Csp-Csp3 Bond Formation via Iron(III)-Promoted Hydroalkynylation of Unactivated Alkenes

Article abstract of DOI:10.1021/acs.orglett.7b00499

An iron(III)-promoted hydroalkynylation of unactivated alkenes toward Csp-Csp³ bond formation has been developed. Various alkenes, including mono-, di-, and trisubstituted alkenes, could all smoothly convert to structural diversified alkynes in this chemoselective protocol. Additionally, the scalability was unraveled and the further divergent transformations of products were conducted to demonstrate the synthetic utility.

Full text of DOI:10.1021/acs.orglett.7b00499

Letter
pubs.acs.org/OrgLett
Csp−Csp3 Bond Formation via Iron(III)-Promoted Hydroalkynylation
of Unactivated Alkenes
Yangyong Shen,† Bo Huang,† Jing Zheng,† Chen Lin,† Yu Liu,‡ and Sunliang Cui*,†
†Institute of Drug Discovery and Design, College of Pharmaceutical Sciences, Zhejiang University, 866 Yuhangtang Road, Hangzhou
310058, Zhejiang, China
‡State Key Laboratory of Plant Physiology and Biochemistry, College of Life Sciences, Zhejiang University, Hangzhou 310058,
Zhejiang, China
S
* Supporting Information
ABSTRACT: An iron(III)-promoted hydroalkynylation of unactivated
alkenes toward Csp−Csp3 bond formation has been developed. Various
alkenes, including mono-, di-, and trisubstituted alkenes, could all smoothly
convert to structural diversified alkynes in this chemoselective protocol.
Additionally, the scalability was unraveled and the further divergent
transformations of products were conducted to demonstrate the synthetic utility.
lkynes are fundamental synthons in organic synthesis and
alkynylation of methylenecyclopropanes and styrenes, while
1
Aimportant structural units of bioactive molecules. There- Kwong and Li reported the asymmetric hydroalkynylation of
norbornadienes and enamides.17 Despite advance in this field, the
fore, numerous efforts have been devoted to the investigation of
development of general hydroalkynylation of alkenes toward
Csp−Csp3 bond formation remains challenging.
alkyne synthesis. For example, the Sonogashira reaction has
gained great success in the construction of Csp−Csp2 bonds.2
However, the coupling of nonactivated alkyl halides has proven
challenging because of competing oxidative addition/β-hydride
elimination. Thus, many explorations have been conducted to
solve this issue. Fu and co-workers have made great progress in
this area and reported a seminal work on the Sonogashira
coupling of primary alkyl halides for the construction of a Csp−
Csp3 bond.3 Afterward, Glorius and Hu independently expanded
this scope.4 Other coupling reactions toward Csp−Csp3 bond
formation, such as photoredox catalysis, oxidative coupling, and
those using organometallic reagents, have also been extensively
explored recently.5 Meanwhile, Wang invented a Cu-catalyzed
cross-coupling of N-tosylhydrazones and trisalkylsilylethynes for
forging a Csp−Csp3 bond.6 Interestingly, Li reported a metal-free
coupling between an alkyne and alkyl iodide with light in water.7
On the other hand, the decarboxylative alkynylation of carboxylic
acids has also been explored.8
The hydroalkynylation of alkenes for Csp−Csp3 bond
formation represents an important and promising transforma-
tion, since alkenes are readily available and structurally divergent.
A two-step process of alkene hydroalkynylation was previously
reported by Renaud and co-workers (Scheme 1).18 In that
Scheme 1. Csp−Csp3 Bond Formation via Hydroalkynylation
of Unactivated Alkenes
Recently, the transition-metal-mediated hydrofunctionaliza-
tionofalkeneshasgrownasapowerfulapproachforC−CandC−
X (N, O, F, S, Cl) bond formation in the construction of diverse
molecules.9,10 A seminal work of Fe-catalyzed hydroamination of
unactivated alkenes with butyl nitrite was reported by
Mukaiyama.11 Afterward, Carreira and co-workers contributed
great work in alkene hydroamination, -cyanation, and -chlorina-
tion.12 Recently, Boger investigated the scope of Fe-catalyzed
alkene hydrofluorination.13 Meanwhile, Shenvi established the
alkene hydroarylation with a combination of Co/Ni catalysts.14
More recently, Baran has invented Fe-mediated alkene conjugate
addition, hydroamination, and hydromethylation reactions.15
Notably, radical hydrofunctionalization has been recently applied
in the total synthesis of natural products.16 With respect to
hydroalkynylation protocols, Suginome established the hydro-
protocol, thealkenes underwenthydroboration withcatecholbor-
ane to give β-alkylcatecholboranes, which underwent sequential
radical addition to phenylethynylsulfone to furnish alkynes. The
process thus delivered anti-Markovnikov products, but few
examples were examined. As part of our interest in functionaliza-
tion of unactivated alkenes,19 we hypothesized that the
hydroalkynylation of alkenes might be possible. Herein, we
would like to report an iron(III)-promoted hydroalkynylation of
unactivated alkenes for rapid construction of a Csp−Csp3 bond.
Received: February 20, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00499
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
We commenced our study by investigating the 2-methylallyl
alcohol 1a. Initially we carried out the reaction using 20 mol %
Fe(acac)3 as the catalyst, PhSiH3 as the reductant, and NaHCO3
as the base inethanol at 60 °C. Various alkynylating reagents were
screened, and all thereagents 2including 2aand 2bwere reported
as alkynylating reagents and tested inthis reaction. The utilization
of 2a showed no alkynylating reaction and the alkenes were
reduced to alkanes, while 2b was slightly unstable to decompose
(Table 1, entries 1 and 2).20 The more electrophilic phenyl-
Scheme 2. Scope of Alkynyl Bromides
a
Table 1. Reaction Optimization
b
entry
metal salt
alkynylating reagent solvent t (°C) yield (%)
c
1
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
Mn(acac)3
Co(acac)2
Ni(acac)2
Fe(acac)3
Fe(acac)3
Fe(acac)3
Fe(acac)3
2a
2b
2c
2d
2e
2d
2d
2d
2d
2d
2d
2d
2d
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
THF
60
60
60
60
60
60
60
60
60
60
25
60
60
−
−
−
26
c
2
a
Reaction conditions: Fe(acac)3 (50 mol %), 1 (0.6 mmol), 2 (0.2
mmol), PhSiH3 (0.3 mmol), NaHCO3 (0.2 mmol), EtOH (2 mL), 60
°C, 2 h; yields refer to isolated products.
c
3
c
4
c
5
−
d
6
76
44
<5
−
d
7
d
8
such as methyl, methoxy, chloro, cyano, and ester. In addition, the
structure of 3d was confirmed by X-ray analysis.24 Moreover, the
2- and 3-substituted phenylethynyl bromides were both
compatible in this transformation to deliver the alkynes in
moderate to good yields (3g−3i). Notably, the heterocyclic
alkynyl bromides, such as 3-pyridine and 2-thiophene, were also
well applicable in this process (3j−3k). Interestingly, the
styrenylethynyl bromide could proceed smoothly to give the
enyne product in moderate yield (3l). When the alkyl alkynyl
bromides were utilized, no hydroalkynylation product was
observed, probably because of no stabilization of the resultant
radical by aromatic rings.
d
9
d
10
25
d
11
EtOH
EtOH
EtOH
−
41
42
de
,
12
13
f
a
Reaction conditions: 1a (0.6 mmol), 2 (0.2 mmol), PhSiH3 (0.3
b
mmol), NaHCO3 (0.2 mmol), solvent (2 mL), 2 h. Yields refer to
isolated products. Metal amount (20 mol %). Metal amount (50 mol
%). Removal of NaHCO3. 1a (0.2 mmol) was used.
c
d
e
f
We next examined the scope of unactivated alkenes (Scheme
3). A series of alkenes, including mono-, di-, and trisubstituted
alkenes, were subject to this process. The terminal monosub-
stituted alkenes, such as allyl alcohol, hex-5-en-1-ol, allyl cyanide,
allyl carboxamide, benzyl pent-4-enoate, and 1-allyl-4-methox-
ybenzene, could engage in this process to afford the products in
moderate to good yields (4a−4f). The internal disubstituted
alkenes, including (Z)-but-2-ene-1,4-diol and cyclic N-Boc-2,5-
dihydro-1H-pyrrole, were well amenable to this protocol to
furnish the products in moderate yields (4g and 4h). With respect
to terminal disubstituted alkenes, such as ketone-tethered alkene
1j and piperidine functionalized alkene 1k, these alkenes could
undergo this transformation smoothly to give the diversified
alkyne products in good yields (4i and 4j). Those trisubstituted
alkenes, such as cyclohexene tethered alcohol 1l and amine 1m,
could proceed well to furnish the alkynes in good yields (4k and
4l). Furthermore, the synthetic utility of this protocol was
demonstrated by hydroalkynylation of natural product deriva-
tives. For example, the (S)-(−)-β-citronellol and ( )-isopulegol
could be easily transferred to the alkyne derivatives in moderate
yields.25 More interestingly, when quinine was used, the alkyne
derivative 4o could be furnished in 52% yield with a 3:1 dr ratio.
And the 3-methyl estrone ether only delivered a single
diastereomer, 4p, in 57% yield, which was confirmed by X-ray
analysis.26 Therefore, this transformation enables the synthesis of
diverse alkynes with high functional group compatibility from
simple alkenes.
ethynyl phenyliodoniumtriflate (2c) was also found to be
ineffective (entry 3).21 Gratifyingly, the employment of phenyl-
ethynyl bromide (2d) led to formation of the expected
alkynylation product 3a in 26% yield (entry 4),22 and the
phenylethynyl benziodoxolone (2e) proved ineffective (entry
5).23 When the amount of Fe(acac)3 was increased to 50 mol %,
the yields were significantly improved to 76% (entry 6). The next
optimization was carried out using different catalysts, solvents,
and reaction temperatures (entries 7−11). The results showed
that the combination of Fe(acac)3/PhSiH3/EtOH was optimal.
The removal of NaHCO3 would lead to a lower yield thus
demonstrating the essential role of NaHCO3 as the base (entry
12). Wealsouseda1:1molar ratio ofalkenes, butthis resulted ina
slightly low yield (entry 13). The use of 3 equiv of 1a could
facilitate the reaction, and excess 1a could be recovered.
With the optimized reaction conditions in hand, we explored
the substrate scope of alkynyl bromides (Scheme 2). The alkynyl
bromides were easily prepared by bromination of alkynes with N-
bromo succinimide (NBS), and various alkynyl bromides were
employed as an alkynylating reagent. For example, the 4-
substituted phenylethynyl bromides, regardless of the electron-
donating or -withdrawing substitution, could engage in this
process to deliver the corresponding homopropargyl alcohols in
good to excellent yields (3b−3f), with a valuable functional group
B
DOI: 10.1021/acs.orglett.7b00499
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 3. Scope of Alkenes
Figure 1. Scale-up reaction and synthetic application.
Figure 2. Reaction mechanism study.
Figure 3. Proposed reaction mechanism.
a
Reaction conditions: Fe(acac)3 (50 mol %), 1 (0.6 mmol), 2 (0.2
converted toFe hydride species A upon treatment of phenylsilane
and ethanol. Then A undergoes hydrogen atom transfer (HAT)
to alkene 1 to form alkyl radical B and Fe(II) species C through
radical cage pairing of B and C,9d which is trapped by alkynyl
bromide 2 to generate the β-bromo radical D. The sequential
elimination of D would furnish the alkynylated product (3 or 4)
and a bromo radical, which would reoxidize C to Fe(III) to enable
the catalytic cycle. We assumed that the bromo radical would
partially deactivate the Fe(III) catalyst; thus, use of 50 mol % Fe-
catalyst could facilitate the reaction and the addition of NaHCO3
as the base is essential to remove HBr. Dependent on the
alkynylating reagent utilized (2a−2c, 2e), either decomposition
or low reactivity was observed, thus disabling the hydro-
alkynylation.
In summary, a facile hydroalkynylation of unactivated alkynes
toward Csp−Csp3 bond formation has been developed. The
alkene materials are readily available, the Fe catalyst is
inexpensive, and the method can be scaled up. This protocol
thus represents a simple and distinct method for alkynes
synthesis, which would demonstrate valuable synthetic utility in
natural products and bioactive molecules modification.
mmol), PhSiH3 (0.3 mmol), NaHCO3 (0.2 mmol), EtOH (2 mL), 60
°C, 2 h, yields refer to isolated products. THF (2 mL) with EtOH
(0.4 mmol) was used as solvent.
b
Furthermore, a scale-up reaction was conducted to unravel the
synthetic potential. A 5 mmol scale reaction was carried out for
allyl alcohol 1b and phenylethynyl bromide 2d, and the
homopropargyl alcohol product 4a was isolated in 42% yield
(Figure 1). Further elaboration of 4a was also conducted. For
example, 4a could be transferred to ketone 5 in the presence of
PdCl2, and the hydroxyl group was found to be converted to
chlorine. Alternatively, 4a could convert to but-3-ynoic acid 6
upon oxidation. Meanwhile, when 4a was subject to the Rh(III)-
catalyzed C−H activation/cyclization with N-pivaloyl-
hydroxamic acid and oxime ester, the heterocycles 7 and 8
could be accessed rapidly.27
To gain insight into the reaction mechanism, control
experiments were carried out (Figure 2). The vinylcyclopropane
1r was prepared and subjected to the standard hydroalkynylation
process, and the ring opening alkynylation product 9 was isolated,
indicating a cyclopropane opening radical pathway.
Based on these results and literature,9d−f,22 a plausible reaction
mechanism is proposed in Figure 3. Initially, the Fe(III) is
C
DOI: 10.1021/acs.orglett.7b00499
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
■
S
Publications website at DOI: 10.1021/acs.orglett.7b00499.
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Full experimental procedures, characterization data, and
NMR spectra data (PDF)
Crystallographic data for 3d (CIF)
Crystallographic data for 4p (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: slcui@zju.edu.cn.
ORCID
Sunliang Cui: 0000-0001-9407-5190
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for the financial support from National Natural
Science Foundation of China (21472163, 31400224).
■
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DOI: 10.1021/acs.orglett.7b00499
Org. Lett. XXXX, XXX, XXX−XXX