Trimethyl Orthoacetate and Ethylene Glycol Mono-Vinyl Ether as Enolate Surrogates in Enantioselective Iridium-Catalyzed Allylation

Article abstract of DOI:10.1002/anie.201803558

Trimethyl orthoacetate and ethylene glycol mono-vinyl ether are employed in iridium-catalyzed enantioselective allylation reactions. The method documented enables their convenient use as surrogates for silyl ketene acetals and silyl enol ethers to prepare

Full text of DOI:10.1002/anie.201803558

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Accepted Article
Title: Trimethyl Orthoacetate and Ethylene Glycol Mono-Vinyl Ether
as Enolate Surrogates in Enantioselective Iridium-Catalyzed
Allylation
Authors: Erick Moran Carreira and Yeshua Sempere
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803558
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Trimethyl Orthoacetate and Ethylene Glycol Mono-Vinyl Ether as
Enolate Surrogates in Enantioselective Iridium-Catalyzed
Allylation
Yeshua Sempere and Erick M. Carreira*
Abstract: Trimethyl orthoacetate and ethylene glycol mono-vinyl
ether are employed in iridium-catalyzed, enantioselective allylation
reactions. The method documented enables their convenient use as
surrogates for silyl ketene acetals and silyl enol ethers to prepare
γ,δ-unsaturated esters and protected aldehydes with excellent
enantioselectivity. The utility of this novel method has been
demonstrated by its implementation in a formal, enantioselective
synthesis of the meroterpenoid (+)-conicol.
nucleophiles in iridium-catalyzed enantioselective allylation
reactions (Scheme 2).[4] However, for compounds with
attenuated C-H acidity, such as esters and ketones, masked
enolate equivalents are necessary. The use of silyl enol ethers
and silyl ketene acetals in enantioselective iridium-catalyzed
allylic substitution reactions has been reported primarily by
Hartwig.[5, 6] Additionally, their use in allylic substitution has been
well-documented
using
palladium[7]
and
ruthenium.[8]
Interestingly, the use of the parent silyl ketene acetals derived
from acetic acid esters in Ir-catalyzed allylation has not been
reported.
Enantioselective, iridium-catalyzed allylic substitution is
a
transformation that has been strategically implemented and
showcased in syntheses of functionally diverse targets, including
natural products and pharmaceutical agents.[1] The versatility of
the reactions stems from the development of numerous robust
processes, involving a wide range of heteroatom and carbon
nucleophiles. Enolates and silyl ketene acetals are particularly
useful nucleophiles because they provide olefin products with
additional functional group handles. Herein, we disclose that
trimethyl orthoacetate 1 may be used as a simple, commercially
available surrogate for acetate enolates in Ir-catalyzed alkylation
of racemic allylic carbonates (Scheme 1). We also disclose the
use of ethylene glycol mono-vinyl ether 3 as a protected
acetaldehyde enolate equivalent. The allylation reactions with
each produces a diverse collection of optically active, -
substituted γ,δ-unsaturated methyl esters along with aldehydes
protected as dioxolane acetals (Scheme 1)
Prior work
Nucleophiles employed in allylic substitution
This work
Scheme 2. Enolates and common surrogates in enantioselective iridium-
catalyzed allylation reactions.
The basis for the study we describe is the hypothesis that
trimethyl orthoacetate could provide access to dimethyl ketene
acetal, which might in turn participate in allylic substitution
reactions with electrophilic η3-allyl Ir(III) intermediates.[9] The
classic use of orthoesters is in the Johnson-Claisen
rearrangement reaction, wherein mixed ketene acetals are
generated from allylic alcohols in the presence of Brønsted
acids.[10] To the best of our knowledge, the use of orthoesters as
nucleophiles in metal-catalyzed allylation reactions is unknown.
The iridium-P, olefin catalyst we have previously described is
notable because of its robustness, and, specifically, its
compatibility with a wide range of Brønsted and Lewis acids.[9b-f]
Accordingly, we initially set out to identify conditions, which
would lead to formation of dimethyl ketene acetal from
MeC(OMe)3 under conditions compatible with Ir-catalyzed allylic
substitution reactions. An important consideration is that 1,1-
dialkoxyalkenes can undergo polymerization when treated with
electrophiles.[11] Moreover, we decided to avoid the use allylic
alcohols as substrates and instead resort to allylic carbonates to
avoid forming mixed ketene acetals involving 1 and the allylic
alcohols.
Scheme 1. Trimethyl orthoacetate and ethylene glycol mono-vinyl ether as
enolate surrogates in enantioselective iridium-catalyzed allylation reactions.
Beyond the initial use of malonates,[2] and derivatives of
amino[3] and aryl acetic acid esters have been reported as
[*]
Y. Sempere, Prof. Dr. E. M. Carreira
Laboratorium für Organische Chemie
HCI H335, Eidgenössische Technische Hochschule Zürich
Vladimir-Prelog-Weg 3, 8093 Zürich (Switzerland)
E-mail: carreira@org.chem.ethz.ch
In initial prospecting experiments, we screened a variety of
conditions, including Lewis acids, such as Zn2+, Fe3+, In3+, and
Supporting information for this article is available on the WWW
under xxx.
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10.1002/anie.201803558
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Sc3+.[12] As shown in Table 1, the observations with Zn(OTf)2
typify the problem faced. Thus, although 4a was formed in high
enantioselectivity, the major product of the reaction was ether 7,
formed by competitive trapping of an allyl-iridium intermediate by
methanol. Interestingly, examination of other Zn(II) salts and a
variety of solvents gave promising results. In particular, ZnBr2 in
1,4-dioxane was found to perform well (Table 1, entries 4-6),with
increasing quantities of ZnBr2 having a marked positive effect on
the yield of the desired product.
Ultimately, the optimal
conditions identified were allylic carbonate 2a (1 equiv),
MeC(OMe)3 1 (2 equiv) and ZnBr2 (2 equiv) in the presence of
[Ir(cod)Cl]2 (2.5 mol%) and (R)-L (10 mol%) in 1,4-dioxane at
room temperature for 12h. As shown in Entry 6, product 4a was
isolated in 80% yield and 97% ee.
Table 1. Optimization of reaction conditions[a]
Yield
4a
Yield
7
ee
Entry
Solvent
Additive
Equiv.
[%][c]
[%][b]
[%][b]
1
2
3
4
5
6
CHCl3
CHCl3
Zn(OTf)2
ZnBr2
ZnBr2
ZnBr2
ZnBr2
ZnBr2
1
1
18
46
68
55
60
80
82
-
86
98
84
88
98
97
toluene
1
<5
9
1,4-dioxane
1,4-dioxane
1,4-dioxane
0.5
1
Scheme 2. Substrate scope of the enantioselective alkylation with trimethyl
orthoacetate (1). Unless otherwise noted, all reactions were performed on a
0.20 mmol scale under the standard conditions (see Table 1, entry 6). Yields
refer to isolated products after purification by chromatography on silica gel.
Enantiomeric excess determined by SFC or HPLC using a chiral stationary
phase. [a] Acetyl underwent cleavage under the reaction conditions. [b]
7
2
<5
[a] Reaction conditions: (±)-2a (1.0 equiv), [Ir(cod)Cl]2 (2.5 mol%), (R)-L
(10 mol%), 1 (2.0 equiv), 0.5 M, rt. [b] Determined by 1H NMR analysis of the
unpurified reaction mixture using 1,4-dimethoxybenzene as internal standard.
[c] Enantiomeric excess determined by supercritical fluid chromatography
Absolute configuration determined by X-ray analysis of
a
derivate
compound.[12] [c] Derivative prepared from 4o after Boc-group hydrolysis and
(SFC) with
a chiral stationary phase. Np = 2-naphthyl, Boc = tert-
reaction of the aniline produced with excess p-BrC6H4NCO. Ts
= p-
butyloxycarbonyl.
MeC6H4SO2, Pin = pinacolate.
We then explored the scope of the enantioselective allylation
reaction with trimethyl orthoacetate (Scheme 2). Allylic
carbonates bearing unsubstituted aromatic rings such as 2-
naphthyl and phenyl underwent the reaction smoothly, and
products were obtained in good isolated yields with excellent
enantioselectivity (4a and 4b). Halogenated and electron-rich
substrates also performed well under the reaction conditions,
leading to the corresponding ester products (4c-d and 4e-g) in
good yields with excellent enantioselectivity in all cases.
Additionally, electron-withdrawing groups on the aromatic ring
were also well tolerated (products 4h-j). The use of
heteroaromatic substrates was achieved, and the transformation
of thienyl and indole allylic carbonates generated the
corresponding γ,δ-unsaturated methyl esters enantioselectively
(4k and 4l). Gratifyingly, 1,4-diene (product 4m) as well as
amine- or boron- substituted aromatic substrates furnished the
desired adducts with high enantioselectivity (4n and 4o).
Encouraged by the use of MeC(OMe)3 as an acetate enolate
equivalent in Ir-catalyzed allylation reactions, we sought to
extend the underlying concept to the use of other similar
nucleophiles. Although aldehyde-derived enamines have been
employed as nucleophiles in iridium-catalyzed allylation
reactions,[9b-e] the use of acetaldehydes is notably absent.
Generally, self-aldolization through C-C bond formation or
oligomerization through C-O bond formation can occur rapidly
and at the expense of product yield. However, we hypothesized
that vinyl ether 3 might be capable of reacting with electrophilic
η3-allyl Ir(III) intermediates I-1 to afford intermediate 8, which
following cyclization would yield acetal-protected aldehyde 5
(Scheme 3).
Commercially available ethylene glycol mono-vinyl ether
(EGME) participated in iridium-catalyzed allylation reactions
under nearly identical conditions to those already established for
orthoacetates.[13] The scope of this transformation was explored
as shown in Scheme 4.
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10.1002/anie.201803558
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section). The intermediacy of I-2 is also supported by the
isolation of and characterization of 9, which was transformed
into methyl ester 4 upon exposure to mild acid (aqueous NH4Cl).
Scheme 3. Proposed mode of action of alkyl enol ether 3.
Unsubstituted as well as alkoxy-substituted substrates provided
the corresponding products (5a-c) in good yields and
enantioselectivity. Substrates with electron-withdrawing groups
proceeded with 83% and 78% yield, respectively and with
greater than 98% enantioseletivity in both cases (5d and 5e).
Heteroaromatic substrates were also tolerated (acetals 5g and
5h) although the substrate possessing a quinoline ring (product
5f) resulted in a diminished yield (40%). 1,4-Enyne substrate
furnished adduct in high yield and good optical purity (5j).
Disubstituted aromatic rings afforded the desired acetals as well,
albeit only in moderate yields and with decreased
enantioselectivity (5k and 5l).
Scheme 5. Proposed mechanistic pathways. 8 and 9 were obtained using 2a
and 2e as substrates, respectively.
In an effort to demonstrate the practicality and utility of the
enantioselective method that we have described, it was applied
to a formal synthesis of (+)-conicol (Scheme 6). (+)-Conicol is a
naturally occurring meroterpene isolated from the ascidian
Aplidum conicum[14] and has shown cytotoxicity against various
carcinomas.[15] Starting from 2,5-dimethoxy phenyl allylic
carbonate 2p, iridium-catalyzed allylic alkylation in the presence
of trimethyl orthoacetate 1 yielded ester 4p in 53% yield with
99% ee on 2 mmol scale (Scheme 6). Alkylation of ester 4p,
followed by ring closing metathesis and subsequent
epimerization of the diastereomeric mixture furnished known
intermediate 10 in 34% yield over three steps. Ester 10 can be
converted into (+)-conicol in
procedures.[16]
a rapid manner via known
Scheme 4. Substrate scope of the enantioselective alkylation with EGME.
Unless otherwise noted, all reactions were performed on a 0.25 mmol scale
under the standard conditions. Yields refer to isolated products after
purification by column chromatography on silica gel. Enantiomeric excess
determined by SFC or HPLC using a chiral stationary phase. [a] Absolute
configuration determined by X-ray analysis of a derivate compound.[12] [b]
Derivative prepared from 5i after TBS removal and reaction of the phenol
produced with excess p-BrC6H4NCO.TBS = t-BuMe2Si; Bn = Benzyl.
Scheme 6. Enantioselective formal synthesis of (+)-Conicol. Conditions: a) 3-
methylbut-3-en-1-yl trifluoromethanesulfonate (2.5 equiv.), LiHMDS (2.0
equiv.), THF, -78 ºC, 52%. b) GC-II (8 mol%), CH2Cl2, 68%. c) NaOMe (5
equiv.), MeOH, 65 ºC, 48 h, 95%. GC-II = Grubbs Catalyst 2nd generation.
A collection of experimental observations enable us to
propose a putative reaction mechanism for the formation of γ, δ-
unsaturated esters from orthoester 1 (Scheme 5). In the first
step, dimethyl ketene acetal 6 attacks the electrophilic allyl
iridium species I-1 to afford I-2, which may suffer either of two
fates: proton loss to give I-3 or trapping by methanol to furnish 9.
Both would lead to the isolated methyl esters following work-up
procedures. The isolation and characterization of trace amounts
of 8 is consistent with the presence of I-3 participating in
allylation reactions in competition with 6 (see supplementary
In summary, we have disclosed an enantioselective allylic
alkylation that enables the preparation of β-substituted γ, δ-
unsaturated esters and protected aldehydes. The method relies
on the use of trimethyl orthoacetate and ethylene glycol mono-
vinyl ether as surrogates for enolates in iridium-catalyzed
enantioselective allylation. This approach fills an existing gap
involving the use of acetate-derived silyl ketene acetals and enol
silanes from acetaldehyde. It thereby complements existing
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10.1002/anie.201803558
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[6]
[7]
For iridium-catalyzed asymmetric allylic substitution with ester-derived
silyl ketene acetals, see: X. Jiang, J. F. Hartwig, Angew. Chem. 2017,
129, 9013-9017; Angew. Chem. Int. Ed. 2017, 56, 8887-8891.
a) A. Saitoh, K. Achiwa, T. Morimoto, Tetrahedron: Asymmetry 1998, 9,
741-744; b) Hegedus, L. S.; Darlington, W. H.; Russell, C. E. J. Org.
Chem.1980, 45, 5193-5196.
approaches by enabling transformations with nucleophiles not
requiring prior preparation. Finally, the synthetic utility of this
methodology was showcased through
synthesis of meroterpenoid (+)-Conicol.
a
concise formal
Acknowledgements
[8]
[9]
N. Kanbayashi, A. Yamazawa, K. Takii, T.-a. Okamura, K. Onitsuka,
Adv. Synth. Catal. 2016, 358, 555-560.
We thank the NMR and X-ray crystallography services for their
assistance. We also thank the Wennemers´ group for their
support with chiral chromatography. ETH Zürich and the Swiss
National Science Foundation (200020_152898) are gratefully
acknowledged for financial support.
The genesis of this idea stems from the use of 2-methoxypropene in
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Schafroth, E. M. Carreira, Science 2013, 340, 1065-1068; c) S.
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S. Krautwald, E. M. Carreira, J. Am. Chem. Soc. 2017, 139, 3603-3606;
for other examples of enamines as nucleophiles in iridium-catalyzed
allylic substitution, see: g) D. J. Weix, J. F. Hartwig, J. Am. Chem. Soc.
2007, 129, 7720-7721; h) L. Næsborg, K. S. Halskov, F. Tur, S. M. N.
Mønsted, K. A. Jørgensen, Angew. Chem. Int. Ed. 2015, 54, 10193-
10197; Angew. Chem. 2015, 127, 10331-10335.
Keywords: Iridium • orthoacetate • allylation • enantioselective •
synthesis
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[12] For more details, see Supporting Information.
[13] We also tested triethyl orthoacetate as nucleophile under identical
reaction conditions with substrate 2b. The β-substituted, γ,δ-
unsaturated ethyl ester (4b’) was obtained in 76% yield and 92%
enantioselectivity; for the use of EGME in aldehyde addition reactions
using MgII/TiIV or NaI/ TiIV , see: a) P. Knochel, P. Quinio, L. Kohout, D.
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Trimethyl orthoacetate and ethylene
glycol mono-vinyl ether are employed
as enolate surrogates in iridium-
catalyzed enantioselective allylation
reactions to prepare γ,δ-unsaturated
esters and protected aldehydes with
excellent enantioselectivity. The utility
of this novel method has been
Yeshua Sempere, Erick M.Carreira*
Page No. – Page No.
Trimethyl Orthoacetate and Ethylene
Glycol Mono-Vinyl Ether as Enolate
Surrogates in Enantioselective
Iridium-Catalyzed Allylation
demonstrated by its implementation in
a formal, enantioselective synthesis of
the meroterpenoid (+)-conicol.
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