Catalytic Diverse Radical-Mediated 1,2-Cyanofunctionalization of Unactivated Alkenes via Synergistic Remote Cyano Migration and Protected Strategies

Article abstract of DOI:10.1021/acs.orglett.6b02960

A catalytic radical protocol for 1,2-cyanofunctionalization of unactivated alkenes involving remote cyano migration triggered by addition of diverse carbon- and heteroatom-centered radicals to alkenes has been developed. This powerful strategy provides a

Full text of DOI:10.1021/acs.orglett.6b02960

Letter
pubs.acs.org/OrgLett
Catalytic Diverse Radical-Mediated 1,2-Cyanofunctionalization of
Unactivated Alkenes via Synergistic Remote Cyano Migration and
Protected Strategies
Na Wang,†,‡,§ Lei Li,‡,§ Zhong-Liang Li,‡ Ning-Yuan Yang,‡ Zhen Guo,*,† Hong-Xia Zhang,†
and Xin-Yuan Liu*,‡
†College of Materials Science & Engineering, Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of
Education, Taiyuan University of Technology, Shanxi, 030024, China
‡Department of Chemistry, South University of Science and Technology of China, Shenzhen, 518055, China
S
* Supporting Information
ABSTRACT: A catalytic radical protocol for 1,2-cyanofunc-
tionalization of unactivated alkenes involving remote cyano
migration triggered by addition of diverse carbon- and
heteroatom-centered radicals to alkenes has been developed.
This powerful strategy provides a diverse platform for the
collection of a variety of synthetically important β-function-
alized alkyl nitriles bearing densely functionalized carbonyl,
cyano, and other various functional groups within the same molecules. The substrates TMS (trimethylsilyl)-protected alkenyl
cyanohydrins are straightforwardly accessible via simple cyanosilylation of the corresponding ketones.
Scheme 1. 1,2-Cyanofunctionalization of Alkenes
he straightforward transformation of unactivated alkenes
Tinto various complex molecules provides an efficient
platform in organic synthesis in the past several decades.1
Among intensive research in this area, the recently explored 1,2-
difunctionalization of unactivated alkenes triggered by addition
of in situ generated radicals to alkenes to realize the halo-, oxy-,
thio-, amino-, and carbo-functionalization has become a hot
topic, providing the most attractive strategy for the simultaneous
incorporation of two functional groups into alkenes.2 Given alkyl
nitriles as one of the most important building blocks owing to
their participation in the straightforward synthesis of aldehydes,
carboxylic acids, and amines, the catalytic cyanofunctionalization
of unactivated alkenes to simultaneously incorporate the cyano
group and diversely functional groups should be of great
significance in organic synthesis. Although significant progress
has been made in cyanofunctionalization of alkenes,3 there are
still limitations in these methodologies. Due to the unavailability
of systematic study in this area, most of them are inherently
restricted to copper-catalyzed cyanotrifluoromethylation of
alkenes3b−d or the substrate scope is only limited to activated
alkenes for azidocyanation and aminocyanation,3a,e thus
hindering their widespread adoption in the collection of diversely
β-functionalized nitriles (Scheme 1a). Therefore, the develop-
ment of a general and efficient method to realize the 1,2-
cyanofunctionalization of unactivated alkenes for the concom-
itant incorporation of a cyano group and multiple functional
groups into organic molecules is highly desirable.
carbon- and heteroatom-centered radicals to alkenes. A unique
feature of the aforementioned hydrogen migration process
resides in the preference for the generation of a lower-energy
radical,5 thus providing a driving force for remote hydrogen atom
abstraction by an inherently high-energy sp3 carbon-centered
radical generated from the addition of a radical to an alkene. On
the other hand, protecting groups are always unavoidable in
natural product synthesis because of the functional group
reactivity and tolerance toward the harsh conditions. To further
modify the functional group, a deprotection step is inevitable.6
Thus, the efficient combination of a deprotection step and
modification of the obtained functional group represents a step-
economical strategy and will shorten the synthetic route. Based
on the above assumption and from the point of step economy, we
rationally design the TMS (trimethylsilyl)-protected alkenyl
In our continuous efforts in radical-initiated remote hydrogen
migration chemistry,4 we envisioned the possibility of developing
a novel cyanofunctionalization of unactivated alkenes through
remote cyano migration triggered by addition of the diverse
Received: September 30, 2016
© XXXX American Chemical Society
A
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Letter
cyanohydrin 1, which are easily obtained from simply
straightforward cyanosilylation of the corresponding ketones,7
as the starting material in our efforts toward the catalytic 1,2-
cyanofunctionalization of unactivated alkenes via the 1,4(5)-
cyano migration process triggered by addition of diverse carbon-
and heteroatom-centered radicals to alkenes (Scheme 2b). To
trifluoromethylation of 1A proceeded well in the presence of
CuBr in EtOAc at 60 °C for 12 h, affording the desired β-
trifluoromethylated nitrile 3A as the sole product via the remote
radical 1,4-cyano migration, albeit in 58% yield (Table S1 in SI,
entry 1). Fortunately, no oxytrifluoromethylation or β-hydride
elimination product was detected, showing the high selectivity of
this strategy. Encouraged by this result, after systematically
screening the reaction parameters, we finally identified the
optimized conditions as the following: the reaction of 1A and 2a
in 1:1.5 molar ratio, with CuI (20 mol %) as the catalyst, in
CH3CN at 60 °C for 12 h, affording 3A in 75% isolated yield
(Table S1, entry 6). It should be noted that under the standard
reaction conditions, the reaction of TMS-removal alkenyl
cyanohydrin 1A′ gave a similar result with 1A, implying the
efficiency of the synergistic deprotective cyanotrifluoro-
methylation strategy was not significantly affected, thus
rendering this strategy more synthetically appealing in terms of
step-economy values to allow the deprotection step to be
omitted (Table S1, entry 13 and Scheme 2).
Scheme 2. Substrate Scope for 1,2-Cyanotrifluoromethylation
Reactions
With the optimal conditions in hand, we next explored the
reaction scope with TMS-protected alkenyl cyanohydrins
(Scheme 2). The cyanotrifluoromethylation reaction of alkenes
via 1, 4-cyano migration was first examined. The linear substrates
possessing aryl or aliphatic groups were found to be suitable
substrates for the reaction to afford β-trifluoromethylated nitriles
3B and 3C in 73% and 77% yields under the standard reaction
conditions. The reaction of 1,1-disubstituted alkenes 1D
provided the desired β-trifluoromethylated nitrile 3D in excellent
yield (86%). In addition to simple linear substrates, the cyclic
substrate 1E was also applicable to this transformation, giving the
desired product 3E in 58% yield. Noteworthy is that the aryl
tethered substrates 1F, where the alkene moiety is conjugated
with an aryl ring, proceeded as well to generate 3F in 45% yield.
Encouraged by the above success, we next turned our attention to
the 1,5-cyano migration process. Gratifyingly, upon treatment
with the standard reaction conditions, 1G furnished 3G in 77%
yield. Further expansion of the substrate scope revealed that the
aryl tethered substrates bearing electron-neutral (1H), -rich (1I),
or -deficient (1J) aryl groups did not affect the reaction efficiency
to deliver 3H−3J in 70−90% yields. Substrates with an aryl ring α
to the alcohol group reacted effectively in this reaction.
Furthermore, substituents that are electron-neutral (1K),
-deficient (1L), or -rich (1M and 1N) on the aryl ring were
tolerated well to afford the corresponding highly functionalized
β-trifluoromethylated nitriles 3K−3N in 65−92% yields.
Unfortunately, the reaction of 1O−1Q afforded almost no
desired product 3O−3Q under the standard reaction conditions.
It is encouraging to note that the diverse substrates containing
both linear and tethered backbones are all tolerated under the
current catalytic system, showing a broader substrate scope than
that for the just reported method.9
a
b
0.1 M substrate 1 in 0.2 mmol scale. Isolated yield.
realize this strategy, several challenges should be overcome: (1)
The unexpected side reactions, such as the competitive 1,2-oxy-
functionalizations of alkenes, β-hydride elimination, and other
transformations, should be overcome.2,8 (2) Synergistic TMS
deprotection and cyanofunctionalization of alkenes should be
well tolerated in the one-pot transformation. (3) The
compatibility between unactivated alkenyl TMS-protected
cyanohydrin and various carbon- and heteroatom-centered
radical precursors with different reactivity properties should be
considered. Herein, we report a general and systematic strategy
of catalytic 1,2-cyanofunctionalization of unactivated alkenes via
the 1,4/5-cyano migration process triggered by trifluoromethy-
lation, phosphonylation, azidation, sulfonylation, perfluoroalky-
lation, or difluoromethylation of easily accessible TMS-protected
alkenyl cyanohydrins to afford diversely β-functionalized alkyl
nitriles.9
Modular synthetic access to these model substrates 1 would be
readily achieved in good yields by straightforward and efficient
cyanosilylation from the inexpensive corresponding ketones (see
details in the Supporting Information (SI)). Considering the
great importance of trifluoromethylated organic molecules in
pharmaceuticals and agriculture chemicals,10 we then initiated
our investigations by examining the radical trifluoromethylation
reactions of unactivated alkenes. As such, the reaction of
substrate TMS-protected alkenyl cyanohydrin 1A with Togni’s
reagent 2a11 was selected for the optimization of reaction
conditions. We were delighted to find that the 1,2-cyano-
The successful synthesis of β-trifluoromethylated nitriles from
unactivated alkenes prompted us to screen other radicals for
straightforward access to more types of β-functionalized nitriles.
Gratifyingly, the reaction of 1 with diphenylphosphine oxide 4
(2.0 equiv) in the presence of AgNO3 (40 mol %) in DMF at 80
°C provided the sole 1,2-cyanophosphonyltion product 5 with
high selectivity (Scheme 3a). Further expansion of the substrate
scope showed that β-phosphonyl nitriles 5A, 5D, and 5H were
obtained efficiently under similar conditions in 67−86% yields
via a 1,4(5)-cyano migration process. Furthermore, the
successful catalytic installation of an azide radical to substrate 1
delivered the desired β-azido nitriles 7A, 7D, and 7H in 58−83%
B
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afford the highly functionalized β-sulfonyl nitrile 9H in excellent
yield (92%). Most importantly, the success of CF3, C4F9, CF2H,
CF2CO2Et radical installation to generate 3H, and 11−13 with
sulfonyl chloride (10a−10d)15 via extrusion of sulfur dioxide
under photoredox catalysis conditions, demonstrated the
generality of the present methodology in simultaneously
incorporating the cyano group and various fluoroalkyl radicals
into an alkene (Scheme 4).
Scheme 3. Radical-Initiated 1,2-Cyanophosphonylation and
Cyanoazidation
It should be emphasized that the 1,2-cyanofunctionalization
strategy showed high selectivity for diverse radicals, with no 1,2-
oxyfunctionalized product or β-hydride elimination product
observed. Overall, these features demonstrate that the radical
mediated 1,2-cyanofunctionalization of alkenes exhibits broad
compatibility for the series of diverse radicals, thus offering a
general strategy for simultaneous installation of diversely
functional groups and the valuable cyano group into the alkenes.
Based on the above results and previous reports,16 we
proposed a general mechanism for this catalytic protocol
(Scheme 5). To initiate the process, the targeted R radicals
a
b
0.1 M substrate 1 in 0.2 mmol scale. Isolated yield based on 1. DMF
= N,N-dimethylformamide.
Scheme 5. Proposed Reaction Mechanism
yields with iodine(III) reagent azidoidinane (6)12 with 20 mol %
CuI as the catalyst, demonstrating the excellent compatibility of
the protocol with the azide radical (Scheme 3b).
To further expand the reaction scope to obtain β-sulfonyl
nitriles, we next examine the in situ generated sulfonyl radical to
probe whether a similar process could be realized. Unfortunately,
our efforts to employ p-toluenesulfinic acid or p-toluenesulfonyl
hydrazide with different oxidants to generate the sulfonyl radicals
for the reaction failed (see Table S2 in SI). Notably, the recently
developed visible-light-mediated photoredox catalysis has
become an environmentally benign tool for the generation of
various radical species under extremely mild conditions.13 We
assumed that the cyanosulfonylation reaction might be realized
under the mild photoredox catalysis system. To our delight, the
reaction of substrate 1A with p-toluenesulfonyl chloride (8) in
the presence of photocatalyst [Ir(ppy)2(dtbbpy)]PF6 (1 mol %)
with 2 equiv of Na2HPO4 under visible light initiation provided
the desired β-sulfonyl nitrile 9A in 82% yield in high selectivity
(Scheme 4a).14 Further examination of the substrate scope
exemplified that disubstituted alkenes 1D proceeded smoothly to
generate 9D in 90% yield under standard reaction conditions.
Notably, the aryl tethered substrate worked more efficiently to
were first generated with the corresponding catalyst. Next, the
substrate 1 would be attacked by the in situ generated radicals R
to give the transient alkyl radical I, which would undergo a 5/6-
exo radical cyclization reaction to afford the alkyliminyl radical
II.16 This high energy radical intermediate would have a driving
force to provide the relatively stable TMS-protected ketyl radical
III via selective β-fission.16d A further SET oxidation and
subsequent removal of TMS group provided the desired β-
functionalized alkyl nitriles to realize the 1,2-cyanofunctionaliza-
tion of an unactivated alkene.17 Further study to disclose the
interesting stereoselectivity of the cyano migration is in progress
in our lab.18
In summary, we have developed a general and efficient
catalytic radical 1,2-cyanofunctionalization of unactivated
alkenes via a remote 1,4(5)-cyano migration process triggered
by trifluoromethylation, phosphonylation, azidation, sulfonyla-
tion, and perfluoroalkylation of unactivated alkenes in the
presence of metal or photoredox catalysis. This protocol exhibits
the use of readily available starting materials with high step
economy, a broad substrate scope, the compatibility of diverse
carbon- and heteroatom-centered radical precursors, and
excellent selectivity. This synergistic strategy provides a
convenient access to diverse β-functionalized alkyl nitriles
bearing densely functionalized valuable carbonyl, cyano, and
other various functional groups within the same molecules,
which are valuable intermediates and should be of great
importance in bulk chemicals. Further mechanistic study is
ongoing in our laboratory.
Scheme 4. Photoredox-Catalyzed 1,2-Cyanosulfonylation and
Cyanoperfluoroalkylation
a
b
0.1 M substrate 1 in 0.2 mmol scale. Isolated yield based on 1.
Catalyst: [Ir(ppy)2(dtbbpy)]PF6. TsCl = tosyl chloride.
C
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02960.
■
S
Experimental procedures, characterization of all new
compounds, Tables S1 and S2 (PDF)
(6) Wuts, P. G. M.; Greene, T. W. Greene’s Protective Groups in Organic
Synthesis; John Wiley & Sons, Inc.: 2006; p 16.
(7) Evans, D. A.; Truesdale, L. K.; Carroll, G. L. J. Chem. Soc., Chem.
AUTHOR INFORMATION
Corresponding Authors
■
Commun. 1973, 55.
*E-mail: liuxy3@sustc.edu.cn.
*E-mail: guozhen@tyut.edu.cn.
ORCID
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During our preparation of this manuscript, Zhu reported a similar
strategy, wherein their transformations are only limited to azide radical
and deproteced linear alkenyl cyanohydrins as substrates, which called
for the TMS removal step. In addition, an excess of TMSN3 (4.0 equiv)
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Xin-Yuan Liu: 0000-0002-6978-6465
Author Contributions
§These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science Foundation
of China (Nos. 21572096, 21302088), Shenzhen overseas high
level talents innovation plan of technical innovation project
(KQCX20150331101823702), Shenzhen special funds for the
development of biomedicine, Internet, new energy, and new
mate-rial industries (JCYJ20150430160022517) is greatly
appreciated. Z.G. is thankful for financial support from the
100-Talent Program and 131-Excellent Talent Plan in Shanxi
Province, and Taiyuan University of Science and Technology of
China.
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whose potential transformation to enantioenriched 3A would be both
synthetically and mechanistically valuable.
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