C(sp3)?H Cyanation Promoted by Visible-Light Photoredox/Phosphate Hybrid Catalysis

Article abstract of DOI:10.1002/chem.201801746

Inspired by the reaction mechanism of photo-induced DNA cleavage in nature, a C(sp³)?H cyanation reaction promoted by visible-light photoredox/phosphate hybrid catalysis was developed. Phosphate radicals, generated by one-electron photooxidation of phosphate salt, functioned as a hydrogen-atom-transfer catalyst to produce nucleophilic carbon radicals from C(sp³)?H bonds with a high bond-dissociation energy. The resulting carbon radicals were trapped by a cyano radical source (TsCN) to produce the C?H cyanation products. Due to the high functional-group tolerance and versatility of the cyano group, the reaction will be useful for realizing streamlined building block syntheses and late-stage functionalization of drug-like molecules.

Full text of DOI:10.1002/chem.201801746

A Journal of
Accepted Article
Title: C(sp3)‒H Cyanation Promoted by Visible-Light Photoredox/
Phosphate Hybrid Catalysis
Authors: Takayuki Wakaki, Kentaro Sakai, Takafumi Enomoto, Mio
Kondo, Shigeyuki Masaoka, Kounosuke Oisaki, and Motomu
Kanai
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To be cited as: Chem. Eur. J. 10.1002/chem.201801746
Link to VoR: http://dx.doi.org/10.1002/chem.201801746
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3
C(sp )H Cyanation Promoted by Visible-Light
Photoredox/Phosphate Hybrid Catalysis
Takayuki Wakaki, Kentaro Sakai, Takafumi Enomoto, Mio Kondo, Shigeyuki Masaoka, Kounosuke
Oisaki*, Motomu Kanai*
Abstract: Inspired by the reaction mechanism of photo-induced DNA-
intramolecular HAT from the deoxyribose moiety to phosphate
radicals generated under high-energy light irradiation (Scheme
1a).18 Our study, inspired by this mechanism, has demonstrated
that in situ-generated phosphate radicals through one-electron
photooxidation act as powerful HAT catalysts capable of cleaving
3
cleavage in nature, a C(sp )–H cyanation reaction promoted by
visible-light photoredox/phosphate hybrid catalysis was developed.
Phosphate radicals, generated by one-electron photooxidation of
phosphate salt, functioned as a hydrogen atom transfer catalyst to
3
3
19
produce nucleophilic carbon radicals from C(sp )–H bonds with a high
strong C(sp )-H bonds with high BDE values (Scheme 1b). This
3
bond dissociation energy. The resulting carbon radicals were trapped
by a cyano radical source (TsCN) to produce the C-H cyanation
products. Due to the high functional-group tolerance and versatility of
the cyano group, the reaction will be useful for realizing streamlined
building block syntheses and late-stage functionalization of drug-like
molecules.
novel HAT catalysis promotes C(sp )-H cyanation at the -
position of heteroatoms.
3
Transformation of unactivated C(sp )H bonds, which are
ubiquitously present in organic compounds, can facilitate
1
streamlined syntheses of complex organic molecules. The
method is also promising for expanding the structural diversity of
2
,3
drug-like lead molecules through late-stage derivatizations.
3
Specifically, C(sp )H functionalization involving a hydrogen atom
transfer (HAT) process driven by photoenergy is attractive due to
the mild reaction conditions and high functional group-tolerance.4
In combination with
a
visible-light photoredox catalysis,
5
structurally distinct HAT catalysts, such as thiols, thiophosphoric
acids and imides, quinuclidines,7 sulfonamides,8 chloride9 and
bromide10 ions, aryl carboxylates,11 and N-hydroxy compounds12
were developed. The currently-available HAT catalysts have
limitations, however, such as (1) limited scope of cleavable C-H
bonds,13 (2) catalyst decomposition due to the inherent
6
14
nucleophilicity and/or reactivity with carbon–carbon multiple
bonds, and (3) insufficient stability,16 thus leaving room for
15
3
improvement. HAT catalysts that can cleave C(sp )–H bonds with
a relatively high bond dissociation energy (BDE: 95–105 kcal/mol)
and high catalyst turnover are in high demand.
Scheme 1. (a) In the natural system: DNA-strand cleavage by a phosphate
radical-mediated HAT process induced by photoenergy. (b) This work: C(sp )-
3
H activation by a phosphate radical-mediated HAT process induced by visible-
light photoredox catalysis.
Photocleavage of DNA chains is a chemical process that may
lead to cell death, mutation, aging, and transformation.17 Sevilla
et al. recently proposed that DNA-photocleavage is initiated by an
Methods for efficient and facile introduction of a cyano group
to organic molecules are valuable due to the high synthetic
[
a]
T. Wakaki, K. Sakai, Dr. K. Oisaki, Prof. Dr. M. Kanai
Graduate School of Pharmaceutical Sciences
The University of Tokyo
versatility of cyano groups.20,21 Many precedents of C(sp3)-H
cyanations, however, often required harsh conditions, such as
high temperature and/or strong oxidizing reagents, and can only
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
E-mail: oisaki@mol.f.u-tokyo.ac.jp; kanai@mol.f.u-tokyo.ac.jp
T. Enomoto, Dr. M. Kondo, Dr. S. Masaoka
Institute for Molecular Science
be applied to substrates bearing electron-donating substituents
on the adjacent heteroatoms.21,22 Inoue23 and Hill24 groups
reported precedents of cyanation targeting C(sp )-H bonds with
[b]
3
National Institutes of Natural Sciences
5
-1 Higashiyama Myodaiji, Okazaki 444-8787, Japan
high BDE values, although the reactions required ultraviolet (UV)
irradiation. C-H cyanation proceeding under mild visible light-
irradiation has not been reported.
Supporting information for this article can be found under:
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We first searched for a suitable combination of a photoredox
Nucleophilic cyanation reagents, TMSCN (4f) or NaCN (4g), did
not produce 5a (entries 10 and 11). As control experiments in the
absence of any elemental factors, blue LED irradiation, the Ir
catalyst and phosphoric acid using
a
mechanism-based
2
5
screening method. Fluorescent quenching was observed when
1
,1’-binaphthyl-2,2’-diyl
hydrogen
bpy)(PF
phosphate
(1, dFCF ppy
(2a)
=
and
2-(2,4-
photocatalyst, K
low (entries 12-15).
2 3
CO , or phosphoric acid 2a, the yield remained
Ir(dFCF ppy) (5,5’-dCF
3
2
3
6
)
3
3
difluorophenyl)-5-(trifluoromethyl)pyridine, 5,5’-dCF bpy = 5,5’-
bis(trifluoromethyl)-2,2’-bipyridine)26 were irradiated with 430-nm
Table 2. Scope and Limitations
LED light in acetonitrile.
Table 1. Optimization of Reaction Conditions
Yields were determined as isolated yields. Yields indicated in parentheses were
1
determined from the
H NMR analysis in the presence of 1,1,2,2-
tetrachloroethane as an internal standard. [a] 4a (3.0 equiv), 20 h.
Having determined the optimal conditions, we next
investigated the substrate scope (Table 2). For protected amines,
pyrrolidines with different carbamate-protecting groups (3a-3d),
N-Boc piperidine (3e), N-Boc azepane (3f), and N-Boc
dibutylamine (3g) produced the corresponding cyanation
products 5a-5g in good to excellent yields (77%-95%). Using N-
Boc secondary amine 3h having methyl and phenylpropyl
substituents on the nitrogen atom, C-H cyanation proceeded at
the methyl group with excellent selectivity (5h: >20:1
regioselectivity). The reaction with N-Boc morpholine (3i)
proceeded selectively at an -C-H bond of the nitrogen atom,
producing 5i in 71% yield. N,N’-Di-Boc-piperazine (3i)
predominantly produced mono-cyanated product 5j in 66% yield,
presumably due to the deactivating effects of the electron-
withdrawing cyano group. For N-Boc-tetrahydroquinoline (3k) and
N-Boc-tetrahydroisoquinoline (3l) having benzylic C-H bonds, the
reaction proceeded selectively at the -position of the nitrogen
atom, giving products 5k and 5l in 52%-53% yields. As
demonstrated in the reaction to isochromane (3m),
tetrahydrothiophene (3n), and thiane (3o), an -C-H bond bound
to an oxygen atom or a sulfur atom was successfully converted to
[
a] Condition A: 1 (2.0 mol%), HAT 2 (20 mol%), 4 (2.0 equiv), CH
Condition B: 1 (1.0 mol%), HAT 2 (5.0 mol%), 4 (1.5 equiv), CH Cl
In the absence of K CO . [c] No light irradiation. [d] In the absence of 1.
3
CN (0.1 M).
2
2
(0.1 M). [b]
2
3
Based on this finding, we next investigated several types of
C-H functionalization reactions using 2 mol% 1 and 20 mol% 2a
under photoirradiation with blue LEDs (Table 1, condition A). The
reaction between N-Boc pyrrolidine (3a) and p-toluenesulfonyl
cyanide (TsCN, 4a) in the presence of K
product 5a in 56% yield (entry 1). When using previously reported
2 3
CO produced cyanation
7
11
HAT catalysts such as quinuclidine (2b), benzoic acid (2c), and
triisoproplysilylthiol (2d) instead of 2a, 5a was obtained in low
5
yield (entries 2-4). Further optimization of the reaction conditions
improved the yield of 5a to 95% when the catalyst loadings of 1
and 2a were reduced to 1 mol% and 5 mol%, respectively, in
dichloromethane solvent (condition B, entry 5). Screening of
cyanation reagents revealed that 4a was optimal (entries 5-11).
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a C-CN bond of 5m-5o in good yield (57%-80%). The reaction
2 3
occurred in a mixture of 1, phosphate 2a, and K CO
.28 (2) The
with N-Boc methionine (3p) afforded two regioisomers, 5p and 5p’, reaction proceeded only under photoirradiation (Table 1, entry 13).
in an excellent combined yield of 97%.
When the reaction was performed under the photoirradiation-
shading cycle, the reaction proceeded only during
photoirradiation.28,29 Those results may suggest that the reaction
does not proceed via a radical chain mechanism. (3) Use of
This reaction was applied to
a
short synthesis of
pharmaceuticals and late-stage cyanation of multifunctional
molecules, taking advantage of the mild reaction conditions and
functional group tolerance. Using commercially available 2-
sodium
p-toluenesulfinate
(NaO
2
Stol)
or
sodium
p-
toluenesulfonate (NaO
3
Stol) instead of 2a did not afford the
chloro-1-(pyrrolidin-1-yl)ethan-1-one (3q) as
a
substrate,
cyanation product. These results suggest that in situ-generated p-
cyanation product 5q was obtained in 53% yield without affecting
the -chloroacetyl moiety, which is sensitive to nucleophiles or
reductants. Amidonitrile 5q is a key intermediate for the synthesis
of dipeptidyl peptidase IV (DPP-IV) inhibitors.27 When this
reaction was applied to N-Boc fluoxetine (3r), C-H cyanation
proceeded regioselectively at the methyl group to produce 5r in
●
toluenesulfonyl radicals (Ts ) or p-toluenesulfonyloxy radicals
●
(
TsO ) do not function as HAT species in this reaction.
Based on the above mechanistic insights, we propose a
possible mechanism for the present reaction, as depicted in
III II
Scheme 3: (1) Photocatalyst 1 (*Ir /Ir = +1.68 V vs. SCE) is
3
8% yield. The product derived from methylene cyanation was not
excited by visible light irradiation. (2) The excited photocatalyst
●

detected at all. When methionine-containing dipeptide 3s was
subjected to the same conditions, however, a mixture of 5s and
oxidizes the phosphate anion (2a /2a : +1.50 V vs. SCE), thus
generating a phosphate radical. Without K CO , the reaction does
2
3
5
s’ was obtained in a 1:1.3 ratio. Functional groups other than the
not proceed because the oxidation of 2a requires +1.72 V (vs.
SCE). (3) The resulting phosphate radical (BDE(OH) = 102.4
kcal/mol)30 selectively abstracts the CH bonds of 3a (BDE(CH)
thioether group, as well as the benzylic C-H bonds, were
unaffected.
=
95 kcal/mol) to produce the corresponding carbon radical. (4)
The resulting carbon radical is trapped by 4a,1
4,23,24,31
producing
cyanation product 5a. (5) The thus-generated p-toluenesulfonyl
●
－
31a
radical (Ts /Ts
= 0.50 V vs. SCE)
is reduced by the
III II
photocatalyst (Ir /Ir = 0.69 V vs. SCE) to give a sulfinate anion
Ts ), thereby closing the whole hybrid catalytic cycle.
－
(
Scheme 3. Proposed Reaction Mechanism
3
In summary, we developed a catalytic C(sp )-H cyanation
reaction that proceeded under mild conditions and visible-light
irradiation. Identification of a hybrid catalysis comprising iridium
photoredox catalyst 1 and phosphate HAT catalyst 2a was key for
the success. Due to the observed functional group tolerance, this
reaction was applicable to late-stage functionalization of a
bioactive compound and peptide. The extension of this novel
phosphate HAT system to other reaction patterns is ongoing.
Acknowledgements
3
Scheme 2. Application of C(sp )-H Cyanation
This work was supported by a JSPS fellowship (T.W.), TOBE
MAKI Scholarship Foundation (K.S.), JSPS KAKENHI Grant
Number JP16H01007 in Precisely Designed Catalysts with
Customized Scaffolding (K.O.), JSPS KAKENHI Grant Number
JP17H06442 in Hybrid Catalysis (M.Kanai).
We collected experimental results to gain some insight into
the reaction mechanism: (1) No fluorescence-quenching occurred
in mixtures of photocatalyst 1 and substrate 3a, 1 and TsCN (4a),
2 3
or 1 and K CO . On the other hand, fluorescence-quenching
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Conflict of interests
The authors declare no conflict of interest.
[18] A. Adhikary, D. Becker, B. J. Palmer, A. N. Heizer, M. D. Sevilla, J. Phys.
Chem. B 2012, 116, 5900-5906.
Keywords: C-H activation • photoredox catalysis • cyanation •
phosphoric acid • late-stage functionalization
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3
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Entry for the Table of Contents
COMMUNICATION
Takayuki Wakaki, Kentaro Sakai,
Takafumi Enomoto, Mio Kondo,
Shigeyuki Masaoka, Kounosuke Oisaki*,
Motomu Kanai*
Page No. – Page No.
3
C(sp )-H Cyanation Promoted by a
3
A C(sp )-H cyanation promoted by a visible-light photoredox/phosphate hybrid
Visible-Light Photoredox/Phosphate
Hybrid Catalysis
catalysis is described. The use of phosphate hydrogen atom transfer (HAT) catalysts
was essential for the reaction. Nitrile compounds were synthesized in moderate to
excellent yields with streamlined synthetic route. This reaction was appreciable to
late-stage functionalization of a bioactive molecules and a peptide.
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