Forming Benzylic Iodides via a Nickel Catalyzed Diastereoselective Dearomative Carboiodination Reaction of Indoles

Article abstract of DOI:10.1002/anie.201900659

A diastereoselective dearomative carboiodination reaction is reported. We report a novel metal-catalyzed approach to install reactive secondary benzylic iodides. Utilizing the unique reactivity of nickel, we have expanded the carboiodination reaction to n

Full text of DOI:10.1002/anie.201900659

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Accepted Article
Title: Forming Benzylic Iodides Via a Nickel Catalyzed Dearomative
Carboiodination Reaction of Indoles
Authors: Austin D. Marchese, Florian Lind, Áine E. Mahon, Hyung
Yoon, and Mark Lautens
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201900659
Angew. Chem. 10.1002/ange.201900659
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10.1002/anie.201900659
Angewandte Chemie International Edition
COMMUNICATION
Forming Benzylic Iodides via a Nickel Catalyzed Diastereo-
selective Dearomative Carboiodination Reaction of Indoles
Austin D. Marchese, Florian Lind[†], Áine E. Mahon[†], Hyung Yoon and Mark Lautens*
Abstract: A diastereoselective dearomative carboiodination reaction
is reported. We report a novel metal-catalyzed approach to install
reactive secondary benzylic iodides. Utilizing the unique reactivity of
nickel, we have expanded the carboiodination reaction onto non-
activated aromatic double bonds forming a previously unattainable
class of iodides. We also report a broadly applicable method to avoid
the use of a metallic reducing agent by utilizing an alkyl phosphite as
the ligand. The reaction is thought to proceed through a syn
intramolecular carbonickelation onto a 2-substituted indole followed
by a diastereoretentive reductive elimination of the carbon–iodine
bond. The complex iodinated indolines generated in the reaction were
obtained in moderate to good yields and good to excellent
diastereoselectivity. The products were easily functionalized via a
variety of synthetic methods.
elimination of a Csp2–X bond8 however, the formation of a Csp3–
X bond is rare.5
Herein we report the diastereoselective dearomative
carboiodination
reaction
using
our
nickel-phosphite
caboiodination catalyst, yielding reactive secondary benzylic
iodides in moderate to good yield and good to excellent
diastereoselectivity. To demonstrate the synthetic versatility of
these benzylic iodides, the products were subjected to a wide
variety of transformations including, oxidation/reduction,
nucleophilic displacements, decarboxylative elimination and
radical cross-coupling reactions. These derivatized products were
isolated in moderate to excellent yields with high chemo- and
stereoselectivity.
Aside from the reduction of the C–I bond, there are no
literature known procedures to generate these derivatized
compounds via
a 1-pot bisfunctionalization, making these
benzylic iodides a useful intermediate to a broad-scope of elusive
1,2-bisfunctionalized indolines.
Since our first report of catalytic reversible oxidative addition
into C–X bonds in 2010,1 we have been exploring the utility of
carbohalogenation as a strategy in synthesis.2 A significant
limitation to the carboiodoination cyclization methodology is the
requirement of a terminal activated alkene (Scheme 1A). All
efforts to expand the scope have been met with failure. These
challenges associated with using an internal alkene in a
carboiodination reaction include the difficulty of the reductive
elimination as well as additional potential decomposition
pathways (e.g. β-elimination).
We and others have explored dearomative functionalization
reactions and have been successful at C–H and various C–C
bond formations (Scheme 1B).3 However, the corresponding C–
X termination step has failed with all palladium catalysts examined
to date. Perhaps this is not surprising due to the limited range of
ligands in the Pd-carboiodination reaction and the sensitive
nature of the resulting benzylic iodides,4 as well as the added
difficulty of the final reductive elimination step to turn over the
catalytic cycle.
Our recent discovery of a nickel catalyzed carboiodination5a
and the potential for an alternative mechanism6 opened the door
to exploring the creation of highly sensitive benzylic iodides. To
the best of our knowledge, there is only one example of nickel
catalyzed dearomative functionalization,7 leaving this concept
largely unexplored. Nickel is known to catalyze the reductive
Scheme 1. Limitation of the Pd-carbiodination reaction and Pd– and Ni–
catalyzed dearomative cascade reactions.
We began the investigation employing NiI2 (10 mol%),
o
P(Oipr)3 (20 mol%), Mn (60 mol%), in toluene (0.2 M) at 100 C
for 24 hours and obtained the product in 15% yield with 20%
recovered starting material. Increasing the ligand load to 40%,
gave 2a in 26% yield with complete consumption of starting
material. We hypothesized that the product might be unstable
under the reaction conditions and showed that the reaction was
complete in only 2 hours, affording 2a in 67% yield with 10:1 d.r
(Entry 2). Changing the ligand:metal ratio did not positively alter
the yield (Entries 3-5). The reaction did not proceed in the
absence of ligand or nickel; however, removing Mn did not impact
the formation of 2a.9
[a]
A. Marchese, F. Lind [†], A. Mahon[†], Dr. H. Yoon, Prof. Dr. M.
Lautens.
Davenport Research Laboratories, Department of Chemistry,
University of Toronto
80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada)
E-mail: mark.lautens@utoronto,ca
These authors contributed equally to this work
Supporting information and the ORCID number(s) for the author(s)
can be found under
[†]
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The use of phosphites to reduce Ni(II) precatalysts
structure of 2e revealed the relationship between the iodine and
represents
carboiodination
phenylacrylamide gave the 3,3-disubstituted oxindole in
a
simpler protocol.10 Our previously reported
the aryl group is syn, which is consistent with
carboiodination-reductive elimination
mechanism15.
a
syn
The
reaction
of
N-(2-iodophenyl)-N-methyl
a
tolerance of a bromide is noteworthy, as it suggests a selective
oxidative addition into the aryl iodide.
comparable yield of 84%, using a phosphite as the ligand and the
reducing agent.11 This discovery adds value to the previously
reported methodology, as the removal of Mn-powder reduces the
amount of waste generated, need for an aqueous workup, as well
as allowing for the opportunity to perform in-situ NMR monitoring
experiments.
Entry
Ni–Cat
Ligand (mol %)
Time(h)
Yield (%)
d.r
1a
2a
3a
4
NiI2
P(Oipr)3 (40)
P(Oipr)3 (40)
P(Oipr)3 (40)
P(Oipr)3 (30)
P(Oipr)3 (50)
P(OEt)3 (40)
N/A
1
2
3
2
2
2
3
3
35
10:1
10:1
10:1
9:1
NiI2
67
NiI2
54
NiI2
61
5
NiI2
28
N.R
10:1
N.R
10:1
6
NiI2
81(80)
10
7
Ni0[P(OEt)3]4
NiI2
Scheme 2. Ni–catalyzed dearomative carboiodination reaction scope. All yields
refer to isolated products. D.r. values were determined by 1H NMR analysis of
the crude reaction mixture. Reactions were run on a 0.2mmol scale. *2f was run
on a 0.166mmol scale.
8b
P(OEt)3 (40)
29
Table 1. Optimization of the Ni–catalyzed dearomative carboiodination
Reaction. Yields determined by 1H NMR using 1,3,5–trimethoxybenzene. a.
reactions were run with Mn present. Yields in bracket is isolated. a. 60 mol% of
Mn was added. b. Reaction was run using the aryl bromide starting material with
2 eqv KI at 110 oC.
Both electron withdrawing and donating groups were
compatible at the 5–position of the indole. An indole bearing the
electron withdrawing fluorine gave a more stable product than the
corresponding 5-methoxy indole, though 2i was obtained in lower
d.r than 2j. With increasing steric bulk at the 2 position of the
indole, the reactivity of the substrate decreased, but the d.r
improved with the higher congeners (2a,2j,2k). This result
supports the hypothesis of a steric influence affecting the
diastereoselectivity. We saw a significant improvement in the
diastereoselectivity of the reaction when an electron withdrawing
ester was placed at the 2 position of the indole, as products 2m
and 2n were obtained as a single diastereomer. The increased
diastereoselectivity could be a combination of the increased steric
bulk on the indole as well as change in electronic bias of the
system. When an aromatic group was placed at the 2–position the
reaction was sluggish, which might be attributed to added π–
conjugation. Both 2o and 2p required the catalyst load to be
doubled, and a higher temperature of 120 ºC in the case of 2p.
Although the yields were lower, the d.r. remained ≥ 20:1.
Substitution of the aryl iodide with a heteroaromatic iodide was
tolerated giving the thiophene-fused product 2q in a 54% yield
and 12:1 d.r.
Many other ligands were screened, with phosphites
emerging as the only viable option.12 Triethyl phosphite was found
to be the optimal ligand, giving the product in an 81% yield and
10:1 d.r (Entry 6). It is noteworthy that Ni0[(POEt)3]4 gave 2a in
only 10% yield, supporting our previous study with added
phosphite. Other solvents and temperatures were screened but
gave no improvement.13
Use of the aryl bromide with 2 equiv of exogenous iodide,
gave 2a in a 29% yield (Entry 8). The same diastereomer, was
formed. Although the yield was considerably lower, the result
demonstrates the ability of the catalyst to insert into the aryl
bromide bond and perform a bromide to iodide exchange.
We explored the scope of the reaction, starting with
modifying electronic effects on the aryl iodide.14 Electron donating
groups at the 4 and 5 position (2b, 2d–2g) were tolerated giving
good yields and d.r of the corresponding products. Introducing an
electron withdrawing –CF3 group para to the iodide gave a slightly
reduced yield and d.r. (2c). An o-methoxy group (2h) was
tolerated, albeit in lower yields, but with
a 17:1 d.r.
Chemoselectivity was observed when the 4–bromo–indoline 2e
was formed in a 60% yield with a 10:1 d.r. A crystal structure was
obtained confirming the stereochemical assignment. The X-ray
To test the scalability of the reaction we employed substrate
1n at 1mmol to give 2n in a somewhat reduced 63% yield, but
with the same diastereoselectivity (>20:1) (Scheme 4). The
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10.1002/anie.201900659
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Employing a copper catalyzed cross-coupling protocol, we
were able to obtain the products using two different radical
coupling partners in moderate yields. When TEMPO was
employed, we obtained the product in a 51% yield as a single
diastereomer (3f). This yield is comparable to the secondary
benzylic halides reported in the literature.18 We also used
allyltributyltin under the same conditions and obtained the
allylated product 3g in a 47% yield as a single diastereomer.
We were also able to obtain selective reduction of the C–I
bond of 2m using Zn/AcOH19, giving the indoline scaffold 3h in
65% yield (see SI for more details). We attempted radical benzylic
iodination of 3h, but all efforts proved unsuccessful. The inability
to instal the iodine via conventional iodination methods supports
the value of the dearomative carboiodination reaction(see SI for
more details).
Scheme 3. Ni–catalyzed dearomative carboiodination reaction scope. 0.2 mmol
scale. All yields refer to isolated products. d.r. values were determined by 1H
NMR analysis of the crude reaction mixture. All reactions were run on a 0.2mmol
scale. a. 20 mol% of NiI2 and 80 mol% of P(OEt)3 was used.
reaction was best run at 105 oC for 4.75h in a more concentrated
solution (0.25 M).
To showcase the reactivity of the product iodides we sought
to derivatize the C–I bond via simple synthetic methods (Scheme
4). Until recently, Pd-catalyzed dearomative-trapping methods in
the literature have been primarily limited to the formation of C–C
and C–H bonds,16 rendering the incorporation of heteroatoms or
other halides difficult.
Reactive nucleophiles gave selective displacement of the
benzylic iodide. Azide and chloride react to give 3a and 3b in
moderate to excellent yields. The nucleophilic displacement
occurred via an SN2 process, giving the anti-diastereomer, which
is not possible to achieve by other metal-catalyzed
methodologies.
When a soft nucleophile was employed, the identity of the
ester plays a key role. With R=CO2Me we obtained exclusively a
Krapcho–type decarboxylation-elimination to form the substituted
indole scaffold (3d). When R=CO2Et, the decarboxylation is
disfavoured and we obtained the displacement product containing
thiophenolate in 57% yield (3c). Although the eliminated product
was not the intended pathway, we note an interesting control
based on the identity of the R group. The formation of this product
is also noteworthy as attempts to form the Heck type product on
the 2-H indole substrate lead to no reaction (see SI for more
details).
Scheme 4. Scalability test and derivatization studies concerning the benzylic
iodide.
In conclusion, we have identified a method to generate
reactive secondary benzylic halides via a diastereoselective
dearomative carboiodination reaction, using an air stable nickel
precatalyst. We have successfully expanded and diversified the
carboiodination reaction to include secondary benzylic carbon–
iodine bond formation. A wide variety of functional groups and
substitution patterns were tolerated giving moderate to good
yields and good to excellent diastereoselectivity.
We have demonstrated subsequent carbon–iodine bond
transformations via nucleophilic attack, elimination, oxidation,
reduction and copper catalyzed cross coupling. This method
further illustrates the utility of nickel catalysis and represents
significant advance in the carboiodination reaction and
dearomative 1,2-difunctionalization reaction in the synthesis of
iodoindoline scaffolds.
We employed an oxidizing agent that is selective for
benzylic alcohols and halide moieties, namely IBX. The desired
dicarbonyl product 3e was isolated in 70% yield, which was
somewhat higher than expected as oxidation reactions of
crowded alkyl halides typically give poor yields.17
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ASSOCIATED CONTENT
Supporting Information
reactions see: a) N. Zeidan, T. Beisel, R. Ross, M. Lautens. Org. Lett.
2018, 20, 7332. b) X. Li, B. Zhou, R. -Z. Yang, F. -M. Yang, R. -X. Liang,
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The
Supporting
Information
Experimental procedures, optimization, characterization and X-
Ray data (PDF)
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Sathyamoorthi, S. Banerjee, J. Du Bois, N. Z. Burns, R. N. Zare. Chem.
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X-ray data for 2e (CIF)
[5] a) H. Yoon, A. D. Marchese, M. Lautens. J. Am. Chem. Soc. 2018,
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AUTHOR INFORMATION
Corresponding Author
[6] For select reviews and references therein see: a) J. A. Milligan, J. P.
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E-mail: mark.lautens@utoronto.ca
Notes
The authors declare no competing financial interests.
[†] These authors contributed equally to this work
ACKNOWLEDGMENT
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We thank the University of Toronto, the Natural Science and
Engineering Research Council (NSERC), and Alphora Research
Inc. for financial support. H.Y and A.D.M thanks the Province of
Ontario for a fellowship (OGS). Á.E.M thanks the Science
Foundation Ireland (SFI) for funding (Grant No.12/RC/2275). We
thank Alan Lough (University of Toronto) for X-ray crystallography
of 2e. We thank the Dr. Darcy Burns and Dr. Jack Sheng
(University of Toronto) for their assistance in NMR experiments.
We thank Adam Tam for helping with the purification of starting
materials 1a-c.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: Carbohalogenation Diastereoselective
Catalysis Dearomative Nickel
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(entry 2 without Mn) yielded 65% product with a 10:1 d.r.
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toluene (1mL). The yield was determined by H1 NMR using 1,3,5-
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[14] All yields are of the isolated major diastereomer14) Due to the
different reactivities of the aryl iodides and stabilities of the products,
reactions were run between 2–6 hours at 100–120oC.
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