Visible-Light Photoredox-Catalyzed C-H Difluoroalkylation of Hydrazones through an Aminyl Radical/Polar Mechanism

Article abstract of DOI:10.1002/anie.201508698

An unprecedented visible-light-induced direct C-H bond difluoroalkylation of aldehyde-derived hydrazones was developed. This reaction represents a new way to synthesize substituted hydrazones. The salient features of this reaction include difluorinated hy

Full text of DOI:10.1002/anie.201508698

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201508698
German Edition: DOI: 10.1002/ange.201508698
Photocatalysis
À
Visible-Light Photoredox-Catalyzed C H Difluoroalkylation of
Hydrazones through an Aminyl Radical/Polar Mechanism
Pan Xu, Guoqiang Wang, Yuchen Zhu, Weipeng Li, Yixiang Cheng, Shuhua Li,* and
Chengjian Zhu*
À
Abstract: An unprecedented visible-light-induced direct C H
bond difluoroalkylation of aldehyde-derived hydrazones was
developed. This reaction represents a new way to synthesize
substituted hydrazones. The salient features of this reaction
include difluorinated hydrazone synthesis rather than classical
amine synthesis, extremely mild reaction conditions, high
efficiency, wide substrate scope, ease in further transformations
of the products, and one-pot syntheses. Mechanistic analyses
and theoretical calculations indicate that this reaction is
enabled by a novel aminyl radical/polar crossover mechanism,
with the aminyl radical being oxidized into the corresponding
aminyl cation through a single electron transfer (SET) process.
venerable synthetic application of radical addition to imine
functional groups is reductive addition for the synthesis of
amines (Scheme 1a).[5,6] In this generic reaction manifold,
=
radical addition to a C N bond first generates an aminyl
radical, which then readily undergoes H atom abstraction
from the solvent or another hydrogen source. Here we
wondered whether this regular pattern could be intercepted,
through a radical-polar crossover strategy,[7] thus considerably
expanding the synthetic utility of imino compounds.
T
he incorporation of a difluoromethyl group (CF2) into
organic molecules is particularly intriguing because of its
significant applications in the life sciences.[1] It is indeed well-
established that the strategic introduction of a difluoromethyl
group can improve the physicochemical properties of organic
compounds. For example, the CF2 group can act as a bio-
isostere for an oxygen atom or a carbonyl group, and also
serve as a lipophilic hydrogen-bond donor.[2] For these
reasons, substantial effort has been devoted to the develop-
ment of synthetic methods for the incorporation of the CF2
group.[3]
Scheme 1. Radical addition to carbon–nitrigen p bonds.
Over the past five years, the addition of a CF2 radical to
a p acceptor has been developed as one of the most
straightforward methods to access structurally diverse
difluorinated compounds.[4] Nevertheless, despite these
important progress, existing methods for radical-type
difluoroalkylation are extremely limited to carbon–carbon
p bonds (alkene, alkyne, or arene). In contrast, and to the best
of our knowledge, no direct difluoroalkylation of carbon–
nitrigen p bonds has been reported. The most common and
Visible-light photoredox catalysis has recently emerged as
a powerful tool in synthetic organic chemistry and features
the unique ability to facilitate radical/polar crossover reac-
tions.[8] We envisioned that the aminyl radical could undergo
a stepwise single-electron oxidation and deprotonation pro-
cess mediated by visible-light photoredox catalysis (Sche-
me 1b). Such a process would restore the imino group and
À
exploit new methodology for C H bond functionalization of
aldehyde-derived imino compounds. As a part of our recent
ongoing efforts on visible-light-promoted direct difluoro-
alkylation,[4j,k] we report below the first successful develop-
[*] P. Xu, Y. Zhu, W. Li, Prof. Dr. Y. Cheng, Prof. Dr. C. Zhu
State Key Laboratory of Coordination Chemistry, School of Chemistry
and Chemical Engineering, Nanjing University
Nanjing 210093 (P. R. China)
À
ment of visible-light-induced C H difluoroalkylation of
aldehyde-derived hydrazones. To our knowledge, the visi-
ble-light photoredox-catalyzed aminyl radical/polar crossover
reaction has not been reported so far, thus representing
a challenge.
E-mail: cjzhu@nju.edu.cn
G. Wang, Prof. Dr. S. Li
Key Laboratory of Mesoscopic Chemistry of Ministry of Education,
Institute of Theoretical and Computational Chemistry School of
Chemistry and Chemical Engineering, Nanjing University
Nanjing, 210093 (P. R. China)
To validate the feasibility of this aminyl radical/polar
process, the readily accessible aldehyde hydrazone 1a was
chosen as the substrate on account of its quick radical
addition rate relative to that of oxime ethers or imines
(Table 1).[9] In an initial test, 1a was reacted with
BrCF2CO2Et (2a) in acetonitrile (0.1m) under visible-light
irradiation from a 25W fluorescent light bulb in the presence
of fac-[Ir(ppy)3] and Na2HPO4 (Table 1, entry 1). Delightfully,
E-mail: shuhua@nju.edu.cn
Prof. Dr. C. Zhu
State Key Laboratory of Organometallic Chemistry, Shanghai Insti-
tute of Organic Chemistry, Shanghai 200032 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508698.
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Table 1: Optimization studies.[a]
Entry
Photocatalyst
Base
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
[Ir(ppy)2(dtbpy)3]PF6
[Ru(bpy)3](PF6)2
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
fac-[Ir(ppy)3]
–
Na2HPO4
Na2HPO4
Na2HPO4
Na2HPO4
Na2HPO4
Na2HPO4
–
CH3CN
CH2Cl2
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
58
40
80
86
83
52
20
52
72
92
0
Scheme 2. Investigation of the effect of the N-substituent. Reaction
conditions: 1 (0.1 mmol), 2a CF2BrCO2Et (0.15 mmol, 1.5 equiv),
Na2HPO4 (0.15 mmol, 1.5 equiv), fac-[Ir(ppy)3] (0.002 mmol, 2 mol%),
DMF (1 mL), at room temperature, 5W LEDs, 12 h. The E-configured
products are shown and the yields are those of the isolated product.
KOAc
9
Na2CO3
Na2HPO4
Na2HPO4
Na2HPO4
À
[a] 1.2 equiv CF2BrCO2Et were used. Trace amounts of the aromatic C
H difluoroalkylation product were detected. [b] Starting materials were
consumed and a complex mixture of products were obtained. Boc=
tert-butoxycarbonyl, Bs=benzenesulfonyl.
10[c]
11
12[d]
fac-[Ir(ppy)3]
0
[a] The reactions were carried out with 1a (0.1 mmol), CF2BrCO2Et
(0.15 mmol, 1.5 equiv), base (0.15 mmol, 1.5 equiv), photocatalyst
(0.002 mmol, 2 mol%), solvent (1 mL), at room temperature, 25W
fluorescent light bulb, 12 h. [b] Yield of isolated product. [c] 5W LEDs.
[d] In dark. bpy=bipyridine, DMF=N,N-dimethylformamide,
DMSO=dimethylsulfoxide, ppy=phenylpyridine, dtbpy=4,4’-di-tert-
butyl-2,2’-bipyridine.
=
are other radical acceptors containing a C N group. How-
ever, no reaction occurred when either oxime ethers (1ak and
1al) or an imine (1am) were used as the substrates.
With these results in hand, we turned our attention to
explore the scope with respect to N,N-dialkyl hydrazones
under the optimized reaction conditions. The representative
examples are shown in Scheme 3. (Hetero)aryl aldehyde-
derivied hydrazones bearing either electron-donating
(methyl, methoxy) or electron-withdrawing (trifluoromethyl,
fluoro) substituents furnished the corresponding products
with good to excellent yields (3b–p). The position of the
substitutents on the phenyl ring has no effect on this reaction.
the desired product 3a was isolated with a satisfactory yield of
58%. After solvent screening, strong polar solvents, such as
DMF and DMSO, were found to be more suitable for this
reaction (entries 2–4). The catalysts [Ir(ppy)2(dtbpy)3]PF6 and
[Ru(bpy)3](PF6)2 showed inferior reactivity toward the con-
version of 1a (entries 5 and 6). The addition of a base is
essential for this reaction and its role is to neutralize the
byproduct HBr. Further screening showed other bases, such
as KOAc and Na2CO3, gave lower yields of this reaction
(entries 7–9). Furthermore, it was found the highest yield of
3a (92%) was obtained when 5W LEDs were adopted instead
of a 25 W fluorescent light bulb (entry 10). Control experi-
ments showed the reaction could not proceed in the absence
of either visible-light irradiation or the photocatalyst
(entries 11 and 12). It is worth mentioning that no difluori-
nated hydrazones were detected in any of the above cases.
With the preliminary realization of visible-light-induced
À
Remarkably, the C H difluoroalkylation reaction of hetero-
cyclic hydrazones also proceeded well to give the desired
products in good yields (3o–p). Encouraged by the successful
difluoroalkylation of (hetero)aryl aldehyde-derived hydra-
zones, we tried to extend the scope of this reaction to aliphatic
aldehyde-derived hydrazones. To our delight, the difluoro-
alkylation proceeded well, thus giving the desired product in
moderate to good yields (3q–s). Bromodifluoroacetamides
were studied as difluorinating reagents,[10] and they proved to
be suitable substrates, thus providing the corresponding
difluorinated hydrazone with excellent yields (3t–u). Addi-
tionally, the phenylalanine-derived bromodifluoroamide also
reacted smoothly, thus offering facile access to complex
fluorinated compounds (3v). To clarify, the main product in
the target hydrazones was determined to be E configured by
À
C H difluoroalkylation of 1a, we next investigated the effect
of the N-substituent (1ab–aj; Scheme 2). The reaction of
1ab–ae all furnished the desired products with moderate to
good yields (53–90%), whereas the N-Boc and N-Bs hydra-
zones, 1af and 1ag, respectively, failed to give the desired
product. Additionally, this reaction did not tolerate secondary
amino groups as a complex mixture of products was obtained
in the cases of 1ah–aj, eventhough the starting materials were
consumed. These experimental results indicated that the N,N-
dialkyl structural motif is crucial for this transformation. We
speculated that this activity difference may be explained by
the three-electron p-bond interaction between amino radical
with the adjacent nitrogen atom. This interaction could affect
stabilization of the transition state. Oxime ethers and imines
a
1H–1H NOESY NMR experiment (see the Supporting
Information for details).[11] In addition, we found that slow E/
Z isomerization is affected by both solvent and temperature.
To demonstrate the synthetic utility of this visible-light-
promoted difluoroalkylation reaction, a gram-scale reaction
was carried out under standard reaction conditions. We were
delighted to find that the synthesis of 3a proceeded smoothly
with a good yield of 86% on a 5 mmol scale (Scheme 4a).
Hydrazones are significant and versatile reagents in organic
chemistry. Simple acidic treatment of 3a readily delivered the
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Scheme 4. Synthetic utility of methodology. LAH=lithium aluminum
hydride.
Scheme 3. Scope of N,N-dialkyl hydrazones in the difluoroalkylation
reaction. Reaction conditions: 1 (0.1 mmol), 2 (0.15 mmol, 1.5 equiv),
Na2HPO4 (0.15 mmol, 1.5 equiv), fac-[Ir(ppy)3] (0.002 mmol, 2 mol%),
DMF (1 mL), at room temperature, 5W LEDs, 12–24 h. The E-
configured products are shown and the yields are those of the isolated
products. [a] 3 equiv phenylalanine derived bromodifluoroamide 2d
were used.
Scheme 5. Plausible mechanism.
corresponding difluorinated b-ketoester, which could react
with hydrazines toward biologically active 4,4-difluoropyra-
zol-5-ones (Scheme 4b).[12] In addition, difluorinated hydra-
zine could be easily obtained in 79% yield by reduction of 3a
with lithium aluminum hydride in diethyl ether (Scheme 4b).
Integration of multistep chemical reactions into a one-pot
reaction represents an atom- and step-economical process.
We wished to incorporate the aldehyde, hydrazine, and
thus further implying a radical reaction. Subsequent radical
=
addition to the C N bond leads to the aminyl radical
intermediate 8, which should be stabilized by the adjacent
nitrogen atom through a possible three-electron p-bonding
interaction. At this point, 8 does not undergo H atom
abstraction to generate corresponding hydrazine 9. Instead,
a key aminyl radical/polar crossover step proceeds between 8
and Ir4+, thus regenerating the photocatalyst and either the
aminyl cation 10 or 11 (path a). Further tautomerization and
deprotonation of aminyl cation would give the product 3.
Alternatively, a light on/off experiment verified the necessity
of continuous irradiation of visible light and suggested that
chain propagation is not the predominant mechanistic path-
way (Figure 1).
In addition to the aminyl radical/polar mechanism
(Scheme 5, path a), another possible carbon radical/polar
process is depicted (Scheme 5, path b). As the oxidation
potential of radical intermediate cannot be measured, DFT
calculations were performed to explore the Gibbs free-energy
profiles of the two different reaction paths (Figure 2). In
path b, the conversion of the aminyl radical 8 into the carbon
radical 12 for substrates 1a, 1af, 1ag, and 1al is endothermic
À
BrCF2CO2Et in a highly efficient one-pot condensation/C
C(CF2) bond-forming reaction. Most remarkably, the one-pot
three-component coupling reaction proceeded smoothly to
give an excellent yield (95%) of product 3e, thus making the
present photoredox catalysis strategy more attractive and
robust (Scheme 4c).
A plausible aminyl radical/polar mechanism is proposed,
but a precise reaction mechanism awaits further study. Under
visible-light irradiation, the photocatalyst fac-[Ir3+(ppy)3]
undergoes a metal to ligand charge-transfer (MLCT) process
to produce the strongly reducing excited state Ir3+* (À1.73 V
vs. SCE in CH3CN; Scheme 5). Then a SET process from this
species to 2a generates the CF2 radical precursor 7 and Ir4+
(+ 0.77 V vs. SCE in CH3CN).[4] It was found no product was
detected when the radical inhibitor 2,2,6,6-tetramethyl-1-
piperidinyloxyl (TEMPO) was added to the reaction system,
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the formation of the aminyl cation is only exothermic by
0.4 kcalmolÀ1, and for substrates 1af and 1al, the correspond-
ing process is endothermic by 1.6 and 17.9 kcalmolÀ1,
respectively. Hence, the formation of 11 from 1a is more
favorable than the processes from the other three substrates.
These results can account for the fact that only 1a exhibits the
reactivity experimentally. Therefore, the aminyl radical/polar
mechanism (path a) is responsible for the visible-light photo-
redox-catalyzed difluoroalkylation of aldehyde-derived
hydrazones.
In conclusion, we have successfully developed the visible-
À
light photoredox-catalyzed C H difluoroalkylation of alde-
Figure 1. Time profile of difluoroalkylation of 1e with and without
light.
hyde-derived hydrazones for the first time. In addition, the
current method is further highlighted by the feasibility of the
one-pot synthesis of a difluorinated hydrazone. More impor-
À
tantly, this unprecedented photoredox protocol for C H bond
by 6.1, 3.4, 1.3, and 7.1 kcalmolÀ1, respectively. Although the
conversion of 8 into 12 for substrates 1af and 1ag is
thermodynamically more favorable than that of substrate
1a, both 1af and 1ag exhibit no reactivity experimentally.
Hence, the path b can be excluded. On the contrary, for
path a, the SET step in the conversion of 8 into 11 for
substrate 1a is exothermic by 13.2 kcalmolÀ1. However, for
the substrate with the electron-withdrawing substituent 1ag,
functionalization of hydrazones is enabled by a novel aminyl
radical/polar crossover mechanism. We anticipate this new
mode of intrinsic reactivity, revealed by the aminyl radical/
polar crossover process, will be used for more transformations
of carbon–nitrogen p bond bonds.
Figure 2. Computed Gibbs free-energy (in kcalmolÀ1) profiles for visible-light photoredox-catalyzed difluoroalkylation of aldehyde-derived
hydrazones.
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Foundation of China (21172106, 21174061, 21474048 and
21372114) for financial support.
=
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Keywords: fluorine · hydrazones · imines · photocatalysis ·
radical reactions
How to cite: Angew. Chem. Int. Ed. 2016, 55, 2939–2943
Angew. Chem. 2016, 128, 2992–2996
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Received: September 17, 2015
Revised: October 14, 2015
Published online: November 23, 2015
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