Ni-Catalyzed Regioselective 1,2-Dicarbofunctionalization of Olefins by Intercepting Heck Intermediates as Imine-Stabilized Transient Metallacycles

Article abstract of DOI:10.1021/jacs.7b06340

We disclose a strategy for Ni-catalyzed dicarbofunctionalization of olefins in styrenes by intercepting Heck C(sp³)-NiX intermediates with arylzinc reagents. This approach utilizes a readily removable imine as a coordinating group that plays a

Full text of DOI:10.1021/jacs.7b06340

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Communication
Ni-Catalyzed Regioselective 1,2-Dicarbofunctionalization of Olefins by
Intercepting Heck Intermediates as Imine-Stabilized Transient Metallacycles
Bijay Shrestha, Prakash Basnet, Roshan K Dhungana, Shekhar
KC, Surendra Thapa, Jeremiah M. Sears, and Ramesh Giri
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06340 • Publication Date (Web): 24 Jul 2017
Downloaded from http://pubs.acs.org on July 24, 2017
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Ni-Catalyzed Regioselective 1,2-Dicarbofunctionalization of Olefins
by Intercepting Heck Intermediates as Imine-Stabilized Transient
Metallacycles
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Bijay Shrestha,† Prakash Basnet,† Roshan K. Dhungana,† Shekhar KC,† Surendra Thapa,† Jeremiah M.
Sears,§ and Ramesh Giri†*
†Department of Chemistry & Chemical Biology, The University of New Mexico, Albuquerque, NM 87131, USA.
§ Sandia National Laboratory, Advanced Materials Laboratory, Albuquerque, NM 87106, USA.
Supporting Information Placeholder
cies (2). In this respect, Sigman and others have shown that 1,2-
dicarbofunctionalization of dienes and styrenes can be effected
with organometallic reagents and organohalides/tri-flates by stabi-
lizing the Heck C(sp3)-MX species by intrinsic -allylation/-
benzylation.9 In simple olefins where C(sp3)-MX species cannot
be stabilized, the reaction generally affords 1,1-difunctionalized
products via a -H elimination/M-H re-insertion cascade.10 Simi-
lar 1,2-dicarbofunctionaliza-tion reactions have also been realized
in cases where radicals are generated by transition metals,11 such
as in the addition of BrCF2CO2Et and ArN2BF4-derived
•CF2CO2Et/Ar• and arylboronic acids to enamides and dienes.12
ABSTRACT: We disclose a strategy for Ni-catalyzed regiose-
lective dicarbofunctionalization of olefins in styrene derivatives
by intercepting Heck C(sp3)-NiX intermediates with arylzinc rea-
gents. This approach utilizes a readily removable imine as a coor-
dinating group that plays a dual role of intercepting oxidative
addition species derived from aryl halides and triflates to promote
Heck carbometallation, and stabilizing the Heck C(sp3)-NiX in-
termediates as transient metallacycles to suppress -hydride elim-
ination and facilitate transmetalation/reductive elimination steps.
This method affords diversely-substituted 1,1,2-triarylethyl prod-
ucts that occur as structural motifs in various natural products.
Catalytic dicarbofunctionalization of olefins via interception of
Heck alkyl-transition metal {C(sp3)-MX} intermediates by cross-
coupling could provide a powerful synthetic tool in organic syn-
thesis.1 This method that installs two carbon-carbon (C-C) bonds
across olefins with organohalides and organometallic reagents
enables to construct rapidly complex molecular architectures with
new stereocenters from simple and readily available feedstock
chemicals. To this end, the majority of these processes have fo-
cused on two-component reactions that generally rely upon cy-
clization into olefins tethered to enolates,2 alkyl halides,3 aryl
halides4 and organometallic reagents.5 Some examples of oxida-
tive dicarbofunctionalization are also known,6 which introduce
two identical aryl groups across an olefin using organometallic
reagents as the aryl source.7
Scheme 1: Strategy to overcome major challenges for regiose-
lective olefin dicarbofunctionalization
In our continued efforts to dicarbofunctionalize olefins,2a,5a we
hypothesized that the two undesired pathways (Scheme 1) could
be simultaneously overcome by installing a removable coordinat-
ing group (CG) in olefin substrates that could function based on
two distinct principles (Path D) – 1) CG could initially intercept
the high valent, oxidative addition intermediate R-M-X (1) as
species 3 via a bidentate coordination mode with the help of the
vinyl group and promote Heck carbometallation onto the olefin
faster than the direct cross-coupling through the intramolecular
insertion process; 2) CG could then intercept and stabilize the
Heck C(sp3)-MX species as transient metallacycles (species 4 and
5), which are expected to slow down the process of -H elimina-
tion due to restricted bond rotations (Scheme 1).13 We envision
that this retardation will enable transmetalation/reductive elimina-
tion to proceed sufficiently faster than -H elimination to furnish
the desired products.14 Herein we report dicarbofuntionalization
In contrast, the three-component dicarbofunctionalization of ole-
fins with organohalides and organometallic reagents remains ex-
ceptionally rare. In this process, an oxidative addition intermedi-
ate (R-M-X) (1) is expected to insert an olefin to generate a new
C(sp3)-MX (2) species (Scheme 1),8 which is subsequently inter-
cepted by organometallic reagents (R’-M’) via a transmeta-
lation/reductive elimination sequence to form a desired product
(Path A). However, the development of this kind of transfor-
mation remains formidably challenging due to two major side
reactions – 1) the cross-coupling between organohalides and or-
ganometallic reagents prior to olefin insertion (Path B); and 2) the
Heck reaction by -hydride (-H) elimination from the C(sp3)-
MX intermediate (2) after olefin insertion (Path C). Therefore,
enabling 1,2-dicarbofunctionalization of olefins by cross-coupling
requires careful strategies to intercept the Heck C(sp3)–MX spe-
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of olefins in styrene derivatives with aryl halides/triflates and
cross-coupling product 15 in significant amounts. We further
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arylzinc reagents that relies upon the concept of intercepting the
transiently stabilized oxidative addition and Heck C(sp3)-MX
intermediates.
Table 1. Optimization of reaction conditionsa
examined the reactions of 2-vinylbenzaldehyde and vinylaldimine
6 separately with 4-iodobenzotrifluoride under the standard condi-
tions but in the absence of PhZnI. Only the reaction of vinylal-
dimine 6 produced the Heck product in 27% yield.17 These results
indicate that coordination of the imine group is indeed required
for both the Heck carbometallation of ArNiI on the vinyl group of
2-vinylaldimine 6 through species 17, and stabilizing the Heck
C(sp3)-NiX intermediates 18 and 19 for further transmetallation
with ArZnI to ultimately deliver the 1,2-diarylated product 14
(Scheme 3).
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Scheme 2. Control experiments for establishing the role of the
imine group
Scheme 3. Proposed mechanistic pathway for regioselective
1,2-dicarbofunctionalization
b
a0.1 mmol scale reactions in 0.5 mL solvent. Yields were de-
1
termined by H NMR using pyrene as an internal standard. Value
in parenthesis is the isolated yield from 0.5 mmol in 18 h.
After optimizing the reaction conditions and establishing the role
of the imine group, we examined the scope of the current reaction
(Table 2). A wide range of electron-rich, neutral and deficient aryl
Table 2. Scope with aryl iodides and bromidesa
cPd(OAc)2, CoCl2, FeCl2 or CuI used.
In order to test the hypothesis, we chose imines as a coordinating
group because of their efficacy of binding to transition metals and
ease of removal by simple aqueous acidic workup. When we re-
acted
2-vinyl-N-phenylbenzylimine
(6)
with
4-
iodobenzotrifluoride and PhZnI in the presence of 2 mol%
Ni(cod)2 in dioxane at 80 °C, we were pleased to observe the ex-
pected product 14 as a single regioisomer in 85% yield after acid-
ic workup (Table 1, entry 1). Scaling up the reaction to 0.5 mmol
required longer time for best product yield (entry 1). The reaction
did not furnish any product in the absence of Ni(cod)2 (entry 2).
The product was formed in lower yields in shorter reaction time
(entry 3). We also changed the imine group in vinylaldimine 6 to
benzylimine (7), n-butylimine (8), t-butylimine (9) and N-
methoxyimine (10), which did not afford the expected product 14
in significant amounts (entries 4-5).15 We then electronically mod-
ified the N-phenyl group in vinylimine 6 with p-F (11), p-Me (12)
and p-OMe (13) groups. Among these vinylaldimines, only the N-
anisolyl-imine 13 furnished the expected product 14 in compara-
ble yield (entries 6-7). The use of a Ni-catalyst was found to be
critical for this transformation as other catalysts based on Pd, Co,
Fe and Cu did not catalyze the reaction (entry 8). (Ph3P)4Ni and
NiBr2 furnished the product 14 in lower yields (entries 9-10). The
reaction can be conducted in NMP or THF, which furnished the
product 14 in comparable yields (entry 11). Other solvents such as
DMF, DMSO, benzene and MeCN afforded the product 14 in
lower yields (entries 12-13).
We conducted further studies in order to probe the role of the
imine group (Scheme 2). We performed the reaction of 2-
vinylbenzaldehyde with 4-iodobenzotrifluoride and PhZnI under
the standard conditions. Despite Ni being a good catalyst for the
Heck reaction,16 2-vinylbenzaldehyde did not afford any Heck or
difunctionalized product. The reaction furnished only the direct
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aIsolated from 0.5 mmol. 80 °C for ArI and 100 °C for ArBr un-
at 100 °C to afford the best product yield. The reaction tolerates
chloride (20-21), and other sensitive functional groups such as
nitrile (22-23), ester (24), ketone (25), dioxolyl (32), thioether
(33), silyl ether (34) and benzyl ether (35). The reaction also
works well with electron-rich aryl iodides containing multiple
electron-donating groups as indicated by the use of 3,5-
dimethyliodobenzene (28), 3,4,5-trimethoxyiodo-benzene (31), 5-
iodobenzo[d][1,3]dioxole (32) and 2-(benzyl-oxy)-4-iodo-1-
methoxybenzene (35). The reaction can also be utilized with aryl
halides containing functional groups such as Cl, CF3 and sterically
bulky i-Pr at the ortho-position (20-22, 26, 27), which furnishes
products in good yields.
b
c
d
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6
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8
less stated otherwise. 2 mol % Ni(cod)2. 5 mol % Ni(cod)2. 10
mol % Ni(cod)2. eNiBr2. f18 h.
Table 3. Scope with vinylaldimines and arylzinc reagentsa
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We further examined the scope of the reaction with functionalized
vinylaldimines and arylzinc reagents (Table 3). Vinylaldimines
containing both electron-withdrawing and donating groups such
as Cl, F, Me and OMe could be used as a substrate with a variety
of electron-rich and deficient aryl halides and arylzinc reagents
(36-58).18 Arylzinc reagents containing F, CF3, CN, CO2Me, Me
and OMe can be utilized in the reaction, which furnishes products
in good to excellent yields. The reaction also tolerates ortho-
substituted vinylaldimines (43) and arylzinc reagents (49-50).
The reaction protocol can be extended to the use of aryl triflates
instead of aryl halides along with a variety of arylzinc reagents
and substituted vinylaldimines, which affords products in good
yields (Table 4). The reaction generally works well with moder-
ately electron-rich, neutral and deficient aryl triflates. The reactiv-
ity of these aryl triflates is similar to that of aryl iodides that was
generally observed at 80 °C requiring 2-5 mol% of catalyst load-
ing.
Table 4. Scope with aryl triflatesa
aIsolated from 0.5 mmol. b5 mol % Ni(cod)2, 100 °C.
We also examined the scope of the current transformation with
vinylimines derived from 2-vinylanilines and benzaldehyde
(Scheme 4). The reaction of vinylimine 62 and (4-(trifluoro-
methyl)phenyl)zinc iodide with iodobenzene, 4-iodotoluene and
2-isopropyliodobenze furnished the corresponding diarylated
imine products 63-65 in good yields.19
aIsolated from 0.5 mmol. 80 °C for ArI and 100 °C for ArBr un-
less stated otherwise. 2 mol % Ni(cod)2. 5 mol % Ni(cod)2. 10
mol % Ni(cod)2. eNiBr2. f18 h. g24 h. h100 °C.
b
c
d
halides (I, Br) and arylzinc reagents can be utilized as coupling
partners with the vinylaldimine 6, which affords the 1,2-diarylated
products 20-35 as the only regioisomers in good to excellent
yields. The regioselectivity of the reaction was confirmed by the
single crystal X-ray structure of product 30, and 1H-1H COSY
experiment of compound 26. The reaction of aryl iodides required
lower catalyst loadings (2-5 mol%) than with that of aryl bro-
mides (5-10 mol%) but the latter generally provided products in
better yields (23-26). Similarly, aryl bromides also required high-
er temperature than aryl iodides (80 °C) and typically proceeded
Scheme 4. Scope with vinylimines derived from 2-vinyl-
anilines
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Seashore-Ludlow, B.; Somfai, P. Org. Lett. 2010, 12, 3732; (f) Ishiyama,
In summary, we have developed a Ni-catalyzed regioselective
dicarbofunctionalization of olefins in styrene derivatives with aryl
halides/triflates and arylzinc reagents, the success of which arise
from the stabilization of Heck C(sp3)-NiX intermediates as transi-
ent metallacycles by imine coordination. The reaction shows high
functional group and steric tolerance, and furnishes products in
good to excellent yields. The current reaction affords an expedient
route to differently-substituted 1,1,2-triarylethyl products that
widely occur as structural scaffolds in a variety of natural prod-
ucts and bioactive molecules.20
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ASSOCIATED CONTENT
Supporting Information
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Experimental procedures, characterization data for all com-
pounds, and crystallographic data (CIF). This material is
available free of charge via the Internet at
http://pubs.acs.org.
(11) For reductive dicarbofunctionalization of olefins by a radical
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Akiyama, S.; Hayama, K.; Ito, H. Org. Lett. 2017, 19, 2614.
AUTHOR INFORMATION
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Chem. Soc. 2016, 138, 15122. For directed olefin dioxygenation/
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(15) We also examined other coordinating groups such as 8-
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benzylquinolin-8-amine, 8-(allyloxy)quinoline and N-allyl-N-(pyridin-2-
yl)benzamide, which did not furnish any difunctionalized product.
(16) (a) Matsubara, R.; Gutierrez, A. C.; Jamison, T. F. J. Am. Chem.
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Angew. Chem. Int. Ed. 2014, 53, 1858; (d) Harris, M. R.; Konev, M. O.;
Jarvo, E. R. J. Am. Chem. Soc. 2014, 136, 7825; (e) Liu, C.; Tang, S.; Liu,
D.; Yuan, J.; Zheng, L.; Meng, L.; Lei, A. Angew. Chem. Int. Ed. 2012,
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Int. Ed. 2016, 55, 11921.
(17) For directed Heck reaction, see: (a) Oestreich, M. In Directed
Metallation; Chatani, N., Ed.; Springer Berlin Heidelberg: Berlin,
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Nokami, T.; Ohashi, Y.; Yoshida, J.-i. Org. Lett. 2004, 6, 3695; (d) Itami,
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A. J. Org. Chem. 1990, 55, 5757.
(18) Vinylaldimine containing internal olefins, such as 2-(1-propenyl)-
N-phenylbenzylimine (trans:cis/2:1), furnished difunctionalized products
only in less than 10% NMR yields.
(19) Unlike the products of vinylimines derived from 2-
vinylbenzaldehydes, these products were surprisingly resistant to acidic
hydrolysis and the product 63 hydrolyzed in <20% GC yield in
dioxane:4N H2SO4 in 15 h at 120 °C.
Corresponding Author
rgiri@unm.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank the University of New Mexico (UNM) and the National
Science Foundation (NSF CHE-1554299) for financial support,
and upgrades to the NMR (NSF grants CHE08-40523 and
CHE09-46690) and MS Facilities. The Bruker X-ray diffractome-
ter was purchased via an NSF CRIF:MU award to UNM (CHE04-
43580).
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