A diastereoselective one-pot, three-step cascade toward α-substituted allylboronic esters

Article abstract of DOI:10.1021/jo5004168

A new highly diastereoselective synthesis of chiral α-substituted allylboronic esters, based on a one-pot, three-step cascade, is presented. The palladium- and acid-cocatalyzed reaction cascade involves a desilylation of a TBS-protected allylic alcohol, borylation, and addition of an allyl group to an aldehyde. Herein we present the first application of a TBS-protected allylic alcohol in a palladium-catalyzed borylation/allylation reaction.

Full text of DOI:10.1021/jo5004168

Note
pubs.acs.org/joc
A Diastereoselective One-Pot, Three-Step Cascade toward
α‑Substituted Allylboronic Esters
Dietrich Bose,† Patrik Niesobski,† Marvin Lubcke,† and Jorg Pietruszka*,†,‡
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̈
†Institut fur Bioorganische Chemie, Heinrich-Heine-Universitat Dusseldorf am Forschungszentrum Julich, 52426 Julich, Germany
̈
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̈
̈
̈
‡Institut fur Bio- und Geowissenschaften (IBG-1: Biotechnologie), Forschungszentrum Julich, 52428 Julich, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: A new highly diastereoselective synthesis of
chiral α-substituted allylboronic esters, based on a one-pot,
three-step cascade, is presented. The palladium- and acid-
cocatalyzed reaction cascade involves a desilylation of a TBS-
protected allylic alcohol, borylation, and addition of an allyl
group to an aldehyde. Herein we present the first application
of a TBS-protected allylic alcohol in a palladium-catalyzed
borylation/allylation reaction.
n the past several years, the direct activation of allylic
alcohols in palladium-catalyzed transformations has attracted
a high level of interest. Given the advantages of these
unactivated substrates over more active species like allylic
carbonates, halides, and esters, with regard to stability and
availability, they are clearly the more preferable substrates.1
mesylate 10, the application of toxic SnCl2 as the metal source,
and the partially unsatisfying diastereomeric ratios of the
products.
I
́
Inspired by Szabo’s results, we decided to develop an
improved synthetic approach toward allylboronates 6−8,
avoiding the aforementioned pitfalls. Therefore, it was
envisaged to use tetrahydroxy diboron (2a) as a boron source
and allylic alcohol 11 or silyl ether 12 as the starting material of
choice to reduce the number of steps and to avoid handling of
unstable compounds. With the choice of this mode of action, a
number of advantages arise. First, toxic allyl tin intermediate
13a would be substituted with nontoxic allylboronate 13b.
Second, one might achieve a better diastereoselectivity of the
allyl addition step by means of formation of a more compact
transition state TS 14 because of the shorter B−O coordination
in comparison to the Sn−O complexation (Scheme 2).
́
Among others, Szabo and co-workers published a number of
procedures toward the catalytic borylation of unactivated allylic
alcohols 1 via application of diboronic species 2 as the boron
source, leading to allylboronic acids or esters 3 as highly active
and valuable products or intermediates for further trans-
formations.2 The same group also showed that such
allylboronic acid intermediates 3 could be transformed into
more stable derivatives, like tetrafluoro borates.2c Also,
allylboronic acids directly react with electrophiles such as
aldehydes 4 or ketones2a,3 to form homoallylic alcohols 5 with
impressive diastereoselectivity (Scheme 1).2a,b,e
Proposed intermediate 13b represents a member of the
group of highly valuable, chiral, 1,3-bifunctionalized allylmetal
reagents, precisely 1,3-diboron compounds,6 with highly
desirable applications7 in double allylboration reactions as
shown by Roush,7a−g Soderquist,6c and others.7h,i In our
previous publications, we demonstrated that the chiral α-
substituted allylboronic esters 6−8 showed unrivaled stability
and are versatile reagents for natural product synthesis.8 Unlike
other examples of 1,3-diboron reagents, we are therefore able to
isolate, purify, and store allylboronates 6−8 after the first allyl
addition, thus also obtaining these reagents in absolutely
enantio- and diastereomerically pure form.
Scheme 1. Palladium-Catalyzed Transformation of Allylic
Alcohols to Allylboronates and Their Addition to Aldehydes
Developed by the Szabo Group
́
The desired reaction was probed by applying the reaction
Asymmetric versions of such palladium-catalyzed allylic
substitution/allyl addition reactions, using nonactivated allylic
alcohols, are particularly rare.1b,e,4 In 2009, our group published
the synthesis of α-substituted, enantiomerically pure allylbor-
onic esters 6−8 via palladium-catalyzed carbonyl allylation
reactions starting from an allylic mesylate 10 (Scheme 2).5 This
three-step method comprises a few drawbacks, namely, the
lengthy synthesis of the very reactive and therefore unstable
conditions published by Szabo’
́
s group.2a Reacting allylic
alcohol 11 in a DMSO/MeOH (1:1) mixture with tetrahydroxy
diboron (2) and cyclohexyl carbaldehyde (4a) and 5 mol %
tetrakisacetonitrile palladium(II) tetrafluoroborate {[Pd-
Received: February 20, 2014
© XXXX American Chemical Society
A
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The Journal of Organic Chemistry
Note
Scheme 2. Palladium-Catalyzed Carbonyl Allylation toward Enantiomerically Pure, α-Substituted Allylboronic Esters 6−8
a
Table 1. Optimization of Carbonyl Allylation Reaction Conditions
b
b
entry
R
[Pd]
[acid]
solvent
temp (°C)
conv (%)
dr
c
1
OH
Pd(CH3CN)4(BF4)2
Pd(CH3CN)4(BF4)2
Pd(CH3CN)4(BF4)2
Pd(CH3CN)4(BF4)2
Pd(dba)2
−
−
A
B
B
B
B
B
B
B
B
B
B
20
20
20
20
20
6
−
−
2
OH
>90
>95
>95
0
6:1
6:1
6:1
3
OH
PTSA
PTSA
−
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
4
OTBS
OH
5
−
d
e
e
6
OH
Pd(dba)2
70 />95
10:1
10:1
10:1
10:1
10:1
−
d
7
OTBS
OTBS
OTBS
OAc
Cl
Pd(dba)2
6
70 />95
8
Pd(CH3CN)4(BF4)2
Pd(CH3CN)4(BF4)2
Pd(CH3CN)4(BF4)2
Pd(CH3CN)4(BF4)2
6
>95
30
30
0
9
0
10
11
6
6
a
b
Reactions were performed on a 0.08 mmol scale using 0.5 mL of the solvent mixture. Conversions and diastereomeric ratios (6:7) were
determined by H NMR analysis of the crude reaction mixture. Full decomposition of starting materials was observed. After 18 h. After 42 h.
c
d
e
1
(CH3CN)4](BF4)2} as a precatalyst (Table 1, entry 1) led to
complete decomposition of the starting material and produced
a complex product mixture. A short solvent screening, including
DMSO, MeOH, CH3CN, THF, toluene, diethyl ether, and
mixtures of them, revealed a DMSO/CH3CN (1:1) mixture as
the solvent of choice (results not shown). Under these
conditions, a high level of conversion (>90%), determined by
1H NMR analysis of the crude reaction mixture, was observed
after 18 h at room temperature (entry 2). The tested solvents
were neither dried nor degassed prior to use because it was
found that intensely dried solvents showed a much lower extent
of conversion and that degassing of solvents did not have any
influence on the reaction outcome. This shows that a small
amount of water is necessary for the reaction to take place, a
finding that is in perfect agreement with those published
previously.2a,9 Nevertheless, recrystallization of tetrahydroxy
diboron2a prior to use proved to be necessary concerning its
oxidation after a short storage time. At this point, the influence
of a Brønsted acid was also studied. The experiments showed
that application of 4 mol % PTSA improved the reaction and
led to an excellent level of conversion of >95% (entry 3). These
conditions were transferred to TBS-protected allylic alcohol 12,
which is the direct precursor of allylic alcohol 11, and we were
very pleased to find full conversion with cyclohexyl
carbaldehyde (entry 4). To the best of our knowledge, this is
the first report of an application of a TBS-protected allylic
alcohol in a palladium-catalyzed transformation. Other Pd
catalysts were tested next (see the Supporting Information for
more details), and experiments revealed that employment of
significantly cheaper bis(dibenzylideneacetone) palladium(0)
[Pd(dba)2] is also possible. However, this precatalyst was found
to be less reactive and led to full conversion only after 42 h.
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The Journal of Organic Chemistry
Note
Additionally, it was found that Pd(dba)2 cannot activate allylic
alcohol 11 and that addition of an acid was necessary to obtain
any conversion (entries 5 and 6). To enhance the
diastereomeric ratio, the temperature was reduced to 6 °C.
Again, both reactions showed full conversion after 18 and 42 h,
respectively, but the diastereomeric ratio could be increased
from 6:1 (entry 7) to 10:1 (entry 8). Further decreasing the
temperature to 0 °C did not lead to a better diastereoselectivity
but resulted in a much slower reaction rate (entry 9).
Application of such optimized reaction conditions to substrates
containing a leaving group, such as acetate 15 or chloride 16,
revealed a quite slow conversion in the case of allyl acetate 15
(entry 10) and no conversion of allyl chloride 16 (entry 11).
With the optimal reaction conditions in hand, we turned our
attention to the substrate scope. To compare our new method
with the previously described carbonyl allylations,5a,b we chose
to examine a number of aldehydes listed in Table 2.
Any attempts to decrease the catalyst load to 2.5 mol %
[Pd(CH3CN)4](BF4)2 or even 1 mol % [Pd(CH3CN)4](BF4)2
led to a significant decrease in the reaction rate and were
therefore rejected. In contrast to these findings, the
concentration of PTSA seemed to have only a minor influence
on the reaction rate. Tested concentrations between 1 and 20
mol % PTSA showed full conversion after 18 h at 6 °C in all
cases (see the Supporting Information for details). However, a
NMR study of the reaction course with 20 mol % [Pd-
(CH3CN)4](BF4)2 and 20 mol % PTSA gave full conversion
after only approximately 3 h (see the Supporting Information
for details) without a significant increase in the level of
decomposition products.
Some of the reactions have also been tested applying the
cheaper but less active Pd(dba)2 precatalyst as shown in Table
3. After 42 h, all reactions were found to reach full conversion
Table 3. Carbonyl Allylation Applying the Pd(dba)2
a
a
Catalyst
Table 2. Probing the Scope of the Carbonyl Allylation
b
c
b
c
entry
R
product
yield (%)
dr
entry
R
product
yield (%)
dr
1
2
3
4
5
6
Ph
6a
6e
6h
6j
88
42
79
42
0
>20:1
20:1
15:1
10:1
−
1
Ph
6a
86
84
83
57
56
72
90
75
89
82
84
>20:1
>20:1
>20:1
>20:1
20:1
CO2Et
2
3,4-F2C6H3
C6F5
6b
6c
(CH3)2CH
C6H11
3
4
furfuryl
6d
6e
furfuryl
6d
6f
5
CO2Et
PhCHCH (E)
0
−
6
PhCHCH (E)
PhCH2CH2
(CH3)2CH
(CH3)2CH2CH
C6H11
6f
>20:1
10:1
a
Reactions were performed on a 0.16 mmol scale using 1.0 mL of the
7
6g
6h
6i
b
solvent mixture. Yield of isolated and purified major diastereomer 6.
8
15:1
c
Diastereomeric ratios (6:7) were determined by 1H NMR analysis of
9
15:1
the crude reaction mixture.
10
11
6j
10:1
d
H
6k/7k
1:1.7
a
and the yields of isolated products as well as diastereomeric
ratios were comparable to those of the reactions shown in
Table 2.
Reactions were performed on a 0.16 mmol scale using 1.0 mL of the
b
solvent mixture. Yield of isolated and purified major diastereomer 6.
c
Diastereomeric ratios (6:7) were determined by 1H NMR analysis of
d
the crude reaction mixture. Reaction performed on a 1.20 mmol
scale; paraformaldehyde was used.
A drawback of the Pd(dba)2 precatalyst is its incompatibility
with certain substrates such as furfuraldehyde (entry 5) or
cinnamyl aldehyde (entry 6) most probably because of the
coordination of these substrates to the catalyst, which inhibited
the reaction and led to a lower level of conversion with a large
amount of decomposition products. Any attempt to separate
the borylation and the allylation reaction by adding the
aldehyde after complete borylation failed. The double borylated
intermediate 13b reacts so fast with an aldehyde during the
With the optimized reaction conditions, heteroaromatic and
unsaturated aldehydes (entries 1−6) show perfect diastereose-
lectivity (dr ≥ 20:1) because of their sterically demanding
nature. As expected, aliphatic aldehydes (entries 7−10) show a
slightly decreased but still very good diastereoselectivity (dr =
10−15:1). Only formaldehyde (entry 11) shows moderate
diastereoselectivity [dr = 1.7:1 (7k:6k)]. However, to the best
of our knowledge, this is the first example in which facial
selectivity, using the smallest aldehyde, was observed at all. It is
worth mentioning that with this aldehyde the reaction could be
scaled up to a 1.2 mmol scale without any loss of reactivity.
Neutral and electron-deficient aldehydes show good to very
good yields (entries 1−3 and 6−11); electron-rich aldehydes
such as furfural (entry 4) show only moderate conversion, and
formation of byproducts, as protodeboronation of intermediate
allylboronic acid 13b, was observed. Ethyl glyoxalate (entry 5)
was found to be a troublesome substrate; the commercially
available solution, containing 40% toluene was found not to be
suitable because the toluene interfered with the reaction and
the redistilled ethyl glyoxalate showed only moderate yield,
probably because of the fast oligomerization of the aldehyde.
1
reaction course that it could not be observed by means of H
NMR when the reaction was performed in a NMR tube in a
reaction monitoring study (see the Supporting Information).
When the reaction was conducted without addition of
aldehyde, only protodeboronation products 15, (E)-16, and
(Z)-16 were isolated as a 1:0.2:0.1 mixture of three inseparable
isomers as shown in Figure 1.
In conclusion, we report the first acid- and Pd-cocatalyzed
deprotection/borylation/allyl addition sequence for TBS-
protected allylic alcohols. We therefore have been able to
expand the substrate scope of such reactions toward even less
activated compounds, thus avoiding an additional deprotection
step. In comparison to the previously published method for the
synthesis of allylboronates 6−8,5b a number of advantages arise.
The method presented here is a more environmentally friendly
C
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The Journal of Organic Chemistry
Note
DMSO-d6) δ 1.50 (d, J = 6.9 Hz, 1 H, 1-H), 2.91 (s, 6 H, OCH3), 4.85
(d, J = 13.6 Hz, 1 H, 3-H), 5.25 (s, 2 H, 4′-H and 5′-H), 6.25 (dq, J =
13.6, 6.9 Hz, 1 H, 2-H), 7.20−7.35 (m, arom. CH). Signals for 15: 13C
NMR (151 MHz, DMSO-d6) δ 17.2 (C-3), 51.3 (OCH3), 77.1 (C-4′
and C-5′), 82.6 (CPh2OMe), 114.7 (C-1), 127.3, 127.4, 127.7, 127.9,
128.0, 129.1 (arom. CH), 133.5 (C-2), 140.5, 140.6 (arom. Cipso). 13
C
NMR signals belonging to (E)-16 and (Z)-16 are too weak to be fully
characterized.
ASSOCIATED CONTENT
Figure 1. Protodeboronation products 15, (E)-16, and (Z)-16 that
were formed when the reaction was conducted without the addition of
an aldehyde.
■
S
* Supporting Information
Detailed catalyst and acid screening, a NMR study of the
1
reaction course, and copies of H and 13C NMR spectra of all
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
and safe reaction because of the application of nontoxic
tetrahydroxy diboron instead of toxic SnCl2 as the metalation
reagent. A one-pot, three-step procedure facilitates the
synthesis of valuable building blocks by reducing the number
of isolation and purification steps and by utilization of nondried
solvents and atmospheric conditions. Additionally, the reaction
proceeds with strongly improved yields and diastereoselectiv-
ities. The synthetic value of such α-substituted, chiral
allylboronic esters for natural product synthesis was already
shown by our group in previous reports. Especially
allylboronates 6k and 7k, which were synthesized on a
synthetically useful scale and are can be separated by means
of MPLC, are highly valuable reagents for the enantioselective
synthesis of dihydro-α-pyrones. They have been used, for
example, in the selective synthesis of both enantiomers of
Rugulactone, Goniothalamin or Massoia lactone.8 With this
more convenient protocol available, the reagents should
become even more generally applicable in future applications.
AUTHOR INFORMATION
Corresponding Author
*E-mail: j.pietruszka@fz-juelich.de.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge the Fonds der Chemischen
Industrie (scholarship to D.B.), the Deutsche Forschungsge-
■
meinschaft, the Heinrich-Heine-Universitat Dusseldorf, and the
̈
̈
Forschungszentrum Julich GmbH for their generous support of
̈
our projects. Furthermore, we thank the analytical departments
the Forschungszentrum Julich as well as Monika Gehsing,
̈
Rainer Goldbaum, Birgit Henßen, Erik Kranz, Vera Ophoven,
Bea Paschold, and Dagmar Drobietz for their ongoing support.
EXPERIMENTAL SECTION
■
REFERENCES
■
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