Neutral Organic Super Electron Donors Made Catalytic

Article abstract of DOI:10.1002/anie.201905814

Neutral organic super electron donors (SEDs) display impressive reducing power but, until now, it has not been possible to use them catalytically in radical chain reactions. This is because, following electron transfer, these donors form persistent radical cations that trap substrate-derived radicals. This paper unlocks a conceptually new approach to super electron donors that overcomes this issue, leading to the first catalytic neutral organic super electron donor.

Full text of DOI:10.1002/anie.201905814

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Accepted Article
Title: CatalySED! Neutral Organic Super Electron Donors Made
Catalytic
Authors: John Murphy, Simon Rohrbach, Rushabh S. Shah, and Tell
Tuttle
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201905814
Angew. Chem. 10.1002/ange.201905814
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10.1002/anie.201905814
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Neutral Organic Super Electron Donors Made Catalytic
Simon Rohrbach,[a] Rushabh S. Shah,[b] Tell Tuttle,*[a] and John A. Murphy*[a]
Abstract: Neutral organic super electron donors (SEDs) display
impressive reducing power but, until now, it has not been possible to
use them catalytically in radical chain reactions. This is because,
following electron transfer, these donors form persistent radical
cations that trap substrate-derived radicals. This paper unlocks a
conceptually new approach to super electron donors that overcomes
this issue, leading to the first catalytic neutral organic super electron
donor.
low to quench substrate-derived radicals effectively. Donor 14

reacts with substrate to form radical R1 and the benzimidazolium

salt 15. The radical evolves to radical R2 and abstracts H from 13
to complete the radical chain. Our plans would meanwhile reduce
the cation in 15 back to the dihydrobenzimidazole 13 in situ with
a mild hydridic reducing agent. Thereby cation 15 would act as an
organocatalyst that is converted into an organic super electron
donor 14 during its catalytic turnover.
Redox reactions occupy a central and rapidly developing role in
organic chemistry. Organic electron donors have moved forward
significantly since the reactions of TTF (tetrathiafulvalene) 1 [1] and
TDAE [tetrakis(diethylamino)ethene]
2
were explored,[2] as
witnessed in the reactivity of the increasingly powerful donors 3 −
6.[3,4] TTF, 1, is a weak electron donor that can reduce
arenediazonium salts, but not aryl halides. Upon oxidation, the π-
system gains aromaticity as illustrated for structure 8 (Scheme 1).
This aromatic driving force is a key determinant of electron donors’
reducing power.[4a] Stronger donors e.g. 2 use nitrogen lone pairs
rather than sulfur lone pairs to stabilise radicals and cations in the
oxidized forms. Combining the benefits of developing aromaticity
and use of N atoms inspired the structural templates for neutral
organic ‘super electron donors’ (SED) 3-6, which are defined as
neutral ground state organic molecules that reduce aryl halides to
aryl radicals or aryl anions.[4] With photoactivation, donors such
as 4 and 5 have been shown to reduce a wide range of difficult
substrates, even including alkylbenzenes.[5,6]
Scheme 1. Organic super electron donors 3-6, and predecessors 1 and 2.
In previous studies, it was established that radical cations of
neutral donors, e.g. 7 or 10, which are formed by electron
donation to a substrate RX, behave as persistent radicals[7] and
.
combine with radicals, R , derived from the substrate (Scheme 2).
In the case of TTF 1, the trapping occurs on the sulfur atom of the
radical cation 7 to give sulfonium salt 9, from which 1 can be
regenerated usefully in situ by radical-polar crossover reaction,[1]
but for the nitrogen-containing radical cations, derived from the
super electron donors 3-6, trapping occurs on carbon (e.g.
1012) and the trapped species 12 is then not available for
further useful chemistry.[8] This impedes the use of donors 3-6 in
radical chain reactions. In this paper, we provide a solution to this
longstanding issue by altering the nature of the super electron
donors.
Scheme 2. The established electron donors afford radical cations that readily
undergo combination reaction with substrate-derived radicals.
The plan is shown in Scheme 3. Dihydrobenzimidazole 13 was
selected as a precursor of the single-electron donor 14.[9] Initiation
by hydrogen atom transfer would afford 14. At any time, species
14 would only be present in trace amounts and could not
accumulate since it would be formed as an intermediate in the
chain reaction shown. Accordingly, its concentration would be too
[a]
S. Rohrbach, Prof. Dr. T. Tuttle, Prof. Dr. J. A. Murphy
Dept. of Pure and Applied Chemistry, University of Strathclyde,
295 Cathedral Street, Glasgow G1 1XL, United Kingdom
E-mail: John.Murphy@strath.ac.uk
[b]
Dr. R. S. Shah
GlaxoSmithKline Medicines Research Centre
Gunnels Wood Road, Stevenage SG1 2NY, United Kingdom
Supporting information for this article is given via a link at the end of the
document.
Scheme 3. Proposal for reaction cycle with organic electron donor 14.
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Since little is known[10] about dihydrobenzimidazoles as reducing
agents in radical reactions, we firstly investigated the chemistry of
13. Later, the aim would be to investigate how the full catalytic
cycle can be closed (Scheme 3).
Compound 13 was obtained in high yield by reacting
benzimidazolium salt 15-I with NaBH4 (Scheme 4).[11a] The
material did not need inert atmosphere or dry conditions, making
13 a convenient precursor of an organic super electron donor.[12]
Scheme 4. Synthesis of dihydrobenzimidazole 13.
In preliminary optimisation studies,[11b] the reactions were left
open to air, and a moderate temperature of 55 °C gave
conveniently high reaction rates. With substrate 16, a more
detailed analysis of the optimal conditions was undertaken (Table
1). Specifically, the effect of dodecanethiol was studied, which
acts as a polarity reversal catalyst (PRC).[12] Entry 3 shows that
0.2 equiv. of dodecanethiol enhances the reaction rate and the
overall yield; this would arise by mediating the hydrogen
abstraction from compound 13 to afford the electron donor 14.[13]
Decreasing [PRC] led to lower yield of 17 and to longer reaction
times (Entries 1 and 2; an equivalent trend was also observed in
MeCN as the solvent[11c]). Performing the reaction under inert
atmosphere (N2 or Ar) markedly decreased the reaction rate,
supporting our hypothesis that air acts as an initiator (Entry 4).
The optimal conditions were then applied to a range of substrates
(Scheme 5, Conditions A). The conditions worked well with 5-exo-
trig reactions involving an unactivated alkene (18a, 18d and 20)
Table 1. Optimisation of a reductive radical cyclisation reaction with 13.
Entry
PRC[a]
Time (h:min)[b]
3:00
Yield of 17[c]
64 % (65 %)
76 %
1
none
Scheme 5. Substrate scope. Conditions A: Substrate (1.0 equiv.), aminal 13
(2.0 equiv.) dodecanethiol (0.2 equiv.), DMF (dimethylformamide, 0.5 M), open
to air. Conditions B: Substrate (1.0 equiv.), catalyst 15-I (0.2 equiv.)
dodecanethiol (0.2 equiv.), NaBH4 (2.0 equiv.), DMF (0.5 M), open to air. [a]
Yields were determined by 1H-NMR vs. an internal standard. [b] directlyreduced
by-product was isolated in small amounts[8]; [c] 70 % recovered starting material.
2
0.05 equiv.
0.2 equiv.
0.2 equiv.
1:00
3
0:50
87 % (86 %)
62 %[e]
4[d]
6:00
or an electron-poor alkene (18c) to give the corresponding
cyclised products 19a, (64 %), 19d (83 %), 21 (72 %) and 19c
(87 %) in very good yields. The electron-rich enol-ester 18b was
less compatible with the protocol and gave only a low yield of
cyclised product 19b. 5-Exo-dig cyclisation of the alkynes 22a and
22b gave the indoline products 23a (84 %) and 23b (90 %) in
[a] Dodecanethiol was used as a polarity reversal catalyst (PRC) [b] The
reaction progress was monitored by GC-FID (gas chromatography with
flame ionization detection) and the time when the reaction reached full
conversion is given. [c] Yields were determined vs an internal standard by
GC-FID. The yields of isolated product 17 are given in brackets. [d] The
reaction was performed under inert atmosphere. [e] Remaining starting
material (22 %) was also isolated.
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excellent yields. Similarly, high yields were obtained for the
substrate 24a. In this case the indoline intermediate isomerised to
the indole product 25a (79 %) during purification. Only the
terminal alkyne 24b gave the cyclised product 25b in a low yield.
The unactivated alkyl iodide and bromide 26a and 26b were
viable substrates, too, and gave product 27 in 87 % yield and 67 %
yield, respectively. No product was detected in the reaction of the
alkyl chloride 26c and 70 % of the starting material was recovered.
The catalytic protocol was then applied to substrates in Scheme
5 (Conditions B). The 5-exo-trig cyclisation with 18a, 18d and 20
gave the corresponding cyclized products 19a (77 %), 19d (67 %)
and 21 (62 %) in good yields. The substrates 22a, 22b, 24a and
24b gave rise to the indolenine products 23a and 23b and indole
products 25a and 25b. The reactivity that was previously
observed with Conditions A was essentially reproduced by the
catalytic Conditions B. Finally, we put the catalytic protocol to the
test with more complex radical cascade reactions where two
carbon-carbon bonds are formed in tandem. From the substrates
28a and 28b, the tricyclic products 29a and 29b were obtained in
satisfactory yields of 51 % and 62 %, respectively. Overall, the
results with the catalytic Conditions B demonstrate that it is
possible to achieve comparably high yields to the Conditions A
which had used 13 in stoichiometric amounts.
Table 2. Optimisation of the catalytic protocol with salt 15-I.
Entry
reducing agent
NaBH4
Time (h:min)[a]
3:00
Yield of 13[b]
1
84 % (83 %)
35 %[c]
2
NaBH(OAc)3
NaBH3CN
NaBH4
30:00
3
20:00
2 %[d]
61 %[f]
4[e]
4:00
5[g]
NaBH4
20:00
9 %[h]
[a] The reaction progress was monitored by GC-FID and the time when
the reaction reached full conversion is given. [b] Yields were determined
vs an internal standard by GC-FID. In brackets the yields of pure isolated
product 17is given. [c] Remaining starting material 65 %. [d] Remaining
starting material 98 %. [e] 0.05 equiv. of salt 15-I were used. [f] Yield
determined by 1H-NMR vs internal standard. No remaining substrate. [g]
Blank reaction in the absence of salt 15-I. [h] Remaining starting material
[87 %] was determined by 1H-NMR vs internal standard.
Table 3. Benchmarking the electron donor 14 against 30 and 3.
Reducing
Entry
Agent[a]
30 [b]
13[c]
To build on these encouraging results, we explored the use of the
electron donor 14 in a catalytic manner. In several of the above
reactions with dihydrobenzimidazole 13, the formation of the salt
15-I was observed.[11b] It was thus natural to address the
conversion of this salt back to 13 in situ with an appropriate
terminal reducing agent. Thereby the catalytic cycle would be
closed as shown in Scheme 3. Model substrate 16 was again
chosen to develop a protocol where the electron donor would be
formed catalytically (Table 2). As a starting point, the optimal
conditions for reactions with 16 were chosen (Table 1, Entry 3)
but 13 was substituted by 15-I (0.2 equiv) and. sodium
borohydride (2.0 equiv) (Table 2, Entry 1). Pleasingly, the product
17 (84 %) was formed in almost the same yield as in the reaction
with 2.0 equiv. of 13 (87 %) (cf. Table 1, Entry 3). Milder terminal
reducing agents than NaBH4, such as NaBH(OAc)3 and
NaBH3CN gave inferior results (Entry 2 and 3). Decreasing the
loading of the organocatalyst 15-I from 0.2. to 0.05 equiv. led to a
much lower yield (Entry 4). In a control reaction without the
catalyst 15-I, the cyclised product 17 was only formed in trace
amounts (Entry 5).
1
2
3
0 %
95 %
99 %
−
−
80 %
< 1 %
40 %
4 %
3[d]
[a] Conversion measured by 1H-NMR. [b] 30 (2.0 equiv.), dodecanethiol (0.2
equiv.), DMF (0.5 M), 55 °C, 4 h, open to air. [c] 13 (2.0 equiv.),
dodecanethiol (0.2 equiv.), DMF (0.5 M), 55 °C, 4 h, open to air. [d]
According to a standard literature procedure:[13] 1 (2.0 equiv., formed in situ),
DMF (0.25 M), 100 °C, 18 h, inert atmosphere, sealed tube .
The radical 14 is structurally related to 3. However, the reducing
power of 14 is markedly greater than the reducing power of the
parent electron donor 3. Through computational studies,[11d] and
by cyclic voltammetry experiments,[14] species 14 was found to be
more reducing than 3 by approximately 1 V. An analogous
observation was made by Giri et al. on their system.[15] In fact, with
a reported[14] oxidation potential of −1.86 V vs SCE (saturated
calomel electrode), electron donor, 14, is amongst the most
potent neutral organic ground state reducing agents known.[3d]
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[3]
(a) J. A. Murphy, T. A. Khan, S. -Z. Zhou, D. W. Thomson and M. Mahesh,
Angew. Chem. Int. Ed., 2005, 44, 1356–1360. (b) J. A. Murphy, S.-Z.
Zhou, D. W. Thomson, F. Schoenebeck, Mohan M., S. R. Park, T. Tuttle
and L. E. A. Berlouis, Angew. Chem. Int. Ed., 2007, 46, 5178–5183. (c)
J. A. Murphy, J. Garnier, S. R. Park, F. Schoenebeck, S.-Z. Zhou and A.
T. Turner, Org. Lett., 2008, 10, 1227–1230 (d) S. S. Hanson, E. Doni, K.
T. Traboulsee, G. Coulthard, J. A. Murphy, C. A. Dyker, Angew. Chem.
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Chem. 2018, 96, 522−525. (f) S. S. Hanson. N. A. Richard, C. A. Dyker,
Chem- Eur J., 2015, 21: 8052–8055.
To assess how this greater reducing power translates into
reactivity, we directly compared the reactivity of the single
electron donor 14 with the previously explored electron donor 3
(Table 3). Additionally, we sought experimental support that it is
actually 14 that acts as a reducing agent in our system and not its
closed-shell precursor 13. Phenylenediamine 13 is electron-rich
and might potentially act as an electron donor even without
undergoing hydrogen atom abstraction. Compounds 13 and 30
are similar in their electronic nature as diamines but 30 can’t give
rise to a radical species analogous to 14 (i.e. a radical species
where a gain in aromaticity can result from one-electron oxidation).
In our hands, 30 was incapable of reducing even the easiest-to-
reduce substrate 31, in the series 31-33. This observation
substantiates our hypothesis that 13 does not act as an electron
donor in its own right towards this substrate. It needs to be
converted to 14 to give rise to a potent reducing agent. With our
optimal conditions (as identified in Table 1, Entry 3), we found that
[4]
[5]
For reviews, see (a) E. Doni, J. A. Murphy, Chem. Commun. 2014, 50,
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(a) E. Cahard, F. Schoenebeck, J. Garnier, S. P. Y. Cutulic, S. Zhou, J.
A. Murphy, Angew Chem. Int. Ed., 2012, 51, 3673–3676; (b) E. Doni, B.
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135, 10934–10937. (c) S. O'Sullivan, E. Doni, T. Tuttle, J. A. Murphy,
Angew. Chem. Int. Ed. 2014, 53, 474–478.
[6]
Organic electron donors are diverse in their structures and application,
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4-phenyliodobenzene
31
was
dehalogenated
almost
quantitatively. Also, the more difficult to reduce 1-
bromonaphthalene 32 was reduced in high yield and the even
more challenging 4-bromoanisole 33 was reduced in 40 % yield.
With the previously established electron donor 3,[1b] the aryl
bromide substrates 32 and 33 could not be reduced even at
elevated temperature. Only the aryl iodide substrate 31 was
susceptible to reduction with electron donor 3. This comparison
clearly shows that the new protocol is superior to the protocol with
electron donor 3 in terms of reducing power.
In conclusion, we have demonstrated that dihydrobenzimidazole
13 is a readily accessible precursor of the potent single electron
donor 14. Mild temperatures, fast reaction rates and no need to
establish an inert atmosphere are the key characteristics of this
protocol. Further, the electron donor 14 can be accessed in a
catalytic cycle starting with the salt 15-I. To the best of our
knowledge, this is the first example where a neutral organic super
electron donor has been used in a catalytic cycle. Viewed from a
more general perspective, we have shown how a suitable
heterocycle can react with a mild hydridic reducing agent to
access a highly reducing intermediate.[16]
[7]
[8]
[9]
J. A. Murphy, F. Schoenebeck, N. J. Findlay, D. W. Thomson, S.-Z. Zhou,
J. Garnier, J. Am. Chem. Soc. 2009, 131, 6475−6479.
(a) H. Chikashita, H. Ide, K. Itoh, J. Org. Chem. 1986, 51, 5400–5405.
(b) D. D. Tanner, J. J. Chen, J. Org. Chem. 1989, 54, 3842–3846. (c) B.
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15018–15025. (d) For pioneering studies combining H atom transfer with
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1989, 54, 4259–4260.
Further investigations in our laboratory will focus on expanding
the principle presented here to other classes of organic electron
donors.
[10] (a) J. R. Ames, M. A. Houghtaling, D. L. Terrian, T. P. Mitchell, Can. J.
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[11] (a) For a typical literature procedure see T. Igarashi, E. Tayama, H.
Iwamoto, E. Hasegawa, Tetrahedron Lett. 2013, 54, 6874–6877. This
procedure was substantially modified to suit our needs, see supporting
information. (b) See supporting information, "Biphenyl 34 – Preliminary
Optimisation Studies of Reaction Conditions” p7-p9. (c) See supporting
information, “tert-Butyl-3-methylindoline-1-carboxylate 17 – Optimisation
for General Procedure A”, p9 – p11. (d) See supporting information,
“Calculated Oxidation Potentials” p37.
Acknowledgements
We thank the EPSRC UK National Mass Spectrometry Facility at
Swansea University for high-resolution mass spectrometry
analysis. We thank The University of Strathclyde and GSK for
funding.
[12] For alternative outcomes of reactions of imidazolium salts with NaBH4,
see (a) S. Gardner, T. Kawamoto, D. P. Curran, J. Org. Chem. 2015, 80,
9794−9797; (b) E. F. Godefroi, J. Org. Chem. 1968, 33, 860−862.
[13] The aminal hydrogen atoms in 13 are hydridic. Consequently, the
abstraction of one of these hydrogen atoms by a nucleophilic carbon
centred radical is slow due to a mismatch in polarity. The thiol acts as a
mediator in this step. The hydrogen abstraction from the thiol by a carbon
centred radical is matched in polarity and fast. The thiyl radical that is
generated is electrophilic and can efficiently abstract an aminal hydrogen
atom. For further information on PRC see (a) B. P. Roberts, Chem. Soc.
Rev. 1999, 28, 25–35; (b) H. S. Dang, M. R. J. Elsegood, K.-M. Kim, B.
P. Roberts, J. Chem. Soc., Perkin Trans. 1 1999, 2061−2068; (c) A. J.
Fielding, B. P. Roberts, Tetrahedron Lett. 2001, 42, 4061−4064.
Keywords: catalysis • organic electron donor • reduction • single
electron transfer • upconversion
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[14] (a) X.-Q. Zhu, M.-T. Zhang, A. Yu, C.-H. Wang, J.-P. Cheng, J. Am.
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Entry for the Table of Contents (Please choose one layout)
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This paper reports an organocatalytic
role for a benzimidazolium salt in
radical chemistry. A highly reducing
intermediate (−1.86 V vs. SCE) is
produced simply by treatment with
NaBH4 and then using air as initiator.
This is the first time that an organic
super electron donor has been used
catalytically. and introduces a novel
S. Rohrbach, R. S. Shah, T. Tuttle,*
and J. A. Murphy*
Page No. – Page No.
CatalySED! Neutral Organic Super
Electron Donors Made Catalytic
catalytic
approach
for
the
upconversion of reducing power.
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