Bidentate auxiliary-directed alkenyl C-H allylation: Via exo-palladacycles: Synthesis of branched 1,4-dienes

Article abstract of DOI:10.1039/c9cc07466j

An alkenyl C-H allylation by an exo-palladacycle intermediate is demonstrated, employing unactivated (Z)-Alkenes and allyl carbonates. With the use of an 8-Aminoquinoline (AQ) derived amide as the directing group, the N,N-bidentate-chelation-Assisted C-H activation protocol proceeded under mild and oxidant-free conditions with excellent selectivity. The utility of this approach is demonstrated by the preparative scale, selective conversion of inseparable Z/E alkenes and ready removal of the amide auxiliary to provide the corresponding ester.

Full text of DOI:10.1039/c9cc07466j

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Bidentate auxiliary-directed alkenyl C–H allylation
via exo-palladacycles: synthesis of branched
1,4-dienes†
Cite this: Chem. Commun., 2019,
55, 13582
Received 23rd September 2019,
Accepted 1st October 2019
Cong Shen, Xiunan Lu, Jian Zhang, * Liyuan Ding, Yaling Sun and Guofu Zhong*
DOI: 10.1039/c9cc07466j
rsc.li/chemcomm
An alkenyl C–H allylation by an exo-palladacycle intermediate is linear skipped dienes (Fig. 1a). Moreover, substrate scope of these
demonstrated, employing unactivated (Z)-alkenes and allyl carbo- methods are generally limited to electron biased alkenes and
nates. With the use of an 8-aminoquinoline (AQ) derived amide as suffer from poor reactivity and/or selectivity when applied to the
the directing group, the N,N-bidentate-chelation-assisted C–H synthesis of branched and multi-substituted 1,4-dienes.
activation protocol proceeded under mild and oxidant-free condi-
Alkenyl C–H functionalizations by endo-metallocycle inter-
tions with excellent selectivity. The utility of this approach is mediates have attracted much attention.9–17 However, there are
demonstrated by the preparative scale, selective conversion of still very limited reports on alkenyl C–H activation by the for-
inseparable Z/E alkenes and ready removal of the amide auxiliary mation of disfavored exo-metallocycles (double bond outside the
to provide the corresponding ester.
metallocycle) due to increased energy requirement.18,19 The Engle
group demonstrated a N,N-bidentate chelation-assisted alkenyl
1,4-Dienes are ubiquitous structural motifs in numerous pharma- C–H alkenylation for the synthesis of 1,3-dienes.19c Very recently,
ceutical molecules, and are also widely used as synthetic building Carreira and co-workers disclosed an elegant C–H iodination of
blocks.1 Many methods have been demonstrated for the efficient unactivated alkenes, using picolinamide as the directing group
preparation of 1,4-dienes,2–6 including cross-coupling3 and ene (Fig. 1b).19d Given the importance of 1,4-dienes and our con-
reactions.4 Recently, several elegant methods have been developed tinuous interests in alkenyl C–H functionalization,9e–i herein,
for catalytic cross-coupling.7 Rajan-Babu and co-workers demon- we focus on C–H allylation using sterically and electronically
strated an enantioselective hetero-dimerization by cobalt catalysis.7a unbiased alkenes and allyl carbonates to selectively construct
The Trost group developed a method for the synthesis of branched skipped dienes, which represents the first example
1,3,6-trienes by sequential coupling with two alkynes.7c How- on chelation-assisted olefinic C–H allylation by exo-palladation
ever, atom/step economic and selective synthesis of structurally (Fig. 1c).
diverse skipped dienes by C–H activation from readily available
chemicals would be highly attractive.
Chelation-assisted C–H allylation by transition metal cata-
lysis has attracted much attention in terms of synthetic and
atom efficiency.8 As we know, while great efforts have been made
on vinylic C–H olefination via cyclometallation to produce
conjugated dienes,9 limited examples have been developed on
directing-group-assisted olefinic C–H allylation to prepare
skipped dienes,10 presumably due to the readily occurring olefin
isomerization.11 In particular, the reported protocols are restricted
to chelation-assisted vicinal C(alkenyl)–H activation via endo-
metallocycle (double bond inside the metallocycle), leading to
College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal
University, Hangzhou 311121, China. E-mail: zhangjian@hznu.edu.cn,
zgf@hznu.edu.cn
Fig. 1 Alkenyl C–H functionalizations via endo-metallocycles and exo-
† Electronic supplementary information (ESI) available: Detailed experimental
metallocycles.
procedures analytical data. See DOI: 10.1039/c9cc07466j
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Table 1 Optimization of alkenyl C–H allylationa
Table 2 Substrate scopea,b
Entry
Catalyst
Additive
Solvent
Yieldb (%)
1
2
3
4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
—
DMSO
MeOH
MeCN
Dioxane
48
58
40
25
13
80
39
47
52
38
0
5
DCE
6c
DMSO/MeOH
DMSO/MeOH
DMSO/MeOH
DMSO/MeOH
DMSO/MeOH
DMSO/MeOH
DMSO/MeOH
7c
8c
AcOH
9c,d
10c,d
11c
12c,e
Ac-Phe-OH
Ac-Ile-OH
PivOH
PivOH
Pd(OAc)2
49
a
Conditions for C–H allylation: 1a (0.1 mmol), 2a (0.4 mmol), [Pd]
(20 mol%), additive (2.0 equiv.), in a solvent (0.1 M) at 40 1C for 16 h.
b
c
d
Isolated yields. DMSO/MeOH = 1/2 (v/v). Mono-Protected amino
e
acid used (50 mol%). Pd(OAc)2 (10 mol %) used. DMSO = dimethyl
sulfoxide; DCE = 1,2-dichloroethane.
Bidentate directing groups have attracted much attention
because they offer the possibility of developing new types of
transformations of C–H bonds.20 Encouraged by Daugulis’s N,N-
bidentate chelation strategy,20a we assumed that 8-aminoquinoline
(AQ) directing group may function as a suitable directing group,
owing to their ability to facilitate the formation of thermodynami-
cally stable 6-membered alkenyl-palladacycle through a concerted
metalation-deprotonation (CMD) pathway.21 So, we commenced
the investigation by surveying the reaction conditions using
(Z)-5-phenyl-4-pentenamide 1a as the model substrate and allyl
carbonate 2a as the coupling partner (Table 1). We found that C–H
a
Conditions for C–H allylation: 1 (0.1 mmol), 2 (0.4 mmol), Pd(OAc)2
(20 mol %), pivalic acid (2.0 equiv.), in DMSO/MeOH = 1/2 (v/v) (0.1 M),
at 40 1C for 16 h. Isolated yields. At 100 1C for 16 h.
b
c
allylation via a six-membered exo-palladacycle took place in the CF3-, and OMe-substituents were all well tolerated to give the
presence of Pd(OAc)2 complex and pivalic acid in DMSO, leading to corresponding products in 62-93% yields, thus showing good func-
desired 1,4-diene product 3aa in 48% yield (entry 1). Replacing tional compatibility (3aa–3ga). Pentenamides bearing meta- and
DMSO with MeOH, MeCN, dioxane or DCE did not improve the ortho-substituted phenyl group were also well converted, regardless
reaction yield (entries 2–5). To our delight, the usage of a mixed of its electron-donating and withdrawing properties (3ha–3ka).
solvent (DMSO/MeOH = 1/2) led to 80% yield and excellent Notably, heterocycles such as furan and even thiophene were well
selectivity (entry 6). Notably, the other potential products generated tolerated in the reaction (3la and 3ma). In contrast, N-Me pyrrole 1n
from competitive allylic and/or alkenyl C–H activation as well as led to 59% yield with decreased stereo-selectivity (3na). Installation
1,3-diene by olefin isomerization were not observed. Further opti- of a larger aromatic ring such as naphthalene to the alkene substrate
mization revealed that a carboxylic acid promoter was crucial, and also converted well (3oa). In addition, incorporation of alkyl groups,
acetic acid was found to be much less beneficial (entries 7 and 8). such as methyl, ethyl, pentyl, cyclohexyl and even bulky tert-butyl
Mono-Protected amino acid (MPAA) such as Ac-Phe-OH or provided successful C–H allylations (3pa–3ta, 74–85% yields). Cyclic
Ac-Ile-OH was also tested, but neither of them improved the substrates embedded with a cyclohexenyl or cyclopentene moiety
reaction (entries 9 and 10). Other palladium salts such as PdCl2 reacted well with excellent selectivity (3ua and 3va). However,
led to no products, exhibiting the key role of carboxylate in terminal alkene 1w afforded a relatively low reactivity even at an
such aminoquinoline amide directed C(alkenyl)–H activation elevated temperature, presumably due to its decreased nucleo-
(entry 11). Lower catalyst loading (10 mol%) led to 3aa in philicity towards transition metals (3wa). Furthermore, both
moderate yields (entry 12).
tri-substituted substrate 1x and E-alkene 1y exhibited no reac-
With the optimized reaction conditions in hand, we next exam- tivity, presumably due to the increased steric repulsion in the
ined the scope and limitation of alkenyl amides 1 (Table 2). Allyla- cyclometallation/olefin insertion pathway. Finally, (Z)-6-ethyl-5-
tion of differently substituted aromatic pentenamide 1 bearing hexenamide 1z showed no reactivity under optimal conditions.
8-aminoquinoline was sufficiently investigated. para-F-, Cl-, Br-, These results show that the reaction is highly sensitive to the
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electronic and steric effects, and a six-membered exo-cyclic
palladium species generated from N,N-bidentate-chelation-
assisted alkenyl C–H bond activation could be proposed as
the possible intermediate (also see eqn (1) and (2)).
Encouraged by these results, we examined highly substituted
allylic carbonates 2b–2f under the optimal conditions. The
g-aromatic- or aliphatic-substituted allylic carbonate 2b–2d were
smoothly converted to give the skipped dienes in moderate to
good yields, albeit with decreased stereo-selectivity at newly
formed allyl moieties (3pb–3pd, 40–82% yield). Notably, these C–H
allylation reactions proceeded readily with complete a-selectivity,
and no migration of the double bond was observed.11 However, the
substitution on the a- or b-carbon of allyl carbonates greatly retarded
the reaction, which may be attributed to the steric interaction in the
alkene insertion step (2e and 2f).
Scheme 1 Competition experiments.
(1)
(2)
Scheme 2 Deuterium labelled experiments.
When alkene 1a was subjected to the corresponding optimal
conditions using mixed DMSO-d6 and CD3OD, significant ole-
finic H/D exchanges were observed, thereby implicating rever-
sible cyclometallation modes (Scheme 2a). However, internal
alkene 1a also underwent competitive Z/E isomerization, and
the absence of deuterium incorporation excluded the possible
p-allylpalladium(II) formation.22 We also performed the deuterium
incorporation experiment in the presence of allyl carbonate 2a.
The lack of D-incorporation to product 3aa and recovered acryl-
amide 1b suggested that the allylation step is sufficiently fast to
outcompete the reversibility of the C(alkenyl)–H activation step
(Scheme 2b). Finally, kinetic isotope effect (KIE) values deter-
mined via an intermolecular competition experiment suggested
the olefinic C–H cleavage to be the rate-determining step in the
C–H allylation reactions (Scheme 2c).23
Some other bidentate- and monodentate-auxiliaries were
also examined. To our delight, Daugulis’s picolinamide (PA)
directing group facilitated analogous C(alkenyl)–H allylation of
bishomoallylic amine substrate under optimal conditions (5a,
52% yield), showing the robustness of the protocol (eqn (1)). In
contrast, simple amide 6 exhibited no reactivity, with most of
the starting material being recovered (eqn (2)).
Further, we examined the reactivity of different allyl deriva-
tives (Table 3). While allyl iodide 2g and allyl phosphate 2i
exhibited limited reactivity towards amide 1q, allyl alcohol 2h
was totally inert. In contrast, other allyl carboxylic esters,
such as allyl acetate 2j and allyl hexanoate 2k led to moderate
yields, albeit with significant double bond migration to form
conjugated dienes.
The cross-coupling reactions can be scaled up to gram scale
with lower catalyst loading without decrease in the product
yield and selectivity, as illustrated by the preparation of 3pa
to demonstrate the robustness of the protocol (Scheme 3a).
Given the good reactivity of the bidentate-chelation-assisted
alkenyl C–H allylation, we attempted to gain preliminary
understanding of the reaction mechanisms. Competition experi-
ments between amides 1c and 1g highlighted that electron-
deficient alkenes are more reactive, exhibiting an electrophilic
C–H bond activation (Scheme 1a).9 Intermolecular competition
experiments between alkene 1a and 1r revealed styrene to be
converted preferentially (Scheme 1b).
Table 3 Scope of different allyl derivatives
Diene Diene
3 (%) 4 (%)
Entry
R
1
2
3
4
5
I (2g)
OH (2h)
OPO(OEt)2 (2i) 30
OAc (2j)
11
0
0
0
0
22
34
28
23
OCOHex (2k)
Scheme 3 Synthetic applications.
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smoothly obtained in 80% yield (Scheme 3c). Also, an inseparable
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ponding alkane 9 via catalytic hydrogenation (Scheme 3d).
In conclusion, we have developed a catalytic method to
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We gratefully acknowledge National Natural Science Founda-
tion of China (NSFC) (21502037 and 21672048), Natural Science
Foundation of Zhejiang Province (ZJNSF) (LY19B020006), and the
Pandeng Plan Foundation of Hangzhou Normal University for
Youth Scholars of Materials, Chemistry and Chemical Engineer-
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