Copper-Catalyzed Hydroalumination of Allenes with Diisobutylaluminum Hydride: Synthesis of Allylic Ketones with α-Quaternary Centers via Tandem Allylation/Oppenauer Oxidation

Article abstract of DOI:10.1021/acs.orglett.0c01876

An efficient and straightforward approach to allylaluminum reagent synthesis through Cu-catalyzed hydroalumination of readily accessible allenes with diisobutylaluminum hydride is described. The N-heterocyclic carbene-based copper complex promotes hydride addition to various functionalized allenes under mild reaction conditions. The catalytic reaction is applied to a highly selective one-pot synthesis of allylic ketones with α-tertiary and α-quaternary centers through tandem nucleophilic addition of in situ-generated allylaluminums to aldehydes/Oppenauer oxidation.

Full text of DOI:10.1021/acs.orglett.0c01876

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Letter
Copper-Catalyzed Hydroalumination of Allenes with
Diisobutylaluminum Hydride: Synthesis of Allylic Ketones with
α‑Quaternary Centers via Tandem Allylation/Oppenauer Oxidation
Sangback Lee, Sanghyun Lee, and Yunmi Lee*
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ABSTRACT: An efficient and straightforward approach to allylalu-
minum reagent synthesis through Cu-catalyzed hydroalumination of
readily accessible allenes with diisobutylaluminum hydride is
described. The N-heterocyclic carbene-based copper complex
promotes hydride addition to various functionalized allenes under
mild reaction conditions. The catalytic reaction is applied to a highly
selective one-pot synthesis of allylic ketones with α-tertiary and α-
quaternary centers through tandem nucleophilic addition of in situ-generated allylaluminums to aldehydes/Oppenauer oxidation.
he addition of allylic organometallic reagents to carbonyl
1b).4 Despite the invention of several methods, some
Tcompounds is a powerful and widely used method for the compelling issues to be addressed for their practical utility
remain: access to certain allylic halides is problematic, and the
substrate scope in the catalytic process is limited to cyclic
dienes.
preparation of homoallylic alcohols and allylic ketones, which
are valuable intermediates in the synthesis of complex
molecules, natural products, and biologically active molecules.1
While numerous examples of allylation using allylstannanes,
allylsilanes, allylboranes, and allylzincs have been reported,
allylmetal reagents based on aluminum, which is the most
abundant metal and has low toxicity, have received relatively
little attention, presumably because of the absence of
convenient and efficient access. Typical strategies for the
synthesis of allylaluminum reagents include the direct insertion
of aluminum metal into allylic halides in the presence of a
Copper-catalyzed hydrometalation of allenes with a hydride
source could be a promising route for synthesizing allylic metal
reagents because it is a simple and atom-economical process,
uses a relatively low-cost and easy-to-handle copper catalyst,
and employs readily accessible allene substrates. To date, a few
examples of copper hydride-catalyzed reactions with allenes
have been reported.5 In all of those studies, only silane was
used as a hydride source, leading to the formation of a copper
hydride complex that undergoes hydrocupration with allenes
to generate a catalytic amount of allylcopper intermediates.
Therefore, the allylcopper reagents served as allylic nucleo-
philes for allylation with electrophiles, including aldehydes,
imines, alkyl triflates, and CO2. Unlike the previous CuH-
catalyzed reactions involving silanes, we proposed that Cu-
catalyzed hydride addition of DIBAL-H6 to allenes would
produce allylaluminum reagents via allylcopper intermediates,
which could be further utilized to develop unexplored
allylation reactions by applying the intrinsic reactivity and
Lewis acidity of aluminum (Scheme 1c).
2
catalytic amount of HgCl2 or InCl3 and transmetalation with
Grignard or lithium reagents (Scheme 1a).3 A complementary
approach reported by Vasse, Szymoniak, and co-workers
involves titanium-catalyzed hydroalumination of conjugated
dienes with diisobutylaluminum hydride (DIBAL-H) (Scheme
Scheme 1. Generation of Allylaluminum Reagents and Their
Reactions with Aldehydes
Tandem nucleophilic addition/Oppenauer oxidation with an
aldehyde has emerged as a mild and efficient method for the
synthesis of ketones.7 This form of oxidation was proposed to
Received: June 4, 2020
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© XXXX American Chemical Society
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proceed by transfer of the hydride to the aldehyde from in situ-
generated metal alkoxide intermediates obtained from
nucleophilic attack of the aldehyde. Various types of
organometallic reagents, such as nucleophiles based on
organomagnesium,8 -zinc,9 -aluminum,10 -tin,11 -boron,12
-indium,13 and -zirconium,14 are involved in this trans-
formation. Among them, allylmetal reagents are particularly
attractive because they can be used to prepare synthetically
useful allylic ketones bearing α-tertiary and even difficult-to-
access α-quaternary stereogenic centers under simple and mild
reaction conditions.15 Although one example of tandem
allylation/Oppenauer oxidation with an aldehyde engaging
allylic chromiums, which afforded α-substituted allylic ketones,
was reported by Wessjohann and co-workers,16 the reaction
was carried out with allylic bromide in the presence of excess
chromium chloride, and the substrate scope was limited to
three kinds of allylic bromides: crotyl, prenyl, and geranyl
bromide. Considering the significance and challenges of the
construction of acyclic all-carbon quaternary centers in the α-
carbon position of allylic ketones, we pursued the exploration
of an efficient and simple method on a synthetically useful
level.17 To the best of our knowledge, access to α-substituted
allylic ketones through Oppenauer oxidation of aldehydes with
allylaluminum reagents has not been reported.18 Furthermore,
most reactions of aldehydes with allylic organometallic
reagents have been used primarily to produce homoallylic
alcohols, not allylic ketones.
Table 1. Optimization of the Cu-Catalyzed
Hydroalumination of Allene 1a
a
equiv of
DIBAL-H
temp.
(°C)
time
(h)
conv. yield of 2
b
b
entry NHC−CuCl
(%)
(%)
1
2
3
4
5
6
7
8
SIPrCuCl
SIPrCuCl
IPrCuCl
IPrCuCl
SIMesCuCl
IMesCuCl
CuCl
1.5
1.2
1.2
1.2
1.2
1.2
1.2
1.2
22
60
60
60
60
60
60
60
2
2
2
3
3
3
3
3
47
87
90
>98
16
8
47
87
90
>98
16
8
<2
<2
<2
<2
none
a
Reaction conditions: allene 1a (0.200 mmol), (i-Bu)2AlH (0.240−
0.300 mmol), NHC-CuCl (5 mol %), THF (0.2 M), under N2.
b
1
Determined by analysis of H NMR spectra of unpurified mixtures
after quenching with H2O using 1,3,5-trimethoxybenzene as an
internal standard; see the Supporting Information for details.
In this study, we developed a new and promising strategy to
access synthetically useful allylaluminum reagents through Cu-
catalyzed hydroalumination of DIBAL-H with easily accessible
allenes in the presence of CuCl based on an N-heterocyclic
carbene (NHC) ligand. In addition, we describe a one-pot
synthesis of versatile α-substituted-β,γ-unsaturated ketones
containing α-quaternary centers, which are difficult and
challenging to synthesize, through regioselective allylation of
in situ-generated allylaluminum reagents with an aldehyde and
sequential Oppenauer oxidation (Scheme 1c).
Scheme 2. Allylation of Allene 1a with Aldehyde 3a
We began our investigations by examining the Cu-catalyzed
hydroalumination of model allene 1a with DIBAL-H, as shown
in Table 1. Treatment of allene 1a with 5 mol % SIPrCuCl and
1.5 equiv of DIBAL-H at room temperature for 2 h provided
the desired allylaluminum 2 in 47% yield (entry 1). After
considerable efforts, the use of 5 mol % IPrCuCl and 1.2 equiv
of DIBAL-H at 60 °C for 3 h was found to be optimal,
affording the corresponding allylaluminum reagents 2 in >98%
yield (entries 2−4). SIMesCuCl and IMesCuCl catalysts were
not effective in promoting the hydroalumination of allene 1a
(8−16% yield; entries 5 and 6). It was noted that Cu-catalyzed
hydroalumination of allene 1a selectively generated allylalumi-
num reagents rather than vinylaluminum reagents.19 As
illustrated in entries 7 and 8, the reaction did not proceed in
the presence of CuCl itself or without a catalyst (<2%
conversion), indicating that an NHC ligand was required to
promote the addition of DIBAL-H to allene 1a. On the basis of
previous studies involving CuH catalysis by Tsuji, Lalic, Bai,
and co-workers,20 we assumed that the reaction of NHC−
CuCl with DIBAL-H would generate a copper hydride species
complexed with the NHC ligand, which might play an
important role in stabilizing the CuH species and enhancing
the activity of hydride addition to allenes.
of 1.2 equiv of aldehyde 3a to allylaluminum reagent 2 derived
directly from the catalytic hydroalumination of allene 1a with
DIBAL-H provided homoallylic alcohol product 4a in 94%
yield (Scheme 2a).21 On the other hand, when 3.0 equiv of
aldehyde 3a was introduced, allylic ketone 5aa was
predominantly and markedly obtained in 94% yield at room
temperature within 30 min, along with the formation of benzyl
alcohol 5aa′, indicating that hydride transfer to the aldehyde
from the aluminum alkoxide intermediate occurred through
Oppenauer oxidation (Scheme 2b). The allylation with
aldehyde 3a took place with exclusive regioselectivity, affording
5aa as a single regioisomer.1,22 It was found that an excess
amount of benzyl alcohol 5aa′ was observed in 200% yield.
This might have occurred because hydride transfer to aldehyde
3a proceeded competitively not only from the aluminum
alkoxide but also probably from the two isobutyl groups on the
aluminum center during the oxidation. Therefore, the use of
3.0 equiv of the aldehyde was required to obtain a high yield of
the ketone product. To gain some insight into the reaction
intermediate, the direct oxidation of homoallylic alcohol 4a
After the optimized reaction conditions were established, we
investigated tandem allylation/Oppenauer oxidation using
allylaluminum reagents, as shown in Scheme 2. The addition
B
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was carried out (Scheme 2c). When 4a was treated with
DIBAL-H to generate an aluminum alkoxide intermediate and
then aldehyde 3a was added as a hydride acceptor, alcohol 4a
was readily oxidized, furnishing allylic ketone 5aa in 95% yield.
This result explained that the copper catalyst used in the
hydroalumination reaction did not play a main role in
facilitating oxidation in the one-pot synthesis and that the
Oppenauer oxidation proceeded via the aluminum alkoxide
intermediate.
Under the established reaction conditions, the Cu-catalyzed
hydroalumination/allylation/Oppenauer oxidation could be
successfully carried out with a wide range of monosubstituted
allenes and aldehydes (Scheme 3). All of the transformations
93%). The reaction with the sterically demanding cyclohexyl-
substituted allene 1n also showed good reactivity, giving the
desired allylic ketone 5na in 92% yield. Next, when various
aromatic aldehydes 3b−i containing fluoro, chloro, bromo,
trifluoromethyl, or methoxy groups at the ortho, meta, or para
position were utilized for tandem Oppenauer oxidation with
allene 1a, the corresponding allylic ketones 5ab−ai were
efficiently obtained in 86−95% yield. However, reaction with
benzaldehydes bearing chloro or bromo substituents at the
ortho position of the phenyl group gave homoallylic alcohol
products in 94−95% yield; the Oppenauer oxidation did not
proceed, presumably as a result of steric hindrance.
Naphthaldehyde (3j) and heteroaromatic aldehydes 3k and
3l bearing a thienyl or furyl group reacted with high efficiency
and regioselectivity (85−95% yield of 5aj−al). Aliphatic
aldehydes, such as valeraldehyde (3m), isovaleraldehyde
(3n), and 3-phenylpropanal (3o), were also suitable for this
transformation, affording allylic ketones 5am−ao in high yields
(82−94%).
Scheme 3. Reaction of Monosubstituted Allenes with
a
Various Aldehydes
Our new copper-catalyzed hydroalumination with 1,1-
disubstituted allenes significantly contributes to the efficient
construction of congested α-quaternary carbon centers on
acyclic β,γ-unsaturated ketones, which are synthetically
challenging moieties. As summarized in Scheme 4, a variety
Scheme 4. Reaction with Various 1,1-Disubstituted Allenes
a
Reaction conditions: allene 1 (0.300 mmol), (i-Bu)2Al−H (0.360
mmol), IPrCuCl (5 mol %), aldehyde 3 (0.900 mmol), THF (0.15
M), under N2. Yields of the isolated products are shown.
were promoted by 5 mol % IPrCuCl catalyst and showed high
efficiency and excellent regioselectivity (>99:<1), affording
versatile α-substituted β,γ-unsaturated ketones. The Cu-
catalyzed hydroaluminations of DIBAL-H with phenethyl-
substituted allenes 1b−i, including chloro, bromo, fluoro,
trifluoromethyl, or methoxy moieties on the phenyl group,
proceeded smoothly to generate the corresponding allylalumi-
num reagents, which directly reacted with aldehyde 3a to
provide the desired ketone products 5ba−ia in 80−98% yield.
Synthetic functional groups such as a benzyl ether, silyl ether,
or chloro substituent on allenes 1j−m were well-tolerated in
the Cu-catalyzed process. The corresponding functionalized
allylic ketones 5ja−ma were obtained in high yields (74−
of 1,1-disubstituted allenes exhibited excellent reactivity and
regioselectivity. Under mild reaction conditions, allenes
substituted with methyl and various alkyl functionalities, such
as chloro-, methoxy-, or fluoro-substituted phenethyl groups
(6a−c), a chloro-substituted alkyl group (6d), a silyl ether
(6e), or a benzyl ether (6f), were successfully converted to the
corresponding α-quaternary-substituted allylic ketones 7aa−fa
in high yields (77−93%). The reactions of silyl ether-
substituted allenes 6g−l, including ethyl, n-butyl, n-hexyl,
C
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isobutyl, phenethyl, and allyl units, all proceeded well and
provided the desired products 7ga−la in 82−87% yield. In
addition, when a variety of aromatic, heteroaromatic, and
aliphatic aldehydes were applied to the process with allene 6f,
high yields of the new functionalized allylic ketones 7ff−fo
were observed with high efficiency.
Scheme 7. Proposed Mechanism
To illustrate the synthetic potential of the newly developed
one-pot Oppenauer oxidation, we applied the α-quaternary-
substituted ketone product 7fa to the synthesis of a pyrazoline
derivative, which can serve as a valuable building block for
biologically active molecules (Scheme 5).23 Allylic ketone 7fa
Scheme 5. Transformation of Ketone 7fa
ment of a new strategy for synthesizing α-quaternary-
substituted allylic ketones with high efficiency and regiose-
lectivity through one-pot Cu-catalyzed hydroalumination/
allylation/Oppenauer oxidation under mild reaction condi-
tions. Further research on the scope of this new copper-
catalyzed hydride addition and mechanistic insights is ongoing
in our laboratory.
was converted to hydrazone 8 in a reaction with phenyl-
hydrazine in the presence of acetic acid. When hydrazone 8
was treated with aniline and Cu(OAc)2, copper-mediated
diamination smoothly proceeded to give pyrazoline 9 in 80%
yield.
This method was also capable of the one-pot gram-scale
synthesis of α-substituted allylic ketones (Scheme 6). The
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01876.
■
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Scheme 6. Gram-Scale Reactions
Experimental details, characterization data, and NMR
spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
■
reactions of more than 1 g of allenes 1k and 6f with aldehyde
3a were efficiently carried out, affording the corresponding
allylic ketones 5ka and 7fa in 81 and 80% yield, respectively.
These results highlight the potential of this mild and simple
one-pot process in significant practical applications.
Yunmi Lee − Department of Chemistry, Kwangwoon University,
Seoul 01897, Republic of Korea; orcid.org/0000-0003-
1315-4001; Email: ymlee@kw.ac.kr
Authors
On the basis of our observations, we propose a plausible
mechanism in Scheme 7. The reaction of IPrCuCl with
DIBAL-H would generate copper hydride species A,20 which
would insert into allene 1 to form allylcopper complex B via
hydrocupration. The allylcopper intermediate B would react
with DIBAL-H to regenerate IPrCuH species A with
concomitant release of the allylaluminum intermediate C.
Subsequently, allylaluminum reagent C would regioselectively
attack aldehyde 3 to give the corresponding aluminum alkoxide
D. Then another aldehyde 3 would coordinate to alkoxide D,
leading to the six-membered-ring transition state E, which
would then undergo hydride transfer to give the final ketone
product F and alcohol G after protonation.
In conclusion, we have developed an efficient method for the
preparation of functionalized allylaluminum reagents from
commercially available and readily accessible DIBAL-H and
allenes through copper-catalyzed hydride addition. The use of
the NHC ligand was key for achieving this catalytic
transformation. The catalytic process allowed for the develop-
Sangback Lee − Department of Chemistry, Kwangwoon
University, Seoul 01897, Republic of Korea
Sanghyun Lee − Department of Chemistry, Kwangwoon
University, Seoul 01897, Republic of Korea
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01876
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Mr. Karthick Govindan from Kwangwoon University
for experimental assistance. This research was supported by the
Basic Science Research Program through the National
Research Foundation of Korea (NRF) (NRF-
2017R1A2B4008310 and 2020R1A2C1005817). This work
was also partially supported by a research grant from
Kwangwoon University in 2019.
D
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Letter
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