Catalytic Borylative Opening of Propargyl Cyclopropane, Epoxide, Aziridine, and Oxetane Substrates: Ligand Controlled Synthesis of Allenyl Boronates and Alkenyl Diboronates

Article abstract of DOI:10.1002/anie.201510132

A new copper-catalyzed reaction for the stereo- and regioselective synthesis of alkenyl diboronates and allenyl boronates is presented. In this process propargyl derivatives of strained three/four-membered rings were employed as substrates and B₂

Full text of DOI:10.1002/anie.201510132

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International Edition: DOI: 10.1002/anie.201510132
German Edition: DOI: 10.1002/ange.201510132
Homogeneous Catalysis
Catalytic Borylative Opening of Propargyl Cyclopropane, Epoxide,
Aziridine, and Oxetane Substrates: Ligand Controlled Synthesis of
Allenyl Boronates and Alkenyl Diboronates
Jian Zhao and Kµlmµn J. Szabó*
Dedicated to Professor Todd B. Marder on the occasion of his 60th birthday
Abstract: A new copper-catalyzed reaction for the stereo- and
regioselective synthesis of alkenyl diboronates and allenyl
boronates is presented. In this process propargyl derivatives of
strained three/four-membered rings were employed as sub-
strates and B2pin2 was used as the boronate source. Selective
formation of the alkenyl diboronate versus the allenyl boronate
products was controlled by the choice of phosphine ligand.
diboration of allenes was also reported.[3i] The groups of
Hoveyda,[8] Tsuji,[9] Ma,[10] and others[11] published several
studies on the efficient synthesis of alkenyl boronates by
copper-catalyzed hydroboration of allenes using diboronates.
However, copper-catalyzed hydroboration of allenyl boro-
nates is an unexplored area in organic synthesis.
Opening of a strained ring bearing a propargylic moiety is
an efficient approach for the synthesis of functionalized
allenes.[12] Recently, we reported[3a] a new method for the
synthesis of allenyl boronates based on catalytic borylation of
propargyl carbonates and related compounds. We also
attempted to prepare allenyl boronates by borylative ring
opening of propargylic epoxides. These efforts remained
fruitless, as the reaction led to formation of bis(borodiene)s,
probably via allenyl boronate intermediates.
We have now found that by appropriate choice of the
catalytic system, in particular the employed phosphine ligand,
the outcome of the borylation reaction can be fully controlled.
When the reaction with a propargylic cyclopropane (1; or
other strained rings) and B2pin2 (2) was carried out with
a copper catalyst in the presence of PCy3 (Cy = cyclohexyl)
the reaction resulted in alkenyl diboronates [Eq. (1)]. How-
ever, when the same reaction conditions were used in the
presence of the bulky P(1-nap)3 (1-nap = 1-naphthyl) ligand,
the reaction led to an allenyl boronate product.
A
llyl, alkenyl, and allenyl boronates are very useful reagents
in stereoselective synthesis, in particular for synthesis of
natural products.[1] However, selective synthesis of these
organoboron compounds is still a very challenging task in
organic synthesis because of the specific properties of the
carbon–boron bonds conjugated with carbon–carbon double
bonds. Synthesis of functionalized allyl boronates and boronic
acids is probably the most developed area in the field of
preparation of unsaturated boronates.[2] Recently, the syn-
thesis and application of alkenyl diboronates, in which one of
the carbon–boron bonds is in the vinylic position and the
other is in the allylic position, has attracted a lot of
attention.[3] The reason for the attraction is that the two
types of carbon–boron bonds may undergo either orthogonal
functionalization or consecutive functionalization, thus creat-
ing molecular complexity in a single reaction step with high
stereoselectivity. Another emerging area is allenylboration of
carbonyl compounds.[4] This powerful synthetic transforma-
tion requires a diversity of allenyl boronates. However,
synthesis of stereodefined functionalized allenylboronates is
still a major challenge in organic synthesis.[3a,5]
The first platinum catalyzed diboration of allenes for the
preparation of alkenyl diboronates was reported by Miyaura
and co-workers.[6] Subsequently, a series of studies based on
palladium-catalyzed reactions was published by the groups of
Cheng[7] and Morken.[3e,g] Recently, transition-metal-free
First we optimized the reaction of the borylative opening
of the propargylic cyclopropane derivative 1a (Table 1).
When 1a was reacted with 3 equivalents of B2pin2 (2) in the
presence of tBuOK and a catalytic amount of CuCl, the
alkenyl diboronate 3a and allenyl boronate 4a were formed
in 3:97 ratio with 61% yield (entry 1). The reaction could be
carried out at room temperature, and is beneficial as the
borylated product may undergo protodeborylation or other
undesired transformations at elevated temperatures. Use of
CuI instead of CuCl led to exclusive formation of 4a, albeit
with a lower yield (entry 2). In this case a large amount of
unreacted starting material, 1a, remained. We found that
addition of alcohols substantially improved the yield. By using
[*] Dr. J. Zhao, Prof. K. J. Szabó
Department of Organic Chemistry, Stockholm University (Sweden)
E-mail: kalman@organ.su.se
Homepage: http://www.organ.su.se/ks
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201510132.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial License, which permits use,
distribution and reproduction in any medium, provided the original
work is properly cited and is not used for commercial purposes.
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Table 2: Borylative opening of propagyl cyclopropanes.
Table 1: Development of copper-catalyzed mono- and diboration of 1a.[a]
Entry [Cu]cat. Ligand
M
Additive (E)-3a/
Yield
(Z)-3a/4a[b] [%][c]
[d]
[d]
1
2
3
4
5
6
7
8
CuCl
CuI
CuI
CuI
CuI
CuI
CuI
CuI
PCy3
PCy3
PCy3
PCy3
K
K
K
K
K
K
K
K
–
–
3:0:97
0:0:100
32:3:65
96:4:0
80:9:11
33:3:64
18:0:82
5:0:95
61
47
93
(88)
94
91
MeOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
PPh3
P(C6H4-p-OMe)3
P(1-nap)3
P(2,4,6-tri-
methylphenyl)3
P(1-nap)3
P(1-nap)3
P(1-nap)3
83
67
9
CuI
CuI
CuI
Li tBuOH
Li tBuOH
Li tBuOH
0:0:>99
0:0:>99
0:0:>99
(76)
(74)
(68)
10[e]
11[f]
[a] Reaction conditions: 1a (0.10 mmol), B2Pin2 (2; 3.0 equiv), Cu
catalyst (10 mol %), ligand (20 mol %), tBuOM (30 mol %), and additive
(3.0 equiv) in toluene (0.2m) were reacted at RT for 24 h under Ar. [b] The
1
ratio was determined from H NMR analysis of the crude reaction
mixture. [c] Combined yield as determined by 1H NMR spectroscopy
using naphthalene as an internal standard. The yields of the isolated
products are shown within parentheses. [d] Without any additive.
[e] B2Pin2 (1.3 equiv) and tBuOH (2.0 equiv) were used. [f] The reaction
was carried out for 72 h.
MeOH the yield was indeed improved, but lowered the 3a/4a
selectivity (entry 3). However, application of tBuOH (instead
of MeOH) maintained the high yield and gave an excellent
3a/4a selectivity and E/Z ratio (entry 4). Application of PCy3
was very important for the selectivity of the reaction. The 3a/
4a selectivity decreased when PCy3 was replaced with either
PPh3 or P(C6H4-p-OMe)3 (entries 5 and 6). When PCy3 was
replaced with a more bulky ligand, such as P(1-nap)3 or
P(2,4,6-trimethylphenyl)3, the 3a/4a selectivity was shifted
toward formation of 4a (entries 7 and 8). Noticeably, by using
P(2,4,6-trimethylphenyl)3, 4a was formed with high selectiv-
ity, but the yield was reduced (entry 8). The high yield and
high selectivity could also be achieved when P(1-nap)3 was
used and tBuOK was replaced by tBuOLi (see entries 7 and
9). Apparently, a slight change in basicity was beneficial for
the allenyl selectivity. In the above optimization studies, we
used 3 equivalents of 2 to allow either disubstitution (3a) or
monosubstitution (4a). However, the amount of B2pin2 can be
reduced to 1.3 equiv without significant change in the yield of
4a (entry 10). The very high allenyl selectivity could be
maintained, even if the reaction time was extended to
72 hours with using 3 equivalents B2pin2 (entry 11). In the
absence of the copper salt and the ligand, the borylated
products 3a/4a were not observed.
[a] Method A: a mixture of 1 (0.10 mmol), 2 (0.30 mmol), CuI (10 mol
%), PCy3 (20 mol %), tBuOK (30 mol %),and tBuOH (3.0 equiv) in
toluene (0.2m) was reacted at RT for 24–48 h under Ar. Method B:
1 (0.10 mmol), 2 (0.13 mmol), CuI (10 mol %), P(1-nap)3 (20 mol %),
tBuOLi (30 mol %), and tBuOH (2.0 equiv) in toluene (0.2m) was
reacted at RT for 24–48 h under Ar. [b] Yield of isolated product. The
E/Z ratio was determined by 1H NMR analysis of the crude reaction
mixture. [c] The reaction was performed at 358C for 48 h. [d] The reaction
was performed for 36 h. [e] The reaction was performed for 48 h. [f] The
reaction was performed at 15–208C for 48 h. [g] CuCl (10 mol %), PCy3
(20 mol %), tBuOK (30 mol %) were used. TBS=tert-butyldimethylsilyl.
With the optimal reaction conditions in hand, we studied
the synthetic scope of the reaction. Similar to 1a, either 1b or
1c reacted with 2 in the presence of catalytic amounts of CuI
and PCy3 (Method A) to give the diborylated products 3b and
3c, respectively, at room temperature with excellent E/Z
ratios (Table 2, entries 1 and 3). By changing the ligand to
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P(1-nap)3 (Method B) the outcome of the reaction was
different (entries 2 and 4), and resulted in the allenyl
boronates 4b and 4c, with no formation of either 3b or 3c.
A bulky alkynyl substituent, such as in 1d, led to slower
borylation, and therefore the reaction was either conducted at
358C (entry 5) or the reaction time was extended (entry 6).
By using PCy3 (Method A) 3d was formed with a high
diastereoselectivity (E/Z = 25:1), while with P(1-nap)3 (Meth-
od B) the reaction resulted in a high yields of the allenyl
boronate 4d. The reaction tolerated several functional
groups, such as chloro, ether, ester groups (entries 9–14).
Gratifyingly, the outcome and the selectivities of the reactions
using 1 f–h as substrates were similar to those for 1b–e. In
most cases, we used disubstituted methy-propargyl-type
cyclopropane derivatives to obtain tetrasubstituted allenyl
boronates. However, trisubstituted allenyl boronates (such as
4i) or less-substituted alkenyl diboronates (such as 3i) can
also be obtained by using the propargyl cyclopropane 1i
(entries 15 and 16). In the case of the synthesis of 4i the
reaction conditions were slightly changed. By using CuI and
P(1-nap)3 (Method B) a protodeborylation of 4i occurred,
therefore we used catalytic amounts of CuCl and PCy3
(Method C) to improve the yield of 4i.
allenyl boronate products, such as 4j (entry 1), 4k (entry 3),
and 4l (entry 4). However, by using PCy3 (Method A), the
reaction led to formation of the diborylated product 3j
(entry 2). It is interesting to point out that according to our
previous studies,[3a] the epoxide 5 gave the diborylated
product when PCy3 (or PPh3) was employed under similar
reaction conditions. The solution for stopping the reaction at
the formation of the allenyl boronate product was to use the
bulky P(1-nap)3 ligand (entry 1). In the above reactions
(Tables 2 and 3), we observed full ligand control. Except for
formation of 4i (Table 2, entry 16) and 4j (Table 3, entry 1),
the reaction resulted in a single borylated product.
We briefly studied the mechanistic aspects of the process.
The reactions proceeded with high yields and selectivities
required tBuOH as an additive. Our isotope-labelling studies
showed that tBuOH served as proton source of the process
[Eqs. (2)–(4)]. When we added tBuOD and PCy3 to the
reaction of 1a and 2 the alkenyl diboronate product [D1]-3a
We found that the borylative opening of the propargylic
cyclopropanes 1a–i can be extended to propargylic substrates
with other strained rings (Table 3), such as the epoxide 5,
oxetane 6, and aziridine 7. The ligand effects on the outcome
of the reaction were identical to those of the reactions for 1.
By using bulky P(1-nap)3 (Method B) the reaction resulted in
Table 3: Extension of the scope of the reaction to propargylic epoxide,
oxetane, and aziridine substrates.
was formed [Eq. (2)]. In this compound we observed deute-
rium uptake at two positions: at the position a to the COOMe
groups and at the allylic positions. In case of using P(1-nap)3
as the ligand under similar reaction conditions the allenyl
boronate [D]-4a was obtained with deuterium uptake only at
the position a to the COOMe groups [Eq. (3)]. The isolated
4a with 2 in the presence of PCy3 resulted in [D2]-3a [Eq. (4)].
In this case the deuterium uptake is somewhat lower than for
the reaction of 1a [Eq. (2)]. A possible reason is that 4a
contains an exchangeable proton (a-position to the carbon-
yls).
Based on the deuterium-labelling studies [Eqs. (2)–(4)]
and the above results in Tables 1 and 2, we constructed
plausible catalytic cycles, using 1a an example, and tBuOH as
the additive (Figure 1). We suggest that CuI in the presence of
tBuOM (M = K, Li) and either P(1-nap)3 or PCy3 undergoes
transmetallation[13] with 2 to give the complex 8. The complex
8 is selectively inserted[8a,9,14] into the triple bond of 1a to give
[a] Method A: a mixture of 1 (0.10 mmol), 2 (0.30 mmol), CuI (10 mol
%), PCy3 (20 mol %), tBuOK (30 mol %), and tBuOH (3.0 equiv) in
toluene (0.2m), was reacted under Ar at RT for 24 h. Method B:
1 (0.10 mmol), 2 (0.13 mmol), CuI (10 mol %), P(1-nap)3 (20 mol %),
tBuOLi (30 mol %), and tBuOH (2.0 equiv) in toluene (0.2m) was
reacted at RT for 24 h under Ar. [b] Yield of isolated product. The
E/Z ratio was determined by 1H NMR analysis. [c] In this reaction about
7% of the bis(borodiene) product was also formed. [d] tBuOK (30 mol
%), MeOH (2.0 equiv) was used instead of tBuOLi (30 mol %) with
tBuOH (2.0 equiv). [e] tBuOK (30 mol %) was used instead of tBuOLi
(30 mol%) and tBuOH (2.0 equiv). [f] The reaction was performed for
48 h. Ts =4-toluenesulfonyl.
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are useful reagents in functionalization of carbonyl com-
pounds for stereoselective synthesis of homoallyl and homo-
propargylic alcohols,[1a,3b,c,f,4,15] and useful precursors for
allenyl derivatives by Suzuki–Miyaura coupling.[16]
Experimental Section
In a typical procedure (Method A): Boronate source B2pin2 (2;
0.30 mmol), CuI (10 mol %), PCy3 (20 mol %), tBuOK (30 mol %)
were mixed in toluene (0.4 mL) and the resulting slurry was stirred for
10 minutes at room temperature under Ar. Then a toluene solution
(0.1 mL) of the mixture of the propargylic cyclopropane 1a
(0.1 mmol) and tBuOH (300 mol%) was added by syringe. The
reaction mixture was stirred at room temperature for 24 hours, and
then diluted by n-pentane (1.5 mL). The precipitate was filtered off by
a silica pad using and washed with EtOAc/n-hexane (1:2 v/v) as an
eluent. The solvent was removed and the alkenyl diboronate product
3a was purified by silica chromatography. The synthesis of the allenyl
boronate 4a was performed in a similar way (Method B), except that
P(1-nap)3 (20 mol %), tBuOLi (30 mol %), and tBuOH (200 mol %)
were used.
Acknowledgments
Support from the Swedish Research Council and the Knut
and Alice Wallenbergs Foundation is gratefully acknowl-
edged. J.Z. thanks the Wenner-Gren Foundations for funding
a postdoctoral fellowship. The generous gift of B2Pin2 from
Allychem is appreciated.
Keywords: allenes · boron · copper · homogeneous catalysis ·
strained rings
Figure 1. Suggested catalytic cycles for the borylative opening of the
propargyl cylopropane 1a.
How to cite: Angew. Chem. Int. Ed. 2016, 55, 1502–1506
Angew. Chem. 2016, 128, 1524–1528
9a. The copper opens the strained cyclopropane ring[12b] to
give the allenyl boronate 9b. The proton arising from tBuOH
replaces the copper in 9b and after tautomerization the
product 4a is formed together with 9c. Transmetallation of 9c
with 2 regenerates the catalyst. When the ligand is P(1-nap)3
the reaction stops at the formation of the allenyl boronate
product (such as 4a). The probable reason is that the
LCuBpin complex with the bulky L = P(1-nap)3 ligand is
not able to undergo insertion of the double bond of the allenyl
boronate 4a. However, in case of PCy3 this insertion is
possible (Figure 1b) and gives the alkenyl diboronate com-
plex 10a. Subsequently, g-protonation of copper[8a,b, 10b] in 10a
provides 3a. According to the deuterium-labelling experi-
ments this g-proton also arises from tBuOH [Eqs. (2) and (4)].
In summary, we have presented new catalytic reactions for
borylative opening of propargyl cyclopropane, oxirane, oxe-
tane, and aziridine substrates. In this process B2pin2 was
employed as the boronate source together with the inex-
pensive copper catalyst CuI and tBuOH additive. The
reaction displays synthetically useful ligand control. In the
presence of PCy3 alkenyl diboronates form with high regio-
and stereoselectivities, while with the bulky P(1-nap)3 ligand
the product is the allenyl boronate. This process provides
a large variety of allyl/alkenyl- and allenyl boronates, which
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Received: October 30, 2015
Published online: December 11, 2015
1506
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