Ir-Catalyzed Intermolecular Branch-Selective Allylic C-H Amidation of Unactivated Terminal Olefins

Article abstract of DOI:10.1021/jacs.9b00237

An efficient method for intermolecular branch-selective allylic C-H amidation has been accomplished via Ir(III) catalysis. The reaction proceeds through initial allylic C-H activation, supported by the isolation and crystallographic characterization of an allyl-Ir(III) intermediate, followed by a subsequent oxidative amidation with readily available dioxazolones as nitrenoid precursors. A diverse range of amides are successfully installed at the branched position of terminal alkenes in good yields and regioselectivities. Importantly, the reaction allows the use of amide-derived nitrenoid precursors avoiding problematic Curtius-type rearrangements.

Full text of DOI:10.1021/jacs.9b00237

Communication
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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Ir-Catalyzed Intermolecular Branch-Selective Allylic C−H Amidation
of Unactivated Terminal Olefins
Honghui Lei and Tomislav Rovis*
Department of Chemistry, Columbia University, New York, New York 10027, United States
S
* Supporting Information
Scheme 1. Catalytic Allylic C−H Amidation
ABSTRACT: An efficient method for intermolecular
branch-selective allylic C−H amidation has been accom-
plished via Ir(III) catalysis. The reaction proceeds through
initial allylic C−H activation, supported by the isolation
and crystallographic characterization of an allyl-Ir(III)
intermediate, followed by a subsequent oxidative
amidation with readily available dioxazolones as nitrenoid
precursors. A diverse range of amides are successfully
installed at the branched position of terminal alkenes in
good yields and regioselectivities. Importantly, the
reaction allows the use of amide-derived nitrenoid
precursors avoiding problematic Curtius-type rearrange-
ments.
ue to the ubiquity of nitrogen-containing functionalities
1
Din both natural and synthetic bioactive molecules, C−N
bond formation reactions are some of the most frequently used
transformations in medicinal chemistry.2 In addition to classic
strategies which usually require prefunctionalization, direct
amination of C−H bonds could dramatically simplify synthetic
routes, providing more straightforward disconnections for
amine synthesis.3 A number of reports have recently
demonstrated chelate-assisted C−H amination; however,
these methods usually employ preinstalled coordinating
directing groups that limit their general application.4 In
contrast, allylic C−H amination only involves a simple alkene
as the “directing group” which is not only prevalent in a variety
of compounds but is also easily manipulated with a diverse
range of robust methods.
recent years,8 offering various choices for the synthesis of linear
allylic amines. On the other hand, the branch-selective allylic
C−H amination of terminal olefins has been achieved through
an intramolecular manner.5c,e,g,i,6b,g,9 Two important excep-
tions are Tambar’s work employing a sulfurdiimide reagent
followed by a Pd-catalyzed asymmetric [2,3]-rearrangement,
and Hartwig’s Pd-catalyzed oxidation followed by asymmetric
Ir-catalyzed amination to deliver the branched amination
product.10 Moreover, the majority of these examples involve
the use of a nitrogen bearing a sulfonyl or carbamate group;
amide-derived nitrenoids are more challenging due to the
potential intervention of a Curtius-type rearrangement.11
Herein, we report an intermolecular Ir(III)-catalyzed branch-
selective allylic C−H amidation of unactivated terminal olefins.
We have recently reported the intramolecular Ir-catalyzed
diamination of alkenyl hydroxamate esters wherein the
transformation was proposed to partially proceed via Ir-
nitrenoid species.12 We wondered whether Ir nitrenoids might
have sufficient lifetime to allow intermolecular aziridination or
amination of feedstock α-olefins. Inspired by previous reports
of group 9 metal-catalyzed C−H amidations,3f together with
our long-term interests in C−H functionalizations,13 the
Current strategies in allylic C−H amination utilize either
C−H insertion by a metal nitrenoid5 or nucleophilic amination
of allyl-metal species generated via C−H activation.6 Despite
these advances, controlling chemoselectivity and regioselectiv-
ity remains challenging in intermolecular allylic C−H
amination reactions. For instance, terminal olefins that react
with a metal-nitrenoid species suffer from poor chemo-
selectivity between aziridination of the alkene and desired
C−H amination.5h,7 (Scheme 1a(i)). An alternative strategy
was independently reported by the White6d and Liu6e groups
in 2008, demonstrating that linear amination could be achieved
under Pd(II) catalysis (Scheme 1a(ii)). The reaction is
proposed to proceed through an allyl-Pd(II) intermediate
that is generated via allylic C−H activation, followed by
reductive quenching with a nitrogen nucleophile at the less
hindered terminal position. Based on these initial reports,
many modifications and improvements have been developed in
Received: January 8, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.9b00237
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
hypothesis was initially tested with [Cp*MCl2]2 as the
precatalysts (M = Rh, Co, Ir), and either sulfonyl azides14 or
dioxazolones15 as nitrene precursors. Cossy and co-workers
have reported that Cp*Rh(III) promotes an intramolecular
allylic C−H amidation reaction proceeding through an allyl-
Rh(III) intermediate.6g We previously demonstrated that the
allylic activation chemistry could be coupled with a C−C bond
formation.16 More recently, Blakey and co-workers reported an
intermolecular allylic C−H amidation of β-substituted styrenes
that exhibits exclusive linear selectivity with allylbenzene.6i In
sharp contrast, the allylic C−H amidation of 1a with
[Cp*RhCl2]2 in combination with methyl dioxazolone 2a
gives the branched amidation product 3a as the major isomer,
albeit with a moderate yield and regioselectivity (Table 1, entry
LiOAc in DCE, this further suggests that excess acetate inhibits
the reaction, supporting our assertion that a monoacetato-Ir is
required. Moreover, tosyl azide was also tested. It was found to
afford moderate yield of branched amidation product with
excellent regioselectivity, when the reaction was heated to 80
°C (entry 11).
Having established the optimal conditions for branch-
selective allylic amidation, we next sought to examine the
scope of this transformation with various terminal alkenes. The
reaction tolerates a broad range of functional groups, giving
rise to various branched amidation products. As shown in
Scheme 2, several commonly used oxygen, nitrogen or arene
containing functional groups are compatible, affording
corresponding amidation products in good yields and
regioselectivities (3b−3h). Even substrates with susceptible
functionalities, like −Br, −CN and −CO2H, participated
smoothly with the standard condition to provide the desired
products (3i−3k). Although a lower yield was observed with
the alkene bearing a carboxylic acid group, the regioselectivity
remains unaffected, giving the branched product 3k predom-
inately. Despite precedent for Ir-catalyzed dehydrogenation of
alkenes bearing homoallylic carbonyl groups under oxidative
conditions,18 exclusive amidation of 4-phenyl-1-butene and
ethyl 4-pentenoate was observed with no evidence of diene
formation (3l, 3m). Although yield and regioselectivity are
moderate with ethyl 4-pentenoate, the alternative conditions
with TsN3 as the nitrene precursor leads to moderate yield and
excellent regioselectivity (3m). Furthermore, substrates with
homoallylic heteroatoms were found to decrease the reactivity.
Moderate conversions and yields were obtained even with
elevated temperature(3n, 3o). Additionally, sterically hindered
allyl-cyclohexane was remarkably well-tolerated (3p). Allylar-
enes with electron-rich (−OMe) and electron-poor (−CF3)
substituents were also examined. It was found that the
regioselectivity decreased from >20:1 to 2.6:1 by changing
the para- substituent from −OMe to −CF3 (3q−3s). Excellent
regioselectivity could be restored with TsN3 as the nitrene
precursor.
The generality of dioxazolone coupling partners were next
explored. Primary, secondary and tertiary alkyl groups on the
dioxazolone are tolerated with no significant decrease in
reactivity (Scheme 3, 4b−4f). Additionally, the cyclopropane
dioxazolone could also be utilized without detectable ring-
opening (4d). Moreover, electron-donating and electron-
withdrawing substituted phenyl dioxazolones are well tolerated
(4g−4j). Even an alkenyl substituted dioxazolone can
participate to afford 4k in moderated yield. In general, these
aryl and alkenyl dioxazolones are slightly less reactive and
require elevated temperature for full conversion. Under all
these reaction conditions, the Curtius/Lossen-type rearrange-
ment is not competitive5i,19 presumably because of a change in
mechanism (vide infra).
a
Table 1. Reaction Development
b
c
entry
catalyst
base
yield
40%
N.D.
B:L
1
2
3
[Cp*RhCl2]2
[Cp*CoCl2]2
[Cp*IrCl2]2
[Cp*IrCl2]2
Cp*Ir(OAc)2
Cp*Ir(OAc)2
[Cp*IrCl2]2
−
LiOAc
LiOAc
LiOAc
LiOAc
−
2.9:1
−
>20:1
>20:1
>20:1
−
d
75% (73%)
24%
e
4
5
f
75%
N.R.
8%
N.R.
f g
6 ,
−
7
8
9
10
11
CsOAc
LiOAc
−
LiOAc
LiOAc
−
−
−
−
[Cp*IrCl2]2
[Cp*IrCl2]2
[Cp*IrCl2]2
15%
g
N.R.
h
i
39%
>20:1
a
Reactions were conducted on a 0.1 mmol scale using 1a (1.0 equiv),
2a (1.5 equiv), [Cp*IrCl2]2 (2.5 mol %), AgNTf2 (15 mol %) and
b
1
LiOAc (20 mol %). Yield of dominant isomer as determined by H
c
NMR. Determined by analysis of 1H NMR of the unpurified reaction
mixture. Isolated yield. AgNTf2 (10 mol %) was used. Cp*Ir-
(OAc)2 (5 mol %) was used. Without AgNTf2. TsN3 instead of 2a
d
e
f
g
h
i
was applied at 80 °C. Yield of the corresponding allylic tosyl amide.
1). We speculated that this was due to the oxidative nitrene
formation and insertion process which favors the more
electron-rich position. Both yield and regioselectivity were
found to be significantly improved with [Cp*IrCl2]2 while
[Cp*CoCl2]2 failed to produce any amidation products
(entries 2 and 3).
The higher reactivity of Cp*Ir(III) is consistent with the
observations and computational arguments advanced by Baik
and Chang.17 Additionally, the optimized conditions include a
catalytic amount of AgNTf2 and LiOAc. We also found that
the use of Li2CO3 instead of LiOAc produces 3a in 62% yield
with only 10 mol % of AgNTf2 (see the Supporting
Information). Cp*Ir(OAc)2, which was postulated to be the
reactive catalyst generated in situ, was ineffective without
adding additional AgNTf2 (entries 5 and 6). Taken together
with the above observations, we suggest that AgNTf2 probably
helps the dissociation of the acetate ligand (or the amide
product) from iridium, which promotes the alkene coordina-
tion and further activation. It is also worth noting that the yield
is dramatically reduced when CsOAc is used instead of LiOAc
(entry 7). Given that CsOAc is much more soluble than
Further mechanistic insight was gained from isotopic
labeling studies and stoichiometric reactions (Scheme 4).
When subjecting 1aa-d2 to the optimized reaction conditions
using acetic acid as the proton source, no deuterium leaching
was observed at the allylic position of the amidation product.
This indicates that the allylic C−H activation is irreversible
under the standard reaction condition. Furthermore, a smaller
kinetic isotopic effect from intermolecular competition
reaction compared to the one from intramolecular reaction
was observed (KIE = 1.9 vs 2.6). We speculate that the allylic
C−H bond activation is probably not the sole rate-determining
B
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Journal of the American Chemical Society
Communication
Scheme 2. Alkene Scope
a
b
c
d
e
60 °C was used. 80 °C was used. TsN3 was used instead of 2a. NMR yield. Yield based on recovered starting material.
Scheme 3. Dioxazolone Scope
a
b
60 °C was used. 80 °C was used.
step.20 Stoichiometric studies were also performed. An allyl-
Ir(III) complex 5 was trapped by an additional p-
toluenesulfonamide ligand which stabilizes the intermediate
for isolation. Interestingly, the subsequent amidation is only
achieved with the assistance of AgNTf2, which delivers the
desired product in high yield and regioselectivity. In this case,
AgNTf2 might also act as a Lewis acid promoting the
dissociation of the p-toluenesulfonamide ligand to regenerate
the active species with an empty coordination site. Finally,
stoichiometric reactions with 1ab-d1 were also conducted,
leading to the same distribution between deuterated and
protonated products of either the allyl-Ir(III) complex (PD/PH
= 2.6) or the amidation product (PD/PH = 2.7) (see
Supporting Information).
On the basis of the above observations, we hypothesize the
following catalytic cycle (Scheme 5). The active catalyst is
presumably the coordinatively unsaturated cationic mono-
acetato Cp*Ir(III) I that is generated from [Cp*IrCl2]2,
AgNTf2, and LiOAc. Alkene coordination and irreversible
metalation form allyl-Ir(III) species III. The oxidation of
C
DOI: 10.1021/jacs.9b00237
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Journal of the American Chemical Society
Communication
position. Finally proto-demetalation regenerates the active
catalyst and produces the amide product. Importantly, the allyl
Ir intermediate III is more oxidizable than I or II and only
undergoes oxidation/nitrene formation once the allyl moiety is
in place. Thus, the Ir nitrenoid undergoes reductive
elimination to form product faster than Curtius/Lossen-type
rearrangement. This is consistent with Chang’s observations in
intramolecular C−H insertion with related Ir nitrenoids.5i
In summary, we have developed an intermolecular branch-
selective allylic C−H amidation reaction via Ir(III) catalysis.
The inner-sphere Ir-nitrenoid insertion after allylic C−H
activation is key for prevention of undesired aziridination and
achieving branched-selectivity. This opens a new pathway for
branched functionalizations of terminal olefins. Further efforts
at elucidating the mechanism and extending this chemistry are
currently underway.
Scheme 4. Mechanistic Studies
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b00237.
■
S
Detailed experimental procedures, characterization data,
copies of 1H NMR and 13C NMR spectra for all isolated
compounds (PDF)
Data for C27H42IrNO2S (CIF)
AUTHOR INFORMATION
Corresponding Author
*tr2504@columbia.edu
■
ORCID
Tomislav Rovis: 0000-0001-6287-8669
Notes
Scheme 5. Proposed Mechanism
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank NIGMS (GM80442) for support. We thank Daniel
Paley (Columbia) for solving the structure of 5.
■
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