Coupling Reaction between Aldehydes and Non-Activated Hydrocarbons via the Reductive Radical-Polar Crossover Pathway

Article abstract of DOI:10.1021/acs.orglett.0c00096

Herein, we describe the generation of an organochromium-type carbanion species from a non-activated C-H bond and its nucleophilic addition to aldehydes. The catalytic carbanion generation occurred through formal deprotonation of a non-activated C-H bond under mild conditions and did not need the prefunctionalization or anion stabilizing group. Carbon radical intermediates generated by decatungstate photocatalyst-mediated hydrogen abstraction were captured by a chromium salt with the reductive radical-polar crossover reaction to produce organochromium carbanions.

Full text of DOI:10.1021/acs.orglett.0c00096

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Letter
Coupling Reaction between Aldehydes and Non-Activated
Hydrocarbons via the Reductive Radical-Polar Crossover Pathway
Kenzo Yahata,* Shu Sakurai, Shuhei Hori, Shin Yoshioka, Yuki Kaneko, Kai Hasegawa, and Shuji Akai
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ABSTRACT: Herein, we describe the generation of an organochromium-type
carbanion species from a non-activated C−H bond and its nucleophilic addition
to aldehydes. The catalytic carbanion generation occurred through formal
deprotonation of a non-activated C−H bond under mild conditions and did not
need the prefunctionalization or anion stabilizing group. Carbon radical
intermediates generated by decatungstate photocatalyst-mediated hydrogen
abstraction were captured by a chromium salt with the reductive radical-polar
crossover reaction to produce organochromium carbanions.
independently reported the generation of allylchromium
nucleophiles via RRPCO of allyl radicals, which were formed
he addition of carbon nucleophiles to carbonyls plays a
Tkey role in C−C bond formation. Among the
nucleophiles used, organometallic reagents in particular have
contributed to the development of organic synthesis.1
However, despite the usefulness of organometallic reagents,
methods for their preparation are limited. In most cases,
preparation of the organometallic reagent requires prefunction-
alization, such as halogenation for halogen−metal exchange, or
stannylation for tin−lithium exchange. Although direct
deprotonation from a C−H bond is one of the most
straightforward methods for generating organometallics,
strongly basic conditions or anion stabilizing groups such as
carbonyl groups, sulfur atoms, or sp carbons are required
(Scheme 1a). Recently, reductive radical-polar crossover
(RRPCO) reactions have emerged as a powerful method for
the generation of carbanions. RRPCO has enabled the
generation of anions from various prenucleophiles, such as
carboxylic acids, olefins, ammonium, etc.2 Glorius and Kanai
through allylic C−H activation by photoredox catalysts
3
̈
(Scheme 1b). Konig developed the generation of benzylic
anions from benzylic C−H bonds by using the RRPCO
pathway.4 In these reports, the RRPCO process enabled the
mild and catalytic preparation of carbanions from allylic and
benzylic C−H bonds. However, the formation of carbanions
from less acidic non-activated C−H bonds, which are hard to
deprotonate selectively under conventional basic conditions, is
still challenging.
The decatungstate anion, typically utilized as the tetrabuty-
lammonium salt [TBADT, tetrabutylammonium decatung-
state, (nBu4N)4[W10O32]], has been recognized as a unique
photocatalyst that enables non-activated C−H bond function-
alization through a hydrogen atom transfer mechanism.5
TBADT is a highly active photocatalyst with excellent site
selectivity. Although a wide range of reactions have been
developed using TBADT, most reports have focused on radical
reactions, such as addition to unsaturated bonds, fluorination,
and oxidation. We report herein that the combination of
TBADT and a chromium salt generated organochromium
compounds from non-activated C−H bonds under mild
conditions (Scheme 1c).
Scheme 1. Methods for the Preparation of Carbanions
We chose the coupling between cyclohexane 1a and
benzaldehyde 2a as a standard reaction for the optimization
of the reaction conditions (Table 1). Chromium(III)
trichloride was found to be the best chromium source
among those tested6 (entries 1−3). The yield was further
improved by increasing the amount of TBADT, cyclohexane,
Received: January 8, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00096
© XXXX American Chemical Society
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A

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a
wavelength from 390 to 370 nm had little influence on the
results (entry 8).
Table 1. Reaction Optimization
b
TBADT
(mol %)
1a
yield
(%)
With the optimized conditions in hand, we then explored
the substrate scope of this coupling reaction (Scheme 2).
Bromo and methoxy substituents did not affect the results (3c
and 3d). Because of the high chemoselectivity of the
organochromium anion, aldehyde groups were preferentially
alkylated in the presence of ester and ketone functional groups,
and no alkylation of the ketone or ester was observed (3e and
3f). Protecting groups such as tert-butyldimethylsilyl (TBS)
and acetyl were also tolerated under the reaction conditions
(3g and 3h). In addition, carbanion nucleophiles were
prepared from cyclopentane, cyclooctane, and norbornane
and reacted with 2a to give products in good yields (3i−3k).
Furthermore, the developed method could be applied to the
generation of functionalized carbanions as follows. Utilization
of N,N-dimethylacetamide (DMA), dimethylformamide, and
N,N-dimethylaniline (1b)7 enabled the aminomethylation of
aldehydes to produce 1,2-aminoalcohol derivatives in good
entry
Cr salt
(equiv)
1
2
3
4
5
6
CrCl2 (2 equiv)
CrCl3·3THF (2 equiv)
CrCl3 (2 equiv)
CrCl3 (2 equiv)
CrCl3 (3 equiv)
CrCl3 (3 equiv)
CrCl3 (3 equiv)
CrCl3 (3 equiv)
3
3
3
20
20
20
20
20
30
20
30
2
9
20
50
10
10
10
10
10
69
75 (79)
64
c
7
d
8
42
a
Standard conditions: 2a (0.20 mmol), CsCO3 (1.5 equiv), MeCN/
EtOAc (1:1, 0.1 M), 40 W 390 nm Kessil lamp, fan, 48 h. NMR
yield. Isolated yield shown in parentheses. A 40 W 370 nm Kessil
lamp was used instead of a 390 nm lamp. No Cs2CO3 was added.
b
c
d
and CrCl3 (entries 4−6, respectively). Removing the base
resulted in a decreased yield (entry 7). Changing the
a
Scheme 2. Substrate Scope for Aldehyde Alkylation
a
Standard conditions: 2 (0.20 mmol, 1 equiv), 1 (30 equiv), Cs2CO3 (1.5 equiv), CrCl3 (3 equiv), TBADT (10 mol %), MeCN/EtOAc (1:1, 0.1
b
c
d
M), 40 W 390 nm Kessil lamp, fan, 48 h. Isolated yield. 1 (20 equiv) was used. 1 (40 equiv) was used. 1 was used as the solvent: 2 (0.80 mmol),
Cs2CO3 (1.5 equiv), CrCl3 (3 equiv), TBADT (10 mol %), MeCN/EtOAc/1 (2:1:3, 0.067 M), two 40 W 390 nm Kessil lamps, fan, 48 h. 1 was
e
used as the solvent: 2 (0.80 mmol), Cs2CO3 (1.5 equiv), CrCl3 (3 equiv), TBADT (10 mol %), MeCN/1 (2:3, 0.08 M), two 40 W 390 nm Kessil
lamps, fan, 48 h.
B
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yields (3l−3v). In the case of DMA, formal deprotonation of
the N-methyl group, the hydrogen of which is much less acidic
than the carbonyl α-proton, proceeded selectively due to the
unique site selectivity of TBADT-mediated hydrogen abstrac-
tion. Aliphatic cyclohexanecarboxaldehyde was also amino-
methylated in good yield (3t). By using anisole, the
oxymethylation of aldehydes was also possible (3w−3ae).
tert-Butyl methyl ether also gave the desired product (3af), but
the yield was lower. Acetals could be used as alternatives to
1,3-dithiane,8 although the yields were low (3ag and 3ah).
Control experiments were carried out to gain mechanistic
insight into this reaction (Scheme 3). No product was
amount (<1%) of 7 was observed when the reaction was
conducted in the dark (eq 7). The generation of 7 is evidence
of the formation of the anionic intermediate, which triggered
the β-alkoxy elimination to form 7.
From the control experiments described above, we propose
the reaction mechanism as depicted in Figure 1. First,
a
Scheme 3. Mechanistic Studies
Figure 1. Proposed mechanism.
abstraction of hydrogen from a non-activated C−H bond by
photoexcited decatungstate B generates carbon radical
intermediate D and reduced decatungstate C. Reduced
decatungstate C [E([W10O32]4−/[W10O32]5− = −0.97 V vs
SCE],11 reduces CrCl3 to CrCl2 [E(Cr3+/Cr2+) = −0.65 V vs
SCE].12 The resulting CrCl2 captures radical intermediate D to
form organochromium carbanion E. Carbanion E reacts with
aldehydes with high chemoselectivity to give final product F.
In summary, we have developed the formal generation of a
catalytic carbanion from a non-activated C−H bond via
decatungstate-catalyzed hydrogen abstraction followed by
RRPCO radical capture with a chromium salt. The method
could be applied to a wide range of substrates, including
cycloalkanes, amides, and ethers, with high site selectivity and
chemoselectivity, and the organochromium carbanions gen-
erated then reacted with aldehydes. By using DMA and anisole
as nucleophile precursors, novel synthetic routes to vicinal
aminoalcohol and diol derivatives, which are useful building
blocks for pharmaceutical and materials chemistry, were
possible. Mechanistic studies supported the generation of
carbanion intermediates.
a
Standard condition A: aldehyde or ketone (0.20 mmol), 1 (30
equiv), Cs2CO3 (1.5 equiv), CrCl3 (3 equiv), TBADT (10 mol %),
MeCN/EtOAc (1:1, 0.1 M), 40 W 390 nm Kessil lamp, fan, 48 h.
Standard condition B: 2a (0.80 mmol), Cs2CO3 (1.5 equiv), CrCl3 (3
equiv), TBADT (10 mol %), MeCN/1b (2:3, 0.08 M), two 40 W 390
nm Kessil lamps, fan, 48 h.
observed for reactions between 1a and 2a without the
chromium salt or without light (eqs 1 and 2). Although the
reaction between 1b and 2a is known to proceed by a radical-
ketyl radical coupling mechanism under irradiation at 300 nm,7
only 2% of 3v was observed in the NMR spectrum of the crude
reaction mixture using the standard reaction conditions but
without CrCl3 (eq 3). Changing 2a to acetophenone (5) or
benzophenone (6) resulted in the recovery of the starting
material in 80% and 81% yields, respectively (eq 5), and the
coupling products were observed in the NMR spectrum of the
crude reaction mixture in <1% yield. These results eliminate
the possibility of the formation of bonds between the alkyl
radical and ketyl radical that are generated from carbonyl
groups by irradiation.7,9 Moreover, the reaction between 2a
and eucalyptol (1c) gave only trace amounts of coupling
product 3ai, but instead, α-terpineol (7) was obtained as the
major product (eq 6).10 On the other hand, only a trace
ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00096.
Experimental Procedures and spectral data for all new
compounds (PDF)
C
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AUTHOR INFORMATION
Corresponding Author
■
Kenzo Yahata − Osaka University, Suita, Japan;
orcid.org/0000-0002-6991-9221; Email: yahata-k@
phs.osaka-u.ac.jp
Other Authors
Shu Sakurai − Osaka University, Suita, Japan
Shuhei Hori − Osaka University, Suita, Japan
Shin Yoshioka − Osaka University, Suita, Japan
Yuki Kaneko − Osaka University, Suita, Japan
Kai Hasegawa − Osaka University, Suita, Japan
Shuji Akai − Osaka University, Suita, Japan;
orcid.org/0000-0001-9149-8745
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00096
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by a JSPS Grant-in-Aid for
Young Scientists (Grant JP18K14868), the Teijin Pharma
Award in Synthetic Organic Chemistry Japan, a Grant-in-Aid
from the Tokyo Biochemical Research Foundation, the
Keihanshin Consortium for Fostering the Next Generation of
Global Leaders in Research (K-CONNEX), established by
Building of Consortia for the Development of Human
Resources in Science and Technology, MEXT, and the
Platform Project for Supporting Drug Discovery and Life
Science Research [Basis for Supporting Innovative Drug
Discovery and Life Science Research (BINDS)] from AMED
(Grant JP19am0101084).
̌
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Nature 2018, 560, 70. (s) Yuan, Z.; Yang, H.; Malik, N.; Colovic, M.;
́
Weber, D. S.; Wilson, D.; Benard, F.; Martin, R. E.; Warren, J. J.;
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