Synthesis of Hindered Anilines: Three-Component Coupling of Arylboronic Acids, tert-Butyl Nitrite, and Alkyl Bromides

Article abstract of DOI:10.1021/acs.orglett.6b02523

The synthesis of sterically hindered amines has been a significant challenge in organic chemistry. Herein, we report a modular, three-component coupling that constructs two carbon-nitrogen bonds including a sterically hindered C_sp³-N bond using commercially available materials. This process uses an earth-abundant copper catalyst and mild reaction conditions, allowing access to a variety of complex aromatic amines.

Full text of DOI:10.1021/acs.orglett.6b02523

Letter
pubs.acs.org/OrgLett
Synthesis of Hindered Anilines: Three-Component Coupling of
Arylboronic Acids, tert-Butyl Nitrite, and Alkyl Bromides
David J. Fisher, James B. Shaum, C. Landon Mills, and Javier Read de Alaniz*
Department of Chemistry & Biochemistry, University of California, Santa Barbara, California 93106, United States
S
* Supporting Information
ABSTRACT: The synthesis of sterically hindered amines has
been a significant challenge in organic chemistry. Herein, we
report a modular, three-component coupling that constructs
two carbon−nitrogen bonds including a sterically hindered
Csp3−N bond using commercially available materials. This
process uses an earth-abundant copper catalyst and mild
reaction conditions, allowing access to a variety of complex
aromatic amines.
ollowing several decades of study, the development of
Fmetal-catalyzed C−N bond formation has had a profound
impact on organic synthesis.1 Often, the synthetic utility of
these methods can be attributed to their chemical robustness,
generality, and availability of the starting materials. Additional
features such as mildness, scalability, and use of earth-abundant
metals also play an important role in their widespread use.
Notable examples include palladium-catalyzed Buchwald−
Hartwig2 and copper-catalyzed Ullman-type3 coupling reactions
between aryl halides and nitrogen nucleophiles, which have
modernized the synthesis of Csp2−N-containing molecules.
Within this context, a number of modern methods for the
synthesis of hindered anilines have been reported. Several
groups have independently reported coupling reactions
involving highly reactive organometallic intermediates, and
more recently Lalic disclosed a milder copper-catalyzed
addition of arylboronic acids to O-benzoylhydroxylamines.4
Furthermore, Buchwald and co-workers have developed several
elegant approaches for the arylation of hindered primary and
secondary amines using a newly developed ligand scaffold for
palladium (Figure 1a).5 In contrast, metal-catalyzed methods
3
for the formation of Csp −N bonds is a less developed field and
Figure 1. Copper-catalyzed C−N bond formation with alkyl halides.
remains a worthwhile objective.6
The use of alkyl halides as a coupling partner has unique
potential in this context, exemplified by the classic substitution
reaction (SN2) between a nitrogen nucleophile and an alkyl
halide.7 Despite their widespread use, N-alkylation reactions
have inherent limitations such as overalkylation and poor yields
with less reactive alkyl halides (e.g., secondary and tertiary
halides).8 Recently, alkyl halides that are not suitable substrates
for classic SN2 reactions due to steric hindrance have emerged
as useful coupling partners for C−N bond formation. For
example, the collaborative approach by Peters, Fu, and co-
workers employed a photoinduced, copper-catalyzed coupling
between alkyl halides and amine nucleophiles (Figure 1b).9 The
amine-coupling partners have thus far been restricted to
carbazoles, indoles, and amides.
We10 and others11 reported an alternative strategy for C−N
bond formation by merging radical reactions with nitroso
chemistry (Figure 1c). This process, which has largely been
limited to the field of polymer chemistry,12 utilizes two
electrophiles, an alkyl halide and a nitroso compound, and
involves the addition of a carbon-centered radical to the
nitrosoarene. In principle, this approach should have broad
opportunities for synthesizing Csp −N bonds by leveraging the
use of readily available alkyl halides and employing a range of
nitroso compounds. In practice, however, progress has been
3
Received: August 23, 2016
© XXXX American Chemical Society
A
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Letter
largely restricted to commercially available nitrosobenzene and
2-methyl-2-nitrosopropane,11 with the exception of the use of
N-arylhydroxylamines under oxidizing conditions.10 However,
N-arylhydroxylamines must be prepared prior to use because
they are not commercially available and are often unstable
when stored for extended periods of time.13 Herein, we report
that this Cu-mediated radical-based strategy has broader
generality and describe the first approach to use a three-
component coupling reaction with arylboronic acids, tert-butyl
nitrite, and various alkyl halides for the synthesis of hindered
aniline derivatives (Figure 1d). This modular process occurs
under mild reaction conditions, constructs two C−N bonds,
uses commercially available starting materials, is compatible
with α-bromocarbonyls, α-bromonitriles, and benzyl bromides,
and employs earth-abundant copper salts.
Scheme 2. Scope of the α-Bromocarbonyls and Benzyl
Bromides with Arylboronic Acids
Initially, conditions independently developed by the groups
of Wu and Yan for the metal-free ipso-nitration of arylboronic
acids with tert-butyl nitrite were evaluated.14 Specifically, we
sought to identify conditions to form exclusively the nitro-
soarene intermediate and prevent the competitive over-
oxidation to the nitroarene. Running the reaction under inert
conditions and reducing the equivalents of tert-butyl nitrite (1.5
equiv proved optimal) afforded the 4-methoxyphenylnitroso-
benzene in >95% yield. Next, a one-pot, three-component
coupling using 4-methoxyphenylboronic acid (0.6 equiv), tert-
butyl nitrite (0.9 equiv), and ethyl α-bromoisobutyrate (1.0
equiv) was attempted using standard stoichiometric Cu(0)
ATRP conditions.15 Unfortunately, after 24 h only a trace
amount of the desired product was produced along with a
substantial amount of the nitroarene. Although simultaneous
mixing of all reagents was problematic, the undesired formation
of the nitroarene could be avoided by sequential reaction of
reagents (Scheme 1). Upon consumption of arylboronic acid
Scheme 1. One-Pot Synthesis of α-Amino Ester
(1), ethyl α-bromoisobutyrate, Cu(0), and PMDETA
(N,N,N′,N″,N″-pentamethyldiethylenetriamine) were added
and allowed to react to completion. Using this protocol and
subsequent treatment of the crude reaction mixture with a
solution of freshly prepared SmI2, furnished the α-amino ester
(3) in 81% yield.
With optimized conditions in hand, we sought to develop the
substrate scope. Electron-rich arylboronic acids were well
tolerated with both α-bromo esters and amides. For example,
those bearing O-alkyl substituents at the 2, 3, and/or 4
positions generated products in high yield (Scheme 2). 4-
Ethoxy-3-chlorophenylboronic acid reacted to give 8, allowing
incorporation of a synthetic handle for further manipulation.
Aromatic thioethers were also amenable to this process as seen
by 10. Electron-neutral arylboronic acids can be used (9);
however, excess arylboronic acid is required because oxidation
of the nitrosoarene intermediate to the corresponding nitro-
arene could not be avoided using optimized, one-pot reaction
a
b
N−O bond cleavage with SmI2/THF 23 °C. N−O bond cleavage
with Zn/HCl in THF at 60 °C. See the Supporting Information for
c
details. 3 equiv of phenylboronic acid and 4.5 equiv of t-BuONO
used. PMDETA = N,N,N′,N″,N″-pentamethyldiethylenetriamine.
conditions. Overoxidation to the corresponding nitroarene is
also observed with electron-deficient arylboronic acids.
Utilizing the same conditions, benzyl bromides also under-
went the desired three-component, Cu-mediated coupling
B
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reaction (Scheme 2, 14−23). However, in contrast to the
carbonyl derivatives, cleavage of the N−O-alkylated adduct that
resulted from the radical-coupling reaction was unsuccessful
with SmI2. As such, to gain access to the desired benzylamine
products, reducing Zn/HCl conditions were used to cleave the
N−O bond. A range of electron-rich arylboronic acids proved
to be suitable coupling partners with the benzyl bromide
substrates. In addition, sterically hindered benzyl bromide
electrophiles that are typically poor substrates for substitution
reactions,16 including those bearing a α-branched isopropyl
(19−21) and cyclohexyl (22 and 23) groups, showed excellent
reactivity under the optimized conditions. Because this reaction
presumably proceeds via radical intermediates, overalkylation
products are never observed.
acids substituted in the ortho, meta, or para position to generate
a range of α-aminonitriles (Scheme 3).
As proof of concept, this copper-mediated, three-component
strategy was used to perform the convergent and facile
synthesis of the 2-thiohydantoin scaffold (Scheme 4 top),
Scheme 4. Application for the Synthesis of a 2-
Thiohydantoin Derivative
To further display the versatility of this method, it was
applied to the synthesis of α-aminonitriles (Scheme 3),17
Scheme 3. Scope of α-Bromonitriles with Electron-Rich
Arylboronic Acids
a
Conditions: (a) 0.9 equiv of t-BuONO, 0.6 equiv of 4-
methoxyphenylboronic acid, 1.0 equiv of Cu(0), 0.5 equiv of
PMDETA, MeCN, rt, then SmI2/HMPA, 60 °C; (b) 2.0 equiv of 4-
nitrophenylisothiocyanate, DMF, rt, then 2 N HCl, MeOH, 80 °C.
which has attracted widespread synthetic interest due in part to
the chemotherapeutic activity of enzalutamide 3520 and
antimicrobial activity of 36.21 In addition, 37 has been
developed as a possible treatment for diabetes.22 As depicted
in Scheme 4, 2-thiohydantoin 39 can be prepared in two steps
from commercially available materials. First, by using copper-
mediated coupling of 4-methoxyphenylboronic acid (2), α-
bromoisobutyronitrile (38), and tert-butyl nitrite, followed by
reduction of N−O adduct, α-aminonitrile 33 was produced in
53% yield. Treatment of this product with 4-nitrophenyl
isothiocyanate followed by HCl afforded 2-phenylthiohydan-
toin 39 in 68% yield.
In conclusion, we have developed a general method for the
synthesis of hindered secondary anilines through a three-
component coupling of electron-rich arylboronic acids with
tert-butyl nitrite and a variety of alkyl halides. The modularity of
the process is illustrated through the synthesis of a variety of α-
amino carbonyls, benzylamines, and α-amino nitriles. This
method utilizes commercially available starting materials, occurs
under mild reaction conditions, and forms two C−N bonds in a
single protocol. Finally, the application of this method was
demonstrated by convergent synthesis of a biologically active 2-
thiohydantoin derivative.
a
b
N−O bond cleavage at 23 °C. N−O bond cleavage at 40 °C. See the
Supporting Information for details. PMDETA = N,N,N′,N″,N″-
pentamethyldiethylenetriamine.
valuable precursors to a wide variety of pharmacologically
relevant molecules.18 Both α-bromopropionitrile and α-
bromoisobutyronitrile reacted with 4-methoxyphenylboronic
acid and tert-butyl nitrite under the optimized reaction
conditions to give the N−O alkylated adducts in 75% and
95% yield, respectively. Like the benzylamine N−O alkylated
adducts, cleavage of the N−O bond for the nitrile derivatives
proved challenging using SmI2. However, the addition of
HMPA, known to increase the reactivity of SmI2,19 enabled the
cleavage and afforded the desired product in good yield. The
amination of secondary (26−31) and tertiary (32−34) α-
bromonitriles was compatible with electron-rich arylboronic
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02523.
Experimental procedures and characterization data for all
compounds (PDF)
C
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(11) A few recent publications have described the use of nitroarenes
as precursors for the arylnitroso compound. For examples, see:
(a) Gui, J.; Pan, C.-M.; Jin, Y.; Qin, T.; Lo, J. C.; Lee, B. J.; Spergel, S.
H.; Mertzman, M. E.; Pitts, W. J.; La Cruz, T. E.; Schmidt, M. A.;
Darvatkar, N.; Natarajan, S. R.; Baran, P. S. Science 2015, 348, 886.
(b) Zhu, K.; Shaver, M. P.; Thomas, S. P. Chem. Sci. 2016, 7, 3031.
(c) Zhu, K.; Shaver, M. P.; Thomas, S. P. Chem. - Asian J. 2016, 11,
977. For examples using nitroso compounds, see: (d) Gingras, B. A.;
Waters, W. A. J. Chem. Soc. 1954, 1920. (e) Hosogai, T.; Inamoto, N. J.
Chem. Soc. C 1971, 3399. (f) Corey, E. J.; Gross, A. W. J. Org. Chem.
1985, 50, 5391. (g) Russell, G. A.; Yao, C.-F. Heteroat. Chem. 1993, 4,
433.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: javier@chem.ucsb.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-1566614)
for financial support. J.B.S. is thankful for a Mellichamp
Academic Initiative in Sustainability Fellowship. NMR
instrumentation was supported by an NIH Shared Instrumen-
tation Grant (1S10OD012077-01A1).
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