Modifying Woodward–Hoffmann Stereoselectivity Under Vibrational Strong Coupling

Article abstract of DOI:10.1002/anie.202013465

Vibrational strong coupling (VSC) has recently been shown to change the rate and chemoselectivity of ground-state chemical reactions via the formation of light–matter hybrid polaritonic states. However, the observation that vibrational-mode symmetry has a large influence on charge-transfer reactions under VSC suggests that symmetry considerations could be used to control other types of chemical selectivity through VSC. Here, we show that VSC influences the stereoselectivity of the thermal electrocyclic ring opening of a cyclobutene derivative, a reaction which follows the Woodward–Hoffmann rules. The direction of the change in stereoselectivity depends on the vibrational mode that is coupled, as do changes in rate and reaction thermodynamics. These results on pericyclic reactions confirm that symmetry plays a key role in chemistry under VSC.

Full text of DOI:10.1002/anie.202013465

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Modifying Woodward-Hoffmann Stereoselectivity under
Vibrational Strong Coupling
Authors: Abhijit Sau, Kalaivanan Nagarajan, Bianca Patrahau, Lucas
Lethuillier-Karl, Robrecht Vergauwe, Anoop Thomas, Joseph
Moran, Cyriaque Genet, and Thomas W. Ebbesen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202013465
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10.1002/anie.202013465
Angewandte Chemie International Edition
Modifying Woodward-Hoffmann Stereoselectivity under
Vibrational Strong Coupling
Abhijit Sau‡, Kalaivanan Nagarajan‡, Bianca Patrahau, Lucas Lethuillier-Karl, Robrecht M.
A. Vergauwe, Anoop Thomas, Joseph Moran*, Cyriaque Genet* and Thomas W. Ebbesen*
University of Strasbourg, CNRS, ISIS & icFRC, 67000 Strasbourg, France
‡ Equal contribution to this study.
Abstract
Vibrational strong coupling (VSC) has recently been shown to change the rate and
chemoselectivity of ground state chemical reactions via the formation of light-matter hybrid
polaritonic states. However, the observation that vibrational mode symmetry has a large
influence on charge transfer reactions under VSC suggests that symmetry considerations could
be used to control other types of chemical selectivity through VSC. Here, we show that VSC
influences the stereoselectivity of the thermal electrocyclic ring-opening of a cyclobutene
derivative, a reaction which follows the Woodward-Hoffmann rules. The direction of the
change in stereoselectivity depends on the vibrational mode that is coupled, as do changes in
rate and reaction thermodynamics. These results on pericyclic reactions confirm that symmetry
plays a key role in chemistry under VSC.
Keywords: pericyclic reaction, Woodward-Hoffmann, strong coupling, orbital symmetry,
electrocyclic ring opening
1
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Introduction:
The Woodward-Hoffmann (WH) rules elaborated in the 1960’s epitomizes the
1-6

importance of symmetry in chemical reactivity. In essence, the conservation of orbital
symmetry between reactants and products affect the reaction landscape and thereby the
products that are favoured. In ideal cases, such as for cis-3,4-disubstitued cyclobutene, the
products that are forbidden in the ground states are allowed in the excited state and vice-versa:
cis-3,4-disubstitued cyclobutene undergoes conrotatory electrocyclic ring opening under
thermal conditions to form cis-trans-1,3-butadiene, while under photochemical condition,
trans, trans-1,3-butadiene is obtained through the disrotatory pathway, as illustrated in Figure
1a. However, in most cases, a mixture of products is obtained due to effects that mitigate the
symmetry-based outcome, such as secondary effects of substituents, even if overall the WH
2-6

rules are followed.
Chemistry under strong coupling has received much attention since the first
7-34

demonstration of a modified chemical landscape. 
Recently, we reported that vibrational
symmetry plays a key role in modifying the charge-transfer complexation equilibrium between
I2 and mesitylene under vibrational strong coupling (VSC).12 To further explore the role of
symmetry under VSC, we have chosen the well-studied thermal electrocyclic ring-opening of
cyclobutene into butadiene, where the reaction path can be predicted by the principle of orbital
symmetry conservation according to the WH rules. Among the various cyclobutene derivatives
reported for this kind of reaction, we have synthesised cyclobutene-cis-3,4-
dimethylcarboxylate (CB), which is suitable for studies under VSC. As expected from the WH
rules,[1] the molecule under study undergoes thermal ring-opening at 90 ºC to yield mostly the
corresponding symmetry-allowed cis-trans product, but also some symmetry-forbidden trans-
trans 1,3-butadiene product. The formation of symmetry-forbidden trans-trans butadiene can
be attributed to secondary orbital interaction from the carbonyl group, which reduces the
3-6

activation energy of the disrotatory transition state.
In addition, it is a liquid at room
temperature, the reaction is monomolecular and gives no products other than to those expected
from the WH rules, all features that simplify the study.
Vibrational strong coupling (Figure 1b) is achieved by placing the molecular solution
inside a Fabry-Perot optical cavity, which is tuned to have a mode in resonance with a selected
vibrational mode of the molecule (Figure 1c). When the conditions for strong coupling are
satisfied, the molecular transition splits into two new hybrid states known as P+ and P-,
2
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10.1002/anie.202013465
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separated by a Rabi splitting energy (ħΩR). The strength of strong coupling is denoted by the
Rabi splitting energy, which is proportional to the transition dipole moment of the
Figure 1. a) Electrocyclic ring-opening reaction of cis-3,4-disubstitued cyclobutene which
undergoes conrotation under thermal condition to form cis-trans butadiene and disrotation in
the presence of light to give trans-trans butadiene ; b) Schematic representation of vibrational
strong coupling between the molecular vibrational transition and Fabry-Perot cavity mode; c)
FT-IR spectra of cyclobutene-cis-3,4-dimethylcarboxylate, with inset showing the molecular
structure.
vibration and square root of the concentration of the coupled molecules inside the cavity. VSC
occurs in the dark due to the coupling of the zero-point energy of the cavity mode with that of
9
the vibrational transition. 
The Fabry-Perot cavity was fabricated by separating two parallel Au (10 nm) mirror-
coated ZnSe substrates with a 6 m Mylar spacer, and having a hollow channel designed to
inject the liquid sample. Such cavities will generate many resonances separated the free spectral
range or FSR and one of them needs to be tuned to a vibration (Figure S1, Supporting
Information). To prevent the reaction mixture from coming in contact with the metal surface,
200 nm of SiOx was sputtered on top of the Au. For the so-called non-cavity condition, the
same SiOx coated windows were used but without the Au coating. Electrocyclic ring-opening
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of CB was performed by heating the sample at 90 ºC for two hours (Figure 2a). To begin, we
followed the reaction under non-cavity conditions (Figure S2, Supporting Information). Neat
CB was injected into the preheated chamber at 90 ºC and the reaction was monitored in the FT-
IR (Figure 2b). The first-order nature of the reaction was confirmed by the exponential growth
of the peak at 1602 cm-1, which corresponds to the C=C stretching mode of the butadiene
derivative (Figure S3a, Supporting Information). After two hours, the reaction mixture was
1
allowed to cool to room temperature (RT) and dissolved in CDCl3 to perform the H-NMR
measurement. The 1H-NMR spectrum shows the formation of the symmetry-allowed cis-trans
butadiene and the symmetry-forbidden trans-trans derivative, along with the unreacted starting
material CB (Figure 2e). The trans-trans butadiene is characterised by the signals a and b,
whereas the cis-trans derivative is identified by the signals c, d, e and f, as shown in Figure 2.
As predicted by the WH rules, the symmetry-allowed cis-trans butadiene is obtained as the
major product (92%) while the symmetry-forbidden trans-trans derivative is present only with
a 8% yield. This leads to a ratio between symmetry-allowed conrotatory (cis-trans) vs
symmetry-forbidden disrotatory (trans-trans) product ratio of 12.7 ±0.5. No other products or
intermediates were observed in the NMR spectra. The overall reaction rate (kobs) at 90 ºC under
non-cavity conditions was 0.0120 min-1 (Figure 2h). The formation of symmetry-forbidden
trans-trans butadiene product is attributed to the secondary orbital interactions arising from the
3-6

carbonyl groups at C3 and C4 of cyclobutene derivative CB.
To perform the reaction under VSC, the FSR of Fabry-Perot cavities were carefully
tuned at 90 °C to have a mode to couple either carbonyl stretching band at 1740 cm-1 or the in-
plane C-H bending at 1436 cm-1. Cavity tuning is easily performed by adjusting the screws that
hold the assembly together, as detailed in the Supporting Information (Figure S1, Supporting
Information). As with the non-cavity experiment, the cavity assembly was equilibrated at 90
ºC for two hours before injecting the sample.
In a first set of experiments, the 5th mode of the Fabry-Perot cavity was tuned to the
C=O stretching mode of CB (Figure 2c) at 1740 cm-1 at 90 ºC. Neat CB was injected and the
reaction progress was monitored by FT-IR. The appearance of equally spaced P+ and P- with
a Rabi splitting of 160 cm-1, corresponding to the molecular transition at 1740 cm-1, confirms
the strong coupling. The other peaks in Fig. 2c are the 4th and 6th cavity modes and are not
interacting with any vibrations. The kinetics of the reaction was followed by monitoring the
shift in the higher order cavity modes, which reflects the difference in refractive index of the
reactant and products (Figure S3, Supporting Information). It should be noted that both the
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Figure 2. a) Electrocyclic ring opening reaction of cyclobutene-cis-3,4-dimethylcarboxylate
(CB) under thermal conditions. FT-IR spectra in transmission mode of b) non-cavity
conditions, c) under VSC of the C=O stretching mode (1740 cm-1) and d) under VSC of the C-
H bending mode (1436 cm-1). e), f) and g) 1H-NMR spectra showing alkenyl proton signals of
cis-trans and trans-trans butadiene products along with the unreacted cyclobutene starting
material obtained respectively from non-cavity conditions, VSC at 1740 cm-1 and VSC at 1436
1
cm-1. Note that the H signal labelled * is from CDCl3 and CB is from unreacted starting
material. h) Observed kinetics (kobs) of the reaction under non-cavity (black circle) and under
VSC conditions (C=O, blue triangles and C-H, red square. i) Superimposed 1H-NMR signals
normalised to the cis-trans signal e showing the change in the yield of the disrotation product
trans-trans (signal a).
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reactant and the products have the same C=O group and therefore as the reaction proceeds the
system remains coupled and the Rabi splitting hardly changes. Under VSC of C=O stretching,
the overall reaction rate (kobs) was 0.0080 min-1. After two hours, the reaction mixture was
1
allowed to cool down to RT and the products were quantified by H-NMR (Figure 2f).
Surprisingly, we observed significant reduction in the symmetry-forbidden trans-trans
butadiene derivative (1.5%) when compared to the non-cavity condition (8%, Figure 2i). This
leads to a ratio between the symmetry-allowed and the symmetry-forbidden products of 28 ±2
under VSC of the C=O stretching mode. As detailed in the Supporting Information, only ~ 60%
area of the Fabry-Perot cavity is homogeneous and tuned under VSC. By incorporating the area
correction, the ratio of cis-trans to trans-trans butadiene is actually 38 ±2 (Table 1). In other
words, coupling the C=O stretching mode increases the selectivity for the symmetry-allowed
product.
In a second series of experiments, we coupled the cyclobutene ring C-H in-plane
bending mode (Figure 2d), which overlaps with the strong CH3 deformation modes at 1436 cm-
1. After injecting the neat sample into the equilibrated cavity at 90 ºC, FT-IR spectra show P+
and P- with a Rabi splitting of 62 cm-1 with the 4th mode of the cavity. It should be noted that
an apparent splitting appears between the 5th mode and the C=O stretching mode but closer
analysis shows that it is detuned by 60 cm-1. It is now well-known that at such detuning, there
is no strong coupling effect on the chemistry12,15,16,22,23. Again, after two hours, the reaction
1
mixture was analysed by H-NMR. Similar to VSC of C=O stretching mode, the overall
reaction rate (kobs) was found to be 0.0083 min-1. Interestingly, we observed a significant
enhancement in the formation of symmetry-forbidden trans-trans butadiene (15%) when
compared to the non-cavity condition (Figure 2g and 2i). This results in a symmetry-allowed
vs symmetry forbidden product ratio of 5 ±1, after correcting the values for the tuned area
(Table 1).
As a final reference, similar experiments were performed at off-resonance condition
(Figure S4, Supporting Information) where none of the cavity modes are strongly coupled with
any of the molecular vibrations. The mere presence of cavity modes in the off resonance
experiments did not show any modification in the product ratio (Table 1), which demonstrates
the importance of strong coupling between the cavity mode and the molecular transition to
observe the changes in the reaction.
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Individual rate constants for the symmetry-allowed conrotatory (kcon) and symmetry-
forbidden disrotatory (kdis) ring-openings were estimated from the observed rate constant (kobs)
of the reaction (Figure 2h) and the product ratio, by combing equations 1 and 2:
푘푐표푛 + 푘푑ꢀ푠 = 푘표푏푠
(1)
(2)
푘
[푐ꢀ푠−푡푟푎푛푠]
푐표푛
=
푘
[푡푟푎푛푠−푡푟푎푛푠]
푑푖푠
The rate of formation of cis-trans and trans-trans butadiene under non–cavity conditions at 90
ºC was 0.011 min-1 and 0.001 min-1 respectively. Under VSC, kcon and kdis varies depending on
the coupled vibration and the values are given in the Supporting Information (Table S1).
The difference between the free energies of activation of the symmetry-allowed
conrotation and symmetry-forbidden disrotation could be calculated from the product ratio by
using the following equation, and the results are shown in the Table 1:
[푐ꢀ푠−푡푟푎푛푠]
∆∆퐺‡ = 푅푇푙푛 [푡푟푎푛푠−푡푟푎푛푠] (3)
Table 1. Cis-trans to trans-trans product ratio (conrotation/disrotation) obtained from 1H-NMR
measurements of electrocyclic ring opening of CB at 90 ºC in non-cavity and cavity tuned to
different vibrational modes. Relative Gibbs free energy change (G‡) between the con and
disrotatory transition states are obtained from the product ratio using equation 3.
G‡ (kJ mol-1)
Experiment
cis-trans/trans-trans
Non-cavity
7.7 ±0.2
7.6 ±0.2
12.7 +0.5
12.4 +0.5
Cavity off resonance
Cavity on resonance
C=O stretching mode (1740 cm-1)
38.0 +2
5.0 +1
11.0 ±0.5
4.9 ±0.5
C-H bending mode, in-plane (1436 cm-1)
In non-cavity conditions, the average free energy change between con- and disrotatory
transition states of CB at 90 ºC is 7.6 ±0.2 kJ mol-1, which is much lower when compared to
2
cis-3,4-dimethylcyclobutene (35  40 kJ mol-1).  This significant reduction in the G‡ can
be attributed to the stabilisation of the disrotatory transition state through secondary orbital
3-6
interactions with the carbonyl group.   VSC of the C=O stretching mode exhibits increased
G‡ (11 ±0.5 kJ mol-1), resulting in reduced trans-trans product formation, which presumably
occurs due to the perturbation of the secondary orbital interaction of the C=O with the
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cyclobutene ring. On the other hand, VSC on C-H bending shows decreased G‡ (4.9 ±0.5
kJ mol-1), which enhances the symmetry-forbidden trans-trans product.
Such modifications can be interpreted by looking at the additional symmetry constraints
that VSC puts on the orbital correlation diagram. Let us start with the case of VSC involving
the in-plane C-H bending mode. The cavity mode strongly coupled to the molecules puts the
orbitals in-phase with respect to the intracavity field, and thus forces the phases of the newly
formed p orbitals to remain symmetric with respect to the mirror plane of the cis-cyclobutene
moiety (Figure 1a). Such symmetries are compatible with the electrocyclic ring-opening
through a disrotation, explaining why VSC on C-H bending favours disrotatory processes and
increases kdis in the reaction path.
To understand the effects of VSC of the C=O stretching mode, it should be recalled the
3-6

peculiar role played by the carbonyl groups in the ring-opening process.
It mediates
secondary, but efficient, orbital interactions with the cyclobutene ring and thereby affecting the
WH rules.
Figure 3. Relative orientation of carbonyl groups in the transition state expected in a)
symmetry allowed conrotation and b) symmetry forbidden disrotation.
As shown in Figure 3, the trans-trans isomer has divergent carbonyl groups while the cis-trans
isomer displays carbonyl groups closely aligned. Considering that the single-mode intracavity
electric field 푬푐푎푣 puts the dipoles in-phase under VSC, these two configurations lead to two
resulting collective dipoles 푫 that are expected to yield different contributions to the C=O
dipolar coupling 푫 ∙ 푬푐푎푣. The well-aligned carbonyl groups of the cis-trans isomer resulting
in the larger resulting permanent dipole moment should increase the coupling strength with the
intracavity electric field relative to the trans-trans isomer. We can then anticipate that under
VSC, the electrocyclic ring-opening is driven accordingly: the conrotation path leads to the cis-
trans isomer that favours VSC over the disrotation path. From a thermodynamic viewpoint, this
corresponds to an increased stabilization of the conrotatory transition state.
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The thermodynamics of the electrocyclic ring-opening reaction under VSC of both the
C=O and the C-H vibrations were determined by measuring the reaction rate as a function of
temperature (Figure S5, Supporting Information). The thermodynamic parameters H‡, S‡
and G‡ were then determined from the Eyring plot, ln(k/T) vs 1/T:
‡
‡
−∆퐻
1
푘
∆푆
ln(푘푇) =
푇 + {ln ( ℎ ) +
} (4)
퐵
푅
푅
where kB, h and R are the Boltzmann, Plank’s and gas constants respectively.H‡ and S‡ are
extracted from the slope and intercept from the plot ln(k/T) vs 1/T (Figure S5, Supporting
Information). The results are summarized in Table 2. When compared to non-cavity
Table 2. Thermodynamic parameters in non-cavity and cavity conditions tuned to different
vibrational modes.
H‡ (kJ mol-1)
S‡ (J mol-1)
G‡ (kJ mol-1)
Experiment
cis-trans trans-trans cis-trans trans-trans cis-trans
trans-trans
Non-cavity
C=O coupling
C-H coupling
93
79
85
92
82
84
5
-18
-46
-37
91
88
93
99
99
98
-24
-21
conditions, VSC on both C=O and C-H vibrations shows reduced H‡ and S‡ for cis-trans and
trans-trans products. However, the relative Gibbs free energy change between the con- and
disrotatory transition states is increased upon VSC on C=O (10.8 ±0.5 kJ mol-1), while it is
reduced under VSC on C-H (4.9 ±0.5 kJ mol-1), when compared to non-cavity condition (7.6
±0.5 kJ mol-1). These thermodynamic changes are represented in Figure 4 and are entirely
consistent with the cavity effects discussed above, in that VSC leads to stabilizing either
symmetry-allowed conrotatory or symmetry-forbidden disrotatory products depending on
which C=O or C-H vibrational band is coupled.
9
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Figure 4. Energy level diagram for the con- and disrotatory electrocyclic ring-opening of CB
under non-cavity and VSC conditions. The relative Gibbs free energy change (G‡) between
symmetry-allowed conrotatory and symmetry-forbidden disrotatory transition states is altered
by coupling to C=O (blue line) and C-H (red line), when compared to the non-cavity condition.
In conclusion, the modification of the stereoselectivity of a pericyclic reaction as
studied here confirms the key role of symmetry in chemistry under VSC. The cavity vacuum
field imposes symmetry constraints that facilitate either the con- or disrotation pathways in
such cyclic ring-opening reaction and thereby the selectivity of the Woodward-Hoffmann rules.
Further such studies should enrich our understanding of the details of chemistry under VSC.
Acknowledgements
We acknowledge support of the International Center for Frontier Research in Chemistry
(icFRC, Strasbourg), the ANR “ERA‐NET QuantERA” – Projet “RouTe” (ANR‐18‐QUAN‐
0005‐01), the ANR Equipex Union (ANR‐10‐EQPX‐52‐01), CSC (ANR‐10‐LABX‐ 0026
CSC) within the Investissement d'Avenir program ANR‐10‐IDEX‐0002‐02. TWE and JM both
acknowledge the support of the ERC (project no 788482 MOLUSC and no 639170 CarbonFix,
respectively). A.S. thanks the EU for a H2020 Marie Skłodowska Curie Fellowship (792101).
Conflict of Interest
The authors declare no conflict of interest.
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Table of Contents
Vacuum field strong coupling with molecular vibrations imposes symmetry constraints
which alter the Woodward-Hoffmann selectivity of cyclobutene ring opening under
thermal conditions.
12
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