Photo-Induced ortho-C-H Borylation of Arenes through in Situ Generation of Rhodium(II) Ate Complexes

Article abstract of DOI:10.1021/jacs.1c05859

Photoinduced in situ "oxidation"of half-sandwich metal complexes to "high-valent"cationic metal complexes has been used to accelerate catalytic reactions. Here, we report the unprecedented photoinduced in situ "reduction"of half-sandwich metal [Rh(III)] complexes to "low-valent"anionic metal [Rh(II)] ate complexes, which facilitate ligand exchange with electron-deficient elements (diboron). This strategy was realized by using a functionalized cyclopentadienyl (CpA3) Rh(III) catalyst we developed, which enabled the basic group-directed room temperature ortho-C-H borylation of arenes.

Full text of DOI:10.1021/jacs.1c05859

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Photo-Induced ortho-C−H Borylation of Arenes through In Situ
Generation of Rhodium(II) Ate Complexes
̂
Jin Tanaka, Yuki Nagashima,* Antonio Junio Araujo Dias, and Ken Tanaka*
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ABSTRACT: Photoinduced in situ “oxidation” of half-sandwich metal complexes to “high-valent” cationic metal complexes has
been used to accelerate catalytic reactions. Here, we report the unprecedented photoinduced in situ “reduction” of half-sandwich
metal [Rh(III)] complexes to “low-valent” anionic metal [Rh(II)] ate complexes, which facilitate ligand exchange with electron-
deficient elements (diboron). This strategy was realized by using a functionalized cyclopentadienyl (CpA3) Rh(III) catalyst we
developed, which enabled the basic group-directed room temperature ortho-C−H borylation of arenes.
hotoinduced transition-metal-catalyzed reactions have
emerged as a new synthetic strategy through in situ
Neutral Rh(II) complexes, which have been widely used for
carbenoid reactions, generally form a dimer [such as
Rh2(OAc)4] to stabilize their unpaired electrons.16−18 In situ
generations of neutral CpRh(II) complexes were also reported
but not used for catalytic reactions.19,20 On the contrary, an
anionic Rh(II) complex, which would be generated by one-
electron reduction of the corresponding Rh(III) complex, is
believed to form an anionic monomer because the rhodium
center is coordinatively saturated (19 electrons). However,
synthesis or in situ generation of anionic CpRh(II) complexes
have not been reported probably due to the low reduction
potential of CpRh(III) complexes.21,22 Thus, their reactivities
and applications to organic synthesis have been unrevealed for
a long time. We hypothesized that CpRh(III) complex A
bearing appropriately tuned arene and Cp ligands would be
photoexcited to its triplet-state 3A*, which would be subjected
to a one-electron reduction. Besides, unlike “high-valent
cationic” metal complexes B, higher-lying HOMO of anionic
CpRh(II) complex C would facilitate ligand exchange with
electron-deficient elements, such as boron, by strong electro-
static interaction, which would allow the photoinduced ortho-
C−H borylation of arenes under mild conditions (Figure 1b).
To test the feasibility of our working hypothesis shown in
Figure 1b, we first performed time-dependent density func-
tional theory (TDDFT) calculations of pentamethylcyclopen-
tadienyl (Cp*) aryl Rh(III) acetates bearing different directing
groups (Figure 1d).23 The TDDFT calculation of the
absorption spectrum of basic pyrazole complex A1 indicated
that the visible absorption is assigned to the HOMO →
LUMO transition, and ligand-to-metal charge-transfer
(LMCT)24−30 character is predominant (Figure S4). In
P
modulation of the oxidation state of metal-containing
intermediates, which provides access to complementary
activation modes to those conventionally used.1−5 Recently,
a series of half-sandwich d6 transition-metal [Ir(III), Rh(III),
and Ru(II)] catalysts, which are widely used for directed ortho-
C−H functionalizations of arenes by concerted-metalation
deprotonation (CMD),6−11 has also been combined with
photoinduced reactions and allowed room temperature C−C
bond formations. Since 2019, Ackermann and Greaney
independently reported elegant systems of photoinduced
Ru(II)-catalyzed C−H alkylation12,13 by in situ oxidation of
neutral Ru(II) complex A to cationic Ru(III) complex B
(Figure 1a, top). In 2020, Chang designed a sophisticated
cyclopentadienyl (Cp) ligand bearing two functional domains
for photosensitization and C−H carbometalation. This novel
ligand enabled photoinduced Rh(III)-catalyzed C−H aryla-
tion14 through in situ generations of cationic CpRh(IV)
complex B (Figure 1a, bottom). Photoinduced in situ
generations of these “high-valent cationic” half-sandwich
transition-metal complexes B1−5 facilitate rate-determining
steps, including ligand exchange (LE)12,13 or reductive
elimination (RE)14 while maintaining smooth C−H activation
by CMD. However, photoinduced in situ generation of a “low-
valent anionic” half-sandwich transition-metal complex that
would facilitate ligand exchange with electron-deficient
coupling partners (e.g., diboron) has not been utilized in the
C−H functionalizations. Here, we report ortho-C−H bor-
ylation of arenes using unprecedented photoinduced in situ
generation of “anionic” CpRh(II) ate complex C through
“reduction” of CpRh(III) complex A (Figure 1b). Basle
recently reported a visible-light-induced rhodium-catalyzed
borylation using a sophisticated N-heterocyclic carbene−Rh(I)
catalyst (Figure 1c),15 while this protocol showed a limited
scope of directing groups (2-pyridyl arenes). Our strategy
using the CpRh(II) ate complex C expands the scope of
directing groups by the facilitation of ligand exchange with
diboron reagents.
́
Received: June 7, 2021
Published: July 20, 2021
https://doi.org/10.1021/jacs.1c05859
J. Am. Chem. Soc. 2021, 143, 11325−11331
© 2021 American Chemical Society
11325

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Communication
a,b
Table 1. Optimization of Reaction Conditions
yield
c
entry
catalyst
base
none
NaOAc
additive
(%)
1
2
3
4
5
6
7
8
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[CpRhI2]n
[CpERhCl2]2
[CpA1RhCl2]2
[CpA2RhCl2]2
[CpA3RhCl2]2
[CpA4RhCl2]2
[Cp*IrCl2]2
none
none
0
11
6
19
0
NaOMe none
NaOtBu none
NaOtBu none
NaOtBu none
NaOtBu none
NaOtBu none
NaOtBu none
NaOtBu none
NaOtBu none
NaOtBu none
0
47
53
65
41
0
9
10
11
12
[RuCl2(p-
cymene)]2
21
d
13
[CpA3RhCl2]2
[CpA3RhCl2]2
[CpA3RhCl2]2
[CpA3RhCl2]2
NaOtBu none
0
58
67
50
14
15
16
NaOtBu [Ir(ppy)3]
NaOtBu [Ir(dtbbpy)(ppy)2]PF6
NaOtBu [Ir(dF(CF3)
ppy)2(dtbbpy)]PF6
Figure 1. Photoinduced in situ oxidation (previous works) and
reduction (this work) strategies. CMD = concerted-metalation
deprotonation. ISC = intersystem crossing. PS = photosensitizer.
17
18
19
20
[CpA3RhCl2]2
[Cp*RhCl2]2
[CpRhI2]n
[RuCl2(p-
cymene)]2
NaOtBu [Ir(dFppy)3]
NaOtBu [Ir(dFppy)3]
NaOtBu [Ir(dFppy)3]
NaOtBu [Ir(dFppy)3]
81
5
0
18
contrast, amide complex A2 showed visible absorption
assigned to the HOMO → LUMO transition that is mainly
described as a metal-centered transition. The experimental
spectra of the complexes A1 and A3 showed good agreement
with the calculated spectra (Figure S2). This observation
indicates that Rh(III) intermediates with basic rather than
neutral directing groups would possess the favorable photo-
physical property for the long triplet lifetime through visible
light irradiation.
a
b
pin = pinacolato. Catalyst (0.0050 mmol Rh, Ir, or Ru), additive
(0.001 mmol), base (0.010 mmol), 1a (0.10 mmol), 2 (0.20 mmol),
c
and THF (1.0 mL) were used. Determined by 1H NMR using
d
mesitylene as an internal standard. Without blue LED irradiation.
improved yield of 65% (entry 9).36 Half-sandwich Ir(III) and
Ru(II) catalysts were ineffective for this borylation (entries 11
and 12). More importantly, the present borylation was
completely shut down without blue LED irradiation (entry
13). These results indicate that the appropriately function-
alized CpRh(III) complex catalyzes the ortho-C−H borylation
through a photoexcited triplet state. We surveyed various
iridium photocatalysts (1 mol %) to facilitate the electron-
transfer process (entries 14−17), which revealed that the use
of [Ir(dFppy)3] gave 3a in the highest yield of 81% (entry 17).
This facilitation using [Ir(dFppy)3] to the present borylation
was unique only to CpA3Rh(III) catalyst, not to Cp*Rh(III),
CpRh(III), and Ru(II) catalyst (entries 18−20).
Based on the above anticipations, we started with a
screening study to find a suitable half-sandwich d6 transition-
metal catalyst (5 mol % metal) by using 1-phenylpyrazole (1a),
bis(pinacolato)diboron (2, 2 equiv), and bases (10 mol %)
under blue LED irradiation (Table 1). When using the
Cp*Rh(III) catalyst, any conditions without base did not give
the desired product 3a, whereas the use of NaOAc, NaOMe,
and NaOtBu slightly promoted the borylation (entries 1−4)
and NaOtBu was the base of choice. Rh(III) complexes with
31,32
electron-deficient ligands (CpE
and Cp33,34) did not
promote any reactions (entries 5 and 6). Our previously
reported CpA1−3Rh(III) complexes35 were also tested (entries
7−10), which revealed that the CpA3Rh(III) complex bearing
the electron-rich N-phenylcarbamoyl moiety afforded 3a in an
The substrate scope of the photoinduced ortho-C−H
borylation is shown in Scheme 1a. The key findings are as
follows: 1) a wide variety of basic directing groups, such as
pyrazoles (1a and 1b), pyridines (1c−1e), and π extended
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a
1l) were also applicable; 4) arenes bearing electron-with-
drawing groups (o-F, m-/p-CF3, m-/p-Cl, p-Br, and p-phenyl)
as well as electron-donating groups (o-/m-/p-Me, m-/p-MeO,
and p-alkyl) were compatible to give the corresponding
borylated products in good to excellent yields; 5) sequential
borylation/oxidation processes were available to give phenol
products 4 without isolation of 3. In addition, arenes bearing
various basic groups (1a, 1c, and 1k) participated in the
present borylation even without a photocatalyst, which
revealed that this broad substrate scope is derived from our
developed CpA3Rh(III) catalyst.
Scheme 1. Photo-Induced ortho-C−H Borylation
Although there are many examples of the transition-metal-
catalyzed ortho-C−H borylation, these reactions suffered from
harsh conditions (reagents)37−44 and/or an excess amount of
substrates.45,46 Thus, the present room temperature borylation
with a broad substrate scope is worthy of note. Furthermore,
the present borylation is highly chemoselective, and only basic
groups act as directing groups because ligand-to-metal charge
transfer of Rh(III) intermediates is necessary (Figure 1d).
Thus, no C−H borylation at the ortho-position of the amide
(1m), CF3 (1r and 1u), halogens (1q, 1s, and 1w), and
methoxy moieties (1t and 1x) were detected. Moreover, the
borylation of 7 bearing both basic and neutral directing groups
exclusively afforded the basic group-directed product 8,
whereas a mixture of multiborylation products was generated
under the previously reported Ir(I)-catalyzed conditions47
(Scheme 1b). To demonstrate the synthetic utility of the
present borylation, we examined the one-pot borylation-
elementalization in the presence of the basic functional groups
(Scheme 2). Pleasingly, not only hydroxylation (4b) but also
Scheme 2. Synthetic Applications
a
[CpA3RhCl2]2 (0.0050 mmol), [Ir(dFppy)3] (0.002 mmol), NaOtBu
(0.020 mmol), 1 (0.20 mmol), 2 (0.40 mmol), and THF (2.0 mL)
were used. Cited yields were determined by 1H NMR (3) or of
b
c
d
isolated products (4). Without [Ir(dFppy)3]. For 72 h. Bis-
(neopentylglycolato)diboron (5) was used instead of 2.
e
[CpA3RhCl2]2 (0.010 mmol), [Ir(dFppy)3] (0.004 mmol), NaOtBu
(0.040 mmol), 1 (0.20 mmol), 2 (0.40 mmol), and THF (2.0 mL)
were used. For 48 h.
chlorination (9b), deuteration (10b), alkynylation (11b),
arylation (12b), alkylation (13b), and dimerization (14b)
reactions smoothly proceeded in one-pot starting from 1b
without isolation of boronate 3b.
We then sought to acquire mechanistic insights into the
present borylation reaction. First, control experiments were
examined as shown in Scheme 3a. No LED irradiation or no
f
heteroaromatics (1i and 1j) could be used to give the
corresponding arylboronates in excellent yields 2) oxygen and
methylene linkers between directing groups and arenes (1f−
1h) were tolerated in the reaction; 3) benzylamines (1k and
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Communication
results suggest that the current reaction might start from in situ
reduction of the Rh(III) complex to generate Rh(I) species,
and the following steps also involve the photoinduced
reduction process facilitated by the base that coordinates to
the diboron reagent.
Scheme 3. Experimental Mechanistic Studies and Plausible
Reaction Mechanisms
Based on the experimental mechanistic studies, plausible
reaction mechanisms are shown in Scheme 3c. First, as shown
in the top scheme, Rh(III) di-tert-butoxide D, generated from
[CpA3RhCl2]2 and NaOtBu, is slowly reduced by borate
complex E to give active Rh(I) complex F along with a
boronate ester [tBuO−B(pin)]. In the presence of a photo-
catalyst [Ir(III)], rapid oxidation of borate complex E with the
triplet state of the photocatalyst [Ir(III)*] results in the
generation of the anionic reductive state of the photocatalyst
[Ir(II)−], which instead reduced Rh(III) complex D to Rh(I)
complex F. Then, as shown in the bottom scheme, the
subsequent oxidative addition (OA, step I) of Rh(I) to the
ortho-C−H bond of an arene affords aryl Rh(III) hydride G.
Although the direct ligand exchange from G with diboron is
difficult, photoexcitation of G generates triplet state 3G* that is
more subject to the second reduction giving Rh(II) ate
complex H (SET, step II-A). A facile ligand exchange (LE,
step II-B) between electron-rich H and electron-deficient
diboron forms the Rh−B bond. Subsequent oxidation with the
photocatalyst affords Rh(III) boryl complex I. Finally,
reductive elimination (RE, step III) affords the desired
borylated product and regenerates Rh(I) complex F.
To gain detailed insight into the proposed mechanisms,
model calculations using 1c, bis(ethyleneglycolato)diboron
[B2(eg)2], and CpA3Rh(I) or CpA3Rh(III)(OMe)2 complexes
were performed. First, the initial reductive process of Rh(III)
to Rh(I) species was evaluated in Figure 2a. Both of the two
oxidations of borate complex INT-X to unstable boryl radical
species (INT-Y) are endothermic, but they are compensated
by a large energy release for the reformation of 1 equiv of
diboron [B2(eg)2]. On the other hand, this process is
dramatically changed to an exothermic process by employing
an iridium photocatalyst. These results clearly support that the
present reaction proceeds even with a single rhodium catalyst
but is accelerated by an iridium cocatalyst.
The calculated pathways from generated Rh(I) species
depicted in Figure 2b show that oxidative addition from
CpA3Rh(I) (step I, INT1 to INT2) and reductive elimination
from ArCpA3Rh(III)B(eg) (step III, INT5 to INT6) smoothly
proceed (ΔG‡ < 10 kcal mol−1). On the other hand, the CMD
process with CpA3Rh(III)(OMe)2 (step I’, INT1′ to INT2′) is
kinetically unfavored (ΔG‡ = 32.2 kcal mol−1) and thus
excluded from the plausible pathway.47,48 In addition, the
interconversion from INT2 to INT3′ is also excluded because
the release of unstable NaH with a large endothermic energy
(35.6 kcal mol−1) is required. In the CpA3Rh(I)/(III) catalytic
cycle, the rate-limiting step is the ligand exchange (step II,
INT3 to INT4), in which the highest activation energy (40.3
kcal mol−1) is required. In contrast, the blue-light induced
Rh(III) catalyst (with or without the Ir photocatalyst) shut
down the reaction. The base is also essential; no reaction was
observed in the absence of NaOtBu with or without Rh(III)
catalyst. In addition, the present borylation was inhibited by a
single electron transfer (SET) inhibitor [nitrobenzene (15)], a
radical oxidant [(2,2,6,6-tetramethylpiperidin-1-yl)oxy (16)],
and oxygen (under air), whereas a radical scavenger [9,10-
dihydroanthracene (17)] was able to coexist. These results
indicate that this reaction does not involve any free radical
species but a reduction process through a single electron
transfer. Since the optimum base (NaOtBu) used in the
present borylation does not promote the CMD pathway,47,48
Rh(III)-catalyzed C−H activation can be excluded. To identify
the possibility of C−H activation with Rh(I) species generated
by in situ reduction of the initial Rh(III) complex in the
presence of possible reducing agents (photocatalyst and/or
diboron), we synthesized Rh(I) complex 18 and examined its
catalytic activity. We found that 18 can catalyze the borylation
(39% yield of 3a) only under blue-light irradiation (Scheme
3b). In addition, the present reaction hardly proceeds in the
absence of NaOtBu with or without the Ir photocatalyst. These
3
excitation of INT2 followed by ISC leads to a triplet INT2*
(Figure 2c). This excitation might also be accelerated with an
excited iridium photocatalyst via the triplet energy transfer
from 3Ir(III) to INT2 [Rh(III)] (ΔG = −19.7 kcal mol−1). In
the excited triplet state, a single electron transfer from the
iridium photocatalyst or borate complex INT-X proceed with a
large energy release (−43.2 or −3.5 kcal mol−1, respectively) to
in situ generate Rh(II) ate complex INT2-Rh(II), which is
preferred to the endothermic direct ligand exchange (3INT2*
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Figure 2. Theoretical mechanistic studies. The changes of the Gibbs free energies calculated at the M06/6-311+G** (C, H, O, and N), SDD (Rh)
levels using PCM(THF)//M06/6-31G* (C, H, O, and N) and LANL2DZ (Rh) levels are shown in kcal mol−1.
to 3INT3*).49 Compared with Rh(III) complex INT3, cationic
aryl Rh(IV) complex INT3-Rh(IV), which could be generated
by a single-electron oxidation,12−14 still requires a high
activation energy (32.2 kcal mol−1) (Figure 2d). Indeed, the
structure of TS2-Rh(IV) is similar to that of TS2-Rh(III), and
stabilization by the cationic complex was not observed (Figure
S3). In contrast, Rh(II) ate complex INT3-Rh(II) smoothly
affords the neutral Rh(II) complex and borate hydride species
INT3-2-Rh(II) (ΔG‡ = 10.5 kcal mol−1), and subsequent
rapid ligand exchange of the boryl group forms the Rh−B bond
with a low activation energy (1.8 kcal mol−1) (Figure 2e). The
formation of borate hydride species is limited to the case of the
in situ generation of the Rh(II) ate complex. Thus, all attempts
to locate a similar borate hydride species from INT3-Rh(III)
or INT3-Rh(IV) resulted in a geometric rearrangement to
starting complexes without a barrier. These comparisons
support that in situ generation of the Rh(II) ate complex
allows the present photoinduced borylation reaction.
In conclusion, we have developed the first protocol for in situ
generations of “low-valent anionic” Rh(II) ate complexes
through photoexcited “reduction” of functionalized Cp (CpA3)
Rh(III) intermediates. This “reductive” strategy enables room
temperature C−H borylation of arenes by access to alternative
reaction pathways [ligand exchange with an electron-deficient
element (boron)], which has never been available by
conventional photoinduced “oxidative” strategies to form
“high-valent cationic” complexes. We believe that the present
“reductive” protocol provides complementary activation modes
to conventionally used “oxidative” ones in photoinduced
transition-metal-catalyzed reactions. Further applications of
this strategy to C−H functionalizations with electron-deficient
elements other than boron, such as silane and tin, and the
synthesis of a range of bioactive and functional compounds are
in progress.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c05859.
Experimental details, analytical data, and spectral
reproductions for all new compounds, and details of
computational studies (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Yuki Nagashima − Department of Chemical Science and
Engineering, Tokyo Institute of Technology, Meguro-ku,
Tokyo 152-8550, Japan; orcid.org/0000-0001-8470-
5638; Email: nagashima.y.ae@m.titech.ac.jp
Ken Tanaka − Department of Chemical Science and
Engineering, Tokyo Institute of Technology, Meguro-ku,
Tokyo 152-8550, Japan; orcid.org/0000-0003-0534-
7559; Email: ktanaka@apc.titech.ac.jp
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Authors
Jin Tanaka − Department of Chemical Science and
Engineering, Tokyo Institute of Technology, Meguro-ku,
Tokyo 152-8550, Japan
pubs.acs.org/JACS
Communication
(14) Kim, J.; Kim, D.; Chang, S. Merging Two Functions in a Single
Rh Catalyst System: Bimodular Conjugate for Light-Induced
Oxidative Coupling. J. Am. Chem. Soc. 2020, 142, 19052−19057.
(15) Thongpaen, J.; Manguin, R.; Dorcet, V.; Vives, T.; Duhayon,
C.; Mauduit, M.; Baslé, O. Visible Light Induced Rhodium(I)-
Catalyzed C−H Borylation. Angew. Chem., Int. Ed. 2019, 58, 15244−
15248.
̂
Antonio Junio Araujo Dias − Department of Chemical
Science and Engineering, Tokyo Institute of Technology,
Meguro-ku, Tokyo 152-8550, Japan
(16) Davies, H. M. L; Beckwith, R. E. J Catalytic Enantioselective
C−H Activation by Means of Metal−Carbenoid-Induced C−H
Insertion. Chem. Rev. 2003, 103, 2861−2903.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c05859
(17) Doyle, M. P. Perspective on Dirhodium Carboxamidates as
Catalysts. J. Org. Chem. 2006, 71, 9253−9260.
Notes
(18) Davies, H. M. L; Denton, J. R. Application of donor/acceptor-
carbenoids to the synthesis of natural products. Chem. Soc. Rev. 2009,
38, 3061−3071.
The authors declare no competing financial interest.
(19) Hu, Y.; Norton, J. R. Kinetics and Thermodynamics of H−/H•/
H+ Transfer from a Rhodium(III) Hydride. J. Am. Chem. Soc. 2014,
136, 5938−5948.
ACKNOWLEDGMENTS
■
The authors are thankful for Grants-in-Aid for Scientific
Research (No. JP20K22521 and JP21K14623 to Y.N. and No.
JP19H00893 to K.T.) from JSPS (Japan). We thank Umicore
for generous support in supplying the palladium and rhodium
complexes. A generous allotment of computational resources
from TSUBAME (Tokyo Institute of Technology) is gratefully
acknowledged.
(20) Moore, W. N. G; Henke, W. C.; Lionetti, D.; Day, V. W.;
Blakemore, J. D. Single-Electron Redox Chemistry on the [Cp*Rh]
Platform Enabled by a Nitrated Bipyridyl Ligand. Molecules 2018, 23,
2857−2873.
(21) Wanniarachchi, S.; Liddle, B. J.; Kizer, B.; Hewage, J. S.;
Lindeman, S. V.; Gardinier, J. R. Syntheses and Electronic Properties
of Rhodium(III) Complexes Bearing a Redox-Active Ligand. Inorg.
Chem. 2012, 51, 10572−10580.
(22) Piou, T.; Romanov-Michailidis, F.; Romanova-Michaelides, M.;
Jackson, K. E.; Semakul, N.; Taggart, T. D.; Newell, B. S.; Rithner, C.
D.; Paton, R. S.; Rovis, T. Correlating Reactivity and Selectivity to
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Activation Reactions: An Experimental and Computational Study. J.
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(23) For detailed calculations of all model complexes showing
several absorption peaks in the 300−550 nm region, in which each
orbital contribution for dominant transitions is different, see Figure
S2.
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3
(49) The direct pathway in the excited state (3INT3* to INT4*)
may be already excluded by the control experiment using Rh(I)
complex 18 without NaOtBu (Scheme 3b). Indeed, while 3TS2*
could not be identified, we obtained the potential energy diagram by
calculating some optimized structures around the transition state
(Figure S6), which indicates that the activation energy would be still
high (>30 kcal mol−1) in the excited state.
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J. Am. Chem. Soc. 2021, 143, 11325−11331