Stereoselective Traceless Borylation–Allenation of Propargylic Epoxides: Dual Role of the Copper Catalyst

Article abstract of DOI:10.1002/chem.201705019

Chiral α-allenols are prepared with high diastereocontrol through an unprecedented and spontaneous β-oxygen elimination of an α-epoxy vinyl boronate. Stochiometric experiments and DFT calculations support a dual role of the copper catalyst, which orchestrates the hydroboration and the syn-elimination step.

Full text of DOI:10.1002/chem.201705019

A Journal of
Accepted Article
Title: Stereoselective Traceless Borylation-Allenation of Propargylic
Epoxides: Dual Role of the Copper-Catalyst.
Authors: Carlos Jarava-Barrera, Alejandro Parra, Laura Amenós, Ana
Arroyo, and Mariola Tortosa
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To be cited as: Chem. Eur. J. 10.1002/chem.201705019
Link to VoR: http://dx.doi.org/10.1002/chem.201705019
Supported by

10.1002/chem.201705019
Chemistry - A European Journal
COMMUNICATION
Stereoselective Traceless Borylation-Allenation of
Propargylic Epoxides: Dual Role of the Copper-Catalyst.
Carlos Jarava-Barrera, Alejandro Parra, Laura Amenós, Ana Arroyo and Mariola Tortosa*
ꢀꢁα-Allenols from propargylic epoxides
(1)
Abstract: Chiral a-allenols are prepared with high diastereocontrol
through an unprecedented and spontaneous b-oxygen elimination of
an a-epoxy vinyl boronate. Stochiometric experiments and DFT
calculations support a dual role of the copper catalyst, which
orchestrates the hydroboration and the syn-elimination step.
R1
R2
OH
R2
OH
Problems:
metal
catalyst
R3M
.
R3
.
H
O
H
H
R1
R1
H
R2
metal = Cu, Pd, Fe, Rh
✓Regioselectivity
✓Functional group compatibility
M = Li, MgX, B(OR)2
(2)
ꢀꢁFormal copper-catalyzed anti hydride addition
R1
R1
H
Allenes have become an important class of building blocks
with a unique reactivity along their axially chiral backbone.[1]
Among chiral allenes, a-hydroxyallenes are particularly
interesting because they can be readily transformed into 2,5-
dihydrofurans,[2] furanones[3] and different hetero-substituted
allenes.[4] Although a number of methods have been recently
developed,[5] the SN2’ substitution of propargylic epoxides with
organometallic reagents is still the most general approach for the
stereoselective preparation of a-allenols (Scheme 1, eq 1).[6]
Despite significant efforts made in this field, the addition of a
hydride anion, the smallest possible nucleophile, has proven a
particular challenge in this space. The use of highly reactive metal
hydrides presents problems regarding regiochemistry and
functional group compatibility.[7] In 2007, Krause developed an
elegant solution using catalytically generated copper hydride
(Scheme 1, eq 2).[8] The reaction proceeded through the formation
of vinyl copper species A that underwent an anti b-oxygen
elimination to produce anti-allenols from trans epoxides, with high
levels of diastereocontrol.
Inspired by this mechanism, we wondered if an a-epoxy vinyl
boronate such as B, would participate in a b-oxygen elimination
to generate allenes (Scheme 1, eq 3). While the b-oxygen
elimination of vinyl metal species to produce allenes is a known
pathway,[9] the use of a metalloid such as boron, has not been
previously reported. Indeed, a-epoxy vinyl boronates B are
elusive structures that had not been described in the literature.
We planned to access these challenging intermediates from
propargylic epoxides using copper-catalysis, by careful reaction
optimization. We reasoned that the boron atom might provide an
opportunity to -
R2
OH
anti
O
.
R1
(cat)
H
R1 = alkyl
anti
LCu
H
O
LCu
R2
H
R2
H
Elimination
H
H
A
stereoselectivity
ꢀꢁThis work: Formal copper-catalyzed syn hydride addition
(3)
Me
O
R2
pinB
CuL
.
syn
H
O
H
R2
R1
H
OH
Elimination
major
pathway
R1
R1
H
syn-B
O
LCu (I) cat
syn
H
stereoselectivity
B2pin2, MeOH
R2
H
R2
anti
.
R1
H
R1
Elimination
OH
H
O
pinB
anti
H
H
stereoselectivity
R2
anti-B
Scheme 1. a-Allenols from propargylic epoxides
Challenges to this approach included potential regioselectivity
issues in the borylation step, competition between the syn- and
anti-elimination pathways and further undesired borylation of the
products. However, if suitable conditions could be identified, this
strategy would offer a route to prepare disubstituted a-allenols
with relative stereochemistry and substitution patterns that
complement those obtainable by existing methods. Importantly,
this synthetic challenge would be tackled while exploring a novel
mechanistic pathway. Herein, we report our efforts towards this
goal.
To test our hypothesis, we prepared racemic propargylic
epoxide 1a and treated it under copper-catalyzed borylation
conditions. We chose a phenyl substituent on the alkyne[10] hoping
to overcome the inherent tendency of the copper-boryl complex
generated in situ to form allenyl boronates from propargylic
electrophiles through a formal SN2’ addition.[11] Moreover, over-
borylation of the allene was a possible pathway that we needed
to avoid.[12] Using B2pin2 (1.1 equiv.), CuCl (10 mol%), xantphos
(11 mol%), NaO-t-Bu (20 mol%) and MeOH (2 equiv.) in THF we
observed the formation of a 41:50:9 mixture of 2a, 3a, and
homolallylic alcohol 4a, with moderate yield (Table 1, entry 1). We
reasoned that compound 4a was probably formed through
hydroboration-protodeboronation of allene 2a under the reaction
conditions. We did not observe formation of allenyl boronate 6,
and, surprisingly, no sign of vinyl boronate intermediate 5a was
[a]
[b]
C. Jarava-Barrera, Dr. A. Parra, L. Amenós, A. Arroyo, Dr. M.
Tortosa
Organic Chemistry Department
Universidad Autónoma de Madrid, 28949 Madrid, Spain
E-mail: mariola.tortosa@uam.es
Dr. A. Parra, Dr. M. Tortosa
1
observed in the H NMR of the crude product. This experiment
suggested that the hydroboration was taking place with the
desired regiochemistry to give 5a, and that the proposed
elimination occurred spontaneously. This result was striking and
encourage us to further optimize the reaction conditions to pursuit
stereocontrol.
Institute for Advance Research in Chemical Sciences (IAdChem)
Universidad Autónoma de Madrid, 28949 Madrid, Spain
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201705019
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Table 1. Optimization of the reaction conditionsa
enantiopurity of the epoxide is transferred completely to the
product. Remarkably, the C-Cl bond in 2j remained intact through
the transformation.[14] The slighty lower dr observed for 2d, 2e and
2j is a reflection of the diastereomeric ratio of the starting
epoxides. Starting from a cyclic epoxide, compound 2k was
prepared in moderate yield but with complete stereocontrol. We
also studied the hydroboration-allenation sequence with tri-
(R3,R4 ¹ H) and tetrasubstituted (R2,R3,R4 ¹ H) epoxides to
prepare compounds 2l-2o. Although there is no stereoselectivity
involved in these examples, they show that the allenation takes
places in good yield despite increased steric hindrance around
the epoxide. Importantly, compounds 2n and 2o prove that the
regioselectivity in the hydroboration step can be controlled not
only with an aromatic ring but also with an ester and an acetal
group.
pinB
Ph
C6H13
OH
.
Ph
O
CuCl, L, B2pin2
O
H
H
H
pinB
base, MeOH, THF
H
Ph
5a
6a
H
C6H13
C6H13
not formed
(±)-1a
Ph
C6H13
Ph
OH
C6H13
.
.
H
C6H13
OH
Ph
H
OH
(±)-2a
(±)-3a
4a
L
Base
2a:3a:4ab
41:50:9
79:9:12
Yieldc (%)
Entry
1
2
xantphos
Ph3P
NaO-t-Bu
NaO-t-Bu
55
70
3
4
5d
Cy3P
t-Bu3P
IMesCuCl
-
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
NaO-t-Bu
LiO-t-Bu
KO-t-Bu
NaOPh
89:7:4
87:10:3
93:5:2
70
75
67
25
38
72
64
Table 2. Copper-catalyzed synthesis of a-allenolsa,b,c
R3
6
87:10:3
93:7:0
R1
R2
CuCl (5 mol%), Cy3P (30 mol%)
B2pin2 (1.1 equiv), NaOt-Bu (20 mol%)
․
H
8
Cy3P
O
R4
OH
R4
R1
9
Cy3P
56:36:8
67:7:26
R3
R2
MeOH (2 equiv), THF
2
1
10
Cy3P
C6H13
C6H13
C6H13
C6H13
OH
․
․
․
․
H
H
H
H
H
11e
Cy3P
NaO-t-Bu
95:5:0
64
OH
OH
OH
12f
Cy3P
NaO-t-Bu
97:3:0
65
2d, 76%
2a, 65%
dr = 97:3
2c, 70%
2b, 55%
dr = 85:15c
aReaction conditions: 1a (0.2 mmol), B2pin2 (0.22 mmol), base (20 mol%),
dr = 92:8
dr = 95:5
CuCl (10 mol%), L (11 mol%), MeOH (0.4 mmol), THF (0.2 M). bDetermined by
OMe
Me
F
1H NMR analysis. cCombined isolated yield. dIMesCuCl (10 mol%) was used
Me
OH
․
․
․
․
H
OBn
H
OBn
H
OBn
instead of CuCl. eCuCl (5 mol%), Cy3P (11 mol%). CuCl (5 mol%), Cy3P (30
f
OH
OH
OH
mol%).
2g, 69%
dr = 94:6
er = 92:8
2e, 89%
2f, 63%
dr = 92:8
er = 91:9
2h, 71%
dr = 93:7
er = 93:7
dr = 90:10d
Switching to monodentate phosphine ligands, the
diastereoselectivity improved dramatically (Table 1, entries 2-4).
Ph3P afforded allene 2a with a 91:9 diastereomeric ratio (Table 1,
entry 2) along with a small amount of homoallylic alcohol 4a.
Electron-rich monodentate phosphines such as Cy3P and t-Bu3P
shut down the second borylation, minimizing the formation of 4a,
while maintaining high levels of diastereocontrol. N-Heterocyclic
carbene ligand IMes gave similar results (Table 1, entry 5). In the
absence of a ligand, allene 2a was obtained in poor yield and
lower diastereocontrol (Table 1, entry 6). The initial use of NaO-t-
Bu as base proved fortuitous in limiting the formation of 3a and 4a
(Table 1, entries 8-10). With LiO-t-Bu the diastereoselectivity was
high, and we did not observe formation of 4a, but the yield
dropped significantly (Table 1, entry 8). However, KO-t-Bu
afforded 3a as the major diastereomer, albeit with low
stereocontrol (Table 1, entry 9), while NaOPh was less effective
at avoiding the formation of homoallylic alcohol 4a. When the
amount of CuCl was reduced to 5 mol% while maintaining the
ligand (11 mol%), the diastereoselectivity was slightly improved
(dr = 95:5) and 4a was not observed (Table 1, entry 11). Finally,
the use of 5 mol% of CuCl and 30 mol% of Cy3P proved optimal
for maximizing the diastereoselectivity while eliminating
byproduct 4a (Table 1, entry 12). The relative stereochemistry of
2a was established by single crystal X-ray crystallography.[13]
F
Br
․
․
․
․
H
OTBDPS
H
Cl
H
H
OH
OH
OH
OH
2j, 51%
2i, 66%
dr = 95:5
er = 91:9
2k, 54%
dr ≥ 98:2
2l, 83%
dr = 90:10e
Ph
Ph
Ph
Me
․
H
Me
Me
․
H
․
HO Me
H
HO Me
HO Me
EtO
EtO2C
OEt
2m, 79%
2n, 59%
2o, 61%
c
aReaction conditions: Table 1, entry 12. bdr determined by 1H NMR analysis.
Starting epoxide 1d was prepared as a 94:6-E:Z mixture. d Starting epoxide 1e
was prepared as a 95:5-E:Z mixture. e Starting epoxide 1j was prepared as a
94:6-E:Z mixture.
From the outset, our mechanistic hypothesis involved an initial
copper-catalyzed hydroboration of the alkyne to form a-epoxy
vinyl boronates such as 5a. However, generating clear evidence
for the formation of 5a was a major challenge. Indeed, we could
not initially rule out the formation an allenyl boronate (6) followed
by stereoselective protodeboronation to produce allene 2.
We carried out a number of experiments to gain some insight
into the mechanism of the reaction (Scheme 2). Using deuterated
methanol, we observed the formation of allene 2a-D (80%-D,
Scheme 2, eq. 1), confirming the location of the copper atom
during the hydroboration step. We also proved that homoallylic
alcohol 4a, observed during the optimization of the reaction
conditions (Table 1), was formed through borylation of allene 2a
(Scheme 2, eq. 2).[15] To gain further evidence on the formation of
a-epoxy vinyl boronates 5, we run an experiment using a
stoichiometric amount of catalyst in toluene-d8 (Scheme 2, eq 3).
Gratifyingly, we observed the formation of vinyl copper
intermediate 7a with 50% conversion (Ha 3.14 ppm, Hb 3.26 ppm).
Next, we prepared a series of propargylic epoxides to test the
scope of our protocol (Table 2). Propargylic epoxides with
different aromatic rings on the alkyne (R1) afforded the desired
allenols with good yield and high diastereoselectivity (compounds
2b-2d). Starting from enantioenriched propargylic epoxides 1f-1i,
benzyl ethers 2f-2h and silyl ether 2i were obtained in good yields
with excellent chirality transfer. These examples show that the
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This intermediate was transformed into 5a after addition of water
to the NMR tube (Ha 3.36 ppm, Hb 3.28 ppm). Importantly, at this
point we did not observe any significant peak at 4.30 ppm that
would indicate allene formation.[16] This observation shows that
protonation of the vinyl copper intermediate occurs prior to the
elimination, precluding the formation of an allenyl copper species.
Epoxide 5a was not stable enough to be isolated and afforded
allene 2a after workup and SiO2 purification. The diasteromeric
ratio was significantly lower than that observed for 2a in Table 1
and varied from 58:42 to 83:17 after four different trials. This
observation revealed that the copper catalyst was playing a key
role controlling the diastereoselectivity in the elimination step.
elimination).[17] Further reaction of copper alkoxide 8 with B2pin2
generates a-allenol 9 and a copper-boryl complex that would re-
initiate the catalytic cycle. a-Allenol 2 would be obtained after the
workup and purification procedure.
Scheme 4. Proposed mechanism
+
LCuOt-Bu
B2pin2
LCuCl NaOt-Bu
pinBOt-Bu
R2
R1
․
H
O
LCu-Bpin
H
OBpin
2
R1
9
R2
H
1
B2pin2
R2
LCu
R1
Bpin
․
R2
OH
H
ꢀꢁDeuteration experiment
․
(1)
LCu
O
OCuL
8
H
R1
Ph
X
H
C6H13
R1
․
7
R2
O
CuCl (5 mol%), PCy3 (30 mol%)
B2pin2 (1.1 equiv), NaOt-Bu (20 mol%)
D
H
syn
elimination
OH
pinBOMe
H
C6H13
Ph
MeOD (2 equiv), THF
2a-D, 55% yield (80%D)
MeOH
1a
H
Bpin
dr = 97:3
LCuOMe
O
ꢀFormation of homoallylic alcohol 4a
CuCl (5 mol%)
(2)
R1
H
LCuOMe
H
OH
R2
PCy3 (30 mol%)
5
C6H13
․
C6H13
H
B2pin2 (1.1 equiv)
NaOt-Bu (20 mol%)
MeOH, THF
Ph
OH
Me
OMe
Ph
4a, 61% yield
E/Z = 6/94
pinB
2a
O
pinB
CuL
anti
syn
H
R2
H
O
H
ꢀStochiometric experiments
(3)
3
H
2
R1
R1
O
Ph
H
R2
B2pin2 (1.1 equiv)
IMesCuCl (1 equiv)
LCu
IMesCu
Bpin
TS2
O
TS1
Hb
50%
O
conversion
Ha
C6H13
NaOt-Bu (1 equiv)
toluene-D8, 1 h
Ph
Hb
7a
Ha
1a
C6H13
In summary, chiral a-allenols are prepared with high
X
H2O
diastereocontrol through an unprecedented and spontaneous b-
oxygen elimination of an a-epoxy vinyl boronate. This
transformation seems to be operating via a dual role of the copper
catalyst. Further applications of this novel rearrangement to other
unsaturated systems are underway.
C6H13
not formed
․
Hc
Hc
Bpin
C6H13
1) Workup
2) SiO2
․
HO Hb
IMesCu
O
Ph
4.30 ppm
2a, 43%
dr = from 58:42 to 83:17
OH
Ph
Hb
Ph
Ha
5a
C6H13
(4)
C6H13
OH
ꢀTrapping the vinyl boronate
Ha
C6H13
Hb
Me3Si
․
pinB
H
H
CuCl (5 mol%)
PCy3 (30 mol%)
O
Hb
O
SiO2
Acknowledgements
TMS
SiMe3
5p
Ha
C6H13
B2pin2 (1.1 equiv)
NaOt-Bu (20 mol%)
MeOH, THF
2p, 48%
dr = 75:25
1p
73% convension
We thank the European Research Council (ERC-337776) and
MINECO (CTQ2016-78779-R) for financial support. M. T. thanks
MICINN for a RyC contract. We acknowledge Dr. Josefina Perles
for X-ray structure analysis. We acknowledge the generous
allocation of computer time at Centro de Computación Científica
at UAM (CCC-UAM).
Scheme 2. Mechanistic studies
Finally, we designed an experiment that would allow us to
slow down the elimination step by preventing the epoxide and the
boron atom from adopting the required conformation for the syn-
elimination. We chose propargylic epoxide 1p with a trimethyl silyl
group on the alkyne (Scheme 2, eq 4). We reasoned that the
silicon atom could still direct the regiochemistry of the
hydroboration and at the same time prevent the syn-elimination
by coordination of the silicon and the oxygen of the epoxide.
Indeed, using the optimized conditions, we observed formation of
vinyl boronate 5p with 73% conversion. Epoxide 5p was stable
through the workup procedure and afforded allene 2p after
column chromatography with moderate diastereoselectivity,
which reinforces the role of the copper in the diastereocontrol.
Keywords: allenes• stereoselective synthesis • boron • copper •
epoxides
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With these results, we propose a mechanistic scenario
consisting of an initial regioselective insertion of the alkyne into
the Cu-B bond to produce vinyl copper intermediate 7.
Protonation with MeOH would afford epoxy boronate 5 and
copper methoxide, which could facilitate the syn-elimination step
through a 6-membered transition state TS1. DFT calculations
support the preference for TS1 (syn-elimination) over TS2 (anti-
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Chemistry - A European Journal
COMMUNICATION
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at
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PCy3 (30 mol%)
OH
C6H13
H
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C6H13
C6H13
OH
․
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NaOt-Bu (20 mol%)
MeOH, THF
Ph
Ph
Ph Bpin
4a
2a
[16] See Supporting Information for details.
[17] For the complete calculated energy profile for the elimination step see
Supporting Information.
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COMMUNICATION
COMMUNICATION
Carlos Jarava-Barrera, Alejandro Parra,
Laura Amenós, Ana Arroyo and Mariola
Tortosa*
Page No. – Page No.
Dual Player: In this report, chiral a-allenols are prepared with high diastereocontrol
through an unprecedented and spontaneous b-oxygen elimination of an a-epoxy
vinyl boronate. Stochiometric experiments and DFT calculations support a dual role
of the copper catalyst which orchestrates the hydroboration and the syn-elimination
step.
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