K2S2O8-HFIP synergistically promoted: Para -selective sp3 C-H bond diarylation of glycine esters

Article abstract of DOI:10.1039/c9ob02489a

A metal-free K₂S₂O₈-HFIP synergistically promoted double Friedel-Crafts alkylation between a glycine derivative and N-substituted aniline was developed to efficiently synthesize diarylmethane derivatives with high para-selectivity. The reaction proceeded smoothly in the absence of any metal and ligand, and exhibited a good tolerance of functional groups.

Full text of DOI:10.1039/c9ob02489a

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K2S2O8-HFIP synergistically promoted para-selec-
tive sp3 C–H bond diarylation of glycine esters†
Cite this: DOI: 10.1039/c9ob02489a
Qingfeng Xu,‡ Bang Li,‡ Yujie Ma, Fei Sun, Yanan Gao and Na Ye
*
Received 19th November 2019,
Accepted 16th December 2019
A metal-free K2S2O8-HFIP synergistically promoted double Friedel–Crafts alkylation between a glycine
derivative and N-substituted aniline was developed to efficiently synthesize diarylmethane derivatives with
high para-selectivity. The reaction proceeded smoothly in the absence of any metal and ligand, and
exhibited a good tolerance of functional groups.
DOI: 10.1039/c9ob02489a
rsc.li/obc
of 3,3′ bisindolylmethanes has been well established by diary-
Introduction
lation of the sp3 C–H bond with indoles via transition metal-
17
The geminal diaryl functionality featuring indoles, anilines, or
,
organo-,18 photo-,19 and radical cation1h catalyzed reactions
other electro-rich aryls is frequently found in alkaloid natural as well as other methods.20 However, few methods have been
products (e.g. streptindole, arundine, vibrindole A) and in developed for diarylation of the sp3 C–H bond in esters with
many pharmaceutically and biologically important com- electron-rich arenes. Most recently, Chandrasekharam and co-
pounds1 with representative agents including muscarinic workers17 reported a Cu-catalyzed double Friedel–Crafts (FC)
antagonists like fesoterodine2 and tolterodine (Detrol),3 anti- alkylation reaction between glycine derivatives and limited
depressants such as sertraline (Zoloft)4 and nomifensine,5 and aniline substrates at 100 °C (Scheme 1A).
the phosphodiesterase type 4 inhibitor CDP-840.6 Owing to
Transition metal-free C–H coupling has always attracted
their intriguing biological activities, many studies have reported much attention21 and remains to be highly desirable for the
on the synthesis of diarylmethane derivatives.7a,b The most construction of diarylmethane derivatives. Very recently, we
common methods involve condensation of electron-rich arenes have demonstrated an effective and concise Michael reaction
with aldehydes/ketones or imines.7c–g Transition-metal catalyzed of N,N-disubstituted anilines as C(sp2)–H nucleophiles with
coupling methods between diazo compounds and electron-rich8 maleimides as electrophiles in 1,1,1,3,3,3-hexafluoro-2-propa-
or pre-functional aryl substrates (e.g. boronic acid,9 boroxines,10
halides,11 or siloxanes12) have also been developed.
The direct cross-dehydrogenative coupling (CDC) reaction
between two hydrocarbons has been considered as one of the
most fundamental strategies to construct C–C bonds which
avoid pre-functionalization of substrates.13 Many impressive
results in the arylation of sp3 carbon centers have been
achieved through the CDC reaction.14 Glycines, the simplest
and the most inexpensive natural amino acids, are abundant
chemical feedstock and have become important building
blocks to form novel and complex organic molecules15 since
the pioneering study of Li.16 A double alkylation reaction of
glycines with different arenes has emerged as an alternative
route to synthesize the geminal diarylmethanes. The synthesis
Jiangsu Key Laboratory of Neuropsychiatric Diseases and College of Pharmaceutical
Sciences, Soochow University, Suzhou, Jiangsu 215123, China.
E-mail: yena@suda.edu.cn
†Electronic supplementary information (ESI) available. CCDC 1950371. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9ob02489a
‡These authors contributed equally to this work.
Scheme 1 Protocols for sp3 C–H bond arylation.
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nol (HFIP) with no need for any additional metal catalysts or desired diarylated product 3a was expectedly obtained in an
reagents (Scheme 1B).22 Inspired by these results, we envi- isolated yield of 32% with para-selectivity (entry 2) and no
saged that a HFIP-promoted double FC alkylation reaction mono-FC alkylation by-product 3a′ (Table 1) was observed
between glycine derivatives and anilines would result in the from TLC plates. The yield was unchanged under open air
straightforward formation of diarylmethane derivatives. (entry 3). To our delight, it improved to 56% at 65 °C (entry 4).
Herein, we report an efficient transition metal-free K2S2O8- Optimization of an equivalent ratio of aniline 2a and K2S2O8
HFIP synergistically promoted double FC alkylation of provided the best proportion (3 : 2) in an excellent yield (88%)
N-substituted anilines with phenylglycine esters under mild (entries 3–10).
conditions (Scheme 1C).
Later, replacing the oxidant with Na2S2O8, DTBP, TBHP,
m-CPBA, H2O2, oxone, or NaIO4, was found to be ineffective in
affording 3a in a better yield (entries 11–17). Additionally,
changing the reaction solvent did not show any improvement
either. Notably, alcohol solvents, TFE and methanol, were sig-
nificantly superior to non-alcohol solvents (e.g. DMSO, THF,
DMF, 1,4-dioxane, and CHCl3) in this transformation (entries
18–24), but still less efficient than HFIP (entry 8), which high-
lights the importance of alcohol solvents in the
transformation.
We also optimized the substitutions on phenylglycine
esters 1 under the optimal reaction conditions in hand
(Table 2). The yield was decreased when the methyl group in
the phenylglycine esters was switched from the para to ortho or
meta position (1a–c), maybe due to the formation of self-
dimeric byproducts.23 Removal of the methyl group (1d) also
lowered the yield. To our delight, electro-donating groups (1a
and 1e) were found to be more effective than electro-withdraw-
ing groups (1f–j) in affording 3a with a high yield. Finally,
ethyl (4-methoxyphenyl)glycinate (1e) achieved 3a in the
highest yield (93%) and thus was used for further analysis of
substrate scope.
Results and discussion
Our initial investigation began with a double FC alkylation
reaction of 4-methylphenylglycines 1a with N,N-dimethyl
aniline 2a in HFIP under an O2 atmosphere at 50 °C for 12 h
(Table 1, entry 1). However, no reaction occurred. Potassium
persulfate (K2S2O8) has emerged as a cost-effective, suitable in-
organic oxidant for a wide array of oxidative transforma-
tions.21b When we added K2S2O8 to our reaction, only the
Table 1 Optimization of reaction conditionsa
Eq.
Entry (2a)
Eq.
Temp/
°C
Yieldb
(%)
With the optimal reaction conditions in hand, we focused
our attention on the aniline substrate scope of the K2S2O8-
mediated double FC reaction in HFIP under open flask con-
ditions. A range of electron-rich anilines were thus converted
under the optimized conditions (Scheme 2). Overall, most of
the aniline substrates were readily converted into the corres-
ponding CDC reaction products with high para-selectivity and
Oxidant (oxidant)
Solvent
1c
2c
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
2
2
2
2
3
2
3
3
3
4
3
3
3
3
3
3
3
3
3
3
3
3
3
—
—
1
1
1
1
2
1
2
3
2
2
2
2
2
2
2
2
2
2
2
2
2
2
50
50
50
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
65
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
TFE
–
d
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
Na2S2O8
DTBP
32
31
56
54
69
83
88
70
78
31
6
10
56
20
44
53
71
47
21
17
13
6
Table 2 Optimization of phenylglycines (1)a
TBHP
m-CPBA
H2O2
Oxone
NaIO4
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
CH3OH
DMSO
DMF
THF
1,4-
Dioxane
CHCl3
No.
R
Yieldb (%)
No.
R
Yieldb (%)
37%
1a
1b
1c
1d
1e
4-Me
3-Me
2-Me
H
88%
43%
51%
30%
93%
1f
1g
1h
1i
4-Br
4-Cl
4-F
3-Br
2-Br
c
—
d
24
3
K2S2O8
2
65
19
—
—
c
a Standard reaction conditions: 1a (0.25 mmol), 2a, oxidant, solvent
(2.0 mL), open air, 65 °C for 12 h. b The isolated yield of 3a is based on
c
4-OMe
1j
—
1a. c Under O2. d No reaction. Na2S2O8, sodium persulfate; DTBP, di- a Standard reaction conditions: 1a–j (0.25 mmol), 2a (0.75 mmol),
tert-butyl peroxide; TBHP, tert-butyl hydroperoxide; m-CPBA, 3-chloro- K2S2O8 (0.50 mmol), HFIP (2.0 mL), open air, 65 °C for 12 h. b The iso-
perbenzoic acid; H2O2, hydrogen peroxide; NaIO4, sodium periodate; lated yield is based on 1a–j. c Complex mixtures and no desired
TFE, 2,2,2-trifluoroethanol.
product. d No reaction.
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double para-substituted product 3m in an 82% yield.
Furthermore, when we investigated the N,N-dimethylaniline
bearing either electron-donating groups (2n, 2p, 2u) or elec-
tron-withdrawing groups (2r–t) on the 3-position of the phenyl
ring, the corresponding gem-diarylacetic derivatives 3n, 3p,
and 3r–u were isolated in good yields (72–92%) and para-selec-
tivities. Moreover, compound 3p was crystallized, and the X-ray
analysis unambiguously established the observed regio-
selectivity (Fig. 1).
Unexpectedly, N,N-dimethylanilines with acyclic electron-
donating groups on the 2-position of the phenyl ring (2o and
2q) failed to provide the expected products (3o and 3q,
Scheme 2). Moreover, if the para-position was blocked, no
ortho substitution was observed likely due to the strong steric
effect, e.g., when N,N,4-trimethylaniline (2v) was used as a sub-
strate. However, anilines cyclizing the substitution on the
2-position, such as N-ethyltetrahydroquinoline (2w), free tetra-
hydroquinoline (2x), and julolidine (2y), all smoothly gave the
corresponding double para alkylated products (3w–3y) in low
to good yields (15–66%). Similarly, the reaction of N,N-
dimethyl-1-naphthylamine (2z) gave the C4-substituted
product 3z in a 64% yield under the standard reaction
conditions.
The scope of this reaction with respect to the functional
group of phenylglycines was further investigated (Scheme 2).
To our satisfaction, the reactions of phenylglycines having
methyl ester (1aa) and N,N-dimethylamide (1ac) functionalities
readily furnished the desired products 3aa and 3ac in 93% and
56% yields, respectively, with the expected para-selectivity. And
the reaction also tolerated phenylglycine acetone (1ab) in a
28% yield. Disappointingly, the reaction of secondary amide
1ad failed to provide the corresponding products 3ad.
To have a better understanding of this transformation,
control experiments were conducted. The reaction of 1aa and
Scheme 2 Scope and limitation for N-substituted aniline substrates (2).
Standard reaction conditions: 1e or 1aa–ad (0.25 mmol), 2a–z
(0.75 mmol), K2S2O8 (0.50 mmol), HFIP (2.0 mL), open air, 65 °C for
12 h. Isolated yields are shown.
2a under
a nitrogen atmosphere was also investigated
[Scheme 3, eqn (1)]; the desired product 3aa was achieved in a
yield comparable to that in the open air (90% vs. 93%), indicat-
ing that oxygen is not crucial for the reaction. When 5 equiv.
good to excellent isolated yields. Aniline derivatives with of tetramethylpiperidin-1-oxyl (TEMPO), a radical scavenger,
acyclic moieties (2a–c and 2e–f) could be smoothly trans- was added into the reaction system under optimized con-
formed into the desired products 3a–c and 3e–f with good ditions, the yield of the coupling product 3aa was dramatically
yields. For example, N,N-diethylaniline (2b) and N,N-dibenzyla-
niline (2c) reacted well with ethyl (4-methoxyphenyl)glycinate
(1e) and gave the corresponding double para-substituted pro-
ducts 3b and 3c in 91% and 81% yields, respectively. Although
the primary aniline 2d unsuccessfully converted into 3d due to
the potential azo reaction,24 secondary amines such as
N-methylaniline (2g) and N-benzylaniline (2h) successfully
converted into the corresponding para alkylated products 3g
and 3h, respectively.
Disappointingly, the reaction of acetanilide (2i) failed to
provide the corresponding product 3i. Aniline derivatives with
cyclic moieties (2j–2m) could also be smoothly transformed
into the desired products 3j–3m with good yields (74–82%).
For example, 4-phenylmorpholine (2m) reacted well with
4-methoxyphenylglycine ester (1e) to give the corresponding Fig. 1 X-ray crystal of 3p.
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ably the decomposition of K2S2O8 to generate the sulfate
radical anion (SO4•−), which abstracts a hydrogen atom from
1aa to afford a free radical A stabilized by the push–pull
•−
effect.1h,25 The subsequent one electron oxidation by SO4
offers an intermediate B, followed by nucleophilic addition
with 2a leading to the intermediate 3aa′ with the aid of
H-bond activation of phenylglycine derivatives by HFIP.
Subsequently, a second oxidation of compound 3aa′ occurs to
form another iminium intermediate C, followed by nucleophi-
lic addition with 2a and elimination of the aniline,1h,26 finally
leading to the product 3aa.
Conclusions
In summary, we developed a robust K2S2O8–HFIP synergisti-
cally promoted FC dialkylation reaction of a glycine derivative
with N-substituted aniline, which provides a straightforward
access to diarylmethane derivatives. The reaction proceeds for
a wide range of substrates with high para-selectivity, and the
desired diversified products are obtained in good to excellent
isolated yields overall. It is notable that the reaction tolerates a
range of functional groups and performs well in the absence
of any transition metal and ligand.
Scheme 3 Control experiments.
decreased from 93% to 9%, and no mono-FC alkylation
product 3aa′ was detected [eqn (2)]. This result suggests that
the reaction may undergo a radical mechanism. Moreover,
intermediate 3aa′ could be detected by LC-MS in the reaction
mixture of 1aa and 2a under the standard reaction conditions
by stopping the reaction after a short time (0.5 h) [eqn (3)].
Furthermore, we proved that the detectable intermediate 3aa′
can provide the same product 3aa in a 91% yield when sub-
jected to standard reaction conditions [eqn (4)].
Conflicts of interest
There are no conflicts to declare.
On the basis of the control experiments and previous work,
a plausible reaction mechanism for the present K2S2O8–HFIP
synergistically promoted double FC dialkylation reactions is
depicted in Scheme 4. The initial step of the reaction is prob-
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (81703330), the Natural Science
Foundation of Jiangsu Province (BK20170347), the Priority
Academic Program Development of the Jiangsu Higher
Education Institutes (PAPD), and the Jiangsu Key Laboratory of
Neuropsychiatric Diseases (BM2013003).
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