Palladium-NHC-Catalyzed Allylic Alkylation of Pronucleophiles with Alkynes

Article abstract of DOI:10.1021/acs.orglett.9b02937

The palladium-N-heterocyclic carbene (NHC)-catalyzed allylic alkylation of various pronucleophiles with alkynes has been accomplished under mild conditions. The protocol exhibits broad functional group compatibility and high atom economy. Moreover, the ca

Full text of DOI:10.1021/acs.orglett.9b02937

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Palladium−NHC-Catalyzed Allylic Alkylation of Pronucleophiles with
Alkynes
Wei Ren,† Qian-Ming Zuo,† Yan-Ning Niu,‡ and Shang-Dong Yang*,†
†State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
‡Department of Teaching and Research, Nanjing Forestry University, Huaian 223003, P. R. China
S
* Supporting Information
ABSTRACT: The palladium−N-heterocyclic carbene (NHC)-catalyzed allylic alkylation of various pronucleophiles with
alkynes has been accomplished under mild conditions. The protocol exhibits broad functional group compatibility and high
atom economy. Moreover, the catalytic process avoids the use of external oxidants and acid as additives.
he allylic unit has always attracted the research interest of
construction of synthetically useful N,O-acetal derivatives are
Tchemists because of its diverse range of transformations performed (Scheme 1, A); however, regrettably, azlactones
in synthetic chemistry. Consequently, a great deal of effort has
been devoted to applying distinct strategies to introduce the
allylic moiety into various molecules over the past decades.1
Among these strategies, transition-metal-catalyzed allylic
substitution reactions2 and direct allyl C−H oxidation
reactions3 have evolved into effective synthetic tools that
have been widely applied in organic synthesis. Although
tremendous progress has been achieved, the efficiency of these
strategies is still limited to prefunctionalization of substrates1c,4
or the use of an external equivalent of oxidant is required.3e,5
Therefore, the development of appropriate methodologies that
avoid these limitations is particularly important. As an ideal
electrophilic π-allyl precursor, alkynes have attracted the
attention of chemists and they have been widely used for sp3
C−X (X = C, N, O, etc.) coupling reactions. This is because of
the high atom economy of the approach, and because the
preinstallation of an allylic leaving group or the use of external
equivalent oxidants is avoided.6 In the past decade, substantial
efforts have been made to develop highly selective allylic C−H
functionalization of alkynes. To date, numerous excellent
results have been achieved, including hydroamination,7
oxygenation,8 alkylation,9 arylation,10 dearomatization,11 and
others.12 Among these, azlactone derivatives as one type of
excellent nucleophile were employed in the allyl alkylation of
alkynes by Breit’s group in 2017.13 In the reaction, rhodium-
catalyzed regioselective addition of azlactones to internal
alkynes and subsequent aza-Cope rearrangement for the
Scheme 1. Alkynes Involved in Allylation of Azlactones
that were substituted by an alkyl group at C-4 position did not
work in the reaction. Herein, we report on the palladium−N-
heterocyclic carbene (NHC)-catalyzed allylic alkylation of
azlactone derivatives with alkynes to afford the C4-allyled
products (Scheme 1, B). Our method provides a useful
complement to the rhodium-catalyzed allylation, and the C-4
alkyl substituted azlactones exhibit favorable reaction activity.
Moreover, the other pronucleophiles such as α-cyanoketone,
oxindole, α-cyanophosphonate, and β-naphthol also suitable
for this transformation. In particular, NHCs, as a type of
Received: August 18, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02937
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Letter
unique ligand, are used for the first time in the catalytic allyl
alkylation of alkynes, wherein they exhibit exclusive activity
and regioselectivity.
IPr·HCl (10 mol %), and KHCO3 (2.0 equiv) in toluene at
110 °C for 11 h.
With the optimized conditions in hand, we then examined
the substrate scope of the reaction with respect to the alkynes;
the results are summarized in Scheme 2. Thus, alkynes bearing
In the initial study, we selected prop-1-yn-1-ylbenzene (1a)
and 4-isopropyl-2-phenyloxazol-5(4H)-one (2a) as model
substrates to optimize the reaction conditions. In the presence
of IPr·HCl, KHCO3, and PhCO2H as the additive, a series of
palladium catalysts were examined in toluene at 110 °C under
argon for 11 h; the results revealed that Pd(OAc)2 was the best
catalyst, giving the allylic alkylated product 3aa in 88% yield
(see Table S1 for details). Control experiments indicated that
the precatalyst, N-heterocyclic carbene ligand, and base were
essential to this transformation; PhCO2H was not required
(see Table S1 for details). Encouraged by these results, we
further screened the reaction parameters including solvents,
ligand, base, and others factors under an argon atmosphere
extensively, in the absence of benzoic acid as the additive.
Screening of bases showed that KHCO3 proved to be most
efficient, giving 3aa in 88% yield (Table 1, entries 1−4; see
Scheme 2. Substrate Scope of of the Reaction with Respect
a b
,
to Alkynes
a b
,
Table 1. Optimization of Reaction Conditions
Entry
Ligand
Base
Solvent
Toluene
Yield [%]
1
L1
K2CO3
83
2
3
4
5
6
7
8
9
10
11
12
13
14
15
L1
L1
L1
L2
Na2CO3
K3PO4
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Chlorobenzene
Dioxane
DME
55
50
88
83
22
62
trace
19
trace
86
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
L3
PPh3
BINAP
DPPP
L1
L1
L1
L1
L1
L1
82
DCE
Toluene
Toluene
trace
c
67
d
78
a
Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), Pd(OAc)2 (10
mol %), ligand (10 mol %), and base (2.0 equiv) in solvent (2.5 mL)
b
c
at 110 °C under Ar for 11 h. Isolated yield. Pd(OAc)2 (5 mol %),
IPr·HCl (5 mol %). 6 h.
d
a
Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), Pd(OAc)2 (10
mol %), IPr·HCl (10 mol %), and KHCO3 (2.0 equiv) in toluene (2.5
b
mL) at 110 °C under Ar for 11 h. Isolated yields.
either electron-donating or electron-withdrawing substituents
on the phenyl ring, such as methoxyl, tert-butyl, methyl, diethyl
(methyl) phosphate, fluoro, chloro, and ester groups, were
compatible with the catalytic system and proceeded smoothly
to give the corresponding allylation products 3aa−ka in good
to excellent yields. When the phenyl group was displaced with
a naphthalene moiety, the linear allylation product 3la was also
isolated in 83% yield. As another reaction partner, 5-(prop-1-
yn-1-yl)benzo[d][1,3]dioxole could also be smoothly con-
verted into the desired product 3ma in 88% yield. Moreover,
heteroaromatic alkynes such as 2-thiophene and 2-pyridine
were well tolerated in this transformation. To our delight,
skipped enynes and estrone-derived alkynes were also suitable
substrates for the allylation under the optimized reaction
Table S2 for details). A range of NHCs were then tested, and
the results showed that L1 was the best while indicating the
importance of the electronic effect of the NHC ligands. We
next screened the phosphine ligands and found that the use of
PPh3 could produce 3aa in 62% yield, whereas both BINAP
and DPPP exhibited inferior catalytic activity (Table 1, entries
7−9). Subsequently, other solvents such as chlorobenzene,
dioxane, DME, and DCE were also examined, and toluene was
established as the best choice (Table 1, entries 10−13). After
screening the reaction time and catalyst loadings (Table 1
entries 14 and 15; see Table S2 for details), the optimized
reaction conditions were established as Pd(OAc)2 (10 mol %),
B
DOI: 10.1021/acs.orglett.9b02937
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Letter
conditions, which afforded the desired products 3pa and 3qa
in moderate yields. Notably, 2-butynoic acid derivatives were
also well tolerated in the practical allylation reaction, affording
the desired products 3ra−va in moderate yields.
We then examined the generality of the reaction with
pronucleophiles under the optimized reaction conditions; the
results are shown in Scheme 3. The substrate scope of the
corresponding N-protected amide 5 in 90% yield (Scheme 4a).
Moreover, 3aa was transformed into N-(5-allyl-5-hydroxy-4-
Scheme 4. Derivatization of Reaction Products
Scheme 3. Substrate Scope of the Reaction with Respect to
a b
,
Nucleophiles
isopropyl-1-phenylocta-1,7-dien-4-yl)benzamide (6) through
the addition of allylmagnesium bromide to azlactone (Scheme
4a). Next, tert-butyl 2-(((tert-butoxycarbonyl)amino)methyl)-
2,5-diphenylpentanoate (7) could be obtained by performing
the reduction reaction of 4al at room temperature with NiCl2·
6H2O/NaBH4 in methanol and then was efficiently converted
into the all-carbon quaternary β-amino acid derivatives 8 upon
treatment with TFA in DCM (Scheme 4b). On the other
hand, with a basic solution of H2O2, the alkylation product 4ao
could be transformed into the α-monoallylation nitrile
compound 9, which is a very important intermediate14 that
could be further transformed into Boc-protected amine 10,
which included the exhaustive reduction of the olefinic and the
cyano groups (Scheme 4c). The spirocyclic oxindole skeleton
is widely found in pharmaceutical molecules and natural
products, and it exhibits unique chemical characteristics.15
Thus, when ethyl 2-but-ynoate (1r) was used as the substrate,
the allylation reaction with 1-methyl-2-oxoindoline-3-carbon-
itrile (2n) could proceed smoothly and produce the
corresponding product in relatively higher yield than 4an.
Meanwhile, reaction of the oxindole 4rn with NiCl2·6H2O/
NaBH4 furnished the amino oxindole 11 in 70% yield;
subsequent hydrolysis of the ester and intramolecular
condensation reaction of the carboxylic acid with the amine
generated the spiro-fused compound 12, which represents one
of the spirocyclic indole frameworks as a versatile compound
(Scheme 4d).
To illustrate the formation of allene intermediates, we
further investigated the reaction of phenyl allene (1x) with 4-
isopropyl-2-phenyloxazol-5(4H)-one (2a) under the optimized
reaction conditions, and found that the target coupling product
3aa was obtained in 78% yield (Scheme 5). This result implied
that the phenyl allene intermediate may be involved in this
transformation. Based on previous reports9a,16 and on the
experimental results, a plausible mechanism was proposed.
Initially, the active palladium catalyst A is generated in situ
through ligand exchange of Pd(OAc)2 with free IPr·HCl in the
presence of base. Subsequently, a hydridopalladium inter-
mediates B is presumably generated in situ and syn-migratory
insertion of B into alkyne 1a furnished the intermediate C.
Next, phenyl allene D is generated via β-hydrogen elimination
a
Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), Pd(OAc)2 (10
mol %), IPr·HCl (10 mol %), and KHCO3 (2.0 equiv) in toluene (2.5
b
mL) at 110 °C under Ar for 11 h. Isolated yields.
reaction with various azlactones derived from amino acids with
1-phenyl-1-propyne (1a) under the optimal conditions was
first examined. Specifically, azlactones containing methyl (2b),
ethyl (2c), tert-butyl (2d), isobutyl (2e), cyclohexylmethyl
(2f), or benzyl (2g) groups participated in this transformation,
affording the corresponding allyl alkylation products 4ab−ag
in moderate to good yields. In contrast, 2,4-diphenyloxazol-
5(4H)-one (2h) as a substrate only gave the expected product
4ah in 14% yield, probably because of the electric effect
imparted by the phenyl group. Meanwhile, substrates bearing a
cyano group were also compatible with the reaction conditions
and provided 4aj−al in 79−93% yields. Furthermore, oxindole
derivatives were also investigated and delivered the target
products 4am and 4an in 82% and 35% yields, respectively.
When diethyl (cyano(naphthalen-2-yl)methyl)phosphonate
(2o) was used as the substrate, the reaction also proceeded
smoothly to furnish the product 4ao in 79% yield. To our
delight, 1,3-dimethylnaphthalen-2-ol (2p) as the reaction
partner under the optimal reaction conditions gave the
dearomatization product 4ap in 63% yield with excellent
selectivity. Unfortunately, when but-1-yn-1-ylbenzene was
applied in the reaction, the result showed that no product
was obtained.
To demonstrate the utility of the reaction products, a range
of transformations of the functionalized allylation products
were carried out. First, the ring-opening hydrolysis of azlactone
3aa by treatment with 6 N HCl in dioxane produced the
C
DOI: 10.1021/acs.orglett.9b02937
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Scheme 5. Possible Catalytic Cycle
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02937.
Experimental details and characterization data for all
new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*yangshd@lzu.edu.cn
ORCID
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Shang-Dong Yang: 0000-0002-4486-800X
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC (No.
21532001) and International Joint Research Centre for Green
Catalysis and Synthesis, Gansu Provincial Sci. & Tech.
Department (No. 2016B01017).
D
DOI: 10.1021/acs.orglett.9b02937
Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b02937
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