Photoredox-catalyzed deaminative alkylation via C-N bond activation of primary amines

Article abstract of DOI:10.1021/jacs.0c08595

Primary amines are often cheap, naturally occurring, and chemically diverse starting materials. For these reasons, deaminative functionalization of amines has emerged as an important area of research. Recent advances in C-N activation transform simple α-1° and α-2° amines into alkylating reagents via Katritzky pyridinium salts. We report a complementary method that activates sterically encumbered α-3° primary amines through visible light photoredox catalysis. By condensing α-3° primary amines with electron-rich aryl aldehyde, we enable an oxidation and deprotonation event, which generates a key imidoyl radical intermediate. A subsequent β-scission event liberates alkyl radicals for coupling with electron-deficient olefins for the generation of unnatural γ-quaternary amino acids and other valuable synthetic targets.

Full text of DOI:10.1021/jacs.0c08595

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Photoredox-Catalyzed Deaminative Alkylation via C−N Bond
Activation of Primary Amines
Melissa A. Ashley and Tomislav Rovis*
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ABSTRACT: Primary amines are often cheap, naturally occurring, and chemically diverse starting materials. For these reasons,
deaminative functionalization of amines has emerged as an important area of research. Recent advances in C−N activation transform
simple α-1° and α-2° amines into alkylating reagents via Katritzky pyridinium salts. We report a complementary method that
activates sterically encumbered α-3° primary amines through visible light photoredox catalysis. By condensing α-3° primary amines
with electron-rich aryl aldehyde, we enable an oxidation and deprotonation event, which generates a key imidoyl radical
intermediate. A subsequent β-scission event liberates alkyl radicals for coupling with electron-deficient olefins for the generation of
unnatural γ-quaternary amino acids and other valuable synthetic targets.
Scheme 1. C−N Bond Activation: Amines as Alkyl
Precursors
rimary amines are present in a wide range of building
blocks, and their use as synthetic handles in synthesis is
P
vast. Traditionally, organic chemists have focused on N−H
functionalization of amines, such as reductive aminations1 or
cross-couplings.2 Additionally, amines can be employed as
directing groups for C−H activation to achieve α,3 β,4 γ,5 and
δ6 C−H functionalization. While amines offer immense
synthetic leverage, harnessing the C−N bond as a functional
handle for deaminative transformations remains underutilized.
To achieve this, additional methods for C−N functionalization
of unactivated primary amines are required. Until recently,
most approaches were limited to allylic or benzylic amines
(Scheme 1A).7 The use of primary amines as alkyl precursors
has garnered tremendous attention with the re-emergence of
Katritzky salts.8 In 2017, the Watson group demonstrated the
transformative power of using C−N bonds for a deaminative
Suzuki−Miyaura, in which the primary amine sources
contained unactivated alkyl groups.9 Since this seminal
publication, many research groups have used pyridinium salts
for a variety of transition-metal-catalyzed10 and photoredox-
catalyzed11,12 deaminative functionalizations.13 Notably, these
salts are predominantly activated via a single-electron
reduction (∼−0.9 V vs SCE14).15 The popularity of Katritzky
salts as a functional handle is attributed to a catalyst-free, trivial
activation step in which primary amines are condensed on
pyrylium salts. However, this is limited to α-1° and α-2°
amines, whereas more sterically encumbered α-3° amines do
not condense.16
Our goal was two-fold: (1) create a method to activate C−N
bonds of sterically congested primary amines and (2) liberate
the alkyl radical via an oxidative event, to provide a
complementary approach to Katritzky salt activation. We
turned our attention to aromatic aldehydes as a potential
activating group with the intention of forming an imidoyl
radical, which could then undergo a β-scission event to liberate
alkyl radical coupling partners.17 Previously, imidoyl radicals
have been formed through radical addition across isocya-
Received: August 10, 2020
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© XXXX American Chemical Society
A

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nides18 often for heteroarene synthesis.19 We speculated that
imidoyl radicals could potentially be generated by a single-
electron oxidation and deprotonation of electron-rich imines,
given that proton-coupled electron-transfer is a known strategy
to activate strong C−H bonds.20 Imines are commonly
activated through a single electron reduction event;21 however,
we postulated if we tune the arene toward an oxidation event,
the imidoyl C−H would be primed for deprotonation. For
example, when considering an analogous system, upon
oxidation of toluene (Ep/2 = 2.36 V vs SCE)22 the resulting
radical cation is an extremely strong π-acid (pKa of −6;
Scheme 1C).23 We aimed to translate this reactivity to imines
via oxidation of the arene activating group followed by
deprotonation of an acidified imidoyl C−H to form imidoyl
radicals.
We first explored the oxidation potential of imines by
condensing adamantylamine on a variety of aldehyde activating
groups. We found 2,4,6-trimethoxybenzaldehyde to be a viable
activating group. Initial conditions gave desired product in a
19% yield (Table 1, entry 1). Switching to a more oxidizing
photocatalyst, [Ir(dF-CF3-ppy)2(dtbbpy)PF6] (PC B; Ir-
(III)*/Ir(II) = 1.21 V vs SCE) leads to an increase in yield
to 46% (entry 2). Trifluorotoluene and 1,2-dichloroethane as
solvents give similar yields, yet DCE is preferred due to
improved photocatalyst solubility. Stoichiometric amount of
TMG as base is tolerated (entry 3); indeed, the reaction works
even in its absence (entry 7; we assume the imine nitrogen is
sufficiently basic to perform the deprotonation), but a
substoichiometric amount is optimal (20 mol %). Further-
more, diluting the reaction results in higher yield of product
(entries 5 and 6 vs optimal conditions). To our delight,
switching to coupling partner 4 provides high yields and
enables access to unnatural γ-quaternary amino acids.
With optimized conditions in hand, we examined the
reaction scope of primary amines (Scheme 2) with adamantyl-
amine-derived substrates affording products in 92% (5) and
75% (6) yields. Acyclic α-3° amines afford the desired product
in high yields (7−9). Small drug molecule phenantramine (10)
and fluorinated derivative (11) demonstrate high reactivity. A
variety of functional groups are well-tolerated forming products
bearing Boc-protected amines (12, 18), ethers (17), esters (13,
14), and protected alcohols (9). A slightly more strained
[2.2.2]-bicycle proceeds in a 57% yield (14). Products bearing
various ring sizes are formed well (12, 13, 15, 16) with more
strained oxetane- and azetidine-derived amines providing
coupling product in 58% (17) and 63% (18) yield.24
We then turned our attention to expanding this method-
ology to a range of electron-deficient olefin coupling partners.
Using the Karady−Beckwith chiral Dha substrate,25 the
reaction proceeds in high diastereoselectivity and 70% yield
(19). Simple acrylates participate in moderate yields (3, 20,
21) and introduction of substitution at the α-carbon leads to
an improved yield (22−26). We hypothesize that substitution
decreases undesired oligomerization of the resulting radical
upon addition into the acrylate, as well as diminishes any group
transfer byproducts.26 Exocyclic acrylates and enones are
coupled in 52% (27) and 70% with 28 isolated as a single
isomer. Vinyl sulfones and methacrylonitrile also form the
desired products (29−30).
We next turned our attention to mechanistic interrogation to
determine the mode of C−N activation and alkyl radical
formation (Scheme 3). In situ LED NMR monitoring suggests
the reaction is first order in the imine and acrylate coupling
partner (Scheme 3B). Based on these findings, we propose that
upon condensation of the primary amine onto the aldehyde
activating group, the redox-active imine successfully quenches
the excited state of the PC B, as evident by Stern−Volmer
quenching studies (Scheme 3C). A proton transfer event forms
the key imidoyl radical intermediate. A subsequent β-scission
liberates the alkyl radical which couples to electron-deficient
olefins. A final reduction and protonation leads to desired
product formation and regenerates the ground state of PC B.
The remaining crucial mechanistic detail to elucidate was
determining the route from imine to imidoyl radical. Spin
density plotting of the imine radical-cation reveals an
appreciable amount of spin density on both the imine nitrogen
and arene carbon (C1; Scheme 3D). This led us to envision
two potential routes to imidoyl radical formation: (1) a
nitrogen radical-cation acting as a hydrogen atom transfer
(HAT) reagent or (2) oxidation of the arene and subsequent
deprotonation of an acidified imidoyl hydrogen. To test the
feasibility of a potential HAT pathway, we conducted a
competition experiment between 34 and 35 (Scheme 3E). The
oxidation of imine 35 (Ep/2 = +1.86 V vs SCE) is well outside
the range of the iridium excited state, yet when comparing
imidoyl C−H bond strength, it is similar to that of 34.
a
Table 1. Optimization and Control Studies
b
entry
deviation
yield (%)
1
2
3
2 equiv 2, 2 equiv K3PO4, PhCF3 (0.1 M), PC A
2 equiv 2, 2 equiv K3PO4, PhCF3 (0.1 M)
1 equiv TMG
19
46
72
54
68
60
40
0
4
2 equiv acrylate
5
0.10 M DCE
6
0.20 M DCE
7
no TMG
8
no 427 nm LEDs
9
no photocatalyst, 427 nm LEDs
no photocatalyst, 75 °C
coupling partner 4
0
10
11
12
0
91
c
coupling partner 4, PC C
55
a
Optimized conditions: 1a (0.1 mmol), 2 (0.11 mmol), [Ir(dF-CF3-
ppy)2(dtbbpy)]PF6 (3 mol %), 1,1,3,3-tetramethylguanidine (TMG,
b
20 mol %) in 1,2-dichloroethane (DCE, 0.05 M). GCMS yield using
c
mesitylene as internal standard. Isolated yield.
B
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a b c
, ,
Scheme 2. Scope
a
Reaction was performed with imine (0.1 mmol), alkene (0.11 mmol), [Ir(dF-CF3-ppy)2(dtbbpy)]PF6 (3 mol %), 1,1,3,3-tetramethylguanidine
b
c
(TMG, 20 mol %) in 1,2-dichloroethane (DCE, 0.05 M). Yields are reported from the premade imine. Imines were synthesized under Dean−
Stark conditions in high to near quantitative yields and used without further purification (see SI for more details). Reaction was performed with
d
adamantylamine (0.1 mmol), 2,4,6-trimethoxybenzaldehyde (0.105 mmol) 2a (0.11 mmol), [Ir(dF-CF3-ppy)2(dtbbpy)]PF6 (3 mol %), TMG (20
mol %), 4 Å mol sieves in trifluorotoluene (0.05 M) at 100 °C.
Therefore, we rationalized if we observe the formation of 5, it
would likely be through a HAT event from a 34 nitrogen
radical-cation. By only detecting 13 and accounting for the
remaining 35, we conclude that for C−N bond activation to
occur, an oxidation event of the substrate is crucial.
Having determined a nitrogen radical-cation promoted HAT
event as unlikely, we propose a mechanism in which the spin
density of the imine radical cation residing on C1 is vital. A
crystal structure of 1 (Scheme 3F) reveals that the C−N
double bond is not in full conjugation with the π-system of the
arene ring, resulting in the imine C−H residing out-of-plane
with the ring system. Oxidation of the arene ring thus leads to
acidification of the imidoyl C−H in parallel to that observed
for toluene, as explained previously. By computationally
determining the imidoyl C−H bond dissociation energy of
our imine and experimentally determining the oxidation
potential (Ep/2 = 1.43 V vs SCE), we estimate the pKa of the
imine radical cation to be ∼15 (see SI for details).27 This
greatly acidified imidoyl C−H is now easily deprotonated. A
spin-centered-shift leads to imidoyl radical formation, and a
C
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Scheme 3. Mechanistic Analysis
a
b
Atomic spin densities obtained from Mulliken analysis. Imine C−H BDEs determined for N-tert-butyl-1-(2,4,6-trimethoxyphenyl)methanimine
c
and N-tert-butyl-1-phenylmethanimine. pKa of radical cation 31 calculated using the computed imine C−H BDE and experimentally obtained
oxidation potential. See SI for more details.
subsequent β-scission event releases an alkyl radical for further
coupling with electron-deficient olefins. Given that the
oxidation is slightly endergonic, and complicated by the fact
our substrate can act as a base itself, we are unable to
differentiate between a stepwise or concerted PCET event.28
We have developed a protocol to activate C−N bonds of
sterically encumbered amines through formation of redox-
active imines. Upon an oxidation, a dramatic acidification of
the imidoyl C−H is observed. Deprotonation of this C−H and
a spin-centered shift forms the key imidoyl radical
intermediate. A subsequent β-scission provides nucleophilic
alkyl radicals which are coupled with electron-deficient alkenes
to access unnatural γ-quaternary amino acid derivatives and
various synthetic targets. Given the large impact of Katritzky
salts for C−N activation of α-1° and α-2° amines, we believe
the complementarity of this reductive method for C−N
D
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c08595.
■
sı
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Experimental procedures, spectral characterization, and
additional data (PDF)
Crystal data (CIF)
AUTHOR INFORMATION
Corresponding Author
■
Tomislav Rovis − Department of Chemistry, Columbia
University, New York, New York 10027, United States;
orcid.org/0000-0001-6287-8669; Email: tr2504@
columbia.edu
Author
Melissa A. Ashley − Department of Chemistry, Columbia
University, New York, New York 10027, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c08595
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank NIGMS (GM125206) for support. M.A.A. thanks
Bristol-Myers Squibb for a Graduate Fellowship. We thank
Daniel Paley (Columbia University) for solving the structure of
1.
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F
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Journal of the American Chemical Society
pubs.acs.org/JACS
Communication
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7998−8003.
(26) The mass of imine and coupling partner was detected.
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(28) We have ruled out energy transfer as a potential pathway.
Please see SI for spectroscopic data (absorbance of N-tert-butyl-1-
(2,4,6-trimethoxyphenyl)methanimine in DCE and emission of PC B
in DCE).
G
https://dx.doi.org/10.1021/jacs.0c08595
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX