ω-Alkenylallylboronates: Design, Synthesis, and Application to the Asymmetric Allylation/RCM Tandem Sequence

Article abstract of DOI:10.1002/ejoc.202100528

The synthesis of allylboronates bearing an alkenyl moiety appended at a remote position is described for the first time. For that aim, the palladium-catalyzed borylation of the corresponding allylic alcohols was used as the key step. The required allylic alcohols were in turn obtained in 3–4 step sequences. The presence of an extra alkenyl moiety at a strategic position allows the triggering of a tandem asymmetric allylation/RCM reaction sequence that efficiently affords different sized cycles featuring two consecutive stereocenters and an exocyclic alcohol function. Products are obtained in moderate to excellent yields and high enantioselectivities, in most of the cases.

Full text of DOI:10.1002/ejoc.202100528

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: ω-Alkenylallylboronates: Design, Synthesis and Application to the
Asymmetric Allylation / RCM Tandem Sequence
Authors: Francisco Garnes-Portolés, Rubén Miguélez, Matthew N
Grayson, and Pablo Barrio
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100528
Link to VoR: https://doi.org/10.1002/ejoc.202100528
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1/2020

European Journal of Organic Chemistry
10.1002/ejoc.202100528
COMMUNICATION
ω-Alkenylallylboronates: Design, Synthesis and Application to
the Asymmetric Allylation / RCM Tandem Sequence
b‡
a
c
a,b
Francisco Garnes-Portolés, Rubén Miguélez, Matthew N. Grayson and Pablo Barrio *
Dedicated to Professor Santos Fustero
[
a]
MSc R. Miguélez, Dr. Pablo Barrio
Departmento de Química Orgánica e Inorgánica
Universidad de Oviedo
Avenida Julian Clavería 8, 33006 Oviedo (Spain)
E-mail: barriopablo@uniovi.es
MSc F. Garnés
Departmento de Química Orgánica
Universidad de Valencia
[
[
b]
c]
46100, Burjassot (Spain)
Dr. M. N. Grayson
Department of Chemistry
University of Bath
Claverton Down, Bath, BA2 7AY (UK)
Supporting information for this article is given via a link at the end of the document.
Abstract: the synthesis of allylboronates bearing an alkenyl moiety
appended at a remote position is described for the first time. For that
aim, the palladium-catalyzed borylation of the corresponding allylic
alcohols was used as the key step. The required allylic alcohols were
in turn obtained in 3-4 step sequences. The presence of an extra
alkenyl moiety at a strategic position allows triggering a tandem
asymmetric allylation/RCM reaction sequence that efficiently affords
different sized cycles featuring two consecutive stereocenters and an
exocyclic alcohol function. Products are obtained in moderate to
excellent yields and high enantioselectivities, in most of the cases.
this, in 2015, we described for the first time the use of an
allylboronate bearing an extra functionality pending at the γ
8
position. Since then, other groups have shown interest in this
field giving rise to a number of interesting synthetic applications.9
Herein, the introduction of an extra alkenyl functionality in a
remote position of the allylboronate instead of the aldehyde
results in the formation of two contiguous stereocenters, now one
endocyclic and the second exocyclic. Moreover, for this new
process the alcohol function would occupy an exocyclic position.
O
OH
OH
1
R2
R
The asymmetric construction of cyclic backbones from acyclic
precursors represents a longstanding challenge in organic
synthesis. The simultaneous control over both endo- and
exocyclic stereocenters results in an especially difficult task.
Among stereoselective methodologies, the allylboration of
carbonyl compounds shows a number of interesting features due
to the highly ordered chair-like transition state that accounts for a
stereospecific transformation rendering complete control over the
relative configuration of the two contiguous stereocenters set up
when the allylborating reagent bears substitution at the γ-
position.1,2 In 2010, Antilla described for the first time the chiral
(R)-TRIP
[Ru-II]
R2 R1
(a)
R1
Bpin
via
R2
Ref. 5a
R
O
OH
OH
(R)-TRIP
[Ru]
R
R
(b)
Bpin
n
n
3
,4
n
Brønsted acid-catalyzed asymmetric allylboration of aldehydes.
During the following decade, a number of synthetic applications
were developed by others and us.5 Among them, our first
contribution in the fied described the tandem allylboration / ring-
closing metathesis (RCM) reaction sequence for the asymmetric
construction of cyclic homoallylic alcohols (Scheme 1a).5a For that
aim, an extra alkenyl functionality was introduced in a strategic
remote position of the aldehyde counterpart. Thus, different sized
carbocycles bearing one or two endocyclic stereocenters were
achieved in high yields and excellent diastereo- and
enantioselectivities, in most cases. The viability of the relay
catalysis process Brønsted acid catalyzed asymmetric
This work
Scheme 1. Precedents and new strategy.
Our first challenge was the synthesis of the hitherto unknown ω-
10
8
alkenylallylboronates 1. Building upon our own experience, we
decided to rely on an allylic alcohol borylation of commercially
available (E)-octa-2,7-dien-1-ol 2a (Scheme 2). The key allylic
borylation afforded model substrate 1a in moderate yield
11
(
Scheme 2). It is worth noting that despite the starting allylic
allylboration / RCM reaction sequence allowed us to envision a
new related process, described herein (Scheme 1b).6,7 Following
alcohol 2a was used as a 6:1 mixture of E/Z isomers product 1a
1
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European Journal of Organic Chemistry
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COMMUNICATION
Scheme 2. Synthesis of model substrate.
was obtained exclusively as the corresponding E-isomer. This
result may be explained by the intermediacy of a palladium π-allyl
complex responsible for the observed stereochemistry
scrambling.12
Once the viability of the synthesis of the allyborating agent 1a was
stablished, we aimed to synthesize other ω-alkenylallylboronates
1
b,c that would afford other ring sizes. First, the synthesis of this
previously unknown allylboronate class will be outlined (Scheme
). The precursors for 5- and 7-membered carbocycles 1b,c were
p-TsOH
DMSO, MeOH
OH
Bpin
3
[
Pd] (2.5 mol%)
2
a
1a (53%)
both synthesized by allylic borylation of the corresponding known
ω-alkenylallylic alcohols 2b,c which were in turn prepared from
commercially available 4-penten-1-ol 3 and propargyl alcohol 4 in
+
[
Pd] =
NMe
2
B pin
2
2
Pd
Cl
14,15
2
three and four steps, respectively (Scheme 3 ).
OH
i-iii
OH
5
1%
2
b
3
viii
Bpin
n
1
b; n = 1 (58%)
OH
1c; n = 3 (52%)
iv-vii
5%
OH
3
4
2c
Reagents and conditions: i. DMSO, oxallyl chloride, CH Cl Et N, -78ºC; ii.
2
2,
3
Ph P=C(H)CO Me; iii. DIBAL-H, CH Cl -78ºC,2h; iv. DHP, PTSA (5% mol), CH Cl 0ºC to
3
2
2
2,
2
2,
rt; v. n-BuLi, THF, 0 ºC / 5, DMSO; vi.p-TsOH, MeOH; vii. LiAlH THF, 0 ºC then reflux; viii.
4
,
B pin [Pd] (2.5 mol%), DMSO/MeOH, p-TsOH, 50 ºC
2
2,
Br
5
Scheme 3. Synthesis of ω-alkenylallylboronates 1b,c.
Trying to increase the complexity of our tandem process, we
designed bis ω-alkenylallylboronate 1d (Scheme 4). We
envisioned that the presence of two alkenyl chains in the open
chain intermediate may result in a diastereoselective RCM
generation Grubbs’ catalyst ([Ru-II]) afforded final product 9a in
moderate yield and with a minor erosion of the optical purity
(Scheme 5). Our experience allowed us being optimistic about
success of the one-pot transformation under relay catalysis
conditions. Indeed, when the reaction is conducted in the
presence of both catalysts at -30 ºC in toluene overnight, followed
by stirring at room temperature for 6 extra hours, product 9a is
obtained in an improved yield and enantioselectivity (Scheme 5).
It is worth noting that our methodology gives rise to the threo
diastereoisomer that is the minor diastereoisomer formed in the
allylation of carbonyl compounds using either classic cyclic
subsequent
step,
that
could
be
regarded
as
a
pseudodesymmetrization process. 1d was obtained in moderate
yield using otherwise identical conditions that for the rest of the
allylboronates 1a-c using alcohos 2d. 2d was in turn synthesized
in three simple steps starting from ethyl formate 6 (Scheme 4).
Bpin
16
allylmetal additions or in a more recent photoredox asymmetric
O
iv
ix-xi
OH
17
allylation with cyclic alkanes.
H
OEt
29%
5
0%
6
2d
1d
iii
OH
Reagents and conditions: ix. 7 THF, 0 ºC to r.t.; x. PCC, DCM, r.t.;
xi. vinylmagnesium bromide, TFH, 0 ºC to r.t.; viii. as above
MgBr
O
7
OH
H
ii
Br
i
Scheme 4. Synthesis of ω-bisalkenylallylboronate 1d.
4
a
Br
Br
+
9
a
Stepwise: (60%, 90%ee)
Tandem: (58%, 91 % ee)
Bpin
With model substrate 1a in our hands, we evaluated the viability
of our asymmetric allylation / RCM strategy, first in a stepwise
manner. To our delight, the allylboration step afforded product 8a
in good chemical yield, as a single diastereoisomer and in high
enantioselectivity without modifying the standard conditions ((R)-
TRIP, toluene, - 30 ºC).13 Similarly, the RCM using second
8a
1a
(86%, 93% ee)
Reagents and conditions: i. (R)-TRIP (5 mol%), toluene, -30ºC, overnight ii. [Ru-II]
5 mol%), rt iii.(R)-TRIP / [Ru-II] (5 mol%), toluene, -30ºC, overnight then rt
(
Scheme 5. Validation of both the stepwise and the relay catalysis strategies.
2
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European Journal of Organic Chemistry
10.1002/ejoc.202100528
COMMUNICATION
Once our relay catalysis strategy has been stablished, we turned
to study the scope of this new transformation (Scheme 6).
Aromatic aldehydes bearing both electron-withdrawing (9a,c) and
electron-donating substituents (9d) afforded the product in good
yield and high enantioselectivity (Scheme 6). Regarding
substitution pattern, while the introduction of substituents at the
para and metha positions did not compromise chemical yield
neither enantioselectivity (9e,f), the presence of a substituent in
the ortho position completely inhibited the reaction (9g) (Scheme
O
OH
H
1
) (R)-TRIP, toluene, -30ºC
2) [Ru-II], rt or 90 ºC
Bpin
R
4
b,d,j
n
R
+
1
0a-c: n = 0
11a: n = 2
n
1
b: n = 0
1c: n = 2
OH
OH
OH
6
). Moreover, alkenyl as well as heteroaromatic aldehydes could
also be succesfully employed (9h,i). While the use of
cinnamaldehyde resulted in a moderate drop both in chemical
yield and enantioselectivity (9h), furfuraldehyde behaved very
well in terms of enantiocontrol, although the yield was also
moderately lower (9i) (Sheme 6). Finally, aliphatic aldehydes
were also used affording the corresponding products (9j,k) in
moderate yields and enantioselectivities (Scheme 6).
MeO
2
O N
1
0c
10a
98%, 89%ee)
10b
(91%, 89%ee)
(
78%, 97% ee)
(
OH
1
1a
(43%, 78%ee)
R
Scheme 7. Use of ω-alkenylallylboronates 1b,c in the asymmetric
allylboration/RCM tandem sequence.
2
) [Ru-II], rt
Bpin
OH
+
9
a-i
1
a
OH
OH
OH
We believe that the decreased enantioselectivity observed for
seven-membered rings may be explained by the higher
temperature at which the RCM step must be conducted in these
cases (Scheme 7). The ability of ruthenium species to catalyze
transfer hydrogenation reactions is well-documented.18 We
believe that the formation of a ruthenium alcoxyde I may result in
a reversible β-hydride elimination / migratory insertion sequence,
accounting for the loss in stereochemical integrity. In order to
shed some light on this issue, the corresponding ring-opened
intermediate was isolated. To our delight, intermediate 12a was
obtained in high yield, as a single diastereoisomer and in an
excellent 93% ee, showing that the erosion of optical purity
observed was due to the RCM step, as expected (Scheme 8).
Br
F
MeO
9
c
9d
(43%, 87% ee)
9
a
9
b
(
58%, 91% ee)
(63%, 91% ee)
(
51%, 93% ee)
OH
OH
OH
9
e
9f
(62%, 94% ee)
9g
(--)
(
62%, 86% ee)
OH
OH
O
9
h
9i
(35%, 80% ee)
(40%, 95% ee)
OH
OH
OH
OH
(
R)-TRIP (5 mol%)
[Ru-II] (5 mol%)
Toluene, 90 ºC
Ph
BnO
4b + 1b
Toluene, -30 ºC
1
2a (84%)
11a (57%)
mixture of diastereoisomers
78% ee
9
j
9k
1
diastereoisomer
93% ee
(
49%, 58%ee)
(54%, 81%ee)
[
Ru]
[Ru] H
O
I
O
Scheme 6. Scope for the asymmetric allylation/ RCM tandem process using 1a.
II
reversible -H elimination / migratory insertion
Next, the extension of the asymmetric allylboration/RCM tandem
sequence was explored using the new ω-alkenylallylboronates
1
b,c (Scheme 7). Reagent 1b successfully afforded five-
Scheme 8. Study of the loss of optical integrity in seven-membered ring
derivatives 10a,b.
membered products 10a-c in excellent yields and high
enantioselectivities (Scheme 7). Seven-membered derivative 11a
was obtained starting from reagent 1c in moderate yield and
enantioselectivity as mixture of diastereoisomers (Scheme 7).
Finally, we challenged our methodology towards the asymmetric
allylboration / psudodesymmetring RCM tandem process using
bisalkenylated allylboronate 1d. Unfortunately, the final product
1
3 was obtained in low yield, a moderate 5:1 diastereomeric ratio
and a poor 36% ee (Scheme 9). Noteworthy, this is one of the
rare examples of chiral Børnsted acid-catalyzed allylboration
19
using a γ,γ-disubstituted boronate. This fact may account both
for the low reactivity and the poor enantioselectivity obtained.
3
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European Journal of Organic Chemistry
10.1002/ejoc.202100528
COMMUNICATION
O
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Scheme 9. Use of ω-alkenylallylboronate 1d in the asymmetric
3
allylboration/RCM tandem sequence.
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In conclusion, allylboronates functionalized at a remote position
with an extra alkenyl moiety have been synthesized for the first
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corresponding dienyl allylic alcohols. Their participation in a
tandem chiral Brønsted acid-catalyzed allylboration / ring-closing
metathesis tandem process has then been studied affording
several sized-cyclic alcohols, bearing the hydroxyl group at an
exocyclic position. The corresponding 5- and 6-membered
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diastereosimers and in high ee’s. However, 7-membered rings
are obtained with poorer results, due to the high temperature
required for RCM in this case that results in partial epimerization.
The use of a bis alkenylated allylboronate does not allow to carry
out the pseudo desymmetrization of the two chains efficiently.
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Acknowledgements
P. B. thanks the Spanish MINECO for a Ramón y Cajal contract
(RyC-2016-20951).
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[
Keywords: asymmetric allylborylation • ring-closing metathesis •
2
relay catalysis • functionalized allylboronates
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4
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European Journal of Organic Chemistry
10.1002/ejoc.202100528
COMMUNICATION
[19] (a) Y. Liu, C. Mazet J. Org. Chem. 2020, 85, 5638. (b) T. Miura, J.
Nakahashi, M. Murakami Angew. Chem. Int. Ed. 2017, 56, 6989.
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European Journal of Organic Chemistry
10.1002/ejoc.202100528
COMMUNICATION
Entry for the Table of Contents
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OH
n
Bpin
tandem
R
O
n
R
allylation
R)-TRIP
OH
RCM
Ru-II]
R
(
[
n
via
The synthesis of ω-alkenylallylboronate derivatives is described for the first time. In addition, their performance in the chiral Brønsted
acid-catalyzed allylboration / RCM tandem sequence is studied. Hence, cyclic alcohols displaying the hydroxyl functionality at an
exocyclic position and two consecutive stereocenters are obtained as single diastereoisomers and in high degree of
enantioselectivity, in most of the cases.
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This article is protected by copyright. All rights reserved.