prof. Marija Sindler-

Kulyk and Prof. Axel Griesbeck for valuable comments and

discussion.

1,2-Bis((1E,3Z)-3-methyl-4-phenylbuta-1,3-dienyl)benzene

(trans,Z-6). Yellow oil; UV (n-hexane) λ_max/nm (ε/dm³mol^ꢀ1cm^ꢀ1

)

294 (8989); ¹H NMR (CDCl₃, 600 MHz, ppm) δ 7.39ꢀ7.34 (m, 1H),

7.33 (d, 2H, J = 7.2 Hz), 7.30 (d, 2H, J = 7.2 Hz), 7.22 (t, 1H, J = 7.2 Hz),

7.20 (d, 1H, J = 16.6 Hz), 7.16 (m, 1H), 7.01 (d, 1H, J = 16.6 Hz), 6.57

(s, 1H), 2.14 (s, 3H, CH₃); ¹³C NMR (CDCl₃, 150 MHz, ppm) δ 137.1

(s), 135.8 (s), 134.3 (s), 130.2 (d), 129.2 (d), 128.9 (2d), 127.7 (d), 127.6

(2d), 126.9 (d), 126.2 (d), 126.1 (d), 20.6 (q); MS m/z (EI) 362

(M⁺, 100%); HRMS (TOF ES⁺) m/z calculated for C₂₈H₂₆362.0907,

found 362.0913.

ꢀ

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dx.doi.org/10.1021/jo200691x |J. Org. Chem. 2011, 76, 8641–8657

Article Doi

Synthesis, photochemistry, and photophysics of butadiene derivatives: Influence of the methyl group on the molecular structure and photoinduced behavior

DOI: 10.1021/jo200691x

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