Synthesis of an elusive, stable 2-azaallyl radical guided by electrochemical and reactivity studies of 2-azaallyl anions

Article abstract of DOI:10.1039/d0sc04822d

The super electron donor (SED) ability of 2-azaallyl anions has recently been discovered and applied to diverse reactivity, including transition metal-free cross-coupling and dehydrogenative cross-coupling processes. Surprisingly, the redox properties of

Full text of DOI:10.1039/d0sc04822d

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Synthesis of an elusive, stable 2-azaallyl radical
guided by electrochemical and reactivity studies of
2-azaallyl anions†
Cite this: Chem. Sci., 2021, 12, 4405
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Grace B. Panetti,
Eric J. Schelter
Patrick J. Carroll, Michael R. Gau, Brian C. Manor,
*
*
and Patrick J. Walsh
The super electron donor (SED) ability of 2-azaallyl anions has recently been discovered and applied to
diverse reactivity, including transition metal-free cross-coupling and dehydrogenative cross-coupling
processes. Surprisingly, the redox properties of 2-azaallyl anions and radicals have been rarely studied.
Understanding the chemistry of elusive species is the key to further development. Electrochemical
analysis of phenyl substituted 2-azaallyl anions revealed an oxidation wave at E1/2 or Epa ¼ ꢀ1.6 V versus
Fc/Fc+, which is ꢁ800 mV less than the reduction potential predicted (Epa ¼ ꢀ2.4 V vs. Fc/Fc+) based on
reactivity studies. Investigation of the kinetics of electron transfer revealed reorganization energies an
order of magnitude lower than commonly employed SEDs. The electrochemical study enabled the
synthetic design of the first stable, acyclic 2-azaallyl radical. These results indicate that the reorganization
energy should be an important design consideration for the development of more potent organic
reductants.
Received 1st September 2020
Accepted 1st February 2021
DOI: 10.1039/d0sc04822d
rsc.li/chemical-science
of reactivity, electrochemical and structural data on these
reactive species remain underdeveloped.8,15–21 Notably, electro-
Introduction
chemical analysis of aryl and alkyl halide substrates reduced by
2-azaallyl anions reveal Epc values approaching ꢀ2.4 V versus
ferrocene (Fc/Fc+) (ꢀ2.0 V vs. SCE).22 The SET reactivity observed
with 2-azaallyl anions is reminiscent of organic super electron
donors (SEDs), which have garnered attention due to their
potential advantages over transition metal reducing agents,
including lower costs and toxicities (Scheme 2A). Additionally,
SEDs have been successfully developed for a variety of applica-
tions, including replacement of stoichiometric metal reducing
agents and use in redox ow batteries.23–25 To develop highly-
reducing organic compounds, chemists have designed SEDs
that become aromatic upon electron transfer (Scheme 2A). As
a result, reported SEDs have attained reduction potentials
approaching E1/2 ¼ ꢀ1.9 V versus Fc/Fc+ (E1/2 ¼ ꢀ1.5 V vs. SCE)
in DMF.26,27 However, even SEDs with potentials as low as ꢀ1.3 V
versus Fc/Fc+ (ꢀ0.8 V vs. SCE) have shown reactivity with aryl
and alkyl iodide substrates.26,28 The high energy barrier of
electron transfer (uphill by 0.6–0.8 V) is proposed to be over-
come by the irreversibility of iodide anion loss from the radical
anions.29–31 With the interesting reactivity of the 2-azaallyl
anion, and their poorly-dened structural and electrochemical
properties, we set out to investigate the electron transfer
behavior of phenyl substituted 2-azaallyl anions.
Over the last century, the diverse reactivity of 2-azaallyl anions
has been mapped, with new and unexpected reactivity being
discovered to date.1–7 Early in the inception of their chemistry, 2-
azaallyl anions were used primarily as umpolung nucleophiles
derived from imines.8 These intermediates react with a variety
of electrophiles resulting in C–C bond formation (Scheme
1A).4,7–12 Recently, however, this umpolung reagent was found to
activate electrophiles through one-electron processes (Scheme
1B). Using EPR spectroscopy, computational studies, and
radical trapping experiments, our team demonstrated the acti-
vation of aryl, alkyl, and vinyl halides through single electron
transfer (SET) pathways.3,13,14 We then applied the 2-azaallyl
anion SET to a radical ring closing method for the synthesis of
3-aminoethyl benzofurans (Scheme 1C, top).5 A method for the
dehydrogenative C(sp3)–C(sp3) coupling via a SET initiated
radical relay (Scheme 1C, middle) was also outlined.6 Recently,
Ohshima and coworkers introduced
a copper-catalyzed
synthesis of hindered amino acids by coupling 2-azaallyl radi-
cals with alkyl radicals (Scheme 1C, bottom).7 Despite over 100
years of research into 2-azaallyl anions and their different forms
Roy and Diana Vagelos Laboratories, Department of Chemistry, University of
Pennsylvania, 231 South 34th Street, Philadelphia, PA, USA. E-mail: pwalsh@sas.
upenn.edu; schelter@sas.upenn.edu
Herein we describe the structures and electron transfer
characteristics of isolated 2-azaallyl anions. Surprisingly, the
experimentally determined oxidation events occur at less
negative potentials than expected from benchmarking in our
† Electronic supplementary information (ESI) available. CCDC 1992767–1992771.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0sc04822d
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Scheme 2 (A) Murphy and co-workers' reported SEDs and their
electrochemical oxidation potentials. Also, the potentials of the elec-
trophiles to be discussed in this work. The potentials are reported
versus Fc/Fc+ and versus SCE in parentheses.27–32 The waves are listed
with the number of electrons reported and the solvent the cyclic
voltammetry was measured in. (B) This work, which provides isolation
of 2-azaallyl anions and a 2-azaallyl radical, measurements of their
electrochemistry, and investigation of their reactivities.
acids to facilitate the synthesis and characterization of the
proposed intermediates. We found that treating ketimine 1 with
a slight excess of lithium diethyl amide (LiNEt2) in 1,2-
Scheme 1 Divergent reactivity pathways reported for the phenyl
substituted 2-azaallyl anion. (A) 2-Electron nucleophilic addition; E ¼
aldehydes, imines, epoxides, carbon dioxide, etc. (B) An alternative 1-
electron SET process; R–X ¼ aryl halides and alkyl halides. (C)
Representative recent reactivity employing the 2-azaallyl anion,
exhibiting the importance of SET in its reactivity.5–7
previous reactivity studies (Scheme 2B).3,8,14,22 To further inves-
tigate this electrochemical event, isolation and full character-
ization of several 2-azaallyl anions and the corresponding
oxidation product, the 2-azaallyl radical, were performed. Single
crystal XRD data of these species revealed only small changes in
the overall structures between redox states, indicating a hidden
feature of the phenyl substituted 2-azaallyl anions: their low
activation energies of SET.
Results and discussion
Fig. 1 (A) Synthetic scheme of 2-azaallyls 2-Li, 2-Na, 2-K and 4-Na
Initial studies into in situ generated 2-azaallyl anions were from the corresponding imines. (B) Thermal elipsoid plots of
[Na(DME)3][1,3,3-triphenyl-2-azaallyl anion] 2-Na at the 30% proba-
bility level, representative of 2-Li and 2-K. Phenyl ring hydrogen atoms
and the counter cation removed for clarity. (C) Thermal elipsoid plots
of [Na(DME)3][1,1,3,3-tetraphenyl-2-azaallyl anion] 4-Na at the 30%
complicated by irreproducibility and unexpectedly high
numbers of waves observed in electrochemical experiments. To
address these issues, we sought to rst isolate the 2-azaallyl
anions and then study these reactive species. We chose a series
probability level. Hydrogen atoms and the counter cation removed for
of weakly nucleophilic bases with low-boiling point conjugate clarity.
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dimethoxyethane (DME) yielded an intense purple solution
(Fig. 1A). This purple compound was determined to be the ex-
pected lithium 2-azaallyl anion DME solvate 2-Li, isolated in
82% crystalline yield. Sodium amide base (NaNH2) generated
a similar dark purple solution upon reaction with 1, albeit more
slowly. From this solution we isolated the analogous sodium 2-
azaallyl anion 2-Na in 76% crystalline yield. An alternative,
rational synthesis of 2-K was realized employing potassium tert-
butoxide (KOtBu) as the base, providing 90% crystalline yield of
2-K from 1. With the goal of studying the electrochemistry, we
sought to prepare a 2-azaallyl anion more likely to exhibit
reversible oxidation. We rationalized that the 1,1,3,3-tetra-
phenyl 2-azaallyl anion would be sterically protected from
dimerization, a commonly observed side reaction in 2-azaallyl
chemistry.8,13,14 Thus, deprotonation of the imine N-benzhydryl-
1,1-diphenylmethanimine (3) using NaNH2 generated 4-Na in
analogy to the synthesis of 2-Na. All 2-azaallyl anions were iso-
lated as beautiful, strongly absorbing purple crystalline solids (3
¼ 14 000 molꢀ1 cmꢀ1 L, Fig. S21†) that were stable under inert
atmosphere. These anions, however, rapidly decolorized and
decomposed upon exposure to trace oxygen or water.
Single crystal X-ray diffraction (SCXRD) was performed on all
four isolated 2-azaallyl anions. X-ray diffraction quality crystals
of compounds 2-Li, 2-Na, and 4-Na grew from cooled, concen-
trated solutions of DME layered with pentane, while compound
2-K crystallized from hot concentrated solutions of toluene
upon cooling to room temperature. All compounds were found
to have two phenyl rings that were almost perfectly co-planar
with the 2-azaallyl moiety, while the other phenyl rings
rotated out of conjugation (Fig. 1B and C). Compounds 2-Li, 2-
Na, and 2-K all have a similar 2-azaallyl anion core, with
a C–N–C angle of 123.9(8)–124.46(16)ꢂ, while 4-Na has an
expanded angle of 131.61(11)–132.84(11)ꢂ. We suspect that this
increased angle is due to the steric repulsion of the two internal
phenyl rings.
Next, we aimed to (1) validate our hypothesis that arylation
and alkylation occur via direct SET between the anions and aryl
and alkyl iodides (Scheme 2B) and (2) determine if any cation
effects are operative. By following our previously optimized
conditions and replacing NaN(SiMe3)2 and imine 1 with the 2-
azaallyl anions (2-M), we compared the reactivity of the isolated
anions with the in situ generated anions.14 When phenyl iodide
was treated with 2 equiv. of 2-M in DME, the yields of 3 and 30
were 59% for 2-Li, 66% for 2-Na, and 6% for 2-K. The yields for
2-Li and 2-Na t well with our results with in situ generation of 2-
azaallyl anions from ketimine 1 and NaN(SiMe3)2 (72%, Scheme
3A). The yields of 3-Ad from the reaction of adamantyl iodide
with either 2-Li or 2-Na were 56% and 48%, respectively. The in
situ generated anion from 1 and NaN(SiMe3)2 (Scheme 3B)
produced the expected alkylated product in 86% yield. The
higher yield from the in situ generated 2-azaallyl anion may be
due, in part, to the cations of 2-Li and 2-Na being bound by DME
rather than the reaction solvent, methyl tert-butyl ether (MTBE).
To test this hypothesis, we modied our in situ conditions by
adding 3 equiv. of DME per sodium cation (9 equiv. total) to the
reaction medium, resulting in a reduction of the yield of 3-Ad to
44% (Scheme 3B). This observation compares well with the use
Scheme 3 (A) Comparison of the reaction products of the 2-azaallyl
anion and phenyl iodide using isolated 2-M and in situ generated 2-Na
to produce 3 and 30. (B) Comparison of the reaction products of the 2-
azaallyl anion and adamantyl iodide using isolated 2-M and in situ
generated 2-Na with varying amounts of dimethoxyethane (DME) to
produce 3-Ad.
of isolated 2-Na (48% yield). Due to the similarity in yields
between the isolated 2-azaallyl anions and those prepared in
situ, we postulated that 2-Li and 2-Na are the active species in
the SET arylation and alkylation chemistry, acting as effective
room temperature SEDs. Despite the success of 2-Li and 2-Na,
we found that isolated 2-K does not perform well under the
reaction conditions. We attribute this decrease in yield
primarily to solubility issues, as 2-K is sparingly soluble in both
MTBE and DME. With the knowledge that the isolated 2-azaallyl
anions are competent intermediates in our previous chemistry
(Scheme 2B), we next examined their electrochemistry.
To mimic the conditions in our prior arylation chemistry
(Scheme 2B), cyclic voltammetry of the 2-azaallyl anions was
performed in DME. Compounds 2-Li, 2-Na, and 2-K exhibit
effectively the same cyclic voltammetry, with one irreversible
oxidation wave at Epc ¼ ꢀ1.61 V versus the Fc/Fc+ couple (Epc
¼
ꢀ1.1 V versus S.C.E.) (Fig. 2). The irreversibility is consistent
with a fast and irreversible dimerization process following
oxidation, in line with our previous observations (see discussion
below).13,14 While measuring the electrochemistry of 4-Na, we
observed two reversible one electron waves E1/2 ¼ ꢀ1.62 V and
ꢀ0.61 V versus the Fc/Fc+ couple (ꢀ1.11 and ꢀ0.1 V versus
S.C.E.) (Fig. 2), in line with our hypothesis that the radical
derived from 4-Na would possess greater lifetime. We assign
these electrochemical events to 2-azaallyl anion oxidation to the
2-azaallyl radical (E1/2 ¼ ꢀ1.61 V vs. Fc/Fc+) and 2-azaallyl
radical oxidation to the 2-azaallenium cation (E1/2 ¼ ꢀ0.61 V vs.
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Fig. 2 Cyclic voltammetry of compounds 2-Li, 2-Na, 2-K, and 4 with
guides for the eye to indicate the redox state regimes of the 2-azaallyl
compound. Solvent ¼ DME, n ¼ 100 mV sꢀ1, [sub] ¼ 0.001 M, [nNPr4]
[BArF4] ¼ 0.1 M. The signal observed at ꢀ2.3 versus Fc/Fc+ is the redox
feature of small quantities of the imine decomposition product.
Fc/Fc+). While the Epc indicates that 2-azaallyl anions are
moderately strong reductants, they are not as strong as many of
the reported SEDs (Scheme 2A).27 Due to the reactivity of the 2-
azaallyl anion with aryl and alkyl iodides, we expected a signif-
icantly more negative reduction potential. To further probe the
electron transfer of 2-azaallyl anions we turned to chemical
oxidants to characterize the products of 1 electron oxidation.
Based on the electrochemistry above, we searched for
appropriate 1-electron chemical oxidants and found that Ag+
gave the cleanest oxidation. Addition of 1 equiv. of AgPF6 to
a purple solution of 2-Na at ꢀ30 ꢂC resulted in a yellow solution
Fig. 3 (A) 1-Electron oxidation of 2-Na by AgPF6 yielding the dimers
51-1-rac, 51-1-meso, and 51-3 in 89% yield. (B) 1-Electron oxidation of
4-Na by AgPF6 yielding compound 6 showing the redox stability of 4-
Na. (C) Thermal elipsoid plots of [1,1,3,3-tetraphenyl-2-azaallyl radical]
6 at the 30% probability level. Hydrogen atoms removed for clarity. (D)
Wireframe overlay of 4-Na and 6 depicting the structural similarity. (E)
Experimental X-band EPR spectra of 6 at 298 K dissolved in benzene (6
mM) (perpendicular mode; g ¼ 2.0080; n ¼ 9.64 GHz; P ¼ 0.0201 mW;
modulation amplitude ¼ 0.2 mT).
2.0080. The only other reported acyclic 2-azaallyl radical
dimerizes over the course of hours in solution, precluding
isolation.32 Importantly, the structure of radical 6 is similar to
anion 4-Na with the only statistically signicant difference
being a slight decrease of the C–N–C angle from 131.61(11)–
132.84(11)ꢂ to 128.87(14)ꢂ (Fig. 3C and D) within the 2-azaallyl
core. With knowledge of this minor structural change, we
shied our focus to computationally probing electron transfer
in the 2-azaallyl anions.
Using a simplied version of Marcus–Hush theory, a method
rst described by the Nelsen group and extensively used by
Murphy and coworkers, we probed the electron transfer barriers
of the 2-azaallyl anion.24,33,34 This simplied model allows for
facile comparison of the transition state energy (DG*) of
different outer sphere electron transfer processes using the
reorganization energies (l) and the free energy of the electron
transfer (DGrel). We optimized the structures of 2-azaallyl anions
2 and 4 and 2-azaallyl radicals 5-mono and 6 (Scheme 4) using
the B3LYP functional, the 6-31+G(d,p) basis set, and a CPCM
solvent eld. The reorganization energy of the 2-azaallyl anions
2 and 4 upon loss of an electron are remarkably low (1.6–
1
and a black solid (likely Ag0). H NMR analysis of the organic
components indicated a clean conversion to a mixture of 2-
azaallyl dimers 5 in 89% yield (Fig. 3A, 2 : 3 ratio of 51-1-rac to 51-
1-meso, similar to that observed in arylation reactions).14 When
running the LCMS of this mixture, we observed a third peak
with the same mass as well as peaks indicative in the NMR of
a third previously undescribed dimer. We propose that this
species is 51-3, which was not previously reported due to the low
concentrations of 2-azaallyl dimers that are normally observed.
We propose that the irreversibility of the electrochemistry in
Fig. 2 is a result of the facile dimerization of the triphenyl 2-
azaallyl radicals. To slow the dimerization processes, we moved
to the more sterically encumbered 4-Na. Thus, addition of 1
equiv. of AgPF6 to 4-Na produces an NMR silent, green solution
along with a similar black solid that was ltered away (Fig. 3B).
X-ray quality crystals of the green compound were grown from
a cooled, saturated solution of acetonitrile, revealing the
compound to be 1,1,3,3-tetraphenyl 2-azaallyl radical (6). EPR
analysis of 6 in benzene exhibited a broad signal centered at g ¼
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Pennsylvania for support of this work. We also would like to
thank Dr Thibault Cheisson of Penn chemistry with help in
mounting particularly difficult crystals, Dr Yexenia Nieves-
Quinones of Penn chemistry for assistance in computation,
and Professor Mike Zdilla and Connor Koellner of Temple
University for their help with EPR spectroscopy.
Notes and references
1 C. I. Onyeagusi, X. Shao and S. J. Malcolmson, Org. Lett.,
2020, 22, 1681–1685.
2 P. E. Daniel, C. I. Onyeagusi, A. A. Ribeiro, K. Li and
S. J. Malcolmson, ACS Catal., 2019, 9, 205–210.
3 Q. Wang, M. Poznik, M. Li, P. J. Walsh and J. J. Chruma, Adv.
Synth. Catal., 2018, 360, 2854–2868.
4 X. Shao, K. Li and S. J. Malcolmson, J. Am. Chem. Soc., 2018,
140, 7083–7087.
Scheme 4 (A) Computed half reaction free energies (DG) and reor-
ganization energies (l) of the oxidation of free anions 2 (top) and 4
(bottom). Values are in kcal molꢀ1 and calculated using a solvent field
of 1,4-dioxane. (B) Kohn–Sham orbitals of the HOMO of 2 and 4 where
the non-bonding character of the orbital is shown.
5 G. Deng, M. Li, K. Yu, C. Liu, Z. Liu, S. Duan, W. Chen,
X. Yang, H. Zhang and P. J. Walsh, Angew. Chem., Int. Ed.,
2019, 58, 2826–2830.
6 Z. Liu, M. Li, G. Deng, W. Wei, P. Feng, Q. Zi, T. Li, H. Zhang,
X. Yang and P. J. Walsh, Chem. Sci., 2020, 11, 7619–7625.
7 Y. Matsumoto, J. Sawamura, Y. Murata, T. Nishikata,
R. Yazaki and T. Ohshima, J. Am. Chem. Soc., 2020, 142,
8498–8505.
8 S. Tang, X. Zhang, J. Sun, D. Niu and J. J. Chruma, Chem.
Rev., 2018, 118, 10393–10457.
9 T. Niwa, T. Suehiro, H. Yorimitsu and K. Oshima,
Tetrahedron, 2009, 65, 5125–5131.
7.8 kcal molꢀ1; Schemes 4A and S1†). This low energy cost is
likely due to the orbital that the electron is removed from. In the
case of 2-azaallyl anions, the orbital is one with primarily
nonbonding character, which leads to minimal differences in
the energy surfaces before and aer electron transfer (Scheme
4B). While the reorganization energies suggest rapid reactivity,
this should be regarded as a simplied model as it does not
account for other factors affecting electron transfer rate, e.g.
donor–acceptor electronic coupling, donor–acceptor associa-
tion, etc.35–37 We are planning on investigating these factors
affecting electron transfer kinetics in future work.
10 M. Li, B. Yucel, J. Jimenez, M. Rotella, Y. Fu and P. J. Walsh,
Adv. Synth. Catal., 2016, 358, 1910–1915.
11 J. A. Murphy, J. Garnier, S. R. Park, F. Schoenebeck,
S.-z. Zhou and A. T. Turner, Org. Lett., 2008, 10, 1227–1230.
12 P. E. Daniel, A. E. Weber and S. J. Malcolmson, Org. Lett.,
2017, 19, 3490–3493.
13 M. Li, O. Gutierrez, S. Berritt, A. Pascual-Escudero,
A. Ye¸silçimen, X. Yang, J. Adrio, G. Huang, E. Nakamaru-
Ogiso, M. C. Kozlowski and P. J. Walsh, Nat. Chem., 2017,
9, 997.
14 M. Li, S. Berritt, L. Matuszewski, G. Deng, A. Pascual-
Escudero, G. B. Panetti, M. Poznik, X. Yang, J. J. Chruma
and P. J. Walsh, J. Am. Chem. Soc., 2017, 139, 16327–16333.
15 P. Veya, C. Floriani, A. Chiesi-Villa and C. Guastini, J. Chem.
Soc., Chem. Commun., 1991, 991–993.
Conclusions
We have described the electron transfer properties of the 2-
azaallyl anion and provided insight into its ability to activate
difficult substrates via 1-electron transfer at room temperature.
By providing the rst X-ray structure of a stable 2-azaallyl
radical, we demonstrated that the structures of the 2-azaallyl
anion and the 2-azaallyl radical are nearly identical by both X-
ray crystallography and DFT calculations. We attribute this to
the 2-azaallyl anion's HOMO having nonbonding character.
This similarity in the structures lead to the computational
determination of a remarkably small reorganization energy.
This study highlights the importance of reorganization energy
as a compliment to reduction potential, as a guiding principle
in the design of SEDs.
¨
16 J. Pauls, S. Chitsaz and B. Neumuller, Organometallics, 2002,
21, 1515–1517.
17 E. C. Volpe, P. T. Wolczanski and E. B. Lobkovsky,
Organometallics, 2010, 29, 364–377.
18 P. C. Andrews, V. L. Blair, E. C. Border, A. C. Peatt,
J. G. MacLellan and C. D. Thompson, Organometallics,
2013, 32, 7509–7519.
Conflicts of interest
There are no conicts of interest to declare.
19 J. Xing, H. Sun, B. Xue, X. Li, O. Fuhr and D. Fenske,
Organometallics, 2017, 36, 975–980.
Acknowledgements
The authors gratefully acknowledge the U. S. National Science 20 J. Xing, H. Sun, T. Zheng, X. Qi, X. Li, O. Fuhr and D. Fenske,
Foundation (CHE-1955724 to E. J. S., CHE-1902509 to P. J. W., J. Organomet. Chem., 2018, 868, 61–65.
and Graduate Research Fellowship Program NSF-GRFP to G. B. 21 B. A. Frazier, E. R. Bartholomew, P. T. Wolczanski, S. DeBeer,
˜
P.) for primary support. We also thank the University of
M. E. Santiago-Berrios, H. D. Abruna, E. B. Lobkovsky,
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˜´˜
S. C. Bart, S. Mossin, K. Meyer and T. R. Cundari, Inorg. 30 J. E. Arguello and A. B. Penenory, J. Org. Chem., 2003, 68,
¨
Chem., 2011, 50, 12414–12436. 2362–2368.
22 H. G. Roth, N. A. Romero and D. A. Nicewicz, Synlett, 2016, 31 M. Pichette Drapeau, I. Fabre, L. Grimaud, I. Cioni,
27, 714–723.
T. Ollevier and M. Taillefer, Angew. Chem., Int. Ed., 2015,
54, 10587–10591.
23 J. P. Barham, G. Coulthard, K. J. Emery, E. Doni, F. Cumine,
G. Nocera, M. P. John, L. E. A. Berlouis, T. McGuire, T. Tuttle 32 R. Kuhn and F. A. Neugebauer, Montash. Chem., 1963, 94, 16–
and J. A. Murphy, J. Am. Chem. Soc., 2016, 138, 7402–7410. 21.
24 G. M. Anderson, I. Cameron, J. A. Murphy and T. Tuttle, RSC 33 S. F. Nelsen, S. C. Blackstock and Y. Kim, J. Am. Chem. Soc.,
Adv., 2016, 6, 11335–11343. 1987, 109, 677–682.
25 T. P. Vaid and M. S. Sanford, Chem. Commun., 2019, 55, 34 J. A. Murphy, S. z. Zhou, D. W. Thomson, F. Schoenebeck,
11037–11040.
M. Mahesh, S. R. Park, T. Tuttle and L. E. A. Berlouis,
26 J. A. Murphy, J. Org. Chem., 2014, 79, 3731–3746.
Angew. Chem., Int. Ed., 2007, 46, 5178–5183.
27 S. Hanson Samuel, E. Doni, T. Traboulsee Kyle, 35 R. Foster, J. Phys. Chem., 1980, 84, 2135–2141.
G. Coulthard, J. A. Murphy and C. A. Dyker, Angew. Chem., 36 G. E. M. Crisenza, D. Mazzarella and P. Melchiorre, J. Am.
Int. Ed., 2015, 54, 11236–11239.
Chem. Soc., 2020, 142, 5461–5476.
28 J. A. Murphy, T. A. Khan, S.-z. Zhou, D. W. Thomson and 37 R. N. Sampaio, E. J. Piechota, L. Troian-Gautier, A. B. Maurer,
M. Mahesh, Angew. Chem., Int. Ed., 2005, 44, 1356–1360.
29 J. Broggi, T. Terme and P. Vanelle, Angew. Chem., Int. Ed.,
2014, 53, 384–413.
K. Hu, P. A. Schauer, A. D. Blair, C. P. Berlinguette and
G. J. Meyer, Proc. Natl. Acad. Sci. U. S. A., 2018, 115, 7248.
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