Transition-Metal-Free α Csp3?H Cyanation of Sulfonamides

Article abstract of DOI:10.1002/chem.202100902

This report describes the site-selective α-functionalization of sulfonylamide derivatives through the in-situ generation of imine intermediates. The N?F sulfonylamides, which could facilitate the elimination to generate imines, are coupled with TBACN to e

Full text of DOI:10.1002/chem.202100902

Communication
doi.org/10.1002/chem.202100902
Chemistry—A European Journal
Transition-Metal-Free α Csp3À H Cyanation of Sulfonamides
Liejin Zhou,[a] Siqi Wei,[a] Ziran Lei,[a] Gangguo Zhu,*[a] and Zuxiao Zhang*[a]
with a δ CÀ H are not compatible with both transformations due
Abstract: This report describes the site-selective α-function-
alization of sulfonylamide derivatives through the in-situ
generation of imine intermediates. The NÀ F sulfonylamides,
to favorable 1,5 HAT. In 2020, Montgomery and Martin group
reported a nickel merged with photoredox catalyst catalyzed α-
arylation and α-alkylation of benzamides which showed broad
which could facilitate the elimination to generate imines,
substrate scope in moderate to good yields.[8] On the other
are coupled with TBACN to efficiently and mildly afford α-
hand, the electron deficient nature of amide makes α CÀ H less
amino cyanides. Comparing with Strecker reaction, this
nucleophilic and possess stronger bond strength which is
transformation offers a complementary strategy to effi-
harder to form carbon radical via directly hydrogen atom
ciently construct α-amino cyanides from direct α CÀ H
transfer. Relying on an electrostatic attraction to achieve α
functionalization of sulfonylamindes. The reaction is also
characterized by broad substrate scope and flash chroma-
tography column free workup. More importantly, the new
selectivity, Rovis group realized α-alkylation of primary aliphatic
amines via in situ generating a negatively charged carbamate
which would allow α CÀ H abstraction by the positively charged
two-electron pathway to generate imines through manipu-
quinuclidinium radical.[9] Inspired by this work, they also
lation of the leaving group allows us to achieve excellent α
developed α CÀ H alkylation of trifluoromethanesulfonamides
site-selectivity.
by deprotonation of NH and then followed
abstraction.[10]
α
CÀ H
We sought to develop an alternative strategy to realize
highly efficient α CÀ H functionalization of amides which could
avoid the problematic electrophilic N centered radical inter-
mediate. On the other hand, α-aminonitriles are important and
versatile intermediates in organic synthesis that can be readily
converted into biologically active compounds and functional
materials, such as α-amino acids, vicinal diamines, imidazoles,
thiadiazoles, and others.[11] Inspired by Strecker reaction which
Selective functionalization of aliphatic CÀ H bonds represents an
important goal of modern synthetic chemistry.[1] Differentiating
between such bonds in organic molecules with high levels of
selectivity remains a considerable challenge.
CÀ H functionalization of amines has drawn increasing
interest due to their presence in drugs and bioactive
molecules.[2] Electron rich tertiary amines could be oxidized to
the radical cation with subsequent deprotonation or directly
hydrogen atom transfer to deliver an α-amino alkyl radical,
therefore α CÀ H functionalization of amines have been well
established.[3] In contrast, the development α CÀ H functionaliza-
tion of sulfonylamides which are frequently found in drugs and
biomolecules is challenging, because the nitrogen atom with an
electron withdrawing protecting group has much lower elec-
tron density than amines.[4] In addition, the key intermediate of
electrophilic N centered radical would prefer 1,5 HAT instead of
1,2 HAT (Figure 1).[5,7] For example, in 2016 Moriyama group had
developed a nitroxyl-radicalcatalyzed oxidative coupling reac-
tion of N-EWG-protected amines (amides) with silylated nucleo-
philes by the N chloro of N-EWG-protected amides with a
halogenation reagent in situ.[6] Later, Muñiz and coworkers
disclosed a bromide catalyzed α CÀ H oxygenation of sulfonyla-
mide to afford N-sulfonyl oxaziridines.[7] However, substrates
[a] Prof. Dr. L. Zhou, S. Wei, Z. Lei, Prof. Dr. G. Zhu, Prof. Dr. Z. Zhang
Key Laboratory of the Ministry of Education for Advanced Catalysis
Materials
Department of Chemistry,Zhejiang Normal University
688 Yingbin Road, Jinhua 321004 (P. R. China)
E-mail: gangguo@zjnu.cn
zhangzx@zjnu.edu.cn
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202100902
Figure 1. Selective α CÀ H cyanation of amides.
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has been studied over 150 years to synthesize α-amino nitrile
via an imine intermediate.[12] We anticipated that a preinstalled
leaving group on sulfonylamides would have more acidic α
CÀ H to facilitate the formation of reactive imine in situ. There-
fore, we can expect to access α-amino nitriles in presence of
cyanide. Hence, we choose to investigate α cyanation of
sulfonylamide as our model reaction. Recently, NF amides have
been extensively studied in achieving δ CÀ H functionalization
of amides via a 1,5 hydrogen atom transfer pathway.[13]
However, to the best of our knowledge, an α CÀ H cyanation of
NÀ F sulfonylamides is still unknown. Herein, we report a
practical transition-metal-free α CÀ H cyanation of sulfonyla-
mides which proceeds by cyanide reacting with in situ
generated imine from NF substrates to afford the correspond-
ing α-aminonitriles in excellent yield. The transformation is
highlighted with mild conditions, excellent regioselectivity, a
broad substrate scope, and flash chromatography column free
workup.
Next we explored the substrate scope (Scheme 1). Satisfy-
ingly, the scope proved quite general regarding both steric and
electronic properties of the fluorosulfonamides. For example,
both electron-deficient or electron-rich arylsulfonamides bear-
ing benzylic CÀ H at δ position provided the corresponding α
cyanation products 2a–h in excellent yields and exclusive
regioselectivity. Substrates bearing more steric groups on the
aromatic ring also underwent α cyanation smoothly and
furnished the expected products 2i–k in excellent yields. Next,
we explored the generality of the protocol with a wide range of
linear amines using 4-toluenesulfonamide as the parent group.
The transformation provided excellent selectivity at the
desired α-CÀ H bond irrespective of the substitution, including
bearing several secondary CÀ H; tertiary CÀ H and benzylic CÀ H
(2l–p). More importantly, the reaction also exhibits very good
functional group tolerance. Such as heteroarene, alkyne, alkene
and acetal could all survive under the reaction condition and
afford the corresponding product in excellent yield (2q–t).
Among them, the substrate bearing a alkene has no cyclization
product at all which further indicate the advantage of this two
electron pathway. Primary CÀ H could be efficiently cyanated as
well (2v). Furthermore, a variety of tertiary CÀ H substrates were
tested. Both cyclic and linear substrates are suitable for this
reaction and afford the desired products in excellent yield (2w–
z). NÀ F amide could also be converted into the α cyanation
product in good yield under the standard condition (2u, 2aa).
More importantly, the α CÀ H cyanation could be implemented
at a late-stage transformation. For example, complex natural
product or drug derivatives were successfully converted into
their α nitriles sulfonylamide analogues (2ab–2ad) site-selec-
tively.
To test our hypothesis, we chose NÀ F sulfonylamide (1a)
which possessed a benzylic CÀ H at δ position as our model
substrate, TMSCN as cyanides source, CH3CN as solvent to start
our investigation of
α
CÀ H cyanation of sulfonylaminde
(Table 1). Considering the importance of base which is needed
to activate TMSCN to release free cyanide and deprotonate of
substrate 1a to form the imine in situ, a variety of bases were
tested (entry 1–4). Fortunately, all the bases we tried could
deliver us the desired product in very good yields (>90%)
except Na2CO3, and in all cases there were no δ cyanation
product observed. To further optimize the conditions to realize
flash chromatography column free workup, we switched our
cyanide source to TBACN (entry 5), and quantitative yield of α
cyanation product was observed. More importantly, simply
washing the reaction with water could remove the byproduct
and afford pure product in quantitative yield. Other solvents
such as acetone, DCM, DMF and DMSO could also furnish the
desired product albeit in diminished yields (entry 6–9). Thus,
the optimized reaction condition for α CÀ H cyanation of NÀ F
sulfonylamide 1a were as follows: 1a (0.2 mmol) and TBACN
(0.3 mmol) were stirred in acetonitrile at room temperature for
2 hours under nitrogen atmosphere.
After investigated reaction substrate scope we turn our
attention to explore the mechanism of the reaction. The NÀ Cl
sulfonamide instead of 1a was subjected to standard con-
ditions, and only starting material was recovered which
indicated the importance of leaving group to get proper acidity
of α CÀ H (Figure 2a). To rule out radical pathway, radical
inhibitors such as TEMPO were added into the reaction and
showed no effect on the reactions (Figure 2b). Furthermore, a
substrate containing cyclopropylmethyl moiety (1ae) could
Table 1. Optimization of α cyanation of NÀ F sulfonylamide.[a]
Entry
CN
Base
Solvent
2a Yield [%][b]
1
2
3
4
5
6
7
8
9
TMSCN
TMSCN
TMSCN
TMSCN
TBACN
TBACN
TBACN
TBACN
TBACN
Na2CO3
K2CO3
TBAF
Cs2CO3
none
none
none
none
none
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
acetone
DCM
69
93
94
96
>99
85
88
94
90
DMF
DMSO
[a] General conditions (unless otherwise specified): 1a (0.2 mmol), [CNÀ ] (0.30 mmol), base (0.20 mmol), solvent (2.0 mL), rt, 2 h. [b] Isolated yield of 2a.
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Figure 2. Mechanistic study.
corresponding imine were isolated which further indicated the
imine intermediate mechanism.
To further explore the utility of this protocol (Figure 3), a
large scale of α cyanation was conducted and the desired
product was obtained in quantitative yield (e.g., 2a, 1.64 gram).
The product could be easily converted into valuable organic
blocks. For example, the reaction of 2a and H2O2 could produce
the corresponding α amino amide 3 compound in 72% yield.
The reduction of 2a in the presence of BH3·THF, then protected
by Boc group, resulted in a 1,2 di-amine scaffold 4 with
acceptable yield. Finally, the Ts group on the nitrogen atom
could be replaced by benzoyl group after deprotection with
methanesulfonic acid, and reprotection with benzoyl chloride
to give 5 in 75% yield.
In conclusion, a flash chromatography column free and
transition metal free cyanation of α CÀ H sulfonylamide was
Scheme 1. Scope of 1a.[a] [a] Reaction conditions as stated in Table 1, entry 5,
Yields refer to flash chromatography column free workup unless otherwise
noted. [b] The reaction time was extended to 12 h, yields are isolated after
purification via column chromatography. [c] Yields are isolated after
purification via column chromatography.
survive and afford α cyanation product in 99% yield without
any ring opening (Figure 2c), which also suggested that the
reaction did not invole an α carbon radical intermediate. Finally,
when NÀ F amide (1af) was subjected to standard conditions,
both the desired product (2af) and the enamine (2af’) from the
Figure 3. Gram scale synthesis and post functionalization. a. H2O2 (30%),
MeOH, NaOH (aq.); b. 1.BH3·THF; 2. Boc2O; c. 1.MeSO3H, TFA-thioanisole; 2.
BzCl, K2CO3.
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Keywords: cyanation
sulfonylamides · synthetic methods
·
CÀ H functionalization
·
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Manuscript received: March 11, 2021
Accepted manuscript online: March 26, 2021
Version of record online: April 8, 2021
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