Exploring the Photocyclization Pathways of Styrylthiophenes in the Synthesis of Thiahelicenes: When the Theory and Experiment Meet

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Exploring the Photocyclization Pathways of Styrylthiophenes in the

Synthesis of Thiahelicenes: When the Theory and Experiment Meet

Bianca C. Baciu, Jos é Antonio Verg é s, and Albert Guijarro*

Cite This: J. Org. Chem. 2021, 86, 5668 −5679

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sı Supporting Information

ABSTRACT: The introduction of thiophene rings to the helical structure of

carbohelicenes has electronic eﬀects that may be used advantageously in

organic electronics. The performance of these devices is highly dependent on

the sulfur atom topology, so a precise knowledge of the synthetic routes that

may aﬀord isomeric structures is necessary. We have studied the

photocyclization pathway of both 2- and 3-styrylthiophenes on their way to

thiahelicenes by experiment and theory. To begin with, the synthesis of

stereochemically well-deﬁned 2- and 3-styrylthiophenes allowed us to register

ﬁrst, and simulate later, the UV−vis electronic spectra of these precursors.

This information gave us access through time-dependent density functional

theory calculations to the very nature of the excited states involved in the

photocyclization step and from there to the regio- and stereochemical

outcome of the reaction. For the widely known case of a 2-styrylthiophene

derivative, the expected naphtho[2,1-b]thiophene type of ring fusion was

predicted and experimentally observed by synthesis. On the contrary, 3-styrylthiophene derivatives have been seldom used in

synthetic photocyclizations. Among the two possible structural outcomes, only the naphtho[1,2-b]thiophene type of ring fusion was

found to be mechanistically sound, and this was actually the only compound observed by synthesis.

INTRODUCTION

Helicenes display a robust, fully conjugated helical architecture

that makes them prototypes of chiral carbon nanostructures,

means of an oxidizer to aﬀord the aromatic naphthothiophene

moiety (Scheme 1a). It became already apparent at that early

stage that 3-styrylthiophenes were far less proliﬁc precursors

■

(

Scheme 1b) to the point that there are barely no examples of

with increasing applications in organic electronics. With the

this photocyclization strategy in the synthesis of thiahelicenes

up to until very recently. Although highly under-represented, it

is a valuable alternative for the construction of unusual

thiophene-terminated helicenes. Focusing on the central

features of this photocyclization and particularly on its

backbone scaﬀold (Scheme 1b), it has been reported to aﬀord

inclusion of thiophene rings into the helical structure, two

noticeable eﬀects occur. First, a stable doped state with

2,3

improved conducting properties arises; second, an enhanced

−5

binding energy with metallic electrodes results. Both are the

consequence of an enriched π-electron density induced by the

sulfur atom. These two aspects are highly dependent on the

sulfur atom position and bond topology generated therefrom

not only naphtho[1,2-b]thiophenes but also its isomeric

within the thiahelicene, so synthetic routes must be explicit

in that respect. In their seminal studies, Wynberg and

coworkers used the photocyclization of 2-styrylthiophene

naphtho[1,2-c] counterpart, which may leave doubts about

the actual natural outcome of this reaction. This is a critical yet

obscure point that gives rise to diﬀerent topologies in the sulfur

atom positioning, and with it, to the entire electronic structure

of the corresponding thiahelicene. This is an important matter

for many reasons, but in particular, if the objective of the

synthesis is to build conductive yet robust nanocontacts

between a helicene and external metallic electrodes. It is

derivatives as a straightforward way to access to thiahelicenes

(

Scheme 1a). Developed half a century ago, it remains a

prevalent, still captivating method to achieve the type of ring

fusion needed to construct helicenes starting from one of the

simplest conceivable precursors, so a large number of

thiophene-containing helicene structures have been obtained

this way since then. The general accepted mechanism does

Received: January 19, 2021

Published: March 26, 2021

not diﬀer much from the classical Mallory reaction of

stilbenes and involves a fast Z−E isomerization of the

styrylthiophene precursor, followed by a key photocycliza-

tion step, namely, photochemical electrocyclic reaction, to

aﬀord 9a,9b-dihydronaphthothiophenes as transient intermedi-

ates, which in turn dehydrogenate in the reaction media by

2021 American Chemical Society

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Scheme 1. Reaction Pathways for (a) 2- and (b) 3-Styrylthiophenes Based on the Widely Accepted Mechanism of

Photocyclization

Inset (a1) framed in green shows the 3D view of a thiophene-terminated helicenic structure originated from reactions in (a), while (b1) and (b2)

framed in magenta would originate from reactions in (b).

known that quantum interference may lead to constructive or

destructive interference, enhancing or suppressing conductance

depending on the sulfur atom positioning. Within this scenario,

we decided to study in depth this photocyclization in the

framework of our studies pursuing diﬀerent thiophene-

terminated helicenes with special regard to the topology

originated from the sulfur atom positioning in the key

photochemical step. In line with those aims, this work is a

combined experimental-theoretical study structured as follows:

RESULTS AND DISCUSSION

■

Synthesis of (E,E)-3,6-bis(2-(Thiophen-2-yl)vinyl)-

phenanthrene (7) and (E,E)-3,6-bis(2-(Thiophen-3-yl)-

vinyl)phenanthrene (8) as Stereochemically Well-

Deﬁned Photochemical Precursors. Stereochemically

well-deﬁned 2- and 3-styrylthiophene precursors were required

to trace the following key photochemical step of the synthesis.

In the simplest approach, the synthesis of an all-trans

conﬁguration of the double bonds was sought. It was carried

out according to Scheme 2. First, the central symmetric

fragment 3,6-dibromophenanthrene (4) was prepared from

(

a) ﬁrst, we synthesized suitable 2- and 3-styrylthiophenes as

stereochemically homogeneous precursors of thiahelicenes,

and then, (b) we studied both experimentally and theoretically

their absorption process (excitation) and subsequent key

photocyclization step. Finally, (c) we isolated the correspond-

ing thiahelicenes as reaction products and characterized them

unequivocally to establish the proper reaction pathway. We

decided to work with thiophene doubly terminated helicenes

,4′-dibromostilbene (3) by means of a Mallory reaction.

Stilbenic precursor 3 was easily prepared in one step from

commercial 4-bromobenzyl bromide taking advantage of the

dual electrophilic-nucleophilic role of this reagent in the

presence of sodium p-toluenesulﬁnate under basic conditions

(

KOH). In situ elimination aﬀorded trans-stilbene 3 in a

good yield, which was photocyclized very eﬃciently using a

00 W high-pressure Hg lamp (see the Supporting

Information ) to 4. Next, two arms containing the desired E-

2- and E-3-thienylvinyl fragments, respectively, were attached

to central fragment 4 via a Suzuki coupling, maintaining the

stereochemical integrity of E-vinylborane reagents 5 and 6

(

or more speciﬁcally with dithiahelicenes 1 and 2; Scheme 3)

throughout all this mechanistic study for practical reasons, in

line with our developing project on molecular solenoids. Their

applications as single-molecule devices in organic electronics

are currently under study.

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Scheme 2. Synthesis of All-trans (i.e., Conﬁgurationally Homogeneous) (E,E)-3,6-bis(2-(Thiophen-2-yl)vinyl)phenanthrene

(

7) and (E,E)-3,6-bis(3-(Thiophen-2-yl)vinyl)phenanthrene (8)

Figure 1. Experimental UV−vis spectrum of compound 7 (red line) in n-hexane. Calculated spectra of the most abundant conformer 7(C ) (blue

line) and second most abundant 7(C ) (green line, with its intensity halved, see the text) superimposed, as well as the calculated transitions

(

vertical lines) and oscillator strengths (thick vertical lines).

chosen for this purpose. These last reagents were conveniently

prepared with a well-deﬁned stereochemistry through a room-

temperature CuCl-xantphos-catalyzed hydroboration of the

commercially available acetylenic starting materials using

bis(pinacolato)diboron (B Pin ) and potassium t-butoxide in

(Thiophen-2-yl)vinyl)phenanthrene (7) and (E,E)-3,6-bis(2-

(Thiophen-3-yl)vinyl)phenanthrene (8). Styrylthiophene

compounds 7 and 8 are the photochemical precursors of the

targeted dithia[7]helicenes 1 and 2 in this study. The

experimental UV−vis spectra of 7 and 8 in n-hexane are

included in Figures 1 and 2 (red lines, see also the Supporting

Information ). In both cases, the absorption patterns share

many similarities. For 2-thienylstyryl derivative 7, it consists of

methanol.

Understanding the Photochemical Reaction Path-

way. Experimental UV−Vis Spectra of (E,E)-3,6-bis(2-

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Figure 2. Experimental spectrum of compound 8 (red line) in n-hexane. Calculated spectra of the most abundant conformer 8(C ) (blue line) and

second most abundant 8(C ) (green line, with its intensity halved, see the text) superimposed, as well as the calculated transitions (vertical lines)

and oscillator strengths (thick vertical lines).

Table 1. Calculated UV−Vis Spectrum of Major Conformations 7(C ) and 7(C ). State Number, Wavelength (λ), Energy (E),

Oscillator Strength (f), and Assignment of the Transitions

state

(C_s)

S₁

S₂

S₃

S₇

S₈

S₉

λ (nm)

E (eV)

assignment

362.6

321.6

313.0

252.0

242.0

239.7

229.9

219.0

206.71

3.419

3.855

3.961

4.920

5.123

5.173

5.392

5.662

5.998

1.04

1.35

0.11

0.46

0.36

0.13

0.18

0.11

H → L (78%)

H − 1 → L + 1 (16%)

H → L + 1 (40%)

H − 2 → L (26%)

H → L + 2 (17%)

H − 1 → L (46%)

H → L + 2 (46%)

H − 2 → L (46%)

H − 2 → L + 2 (44%)

H − 4 → L (27%)

H − 1 → L + 2 (21%)

H − 1 → L + 1 (28%)

H − 3 → L + 2 (27%)

H − 1 → L (12%)

H − 5 → L + 1 (18%)

H − 2 → L +1 (21%)

H − 3 → L (25%)

H − 4 → L + 1 (16%)

H → L + 10 (13%)

H − 6 → L + 1 (16%)

S₁₂

S₁₈

S₂₂

(C )

S₁

S₂

S₃

S₈

S₉

S₁₈

356.4

321.2

312.7

245.4

239.2

214.3

3.478

3.861

3.965

5.053

5.183

5.786

1.43

1.01

0.29

0.91

0.12

0.28

H → L (77%)

H − 1 →L + 1 (18%)

H − 1 →L (21%)

H − 1 →L (29%)

H − 2 → L + 1 (29%)

H − 4 →L + 1 (33%)

H − 1 →L + 1 (21%)

H → L + 1 (57%)

H → L + 2 (36%)

H − 2 → L + 2 (29%)

H − 5 → L (38%)

H − 3 → L (22%)

H − 2 →L (18%)

H − 1 → L + 1 (21%)

H − 5 → L + 4 (10%)

H − 6 → L + 1 (14%)

three main absorptions, the ﬁrst centered at λ

= 368.0 nm,

and 280.7 nm, and an additional smaller band at λ_m_a_x₃= 244.4

nm that is nonrelevant for our purposes, complete the

experimental spectrum.

With this information in hands, we calibrated our quantum

chemistry methods and began the theoretical simulation of the

electronic spectrum.

max1

with a side peak at 385.8 nm (i.e., 1253.7 cm⁻away from it)

and a nearly symmetrical shoulder at higher energy evidencing

a vibrational ﬁne structure. There is another absorption at λ_m_a_x₂

328.1 nm, also with a shoulder at slightly higher energy, and

additional absorptions at lower wavelengths, for example, λ_m_a_x₃

250.0 nm, less relevant for us since they are out of the reach

Conformational Equilibria of 7 and 8. First, we studied

the conformational equilibria of 7 and 8. There are four single

bonds in each molecule, giving rise to a quite signiﬁcant

of the reactor lamp.

For 3-thienylstyryl derivative 8, the spectrum also consists of

three main absorptions, the ﬁrst two displaying a clear

vibrational ﬁne structure again. The ﬁrst band centered at

amount of 2 = 16 conformations by rotation, which ﬁnally get

reduced by symmetry arguments to 10 nonequivalent

molecular conformations (only ﬂat, fully delocalized structures

were taken into account at this stage). A complete account of

all the conformers, their energies, and vibrational analysis can

be found in the Supporting Information . From them, a

−

λ_m_a_x₁= 350.9 nm has a side peak at 366.9 nm (i.e., 1242.7 cm

away from it) and also a nearly symmetrical shoulder at higher

energy. A second main absorption occurs at λ_m_a_x₂= 306.0 nm,

with two consecutive well-resolved vibrational peaks at 295.5

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Table 2. Calculated UV−Vis Spectrum of Major Conformations 8(C ) and 8(C ). State Number, Wavelength (λ), Energy (E),

Oscillator Strength (f), and Assignment of the Transitions

state

(C_s)

S₁

S₃

S₄

S₅

S₆

S₈

λ (nm)

E (eV)

assignment

344.1

295.8

265.3

259.0

253.3

239.1

236.5

228.4

196.6

3.603

4.192

4.673

4.787

4.896

5.185

5.242

5.429

6.307

1.05

1.61

0.34

0.15

0.20

0.17

0.40

0.12

0.38

H → L (82%)

H → L + 1 (50%)

H − 1 → L + 2 (30%)

H → L + 2 (32%)

H − 2 → L + 1 (28%)

H → L + 8 (21%)

H − 2 → L + 2 (36%)

H − 2 → L + 1 (28%)

H − 5 → L + 8 (25%)

H − 1 → L + 1 (10%)

H − 1 → L (38%)

H − 2 → L + 2 (16%)

H − 2 → L (16%)

H − 2 → L + 2 (13%)

H − 1 → L + 6 (19%)

H − 1 → L + 1 (27%)

H − 1 → L + 2 (18%)

H − 4 → L + 6 (19%)

H − 3 →L (10%)

H − 1 →L (15%)

S₁₀

S₁₁

S₃₀

H − 3 →L (12%)

H − 2 →L + 2 (15%)

H − 4 →L (11%)

(C )

S₁

S₃

S₄

S₅

S₈

S₁₂

336.6

292.7

259.4

257.6

241.2

225.0

3.684

4.236

4.780

4.813

5.142

5.511

1.67

1.25

0.52

0.29

0.37

0.13

H → L (82%)

H − 1 → L + 1 (11%)

H →L + 1 (27%)

H − 2 →L + 2 (18%)

H →L + 12 (16%)

H − 2 → L + 1 (28%)

H − 4 → L + 6 (13%)

H − 1 → L (52%)

H − 1 → L + 2 (47%)

H − 2 → L + 1 (39%)

H − 1 → L + 1 (38%)

H − 4 → L (27%)

H → L + 2 (11%)

H − 3 → L (11%)

H − 1 → L + 1 (14%)

H − 3 → L (13%)

H − 5 → L + 1 (11%)

Maxwell−Boltzmann distribution reveals two prevailing

conformations very close in energy (<0.4 kcal/mol, see the

Supporting Information) that stand out, accounting for more

than 60% of the overall conformational population. They are

represented in Figures 1 and 2 (inset).

occupied molecular orbital (HOMO) → lowest unoccupied

molecular orbital (LUMO) transition combined with a

HOMO − 1 → LUMO + 1 in a much smaller weight along

with other even minor unreported contributions (Table 1). In

Figure 3a, we can see this transition for the major conformer

The calculated spectral shapes of 7(C ) and 8(C ) ﬁt

7(C ) at λ = 362.6 nm as a natural transition orbital (NTO),

v 1

reasonably well with the experimental spectra, reproducing

λ_m_a_x₁, λ_m_a_x₂, and somewhat less the more complex band at λ_m_a_x₃

as well as the intensities of their absorbances. Their

populations are doubled with respect to the minor C₂_v

conformers since they have degenerated structures and beneﬁt

representing the best hole/particle pair representation of the

transition density matrix for that excited state. The MO

diagram in the middle shows the electronic conﬁgurations that

span the actual excited-state wavefunction. The composition of

this peak is very similar for both conformers C and C (and

from the statistical eﬀect of a smaller symmetry number (C

indeed for the rest of the less representative conformers

studied) and corresponds to the experimental λ_m_a_x₁= 368.0

nm. As can be seen in Figure 3, the binding pair of carbons are

the C(4) phenanthrene position and the C(3) of thiophene

(green arrows), with a typical supra-antara stereochemistry as

well as a weakened stilbenic double bond (antibonding

interaction). This will enable its dihedral rotation and meeting

of the reacting pair of carbons as the reactions further progress

compared to the C point group). For the minor conformers

(C ) and 8(C ), the calculated transitions are of similar

2v 2v

energy to the C ones, while their intensities are mismatched

(

green vs blue oscillator strengths, vertical thick lines, and

overall calculated spectral shapes). Thus, by comparison with

the experimental spectrum, it is evident that the statistical

eﬀect due to the symmetry overcomes the rather negligible

diﬀerence in energy calculated between both conformers. We

halved the simulated spectrum for C conformers in Figures 1

in the S surface. The main characteristics of these NTOs are

conserved for both the E- and Z-isomers, the excited state of

which will eventually drive to the expected naphtho[2,1-

b]thiophene type of ring fusion as we will see later.

and 2 to stress this argument. We have therefore identiﬁed the

transitions that give rise to the relevant absorption bands at

λ_m_a_x₁and λ_m_a_x₂, always keeping in mind that the experimental

absorptions have a more complex pattern originated from

vibronic coupling; this simulation is out of the reach of this

work.

The following calculated vertical transition is S , located at

λ = 321.6 nm for the major conformer 7(C ) and nearly the

same for the minor, and corresponds to the experimental λ_m_a_x₂

= 328.1 nm. A description of the MOs involved in this vertical

Calculated Electronic Spectra of the Major Con-

formations of 7 and 8. The assignment of the electronic

transition to the S state for 7 can be found in the Supporting

Information . This state also provides an analogous reaction

pathway with a regio- and stereochemical outcome akin to the

above explained for S . Both S and S states can explain the

spectra for the most representative conformations 7(C ) and

(C ) and minor counterparts 7(C ) and 8(C ) calculated by

s 2v 2v

time-dependent density functional theory (TDDFT) at the

wB97XD/6-311++G(2d,2p) level of theory in n-hexane as the

polarizable continuum model (PCM) is collected in Tables 1

and 2. We conﬁne our discussion to the transitions with

oscillator strengths f > 0.10 and contributions with more than

photochemical reaction outcome, although the lowest excited

state is usually the relevant one in terms of photochemical

production based on half-life arguments (Kasha’s rule,

extended to chemical deactivation).

We now move to the more interesting case 8. For 8, again

the vertical transition S₀→ S₁for any of its major

conformations is a π → −π* type of excitation, mainly

described by a HOMO → LUMO transition combined with a

HOMO − 1 → LUMO + 1 in a much smaller weight (Table

2). In Figure 3b, we can see this transition for the major

0% weight.

Let us focus now on the key transitions of the simulated

spectra. For 7, the vertical transition to the S state from the

ground state S for any of its major conformations is a π →

−π* type of excitation, mainly described by the highest

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very close for the minor one (C ), and corresponds to the

experimental λ_m_a_x₂= 306.0 nm. Similar arguments as those

explained before hold in here. A description of the MOs

involved in this vertical transition to the S state for 8 can be

found in the Supporting Information . Again, this state also

provides an analogous reaction pathway with a regio- and

stereochemical outcome akin to the above explained for S₁.

Calculated Excited States of 7 and 8 and the

Photochemical Reaction Pathway. The above-described

vertical transitions may evolve within the excited-state

hypersurface to minimal energy conformations and eventually

to a suitable excited Z-conﬁguration for the cyclization step.

We have tracked this evolution in the excited state S for 7 and

, starting from the most symmetric conformations (C ) for

simplicity. Calculations were done by density functional theory

DFT) and TDDFT at the wB97XD/6-311++G(2d,2p) level

(

of theory in n-hexane as the PCM. Results are collected in

Figure 4 and the Supporting Information. Upon vertical

excitation, 8 (C ) (I in Figure 4) in the ﬁrst place evolves to a

planar excited state I* (relative minimum) that further relaxes

to IIa* (absolute minimum in S ), while IIb* (also a relative

minimum) is some 5.1 kcal/mol above the former. Both are

the two relevant conformers of the excited state S with a

twisted but near to Z-conﬁguration of the stilbenic double

bond. The study of the changes in the electron density upon

excitation in IIa* and IIb* by NTO analysis is revealing. IIa*

has an important binding interaction between the carbons

C(4) of phenanthrene and C(2) of thiophene, which imparts

extra stability to this excited state and entails an incipient sigma

bond formation (Figure 5a, green arrow). After internal

conversion and vibrational relaxation to the ground state, it

may lead to the dihydro-intermediate III or alternatively the Z-

isomer IIa (Figure 4, green reaction pathway). A typical supra-

antara stereochemistry and a weakened stilbenic double bond

(

C−CC−C dihedral angles of 30 and 31° for IIa* and IIb*,

compared to nearly 0° for the ground-state Z-conﬁgurations)

are in agreement with the expected reaction course.

Conversely, for IIb*, the lack of an eﬃcient overlap

destabilizes this conformation and prevents the evolution

toward sigma-bonded products (Figure 4, red reaction pathway

and Figure 5b, red arrow). Binding through the C(4) of

thiophene would require a dihydro-intermediate having an

expanded valence shell for sulfur (represented in Scheme 1b by

a dotted bond), a rare pattern in organosulfur chemistry that

has deﬁed every attempt we made trying to model it, as it

always evolves toward the stable Z-double bond conformation

IIb.

Figure 3. Vertical transition to the ﬁrst excited state S for the major

conformations of (a) 7(Cs) and (b) 8(Cs), represented as the hole

and particle NTO pair [in this case, HOMO → LUMO weighing 78

and 82% of the transition for (a,b), respectively], and the diagram of

MOs involved in the excited states. Green arrows signal the reacting

pair of carbons.

Starting from 7, an allowed reaction pathway analogous in

every way to that described above is found. The corresponding

energy diagram for 7 can be found in the Supporting

Information. Overall, these mechanistic scenarios corroborate

the anticipated reaction pathways outlined in Figure 3 at the

vertical transition stage.

conformer 8(C ) at λ = 344.1 nm as an NTO, along with the

MO diagram of the transition. Just as in the former case, the

composition of this peak is very similar for both conformers C_s

and C and the rest of the less representative conformers and

corresponds to the experimental λ_m_a_x₁= 350.9 nm. It can be

deduced from Figure 3b that the binding pair of carbons are

the C(4) phenanthrene position and the C(2) of thiophene

Final Photochemical Synthesis of Dithia[7]helicenes

Starting from 7 and 8. Bis-stilbenic compounds 7 and 8

were subjected to irradiation using a 400 W high-pressure Hg

lamp to undergo a sequential double photocyclization under

(

green arrows), while the potentially competing C(4) is

kinetically unreactive (red arrow) through this excitation

pathway. Once evolved to the Z-conformation, its excited state

will eventually lead to the expected naphtho[1,2-b]thiophene

type of ring fusion but not to the naphtho[1,2-c] one (see also

structures in Scheme 1), as we will see later.

typical Mallory −Katz conditions (Scheme 3 and the

Supporting Information ). The reaction aﬀorded the expected

3,14-dithia[7]helicene-1 (or exo-dithia[7]helicene-1) for 7. A

distinctive ﬁngerprint useful in the structural elucidation of this

The following calculated vertical transition of relevance is S₃,

located at λ = 295.8 nm for the major conformer 8(C ) and

type of thiahelicenes by H NMR is the two doublets

originated by the coupling of the vicinal hydrogens of the

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Figure 4. Diagram of relative energy showing the photochemical pathways that 8 (C ) (represented here as I) may follow upon S → S excitation.

Calculated absolute and relative minima located in the ground and excited states (*) are also represented. An allowed route that goes through IIa*

leading both to E ⇆ Z isomerization (I ⇆ IIa) and to the cyclized product (I ⇆ III) is shown in green. Conformer IIb* leading only to E ⇆ Z

isomerization (I ⇆ IIb) is shown in red. No stationary cyclized intermediate was found that could lead to the helicenic ﬁnal product.

Scheme 3. Photochemical Synthesis of Dithia[7]helicenes 1 and 2 via a Double Photocyclization of bis-Stilbenic Precursors 7

and 8

thiophene ring. They are easily spotted in the spectrum for two

reasons. Even though the chemical shifts of thiophene

hydrogens fall within the chemical shift of a typical aromatic

hydrogen (e.g., benzene), within the framework of a helicenic

structure, they are shifted upﬁeld as a consequence of the

anisotropic eﬀect of the rest of the underlying aromatic

structure. Also, the coupling constant (J) of these vicinal

hydrogens is substantially smaller and easily identiﬁed from the

rest of the aromatic signals. The result is a pair of doublets in

the 6−7 ppm region with J around 5−6 Hz. In the case of 1,

there are two doublets at 6.60 (d, J = 5.6 Hz, 2H) and 6.24

(dd, J = 5.6, 0.5 Hz, 2H), clearly identifying a naphtho[2,1-

b]thiophene type of ring fusion (Figure 6a). For 8, the only

compound that could be isolated has two doublets at 7.06 (d, J

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Figure 5. Relevant minima IIa* and IIb* of the ﬁrst excited state S₁

of 8 and changes in the electron density represented as the particle

NTO. A developing sigma-binding interaction in IIa* (green arrow)

and the lack of the same in IIb* (red arrow) is shown.

5.4 Hz, 2H) and 6.80 (d, J = 5.4 Hz, 2H), consistent with a

naphtho[1,2-b]thiophene type of ring fusion as in 2 (Figure

b), but not with a naphtho[1,2-c]thiophene one, which would

display a much smaller J (estimated J = 2.7−2.8 Hz from

Figure 6. Terminal thiophene hydrogens show a distinctive pattern by

H NMR when forming parts of helicenes, consisting of an upﬁeld

chemical shift and a characteristic coupling constant of these vicinal

hydrogens, which makes them easily recognizable. This pattern is

shown in (a) for 1 and in (b) for 2, corroborating their naphtho[2,1-

b]thiophene and naphtho[1,2-b]thiophene type of ring fusion,

respectively.

gauge-invariant atomic orbital DFT calculations, scaled using

experimental J from Figure 6). The isolated compound was

hence identiﬁed as 1,16-dithia[7]helicene-2 (or endo-dithia-

[7]helicene-2). Other than the dithia[7]helicene or starting

material, only intractable, unidentiﬁable products by NMR

were obtained in diﬀerent trials whether the reactions were

taken to completion or not (i.e., to the disappearance of the

starting material). An unequivocal characterization of 1 and 2

by means of X-ray crystallography was recently reported by our

structure of which could be unambiguously assigned by H

NMR spectroscopy. Full structural characterization of the

resulting dithiahelicenes leaves little doubts about the ﬁndings

here reported, which show the potential of excited-state

quantum mechanical methods in structural and mechanistic

elucidation tasks.

group.

CONCLUSIONS

■

In summary, the photocyclization mechanism of 2- and 3-

styrylthiophenes to aﬀord naphtho[2,1-b]thiophene and

naphtho[1,2-b]thiophene type of ring fusions, respectively,

has been studied combining the conclusive information

resulting from synthesis/structural elucidation with DFT and

TDDFT methods to unveil mechanisms. In the ﬁrst place, we

carried out the syntheses of (E,E)-7 and (E,E)-8 and obtained

their experimental UV−vis spectra. Then, we simulated these

electronic spectra and analyzed the nature of their vertical

electronic transitions by TDDFT. This gave us access to the

excited states involved in the reaction. These excited states

were geometrically optimized and analyzed again to gain

information on the photochemical intermediates of the

reaction. The regiochemical and stereochemical outcome of

the photocyclization step is revealed at this point. Finally, back

to the synthesis, the actual photocyclization of 7 and 8 aﬀorded

the theoretically anticipated dithiahelicenes 1 and 2, the

EXPERIMENTAL SECTION

■

General Methods. Unless otherwise stated, commercially

available starting materials and solvents for chromatography and

recrystallization were used without further puriﬁcation. We dried and

distilled over Na/K alloy tetrahydrofuran (THF), benzene, and

cyclohexane immediately before using them in synthesis or photo-

chemistry. Commercially unavailable reagents were synthetized by

diﬀerent means; they are explained below each one separately.

Gas chromatography analyses (GLC) were carried out with a

Hewlett Packard HP-5890 instrument equipped with a ﬂame

ionization detector and a 30 m HP-5 capillary column (0.32 mm

diameter and 0.25 μm ﬁlm thickness) using nitrogen as carrier gas (12

psi). Column chromatography was performed with Merck silica gel 60

(

0.040−0.063 μm, 240−400 mesh). Thin-layer chromatography

(TLC) was performed on precoated silica gel plates (Merck 60,

F254, 0.25 mm). TLC detection was done by UV₂₅₄light, and R_f

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J. Org. Chem. 2021, 86, 5668−5679

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Article

values are given under these conditions. NMR spectra were recorded

on a Bruker Avance 300 and a Bruker Avance 400 (300 and 400 MHz

for ¹H NMR and 75 and 100 MHz for ¹³C NMR, respectively,

broadband proton decoupling was applied during the acquisition of

suspension of (E)-1,2-bis(4-bromophenyl)ethene (33.6 mg, 0.1

mmol; 1 equiv) and KI (16.6 mg, 0.1 mmol; 1 equiv) in cyclohexane

(100 mL) was prepared in a Schlenk tube provided with a vertical

condenser open to the air connected to a chiller. The recirculating

chiller was turned on, and the mixture was irradiated with a 400 W

high-pressure Hg lamp for 3−4 h under reﬂux. The progress of the

reaction was monitored by GLC. After the reaction was completed,

C{ H} spectra) using CDCl as a solvent and tetramethylsilane

(

TMS) as an internal standard. Chemical shifts (δ) are given in ppm

versus TMS. Infrared (IR) analysis was performed with a JASCO FT/

IR 4100 spectrophotometer equipped with an attenuated total

reﬂection component. Low-resolution mass spectrometry was

performed using the electron impact (EI) mode at 70 eV in an

AGILENT 5973N mass spectrometer coupled with an AGILENT

the crude mixture was washed with NaHSO , dried under magnesium

sulfate, and ﬁltered, and the solvent was evaporated under reduced

pressure (15 Torr). The product was puriﬁed by column

chromatography of silica gel (hexane) to aﬀord a white solid (26.8

890N gas chromatographer. High-resolution mass spectrometry

mg, 80% puriﬁed yield).

(

HRMS) analyses were carried out in an AGILENT 7200 using the EI

White solid; mp 191.6 °C (corrected); R = 0.53 (hexane); H

mode at 70 eV by quadrupole time-of-ﬂight (Q-TOF). Melting points

were performed with a Reichert Thermovar polarizing light

microscope and a melting point apparatus and have been corrected.

A double-beam UV−vis spectrophotometer (Shimadzu UV-1603)

was used for recording the electronic spectra.

NMR (CDCl , 300 MHz): δ = 8.72 (d, J = 1.7 Hz, 2H), 7.76 (d, J =

8.3 Hz, 2H), and 7.73−7.69 (m, 4H). C{ H} NMR (CDCl , 75

MHz): δ = 131.0, 130.9, 130.6, 130.2, 126.9, 125.7, and 121.4. MS

(EI) m/z: 337.90 (M + 4, 50.0), 335.90 (M + 2, 100), 333.90 (M ,

50.2), 176.10 (8.1), 150.10 (10), and 88.05 (29). IR (neat) ν_m_a_x

2923, 2851, 1901, 1586, 1494, 1429, 1407, 1381, 1153, 1106, 1069,

Photochemistry. A 400 W high-pressure mercury lamp (Osram

HQL MBF-U) was modiﬁed by cutting away the outer glass envelope

from the screw base (preserving the inner quartz arc tube containing

Hg) and was mounted in a porcelain lamp holder provided with an

aluminum reﬂector. The lamp was connected to a corresponding

power unit, and the light beam was focused to a number of 100 mL

Schlenck’s single-wall borosilicate tubes (0.6 mm wall thickness)

placed some 10 cm away from the source, provided with magnetic

stirring and a vertical condenser refrigerated with a recirculating

chiller (Huber MPC-K6) using a 30% ethylene glycol−water mixture

as a coolant. The chemical hood was lined with aluminum foil to

avoid unwanted exposure to UV radiation. We used cyclohexane or

benzene under reﬂux as solvents.

−

1017, 856, 846, 835, and 769 cm .

(E)-4,4,5,5-Tetramethyl-2-(2-(thiophen-2-yl)vinyl)-1,3,2-dioxo-

borolane (5 ). This compound was prepared by adapting a

procedure from the literature. In an oven-dried Schlenk tube

CuCl (2.96 mg; 0.03 mmol; 0.06 equiv), NaOt-Bu (5.76 mg; 0.06

mmol; 0.12 equiv) and xantphos ligand (17.35 mg; 0.03 mmol; 0.06

equiv) were added, and the reaction mixture was then subjected to

three cycles of vacuum/argon. Then, 1 mL of dry THF was injected,

and the solution was stirred for 30 min at room temperature. Next,

bis(pinacolato)diboron (253.94 mg; 1 mmol; 2 equiv) in 0.5 mL of

dry THF was added. The solution was stirred for 10 min at room

temperature, and 2-ethynylthiophene (54.08 mg; 0.5 mmol; 1 equiv)

was added, followed by MeOH (42 μL, 1 mmol). The reaction

mixture was stirred at room temperature overnight (no starting

material was detected by TLC). Then, it was ﬁltered through a pad of

celite, and the residue was puriﬁed by preparative TLC (silica gel,

Synthesis and Characterization of Compounds. Experimental

Procedures for the Synthesis of Intermediates and Final Products

(

E)-1,2-bis(4-Bromophenyl)ethene (3). This compound was

prepared by the procedure described in the literature. In a 100

mL round ﬂask, 4-bromobenzyl bromide (5.00 g, 20 mmol, 2 equiv),

sodium p-toluenesulﬁnate (1.78 g, 10 mmol, 1 equiv), and KOH

hexane−EtOAc 9:1) obtaining a pale yellow oil (47.2 mg, 40% yield).

Pale yellow oil; R

= 0.51 (hexane−EtOAc 9:1); H NMR (CDCl

(

1.68 g, 30 mmol, 3 equiv) were added, and then 25 mL of dimethyl

300 MHz): δ = 7.47 (d, J = 18.1 Hz, 1H), 7.24 (d, J = 5.1 Hz, 1H),

7.08 (d, J = 3.5 Hz, 1H), 6.99 (dd, J = 5.0 3.6 Hz, 1H), 5.91 (d, J =

sulfoxide (DMSO) was added. The mixture was heated in an oil bath

under reﬂux (100 °C) for 24 h. After cooling to room temperature,

the solvent (DMSO) was removed by vacuum distillation, and a

brown solid was obtained. The product was puriﬁed by column

chromatography on silica gel (hexane) to aﬀord a white solid (2.772

g, 82% yield). A diﬀerent way to purify the product was by

recrystallization from EtOH/CHCl₃(3:1), obtaining 3 as white

18.1 Hz, 1H), and 1.30 (s, 12H). C{ H} NMR (CDCl

= 144.1, 141.9, 127.8, 127.8, 126.4, 83.5, and 24.9. MS (EI) m/z:

, 75 MHz): δ

238.1 (M + 2, 5.6), 237.1 (M + 1, 13.4), 236.1 (M , 88.8), 235 (M

− 1, 23.2), 221.1 (20.5), 193.05 (7.3), 178.05 (10.7), 163.00 (26.8),

151.10 (41.1), 136.00 (100), 111.00 (27.9), 85.10 (8.9), and 57.10

(5.2). IR (neat) νmax: 2974, 2911, 2168, 1616, 1520, 1458, 1423,

−1

crystals.

1373, 1327, 1234, 1146, 976, 910, 849, and 733 cm .

White solid; mp 195.3 °C (corrected); R = 0.53 (hexane); H

(E)-4,4,5,5-Tetramethyl-2-(2-(thiophen-3-yl)vinyl)-1,3,2-dioxo-

borolane (6 ). This compound was prepared following the previous

NMR (CDCl , 300 MHz): δ = 7.51−7.46 (m, 4H), 7.39−7.34 (m,

H), and 7.02 (s, 2H). C{ H} NMR (CDCl , 75 MHz): δ = 136.1,

32.0, 128.3, 128.2, and 121.8. MS (EI) m/z: 339.90 (M + 4, 31.7),

37.90 (M + 2, 61.2), 335.90 (M , 32.5), 281.05 (22.2), 258.00

(

6.8), 207.05 (53.4), 180.00 (2.7), 179.10 (17.9), 178.10 (100),

76.10 (24.7), 152.15 (10.3), 126.05 (3.3), 89.05 (23.0), and 88.10

21.0). IR (neat) ν_m_a_x: 3020, 2924, 1581, 1481, 1404, 1323, 1215,

−

068, 999, 968, 822, and 710 cm .

,6-Dibromophenanthrene (4). This compound was prepared

procedure, replacing the terminal alkyne by 3-ethynylthiophene as a

pale yellow oil (54.3 mg, 46% yield).

using the main photochemical setup described above. A solution/

Pale yellow oil; R = 0.52 (hexane−EtOAc 9:1); H NMR (CDCl ,

00 MHz): δ = 7.38 (d, J = 18.4 Hz, 1H), 7.32−7.28 (m, 2H), 7.28−

13 1

.24 (m, 1H), 5.95 (d, J = 18.3 Hz, 1H), and 1.30 (s, 12H). C{ H}

NMR (CDCl , 75 MHz): δ = 143.3, 141.3, 126.2, 125.1, 125.0, 83.4,

and 24.9. MS (EI) m/z: 238.10 (M + 2, 4.1), 237.10 (M + 1, 10.4),

36.10 (M , 70.2), 235.10 (M − 1, 15.6), 221.10 (17.5), 192.10

7.2), 178.10 (26.5), 163.05 (54.8), 150.10 (26.2), 136.00 (100),

(

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J. Org. Chem. 2021, 86, 5668−5679

The Journal of Organic Chemistry

https://pubs.acs.org/doi/10.1021/acs.joc.1c00147.

Article

10.05 (12.1), 85.05 (9.4), and 57.10 (5.5). IR (neat) ν_m_a_x: 2927,

858, 2167, 2025, 1724, 1624, 1516, 1458, 1331, 1261, 1146, 991,

−

49, 771, and 690 cm .

,6-bis((E)-2-(Thiophen-2-yl)vinyl)phenanthrene (7). This com-

pound was prepared by adapting to our substrates a Suzuki coupling

recirculation chiller was turned on, and the mixture was irradiated

with a 400 W high-pressure Hg lamp for 3 h under reﬂux. The

progress of the reaction was monitored by TLC. After the reaction

was completed, it was washed with aqueous NaHSO , dried over

magnesium sulfate, and ﬁltered, and the solvent was evaporated under

a reduced pressure (15 Torr). The residue was puriﬁed by column

chromatography on silica gel (hexane−CH Cl 8:2) to obtain a yellow

described in the literature. In an oven-dried pressure tube, PdCl₂

17.73 mg; 0.1 mmol; 0.20 equiv), PPh (52.45 mg; 0.2 mmol; 0.40

solid. A 54% yield was calculated by H NMR using 1,3,5-

trimethoxybenzene (99%) as the internal standard. Recrystallization

from EtOAc aﬀorded slightly brown, near colorless crystals (6.8 mg,

(

^e^q^uⁱ^v⁾^,^C^s^C^O3 (977.46 mg; 3 mmol; 6 equiv), and 3,6-

bibromophenanthrene (168.3 mg; 0.5 mmol; 1 equiv) were added.

The tube was sealed with a septum, and after being subjected to three

cycles of vacuum/argon, 4,4,5,5-tetramethyl-2-(2-(thiophen-2-yl)-

vinyl)-1,3,2-dioxoborolane (360.3 mg; 1.5 mmol; 3 equiv) dissolved

in 3.6 mL of THF was added with a syringe, followed by 0.4 mL of

5% yield).

Light brown, near colorless crystals; R = 0.48 (hexane/AcOEt

:1); H NMR (CDCl , 300 MHz): δ = 8.07 (d, J = 8.9 Hz, 2H), 8.06

(

s, 2H), 7.98 (d, J = 8.4 Hz, 2H), 7.93−7.84 (m, 4H), 6.60 (d, J = 5.6

Hz, 2H), and 6.24 (d, J = 5.6, 2H). C{ H} NMR (CDCl , 75

H O. The threaded tube was closed and heated in an oil bath at 85 °C

MHz): δ = 138.6, 135.3, 132.2, 129.9, 127.9, 127.9, 127.1, 125.4,

for 20−24 h. The reaction was monitored by TLC. After the reaction

25.1, 124.6, 124.2, 123.0, and 121.4.

was completed, it was extracted using 10 mL of H O and 3 × 10 mL

1,16-Dithia[7]helicene (2). In this case, a suspension of 3,6-

bis((E )-2-(thiophen-3-yl)vinyl)phenanthrene (19.72 mg; 0.05 mmol;

of CH Cl . A rather insoluble solid in suspension was observed in the

organic phase. The product, yellow solid, was separated by ﬁltration.

On the other hand, the combined extracts were solvent-evaporated

and puriﬁed by column chromatography with silica gel (hexane−

EtOAc 9:1) to obtain an additional fraction of the same yellow solid

which was incorporated to the formerly ﬁltered solid. The product

was obtained as a yellow solid (122.3 mg, 62% yield).

Yellow solid; mp 203.8 °C (corrected); R = 0.55 (hexane−EtOAc

:1); H NMR (CDCl , 400 MHz): δ = 8.66 (d, J = 0.65 Hz, 2H),

.84 (d, J = 8.3 Hz, 2H), 7.77 (dd, J = 8.4, 1.4 Hz, 2H), 7.67 (s, 2H),

.46 (d, J = 16.0 Hz, 2H), 7.26 (d, J = 3.5 Hz, 2H), 7.23 (d, J = 16.0

equiv), I (38.07 mg; 0.15 mmol, 3 equiv), and 1,2-epoxybutane

(360.50 mg; 5 mmol; 100 equiv) in benzene was irradiated following

Hz, 2H) 7.17 (d, J = 3.5 Hz, 2H), and 7.06 (dd, J = 5.1, 3.5 Hz,

the same overall methodology described above for the exo isomer. A

50% yield was calculated by H NMR using 1,3,5-trimethoxybenzene

H). MS (EI, DIP) m/z: 396.2 (M + 2, 24.7), 395.2 (M + 1,

8.6), 394.2 (M , 100), 359.1 (7.1), 308.1 (10.3), 276.1 (8.7), 197.1

(

99%) as the internal standard. Recrystallization from EtOAc aﬀorded

yellow crystals (5.8 mg, 30% yield).

Yellow crystals; R = 0.35 (hexane/AcOEt 9:1); H NMR (CDCl ,

(

18.8), 179.3 (5.8), and 154.2 (3.4). IR (neat) ν_m_a_x:₋3097, 1786, 1608,

508, 1423, 1342, 1192, 949, 887, 845, and 698 cm . HRMS (EI/Q-

TOF) m/z: [M] calcd for C H S , 394.0850; found, 394.0853.

18 2

00 MHz): δ = 8.18 (d, J = 8.4 Hz, 2H), 8.07−7.99 (m, 6H), 7.91 (d,

,6-bis-((E)-2-(Thiophen-3-yl)vinyl)phenanthrene (8). This com-

J = 8.4 Hz, 2H), 7.06 (d, J = 5.4 Hz, 2H), and 6.80 (d, J = 5.4 Hz,

pound was prepared following the previous procedure but using

H). C{ H} NMR (CDCl , 75 MHz): δ = 138.2, 136.5, 132.8,

30.6, 128.8, 128.3, 126.8, 125.6, 125.2, 124.1, 123.1, 122.8, and

22.6.

Calculation Details. For the UV−vis spectral simulation, we

performed a series of DFT geometry optimizations to understand the

conformational equilibria of 7 and 8, followed by TDDFT on the

major conformers in order to ﬁnd the most appropriate functional for

this job. Our experimental UV−vis spectra were used as a benchmark

in all cases for spectral ﬁtting. In general, all the methods tested

provided comparable results and a reasonable ﬁt, indicating that this is

not an intrinsically diﬃcult spectral simulation. The double hybrid

,4,5,5-tetramethyl-2-(2-(thiophen-3-yl)vinyl)-1,3,2-dioxoborolane as

the starting reagent. The product was obtained as a yellow solid

(

146.0 mg, 74% yield).

functional with a dispersion correction term wB97X-D, in n-hexane

Yellow solid in a 74% yield; mp 193.4 °C (corrected); R = 0.60

as the PCM, was ﬁnally chosen for both DFT and TDDFT

calculations. Pople’s split-valence quasi triple-ζ, in the valence shell

basis set and addition of both polarization and diﬀuse functions, 6-

hexane−EtOAc 9:1); H NMR (CDCl , 400 MHz): δ = 8.66 (s,

H), 7.84−7.75 (m, 4H), 7.65 (s, 2H), 7.45 (d, J = 4.2 Hz, 2H),

.40−7.29 (m, 6H), and 7.23 (d, J = 16.0 Hz, 2H). MS (EI, DIP)

11++G(d,p) was used in the ground state for geometry optimization

m/z 396.1 (M + 2, 13.0), 395.1 (M + 1, 30.3), 394.1 (M , 100),

60.1 (9.8), 309.1 (14.1), 284.1 (9.4), and 197.0 (9.1). IR (neat)

and vibrational analysis, while a more expanded basis set, 6-311+

G(2df,2pd), was used for UV−vis spectral calculations. These

−

ν_m_a_x: 3089, 1608, 1408, 1203, 1087, 957, 845, 771, and 690 cm .

HRMS (EI/Q-TOF) m/z: [M] calcd for C H S , 394.0850; found,

calculations were performed using Gaussian09 suite of programs.

For the excited states, optimization and vibrational analysis were

18 2

94.0848.

,14-Dithia[7]helicene (1).

carried out with Gaussian16, which provides the tools to perform

,14

The reaction was run in parallel in

analytical vibrational analysis to TDDFT-optimized excited states.

two oven-dried Schlenk tubes, loaded with a suspension of 3,6-

bis((E)-2-(thiophen-2-yl)vinyl)phenanthrene (19.73 mg; 0.05 mmol;

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

equiv), I (38.07 mg; 0.15 mmol; 3 equiv), and 1,2-epoxybutane

(

360.50 mg; 5 mmol; 100 equiv) in 100 mL of benzene each. The

tubes were provided with vertical condensers connected to a chiller,

and Ar was bubbled into the solution during the reaction. The

677

J. Org. Chem. 2021, 86, 5668−5679

The Journal of Organic Chemistry

T.; Carlotti, M.; Sauter, E.; Zharnikov, M.; Chiechi, R. C. Controlling

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of the relevant UV−vis bands, energies of the optimized

excited states, and NTO analysis (PDF )

(

7) Zhang, Y.; Ye, G.; Soni, S.; Qiu, X.; Krijger, T. L.; Jonkman, H.

destructive quantum interference in tunneling junctions comprising

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AUTHOR INFORMATION

■

(11) Bartocci, G.; Galiazzo, G.; Ginocchietti, G.; Mazzucato, U.;

Corresponding Author

Albert Guijarro − Departamento de Química Orgánica and

Instituto Universitario de Síntesis Orgánica, Campus de San

Vicente del Raspeig, Universidad de Alicante, 03080 Alicante,

Spain; orcid.org/0000-0001-9196-8436;

Email: aguijarro@ua.es

knowledge to applications. Adv. Heterocycl. Chem. 2016, 118, 1−46.

(10) Mallory, F. B.; Mallory, C. W. Photocyclization of stilbenes and

related molecules. Organic Reactions; John Wiley & Sons, Inc., 1984;

Vol. 30, Chapter 1, p 3.

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rotamerism of symmetric and asymmetric diaryl-ethenes and diaryl-

butadienes. Photochem. Photobiol. Sci. 2004, 3, 870−877.

Authors

Bianca C. Baciu − Departamento de Química Orgánica and

Instituto Universitario de Síntesis Orgánica, Campus de San

Vicente del Raspeig, Universidad de Alicante, 03080 Alicante,

Spain

José Antonio Vergés − Departamento de Teoría y Simulación

de Materiales, Instituto de Ciencia de Materiales de Madrid

(12) Shi, Y.-G.; Mellerup, S. K.; Yuan, K.; Hu, G.-F.; Sauriol, F.;

Peng, T.; Wang, N.; Chen, P.; Wang, S. Stabilising fleeting

intermediates of stilbene photocyclization with amino-borane

functionalisation: the rare isolation of persistent dihydrophenan-

threnes and their [1,5] H-shift isomers. Chem. Sci. 2018, 9, 3844−

(

855.

N.; Lee, M. L. The synthesis of naphtho[1,2-b]thiophene and all of

CSIC), 28049 Madrid, Spain; orcid.org/0000-0002-

937-4802

13) (a) Tedjamulia, M. L.; Stuart, J. G.; Tominaga, Y.; Castle, R.

the eight isomers of monomethylnaphtho[1,2-b]thiophene. J.

Heterocycl. Chem. 1984, 21 , 1215 −1219. (b) Song, K.; Wu, L.-Z.;

Yang, C.-H.; Tung, C.-H. Photocyclization and photooxidation of 3-

styrylthiophene. Tetrahedron Lett. 2000, 41, 1951−1954.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.joc.1c00147

Notes

14) Moussa, S.; Aloui, F.; Ben Hassine, B. Synthesis and

optoelectronic properties of some new thiahelicenes. Synth. Commun.

011, 41, 1006−1016.

15) Lambert, C. J. Basic Concepts of Quantum Interference and

Electron Transport in Single-Molecule Electronics. Chem. Soc. Rev.

015, 44, 875−888.

16) Matsushima, T.; Kobayashi, S.; Watanabe, S. Air-driven

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

Financial support by the Spanish Ministerio de Economía y

Competitividad (MAT2016-78625-C2-2-P), the Ministerio de

Ciencia, Innovación y Universidades (PID2019-109539GB-C4

and PGC2018-096955-B-C44), the Generalitat Valenciana

potassium iodide-mediated oxidative photocyclization of stilbene

derivatives. J. Org. Chem. 2016, 81, 7799−7806.

(

PROMETEO/2017/139), and ﬁnally, the University of

(17) Zhao, F.; Luo, J.; Tan, Q.; Liao, Y.; Peng, S.; Deng, G.-J.

Alicante (VIGROB-285) is gratefully acknowledged. The

computational resources provided by the Department of

Applied Physics of the University of Alicante are greatly

appreciated.

Sodium sulfinate-mediated trans-stilbene formation from benzylic

halides. Adv. Synth. Catal. 2012, 354, 1914 −1918.

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diboron reagents to α ,β -acetylenic esters: efficient synthesis of β -

Boryl-α ,β -Ethylenic Esters. Chem. Commun. 2008, 733−734.

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