Iridium-catalyzed alkenyl C-H allylation using conjugated dienes

Article abstract of DOI:10.1039/c9cc04419a

An iridium-catalyzed C-H allylation of acrylamides with conjugated dienes was developed, using NH-Ts amide as the directing group. The ligand- and additive-free protocol provided a convenient and atom economic synthesis of branched 1,4-diene skeletons, enabling the tolerance of a wide scope of functionalities such as OMe, F, Cl, Br and CF₃. The utility of this protocol is also demonstrated by a preparative scale, as well as C-H functionalization of artemisic amide. Furthermore, NH-Ts amide was efficiently removed by methylation and hydrolysis procedures to provide 1,4-dienoic acid.

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Iridium-catalyzed alkenyl C–H allylation using
conjugated dienes†
Cite this:DOI: 10.1039/c9cc04419a
Liangyao Xu, Keke Meng, Jian Zhang, * Yaling Sun, Xiunan Lu, Tingyan Li,
Yan Jiang and Guofu Zhong*
Received 9th June 2019,
Accepted 18th July 2019
DOI: 10.1039/c9cc04419a
rsc.li/chemcomm
An iridium-catalyzed C–H allylation of acrylamides with conjugated skipped (Z/E)-dienes with the exclusive formation of a cis double
dienes was developed, using NH-Ts amide as the directing group. bond.10c However, these protocols employed various olefins
The ligand- and additive-free protocol provided a convenient and bearing allylic surrogates to be proceeded via the b-O or b-X
atom economic synthesis of branched 1,4-diene skeletons, enabling elimination over b-H elimination with the liberation of by-
the tolerance of a wide scope of functionalities such as OMe, F, Cl, products such as acetic acid and CO2. Moreover, the substrate
Br and CF3. The utility of this protocol is also demonstrated by a scope of these methods usually suffered from poor selectivity
preparative scale, as well as C–H functionalization of artemisic amide. and/or limited reactivity when applied to the synthesis of
Furthermore, NH-Ts amide was efficiently removed by methylation branched and multi-substituted skipped dienes.
and hydrolysis procedures to provide 1,4-dienoic acid.
Conjugated dienes (1,3-dienes) are widely recognized synthons
in synthetic chemistry,9,12 and there have been examples of
Skipped dienes (1,4-dienes) represent ubiquitous components allylation using 1,3-dienes.13 However, very limited examples
in countless biologically active molecules such as jerangolid, have been reported on direct alkenyl C–H allylation using
ripostatin and biselyngbyolide analogues (Scheme 1).1 They are conjugated dienes with atomic economy, providing site- and
also versatile synthetic building blocks in synthetic organic stereo-selective preparation of skipped dienes.14 Recently, the Chirik
chemistry.2 Consequently, many powerful synthetic methods group reported a selective [1,4]-hydrovinylation of 1,3-dienes
have been developed for the construction of 1,4-dienes,3–7 including with inactivated olefins enabled by iron diimine catalysts.14a
catalytic cross-couplings,4 ene reactions,5 olefinations,6 as well RajanBabu also demonstrated a cobalt-catalyzed enantioselective
as Morita–Baylis–Hillman reactions.7 Despite their effectiveness, hetero-dimerization of acrylates and 1,3-dienes.14b
developing regio-/stereo-selective, practical and atom economic
synthesis of structurally diverse 1,4-dienes from readily available aromatic C–H functionalizations,15 examples of alkenyl C–H
starting materials still remains a challenge.
functionalization continue to be scarce,10c,16 presumably due to
Compared to the remarkable progress made in Ir-catalyzed
Recently, direct C–H allylation of aromatic or alkyl C–H bonds the lability of alkenes under highly active iridium catalysis.
catalyzed by transition metal catalysis has attracted much attention Furthermore, competitive C(alkenyl)–H and C(allyl)–H cleavage
in terms of synthetic and atom efficiency.8 Remarkable progress sites, as well as ready alkene isomerisation, can make alkenyl
has been made in alkenyl C–H alkenylation via metallocycle
intermediates, leading to valuable 1,3-diene derivatives.9 In con-
trast, there are limited examples of directed alkenyl C–H allylation
to provide versatile 1,4-dienes,10 presumably due to thermodyna-
mically controlled olefin isomerization.11 The Loh group developed
a rhodium(III)-catalyzed C–H allylation of electron-deficient alkenes
with allyl acetates.10a You and co-workers described the direct
allylation of o-amino styrenes with allylic carbonates, affording
College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal
University, Hangzhou 311121, China. E-mail: zhangjian@hznu.edu.cn,
zgf@hznu.edu.cn
† Electronic supplementary information (ESI) available: Detailed experimental
Scheme 1 Alkenyl C–H allylation by iridium catalysis.
procedures and analytical data. See DOI: 10.1039/c9cc04419a
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Table 2 Substrate scopea
C–H allylation a significant challenge.11,15,16 Given the importance
of 1,4-dienes and our continuous interest in chelation-assisted
alkenyl C–H functionalization,9e–h herein, we focus on the iridium-
catalyzed alkenyl C–H allylation using conjugated dienes to
selectively construct branched skipped dienes (Scheme 1).
The initial optimization experiments were performed with
N-Ts acrylamide (Ts = p-toluenesulfonyl, 1a) and 1-phenyl-1,3-
butadiene (2a), catalyzed by various iridium complexes. It is
supposed that the deprotonation of N-sulfonyl acrylamide
bearing an acidic N–H bond with the iridium complex gives
an amidoiridium(I) species, serving as a key intermediate for the
subsequent C–H activation and alkene migratory insertion.15n–p
Various iridium complexes were examined using methanol as
a solvent. Although [IrCp*Cl2]2 exhibited no catalytic activity,
[IrCl(COD)]2 led to 3aa in 54% yield (Table 1, entries 1 and 2).
Next, other iridium complexes bearing a 1,5-cyclooctadiene (COD)
ligand were examined. Much to our delight, both [Ir(cod)]BF4 and
[IrOMe(cod)]2 greatly promoted the reaction, leading to 3aa in 83%
and 90% yields respectively (entries 3 and 4). Other representative
solvents such as toluene, DCE, MeCN, dioxane, DME, and hexane
could not further improve the reaction with [IrOMe(cod)]2 (entries
5–10). Notably, cross-coupling even proceeded in water, albeit with
reduced yield (entry 11). Decreasing the amount of [IrOMe(cod)]2 to
1.0 mol% also led to satisfactory results with longer reaction time
(entry 12). In contrast, neither [RhCp*Cl2]2 nor [Ru( p-cymene)Cl2]2
exhibited catalytic reactivity, although both of them were robust in
C–H functionalizations (entries 13 and 14).9 A palladium complex
such as Pd(OAc)2 also showed no efficacy (entry 15).
With the optimized catalytic conditions in hand, we next
examined the scope of this amide directed alkenyl C–H allyla-
tion reaction (Table 2). Firstly, representative aliphatic and
a
Unless otherwise noted, the reactions were carried out using acryl-
amide 1 (0.24 mmol), diene 2 (0.2 mmol), [IrOMe(cod)]2 (5 mol%), in
MeOH (0.2 M) at 70 1C for 16 h under an argon atmosphere (1 atm). The
yields are isolated yields.
Table 1 Optimization of catalytic conditionsa
aromatic group tethered a- and/or b-substituted acrylamides
1a–1n were investigated. Installation of long alkyl chains to the
a-position of acrylamides still led to excellent yields (3ba and
3ca). Benzyl substituted acrylamide 1d reacted successfully to
provide 3da in 74% yield, and no olefin isomerization was
observed. Aromatic acrylamides bearing sensitive F, Cl, CF3,
and OMe gave the corresponding products in 50–80% yields,
thus exhibiting good functional-group compatibility (3ea–3ja).
While angelic acid derived amide 1k led to 58% yield, acrylamide
embedded with a cyclopentenyl or cyclohexenyl unit produced 3la
or 3ma in excellent yield. However, b-substituted acrylamide 1n
showed limited reactivity, with the formation of an inseparable
conjugated diene by olefin isomerization (3na and 3na0). Differ-
ently N-substituted acrylamides were also investigated. The reac-
tion of N-Ms substituted acrylamide (Ms = methysulfonyl, 1o)
that was converted smoothly provides 3oa in 83% yield. Not only
N-sulfonylamides but also some other amides bearing acidic N–H
bonds were capable of undergoing the C–H allylation. Acryl-
amides bearing NH-OMe were efficiently converted, leading to
methyl or phenyl 1,4-dienes in excellent yield (3pa and 3qa). In
contrast, NH-Me substituted acrylamide 1r bearing a less acidic
N–H bond provided the allylation product in only 17% yield due
Entry
Catalyst
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
[IrCp*Cl2]2
[IrCl(cod)]2
MeOH
MeOH
MeOH
MeOH
Toluene
DCE
MeCN
Dioxane
DME
Hexane
H2O
MeOH
MeOH
MeOH
MeOH
0
54
83
90
89
79
69
84
84
54
54
84
0
[Ir(cod)2]BF4
[IrOMe(cod)]2
[IrOMe(cod)]2
[IrOMe(cod)]2
[IrOMe(cod)]2
[IrOMe(cod)]2
[IrOMe(cod)]2
[IrOMe(cod)]2
[IrOMe(cod)]2
[IrOMe(cod)]2
[RhCp*Cl2]2
9
10
11
12c
13
14
15
[Ru(p-cymene)Cl2]2
Pd(OAc)2
0
0
a
Unless otherwise noted, the reactions were carried out using acrylamide
1a (0.24 mmol), diene 2a (0.2 mmol), [Ir] (10 mol%), in a solvent (0.2 M,
1.0 mL) at 70 1C for 16 h under an argon atmosphere (1 atm). The yields
indicated in the table are isolated yields. 1.0 mol% [IrOMe(cod)]2 was
used, 48 h. DCE = 1,2-dichloroethane; DME = 1,2-dimethoxyethane.
b
c
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to low reactivity (3ra). These results exhibited the key effect of
The scalability of the process is demonstrated by the allylation of
acrylamide 1a with diene 2a on a gram scale, which produced 3aa in
acidic N–H bonds in the catalytic C–H allylation reaction.17
A variety of conjugated dienes were examined to give the 90% yield using only a 2.5 mol% Ir-catalyst (Scheme 4a). The amide
corresponding 1,4-diene products. Olefinic C–H allylation group can be conveniently removed by methylation and then hydro-
using aliphatic diene 2b still proceeded well, giving 3ab in lysis procedures, providing carboxylic acid 4 without any erosion of
69% yield, without the formation of any other isomers by olefin the Z/E configuration (Scheme 4b). A particularly useful application
migration.11 The reactions of 1-aryl-1,3-butadienes bearing F, of this mild protocol is in the late-stage C–H functionalization of
Br and OMe led to products 3ac–3af in 89–94% yields, regardless bioactive molecules such as artemisic acid derived amides, leading
of their electron-withdrawing or electron-donating properties. to alkenylation analogue 3sa in good yield (Scheme 4c). So, we
Significantly, the protocol was extended to heterocycles such as expect our methodology to provide valuable opportunities to accele-
furan, which provided 3ag in good yield. Introduction of a large rate structure–activity relationship studies in drug discovery.
aromatic ring such as anthracene into the diene also led to 3ah in
The possible catalytic cycle is postulated as illustrated in
78% yield. The reaction of an internal diene 2i proceeded with Scheme 5. A methoxoiridium complex reacts with acrylamide
high regioselectivity to give 3ai, albeit with lower yield. Notably, 1 to form amidoiridium(I) species A. Oxidative addition of a
branched butadienes such as 2-methyl-1-phenyl-1,3-butadiene still vicinal C(alkenyl)–H bond to iridium gives hydridoiridium(III)
converted well to give 3aj in moderate yield. Aromatic acrylamide intermediate B, which reacts with 1,3-diene 2 to generate
1e also gave the products 3ec and 3ef in excellent yields.18
p-allyliridium(III) species C by a branch-selective alkene insertion.
Considering the remarkable catalytic efficacy of the iridium- Irreversible reductive elimination from allyliridium C and ligand
catalyzed cross-couplings, we attempted to gain some preliminary exchange by acrylamide 1 give the corresponding branched 1,4-
understanding of the reaction mechanisms. Competition experi- diene 3 and regenerate amidoiridium species A. On the other
ments between acrylamides 1j and 1g revealed the more electron- hand, a linear selective alkene insertion into the Ir–H bond
deficient acrylamide to be converted preferentially, hence rendering proceeds to form E which does not undergo reductive elimina-
an electrophilic C–H bond activation less likely to be operative tion and returns to B via b-hydrogen elimination. The regioselec-
(Scheme 2a). These results are consistent with prior observations tivity is controlled by a combination of electronic and steric
in ruthenium- or rhodium-catalyzed C–H alkenylation.9 Intermo- effects in the migratory insertion of dienes into the Ir–C(alkenyl)
lecular competition experiments between dienes 2f and 2c high- bond.15,20 An alternative reaction pathway involves alkene inser-
lighted the electron-deficient one to be more reactive (Scheme 2b). tion into an Ir–C bond (metallocycle intermediate F) and reduc-
The results of deuterium-labeling experiments provided us tive elimination of the C–H bond was less likely to occur.21
with mechanistic insights (Scheme 3). If substrate 1e was
In conclusion, we have developed an iridium-catalyzed alkenyl
treated with CD3OD under iridium catalysis, deuterium scram- C–H allylation reaction between acrylamides and conjugated
bling was observed at both cis and trans alkenyl C–H bonds dienes. With the assistance of a NH-Ts amide, this allylation
(Scheme 3a). Reaction of 1e with diene 2a in the presence of process enables mild and atom economic preparation of branched
[Ir(OMe)(cod)]2 (5 mol%) in CD3OD (0.2 M) at 70 1C for 11 min 1,4-dienes with excellent regio- and stereo-selectivities. The addi-
gave product 3ea in 42% yield, where deuterium incorporation tive- and ligand-free protocol is applicable to a broad range of
was observed for 3ea as well as recovered 1e and 2a (Scheme 3b).19 acrylamides and conjugated diene substrates, displaying a wide
These results indicated C–H activation and hydro-metalation functional group tolerance. The utility of this protocol is also
steps to be fast and reversible, and the reductive elimination step demonstrated by the preparative scale, as well as late-stage C–H
determines the regioselectivity.15
functionalization of artemisic amides. Furthermore, dienamide
was efficiently converted to 1,4-dienoic acid by removal of NH-Ts
amide under methylation/hydrolysis conditions, indicating the
great synthetic value of this protocol.
We gratefully acknowledge the National Natural Science
Foundation of China (NSFC) (21502037 and 21672048), the
Scheme 2 Intermolecular competition experiments.
Scheme 3 Deuterium-labeled experiments.
Scheme 4 Synthetic applications.
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Scheme 5 Plausible mechanism.
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 Engineering for financial support.
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Conflicts of interest
There are no conflicts to declare.
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These results also support a direct addition of Ir–H species to alkene
by the p-allyliridium(III) intermediate.
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