Palladium-Catalyzed Asymmetric Decarboxylative Addition of β-Keto Acids to Heteroatom-Substituted Allenes

Article abstract of DOI:10.1002/anie.202107990

The Pd-catalyzed asymmetric addition reaction of β-keto acids to heteroatom-substituted allene is reported. This reaction generates β-substituted ketones in an asymmetric manner through a branch-selective decarboxylative allylation pathway. The reaction accommodates various alkoxyallenes as well as amidoallenes.

Full text of DOI:10.1002/anie.202107990

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Accepted Article
Title: Palladium-Catalyzed Asymmetric Decarboxylative Addition of β-
Keto Acids to Heteroatom-substituted Allenes
Authors: Dong-Jin Jang, Sukhyun Lee, Juyeol Lee, Dohyun Moon, and
Young Ho Rhee
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Palladium-Catalyzed Asymmetric Decarboxylative Addition of β-
Keto Acids to Heteroatom-substituted Allenes
†
†
Dong-Jin Jang, [a] Sukhyun Lee, [a] Juyeol Lee, [a] Dohyun Moon[b] and Young Ho Rhee*[a]
[a]
[b]
D.-J. Jang, S. Lee, J. Lee, Prof. Y. H. Rhee
Department of Chemistry
Pohang University of Science and Technology
Cheongam-ro 77, Nam-gu, Pohang, Gyeongbuk 37673, Republic of Korea
E-mail: yhrhee@postech.ac.kr
†These authors contributed equally to the manuscript
Dr. D. Moon
Beamline Department
Pohang Accelerator Laboratory
80 Jigokro-127-beongil, Nam-gu, Pohang, Gyeongbuk 37673, Republic of Korea
Supporting information for this article is given via a link at the end of the document.
Abstract: Pd-catalyzed asymmetric addition reaction of β-keto acids
to heteroatom-substituted allene is reported. This reaction generates
β-substituted ketones in an asymmetric manner by way of a branch-
selective decarboxylative allylation pathway. The reaction
accommodates various alkoxyallenes as well as amidoallenes.
unsaturated ketones possessing stereogenic heteroatom
substituents at the β-position.
Over the last decades, metal-catalyzed asymmetric allylic
alkylation (AAA) has become one of the most powerful carbon-
carbon bond forming reactions.[1] Traditionally, the reaction
accomodates structurally diverse soft carbon nucleophiles. In
recent years, however, some examples of unactivated carbonyl
compounds have been reported. Decarboxylative allylation
reaction has emerged as a new and highly useful variant of the
AAA reaction utilizing carbonyl groups as nucleophiles.[2,3] In
general, this method employs allylic β-keto esters[4] or enol allyl
carbonates[5] as the substrates, which needs to be prepared in
separate steps prior to the desired reaction (Scheme 1). In this
regard, a more direct protocol would be to initiate the reaction
with the addition of β-keto acid to allene precursors. Moreover,
the use of substituted allene offers potential to introduce a
stereogenic center via branch-selecive C-C bond formation.
Despite intriguing features of this method discussed above,
development of an asymmetric reaction is limited, particularly
with carbon-substituted allenes.[6,7] This is presumably due to the
complicated kinetic behavior of π-allyl complexes associated
with the generation and utilization of the allylic β-keto ester
intermediates.
Scheme 1. Basic concept of the research
Since the seminal work of Trost, numerous asymmetric Pd-
catalyzed hydrofunctionalization reactions of alkoxyallene have
been developed.[8,9] Within this context, we reported asymmetric
addition reaction of heteroatom nucleophiles to alkoxyallene,
which emerged as a unique de novo glycosylation method.[10]
Here, we disclose an unprecedented Pd-catalyzed asymmetric
decarboxylative addition reaction of β-keto acid to hetereoatom-
substituted allenes such as alkoxy- and amido-allenes.[11-13] The
latter is noteworthy because asymmetric hydrofunctionalization
of amidoallene is rare. Extensive mechanistic studies
established the initial hydrocarboxylation as the key event
(Scheme 1), which facilitates the subsequent asymmetric
decarboxylative allylation. From a synthetic viewpoint, the
proposed reaction produces highly functionalized γ,δ-
In a preliminary reaction using β-keto acid 1 and allene 2a at -
15oC in CH2Cl2 along with Pd2(dba)3 (2.5 mol%), standard Trost-
type ligand L1 (5 mol%) and triethylamine (3 eq), the desired
product 3a was obtained in low 34% yield but with significant
53% ee (entry 1, Table 1). Switching the ligand to L2 improved
the yield of 3a to 61% with 91% ee (entry 2). Further increase of
the yield (91%) and ee (97%) arose when L3 was employed
(entry 3). Employing more polar solvents such as THF and
acetonitrile as well as using benzyloxyallene 2b dropped the
conversion with slight decrease of ee (entries 4-6). [14] Notably,
no linear isomer was observed, as with the other
hydrofunctionalization of alkoxyallenes.
1
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Table 1. Optimization of the reaction of alkoxyallene
amidoallene 17 was slower than 8. Full conversion required
higher loading of Pd2(dba)3 (5 mol%) and the use of excess β-
keto acids (2 eq). Nevertheless, the corresponding β-amido-
ketones 18~22 were obtained in high yield and
enantioselectivity.[15] Interestingly, small amount linear isomer
was obtained, presumably due to the increased steric effect of
the amide substituents on 17.
entry
ligand
L1
allene
2a
solvent
CH2Cl2
CH2Cl2
CH2Cl2
THF
Yield[a] (%)
ee[b](%)
53
1
2
3
4
5
6
34
61
91
55
37
42
L2
2a
91
L3
2a
97
L3
2a
89
L3
2a
CH3CN
CH2Cl2
74
L3
2b
87
[a] Isolated yield. [b] Determined by HPLC analysis
With the optimized condition described (entry 3, Table 1) in
hand, we explored the scope of this reaction. Varying the para-
substituents of the phenyl ring provided the products 4 and 5 in
comparable yield and ee to that of 3a. In addition, β-keto acids
containing bulkier naphthyl and heterocyclic thiophenyl group
were well accommodated into the reaction, as demonstrated by
the high yield and ee of 6 and 7 (Scheme 2).
Scheme 3. Reaction of the amidoallenes 8 and 17
Next, we tested malonic acid monoester to further broaden the
scope of the reaction (Scheme 4). Initial attempts to use tert-
butyl ester 23 with alkoxyallene 2a failed to provide the desired
β-alkoxy ester 24. Notably, switching to phenyl ester 25
produced 26 in significant 72% yield and 91% ee. Also, an
analogous reaction of 25 and 8 performed with ligand L4 at 40oC
in THF furnished β-amido ester 27 in 92% yield and 87% ee with
~14:1 branch selectivity. Thus, the proposed reaction could be
used in preparation of new functionalized β-amino acid
derivatives. [16] We also tested ene-alkoxyallenes 28 and 30 to
explore the synthesis of cyclic ethers. The reaction of allene 28
with 1 under the method A followed by the ring-closing-
metathesis reaction using 2nd-generation Hoveyda-Grubbs
catalyst (10 mol%) provided enantioenriched dihydrofuran 29 in
58% yield (over two steps) and 93% ee. Analogously,
dihydropyran 31 was prepared in 62% yield and 93% ee from 30.
Based upon these results, the current method may find further
use in the total synthesis of various bioactive C-glycoside natural
products. [17]
Scheme 2. Scope of nucleophiles with alkoxyallene 2a
Encouraged by this promising result with the alkoxyallene, we
then decided to explore oxazolidinone-allene 8. After further
modification, the reaction of 8 (1 eq), acid 1 (1 eq), Pd2(dba)3
(2.5 mol%) and ligand L2 (5 mol%) in THF generated the desired
product 9 in 95% yield and 87% ee with no formation of the
linear isomer (Scheme 3). Interestingly, addition of 5 mol%
trifluoroacetic acid slightly improved the ee to 93%. Under this
new condition, a number of β-keto acids bearing aryl groups
worked well to provide the corresponding β-amido-arylketones
10~15 in high yield and ee. In addition, β-amido-alkylketone 16
was also obtained in significant 86% yield and 89% ee. [14] The
absolute configuration of 14 verified by the X-ray
crystallographic analysis (CCDC deposition No. 2084084)
proved consistent with β-alkoxyketone 3a (For the detailed
analysis, see the SI). Meanwhile, the reaction of acylic
2
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product. Finally, maintaining a tight ion pair between the Pd
complex and carboxylate counteranion (A- in Scheme 6) is
crucial in achieving high enantioselection.
Scheme 4. Synthetic applications
From a mechanistic viewpoint, the initial hydrocarboxylation
seems crucial for the desired transformation.[18] This is
highlighted by the competition reaction of methyl ester 32 with
amidoallene 8. Under the method B, 32 showed no reactivity
towards formation of 33 (Scheme 5), which strongly suggests
1
that the ion pair 34 could not be formed. We also conducted H
NMR monitoring studies for the reaction of 1 and 2a. In this case,
only 37 was observed with no detection of adduct 35 nor 36.
Interestingly, the reaction of tert-butyl ester 23 and 2a described
scheme 4 showed formation of analogous adducts 38 and 39.
We further tested the addition of benzoic acid to allene 2a to
determine the regioselectivity of the initial hydrocarboxylation
step. Under the method A, this reaction quickly generated
unstable branch adduct 40 as the major regioisomer, which
migrated to the more stable 41.[19] Based upon these results, it is
reasonable to assume that branch adduct such as 35 is formed
as the kinetic product in the proposed reaction. To gain further
insight into the C-C bond formation, we tested (E)-36, which
could be readily prepared by alternative route (For the details,
see the supporting information). This experiment generated 3a
in 30% yield and 63% ee. Interestingly, (Z)-36 provided the
same enantiomer 3a in comparable yield and ee. This
enantioconvergent result reasonably excludes a potentially
competing concerted migration pathway from 36.[20] Furthermore,
it confirms that the rapid equilibration of kinetically formed π-allyl
intermediates generated from 36 precedes the C-C bond
formation.[19] This hypothesis is further supported by the reaction
of 41 with 1, which again exhibited comparable yield and ee.
Interestingly, ee of 3a from (E)-36 could be improved by the use
of additional amount of 1. Moreover, crossover experiment using
37 as the exogeneous nucleophile generated a mixture of the
Scheme 5. Mechanistic studies
In summary, we developed
a Pd-catalyzed asymmetric
decarboxylative addition of β-keto acids to alkoxy- and
amidoallenes, which enabled hydrocarboxylation-induced
decarboxylative allylation. This reaction significantly broadens
the scope of asymmetric hydrofunctionalization of heteroatom-
substituted allenes. Currently, we are working on expanding the
scope of the reaction as well as applying this method into the
total synthesis of complex bioactive natural products.
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIT)
(No. NRF-2020R1A3B2079988 and NRF-2018R1A4A1024713).
product 3a and
4
possessing the identical absolute
stereochemistry as that from the reaction of alkoxyallene
depicted in Scheme 2.
Based upon the experimental results and a related working
model,[21] a plausible mechanism is proposed (Scheme 6). Pd-H
species (generated by the oxidative addition of Pd(0) into
carboxylic acids) adds to the allene to form thermodynamically
favorable syn-π allyl complex A, which is in rapid equilibrium
with the adduct B. The subsequent intramolecular C-C bond
formation via enol Pd-carboxylate complex C is likely to be the
enantio-determining (and rate-determining) event, which
generates D by way of inner-sphere mechanism. This picture is
consistent with a related study[22] and our own report.[10d,23]
Decarboxylation afterwards generates the desired ketone
3
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Scheme 6. Proposed catalytic cycle
Keywords: Allene • Addition • Palladium • Catalysis •
Decarboxylation
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4
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[18] For early examples on hydrocarboxylation of allenes, see: a) M. Al-
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Entry for the Table of Contents
We report here an asymmetric decarboxylative allylation reaction initiated by the hydrocarboxylation reaction of heteroatom-
substituted allene. The reaction works particularly well with heteroatom-substituted allenes such as alkoxyallenes and amidoallenes.
A number of aryl and alkyl β-keto acids as well as malonic acid mono-phenyl ester participates in the reaction to generate γ,δ-
unsaturated carbonyl compounds possessing stereogenic heteroatom substituents at the β-position.
Institute and/or researcher Twitter usernames: @CPostech
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