Photo-mediated [1, 3]-Carbonyl shift of β-Ketocarbonyls

Article abstract of DOI:10.1016/j.jphotochem.2020.112553

An organic amine mediated photolytic [1,3]-benzoyl migration of β-benzoyl carbonyl compounds was reported. This migration was achieved by Norrish-Yang cyclization and retro-aldol reaction under black light (365 nm) or visible light irradiation. This photolytic protocol provides an alternative approach to the synthesis of 1,5-dicarbonyl compounds. By chiral primary amine catalysis, a kinetic resolution was also developed to afford enantioenriched 1,5-dicarbonyls.

Full text of DOI:10.1016/j.jphotochem.2020.112553

JournalofPhotochemistry&PhotobiologyA:Chemistry396(2020)112553
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photo-mediated [1, 3]-Carbonyl shift of β-Ketocarbonyls
Wenzhao Zhanga,b, Yao Lic, Sanzhong Luoc,*
a Key Laboratory for Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China
b School of Chemical Science, University of Chinese Academy of Sciences, Beijing, 100049, China
c Center of Basic Molecular Science, Department of Chemistry, Tsinghua University, Beijing, 100084 China
A R T I C L E I N F O
A B S T R A C T
Keywords:
An organic amine mediated photolytic [1,3]-benzoyl migration of β-benzoyl carbonyl compounds was reported.
This migration was achieved by Norrish-Yang cyclization and retro-aldol reaction under black light (365 nm) or
visible light irradiation. This photolytic protocol provides an alternative approach to the synthesis of 1,5-di-
carbonyl compounds. By chiral primary amine catalysis, a kinetic resolution was also developed to afford en-
antioenriched 1,5-dicarbonyls.
Norrish-Yang
Retro-Aldol
[1,3]-shift
1,5-dicarbonyl compounds
1. Introduction
oped with our chiral primary amine catalysts [9,10]
We recently investigated an enamine-version de Mayo reaction
using chiral primary amine catalysis (Scheme 1, I) [1]. Unfortunately,
the reaction did not afford the desired de Mayo adduct 3a, but an [1,3]-
benzoyl shift product 4a together with its further cyclic-condensation
product (vide infra). Mechanistically, the rearrangement adduct 4a was
formed via a retro-aldol process following the typical carbonyl Norrish-
Yang cyclization (Scheme 1, I) [2,3]. In this case, the inherent structural
feature of 1,3-diketones facilitates a facile retro-aldol C-C cleavage of
the Norrish-Yang cyclobutanol (Scheme 1, II) to give 1,5-diketones
[2,3,4]. From the synthetic point of view, this Norrish-Yang cyclization
and retro-aldol sequence provides a facile access to 1,5-diketones,
which are recognized as versatile synthetic intermediates for their
widely applications in organic synthesis [5]. Though known in carbonyl
photo-processes, [2] the synthetic potential of this Norrish-Yang-retro
aldol process remained surprisingly much less explored [3]. The major
challenge comes from the always accompanied α- and β-cleavage
competing pathways,[4,6,7] diminishing its synthetic applicability. The
observation of sole-production of benzoyl-shift adduct 4a in the pre-
sence of aminocatalyst promoted us to further investigate this reaction.
Herein, we’d like to report an amine-promoted and photo-mediated
protocol that complements the typical Michael addition procedure in
accessing 1,5-diketones [8]. In addition, a kinetic resolution of the 1,5-
ketones via an intramolecular aldol condensation has also been devel-
2. Experimental section
2.1. Materials
The corresponding 1,3-diketones 2a-2u were prepared by alkylation
of the corresponding α‐unsubstituted 1,3-diketone with alkyl bromide
iodide [11] or addition of aldehydes to enones;[12] β‐ketoesters 2v-2x
were prepared by alkylation of the corresponding α‐unsubstituted
β‐ketoesters with alkyl bromide;[13] cyclic substrates 6 were prepared
by direct alkylation of corresponding β‐ketoesters or 1,3-diketones with
activated cycloalkanes [14].
2.2. Procedure
An oven-dried 10 mL schlenk tube was charged with 2a (0.1 mmol,
1.0 equiv) and additive. The tube was purged with a stream of nitrogen,
solvent was added via syringe. The resultant mixture was degassed
three times. Then the tube was placed approximately 2 cm to 15 W
365 nm LED (black light) and stirred at room temperature for given
time. The reaction mixture, upon concentration, was purified directly
by silica gel column to give the target products 4a as the major product.
General procedure of reactions of other substrates see SI.
⁎ Corresponding author.
E-mail address: luosz@iccas.ac.cn (S. Luo).
https://doi.org/10.1016/j.jphotochem.2020.112553
Received 7 February 2020; Received in revised form 30 March 2020; Accepted 6 April 2020
Availableonline24April2020
1010-6030/©2020ElsevierB.V.Allrightsreserved.

W. Zhang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry396(2020)112553
Scheme 1. Typical Carbonyl Photochemistry and our unexpected finding of a Norrich-Yang-retro Aldol process.
3. Result discussion
(Table 1, entries 2–5). Protonated diamines 1a-1d as catalysts result in
extremely low yields, although no dealkylated adduct 5 was observed
(Table 1, entries 6–10). In these cases, notable enantioselectivity was
observed, and this was later ascribed to a following-up kinetic resolu-
tion process (see Scheme 4). The screening of other organic base led to
the identification of quinuclidine as the optimal additive. The use of one
equivalent quinuclidine in p-xylene gave 64 % yield of [1,3]-benzyl
shifted adduct 4b as the major product (Table 1, entries 11–15). Under
this condition, minor dealkylated product 5 was still isolated and this
compound in fact also served as the starting material in preparing 2a,
thus could be recycled and reused.
3.1. Optimization of conditions
In our initial experiments, we found the photoexcitation of 2a in the
absence of chiral primary amine 1a led to a dealkylated 5 as the major
product, via the typical Norrish Type-II process (Scheme 1, II). It is
known that amine could retard the Norrish cleavage process by stabi-
lizing the excited carbonyl in the form of radical ion pair (as shown in
Scheme 1, II) [15]. This result promoted us to further optimize the
reaction by screening different base in this photochemical process
(Table 1). Under the irradiation of 365 nm LED, the reaction afforded
mainly dealkylated adduct 5 together with a minor [1,3]-acyl shifted
adduct 4a (Table 1, entry 1) in the absence of any additive. The addi-
tion of both inorganic acid and base didn’t lead to any improvement
3.2. Substrate scope
With optimized conditions in hand, we investigated the substrate
2

W. Zhang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry396(2020)112553
Table 1
a
Optimization of Reaction Conditions
.
Entry
additive
Yield of 4b (%)
Yield of 5 (%)
1
No
30
52
53
51
30
34
0
2
K2CO3
NaOH
FeBr2
31
3
40
4
25
5
InCl2
35
6
1a
10 (58 % ee)
7
1ab
12 (51 % ee)
0
8
1b
7 (50 % ee)
0
9
1c
12 (1% ee)
7
10
11
12
13c
14d
15e
1d
8 (4% ee)
3
DABCO
Quinuclidine
Quinuclidine
Quinuclidine
Quinuclidine
40
52
55
61
64
42
43
40
36
34
a
Reactions were performed at room temperature in 0.2 mL MeCN with b2
(0.1 mmol), additive (20 mol %) under 15 W 365 nm LED, N2, 24 h. Yield of
isolated product.
b
Tf2N− instead of TfO−
1.0 eq additive.
.
c
d
e
1.0 eq additive, in p-xylene.
1.0 eq additive, in p-xylene, conc. = 0.3 M.
scopes of this reaction (Schemes 2 and 3). 1,3-Diketones bearing dif-
ferent 2-alkyl (R2 = alkyl) groups were well tolerated to give the
benzoyl-shifted adducts in moderate to good yields (4a-4h). When 2-
phenethyl substituted 1,3-diketones 2 were used (R2 = aryl), the re-
action afforded α-aryl substituted 1,5-diketones (4i-4m) in good yields.
Substitutions on the benzoyl moiety (R1) with either electron-with-
drawing or electron-donating group were both tolerated (4p-4s). Be-
sides methyl ketones (R3), larger ethyl ketone (4s) and phenyl ketone
(4t) as well as esters (4u-4w) could also be applied with moderate to
good reactivity.
When 2-cycloalkanyl 1,3-ketocarbonyls were examined, the reac-
tion showed better activity compared with its linear acyclic counter-
parts (Scheme 3, 12 h vs 24 h). In these cases, di-substituted ring
compounds were obtained in good yields with high diastereoselectivity.
Both cyclopentanyl and cyclohexanyl ring could be applied and the
diastereoselectivity was determined to be anti- for both cases [16].
A concentration-depended red-shift of the ketocarbonyls were ob-
served in UV–vis spectra (see SI, Figure S1 and S2) [17]. At reaction
concentrations, significant absorption above 360 nm were clearly noted
(e.g. 2n) and some even shifted to the visible light range (e.g. 4n, 4v
and 7e). In the latter cases, conversion to the desired product could be
observed under visible light irradiation (Schemes 2 and 3).
Scheme 2. Substrate scope of linear ketonesa.
aReactions were performed at room temperature in 0.33 mL p-xylene with 2
(0.1 mmol), quinuclidine (0.1 mmol) under 15 W 365 nm LED, N2, 24 h. Yield of
isolated product.
3.3. Further transformation: kinetic resolution
intramolecular aldol condensation [9,10]. Chiral primary amines cat-
alyst such as 1a-1d have been widely applied in a number of direct
aldol reactions, [9d] but their application in kinetic resolution remains
We further explored a kinetic resolution protocol to access chiral
1,5-diketones by taking advantage of chiral primary amine catalyzed
3

W. Zhang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry396(2020)112553
Scheme 3. 1,3-Benzoyl shift onto cyclic compoundsa.
aReactions were performed at room temperature in 0.33 mL p-xylene with 6 (0.1 mmol), quinuclidine (0.1 mmol) under 15 W 365 nm LED, N2, 12 h. Yield of isolated
product
underdeveloped. After a brief screening, 1b was identified as the op-
timal catalyst and the kinetic resolution worked favorably in diethyl
ether (conc. = 0.5 M) under -10 ℃ to give enantio-enriched 1,5-dike-
tone in 91 % ee. The resolution factor s was determined to be 15 in this
case (4b) (Scheme 4). Good kinetic resolution was also achieved in
several other cases (Scheme 4). Unfortunately, the use of cylic 1,5-di-
ketone such as 7b showed rather poor resolution. The stereoncontrol
with the protonated NeH bonding, in consistence of our previous stu-
dies, [9,10] could be invoked to account for the stereoselectivity and
this was further verified by DFT calculations.
conformation 9a was disfavored over 9b with the latter bearing an
equatorial alkyl radical, which would facilitate β-cleavage instead of
the more strained cyclization to form cyclobutane. This conformational
bias may explain the dominant formation of dealkylated product (e.g.
5) via β-cleavage (Scheme 1, II and Table 1, entry 1). On the other
hand, the presence of amine may interrupt the intramolecular H-
bonding via strong acid-base interaction. The strong basic and bulky
nature of quinuclidine may enforce this interruption and the acyclic
biradical 10 may now prefer a conformer 10a due to steric effect, which
favors the cyclobutane formation pathway (Scheme 5, II).
The stereoselectivity in the kinetic resolution could be accounted by
a H-bonding mode with our chiral primary amine catalyst [15,18]. The
favored S-selective transition state Ts-1a features a strong H-bonding
between protonated NeH and carbonyl O (1.59 Å in Ts-1a vs 1.69 Å in
Ts-1b) and diminishing close contact between CeH of isopropyl and
one CeH of methylene (Fig. 1, Ts-1b, red ellipse).
3.4. Discussion
Mechanistically, the role of amine in this 1,3-carbonyl shift is quite
intriguing particularly regarding the control of chemoselectivity. It’s
known that amine could stabilize the excited carbonyl in the form of
radical ion pair (Scheme 1) and facilitate the formation of 1,4-biradical
(e.g. 9) via 1,5-H abstraction [15]. In the absence of amine, the forming
biradical intermediate 9 would mainly exist in chair formations due to
the favored intramolecular H-bonding (Scheme 5, I). Chair
4. Conclusions
In conclusion, we have developed an amine mediated [1,3]-
4

W. Zhang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry396(2020)112553
Scheme 4. Kinetic resolution of 1,5-diketones by chiral primary amine catalysisa.
aReactions were performed in diethyl ether with rac-4 (0.1 mmol), 1b (0.02 mmol) and m-nitrobenzoic acid (0.02 mmol) at −10 °C, N2. Yield of isolated product.
b72 h.
carbonyl migration of β-benzoyl carbonyl compounds under photolytic
conditions. The reaction undergoes a sequence of Norrish-Yang cycli-
zation and retro-aldol process to give 1,5-diketones or δ-benzoyl esters.
This study enlarged the scope of Norrish-Yang reaction to enable effi-
cient synthesis of 1,5-dicarbonyls.
Author statements
The submission requires this file. I checked the website for what-
about this statement and didn’t find any information. I guess this is to
state:
5

W. Zhang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry396(2020)112553
Scheme 5. Conformational analysis in the absence (I) and presence of quinuclidine.
6

W. Zhang, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry396(2020)112553
Fig. 1. Calculated transition states in kinetic
resolution.a.
aRelative free energies of activation are given
in kcal/mol. Interatomic distances are denoted
in Å. The close H–H contacts (< 2.2 Å) are la-
beled in blue.
All the authors are acknowledged about the submission of this
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published elsewhere and the manuscript is solely submitted for pub-
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Declaration of Competing Interest
We declare no conflict of interests.
Acknowledgments
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We thank the Natural Science Foundation of China (21861132003,
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