Formation of Carbonyl Compounds from Amines through Oxidative C-N Bond Cleavage using Visible Light Photocatalysis and Applications to N-PMB-Amide Deprotection

Article abstract of DOI:10.1002/adsc.201500257

A method has been developed for C-N bond cleavage that utilizes visible light photocatalysis. The process, which utilizes 1 mol% of the ruthenium complex Ru(bpy)₃Cl₂ as the photocatalyst, potassium persulfate (K₂S₂O₈) as the oxidant and water/acetonitrile (H₂O/CH₃CN) as the solvent, transforms a variety of primary, secondary and tertiary amines to the corresponding carbonyl compounds. In addition, this method was applied to the removal of a p-methoxylbenzyl (PMB) group from N-PMB protected amides.

Full text of DOI:10.1002/adsc.201500257

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DOI: 10.1002/adsc.201500257
Very Important Publication
Formation of Carbonyl Compounds from Amines through
À
Oxidative C N Bond Cleavage using Visible Light Photocatalysis
and Applications to N-PMB-Amide Deprotection
a,b
b,
Naeem Iqbal and Eun Jin Cho *
a
Department of Applied Chemistry, Hanyang University, Ansan, Gyeonggi-do 426-791, Republic of Korea
Department of Chemistry, Chung-Ang University, Seoul 156-756, Republic of Korea
b
Fax: (+82)-2-825-4736; e-mail: ejcho@cau.ac.kr
Received: March 14, 2015; Revised: April 29, 2015; Published online: July 14, 2015
th
Dedicated to Prof. Stephen L. Buchwald on the occasion of his 60 birthday.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500257.
[
3]
Abstract: A method has been developed for CÀN
bond cleavage that utilizes visible light photocataly-
sis. The process, which utilizes 1 mol% of the ruthe-
nium complex Ru(bpy) Cl as the photocatalyst, po-
ganic transformations. As a result, a demand exists
for new, facile and mild methods to prepare substan-
ces that contain the carbonyl functional group. An al-
ternative to the conventional method for producing
3
2
[4]
tassium persulfate (K S O ) as the oxidant and
aldehydes and ketones by oxidation of alcohols, in-
volves oxidations of readily abundant amines. Several
protocols have been developed for this purpose
2
2
8
water/acetonitrile (H O/CH CN) as the solvent,
2
3
transforms a variety of primary, secondary and terti-
ary amines to the corresponding carbonyl com-
pounds. In addition, this method was applied to the
removal of a p-methoxylbenzyl (PMB) group from
N-PMB protected amides.
[5]
[Scheme 1, Eq. (1)], however, despite their efficien-
cy these methods suffer from the required use of stoi-
chiometric amounts of toxic metal-containing re-
agents and incompatibility with other readily oxidized
functional groups.
Keywords: carbonyl compounds; CÀN bond cleav-
a-CÀH activation of amines (1) via formation of
age; photocatalysis; PMB deprotection; visible light
iminium ion intermediates (2) has been employed in
[6]
a wide variety of organic transformations. Recently,
visible light photocatalysis
[7,8]
has been used to pro-
In living systems, enzyme-catalyzed reactions are in-
volved in a large number of pathways leading to
structurally diverse and complex natural products.
The efficiencies and mild nature of these enzymatic
processes have encouraged organic chemists to devel-
op transformations which mimic those promoted by
[
1]
enzymes. A well-known biological transformation
involves the enzyme-promoted oxidative deamination
of amines to form the corresponding carbonyl prod-
[
2]
ucts. An important example of this process is the
monoamine oxidase-catalyzed reaction of neurogenic
primary amines, which employs FAD cofactors as oxi-
dants and proceeds through pathways involving oxida-
tive formation and hydrolysis of iminium ion interme-
[
2d]
diates. These reactions generate ammonia and car-
bonyl compounds.
Carbonyl groups, especially those of aldehydes and
ketones, are not only found in a wide range of natural
products but they also participate in an array of or- Scheme 1. Oxidative CÀN bond cleavage.
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Naeem Iqbal and Eun Jin Cho
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[9]
tion of a-functionalized amines [Scheme 2, Eq. (4)].
We planned to use water as the nucleophile to cap-
ture iminium ions formed in this manner which would
[9i,10]
result in CÀN bond cleavage
carbonyl products (3) and truncated amines (4)
Scheme 2, Eq. (5)]. Importantly, in order for
and production of
[
a method which follows this mechanistic pathway to
be synthetically applicable, over-oxidation of the car-
bonyl and amine products needs to be avoided
[
Scheme 2, Eqs. (6) and (7)].
In the investigation described below, we developed
a green, mild, efficient and operationally simple
method for oxidative CÀN bond cleavage of amines,
which utilizes visible light photoredox catalysis and
that generates carbonyl compounds [Scheme 1, Eq.
Scheme 2. a-CÀH activation of amines.
(
2)]. In addition, we have shown that this procedure
mote the formation of iminium ions that are captured can be applied to the efficient removal of p-methoxy-
by nucleophiles in processes that lead to the genera- benzyl (PMB) groups from N-PMB protected amides
Table 1. Optimization of the reaction conditions for the CÀN bond cleavage re-
[a]
action of 1a.
[
[
a]
b]
Reaction scale: 1a (0.1 mmol).
The yield was determined by using gas chromatography with dodecane as the
internal standard.
No visible light irradiation.
[c]
2
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Formation of Carbonyl Compounds from Amines
[
11]
ÀN bond cleavage of primary amines to
[
Scheme 1, Eq. (3)]. The green nature of these pro- Table 2. Oxidative C
[a,b]
form carbonyl compounds.
cesses is further heightened by the fact that water is
a component of the solvent system used.
[
12]
Our initial exploratory studies, targeted at the de-
velopment of a photocatalytic method for transform-
ing amines to carbonyl compounds, employed benzyl-
amine 1a as a model amine, the widely used photoca-
talyst Ru(bpy) Cl , and visible light irradiation. We
3
2
observed that irradiation of a solution of 1a and
Ru(bpy) Cl in H O under an oxygen atmosphere
3
2
2
leads to generation of N-benzylbenzaldimine (5a) as
the major product along with a minor amount of ben-
zaldehyde (3a) (Table 1, entry 1). Although imine 5a
was not the desired product, its formation by conden-
sation of amine 1a with aldehyde 3a shows that the
anticipated CÀN bond cleavage process is proceeding
efficiently under the photocatalytic, oxidative condi-
[
13]
tions employed.
In order to enhance the formation of carbonyl
product 3a in this reaction, the effects of several pa-
rameters, including solvent, photocatalyst and oxidant
were evaluated. Although the use of methanol as sol-
vent does not alter the outcome of the reaction
(
Table 1, entry 2), a significantly increased yield of 3a
occurs when the aqueous medium contains organic
co-solvents such as CH CN (Table 1, entries 3–6).
3
While a change of the photocatalyst does not lead to
an enhanced efficiency (Table 1, entries 7–13),
a change in the oxidant from molecular oxygen to
K S O results in a highly selective CÀN bond cleav-
[a]
2
2
8
Reaction scale: 1 (0.5 mmol).
b]
[
age reaction that forms 3a exclusively (Table 1,
The yields were isolated or GC yield (due to the volatili-
ty of the product).
[
14]
entry 15). The results of control experiments dem-
onstrated that the process requires visible light irradi-
[
15]
ation, a photocatalyst, and an oxidant (Table 1, en-
tries 17–19).
showed that a wide range of these types of amines un-
The amine scope of the process was explored next dergo oxidative CÀN bond cleavage exclusively at
(
Table 2). The results show that a variety of amines their benzylic positions to produce benzaldehyde de-
containing both aromatic and aliphatic substituents rivatives. This regioselectivity of these reactions is
are converted in high yields to aldehyde and ketone a consequence of the comparatively higher acidity of
products when the optimized reaction conditions are the benzylic protons in amine cation radical inter-
utilized. Various functional groups, such as an acetal mediates formed by single electron transfer (SET) in
(
3f) and aromatic halides (3g, 3h, and 3i), are unaf- this process. CÀN bond cleavage reactions of cyclic
fected under the reaction conditions. In addition, sub- and acyclic tertiary amines also take place smoothly.
stituents containing benzylic hydrogens, like in 3b, 3c, Moreover, the reactivity of the amines is not affected
and 3k, remain unaltered in this process. Benzyl- significantly by the electronic properties of aryl ring
amines with ortho-substitution are also suitable sub- substituents as demonstrated by the fact that benzyl-
strates for the process (3b and 3h). Reactions of sub- amines with both electron-donating (1s) and electron-
strates having electron-donating substituents (3d, 3e, withdrawing (1t) substituents undergo the photocata-
3
f) on the aromatic ring give comparatively higher lytic reaction in high yields (Table 3, entries 6 and 7).
yields than those with electron-withdrawing groups Interestingly, the styrene-containing tertiary amine
3g, 3h, 3i, 3j). Application of this oxidation method (1u) reacts to form cinnamylaldehyde (3u) in moder-
to aliphatic amines (3k, 3l, 3m, 7a, and 7b) show the ately high yield (Table 3, entry 8).
(
generality and practicality of the method.
Observations made in this effort suggested that the
The facile nature and efficiency of the photocatalyt- photocatalytic protocol might be applicable to the
ic reaction prompted a study aimed at probing its use design of a new N-deprotection strategy. However, we
for CÀN bond cleavage of N-benzyl secondary and envisaged that utilization of this method to deblock
tertiary amines (Table 3). The results of the effort N-benzyl-protected amines might be complicated by
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Naeem Iqbal and Eun Jin Cho
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Table 3. Oxidative CÀN bond cleavage reactions of secon-
dary and tertiary amines.
Table 4. PMB deprotection of amides oxidative CÀN bond
[a]
[a,b]
cleavage.
[
[
a]
b]
Reaction scale: 6 (1.0 mmol).
Yields of isolated products are given.
formed as a side product. Interestingly, the strategy
also appears to be applicable to the removal of the
PMB group from protected sulfonamides as exempli-
fied by the conversion of 10 to benzenesulfonamide
(
11) under the conditions (Scheme 3).
[
[
a]
b]
Reaction scale: 1 (0.5 mmol).
The yields were isolated or GC yield (due to the volatili-
ty of the product).
Scheme 3. Deprotection of N-PMB benzenesulfonamide.
A plausible mechanism for the oxidative CÀN bond
secondary CÀN bond cleavage reactions of the secon-
dary and primary amines produced in these processes.
2
À
In contrast, we reasoned that the photocatalytic cleavage reaction begins with SET to [S O ] from
2
8
method would be ideally suited to the deprotection of the metal-to-ligand charge transfer excited state of
2+
amides because the formed free amides would be less [Ru(II)(bpy) ] , generated by visible light irradiation
3
À
susceptible to over-oxidation. Moreover, an N-p-me- (Scheme 4). This process leads to formation of SO C
4
2À
thoxybenzyl (PMB) group was selected for the pro- and SO , through OÀO bond cleavage of intermedi-
4
3À
3+
posed blocking/deblocking method owing to its ex- ate [S O ] . Reduction of [Ru(III)(bpy) ] by SET
2
8
3
2+
pected higher susceptibility towards CÀN bond cleav- from the amine or amide regenerates [Ru(II)(bpy)3]
[
16]
age. Indeed, in studies aimed at evaluating the new along with the radical cation intemediate 1A. Loss of
protocol, we observed that PMB groups are efficiently an a-proton from 1A followed by oxidation of the re-
removed from N-PMB protected aromatic and ali- sulting a-amino radical generates the imine 2A, which
phatic amides by utilizing the photocatalytic condi- through hydrolysis produces the carbonyl compound
tions [1 mol% Ru(bpy) Cl , 1 equivalent K S O in 3.
3
2
2
2
8
H O/CH CN] (Table 4). In these processes, minor
The study described above led to the development
2
3
amounts of N-acyl-4-methoxybenzaldimines are of a mild, eco-friendly and practical method for carry-
2
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Formation of Carbonyl Compounds from Amines
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1
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An oven-dried re-sealable tube, equipped with a magnetic
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3 2 2 8
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2
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room temperature. After 12 h irradiation, the tube was
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5
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Acknowledgements
[
This work was supported by the National Research Founda-
tion of Korea (NRF) [NRF-2014R1A1A1A05003274, NRF-
2014-011165, and NRF-2012M3A7B4049657] and the TJ Sci-
ence Fellowship of the POSCO TJ Park Foundation.
2
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[
[
15] In this transformation, a blue LED strip was a better
visible light source than a compact fluorescent lamp.
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2
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