Cyanohydrin-Mediated Cyanation of Remote Unactivated C(sp3)-H Bonds

Article abstract of DOI:10.1021/acs.orglett.8b04104

A new approach for generation of alkoxy radical from the O-H bond of cyanohydrin promoted by visible-light photoredox catalysis is reported. The alkoxy radical triggers the successive remote HAT and intramolecular cyano migration, leading to the regiosele

Full text of DOI:10.1021/acs.orglett.8b04104

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cyanohydrin-Mediated Cyanation of Remote Unactivated C(sp3)−H
Bonds
Min Wang,†,§ Leitao Huan,†,§ and Chen Zhu*,†,‡
†Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, 199 Ren-Ai Road, Suzhou Jiangsu 215123, China
‡Key Laboratory of Synthesis Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese Academy of
Science, 345 Lingling Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A new approach for generation of alkoxy radical from the O−H bond of cyanohydrin promoted by visible-light
photoredox catalysis is reported. The alkoxy radical triggers the successive remote HAT and intramolecular cyano migration,
leading to the regioselective cyanation of remote C(sp3)−H bonds. The reaction exhibits a broad functional group tolerance
that allows many sensitive groups to remain intact under mild conditions. To demonstrate the utility of method, the ketonitrile
products are converted to other synthetically valuable compounds.
Scheme 1. Radical-Mediated HAT for C(sp3)−H Activation
ransformation of inert C(sp3)−H bonds to other
T
chemical bonds represents an ideal synthetic strategy
upon consideration of the atom- and step-economy of reaction.
Nevertheless, it remains a long-term challenge in synthetic
chemistry.1 Despite the great progress made by transition-
metal catalysis, radical-mediated hydrogen atom transfer
(HAT) provides an efficient approach for C(sp3)−H
activation.2 This process is largely dependent on substrates
where the tertiary C−H bonds are more preferred than the
secondary and primary C−H bonds. In the presence of
multiple reactive sites, the reaction usually loses regioselective
control, leading to a mixture of regioisomers (Scheme 1a).
The radical translocation process mediated by alkoxy
radicals has long been established to gain distinct chemo-/
regioselectivities.3 However, this method is still underexplored.
It is a formidable challenge to directly oxidize alcohol to alkoxy
radical owing to the high bond dissociation energy (BDE) of
O−H bond (ca. 105 kcal/mol). In this context, many
alternative pathways are sought to transform alcohol to various
precursors (e.g., metal alkoxides, nitrite esters, hypohalites,
etc.) that could readily generate alkoxy radicals under mild
conditions, but these surrogates are sometimes difficult to
prepare or handle (Scheme 1b).4 Photoredox catalysis proved
to be a powerful tool for converting alcoholic O−H bonds to
alkoxy radicals.5 Recently, we reported the visible-light-
promoted heteroarylation of C(sp3)−H bonds via alkoxy
radical-triggered 1,5-HAT.6 Later, Zuo disclosed the CeCl3-
catalyzed C(sp3)−H amination of alcohols under visible-light
irradiation.7 Considering that alcohols are abundant and
commodity chemicals, the alcohol-mediated regioselective
C(sp3)−H functionalization via radical translocation is of
particular synthetic value.
The cyanohydrin−ketonitrile reaction was developed by
Kalvoda half a century ago to functionalize unactivated C−H
bonds in steroids. The early studies harnessed harsh conditions
such as the combination of lead tetraacetate, iodine, and UV
Received: December 23, 2018
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.8b04104
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Letter
irradiation or strong metal oxidants (e.g., silver and cerium
salts, etc.) that usually led to low yields and poor compatibility
with susceptible groups.8 To improve the reaction outcome,
Watt employed the elaborate peracetoxynitrile instead of
cyanohydrin as the precursor of alkoxy radical.9 However,
these pioneering reports have not received much attention, and
the synthetic value of this chemistry has been scarcely explored
over the past few decades. Herein we disclose the visible-light-
promoted regioselective cyanation of remote C(sp3)−H bonds
(Scheme 1c). The alkoxy radical is directly generated from the
O−H bond of cyanohydrin, which triggers the sequence of
remote HAT and intramolecular cyano migration.10 A broad
range of functional groups are well tolerated under the mild
reaction conditions. The versatile ketonitrile products can be
conveniently converted to other synthetically valuable
molecules, illustrating the utility of the protocol.
Scheme 3. Scope of Cyanohydrins and Generality of
Protocol
a
At the outset, cyanohydrin 1a was prepared as the model
substrate to investigate reaction parameters. After a systematic
survey of photocatalysts, oxidants, and solvents, the reaction
was facilitated by visible-light irradiation to afford the desired
ketonitrile 2a in 82% yield on a 0.3 mmol scale through the
cyanation of unactivated γ-C(sp3)−H bond (Scheme 2; for
Scheme 2. Optimized Conditions for the Cyanohydrin−
Ketonitrile Reaction
detailed reaction conditions survey, see the Supporting
Information). Remarkably, the reaction was not interfered by
the commonly occurring β-C−C bond fission of the alkoxyl
radical or the Norrish type II fragmentation.11 It was found
that only persulfates proved to be efficient oxidants, and the
reaction was halted while altering the photoredox catalyst.
Among the solvents examined, a biphasic mixture consisting of
PhCF3/H2O delivered the best yields. The addition of
ammonium salt was indispensable to the transformation,
presumably acting as a phase-transfer catalyst (PTC) in the
reaction. Performing the reaction on a 1.0 mmol scale
compromised the reaction outcome, probably due to the
sharply decreased light penetration in the reaction at larger
scale.
With the optimized reaction conditions in hand, we set
about defining the substrate scope (Scheme 3). Initially, a
variety of functional groups were tested. Both electron-rich and
-deficient groups were well tolerated in the reaction to deliver
good yields (2b−j). The examples of 2j and 2n were
noteworthy, as the existence of aryl bromide reserved a
platform for later modification of products by cross couplings.
The change of substituents from the para- to the meta- or the
ortho-position did not have much impact on reaction outcome
(2k−n). Notably, some groups such as alkynyl and alkenyl
susceptible to radical conditions remained intact in the
reaction (2o and 2p). Apart from aryl cyanohydrins, aliphatic
cyanohydrins were also suitable substrates (2q). The radical
abstraction of tertiary C−H bonds is, in general, prior to other
C−H bonds in accordance with BDEs, whereas the cases of 2s
and 2t showed a reversed selectivity of secondary C−H bonds.
a
Reaction conditions: cyanohydrin 1 (0.3 mmol), [Ir(dF(CF3)-
ppy)2(dtbbpy)]PF6 (3 mol %), K2S2O8 (2.2 equiv), and TBAC (40
mol %) in PhCF3/ H2O (3 mL/0.3 mL) under 14W blue LED
irradiation at rt. Yields of isolated products are given. TBAC =
tetrabutylammonium chloride.
The intramolecular cyano migration underwent through a
cyclic transition state that rendered a unique stereocontrol for
cyanation of cyclic C−H bonds, affording the cis-product 2s.
Moreover, benzylic C−H bonds (2u−w) and the C−H bonds
adjacent to the heteroatom (2x and 2y) were also apt to
furnish the corresponding cyanated products. In the case of
2u−w, most of the unreacted cyanohydrins were recovered. In
the presence of multiple reactive methylenes, the cyanation of
γ-C−H bonds was superior to the δ-ones (2z), ascribing to
that 1,5-HAT via a six-membered cyclic transition state is
kinetically more favorable in this reaction.
Several well-designed experiments were conducted to shed
lights on the mechanistic pathways. To understand the
reactivity of different types of C(sp3)−H bonds in the reaction,
a set of competition experiments were first performed (Scheme
4a). The observed trend, tertiary > benzylic > secondary >
primary C−H, was identical to the routine on the basis of
BDEs. The kinetic isotope effect value (kH/kD = 2.33) gained
from the intramolecular isotope competition experiment of 7
suggested that the C−H cleavage might be involved in the rate-
determining step of reaction (Scheme 4b). The free O−H
bond of cyanohydrin was crucial to the direct formation of the
key alkoxy radical intermediate from the O−H bond which
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oxidation barrier of O−H bond and enables the formation of
alkoxy radical via PCET.12 Two pathways, the photocatalytic
IrIII/IrIII*/IrIV cycle (path a) and persulfate-mediated chain
reaction (path b), might be concurrently engaged in the PCET
process. Furthermore, a competitive pathway, abstraction of
the alcoholic hydrogen from cyanohydrin by sulfate radical
anion to generate alkoxy radical, also could not be ruled out.
The synthetic utility of method was manifested by
transformations of the product into other valuable molecules
(Scheme 6). The product consists of ketone and nitrile; both
Scheme 4. Mechanistic Investigation
Scheme 6. Synthetic Value of Products
initiated the subsequent cascade. To provide support for the
hypothesis, the TMS-protected cyanohydrin 9 was tested. As
expected, only a trace amount of product 2a was obtained in
the reaction (Scheme 4c). Additional experiments including
the optical and electrochemical measurements were carried out
to elucidate the oxidation process from cyanohydrin to alkoxy
radical. The control experiments revealed that the reaction
could also proceed in the absence of photocatalyst and visible-
light irradiation, albeit affording the desired product in a low
yield (see the Supporting Information). Along with the
detected quantum yield (ϕ = 4.7) of the reaction, these
results suggested that the overall process of alkoxy radical
formation might be the outcome of photocatalytic pathway and
the persulfate-mediated radical- chain reaction. The Stern−
Volmer analysis indicated that the excited state of IrIII
photocatalyst was readily oxidatively quenched by K2S2O8 to
form the IrIV species (see the Supporting Information).
However, cyclic voltammograms displayed that neither IrIV
(E1/2IV/III = 1.69 V in MeCN vs SCE) nor persulfate (E0 = 2.01
are versatile feedstocks in synthetic chemistry. First, the ketone
could be readily converted to ester 10 by the Baeyer−Villiger
oxidation. Then, the nitrile could be hydrolyzed to amide that
underwent the intramolecular annulation with ketone to afford
lactam 12. Alternatively, reduction of 2a resulted in 1,5-amino
alcohol 13 which went through the intramolecular cyclization
to furnish piperidine 14. Notably, complete hydrolysis of nitrile
led to a high yield of carboxylic acid 15, which could be
harnessed as precursor for incorporation of various functional
groups through the radical decarboxylative functionalization
according to Li’s methods.13
In summary, we have described a synthetically useful
cyanohydrin-mediated regioselective cyanation of remote
C(sp3)−H bonds. The alkoxy radical is readily generated
from O−H bond under visible-light irradiation, which triggers
the sequential HAT and distal cyano migration without the
undesired β-C−C cleavage and the Norrish type II
fragmentation. The generality and mechanism of the reaction
have been investigated. A variety of functional groups including
many susceptible groups such as alkenyl and alkynyl are well
compatible with the mild conditions. The ketonitrile products
are easily transformed into various valuable molecules,
demonstrating the synthetic utility of method. Since
cyanohydrins originate from ketones and lead to the cyanated
ketones, this protocol can be regarded as a formal late-stage
cyanation of aliphatic ketones.
V) was capable of directly single-electron oxidizing 1a (Ep/2
=
2.5 V in MeCN vs SCE; see the Supporting Information).
Based on the above results, we thus postulated a proton-
coupled electron transfer (PCET) process for the generation of
alkoxy radical (Scheme 5). The interaction of cyanohydrin
with a conjugate base such as sulfate probably reduces the
Scheme 5. Plausible Reaction Mechanistic Pathways
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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.or-
glett.8b04104.
■
S
Experimental details, compound characterization data,
and NMR spectra (PDF)
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■
Corresponding Author
*E-mail: chzhu@suda.edu.cn.
ORCID
Chen Zhu: 0000-0002-4548-047X
Author Contributions
§M.W. and L.H. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.Z. is grateful for financial support from Soochow University,
the National Natural Science Foundation of China
(21722205), the Project of Scientific and Technologic
Infrastructure of Suzhou (SZS201708), and the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD).
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