Electron-Transfer and Hydride-Transfer Pathways in the Stoltz–Grubbs Reducing System (KOtBu/Et3SiH)

Article abstract of DOI:10.1002/anie.201707914

Recent studies by Stoltz, Grubbs et al. have shown that triethylsilane and potassium tert-butoxide react to form a highly attractive and versatile system that shows (reversible) silylation of arenes and heteroarenes as well as reductive cleavage of C?O bonds in aryl ethers and C?S bonds in aryl thioethers. Their extensive mechanistic studies indicate a complex network of reactions with a number of possible intermediates and mechanisms, but their reactions likely feature silyl radicals undergoing addition reactions and S_H2 reactions. This paper focuses on the same system, but through computational and experimental studies, reports complementary facets of its chemistry based on a) single-electron transfer (SET), and b) hydride delivery reactions to arenes.

Full text of DOI:10.1002/anie.201707914

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Accepted Article
Title: Electron Transfer and Hydride Transfer Pathways in the Stoltz-
Grubbs Reducing System (KOtBu-Et3SiH)
Authors: John Murphy, ANDREW SMITH, ALLAN YOUNG, SIMON
ROHRBACH, ERIN O'CONNOR, MARK ALLISON, HONG-
SHUANG WANG, DARREN POOLE, and TELL TUTTLE
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10.1002/anie.201707914
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Electron Transfer and Hydride Transfer Pathways in the Stoltz-
Grubbs Reducing System (KOtBu-Et3SiH)
Andrew J. Smith,[a] Allan Young,[a] Simon Rohrbach,[a] Erin F. O’Connor,[a] Mark Allison,[a] Hong-
Shuang Wang,[a] Darren L. Poole,[b] Tell Tuttle*[a] and John A. Murphy*[a]
Abstract: Recent studies by Stoltz, Grubbs et al. have shown that
triethylsilane and potassium tert-butoxide react to form a highly
attractive and versatile system that shows (reversible) silylation of
arenes and heteroarenes as well as reductive cleavage of C-O bonds
in aryl ethers and C-S bonds in aryl thioethers. Their extensive
mechanistic studies indicate a complex network of reactions with a
number of possible intermediates and mechanisms, but their reactions
likely feature silyl radicals undergoing addition reactions and SH2
reactions. This paper focuses on the same system, but through
computational and experimental studies, reports complementary facets
of its chemistry based on (a) single electron transfer (SET), and (b)
hydride delivery reactions to arenes.
formation of the silyl radicals occurs, but rational working
hypotheses have been advanced.1e
Recently, Stoltz, Grubbs et al.1 have discovered a simple and
elegant system comprising Et3SiH 2 and KOtBu that achieves a
number of remarkable reactions: (i) converting arenes and
heteroarenes and their alkylated counterparts to silyl-substituted
products, often with excellent regiocontrol1a-c (e.g. 13); (ii)
achieving reductive C-S bond cleavages in aryl thioethers (e.g.
45) in a reaction that has potential importance in removing sulfur
traces from hydrocarbon fuels;1d (iii) triggering reductive CO bond
cleavages in aryl ethers (e.g. 67) in a reaction with potential
applications to controlled lignin degradation.1a,d A number of
intermediates likely arise from reaction of these two reagents, and
spectroscopic evidence has resulted in informed proposals being
made for their structures. These reactions have proved puzzling,
but a recent coordinated study by synthetic, mechanistic and
computational chemists has allowed significant advances to be
made.1e,f The conclusions are: (i) the combination of Et3SiH and
KOtBu leads to triethylsilyl radicals that have a major role to play in
the reductive cleavages of the C-O and C-S bonds,1d (ii) triethylsilyl
radicals are also likely to be involved in the silylation reactions,
although non-radical routes to the silylation have also been
considered in depth and may also play a central role.1e,f The
mechanistic details are not fully in place, for example, on how
Scheme 1. Selected transformations of the KOtBu-Et3SiH system.1
We had wondered if single electron transfer mechanisms were
playing a significant role in some of these reactions, notably the
cleavages of C-O and C-S bonds. An early suggestion1a mentioned
pentavalent silicates (e.g. 13b below) as reagents that were likely
involved in the C-O cleavages, but the more recent computational
studies on substrates 4 and 6 instead support an alternative
mechanism.1d In this regard, Scheme 1 shows ipso-addition to the
carbon of the C-O bond by triethylsilyl radicals, followed by C-O
bond cleavage in conversion of 6 to 7.
Our recent interest in reductive chemistry carried out by reactions
that involve potassium tert-butoxide attracted us to this area.2
Studies mentioned above1e suggest that the reactive species
produced could include the radical anion 12b (Figure 1) and the
silicate anion 13b.1a,e [Because of their subsequent importance in
this paper, we mention here that radical anions 12 may be formed
in a number of ways, two of which are shown (inset) in Scheme 2.
(See fig. 14 in ref. 1e for an additional route) For these studies, we
used the computationally less costly trimethylsilyl group instead of
the triethylsilyl group.1d,e] To these, we add the triethylsilyl anion
14b as another putative intermediate. At first sight, these
[a]
Andrew J. Smith, Allan Young, Simon Rohrbach, Erin O’Connor,
Mark Allison, Hong-Shuang Wang, Dr. Tell Tuttle* and Prof. Dr.
John A. Murphy*
Department of Pure and Applied Chemistry, University of
Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, United
Kingdom
e-mail: john.murphy@strath.ac.uk; tell.tuttle@strath.ac.uk
Dr. Darren L. Poole
Flexible Discovery Unit, GlaxoSmithKline Medicines Research
Centre, Gunnels Wood Road, Stevenage SG1 2NY, United
Kingdom
[b]
Supporting information for this article is given via a link at the end of
the document.
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compounds are potentially excellent electron donors, although, as
will be seen below, computational chemistry is very helpful in
eliminating species and mechanisms that are unlikely to contribute.
In recent years, we have reported on many highly reducing organic
electron donors that demonstrate remarkable behaviour.3 We were
therefore keen to test the KOtBuEt3SiH system for evidence of
single electron transfer (SET) activity and, if found, to calibrate the
system’s reactivity.
We now used computational methods to compare the cleavage of
the N-benzyl group of 15 by an SET mechanism (Scheme 3A) with
the three potential electron donors 12a–14a. Here it is seen that
electron transfer from 12a to 15 is almost barrierless and is
exergonic (Table 2, entry 1; the scheme also shows facile
fragmentation of the radical anion 31), while the electron transfer
reactions from 13a and 14a (Table 2, entries 2 and 3) show
prohibitive energy profiles
Table 1. Cleavage of benzyl groups from indole derivatives.
Entry
Substrate
Silane
(3 eq
or 0 eq)
Base
(3 eq)
Product
(Yield %)
Recovered
Substrate
(Yield %)
1
2
15
15
16
16
17
17
17
18
18
19
19
20
20
21
21
22
22
23
23
Et3SiH
blank
Et3SiH
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
NaOtBu[a]
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
24 (29)

(85)

(99)

(98)
(88)

(98)
Trace
(86)
Trace
(93)


3
25 (49) + 29 (15)
4
blank
Et3SiH

26 (73)
5
6
Et3SiH


26 (76)
7
blank
Et3SiH
8
9
blank
Et3SiH

26 (63)
10
11
12
13
14
15
16
17
18
19
blank
Et3SiH

26 (47)
blank
Et3SiH

27 (80)
blank
Et3SiH

28 (57)
(100)
(26)
(99)
(23)
(88)
blank
Et3SiH

26 (55) + 30 (23)
blank


[a]
As in ref. 1, NaOtBu is not a successful substitute for KOtBu.
Scheme 2. Indole-based substrates as probes of electron transfer activity. [a] See
SI file for discussion of mechanism of formation of this compound.
A literature search reveals that N-benzylindole substrates are
reductively cleaved to indoles and toluenes with two reagents –
both involving electron transfer. The first uses Birch chemistry4 and
the second uses low-valent titanium reagents.5 Accordingly, we
prepared a range of N-benzylindole substrates (15–23, Scheme 2),
to test for cleavage with silane and tert-butoxide, and the outcomes
are shown in Table 1. In each case, reactions afforded the
debenzylated products, while blank reactions (no silane) led to
excellent recovery of starting materials. Examples 15-22 also
afforded volatile products from the benzyl unit. To counteract this,
naphthylmethyl substrate 23 was subjected to the reaction and
afforded 1-methylnaphthalene 30, in addition to 3-methylindole 26,
and recovered 23 (Table 1, entry 18).
To understand the site of electron transfer in these reactions, we
modelled the formation and reaction of two radical anions – those
arising by electron transfer to indole 17 and carbazole 22. In both
cases (Figure 1), the SOMO showed spin density on the
heterocycle, rather than on the benzyl group. This is consistent with
the greater delocalisation available in the bicyclic or tricyclic
heterocycle for the transferred electron.
(a)
(b)
Figure 1. Representations of (a) spin density of the SOMO of the radical anion
of (a) N-benzyl-3-methylindole 17 and (b) N-benzylcarbazole 22. Geometry
Optimisations and frequency calculations were carried out in Gaussian13 at
M062X/6-31++G(d,p) level of theory,14,15 with solvation modelled implicitly using
the C-PCM model16 (For full computational details, see SI).
We also tested energy profiles for the debenzylation reaction with
two possible competing pathways (Scheme 3B and 3C). The first
of these recognises that 13a could be a very powerful hydride
transfer agent that might perform an SN2 reaction, although an
unusual one, at the benzylic carbon. However, transfer of hydride
from 13a to substrate 15 shows a barrier of 36.9 kcal/mol for the
benzyl cleavage (Scheme 3B), and so this type of reaction will not
occur under our conditions in the laboratory. The second competing
reaction type would involve an SH2 reaction by a R3Si radical on the
benzylic carbon (Scheme 3C). This would also be an unexpected
reaction, as radical displacements at tetrahedral carbons are
almost unknown, and indeed the kinetic barrier (44.3 kcal/mol) is
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again insurmountable. From these results, SET from intermediate
12a is overwhelmingly the most likely of the computed candidate
mechanisms for benzyl group cleavage. In effect, cleavage
occurred to afford N-methylaniline, 41, which was converted to the
more easily isolated 42 following acetylation (56 % over 2 steps).
When the reaction was repeated, but in the absence of Et3SiH, no
cleavage was observed, with starting material 40 recovered (97 %).
We next varied the 'protecting group‘ on our indole substrates from
benzyl to allyl. Given that the computational results showed
electron transfer to the indole group in substrates 17 and 22, rather
than to the benzyl group, then the reagent ought also to be able to
cleave N-allylindoles by an SET mechanism, because of the
stabilisation of the allyl radical leaving group.6 Accordingly,
substrates 43and 45 were prepared. The indole products 26 and
46 were indeed formed from these substrates (35 % and 33 %
respectively). The low yields may indicate the wealth of alternative
reactions open to this reagent system. Indeed, a second product
was isolated from the reaction of 43, namely o-isopropylaniline 44
(18 %), although we have not explored the mechanism of its
formation as yet. It was clear that the KOtBuEt3SiH system is a
more than competent electron-donating system.
In a more challenging probe for electron transfer potency, we
subjected the benzyl methyl ether 47 to reduction by this sytem. A
close analogue of this substrate had proven a very tough substrate
in previous studies;3h it did not undergo fragmentation until two
electrons had been transferred. In this case, the reduced product
48 was produced in 52 % yield. [A blank reaction afforded
recovered starting material exclusively (62 %)]. Additionally,
subjecting nitrile 497 to the reaction afforded hydrocarbon 50 as
sole product, consistent with electron transfer followed by loss of
cyanide anion.
We calculated the oxidation potential of 12a8); as E= 3.74 V vs.
SCE (MeCN). This makes it much more powerful than alkali metals.
Such a powerful electron donor should provide a good probe for
the Marcus inverted region of SET reactions with substrates that
show low reorganisation energies, (e.g. polycyclic arenes).9 Stoltz,
Grubbs et al. reported1d small amounts of partially reduced arenes
from reduction of naphthalenes. In our hands, and in the presence
of excess of KOtBuEt3SiH, anthracene, phenanthrene and
naphthalene all afforded significant amounts of their dihydro
counterparts (Scheme 5).
Table 2 Energy profiles for candidate electron transfers to 15.
Entry
Electron
Donor
Energy Profile
(kcal/mol)
Byproduct of
Electron Donor

1
G* = 0.3
Grel = 8.1
12a
34
G* = 53.6
Grel = 49.4
Scheme 4. Reductive cleavages induced by the Et3SiH/KOtBu system.
2
3
13a
14a
35
36
These compounds would be expected products from Birch-type
electron transfer processes, but to probe the mechanism we
undertook computational studies of electron transfer from 12a to
the hydrocarbons 5153 to yield the radical anions 6062
respectively. (Table 3) Here, the expected normal order of reactivity
is 51 > 52 > 53.10 This is also reflected in the Grel values shown in
Table 3. However, the reverse pattern is seen for the G* values.
SET to anthracene 51 from radical anion 12a shows an
extraordinary barrier of 90 kcal/mol,11 while reduction of arenes 52
and 53 show progressively lower barriers – if this can be verified by
detailed experimental studies, it will be a very rare intermolecular
ground-state illustration of the Marcus inverted region, (stronger
driving force leads to retarded electron transfer).
G* = 44.8
Grel = 38.7
In comparison, hydride transfer from 13a to afford the
corresponding anions 6365 featured low barriers and favourable
thermodynamics. At least for the reduction of anthracene, hydride
transfer from 13a is indeed likely to occur. With the other substrates,
hydride transfer reactions again show lower barriers than electron
transfer from 12a – this will of course be modulated by the
Scheme 3. Computational assessment of possible mechanisms for reductive
cleavage of 15.
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Scheme 5 Reductions of polycylic arenes by KOtBu-Et3SiH. [a] NMR yield.
In summary, the KOtBuEt3SiH system provides access to a broad
range of mechanisms for reductive chemistry, now including
electron transfer and hydride delivery to arenes. Electron donor 12b
is identified as a uniquely powerful agent.
[4]
J. Fujiwara, Y. Fukutani, H. Sano, K.Maruoka, H. Yamamoto, J. Am. Chem.
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Table 3 Energy profiles: SET from 12a
[5]
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Energy Profile (kcal/mole)
Substrate
Radical Anion Product
51
52
53
60
61
62
G*: 90.0; Grel: 37.8
G*: 28.3; Grel: 25.0
G*: 25.7; Grel: 22.3
[7]
Table 4 Energy profiles: Hydride Transfer from 13a
Anionic Product
Energy Profile (kcal/mole)
Substrate
[8]
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51
52
53
63
64
65
G*: 16.7; Grel: 29.4
G*: 20.0; Grel: 14.8
G*: 21.7; Grel: 13.2
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Acknowledgements
We thank (i) Univ of Strathclyde, GSK and EPSRC for funding, and
SFC and WestCHEM for PECRE bursary funding; (ii) EPSRC
National Mass Spectrometry Service, Swansea for HRMS. Results
were obtained using ARCHIE-WeSt High Performance Computer
(www.archie-west.ac.uk). EPSRC grant no. EP/K000586/1
Keywords: SET • silyl • Marcus inverted region • DFT• hydride
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A. J. Smith, A. Young, S. Rohrbach, E.
O’Connor, M. Allison, H.-S. Wang,
T. Tuttle*, J. A. Murphy*
Reductive transformations cause (i) C-
N bond cleavage in N-benzyl and N-
allylindoles and (ii) reduction of
polycyclic arenes to their dihydro
derivatives
Page No. – Page No.
Electron Transfer and Hydride Transfer
Pathways in the Stoltz-Grubbs Reducing
System (KOtBu-Et3SiH)
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