Carboboration of Alkynes with Cyclodextrin-Encapsulated N-Heterocyclic Carbene Copper Complexes

Article abstract of DOI:10.1002/ejoc.201900246

The copper-catalyzed carboboration of various alkynes was investigated with a modified N-heterocyclic carbene-capped α-cyclodextrin copper(I) complex in which the reactive copper center is deeply encapsulated in the cyclodextrin (CD) cavity. The methylborylation of terminal alkynes was found to give linear (L) (E)-vinyl boron isomers as the major isomers, as expected from the previously proposed “perpendicular” approach of the alkyne to the Cu–B bond, and methylation of the vinyl boron copper intermediate. Under similar conditions, the intramolecular carboboration reaction with terminal alkynes functionalized by alkyl halides, led to exocyclic vinyl boronate species as the major isomer. However, an endocyclic (Z)-isomer was also observed in some cases. This isomer was not previously observed and is unexpected considering the “classical” mechanism. The direct generation of boron functionalized (Z)-alkenes by carboboration of alkynes is unprecedented.

Full text of DOI:10.1002/ejoc.201900246

A Journal of
Accepted Article
Title: Carboboration of Alkynes with Cyclodextrin-Encapsulated N-
Heterocyclic Carbene Copper Complexes
Authors: Zhonghang Wen, Yongmin Zhang, Sylvain Roland, and
Matthieu Sollogoub
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900246
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10.1002/ejoc.201900246
European Journal of Organic Chemistry
FULL PAPER
Carboboration of Alkynes with Cyclodextrin-Encapsulated N-
Heterocyclic Carbene Copper Complexes
Zhonghang Wen,[a] Yongmin Zhang,[a] Sylvain Roland,*[a] and Matthieu Sollogoub*[a]
Abstract: The copper-catalyzed carboboration of various alkynes
Figure 1. Representations of cyclodextrin-N-heterocyclic carbene copper(I)
was investigated with a modified N-heterocyclic carbene-capped α-
cyclodextrin copper(I) complex in which the reactive copper center is
deeply encapsulated in the cyclodextrin (CD) cavity. The
methylborylation of terminal alkynes was found to give linear (L) (E)-
vinyl boron isomers as the major isomers, as expected from the
previously proposed "perpendicular" approach of the alkyne to the
Cu–B bond, and methylation of the vinyl boron copper intermediate.
Under similar conditions, the intramolecular carboboration reaction
with terminal alkynes functionalized by alkyl halides, led to exocyclic
vinyl boronate species as the major isomer. However, an endocyclic
(Z)-isomer was also observed in some cases. This isomer was not
previously observed and is unexpected considering the "classical"
mechanism. The direct generation of boron functionalized (Z)-
alkenes by carboboration of alkynes is unprecedented.
complex (α-ICyD)CuCl: (a) Developed structure showing the α-CD
macrocycle; (b) 3-Dimensional side-view of the complex; (c) Schematic
representation of the CD cavity with the encapsulated copper(I) center.
(a)
RO
RO
(b)
OR
RO
OR
O
O
O
O
O
RO
N
OR
OR
O
O
N
RO
RO
CuCl
O
OR
RO
O
OR
O
OR
(c)
O
RO
O
Cu
Cl
OR
(α-ICyD)CuCl (R = Bn)
Introduction
Herein we present our results concerning the carboboration
reaction catalyzed by (α-ICyD)CuCl (Scheme 1b).3,6,7,8 This
includes investigation of the methyl borylation of a series of
terminal alkynes and examples of intramolecular carboboration
reactions, the latter giving in some cases unexpected results in
terms of regioselectivity.
We previously described (ICyD)CuCl complexes in which the
copper(I) center is deeply encapsulated in the cavity of a
protected cyclodextrin (α or β-CD) (Figure 1).1 The copper
center is maintained in the cavity in a well-defined position
through ligation by an N-heterocyclic carbene (NHC) ligand
which covalently caps the primary rim of the CD.
(a)
Bpin
H
(α-ICyD)CuCl cat.
(Bpin)2, tBuOK
(previous work)
β
R
Cavitand-based and other encapsulated metal complexes
offer interesting opportunities to control selectivity in metal-
catalyzed reactions.2 (α-ICyD)CuCl, derived from benzyl-
protected α-CD as cavitand, was previously shown to catalyze
the formal hydroboration of terminal alkynes in the presence of
(Bpin)2 and MeOH, 3 to give linear (L) vinyl boron species as the
major isomers (Scheme 1a).4 Due to the high steric constraint
imposed by the α-CD cavity, the regioselectivity in favor of the
linear isomer observed with (α-ICyD)CuCl could not be
explained by the classical "parallel" approach of the alkyne to
the Cu–B bond.5 To account for the observed regioselectivity, an
alternative "perpendicular" approach was proposed which was
supported by DFT calculations (Scheme 1a).4 Interestingly,
changing the nature/size of the CD from α-CD to the larger β-CD
was found to induce a switch in mechanism and regioselectivity.
THF, MeOH
R
Linear (L)
MeOH
H
Bpin
Cu
Cu
R
R
R
Bpin
Bpin
vinyl copper
intermediate
« perpandicular »
approach
(b)
R1
Bpin
or
Bpin
R
(α-ICyD)CuCl cat.
(this work)
(Bpin)2, tBuOK
THF, R1X
R
R1
R
(Z)-isomer (unexpected)
Scheme 1. Copper-catalyzed borylation of alkynes with CD-NHC-based
ligands: (a) Previously reported formal hydroboration and perpendicular
approach of the alkyne with (α-ICyD)CuCl; (b) Carboboration reactions
described in the present study.
Results and Discussion
[a]
Z. Wen, Dr. Y. Zhang, Dr. S. Roland and Prof. M. Sollogoub
Institut Parisien de Chimie Moléculaire UMR CNRS 8232
Sorbonne Université – Campus P. et M. Curie
4, Place Jussieu, 75005 Paris (France)
E-mail: sylvain.roland@sorbonne-universite.fr
matthieu.sollogoub@sorbonne-universite.fr
A few examples of application of (NHC)CuCl complexes in the
carboboration reaction of internal or terminal alkynes have been
reported.9,10 To explore the efficiency and regioselectivity of the
(Bpin)2-mediated carboboration of alkynes in the cavity of (α-
ICyD)CuCl, we first investigated the methylboration of phenyl
acetylene as a model reaction (Scheme 2, R = Ph). Under the
optimized conditions, a conversion of 96% was obtained by
Homepage:http://ipcm.fr/-Glycochimie-Organique
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201900246
European Journal of Organic Chemistry
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of the crude reaction mixture; [d] Reaction performed at 20 °C; [e] Reaction
performed with 2 mol% of (α-ICyD)CuCl; [f] Control experiment performed
with 4 mol% of (IPr)CuCl.
using 4 mol % of (α-ICyD)CuCl and 4 equiv. of CH3I. The linear
(L) vinyl boron isomer 2a was obtained with high regioselectivity
(>99/1) with no detectable trace of the branched (B) isomer in
the crude reaction mixture (Table 1, entry 1). To accelerate the
conversion, the reaction was run at 60 °C. After 24 h of reaction
and standard work-up, 2a was isolated in 74% yield. Decreasing
the temperature to 20 °C led to lower conversion and yield but
the high regioselectivity in favor of the linear isomer is
maintained (Table 1, entry 2). In contrast, with a lower catalyst
charge of 2 mol%, a noticeable drop in selectivity was observed
(96/4, Table 1, entry 3), although the isolated yield is slightly
improved (78%).
Under these optimized conditions, the scope of the
methylboration reaction was investigated with
a series of
aromatic terminal alkynes (1b-n) substituted by various electron
withdrawing or donating groups at the o, m, or p positions of the
phenyl ring. In all these examples, the linear (L) isomer 2 was
consistently obtained as the major isomer with selectivities
ranging from 91/9 to >99/1 (ratio 2/3). It is worth to note, that for
the lowest selectivity, observed with p-OMe-phenyl acetylene
(Table 1, entry 6), the linear isomer
2 is still strongly
(α-ICyD)CuCl (4 mol%)
Bpin
CH3
H3C
Bpin
predominant (2d/3d 91/9). Furthermore, the electronic
=
(Bpin)2, tBuOK
properties of the aromatic substituent seem to have no direct
effect on conversions and isolated yields, the best results being
reached with phenylacetylene (1a, R = H, 74%) and m-methyl-
substituted substrate 1c (70%) (Table 1, entry 5), whereas 1g (R
= p-Cl) (Table 1, entry 9) and 1k (R = o-F) (Table 1, entry 13),
for instance, gave very close yields (58-59%) and similar
selectivities. Substitution at the ortho position led in several
cases (1f, 1i, 1n) to a decrease in yields but without effect on
selectivity. The most significant drop in selectivity (87/13) was
observed with 1o for which the alkyne is substituted by an alkyl
group (R = CH2OBn, Table 1, entry 17). A control experiment
performed with the classical bulky NHC ligand IPr (1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) instead of the CD-based
NHC ligand α-ICyD, led to a significant drop in selectivity (2a/3a
= 82/18) (Table 1, entry 18), thus demonstrating the importance
of α-ICyD structure for controlling selectivity. The low sensitivity
to alkyne electronic properties observed here with α-ICyD as
ligand was already observed in the formal hydroboration
reaction catalyzed by (α-ICyD)CuCl and with other bulky N-
alkyl-substituted (NHC)CuCl complexes.4,5a To account for this
effect, it was suggested that copper–alkyne coordination is the
product-determining step.11
Furthermore, the low reaction rate observed with CH3I as the
electrophile, compared with MeOH (previous work),4 suggests
that the reaction of the CD-embedded vinyl boron copper
intermediate with the alkyl halide might be important in the
kinetics of the reaction.12
Next, the potential of (α-ICyD)CuCl was examined in the
intramolecular copper-catalyzed carboboration reaction.3,13 This
cyclization reaction can lead either to cyclic alkylboronates or to
alkenylboronates starting from alkenes or alkynes, respectively.
Both structures are relevant for further synthetic transformations
and are versatile intermediates for organic synthesis. A few
copper-catalyzed intramolecular carboboration reactions have
been described from both internal and terminal alkynes.14 The
reaction, initially reported with propargyl ethers by Lin's group in
2013,14a was found to give exocyclic alkenyl boronates through
intramolecular reaction of the boron vinyl copper intermediate
with the internal electrophile (Figure 2).
R
+
CH3I (4 equiv.)
R
R
1a-q
THF, 60 °C
Linear (L)
Branched (B)
2a-q
3a-q
Scheme 2. Copper-catalyzed methylboration of alkynes with the α-CD-NHC-
based ligand α-ICyD.
Table 1. Copper-catalyzed methylboration of alkynes.[a]
Entry Alkyne (R)
Convn %[b] Yield % (2+3)[b]
ratio 2/3[c]
1
1a (Ph)
96
79
93
87
92
60
74
51
78
62
70
44
49
23
59
49
38
56
58
50
51
43
54
56
>99/1 (2a:3a)
>99/1 (2a:3a)
96/4 (2a:3a)
94/6 (2b:3b)
95/5 (2c:3c)
91/9 (2d:3d)
97/3 (2e:3e)
98/2 (2f:3f)
2[d]
3[e]
4
1a (Ph)
1a (Ph)
1b (p-Me-Ph)
1c (m-Me-Ph)
1d (p-OMe-Ph)
5
6
7
1e (m-OMe-Ph) 65
8
1f (o-OMe-Ph)
1g (p-Cl-Ph)
1h (m-Cl-Ph)
1i (o-Cl-Ph)
1j (p-F-Ph)
47
99
90
>99
83
90
83
91
>99
89
82
9
98/2 (2g:3g)
96/4 (2h:3h)
99/1 (2i:3i)
10
11
12
13
14
15
16
17[e]
18[f]
99/1 (2j:3j)
1k (o-F-Ph)
1l (p-CF3-Ph)
1m (m-CF3-Ph)
1n (o-CF3-Ph)
1o (BnOCH2)
1a (Ph)
96/4 (2k:3k)
96/4 (2l:3l)
95/5 (2m:3m)
>99/1 (2n:3n)
87/13 (2o:3o)
82/18 (2a:3a)
[a] 1.3 Equiv. of (Bpin)2 and tBuOK were used, reaction time 24 h; [b]
Conversion of the alkyne and NMR yield determined by 1H NMR analysis of
the crude reaction mixture by comparison with the internal reference (1,3,5-
trimethoxybenzene, 0.33 equiv.); [c] L/B ratio determined by 1H NMR analysis
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10.1002/ejoc.201900246
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Figure 2. Intramolecular carboboration reaction. Schematic representation of
the cyclization of electrophile-containing alkynes.
We first investigated the cyclization of the simple iodoalkyl-
substituted terminal alkyne 4a (6-iodo-1-hexyne, Scheme 3).
The reaction of 4a in the presence of 8 mol % of (α-ICyD)CuCl
was found to give cyclized vinyl boronates 5a/6a in 50 %
isolated yield. The reaction is slow, and requires about 24 h of
heating at 60 °C in THF to get optimized yields. Interestingly, the
reaction consistently gave, in repeated experiments, a ratio of
c.a. 60:40 of the expected exocyclic vinyl boronate 5a and an
endocyclic six-membered (Z)-vinyl boronate 6a (Table 2, Entry
1).
The formation of 5a (already observed in low yield from 6-
bromo-1-hexyne),14c is expected from "exo" cyclization of the
intermediate linear (L) copper vinyl boronate, which is expected
to be the major isomer in the first reaction step (Scheme 4a). In
contrast, the formation of 6a is unexpected and unprecedented.
The cyclohexyl boronate 6a formally results from an "endo"
cyclization of the linear (L) copper vinyl boronate. A radical
mechanism might be proposed for its formation as shown in
Scheme 4b. The formation of an alkyl radical from acetylenic
halide reduction with concomitant oxidation of a copper(I)-Bpin
species to copper(II) was previously proposed for the formation
of 5a.14e However, the primary radical that may be generated
from 6-iodo-1-hexyne 4a, is not expected to proceed into a 6-
endo cyclization process to give 6a. Consequently, we proposed
here that an intramolecular radical cyclization takes place in the
cavity of the (α-ICyD)CuCl catalyst from the copper vinyl
boronate intermediate. This mechanism allows for regeneration
Bpin
Bpin
R
CuI cat.
(Bpin)2
E
Cu
R
R
cyclization
E
exocyclic vinyl boronates
Our previous observations in the formal hydroboration reaction
of alkynes catalyzed by (α-ICyD)CuCl, showed that internal
alkynes reacted much slower than terminal alkynes due to
important steric interactions with the CD cavity.4 In the present
study, we focussed on the cyclization of terminal alkynes for
which very few examples have been reported in the
literature.14c,d
Using simple halogenoalkyl chains (non activated electrophiles)
as internal electrophiles was previously found not favourable for
the intramolecular carboboration to take place. For instance, the
reaction with a terminal alkyne such as 6-bromo-1-hexyne was
reported by Ito's group to give low yields (8%) of cyclized
compound 5a, by using CuCl/(o-tol)3P as catalytic system (THF,
30 °C).14c Similarly, the Wang and Zhao's group initially noticed
that the cyclization of internal acetylenic iodides such as 6-iodo-
1-phenyl-1-hexyne did not proceed with CuCl/(n-Bu)3P as
catalyst (THF, rt).14e The cyclization of this internal phenyl-
substituted iodo alkyne (R = Ph, Figure 2) has been optimized to
give the expected vinylboronate in 64% yield. However, the
application of the same catalytic system to terminal alkynes was
not reported.
of (α-ICyD)CuI as catalyst. It goes through
a copper(II)
intermediate whose formation could be favoured by the strong
electron donating NHC ligand. We also observed, that
decreasing the concentration of tBuOK in the medium by slow
addition over 1 h, led to a dramatic decrease of the amount of 6a
(Table 2, Entry 2).15 This effect of tBuOK concentration on 5a/6a
ratio could not be clearly rationalized.16 Another possible
pathway for the formation of the six-membered cycle 6a involves
(E)→(Z) isomerization of the minor branched (B) vinyl copper
boronate isomer followed by cyclization (Scheme 4c).17 However,
our attempts to promote 6a formation under photocatalytic
conditions to favor isomerization,17 were unsuccessful, the ratio
5a/6a remaining unchanged (data not shown). To the best of our
knowledge, the application of classical NHC ligands such as IPr
in the intramolecular copper-catalyzed carboboration reaction of
terminal alkynes has not been reported.9,10 Therefore, the
cyclization of 4a was also investigated with (IPr)CuCl as catalyst.
The reaction was found to give a mixture of cyclized vinyl
boronates 5a/6a in 61% yield. More interestingly, a 5a/6a ratio of
41:59 was obtained, showing that the NHC ligand IPr not only
efficiently promotes cyclization of simple terminal alkynes but
also favors the formation of the unexpected six-membered
vinylboronate 6a.
Bpin
Bpin
I
(NHC)CuCl (8 mol%)
(Bpin)2, tBuOK
4a
5a
THF, 60 °C
O
6a-(Z)
X
Bpin
4b: X = I
4c: X = Br
NHC = α-ICyD or IPr
O
5b
Scheme 3. Intramolecular carboboration of alkynes catalyzed by (α-
ICyD)CuCl or (IPr)CuCl. IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene.
Table 2. (α-ICyD)CuCl-catalyzed intramolecular carboboration of alkynes.[a]
Entry
Alkyne
4a
NHC
Convn %[a]
>99
Yield %[b]
50
Ratio 5/6[c]
60:40
94:6
1
α-ICyD
α-ICyD
IPr
2[d]
3
4a
>99
49
4a
>99
61
41:59
>99:1
>99:1
>99:1
4
4b
α-ICyD
α-ICyD
IPr
>99
40
5
4c
>99
51
Finally, based on recent results showing that intramolecular
carboboration reactions gave improved yields with propargyl
ether derivatives,14c we studied the cyclization of 4b/4c with (α-
ICyD)CuCl or (IPr)CuCl as catalysts. In all cases, we observed
exclusively the formation of the exocyclic five-membered vinyl
boronates 5a or 5b, with no detectable traces of six-membered
endocyclic vinyl boronate (Table 2, entries 4–6).
6
4b
>99
>99
[a] 2 Equiv. of (Bpin)2 and tBuOK were used; [b] Conversion of the alkyne
determined by 1H NMR analysis of the crude reaction mixture by comparison
with the internal reference; [b] Isolated yield; [c] 5/6 ratio determined by 1H
NMR analysis of the crude reaction mixture; [d] The solution of base (tBuOK 1
M in THF) was slowly added over 1 h.
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Experimental Section
(a)
« exo »
cyclization
I
Cu
Cu
General: All reactions were performed under an argon atmosphere using
well-dried reaction flask. Unless stated otherwise, reactants were
purchased from commercial sources and used as received without
further purification. All the solvents used as reaction media were distilled.
Column chromatography was performed with silica gel (100–200 mesh).
NMR spectra were recorded on a Bruker 300 MHz or 400 MHz, using the
signal of the residual solvent as an internal reference. The high-
resolution mass spectroscopy (HRMS) was performed on a Bruker
micrOTOF spectrometer, using Agilent ESI-L Low Concentration Tuning-
Mix as reference.
5a
Bpin
Bpin
I
(α-ICyD)CuI
Linear (L)
« endo »
cyclization
(b)
CuI
CuI
Bpin
Bpin
I
I
Linear (L)
Starting Materials and Reagents
CuI
CuII
(α-ICyD)CuCl was synthesized according to our previously published
procedure.1a Alkynes 1o,18 4b,19 and 4c,20 were prepared according to
reported procedures.
Bpin
Bpin
HI
H
I
Standard procedure for the methylboration of alkynes catalyzed by
(α-ICyD)CuI
6a +
(α-ICyD)CuCl (Table 1).
(c)
An oven-dried screwed tube was charged with (α-ICyD)CuCl (0.01 mmol,
29.0 mg) and bis(pinacolato)diboron ((Bpin)2, 0.30 mmol, 75.2 mg). After
being sealed with a septum, the tube was purged by 3 vacuum-argon
cycles. THF (0.5 mL) was added and the mixture was stirred for 5 min at
20 °C. A solution of tBuOK 1.0 M in THF (0.3 mL) was added. After
stirring for 30 min, the alkyne (0.23 mmol) was added (neat or dissolved
in the minimal amount of THF if solid) followed by MeI (0.91 mmol, 56.7
µL). The tube was sealed with a screw cap and the mixture was stirred at
60 °C for 24 h. After cooling, the crude reaction mixture was filtered
(E)
vinylcopper
isomerization
(Z)
I
Cu
Cu
I
Bpin
6a
Bpin
(Z)-Branched
(E)-Branched (B) isomer
Scheme 4. Possible mechanisms for intramolecular carboboration reactions
catalyzed by (α-ICyD)CuCl: (a) "Classical" exo cyclization leading to isomer
5a; (b) Proposed radical pathway involving endo cyclization to get the
unexpected isomer 6a; (c) Isomerization of the (E)-branched isomer and
cyclization of the as-formed (Z) isomer to give 6a.
through
a short pad of celite by rinsing with diethyl ether, and
concentrated under vacuum. The crude residue was purified by silica gel
chromatography (cyclohexane/diethyl ether). The conversion of the
alkyne and NMR yield of the product were determined by 1H NMR
analysis of the crude reaction mixture by comparison with the internal
reference (1,3,5-trimethoxybenzene). The regioselectivity of the reaction
was determined by 1H NMR analysis of the crude reaction mixture.
Conclusions
(E)-4,4,5,5-Tetramethyl-2-(2-phenylprop-1-enyl)-1,3,2-dioxaborolane
(2a): Rf = 0.5 (cyclohexane/diethyl ether, 12:1). 1H NMR (400 MHz,
CDCl3): δ = 7.53–7.47 (m, 2H), 7.36–7.26 (m, 3H), 5.76 (d, J = 0.9 Hz,
1H), 2.42 (d, J = 0.9 Hz, 3H), 1.32 (s, 12H) ppm. The NMR data are in
agreement with the literature.21
We have shown here that the α-CD-based NHC–copper(I)
complex (α-ICyD)CuCl, in which the reactive metallic center is
deeply encapsulated in the cavity of α-CD, catalyzed the methyl
boration of terminal alkynes with high selectivity in favor of linear
(L) (E)-vinyl boron isomers. This selectivity is similar to that
previously observed in the formal hydroboration reaction, and is
consistent with the previously proposed "perpendicular"
approach of the alkyne to the Cu–B bond. The reaction is
relatively insensitive to the electronic properties of the alkynes
and significantly slower than the formal hydroboration reaction,
suggesting strong substrate/cavity interactions in the initial
alkyne approach and in the reaction of alkyl halides with the
encapsulated vinyl boron copper intermediate. In addition, we
have demonstrated that (α-ICyD)CuCl catalyzed the
intramolecular carboboration of challenging alkynes substrates
such as simple acetylenic halides to give cyclized vinyl boron
species in moderate yields. An unprecedented formal endo-type
cyclization of a copper vinyl boronate intermediate to give a
cyclohexenyl boronate was observed. This intriguing
regioselectivity was also observed with the bulky NHC ligand IPr
suggesting an effect of the NHC ligand capping the cavity. The
mechanism remains to be elucidated and should be further
explored.
(E)-4,4,5,5-Tetramethyl-2-(2-p-tolylprop-1-enyl)-1,3,2-dioxaborolane
(2b): Rf = 0.35 (cyclohexane/diethyl ether, 20:1). 1H NMR (300 MHz,
CDCl3): δ = 7.41 (d, J = 8.2 Hz, 2H), 7.13 (d, J = 8.0 Hz, 2H), 5.74 (s, 1H),
2.40 (s, 3H), 2.34 (s, 3H), 1.31 (s, 12H) ppm. The NMR data are in
agreement with the literature.22
(E)-4,4,5,5-Tetramethyl-2-[2-(m-tolyl)prop-1-en-1-yl]-1,3,2-dioxaboro-
lane (2c): Rf = 0.35 (cyclohexane/diethyl ether, 20:1). 1H NMR (300 MHz,
CDCl3): δ = 7.30 (d, J = 9.3 Hz, 2H), 7.21 (t, J = 7.5 Hz, 1H), 7.10 (d, J =
7.5 Hz, 1H), 5.75 (d, J = 0.8 Hz, 1H), 2.40 (d, J = 0.8 Hz, 3H), 2.35 (s,
3H), 1.32 (s, 12H) ppm. The NMR data are in agreement with the
literature.21
(E)-2-[2-(4-Methoxyphenyl)prop-1-en-1-yl]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (2d): Rf = 0.4 (cyclohexane/diethyl ether, 10:1). 1H NMR
(400 MHz, CDCl3): δ = 7.47 (d, J = 9.0 Hz, 2H), 6.85 (d, J = 9.0 Hz, 2H),
5.70 (d, J = 0.9 Hz, 1H), 3.81 (s, 3H), 2.39 (d, J = 0.9 Hz, 3H), 1.31 (s,
12H) ppm. The NMR data are in agreement with the literature.21
(E)-2-[2-(3-Methoxyphenyl)prop-1-en-1-yl]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (2e): Rf = 0.4 (cyclohexane/diethyl ether, 10:1). 1H NMR
(300 MHz, CDCl3): δ = 7.22 (d, J = 7.9 Hz, 1H), 7.14–7.01 (m, 2H), 6.83
(ddd, J = 8.1, 2.5, 0.8 Hz, 1H), 5.76 (d, J = 0.8 Hz, 1H), 3.81 (s, 3H), 2.40
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10.1002/ejoc.201900246
European Journal of Organic Chemistry
FULL PAPER
(d, J = 0.8 Hz, 3H), 1.32 (s, 12H) ppm. 13C NMR (75 MHz, CDCl3): δ =
159.58, 157.80, 145.55, 129.19, 118.55, 113.76, 111.53, 83.11, 55.37,
25.04, 20.33 ppm. HRMS (ESI, micro TOF) m/z: calcd for C16H23BNaO3
[M+Na]+ 297.1635, found 297.1635.
(E)-4,4,5,5-Tetramethyl-2-{2-[2-(trifluoromethyl)phenyl]prop-1-en-1-
yl}-1,3,2-dioxaborolane (2n). Rf = 0.5 (cyclohexane/diethyl ether, 12:1).
1H NMR (400 MHz, CDCl3): δ = 7.61 (d, J = 7.9 Hz, 1H), 7.46 (t, J = 7.6,
0.6 Hz, 1H), 7.33 (t, J = 7.7 Hz, 1H), 7.20 (d, J = 7.7 Hz, 1H), 5.25 (d, J =
0.7 Hz, 1H), 2.30 (d, J = 0.7 Hz, 3H), 1.31 (s, 12H) ppm. 13C NMR (100
MHz, CDCl3): δ = 158.33, 146.17, 131.48, 129.11, 127.04, 126.89 (q, J =
30.2 Hz), 126.09 (q, J= 5.2 Hz), 124.40 (q, J = 273.7 Hz), 83.16, 25.04,
23.74 (q, J = 1.9 Hz) ppm. HRMS (ESI, micrOTOF) m/z: calcd for
C16H20BF3NaO2 [M+Na]+ 335.1404, found 335.1389.
(E)-2-[2-(2-Methoxyphenyl)prop-1-en-1-yl)]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (2f): Rf = 0.4 (cyclohexane/diethyl ether, 10:1). 1H NMR
(400 MHz, CDCl3): δ = 7.29 – 7.20 (m, 1H), 7.18 (dd, J = 7.4, 1.9 Hz, 1H),
6.97– 6.86 (m, 1H), 5.41 (q, J = 1.0 Hz, 1H), 3.80 (s, 3H), 2.34 (d, J = 1.0
Hz, 3H), 1.30 (s, 12H) ppm. The NMR data are in agreement with the
literature.21
2o and 3o have been isolated as a mixture:
(E)-2-[2-(4-Chlorophenyl)prop-1-en-1-yl]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (2g): Rf = 0.45 (cyclohexane/diethyl ether, 12:1). 1H NMR
(400 MHz, CDCl3): δ = 7.42 (d, J = 8.8 Hz, 2H), 7.28 (d, J = 8.8 Hz, 2H),
(E)-2-[3-(Benzyloxy)-2-methylprop-1-en-1-yl]-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (2o) (major isomer): Rf = 0.3 (cyclohexane/diethyl
ether, 10:1). 1H NMR (400 MHz, CDCl3): δ = 7.39–7.24 (m, 5H), 5.50 (d, J
= 0.5 Hz, 1H), 4.51 (s, 2H), 3.97 (d, J = 0.8 Hz, 2H), 2.00 (s, 3H), 1.29 (s,
12H) ppm. 13C NMR (100 MHz, CDCl3): δ = 157.61, 138.52, 128.42,
127.67, 127.58, 82.92, 76.18, 72.18, 24.98, 18.21 ppm. HRMS (ESI,
micrOTOF) m/z: calcd for C17H25BKO3 [M+K]+ 327.1531, found 327.1538.
5.73 (d, J = 0.9 Hz, 1H), 2.38 (d, J = 0.9 Hz, 3H), 1.31 (s, 12H) ppm. 13
C
NMR (100 MHz, CDCl3): δ = 156.45, 142.34, 133.91, 128.42, 127.27,
83.21, 25.03, 20.13 ppm. HRMS (ESI, micrOTOF) m/z: calcd for
C15H20BClNaO2 [M+Na]+ 301.1140, found 301.1149.
(E)-2-[2-(3-Chlorophenyl)prop-1-en-1-yl]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (2h): Rf = 0.45 (cyclohexane/diethyl ether, 12:1). 1H NMR
(300 MHz, CDCl3): δ = 7.50–7.43 (m, 1H), 7.39–7.33 (m, 1H), 7.26–7.22
(m, 2H), 5.75 (d, J = 0.8 Hz, 1H), 2.38 (d, J = 0.8 Hz, 3H), 1.31 (s, 12H)
ppm. The NMR data are in agreement with the literature.21
(E)-2-[1-(Benzyloxy)but-2-en-2-yl]-4,4,5,5-tetramethyl-1,3,2-dioxabo-
rolane (3o) (minor isomer): Rf = 0.3 (cyclohexane/diethyl ether, 10:1). 1H
NMR (400 MHz, CDCl3): δ = 7.39–7.24 (m, 5H), 6.41 (q, J = 6.7 Hz, 1H),
4.52 (s, 2H), 4.08 (s, 2H), 1.97 (dt, J = 6.9, 1.3 Hz, 3H), 1.29 (s, 12H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 143.54, 139.12, 128.28, 127.71,
127.33, 83.13, 74.31, 71.92, 24.98, 17.33 ppm. HRMS (ESI, micrOTOF)
m/z: calcd for C17H25BKO3 [M+K]+ 327.1531, found 327.1538.
(E)-2-[2-(2-Chlorophenyl)prop-1-en-1-yl]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (2i): Rf = 0.45 [cyclohexane/diethyl ether (12:1)]. 1H NMR
(400 MHz, CDCl3): δ = 7.35–7.31 (m, 1H), 7.21–7.16 (m, 3H), 5.34 (q, J =
1.0 Hz, 1H), 2.33 (d, J = 1.1 Hz, 3H), 1.32 (s, 12H) ppm. 13C NMR (100
MHz, CDCl3): δ = 158.83, 145.70, 131.06, 129.68, 129.27, 128.24,
126.74, 83.18, 25.08, 22.23 ppm. HRMS (ESI, micrOTOF) m/z: calcd for
C15H20BClNaO2 [M+Na]+ 301.1140, found 301.1134.
Standard procedure for the intramolecular carboboration of alkynes
catalyzed by (α-ICyD)CuCl (Table 2).
An oven-dried screwed tube was charged with (α-ICyD)CuCl (0.008
mmol, 19.4 mg) or (IPr)CuCl (0.008 mmol, 3.9 mg) and
bis(pinacolato)diboron (0.20 mmol, 50.2 mg). After being sealed with a
septum, the tube was purged by 3 vacuum-argon cycles. THF (0.3 mL)
was added and the mixture was stirred for 5 min at 20 °C. A solution of
tBuOK 1.0 M in THF (0.2 mL) was added. After stirring for 30 min, the
alkyne (0.1 mmol) was added. The tube was sealed with a screw cap and
the mixture was stirred at 60 °C for 24-27 h. After cooling, the crude
reaction mixture was filtered through a short pad of celite by rinsing with
diethyl ether, and concentrated under vacuum. The crude residue was
purified by silica gel chromatography (cyclohexane/diethyl ether). The
conversion of the alkyne and NMR yield of the product were determined
by 1H NMR analysis of the crude reaction mixture by comparison with the
internal reference (1,3,5-trimethoxybenzene), and the regioselectivity of
the reaction was determined by 1H NMR of the crude reaction mixture.
(E)-2-[2-(4-Fluorophenyl)prop-1-en-1-yl]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (2j): Rf = 0.45 (cyclohexane/diethyl ether, 12:1). 1H NMR
(400 MHz, CDCl3): δ = 7.50–7.43 (m, 2H), 7.03–6.96 (m, 2H), 5.70 (d, J =
0.7 Hz, 1H), 2.38 (d, J = 0.9 Hz, 3H), 1.31 (s, 12H) ppm. The NMR data
are in agreement with the literature.21
(E)-2-[2-(2-Fluorophenyl)prop-1-en-1-yl]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (2k): Rf = 0.45 (cyclohexane/diethyl ether, 12:1). 1H NMR
(400 MHz, CDCl3): δ = 7.28 (td, J = 7.7, 1.8 Hz, 1H), 7.22 (dddd, J = 8.1,
6.9, 5.0, 1.8 Hz, 1H), 7.07 (td, J = 7.5, 1.2 Hz, 1H), 7.01 (ddd, J = 11.1,
8.2, 1.1 Hz, 1H), 5.55 (s, 1H), 2.38 (dd, J = 1.6, 1.1 Hz, 3H), 1.31 (s,
12H) ppm. The NMR data are in agreement with the literature.21
(E)-4,4,5,5-Tetramethyl-2-{2-[4-(trifluoromethyl)phenyl]prop-1-en-1-
yl}-1,3,2-dioxaborolane (2l): Rf = 0.5 (cyclohexane/diethyl ether, 12:1).
1H NMR (400 MHz, CDCl3): δ = 7.57 (s, 4H), 5.79 (d, J = 0.9 Hz, 1H),
2.41 (d, J = 0.9 Hz, 3H), 1.32 (s, 12H) ppm. The NMR data are in
agreement with the literature.23
2-(Cyclopentylidenemethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(5a): Rf = 0.4 (cyclohexane/diethyl ether, 10:1). 1H NMR (400 MHz, C6D6)
δ = 5.68 (p, J = 2.1 Hz, 1H), 2.76 (t, J = 7.3 Hz, 2H), 2.26 (t, J = 7.3 Hz,
2H), 1.60–1.51 (m, 2H), 1.47–1.39 (m, 2H), 1.12 (s, 12H) ppm. The NMR
data are in agreement with the literature.24
(E)-4,4,5,5-Tetramethyl-2-{2-[3-(trifluoromethyl)phenyl]prop-1-en-1-
yl}-1,3,2-dioxaborolane (2m): Rf = 0.5 (cyclohexane/diethyl ether, 12:1).
1H NMR (400 MHz, CDCl3): δ = 7.73 (s, 1H), 7.65 (d, J = 7.8 Hz, 1H),
7.53 (d, J = 7.8 Hz, 1H), 7.43 (t, J = 7.8 Hz, 1H), 5.79 (d, J = 0.9 Hz, 1H),
2.41 (d, J = 0.9 Hz, 3H), 1.32 (s, 12H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 156.18, 144.73, 130.73 (q, J = 32.1 Hz), 129.16 (d, J = 1.1 Hz),
128.77, 124.62 (q, J = 3.7 Hz), 124.33 (d, J = 272.3 Hz), 122.81 (q, J =
3.8 Hz), 83.32, 25.04, 20.12 ppm. HRMS (ESI, micrOTOF) m/z: calcd for
C16H20BF3NaO2 [M+Na]+ 335.1404, found 335.1406.
2-(Cyclohex-1-en-1-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6a):
Rf = 0.4 (cyclohexane/diethyl ether, 10:1). 1H NMR (400 MHz, CDCl3): δ =
6.59–6.53 (m, 1H), 2.13–2.04 (m, 4H), 1.60–1.56 (m, 4H), 1.25 (s, 12H).
The NMR data are in agreement with the literature.25
(E)-2-[(Dihydrofuran-3(2H)-ylidene)methyl]-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (5b): Rf = 0.4 (cyclohexane/diethyl ether, 10:1). 1H NMR
(400 MHz, CDCl3): δ = 5.31–5.26 (m, 1H), 4.29 (d, J = 1.5 Hz, 2H), 3.93
(t, J = 7.0 Hz, 2H), 2.83–2.76 (m, 2H), 1.27 (s, 12H) ppm. 13C NMR (100
MHz, CDCl3): δ = 165.04, 83.10, 73.24, 69.30, 33.39, 25.03 ppm. HRMS
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10.1002/ejoc.201900246
European Journal of Organic Chemistry
FULL PAPER
(ESI, micrOTOF) m/z: calcd for C16H20BF3NaO2 [M+Na]+ 233.1321, found
233.1305.
than alkyne insertion into Cu–B bond. This could account for the low
sensitivity of regioselectivity to substrate electronic effect, see ref. 5a.
[12] Previous calculations on the copper-catalyzed alkene carboboration
reaction by Yu and Fu's group suggested that the reaction with alkyl
halides is the rate-determining step see: Z.-Y. Xu, Y.-Y. Jiang, W. Su,
H.-Z. Yu, Y. Fu, Chem. Eur. J. 2016, 22, 14611-14617.
Acknowledgments
[13] For initial examples of alkene cyclization, see: a) C.-T. Yang, Z.-Q.
Zhang, H. Tajuddin, C.-C. Wu, J. Liang, J.-H. Liu, Y. Fu, M. Czyzewska,
P. G. Steel, T. B. Marder, L. Liu, Angew. Chem. Int. Ed. 2012, 51, 528-
532; b) K. Kubota, E. Yamamoto, H. Ito, J. Am. Chem. Soc. 2013, 135,
2635-2640.
ZW thanks the China Scholarship Council for PhD grant.
Keywords: Catalysis • copper • cyclodextrins • regioselectivity •
supramolecular chemistry
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10.1002/ejoc.201900246
European Journal of Organic Chemistry
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Entry for the Table of Contents
FULL PAPER
Me
Bpin
(Bpin)2, tBuOK
The carboboration of terminal
Encapsulated Catalyst*
R
alkynes was studied with
a N-
+
intermolecular
carboboration
Zhonghang Wen, Yongmin Zhang,
Sylvain Roland,* Matthieu Sollogoub*
MeI
heterocyclic carbene-capped α-
cyclodextrin copper(I) complex as
catalyst. The intermolecular reaction
with CH3I gave linear (L) vinyl boron
isomers with high selectivity. The
intramolecular reaction with terminal
alkynes led to exocyclic vinyl
boronates and to an unexpected
endocyclic (Z)-isomer, suggesting
an unusual mechanism promoted by
the ligand.
R
(α-ICyD)CuCl
4-8 mol %
Cu
Cl
Page No. – Page No.
Carboboration of Alkynes with
Cyclodextrin-Encapsulated
R = Bn
Bpin
N-
Copper
I
Heterocyclic
Complexes
Carbene
intramolecular
carboboration
+
Bpin
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