Switchable Synthetic Strategy toward Trisubstituted and Tetrasubstituted Exocyclic Alkenes

Article abstract of DOI:10.1021/acs.orglett.8b02801

An efficient and facile method for the construction of tri- or tetrasubstituted exocyclic alkenes is achieved via a Cu(I)-catalytic system. This protocol exhibits mild conditions, low-cost catalyst, good functional group tolerance, and good yields. The selectivity toward tri- or tetrasubstituted alkenes can be delicately controlled by adjustment of base and solvent. A preliminary mechanism study manifested that the reaction undergoes a radical process, where B₂pin₂ plays an indispensable role.

Full text of DOI:10.1021/acs.orglett.8b02801

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Switchable Synthetic Strategy toward Trisubstituted and
Tetrasubstituted Exocyclic Alkenes
Sen Zhou,† Fangyuan Yuan,† Minjie Guo,‡ Guangwei Wang,*,† Xiangyang Tang,† and Wentao Zhao*,†
†Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, School of Science, Tianjin University,
Tianjin 300072, P. R. China
‡Institute for Molecular Design and Synthesis, School of Pharmaceutical Science and Technology, Tianjin University, Tianjin
300072, P. R. China
S
* Supporting Information
ABSTRACT: An efficient and facile method for the construction of tri- or
tetrasubstituted exocyclic alkenes is achieved via a Cu(I)-catalytic system.
This protocol exhibits mild conditions, low-cost catalyst, good functional
group tolerance, and good yields. The selectivity toward tri- or
tetrasubstituted alkenes can be delicately controlled by adjustment of base
and solvent. A preliminary mechanism study manifested that the reaction
undergoes a radical process, where B2pin2 plays an indispensable role.
xocyclic multisubstituted alkenes are frequently encoun-
tered motifs in diverse organic materials, pharmaceuticals,
Scheme 1. Previous Methods for the Synthesis of Exocyclic
Multisubstituted Alkenes
E
and agrochemicals.1 Additionally, further derivatization of
exocyclic alkenes renders these compounds versatile precursors
for the construction of more complex molecules.2
Retrosynthetic analysis reveals that cyclization from a
functionalized alkyne precursor should provide the most
straightforward approach to this type of compound. During
past several years, chemists have made great efforts in the
construction of exocyclic multisubstituted alkenes.3,4 For
example, Kambe et al. reported a silver-catalyzed carbomagne-
siation of alkynes with alkyl halides and a stoichiometric amount
of Grignard reagents in 2011 (Scheme 1a).5 In 2013, Cook et al.
developed a palladium-catalyzed alkyne insertion/reduction
reaction of unactivated alkyl iodides (Scheme 1b).6 In 2017,
Martin et al. described a visible-light-promoted atom transfer
radical cyclization of unactivated alkyl iodides (Scheme 1c).7
Although great progress has been made in this field, more
practical protocols involving cheap catalysts still need to be
developed. During the past decade, copper catalysts have been
extensively exploited due to their low cost, stability, and easy
operability. In recent years, our and other groups have developed
a series of efficient copper-catalyzed conversions for the
construction of various important organic synthetic motifs.8,9
Although the mechanisms for copper-catalyzed reactions remain
elucidated, copper salts proved to be effective and promising
catalysts in various chemical transformations. Herein, we present
an efficient, practical, and most important of all, switchable
synthetic approach toward trisubstituted and tetrasubstituted
exocyclic alkenes through a copper-catalyzed intramolecular
cyclization in the presence of B2pin2 (Scheme 1d).
alkyl halides instead of unactivated alkyl halide as electrophiles.
Actually, when we operated this reaction with acetylenic iodides
based on our previous studies, this transformation did not
proceed.
Therefore, we need to develop a new reaction system for the
intramolecular carboboration of alkynes so that the exocyclic
alkenes can be conveniently established.
Our study was initiated by examining the intramolecular
annulation of acetylenic iodides with (6-iodohex-1-yn-1-yl)
We proposed the copper-catalyzed intramolecular carbobora-
tion for the construction of multisubstituted exocyclic alkenes
based on our previous report on copper-catalyzed carboboration
of alkynes.10 Notably, most of the reaction involved activated
Received: September 2, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02801
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
benzene (1a) being the benchmark substrate under a copper
catalytic system. As mentioned earlier, under our previously
optimized condition for intermolecular alkyne carboboration,
which involves a Cu(I)/n-Bu3P catalytic system, no desired
product was detected (Table 1, entry 1). However, when this
Then, various dry solvents were screened, and eventually, the
results identified N,N-dimethylformamide (DMF) as the
optimal solvent, giving the desired product 2a with 71% yield
(entries 11−14). However, there is still 13% of trisubstituted
olefin 3a formedalongwith2a (entry13). Itisnoteworthythatin
theabsenceofB2pin2 orCuCl, noexpected2aor3awasobserved
(entries 15−16).
a
Table 1. Optimization of Reaction Conditions
From Table 1, we can see that 3a was detected all the time
duringthe optimizationprocess, whichmightbe attributedtothe
presence of H2O in the system. However, the addition of
exogenous water or tert-butanol had little effect on the product
distribution (see Supporting Information (SI), Optimization
DetailsandTableB). Nevertheless, wecontinuedtooptimizethe
conditions to afford 3a by selecting a suitable hydride source and
reducing the amount of B2pin2 (see SI, Optimization Details and
Table C). Further screening of different bases was conducted,
and Ph2SiH2 was chosen as hydride source. However, when we
used NaOtBu as base, the desired product 3a was obtained with
34% yield along with plenty of terminal elimination product
(entry17). When2.0equivofNaOHwereemployed, thedesired
product 3a was obtained with only 25% yield with large amounts
of 1a being recovered (entry 18). To our delight, the yield for 3a
increased significantly with 4.0 equiv of NaOH as base, giving the
desired product 3a with 84% yield. Finally, we obtained the
optimization reaction conditions for 2a and 3a, respectively.
Using a CuCl/B2pin2-catalytic system, LiOtBu as base, and dry
DMF as solvent under room temperature (Condition A), the
product 2a can be obtained with optimized yield. When using a
CuCl/B2pin2-catalytic system, NaOH as base, Ph2SiH2 as
hydride source, and dry THF as solvent under room temperature
(Condition B), product 3a can be afforded with good yield.
With optimized reaction conditions in hand, we evaluated the
substrate scope of acetylenic iodides. Under Condition A
(Scheme 2), both electron donating groups, such as methyl
and methoxyl, and electron withdrawing groups, such as
trifluoromethyl and cyano, can be well tolerated in spite of
their positions on the aromatic rings, resulting in the
b
yield (%)
entry
catalyst
base
solvent
2a
n.d.
16
0
3a
n.d.
52
0
c
1
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuBr
CuI
CuBr2
CuCl
CuCl
CuCl
CuCl
CuCl
−
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
NaOtBu
NaOtBu
K2CO3
DBU
THF
THF
THF
THF
THF
THF
THF
THF
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
<5
12
33
53
48
51
47
26
33
71(64)
<5
n.d.
n.d.
0
0
NaOH
NaOCH3
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
LiOtBu
−
37
31
22
28
30
32
34
20
13
14
n.d.
n.d.
34
25
84(80)
39
52
29
12
THF
THF
dioxane
MeCN
DMF
DMSO
DMF
DMF
THF
THF
THF
DMSO
MeCN
dioxane
toluene
LiOtBu
NaOtBu
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
d
17
d
18
0
0
23
0
<5
0
de
,
19
20
21
22
23
de
,
de
,
de
,
de
,
a
Reaction conditions: 1a (0.5 mmol, 1 equiv), [Cu] (0.05 mmol, 10
mol %), B2pin2 (0.75 mmol, 1.5 equiv), base (1 mmol, 2 equiv).
Scheme 2. Scope of Exocyclic Tetrasubstituted
Alkenylborons
b
a,b
Yields determined by GC. The value in parentheses is the isolated
c
d
yield. n-Bu3P (24 mol %). Ph2SiH2 (0.75 mmol, 1.5 equiv), B2pin2
e
(0.25 mmol, 0.5 equiv), rt, 4 h. NaOH (2 mmol, 4 equiv). n.d. = not
detected. For details of the optimization, see the Supporting
Information.
reaction was conducted without ligand, exocyclic tri- and
tetrasubstituted alkenes were isolated separately (Table 1,
entry 2). In view of the resulting tri- or tetrasubstituted
alkenylboron derivatives being utilized as highly versatile
intermediates to access diverse tri/tetrasubstituted alkenes via
manifold methods, such as Suzuki−Miyaura coupling,11 Petasis
reaction,12 transition metal-catalyzed conjugate addition,13 etc.,
wearedevotedtooptimizingthetransformationprocesstoafford
both trisubstituted and tetrasubstituted alkenylboron derivatives
selectively. At the beginning, we screened a variety of bases
(Table 1, entries 3−7). When using K2CO3 as base, no desired
product tetrasubstituted 2a was obtained. When stronger base
NaOH was used, product 2a was isolated with 12% yield (entry
5). Subsequently, several other strong bases have been tested,
and LiOtBu was confirmed as the optimal base to afford 2a with
53% yield (entry 7), although trisubstituted olefin 3a was formed
in 22% yield. Next, various copper catalysts were screened, and
no better yield was afforded compared to CuCl (entries 8−10).
a
Reaction Condition A: 1 (0.5 mmol, 1 equiv), CuCl (0.05 mmol, 10
mol %), B2pin2 (0.75 mmol, 1.5 equiv), LiOtBu (1 mmol, 2 equiv),
DMF (0.1 M), rt, 4 h. Isolated yields.
b
B
DOI: 10.1021/acs.orglett.8b02801
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
tetrasubstituted alkene with 50−70% yields (2a−r). Naphthyl
and heteroaryl substrates can also well participate in the reaction
(2p, 2q). What is more, when (E)-(8-iodooct-1-en-3-yn-1-
yl)benzene was subjected to this condition, 2r was afforded with
70% yield. However, extending the alkyl chain with more than
one carbon did not result in the corresponding six-membered
exocyclic alkenes (see SI for details).
Meanwhile, various trisubstitutedexocyclic alkenescanalso be
obtained efficiently from the corresponding acetylenic halides
with up to 91% yield under Condition B (as depicted in Scheme
3). The electron donating and electron withdrawing groups
optimized Condition A or B. In order to further expand the
application of a CuI/B2pin2 system, we conducted the reaction
with alkyl acetylenic halides. Under Condition A, no product 6
was detected, while under Condition B, tetrasubstituted alkenyl
iodide was isolated together with exocyclic trisubstituted alkenes
(Scheme 5, eqs 1−2). After optimizing this transformation
process (see SI, Optimization Details, Table D), we obtained the
reaction conditions for 5 using a Cu(I)/B2pin2 system, NaOH (2
equiv) as base, and dry DMSO as solvent under 50 °C
(Condition C). Under Condition C, various tetrasubstituted
alkenyl iodides can be synthesized efficiently with moderate to
good yields (Scheme 4). For example, alkyl iodides possessing n-
a,b
Scheme 3. Scope of Exocyclic Trisubstituted Alkenes
Scheme 4. Scope of Exocyclic Tetrasubstituted Alkenyl
a,b
Iodides
a
Reaction Condition C: 4 (0.5 mmol, 1 equiv), CuI (0.05 mmol, 10
mol %), B2pin2 (0.1 mmol, 0.2 equiv), NaOH (1 mmol, 2 equiv),
DMSO (0.1 M), 50 °C. Isolated yields.
b
a
Reaction Condition B: 1 (0.5 mmol, 1.0 equiv), CuCl (0.05 mmol,
10 mol %), B2pin2 (0.25 mmol, 0.5 equiv), Ph2SiH2 (0.75 mmol, 1.5
b
equiv), NaOH (2 mmol, 4 equiv), THF (0.1 M), rt, 4 h. Isolated
yields.
butyl (5a), cyclopropyl (5b), phenylethyl (5c), tert-butyl (5d),
and cyclohexenyl (5e) all proceeded well with 45−75% isolated
yields. In addition, ester and ether motifs can also be tolerated
(5f−n). Unfortunately, nitro, hydroxyl, amine, and aldehyde
groups are not compatible under current conditions.
To gain mechanistic insight into the reaction, several control
experiments were carried out (Scheme 5, eqs 3−5). When
2,2,6,6-tetramethyl-1-piperidyloxy (TEMPO), as the radical
scavenger, was added into the reaction under standard
Conditions A, B, and C (Scheme 5, eqs 3−5), these reactions
were remarkably inhibited. In addition, the TEMPO-adduct 8
was obtained in 50% isolated yield (Scheme 5, eq 4). These
results indicate that this transformation might involve a radical
pathway.
Based on these results, a possible mechanism was proposed as
described in Scheme 6, which involved a Cu-catalyzed radical
pathway. First, reaction of Cu(I) catalyst with B2pin2 affords
copper species I through σ-bond metathesis in the presence of
base. Then, oxidation of copper(I) species by acetylenic halide
reagent through a single electron transfer (SET) process results
in a copper(II) species II and alkyl radical intermediate III. Next,
morestableradicalintermediateIVformsthroughthecyclization
of III. The final fate of intermediate IV depends on its
competitive reaction with various coexisting reagents. In the
substituted at the para-position of the aryl group reacted
smoothlywithhighyieldsfrom70to91%,includingmethyl(3b),
methoxyl (3d), fluoro (3e), chloro (3f), bromo (3g),
trifluoromethyl (3h), cyano (3i), and so on. Various (6-
iodohex-1-yn-1-yl)benzene bearing substituents at the ortho-
and meta-positions of the aryl group were also successfully
converted into target products with good yields (3j−o).
Additionally, heteroaromatic ring-substituted acetylenic halide
1p also can be used for the current reaction, and 3p was obtained
in 76% isolated yield. 2-(6-Iodohex-1-yn-1-yl)naphthalene can
also be compatible with the transformation affording 3q with
82% yield. These results show that steric hindrance of the
substituents on the aromatic ring has little impact on this
transformation. When we introduce an oxygen atom to the
unactivated alkyl chain, desired product (isomers) 3r was
isolated with 86% yield. It is noteworthy that various (7-
iodohept-1-yn-1-yl)benzenes could also proceed smoothly and
provide the corresponding six-membered exocyclic alkenes in
moderate yields (3s−u).
As the above result shows, multisubstituted aromatic alkenes
2a or 3a can be obtained, respectively, with good yields under
C
DOI: 10.1021/acs.orglett.8b02801
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 5. Control Experiments
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02801.
Experimental procedures, spectral and analytical data,
1
copies of H and 13C NMR spectra for new compounds
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: wanggw@tju.edu.cn.
*E-mail: wentao_zhao@tju.edu.cn.
ORCID
Guangwei Wang: 0000-0003-4466-9809
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Science Foundation of Tianjin (No.
16JCYBJC20100) and Tianjin University for support of this
research.
Scheme 6. Proposed Mechanism
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