“π-Hole?π” Interaction Promoted Photocatalytic Hydrodefluorination via Inner-Sphere Electron Transfer

Article abstract of DOI:10.1021/jacs.6b08620

We describe a metal-free, photocatalytic hydrodefluorination (HDF) of polyfluoroarenes (FA) using pyrene-based photocatalysts (Py). The weak “π-hole?π” interaction between Py and FA promotes the electron transfer against unfavorable energetics (ΔG_ET up to 0.63 eV) and initiates the subsequent HDF. The steric hindrance of Py and FA largely dictates the HDF reaction rate, pointing to an inner-sphere electron transfer pathway. This work highlights the importance of the size and shape of the photocatalyst and the substrate in controlling the electron transfer mechanism and rates as well as the overall photocatalytic processes.

Full text of DOI:10.1021/jacs.6b08620

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Communication
“#-Hole–#” Interaction Promoted Photocatalytic
Hydrodefluorination via Inner-Sphere Electron Transfer
Jingzhi Lu, Navneet Singh Khetrapal, Jacob A. Johnson, Xiao Cheng Zeng, and Jian Zhang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b08620 • Publication Date (Web): 28 Nov 2016
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“π-Hole–π” Interaction Promoted Photocatalytic Hydrodefluorina-
tion via Inner-Sphere Electron Transfer
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Jingzhi Lu, Navneet S. Khetrapal, Jacob A. Johnson, Xiao Cheng Zeng, and Jian Zhang*
Department of Chemistry, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States
Supporting Information Placeholder
Polyfluoroarene–arene (also known as “π-hole14–π”) inter-
ABSTRACT: We describe a metal-free, photocatalytic hy-
action is a directional and non-covalent intermolecular force.
It originates from the weak electrostatic interaction between
arenes (negative surface potential) and polyfluoroarenes
(positive surface potential due to the flipped quadruple mo-
ment)15 and has found applications in crystal engineering,16
controllable reaction,17 and in some cases, regioselective ca-
talysis.18 Here, we chose three highly fluorescent pyrene de-
rivatives Py1, Py2, and Py3, as the photocatalysts and three
polyfluoroarenes, namely, HFB (hexafluorobenzene), PFB
(pentafluorobenzene), and TFB (1,2,3,4-tetrafluorobenzene),
as the substrates (Figure 1). The three Py photocatalysts pro-
vide a systematic variation of steric hindrance, a critical fac-
tor for inner-sphere ET.19 Py2 and Py3 also appreciably ab-
sorb visible light (λ>390 nm, Figure 1a). Importantly, the
LUMO energies of Py are lower than those of FA (Figure 1b),
suggesting not only it is more difficult to directly reduce FA
(from an external electron donor), but also the ET from *Py
to FA is unfavorable. Indeed, according to the outer-sphere
model,20 ∆GET was calculated ranging from 0.12 eV to 0.72 eV
(based on the Weller equation, see Supporting Information
Table S1 for details), which is also indicated by the negligible
Stern-Volmer quenching constants (KSV < 0.1 M–1, Figure S8).
We envisioned that, however, upon the formation of the “π-
hole–π” Py:FA complexes, a good molecular orbital overlap
could become possible to facilitate the inner-sphere ET to
form FA•–. Followed by the expulsion of fluoride and hydro-
gen atom abstraction, a HDF process of FA shall be devel-
oped.
drodefluorination (HDF) of polyfluoroarenes (FA) using py-
rene-based photocatalysts (Py). The weak “π-hole–π” interac-
tion between Py and FA promotes the electron transfer
against unfavorable energetics (∆GET up to 0.63 eV) and ini-
tiates the subsequent HDF. The steric hindrance of Py and
FA largely dictates the HDF reaction rate, pointing to an
inner-sphere electron transfer pathway. This work highlights
the importance of the size and shape of the photocatalyst
and the substrate in controlling the electron transfer mecha-
nism and rates as well as the overall photocatalytic processes.
Electron transfer (ET) is a key step reaction in many organ-
ic transformations.1 For example, photoredox catalysis typi-
cally involves light-induced, non-adiabatic outer-sphere ET
within a loose encounter complex formed between the excit-
ed photocatalyst and the substrate,2 during which a suitable
overpotential, according to Marcus theory, is generally desir-
able to achieve fast ET kinetics.3 If the ET step is energetical-
ly unfavorable, it is necessary to either non-covalently (e.g.
via Lewis acid,4 Brønsted acid,5 or hydrogen bond6) or cova-
lently (e.g. via organocatalysis7) modulate the substrate’s
redox potentials. Inner-sphere ET,8 on the other hand, occurs
adiabatically within an electron donor-acceptor (EDA) com-
plex, for instance, where the strong electronic coupling cir-
cumvents the crossing of high potential energy surface, and
thus proceeds significantly faster than that predicted by the
outer-sphere model.9 Moreover, since the charge-transfer
(CT) transition of the EDA complex often appears in the visi-
ble region, the use of photocatalyst can be avoided.10 In the
past, EDA complexes formed between two substrates that
constitute the ET partners have been utilized for synthesis.11
Recent work by Melchiorre et al. on asymmetric pho-
to(organo)catalysis further significantly expanded their ap-
plication scope.12
Herein, we describe a new example in which the inner-
sphere ET between substrate and photocatalyst plays a criti-
cal role to overcome the unfavorable ET energetics. Due to
the “π-hole–π” interaction between polyfluoroarenes (FA)
and pyrene-based photocatalysts (Py), photo-induced ET
proceeds smoothly against a large underpotential (∆GET up
to 0.63 eV) and can be best described as an inner-sphere pro-
cess, which is subsequently utilized to promote a hydro-
defluorination (HDF) reaction to afford partially fluorinated
arenes.13
Figure 1. (a) UV-vis absorption and fluorescence emission spectra
(λex = 360 nm) of Py in DMA (dimethylacetamide). (b) Reduction
potentials (all potentials mentioned hereafter are against Fc+/Fc0, Fc
= ferrocene, see Figure S1-2 for cyclic voltammograms) and density
functional theory (DFT) computed electrostatic surface potential
maps (inset) of Py and FA.
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We first use 1H NMR titration to evaluate the intermolecu-
resulted in a diminished yield (18%, 48 h), which however is
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lar interaction of the nine Py-FA pairs in the ground state.
The formation constant (Kc, based on a 1:1 stoichiometry) was
obtained for Py3:HFB (1.11 M–1), Py3:PFB (0.64 M–1), Py2:HFB
(0.82 M–1), and Py2:PFB (0.64 M–1) in CDCl3 (Figure 2a, see
Supporting Information S-4 for details).21 Kc of Py3:TFB and
Py2:TFB were not reliably detectable, likely due to the rela-
tively weak “π-hole” character of TFB. DFT-based structure
optimization also confirmed that the binding energy de-
creases in the order of Py3:HFB>Py3:PFB>Py3:TFB (see Sup-
porting Information S-14). As for three Py1:FA pairs, a small
Kc (0.024 M–1) was obtained for Py1:HFB, which however
should not be considered as a real “π-hole–π” complex since
the large steric hindrance of Py1 makes the good contact be-
tween the two π planes unlikely.
attributed to the low power of the light source and the con-
siderably smaller absorption coefficient (ε) of Py3 in this op-
tical window (Figure S9). As a comparison, with the irradia-
III/II
tion of the blue LED, strongly reductive Ir(ppy)3 (E1/2
=
−2.77 V, an overpotential of +0.21 V, Figure S1) afforded PFB
with a similar yield (21%, Figure 3a).24 Since the absorbance
coefficients of Py3 and Ir(ppy)3 are comparable around 465
nm (Figure S9), their similar photocatalytic efficiency sug-
gests the “π-hole–π” interaction in Py3:HFB complex is in-
deed important to overcome the endergonic ET energetics.
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H
F
Photocatalyst (1 mol%)
DIPEA (1.1 eq)
(a)
F
F
F
F
F
F
F
F
DMA, Ar
26 W CFL, 12 h, 45 oC
F
F
entry
1
photocatalyst
Py3
light source
CFL
% yield (19F NMR)
51
0a
2
3
4
5
6
7
Py3
Py3
Py3
Py3
Py3
Ir(ppy)3
CFL
CFL
CFL
CFL
Blue LED
Blue LED
0b
0c
92 (24 h)
18 (48 h)
21 (48 h)
aNo light. bNo photocatalyst. cNo DIPEA
Figure 3. (a) HDF reaction for Py3:HFB. (b) HDF yields and (c) ∆GET
for nine Py:FA pairs under the reaction condition shown in Figure
3a.
Figure 2. (a) Formation constants of nine Py:FA pairs. (b) Top and
side view of single crystal structure of Py3:HFB. (c) UV-vis spectra of
Py3, Py2, and their mixtures with HFB and PFB in DMA (at the Py:FA
molar ratio < 1:30,000).
We next screened the HDF reaction for nine Py:FA pairs.
Overall, the four Py:FA pairs with relatively large Kc gave the
HDF product in good yields (12 h, 32-66%, unoptimized)
(Figure 3b). In particular, the good reactivity of Py3:PFB (Kc =
0.64 M–1, 33% yield) is highly remarkable considering the
large ET underpotential (∆GET = 0.63 eV). In contrast, the
HDF of Py3:TFB and Py2:TFB was drastically sluggish (3%
and 2%, respectively), which is attributed to the significantly
smaller Kc of the Py:TFB complexes (Figure 2a). As for the
three weak Py1:FA pairs with large steric hindrance, a de-
crease of HDF yield from 38% (Py1:HFB) to 7% (Py1:TFB) was
observed as expected on the basis of the increased ∆GET (Fig-
ure 3b and 3c).
Single crystal structure analysis confirmed the expected “π-
hole–π” interaction in three Py3:FA pairs, including the weak
Py3:TFB complex. All three Py3:FA crystal structures reveal
the 1:2 stoichiometry and the alternating parallel stacking
with a small dihedral angle (1.05°–2.37°) (Figure 2b and S7).22
Most importantly, the average inter-plane distances (3.33–
3.35 Å) are all below the sum of the individual vdW radii of
two aromatic molecules (~3.45 Å23), suggesting the stronger
electrostatic interaction in Py3:FA than the typical π–π stack-
ing force. UV-vis spectra provide further support for the elec-
trostatic nature of the “π-hole–π” interaction: no apparent
bathochromic shift (an indication of CT transition) was ob-
served (Figure 2c and S6).
We next chose Py3:HFB pair (Kc = 1.11 M–1, ∆GET = 0.36 eV)
to test the proposed HDF. To our delight, the reaction in the
presence of a sacrificial electron donor and white light irradi-
ation (26 W compact fluorescent lamp, CFL) at 45oC in 12 h
achieved the desired product PFB with a 51% yield (Figure
3a). Using the optimized condition (solvent DMA and amine
DIPEA: diisopropylethylamine) (see Supporting Information,
Table S6-7), we further confirmed the essential role of light
irradiation, photocatalyst, and amine (Figure 3a). Extending
the reaction time to 24 h gave rise to an excellent yield (92%)
(Figure 3a). The use of a blue LED (0.135 W, λmax = 465 nm)
Since the injection of an electron to FA is presumably the
rate-limiting step of the overall HDF when the fragmentation
of C-F is a sufficiently fast process,25 a systematic comparison
of the relative initial reactions rates should provide insights
into the elemental ET kinetics. If inner-sphere mechanism is
operative, one would expect that a high concentration of
substrate FA would result in a fast initial reaction rate due to
the availability of a larger amount of Py:FA complex. This is
indeed the case for Py3:PFB and Py2:PFB, where the relative
initial reaction rate (see Supporting Information S-11 for de-
tails) nearly doubled when [PFB] increased from 0.075 M to
0.30
M
(Figure 4a). Surprisingly, such substrate-
concentration-dependent reaction rate was also observed for
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weak Py3:TFB and Py2:TFB pairs albeit at much higher FA
(1c) and octafluoronaphthalene (OFN, 1d), and it is compati-
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concentrations (e.g. 1.56 M) (Figure S11). In contrast, no sig-
nificant increase of reaction rate was observed for three
Py1:FA pairs including Py1:HFB with a measurable Kc (Figure
4a and S11),26 suggesting the outer-sphere ET pathway being
dominant in these cases. The importance of “π-hole–π” inter-
action is further supported via the systematic change of the
steric hindrance of substrate. For example, the reaction be-
tween Py2 and p-tolyl-PFB gives a significant higher yield of
the HDF product (70%) than sterically hindered o-tolyl-PFB
(31%) and mes-PEB (22%) that have comparable E1/2 (Figure
4b and S2).
ble with an array of functional groups including aryl (1e),
ester (1f), CF3 (1g), and ether (1h) with the regioselectivity
that can be rationalized based on the maximum spin density
of the C-F bond in FA•– (Table 1).30 Notably, under the opti-
mized reaction condition, HDF of PFB afforded the corre-
sponding 1,2,4,5-TFB (2b) with good yield (80%). It is also
possible to synthesize 2b directly from HFB via one-pot di-
HDF (4 equiv of DIPEA, 76% yield). Monitoring the reaction
process via 19F NMR revealed that the second HDF reaction
did not start until most of HFB was converted to PFB (Figure
S10), demonstrating a minimum effect of product inhibition.
To the best our knowledge, this is the first example of metal-
free catalytic HDF of PFB under mild condition with high
efficiency. Py2 is highly robust. A TON of 24,250 was ob-
tained when 0.002 mol% of Py2 was used for the HDF of HFB
(see Supporting Information S-13 for details). Interestingly,
Py2 can convert OFN (a strong π-hole) into the HDF product
2d with a lower yield (63%) along with two di-HDF products
(see Supporting Information S-16), suggesting a possible
product inhibition, that is, the mono-HDF product 2d now
effectively competes with OFN for the subsequent di-HDF
reaction due to its large π-hole strength.
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Figure 4. (a) Normalized relative initial reaction rate of Py:PFB and
Py1:HFB at different substrate concentration. (b) HDF reaction yield
of p-tolyl-PFB, o-tolyl-PFB, and mes-PFB with Py2 at reaction condi-
tion outlined in Figure 3a.
Table 1. Scope of Photocatalytic HDF of FAa
Py2 (1 mol%)
DIPEA (1.1 eq)
R
R
Fn-1
Fn
Based on the HDF activities described above, a photocata-
lytic cycle is proposed (Scheme 1). Sterically less hindered
Py2 and Py3 can, in principle, undergo the inner-sphere
pathway as soon as the “π-hole–π” complex is formed and a
good orbital overlap is achieved. Upon light irradiation, the
“local” excitation of Py2/Py3 in the “π-hole–π” complex is first
reductively quenched by a sacrificial electron donor (i.e.
DIPEA, see Figure S8 for Stern-Volmer quenching experi-
ments), forming an anionic radical complex intermediate
[Py:FA]•–. Alternatively, it is also possible for *Py to form the
excited encounter “π-hole–π” complex [Py:FA]* (with for-
mation constant KEC).27 An inner-sphere ET within [Py:FA]•–
and subsequent complex dissociation regenerates Py and
affords FA•–,28 which undergoes the fast intramolecular disso-
ciative ET to form the productive aryl radical by expulsion of
the fluoride anion.29 Following a hydrogen atom abstraction
from DIPEA radical cation, the HDF product is obtained.
DMA, Ar
26 W CFL, 24 h, 45 o
H 2
C
1
b
c
a19F NMR yield. 3.0 equiv DIPEA. 4.0 equiv DIPEA,
HFB as the substrate, d2.0 equiv DIPEA.
Scheme 1 Proposed Photocatalytic Cycle for HDF
We further demonstrated the potential utility of Py in the
metal-free C–F reductive alkylation (eq 1). Weaver et al. re-
ported this reaction using Ir(ppy)3 as the photocatalyst.31
Here, Py3 was used as a metal-free photocatalyst to generate
the perfluorophenyl radical that is intercepted by 6.0 equiv
of cyclohexene to afford the C-C coupled product 3a with a
good yield (60%, determined by 19F NMR).
F
F
Py3 (1 mol%)
DIPEA (1.1 eq.)
F
F
F
F
F
+
(1)
EtCN, Ar
26 W CFL, 24 h, 45 o
F
F
F
C
F
1 eq.
6 eq.
3a, 60% yield
We next turned our attention to exploit the utility of Py for
the HDF of FA. Based on the proposed mechanism,
polyfluoroaromatics with small steric hindrance and appro-
priate binding capabilities with Py are expected to be suitable
substrates. Indeed, Py2 works well with common polyfluoro-
aromatics including HFB (1a), PFB (1b), pentafluoropyridine
In summary, we have described a new example showing
the inner-sphere ET between photocatalyst and substrate
plays an important role in the overall photocatalytic reaction.
The appreciable formation constant and favorable steric hin-
drance within the “π-hole–π” Py-FA complexes facilitate an
inner-sphere ET despite unfavorable ET energetics. This pro-
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1979, 101, 5961; (c) Fox, M. A.; Younathan, J.; Fryxell, G. E. J. Org.
cess is utilized in the hydrodefluorination reaction to access
partially fluorinated arenes. Our work points to the further
development of the design paradigm for photoredox catalysis
where the size32 and shape of photocatalyst can be fine-tuned
to enhance the overall catalytic activity. This work also con-
stitutes a new example of the utility of weak, non-covalent
interaction in small molecule catalysis.33
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ASSOCIATED CONTENT
Supporting Information
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2009, 11, 942; (b) Kuehnel, M. F.; Lentz, D.; Braun, T. Angew. Chem.
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Materials, general procedures, synthesis, physical measure-
ments, spectroscopic characterizations, CV diagrams, and
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Bauza, A.; Mooibroek, T. J.; Frontera, A. ChemPhysChem
2015, 16, 2496.
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(a) Gdaniec, M.; Jankowski, W.; Milewska, M. J.; Polonski,
AUTHOR INFORMATION
Corresponding Author
T. Angew. Chem. Int. Ed. 2003, 42, 3903; (b) Hori, A.; Shinohe, A.;
Yamasaki, M.; Nishibori, E.; Aoyagi, S.; Sakata, M. Angew. Chem. Int.
Ed. 2007, 46, 7617.
jzhang3@unl.edu
(17)
(a) Xu, R.; Gramlich, V.; Frauenrath, H. J. Am. Chem. Soc.
Notes
2006, 128, 5541; (b) Xu, R.; Schweizer, W. B.; Frauenrath, H. J. Am.
Chem. Soc. 2008, 130, 11437.
The authors declare no competing financial interests.
(18)
(a) El-azizi, Y.; Schmitzer, A.; Collins, S. K. Angew. Chem.
ACKNOWLEDGMENT
Int. Ed. 2006, 45, 968; (b) Holland, M. C.; Paul, S.; Schweizer, W. B.;
Bergander, K.; Muck-Lichtenfeld, C.; Lakhdar, S.; Mayr, H.; Gilmour,
R. Angew. Chem. Int. Ed. 2013, 52, 7967.
The authors acknowledge the support from the University of
Nebraska−Lincoln. We also thank the Donors of the Ameri-
can Chemical Society Petroleum Research Fund (53678-
DNI10) and National Science Foundation (DMR-1554918) for
partial support of this research.
(19)
(a) Hubig, S. M.; Rathore, R.; Kochi, J. K. J. Am. Chem. Soc.
1999, 121, 617; (b) Luo, P.; Dinnocenzo, J. P.; Merkel, P. B.; Young, R.
H.; Farid, S. J. Org. Chem. 2012, 77, 1632.
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1617.
(21)
Tucker, J. W.; Stephenson, C. R. J. Org. Chem. 2012, 77,
Systematic measurement of Kc and reaction rate (vide infra)
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