Concurrent Formation of N-H Imines and Carbonyl Compounds by Ruthenium-Catalyzed C-C Bond Cleavage of β-Hydroxy Azides

Article abstract of DOI:10.1021/acs.orglett.0c01145

A commercial cyclopentadienylrutenium dicarbonyl dimer ([CpRu(CO)2]2) efficiently catalyzes the formation of N-H imines and carbonyl compounds simultaneously from β-hydroxy azides via C-C bond cleavage under visible light. Density functional theory calculations for the cleavage reaction support the mechanism involving chelation of alkoxy azide species and liberation of nitrogen as the driving force. The synthetic utility of the reaction was demonstrated by a new amine synthesis promoted by chemoselective allylation of imine and synthesis of isoquinoline.

Full text of DOI:10.1021/acs.orglett.0c01145

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Letter
Concurrent Formation of N−H Imines and Carbonyl Compounds by
Ruthenium-Catalyzed C−C Bond Cleavage of β‑Hydroxy Azides
Jeong Min Lee,† Dae Young Bae,† Jin Yong Park, Hwi Yul Jo, Eunsung Lee,* Young Ho Rhee,*
and Jaiwook Park*
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ABSTRACT: A commercial cyclopentadienylrutenium dicarbonyl
dimer ([CpRu(CO)2]2) efficiently catalyzes the formation of N−H
imines and carbonyl compounds simultaneously from β-hydroxy azides
via C−C bond cleavage under visible light. Density functional theory
calculations for the cleavage reaction support the mechanism involving
chelation of alkoxy azide species and liberation of nitrogen as the driving
force. The synthetic utility of the reaction was demonstrated by a new
amine synthesis promoted by chemoselective allylation of imine and
synthesis of isoquinoline.
arbon−carbon bond-cleavage reactions have attracted
Scheme 1. Cleavage of 1,2-Azido Alcohols
C
much attention in organic synthesis because the methods
enable to access targeted complex organic molecules for
valuable chemical feedstock.1 Most of the methods are driven
by ring strain or directing groups, which includes C−C
oxidative addition of strained cyclopropenes, β-carbon
elimination of various cyclized substrates, C−C bond
activations assisted by chelation, decarbonylation, etc.1 In
particular, cleavage of unstrained C−C bonds is recognized as
one of the most challenging transformations.2 To this trend,
several metal-catalyzed processes such as oxidative addition
and β-carbon elimination or photocatalytic redox neutral
processes have been developed to induce C−C bond cleavage.3
In the course of our continuing studies on the development
of a new Ru-catalyzed process under photolytic conditions, we
became interested in cleavage reactions of 1,2-azido alcohols
because C−C bond cleavage reactions can proceed without
external oxidants. Most of the known cleavage reactions of 1,2-
azido alcohol utilize strong oxidants to generate nitriles with
the concomitant formation of carbonyl compounds (Scheme
1).4−7 For example, Suarez and co-workers described the
́
synthesis of nitriles by radical fragmentation with hypervalent
iodine reagent.4 Ye and co-workers reported PCC (pyridinium
chlorochromate) oxidation to nitriles accompanied by a one-
carbon loss.5 Unlike these precedents, little is known about the
cleavage under nonoxidative conditions, which will concur-
rently generate carbonyl compounds and their unstable N−H
imine derivatives. Recently, Chiba and co-workers disclosed a
Pd(II)-catalyzed ring expansion reaction of cyclic 1,2-azido
alcohols to azaheterocycles, where imino-aldehyde was
assumed to be a plausible intermediate (Scheme 1).6 In
addition, the Knowles group also reported a novel photo-
catalytic conversion of cyclic alcohols to linear ketones via C−
C bond cleavage of the alcohols, which involves homolytic
activation of alcohol O−H bonds followed by the generation of
alkoxy radical intermediates via an intramolecular proton-
coupled electron transfer (PCET) process.3c,d Motivated by
these examples and our previous studies on the activation of
Received: March 30, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01145
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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alkyl azides and racemization of alcohols by a ruthenium
catalyst under visible light,8 we have been investigating
cleavage reactions of 1,2-azido alcohols. Here, we report that
Ru-catalyzed reaction under photolytic conditions generates
carbonyl compounds and their corresponding N−H imines as
stable entities via effective C−C bond cleavage of 1,2-azido
alcohols. The system produces NH-imine in a short timeframe
under mild conditions with a wider substrates scope including
four-, five-, and eight-membered azido alcohols. The synthetic
utility of this unprecedented process was demonstrated by the
new amine synthesis utilizing chemoselective allylation of
imines and the synthesis of ketonimine, isoquinoline, and
amino alcohol from cyclic 1,2-azido alcohols.
entry 2) were further applied for other 1,2-azido alcohol
substrates.
The transformation to imine and carbonyl products was
applicable for a broad range of substrates bearing primary azide
(Table 2). The electronic effect of the substituents of aromatic
Table 2. Ruthenium-Catalyzed Cleavage of 1,2-Azido
Alcohol Substrates
Initially, the C−C bond cleavage reaction of 1-azido-1,2-
diphenylpropan-2-ol (2a) was examined under various
conditions. The bulky ruthenium dimer 1a, which is active
for N−H imine formation of alkyl azides, is not effective for
the C−C bond-cleavage reaction, while a simple and
commercial ruthenium dimer (1b) catalyzes the reaction to
afford great yields of the desired products (3a and 4a) in the
presence of light (Table 1). However, in the absence of light,
Table 1. Cleavage of 1,2-Azido Alcohol: Catalyst Screening
and Condition Optimization
a
a
a
a
conv.
(%)
3a
4a
5a
entry
catalyst
solvent
(%)
(%)
(%)
1
2
3
4
5
6
7
8
1a
1b
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
PhMe-d8
CD3CN
CD2Cl2
THF-d8
THF-d8
THF-d8
10
>99
41
2
<1
>99
>99
>99
<1
29
>99
8
99
41
<1
<1
99
99
99
0
3
92
38
0
0
0
0
0
0
0
0
79
0
5
[Cp*Ru(CO)2]2
Ru(PPh3)3(CO)H2
Ru3(CO)12
1b
1b
1b
1b
1b
0
92
91
5
0
21
92
b
9
10
11
c
28
99
d
1b
0
a
1
Determined by H NMR using mesitylene as an internal standard.
b
c
In the absence of light. 0.1 mol % of [CpRu(CO)2]2 was used for 24
d
h. Cp* = pentamethylcyclopentadienyl. Photoirradiation for 5 min.
a
A solution of azide 2 (0.25 mmol) and [CpRu(CO)2]2 (2.0 mol %)
in THF-d8 (0.5 mL) was stirred at rt for 1 h under 30 W fluorescent
b
1
light. The yields were determined after isolation, while H NMR
yields (in parentheses) were determined using mesitylene and CH2Br2
the reaction does not proceed. Compared with 1a, a slightly
less bulky pentamethylcyclopentadienyl ruthenium dimer
affords 41% yield for both 3a and 4a. However, a monomeric
ruthenium complex (Ru(PPh3)3(CO)H2) and triruthenium
carbonyl complex (Ru3(CO)12) turned out to be inefficient
catalysts for the C−C bond cleavage reaction. Interestingly, the
reaction can be operated in various organic solvents such as
THF, toluene, and MeCN without any loss of the yield for 3a
and 4a. In dichloromethane, a condensed product (5) was
formed in high yield probably due to basic impurities.9 The
reduced amount of active catalyst 1b also decreases the yields
of 3a and 4a. Even though the reaction of 2a completes upon
exposure to light for only 5 min (Table 1, entry 11), we
irradiated the reaction for 1 h to secure enough activation time
for the ruthenium catalysts for various other substrates. From
the optimization processes, the standard conditions (Table 1,
c
d
as an internal standard. 1.0 mmol scale (2b). 5.0 mol % of
[CpRu(CO)2]2 was used for 5 h. 10 mol % of [CpRu(CO)2]2 was
e
used for 5 h.
rings of 1,2-azido alcohols was not affected since both methoxy
and fluoride groups gave an almost quantitative yield for the
corresponding aldehyde or ketones. However, aliphatic
disubstituted alcohol, 1-azidooctan-2-ol (2k, Table 2, entry
10), did not produce a desired product, while trisubstituted 1-
azido-2-methylhexan-2-ol (2e, Table 2, entry 4) afforded the
desired product in 80% yield.
The transformation was also applicable for more densely
substituted azido alcohols (Table 3). Monosubstituted 1,2-
azido alcohols with methyl or phenyl are effective for C−C
B
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Table 3. Ruthenium-Catalyzed Cleavage of 1,2-Azido
Alcohol Substrates
Table 4. Ruthenium-Catalyzed Cleavage of Cyclic 1,2-Azido
Alcohol Substrates
a
1
a
The yields were determined by H NMR using mesitylene, CH2Cl2,
The yields were determined after isolation, while others were
1
and residual THF as an internal standard. 8 was isolated by 80% yield.
determined by H NMR using mesitylene and CH2Br2 as an internal
standard. Due to the difficulty of isolation of 3m (acetone), 3m was
b
b
The imine transformed into a cyclic pyrrole (2-phenyl-1H-pyrrole
c
(7a)), which was isolated by 49% yield, after 40 h at 70 °C. The
amine was not observed due to decomposition. 1b (6 mol %), 30
min, conversion (14%). The initial product carbonyl imine was
further converted to 2-phenylpropan-2-ol (3m′, isolated in 12% yield)
d
by reaction with PhMgBr.
e
reduced to amine using NaBH4 (4.0 equiv)/MeOD-d4. (1.0 mL). The
amino alcohol (9) was further isolated as a carboxybenzyl (Cbz)-
protected product by 48% yield.
bond-cleavage reactions. However, a disubstituted substrate at
azido carbon with phenyl group did not produce the desired
products, probably due to the steric effect. In addition, a
tetrasubstituted substrate even methyl group shows no reaction
with the same steric reason.
To investigate the efficiency of 1b as a photocatalyst, 2l was
screened for checking the effective duration of the photo-
irradiation. Compound 1b catalyzes the conversion of 2l to the
desired products with irradiation for only 5 min. However,
further conversion proceeds only when the light irradiates,
probably due to the rebound of CO to the metal center (Table
S1). This demonstrates clearly that the photoirradiation is a
key factor for the catalysis.
From a synthetic viewpoint, the reaction concurrently
generates two reactive functional groups and thus should be
highly useful. In particular, C−C bond cleavage reactions of
cyclized β-hydroxy azides can provide a corresponding
carbonyl imine, which can be further utilized for the synthesis
of N-heterocycles or linear amino alcohols as described in
Table 4. A four-membered cyclic β-hydroxy azide (2s)
afforded a linear benzoyl imine (7) in high yield (95%).
Interestingly, the reaction of cis-2-azido-1-indanol (2t) and
trans-2-azido-1-indanol (2u) under the optimized conditions
generated isoquinoline (8) in a high 94% yield at room
temperature (Table 4). However, six- and seven-membered
cyclic β-hydroxy azides (2v, 2w, 2x) did not afford any desired
product under the standard conditions even with a higher
loading of the catalyst (Table 4, entry 6). Most interestingly,
the reaction catalyzes the C−C bond cleavage of eight-
membered cyclic β-hydroxy azide (2y) to generate the
corresponding carbonyl imine, and the subsequent reduction
of the compound affords an amino alcohol (9).
We further envisioned that the chemoselective reaction of
imine in the presence of aldehyde would offer an
unprecedented synthetic method for highly substituted
amine. For example, the treatment of the crude mixture
generated from 2-azido-1,2-diphenylethan-1-ol (2o) with
allylboronic acid pinacol ester (10) generated imine in 87%
yield (Scheme 2). The allylborane species reacted selectively
first with phenylmethanimine, and the condensation reaction
Scheme 2. Combination of the Cleavage Reaction of 1,2-
Azido Alcohol with Selective Allylation Reaction of N−H
Aldimine*
*
Key: (i) 1b (1.0 mol %)/THF, 30 W fluorescent light; (ii)
allylboronic acid pinacol ester (10, 1.1 equiv); (iii) NaBH3CN (4.0
a
equiv)/MeOH. Isolated yield.
C
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between the allylation product and benzaldehyde proceeded
subsequently to afford (Z)-1-phenyl-N-(1-phenylbut-3-en-1-
yl)methanimine (11). In this reaction, allylic alcohol was
obtained only in a trace amount, thus verifying the affinity of
the allyl boronate with the imine over the aldehyde. The
reduction of 11 with NaBH3CN gave the known secondary
amine 12 in 63% isolated yield.
The feasible mechanism proposed was that the formation of
N−H imine and carbonyl compound proceeds by (i)
activation of the precatalyst, (ii) formation of a ruthenium(II)
azido alkoxide complex, and (iii) formation of a carbonyl
compound and an iminyl ruthenium complex via C−C bond
cleavage. The precatalyst (1b) has been studied and known for
photoactivation to afford a reactive ruthenium dimer (1bact)
via the release of CO (Scheme 3).10 Subsequently, the model
Scheme 3. Proposed Reaction Pathway for Ruthenium-
Catalyzed C−C Bond Cleavage of β-Hydroxy Azides
substrate (1-azido-1,2-diphenylpropan-2-ol, 2o) reacts to
cleave the reactive dimer generating ruthenium(II) alkoxide
complex (13) and a ruthenium(II) hydride (14) (Scheme 3).
The alkoxide complex (13) was assumed to afford a
ruthenium(II) azido alkoxide complex (15), which is an active
catalyst for the C−C bond cleavage reaction by photo-
irradiation. Once 15 was formed, the release of dinitrogen was
assumed to derive the formation of a highly reactive
ruthenium(II) alkoxy nitrene complex (16), followed by the
cleavage of C−C bond in the ruthenium β-hydroxy nitrene
complex via a transition state (TS1) to afford a carbonyl
compound and an iminyl ruthenium complex (17). The
substitution of the carbonyl compound and the iminyl ligand
with the substrate closes the catalytic cycle to generate the
ruthenium(II) azido alkoxide complex (15).
To evaluate the feasible mechanism for the transformation,
the proposed ruthenium complexes were computed by the
density functional theory (DFT) calculations using the
Gaussian 09 program11 package at the B3PW91/BSI//BSII,
SMD level (see the Supporting Information for details of the
calculation)12 in the gas and solution (THF) phase to obtain
an energy profile for the ruthenium catalysis. Figure 1
summarizes structural and free energy data for the proposed
pathway suggested in Scheme 3. For activation of the
precatalyst (1b) with the model substrate (2o), the formation
of two ruthenium intermediates (13 and 14) requires 19.2 and
21.4 kcal/mol in the gas and solution phase, respectively
(Figure 1b). The disproportionation of cyclopentadienylrute-
nium dicarbonyl dimer (1b) involves photodissociation of CO,
Figure 1. (a) Net thermodynamic energy for the formation of N−H
imine and carbonyl compound from β-hydroxy azide, (b) activation of
precatalyst, and (c) thermodynamic profile of a feasible mechanism
for ruthenium-catalyzed C−C bond cleavage of β-hydroxy azides with
schematic or optimized structures and relative free energies (kcal/
mol) computed at the B3PW91/BS I//BS II, SMD level in the gas
and solution (THF) phase.
subsequent ligand substitution, and cleavage of ruthenium
dimer by the substrates.10 After the ruthenium(II) alkoxide
complex (13) is generated, the substitution of CO with a
dangling azido group requires 11.2 and 15.9 kcal/mol in the
gas phase and THF solution, respectively (Figure 1c), to
produce the active catalyst (15). The catalyst (15) then release
N2 immediately to form a thermodynamically unstable
intermediate, the ruthenium(II) alkoxynitrene complex (16),
which produces a κO-carbonyl and iminyl ruthenium complex
(17) via a transition state of C−C bond cleavage with a barely
higher barrier (only 0.5 and 0.2 kcal/mol in gas phase and
THF solution, respectively). The κO-carbonyl and iminyl
ruthenium complex (17) converts to the active catalyst (15)
by deprotonation of the iminyl ligand subsequent ligand
substitution with the substrate (2o). Since the driving force for
this transformation involves the release of dinitrogen gas and
subsequent formation of the unstable nitrene ruthenium
complex, a net energy gain was calculated by 65 and 68
kcal/mol in the gas phase and THF solution, respectively.
D
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Therefore, we believe that the formation of the active catalyst
(13) is the rate-determining step.
ACKNOWLEDGMENTS
■
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIP) (NRF-2018R1A4A1024713).
In summary, we developed a new ruthenium catalysis for the
synthesis of N−H imines and carbonyl compounds from C−C
bond cleavage of 1,2-azido alcohols, of which reaction is
thermodynamically driven by the liberation of molecular
nitrogen under ruthenium catalysis and light irradiation. The
theoretical study clearly demonstrated that the liberation of
dinitrogen and subsequent C−C bond cleavage of an unstable
alkoxy nitrene ruthenium complex offers a strong driving force
right after the formation of the active ruthenium catalyst, of
which formation requires photoactivation. As demonstrated by
the new amine synthesis, we anticipate that the current method
will lead to the development of a new reaction sequence. We
continue to discover new reactivity of different azido substrates
under similar reaction conditions.
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The Supporting Information is available free of charge at
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Characterization including NMR spectroscopic data
(PDF)
AUTHOR INFORMATION
■
Corresponding Authors
Eunsung Lee − Department of Chemistry, POSTECH, (Pohang
University of Science and Technology), Pohang 37673, Republic
of Korea; orcid.org/0000-0002-1507-098X;
Email: eslee@postech.ac.kr
Young Ho Rhee − Department of Chemistry, POSTECH,
(Pohang University of Science and Technology), Pohang 37673,
Republic of Korea; orcid.org/0000-0002-2094-4426;
Email: yhrhee@postech.ac.kr
Jaiwook Park − Department of Chemistry, POSTECH, (Pohang
University of Science and Technology), Pohang 37673, Republic
of Korea; orcid.org/0000-0002-1135-3317; Email: pjw@
postech.ac.kr
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Dae Young Bae − Department of Chemistry, POSTECH,
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Republic of Korea
Jin Yong Park − Department of Chemistry, POSTECH,
(Pohang University of Science and Technology), Pohang
37673, Republic of Korea
Hwi Yul Jo − Department of Chemistry, POSTECH, (Pohang
University of Science and Technology), Pohang 37673,
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Complete contact information is available at:
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Author Contributions
†J.M.L. and D.Y.B. contributed equally to this work.
Notes
The authors declare no competing financial interest.
E
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