Chemoselective Photoredox Synthesis of Unprotected Primary Amines Using Ammonia

Article abstract of DOI:10.1021/acs.orglett.8b01637

Unprotected α-amino carbon radicals are produced as novel intermediates via a transformation that merges acid-promoted N-H imine generation and chemoselective photocatalytic single-electron reduction. Coupling ammonia and aldehydes/ketones allows the generation of primary amines under mild conditions without the need for protecting groups. The key intermediate can be efficiently transformed into primary (di)amines by a formal dimerization, reductive amination via hydrogen atom transfer, and arylation through radical-radical coupling.

Full text of DOI:10.1021/acs.orglett.8b01637

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Chemoselective Photoredox Synthesis of Unprotected Primary
Amines Using Ammonia
Jiawei Rong,† Peter H. Seeberger,†,‡ and Kerry Gilmore*,†
†Max Planck Institute for Colloids and Interfaces, Am Mu
̈
hlenberg 1, 14476 Potsdam, Germany
‡
̈
Freie Universitat Berlin, Institute for Chemistry and Biochemistry, Arnimallee 22, 14195 Berlin, Germany
S
* Supporting Information
ABSTRACT: Unprotected α-amino carbon radicals are produced as novel intermediates via a transformation that merges acid-
promoted N−H imine generation and chemoselective photocatalytic single-electron reduction. Coupling ammonia and
aldehydes/ketones allows the generation of primary amines under mild conditions without the need for protecting groups. The
key intermediate can be efficiently transformed into primary (di)amines by a formal dimerization, reductive amination via
hydrogen atom transfer, and arylation through radical−radical coupling.
Unlike primary amines, which form imines rapidly, ammonia
rimary amines play an essential role in natural and
Psynthetic compounds, including pharmaceuticals, agro- is less nucleophilic, resulting in a slow equilibrating
chemicals, dyes, and materials.1 Although numerous catalytic
methods have been developed to access primary amines,1−3
typically, protected amino groups are used that necessitate a
subsequent−sometimes challenging−deprotection step. One
common method for the synthesis of primary amines is via the
protected imine, which is further functionalized via nucleo-
philic addition,2b,c imine reduction,2d or single-electron
reduction to the α-amino carbon radical.2e,f
This latter approach offers the greatest synthetic potential,
with a range of efficient coupling methods available including
aminopinacol couplings,4a−d Michael additions,4e−g aryl/
alkylations,4h−l and reductive aminations.4m−o A powerful
method to generate these α-amino carbon radicals is
photoredox chemistry, where a photocatalyst and sacrificial
condensation (see the Supporting Information (SI)). This
equilibrium presents a significant challenge for the process due
to the differences in the reduction potentials. For example, the
N−H ketamine 1-(4-fluorophenyl)ethan-1-imine (Scheme 1
and SI) has a significantly lower potential (E1/2red = −2.384 vs
Ag/AgCl in MeCN) than the corresponding 4-fluoroaceto-
phenone (E1/2red = −2.088 vs Ag/AgCl in MeCN), meaning
that the carbonyl should be selectively reduced instead of the
imine.
Scheme 1. Reduction Potential of an N−H Imine Is More
Strongly Affected by Acid Activation than the Respective
a
Ketone
electron donor, typically a Hantzsch ester or Hunig’s base, are
̈
used to reduce a preformed imine. Select multicomponent
examples exist, which circumvent the necessity of a two-step
approach, efficiently forming an imine in situ from aldehydes
and electron-rich primary amines.4e,f,m However, there are two
main challenges in this multicomponent reaction; the slow,
reversible formation of the imine and the chemoselective
reduction of an imine in the presence of a carbonyl, which
typically exhibit competitive reduction potentials.5
To date, no unprotected imines have been generated and
utilized to generate functionalized primary amines directly.
However, reductive amination reactions illustrate that it is
possible to selectively react N−H imines formed in situ by
ammonia condensation.3,6 Herein, we present a method for the
chemoselective formation and utilization of primary amine α-
amino radicals by the photocatalytic reduction of N−H imines
generated in situ from ammonia and an aldehyde/ketone.
a
R1 = 4-fluorophenyl, R2 = CH3.
Received: May 24, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01637
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
The reduction potential of carbonyls is raised in the
presence of Lewis7 or Brønsted acids.8 Circumstantial evidence
suggests that a similar effect is observed with imines activated
by the oxidized sacrificial electron donor.4a,c However, direct
evidence of this effect, as well as its degree of influence as
compared to the analogous carbonyl, was lacking. When we
measured the reduction potential of 4-fluoroacetophenone in
the presence of 1 equiv of trifluoroacetic acid (TFA), a
significant increase in the potential was observed compared to
the nonacidic conditions (E1/2red = −1.586 vs −2.088, Ag/
AgCl in MeCN). However, an even more dramatic change is
observed for the corresponding N−H imine, increasing more
than 1.3 V (E1/2 red = −1.052 vs Ag/AgCl in MeCN), making
the N−H imine more energetically favorable to reduce than
the respective ketone (Scheme 1).
To test whether this difference in reduction potentials of
activated carbonyls/imines is sufficient for a chemoselective
reduction, we first examined the reductive formal dimerization
of a 4-fluorobenzaldehyde and methanolic ammonia to
generate the useful 1,2-diamine motif.1,9 The aryl aldehyde
was chosen to develop the reaction because it exhibits a higher
rate of imine formation compared to the respective ketone (see
Table S6) and the conjugative stabilization of the intermediate
α-amino benzylic radical.
fluorobenzaldehyde, Hu
̈
nig’s base (2 equiv), ammonia (7 N in
MeOH, 10 equiv), and Ru(bpy)3Cl2·6H2O (PC1) (1 mol %)
in acetonitrile was irradiated with blue light in a sealed vial for
15 h at room temperature (for the full experimental procedure,
see the SI). Stronger reducing Ir photocatalysts such as
Ir(dtbbpy)ppy2PF6 (PC2) and Ir(dF(CF3)ppy)2dtbbpy PF6
(PC3) resulted in high yields of the diol product (entries 2 and
3).
The selectivity of the reaction drastically changes when 20
mol % of Sc(OTf)3 is used. Complete conversion is observed
with the Ru catalyst, chemoselectively giving the desired
diamine product in 79% yield as a ∼1:1 mixture of
diastereomers (Table 1, entry 4). The chemoselectivity
diminishes with the stronger reducing photocatalysts, with
PC3 providing only 3:2 selectivity for the diamine (entries 5
and 6). A range of Lewis and Brønsted acids were screened
next; they exhibited either poor-to-moderate yields of diamine
or were unable to promote the reaction at all (entries 7−9 and
Table S2). Lower loadings of Sc(OTf)3 saw diminished yields
(SI). Methanol was the only solvent examined where no
reductive dimerization occurred, with most solvents giving
poor-to-moderate yields (entries 10−12 and Table S3).
Finally, ammonia sources were screened. Solvated ammonia
gave better results than ammonium salts, and the equivalents of
NH3 in MeOH can be dropped to three without any loss in
yield (entries 13−16 and Table S4). The requisite control
studies demonstrated that both light and photocatalyst were
essential to the reaction, and the transformation proved
sensitive to oxygen (Table S1).
As anticipated based on the reduction potentials, no reaction
occurred (Table 1, entry 1) when a degassed solution of 4-
Table 1. Optimization of the Selective Aminopinacol
a,b
Coupling of in Situ Formed Aldimine
With the optimized conditions in hand (Table 1, entry 4), a
series of (hetero)aromatic aldehydes were examined to
determine the versatility of the transformation (Scheme 2).
The unsubstituted benzyl aldehyde gave 2b in 81% isolated
yield. Halogen substituents (F, Cl, Br) were well tolerated in
the ortho, meta, and para positions (2a, 2c−h, 55−82%), with
fluorinated and chlorinated derivatives giving higher yields.
The reaction also works well with electron-rich benzaldehydes.
Alkyl substituents in the para-, meta-, and ortho-positions gave
similar yields to the halogenated derivatives (2i−l, 54−78%).
Excellent yields of the diamine were obtained with stronger
electron donating groups such as OMe and SMe (2m: 62%,
2n: 82%, 2o: 82%). Other aryl aldehydes, such as 2-
naphthalene aldehyde (2p, 62%) and 5-methylfuran-2-
carbaldehyde (2q, 73%), can also be used. No diastereose-
lectivity was observed for all substrates, with dr values around
1:1. Unfortunately, when aliphatic aldehydes were employed in
the reaction, the photocatalytic reduction did not occur and
only trimerized imine was obtained (SI). This result is
presumably caused by the lack of stabilization of the
corresponding α-amino carbon radical, allowing for the slower
side reaction of the activated imine to occur.
Initial attempts to extend these conditions to monoaryl
ketones were unsuccessful, despite the favorable differences in
reduction potentials of activated ketones vs ketimines (Scheme
1). Switching to gaseous NH3 as an alternate ammonia source,
as well as raising the reaction temperature to 60 °C, was found
to be necessary to effect the transformation, due to the slower
rate of N−H ketimine formation (Tables S5 and S6).
Moreover, while at room temperature a small amount of N−
H imine was detected, no product formed, suggesting that the
higher temperature is also required for the dimerization
reaction. Reexamination of reductants and activators revealed
favorable conversions with 1.2 equiv of Hanstzch ester and 1
c
d
ammonia
source
2
3
entry
PC
additive
solvent
(%)
(%)
1
2
3
4
5
6
7
8
PC1
PC2
PC3
PC1
PC2
PC3
PC1
PC1
PC1
PC1
PC1
PC1
PC1
MeCN NH3/MeOH
MeCN NH3/MeOH
MeCN NH3/MeOH
MeCN NH3/MeOH
MeCN NH3/MeOH
MeCN NH3/MeOH
MeCN NH3/MeOH
MeCN NH3/MeOH
MeCN NH3/MeOH
0
0
0
79
24
33
55
0
0
97
92
0
8
22
0
0
0
0
0
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
In(OTf)3
LiBF4
(PhO)2PO2H
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
9
0
10
11
12
13
DMF
NH3/MeOH
MTBE NH3/MeOH
MeOH NH3/MeOH
57
67
0
0
0
MeCN NH3/
78
e
MeOH
14
15
16
PC1
PC1
PC1
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
MeCN NH3/H2O
63
12
8
0
0
0
e
MeCN NH4Cl
e
MeCN NH4CO2H
a
For a full list of additives, solvents, and ammonia sources examined,
see the Supporting Information. Reactions were carried out with 1a
b
c
(0.2 mmol); for full experimental details, see the SI. Yields were
determined after workup by 1H NMR spectra using 1,3,5-
trimethoxybenzene as the internal standard. Product was obtained
d
as a ∼1:1 mixture of diastereomers Yields were determined by 19F
e
NMR spectra using trifluorotoluene as an internal standard. 3.0
equiv. PC1: Ru(bpy)3Cl2·6H2O. PC2: Ir(dtbbpy)ppy2PF6. PC3:
Ir(dF(CF3)ppy)2dtbbpyPF6.
B
DOI: 10.1021/acs.orglett.8b01637
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Organic Letters
Letter
a
Scheme 2. Scope of Aryl Aldehydes Examined*
Scheme 3. Scope of Ketones Examined
a
For full experimental details and a description of aryl and aliphatic
ketones examined, see the Supporting Information.
observed with the protected derivatives due to steric
constraints, as has been noted in SmI2 mediated iminopinacol
coupling reactions.10 However; conditions were found where
the dimerization can be out-competed.
The one electron reduction of N−H imines can be merged
with hydrogen atom transfer catalysis to produce the reductive
amination product.4m−o Adapting the conditions developed for
ketones, catalytic amounts of thiophenol (20 mol %), with the
Hantzch ester already present in the solution, yielded primary
alkyl amines efficiently in 71−96% yields (Scheme 4a). The
Brønsted acid catalyzed reduction of premade N−H ketimines,
employing Hantzch ester as reductant, had been reported to
fail due to the rapid condensation of the primary amine
products and the N−H ketimines.6,11 Under our conditions,
*
For full experimental details and a description of aryl and aliphatic
a
aldehydes examined, see the Supporting Information. Hanstzch ester
̈
used instead of Hunig’s base.
equiv of TFA, respectively. When 4a was reacted with PC1,
Hanstzch ester, and TFA in MeCN under an atmosphere of
ammonia gas at 60 °C, quantitative conversion to the desired
vicinal diamine (5a, Scheme 3) was observed.
Scheme 4. Additional Reactions Pathways Involving
Unprotected α-Amino Carbon Radicals: (a) Photoredox
Reductive Amination of Alkyl Ketones; (b) Reductive
Arylations of Aryl Ketones to Generate Primary Amines on
In general, the reductive dimerization proceeded cleaner and
with higher yields for the ketones than with the respective
aldehydes, presumably due to the enhanced stability of both
the N−H ketimines and the corresponding radical anions. Aryl
alkyl ketones exhibited excellent functional group tolerance,
and good-to-quantitative yields were observed in the presence
of electron-deficient and electron-rich substituents at various
positions (5a−h, 53%-quant). 1-(Naphthalen-2-yl)ethan-1-one
and α-tetralone gave products 5i and 5j in 95% and 74% yields,
respectively. Heteroaryl alkyl ketones, such as 1-furyl (5k), 1-
thienyl (5l), and 2-thienyl (5m) also exhibited good-to-
excellent yields (73−91%). At lower temperatures (40 °C),
ethyl pyruvate can also be reacted, generating the correspond-
ing amino ester dimer in 44% isolated yield (5n). However,
efforts to extend this method further to dialkyl ketones without
adjacent stabilizing groups were unsuccessful (SI).
a
Quaternary Centers
The potential for α-amino carbon radicals to access
functionalized, unprotected primary amines beyond reductive
dimerizations directly was subsequently investigated. Common
transformations of protected α-amino radicals such as
vinylation4e,f and alkylation4g by reaction with various Michael
acceptors were not successful, as only vicinal diamine was
obtained. Addition of the unprotected α-amino carbon radical
to the activated imine intermediate is likely faster than what is
a
For full experimental conditions, see the Supporting Information.
C
DOI: 10.1021/acs.orglett.8b01637
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
with the in situ generation of N−H aldimines and ketimines,
this condensation was never observed.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01637.
■
S
Finally, radical−radical coupling was achieved with 4-
cyanopyridine.4l,12 In the presence of an increased amount of
Hantzch ester, 2 equiv of the cyanopyridine were found to be
sufficient to give unprotected 1° amines on quaternary centers
(α-diarylalkylamines) in high to near-quantitative yields
(Scheme 4b).
The first step of our method is a Lewis acid/Brønsted acid
promoted N−H imine formation between ammonia and the
carbonyl group (Scheme 5). Simultaneously, visible-light
Experimental details, full and additional optimization
tables, characterization data, and associated NMR
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kerry.gilmore@mpikg.mpg.de.
ORCID
Scheme 5. Proposed Mechanism for the Formation and
Trapping of Unprotected Primary α-Amino Radicals
Peter H. Seeberger: 0000-0003-3394-8466
Kerry Gilmore: 0000-0001-9897-6017
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Max-Planck Society and
DARPA (Contract No. W911NF-16-1-0557) for generous
financial support. We thank Ms. Sooyeon Moon (MPIKG) for
analytical support, Ms. Mara Guidi (MPIKG) for the scale-up
reaction, as well as Dr. Bart Pieber (MPIKG) and Dr. Matthew
Plutschack (MPIKG) for fruitful discussions.
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