Deaminative Borylation of Aliphatic Amines Enabled by Visible Light Excitation of an Electron Donor–Acceptor Complex

Article abstract of DOI:10.1002/chem.201804246

A deaminative strategy for the borylation of aliphatic primary amines is described. Alkyl radicals derived from the single-electron reduction of redox-active pyridinium salts, which can be isolated or generated in situ, were borylated in a visible light-mediated reaction with bis(catecholato)diboron. No catalyst or further additives were required. The key electron donor–acceptor complex was characterized in detail by both experimental and computational investigations. The synthetic potential of this mild protocol was demonstrated through the late-stage functionalization of natural products and drug molecules.

Full text of DOI:10.1002/chem.201804246

A Journal of
Accepted Article
Title: Deaminative Borylation of Aliphatic Amines Enabled by Visible
Light Excitation of an Electron-Donor-Acceptor Complex
Authors: Frederik Sandfort, Felix Strieth-Kalthoff, Felix Klauck, Michael
James, and Frank Glorius
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To be cited as: Chem. Eur. J. 10.1002/chem.201804246
Link to VoR: http://dx.doi.org/10.1002/chem.201804246
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Deaminative Borylation of Aliphatic Amines Enabled by Visible
Light Excitation of an Electron-Donor-Acceptor Complex
Frederik Sandfort+, Felix Strieth-Kalthoff+, Felix J. R. Klauck, Michael J. James, and Frank Glorius*
Abstract: A deaminative strategy for the borylation of aliphatic
primary amines is described. Alkyl radicals derived from the single-
electron reduction of redox-active pyridinium salts, which can be
isolated or generated in situ, were borylated in a visible light-
mediated reaction with bis(catecholato)diboron. No catalyst or
further additives were required. The key electron-donor-acceptor
complex was characterized in detail by both experimental and
computational investigations. The synthetic potential of this mild
protocol was demonstrated through the late-stage functionalization
of natural products and drug molecules.
used in two-electron pathways.[11] These salts are readily formed
by simple condensation of the corresponding primary amine with
the commercially available pyrylium salt 1 and can usually be
isolated by filtration. The synthetic use of Katritzky salts has
recently been demonstrated by the Watson group in the context
of Nickel catalysis,[12] as well as under photocatalytic conditions
by our group.[13] In both cases, single-electron reduction of the
Katritzky salt is proposed to give rise to an alkyl radical
intermediate.
Organoboronic acids and their esters are among the most
prominent molecules used in organic chemistry. Their value as
synthetic intermediates, as well as target molecules, has been
proven over the last decades, emphasized by the Nobel prize for
the Suzuki-Miyaura coupling in 2010. While the synthesis of
aromatic boronic acids has been widely established,[1] chemists
have recently turned their focus towards the development of
methods for the synthesis of aliphatic boronates.[2] Besides their
use in organic synthesis, these molecules may also possess
biological activity and have consequently been utilized in
medicinal chemistry programs.[3,4] Traditionally, Csp3–B bonds
are formed either by trapping of organometallic intermediates
with borates, or from the corresponding olefins via
hydroboration.[1] More recent borylation methods have sought to
use milder conditions and precursors derived from naturally
abundant and inexpensive feedstock chemicals. In this context,
radical-mediated processes, in which alkyl radicals are borylated
with (activated) diboranes, have become increasingly
important.[5] A number of prominent examples of this approach
using activated carboxylic acids, namely redox-active N-
hydroxyphthalimide esters, as precursors for borylation have
been independently reported by the groups of Baran, Li and
Aggarwal.[6] In this regard, primary aliphatic amines also present
a potentially interesting precursor for borylation (Scheme 1), as
they are a natural feedstock of similar abundance to carboxylic
acids and are present in numerous natural products and drugs.[7]
Thus, their synthesis and use as versatile building blocks in
organic synthesis has already been widely explored.[8] The
borylation of primary amines has previously been reported for
aromatic and benzylic amines through the use of the
corresponding diazonium or trimethylammonium salts.[9]
However, the borylation of simple aliphatic amines remains an
unresolved challenge in organic synthesis.[10] One method to
activate aliphatic primary amines is via the formation of redox-
active pyridinium salts (Katritzky salts), which were originally
Scheme 1. General outline of deaminative borylation protocols and the
deaminative borylation of aliphatic primary amines (this work). [a] Geometry
determined by DFT calculations.
In modern organic synthesis, single-electron-transfer processes
are often mediated by photoredox catalysis.[14] Whilst these
methods have been used to enable a number of challenging
transformations, the photocatalysts are still typically based on
expensive transition metal complexes. Thus, photocatalyst-free
methods, which can directly use visible light as one of the
mildest, cheapest and greenest forms of energy available, are
highly desirable.[15] To meet this end, single-electron-transfer
processes can be promoted in the absence of a photocatalyst if
either one reaction partner or an encounter complex of two or
more molecules is excited. The latter is generally described as
an electron-donor-acceptor (EDA) complex.[16,17]
Considering the above, and based on our previous work, we
questioned whether Katritzky salts could serve as acceptors in
EDA-promoted reactions to enable challenging functional group
interconversions under catalyst-free conditions. Herein, we
describe the visible light-mediated deaminative borylation of
primary aliphatic amines under catalyst- and additive-free
[*]
F. Sandfort[+], F. Strieth-Kalthoff[+], F. J. R. Klauck, Dr. M. J. James,
Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster (Germany)
E-mail: glorius@uni-muenster.de
[+]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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conditions. This reaction was enabled by the formation and
irradiation of a ternary EDA complex of the Katritzky salt,
Table 1. Visible light-mediated deaminative borylation of Katritzky salt 2a.
bis(catecholato)diboron
(B2cat2)
and
DMA
(N,N-
dimethylacetamide). A range of primary, benzylic and secondary
Katritzky salts, including those derived from natural products,
were all converted into high value aliphatic boronic esters.
Our studies began by examining known boron-based EDA
complex donors in combination with Katritzky salt 2a.[6c,6d]
Pleasingly, we observed the appearance of a significant color
change when dissolving Katritzky salt 2a and B2cat2 in DMA,
which was a strong indicator that the Katritzky salts could indeed
undergo the desired EDA complex formation. Through
optimization studies we found that the desired borylated product
3a could be isolated in 94% yield by irradiating a mixture of
Katritzky salt 2a and 1.5 equivalents of B2cat2 in DMA (0.1 M)
under an atmosphere of argon with visible light (λmax = 455 nm, 5
W blue LEDs) for 16 h followed by transesterification with
pinacol and Et3N (table 1, entry 1). Other boron sources such as
B2pin2, or the use of non-coordinating solvents such as CH2Cl2,
did not lead to any product formation (table 1, entries 2,3).
However, when adding 5.0 equivalents of DMA to the reaction
mixture in CH2Cl2 the desired product could still be formed in
77% yield (table 1, entry 4). Through further optimization we
found that both the amount of B2cat2 and the reaction time could
also be decreased to only 1.2 equivalents and 6 h, respectively
(table 1, entry 5). A control experiment without light revealed
formation of the desired product in 60% yield after 16 h reaction
time (table 1, entry 6). However, further studies showed that the
thermal reactivity was strongly substrate-dependent.[18]
Entry
Variation from standard conditions[a]
Yield [%][b]
1
2
3
4
5
6
7
8
none
B2pin2 (1.5 equiv) instead of B2cat2
CH2Cl2 instead of DMA
CH2Cl2 + DMA (5.0 equiv)
B2cat2 (1.2 equiv), 6 h reaction time
no irradiation
94
0
0
77
95 (91)[c]
60
0
under air
TEMPO (2.0 equiv)
0
[a] Conditions: 2a (0.10 mmol), B2cat2 (0.15 mmol) and DMA (0.1 M) under
argon. [b] Yields determined by 1H NMR spectroscopic analysis using 1,3,5-
trimethoxybenzene as internal standard. [c] Isolated yield on a 0.3 mmol scale
in parentheses.
Scheme 2. A. Proposed reaction mechanism. B. UV/vis spectra of Katritzky salt 2b, B2cat2 and the reaction mixture in DMA. C. Job plot of Katritzky salt 2b and
B2cat2 in DMA. [a] Calculated redox potentials in MeCN vs SCE. [b] TDDFT-computed donor and acceptor orbitals of a charge transfer transition in the EDA
complex.
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To fully confirm and establish the use of Katritzky salts as
acceptors in EDA complexes, we next sought to characterize the
true nature of the complex and the reaction mechanism itself.
While neither Katritzky salt 2b, derived from cyclohexylamine,
nor B2cat2 showed significant UV/vis absorption at 550 nm, an
absorption shoulder appeared in the spectrum of the reaction
then undergoes borylation with DMA-activated B2cat2 IV to form
boronic ester V and open-shell boron species VI. Finally,
electron transfer from open-shell species VI to the Katritzky salt
enables chain propagation. Computationally determined redox
potentials suggest that this electron transfer step is
thermodynamically feasible.
mixture, supporting the formation of an EDA complex (Scheme 2, Having concluded mechanistic investigations, we began scoping
B). However, this shoulder was not present when the reagents
were dissolved in CH2Cl2, but appeared and grew stronger upon
the successive addition of DMA,[18] suggesting the formation of a
ternary EDA complex. A subsequent Job plot analysis revealed
a 1:1 adduct formation of Katritzky salt 2b and B2cat2 in DMA
(Scheme 2, C). Based on these results we propose EDA
complex I, primarily stabilized by π-π interactions.
Computational investigations using time-dependent density
functional theory (TDDFT) calculations revealed three significant
excitations with λ > 400 nm, which can all be assigned to
charge-transfer-type transitions.[18] The involved molecular
orbitals (HOMO & LUMO) are depicted in Scheme 2. A transition
between these orbitals directly corresponds to the proposed
single-electron transfer from the B2cat2 to the Katritzky salt.
Further evidence for the radical nature of the reaction was
obtained when all product formation was inhibited by the
presence of air or TEMPO (table 1, entries 7,8). Next, quantum
yield determination (Φ = 27.8) suggested a radical chain
mechanism to be operative. Consequently, we propose the
following mechanistic scenario (Scheme 2, A): First, the radical
chain is initiated by photochemical or thermal electron transfer
within EDA complex I to form pyridinyl radical II. Fragmentation
of intermediate II leads to the formation of alkyl radical III, which
studies for this deaminative borylation reaction (Scheme 3).
Pleasingly, we found a variety of previously unreactive sterically
different primary Katritzky salts[13] could all undergo borylation in
good yield (3c–e).[18] Different functional groups, such as
carboxylic acid esters or protected amines were well-tolerated
(3f–g). Benzylic boronic esters were also isolated in moderate to
good yield (3h–j). Notably, aryl bromides and aryl chlorides were
tolerated, providing opportunity for further functionalization.
However, when electron poor benzylic Katritzky salts were used,
dimerization of the corresponding benzylic radical was observed
as a major byproduct. Acyclic and cyclic secondary Katritzky
salts with varying ring sizes all underwent borylation efficiently
(3k–o). Furthermore, pharmaceutically relevant saturated
heterocyclic amines afforded the desired products in generally
very good yield (3p–s). The facile transesterification to a MIDA
boronate was also demonstrated (3q) and even a free alcohol
could be tolerated in this reaction process (3t). Having
demonstrated the effectiveness of this borylation protocol for a
variety of primary and secondary Katritzky salts, we sought to
explore the late-stage functionalization of natural products and
drugs. Pleasingly then, dopamine derivative 3u as well as the
highly functionalized boronic ester 3v derived from a Lipitor
intermediate were both isolated in good to excellent yield.
Scheme 3. Scope of the deaminative borylation. Conditions: 2 (0.30 mmol), B2cat2 (0.36 mmol) and DMA (0.1 M) under argon. [a] Irradiation with 30 W blue LEDs
for 24 h at ~60 °C. [b] Irradiation with 5 W blue LEDs for 6 h at ~30 °C. [c] Transesterification: Methyliminodiacetic acid (MIDA), 60 °C, 24 h.
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Furthermore, sterically demanding natural products pinanamine
and myrtanylamine were also efficiently borylated as was
unprotected tryptamine containing a free N–H group (3w–y).
From natural amino acids derived protected leucinol and lysine
afforded the corresponding products 3z and 3aa in moderate
yield. As mentioned previously, the Katritzky salts are formed via
a simple and efficient condensation reaction, which can also be
carried out at room temperature in CH2Cl2, if acetic acid is used
as a Brønsted acid catalyst.[19] However, in some cases isolation
can be difficult if the Katritzky salt does not precipitate. Thus, we
sought to develop a one-pot procedure in which the Katritzky
salt is generated and reacted in situ. As a solvent mixture of
CH2Cl2 and DMA was already successful for borylation during
the optimization process, we were confident that such a protocol
could be realized. Pleasingly, we found that simply through the
addition of a solution of B2cat2 in DMA to a pre-formed Katritzky
salt solution in CH2Cl2, followed by the standard borylation
procedure, the desired product 3a could be isolated in 76% yield
directly from amine 4a (Scheme 4). Compared to isolation of
Katritzky salt 2a (70%)[13] and successive borylation with our
method, the one-pot borylation even offers a slight increase in
yield over two steps (76% vs. 64%). This one-pot protocol is
significant for enabling the first direct deaminative borylation of
primary amines.
Keywords: borylation • deaminative strategy • visible light •
electron-donor-acceptor • DFT calculations
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Scheme 4. One-pot borylation procedure via in situ generation of Katritzki
salt 2a, performed on a 0.3 mmol scale. [a] Performed on a 2.5 mmol scale.
In summary, we report the deaminative borylation of primary
aliphatic amines via the use of Katritzky salts as acceptors in
EDA complexes. The reaction proceeds under mild visible light-
mediated conditions and requires only a coordinating solvent
and no further catalysts or additives. We propose a radical chain
mechanism initiated by an EDA complex based on experimental
and computational studies. The method efficiently converts
primary, benzylic and secondary amines into the corresponding
borylated products. The synthetic value of this methodology is
further demonstrated through the late-stage functionalization of
natural products and drug molecules. Notably, direct amine to
boronate interconversion can also be performed in a one-pot
process, whereby the Katritzky salt is not isolated but reacted in
situ. Further studies into the full synthetic potential and use of
Katritzky salts as acceptors in EDA complexes are ongoing in
our laboratory.
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Acknowledgements
This work was supported by the Fonds der Chemischen
Industrie (F.S.), the Deutsche Forschungsgemeinschaft (SFB
858, F.S.-K.) and the Alexander von Humboldt Foundation
(M.J.J.). We also thank Philipp Pflüger for experimental support
and Lena Pitzer for helpful discussions.
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Entry for the Table of Contents (Please choose one layout)
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F. Sandfort+, F. Strieth-Kalthoff+, F. J. R.
Klauck, M. J. James, F. Glorius*
Page No. – Page No.
Deaminative Borylation of Aliphatic
Amines Enabled by Visible Light
Excitation of an Electron-Donor-
Acceptor Complex
Deaminative Borylation. A deaminative strategy for the borylation of aliphatic
amines is described. Alkyl radicals derived from redox-active pyridinium salts were
borylated in a visible light-mediated reaction with bis(catecholato)diboron. The
synthetic potential of this method was demonstrated by late-stage functionalization
of natural products and drug molecules. The key electron-donor-acceptor complex
is characterized in detail by various experiments as well as computational
investigations.
This article is protected by copyright. All rights reserved.