Photo-auxiliary approach to control excited state reactivity: Cross [2+2]-photocycloaddition of oxazolidinone based hydrazides

Article abstract of DOI:10.1016/j.jphotochem.2019.111883

Chiral oxazolidinone based hydrazides undergo efficient cross [2 + 2]-photocycloaddition upon visible light illumination. Oxazolidinone functionality acted as an energy harvesting photo-auxiliary. The cross [2 + 2]-photocycloaddition proceeded efficiently from the excited state with moderate to excellent isolated yield of the photoproduct. The photo-auxiliary can be conveniently removed post-photoreaction, which highlights the versatility of this strategy.

Full text of DOI:10.1016/j.jphotochem.2019.111883

Accepted Manuscript
Title: Photo-Auxiliary Approach To Control Excited State
Reactivity: Cross [2+2]-Photocycloaddition Of Oxazolidinone
Based Hydrazides
Authors: Sapna Ahuja, Akila Iyer, Sunil Kumar Kandappa,
Jayaraman Sivaguru
PII:
S1010-6030(19)30496-4
DOI:
Article Number:
https://doi.org/10.1016/j.jphotochem.2019.111883
111883
Reference:
JPC 111883
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
23 April 2019
25 May 2019
29 May 2019
Please cite this article as: Ahuja S, Iyer A, Kumar Kandappa S,
Sivaguru J, Photo-Auxiliary Approach To Control Excited State Reactivity:
Cross
[2+2]-Photocycloaddition
Of
Oxazolidinone
Based
Hydrazides,
Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.111883
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Photo-Auxiliary Approach To Control Excited State Reactivity: Cross [2+2]-
Photocycloaddition Of Oxazolidinone Based Hydrazides¥
Sapna, Ahuja,a,‡ Akila Iyer,a,‡ Sunil Kumar Kandappa and Jayaraman Sivagurua,*
a Center for Photochemical Sciences and Department of Chemistry, Bowling Green State
University, Bowling Green, OH 43403, USA
‡ The authors are considered co-first authors in this manuscript.
¥ The manuscript is part of the Rajendra Rathore Memorial special issue.
Graphical abstract
Highlights
•
•
•
Chiral oxazolidinone based hydrazides are employed as a photo-auxiliary that
promotes excited state reaction.
Cross [2+2]-photocycloaddition of oxazolidinone based hydrazides occurs efficiently
upon both direct excitation and sensitization with and/or visible light.
Photophysical data provides insights to the mechanistic details of reactivity leading
to efficient reactivity through a bifurcated electron and/or energy transfer pathways.
Abstract
Chiral oxazolidinone based hydrazides undergo efficient cross [2+2]-photocycloaddition
upon visible light illumination. Oxazolidinone functionality acted as an energy harvesting photo-
auxiliary. The cross [2+2]-photocycloaddition proceeded efficiently from the excited state with
moderate to excellent isolated yield of the photoproduct. The photo-auxiliary can be conveniently
removed post-photoreaction, which highlights the versatility of this strategy.
1

1.0 Introduction
Photocycloaddition has played a seminal role to build versatile building blocks in different
fields.1-4 In particular [2+2]-photocycloaddition from the excited state(s) results in cyclobutanes
featuring inherent ring strain that can be exploited for ring enlarging reaction leading to diverse
set of molecular scaffolds.5 Additionally, cyclobutane ring is present in numerous compounds
ranging from biological, medicinal and natural products.6,7 Consequently, a number of approaches
have been developed in organic chemistry to access this simple yet extremely useful building
blocks. [2+2]-Photocycloaddition is the most common approach to access cyclobutanes that can
lead to either straight addition product (typically observed) or cross addition product (observed in
special cases).8 Though photochemical reactions are particularly attractive in their ability to afford
complex scaffolds with multiple stereocenters from relatively simple reactants,9,10 the short lifetime
of the excited state(s) produced during the course of the reaction often leads to a number of
products limiting the scope of the strategy. This issue has been addressed by employing different
strategies in particular supramolecular approaches utilizing zeolites, crystals and molecular cages
to name a few.11-13 While significant advancement have been made to control of photo-reactivity
by utilizing supramolecular systems,11-13 extrapolating their success to isotropic media still
presents significant bottle neck not to mention the change in the product stereo-/regio-chemistry
when one changes the reaction media. This necessitates evaluation of new strategies to control
excited state reactivity in photoreactions to achieve product selectivity in solution. Over the past
decade, one of the strategies developed by our group with significant success is based on
employing atropisomers to control excited state reactivity of various photochemical reactions.14-17
Building on the success of employing atropisomers, our group developed hydrazide based
chromophores as "photo-auxiliary" to control excited state reactivity.18,19 To expand the scope of
this strategy by
combining the power of photochemistry with chiral auxiliaries, we report our findings on
utilizing hydrazide based chiral oxazolidinones20-23 as photo-auxiliaries for controlling the
photocycloaddition of 1a-c leading to cross [2+2]-cycloadducts 2a-c (Scheme 1).24
Scheme 1: Cross [2+2] photocycloaddition of oxazolidinone based hydrazides 1a-c
2

2.0 Results and discussion
Oxazolidinones are well known chiral auxiliaries both in thermal reactions25,26 and
photochemical reactions.21,27,28 We envisioned utilizing them as "photo-chiral auxiliaries" in which
it played a dual role as a chiral perturber and as an excited state reactant. Based on prior results
from our group that established hydrazide as an effective photo-auxiliary,18,19 we envisioned
utilizing oxazolidinones based hydrazides as excited state scaffolds to direct and control [2+2]-
photocycloaddition. To realize this, we synthesized oxazolidinone based hydrazides 1a-c from N-
amino S-(+)-4-phenyl-2-oxazolidinone 6 (Scheme 2). Oxazolidinone based hydrazides 1a-c were
1
characterized by H and 13C NMR spectroscopy and high resolution mass spectrometry.24 The
synthesized hydrazides 1a-c were evaluated for [2+2] photocycloaddition (Table 1). To optimize
the reaction conditions, 1a was chosen as model substrate and the photoreaction was evaluated
under both direct and sensitized irradiation conditions (Table 1). After the completion of
photoreaction, the reaction mixture was concentrated under reduced pressure and photoproduct
1
were purified by column chromatography and characterized by H and 13C NMR spectroscopy,
high resolution mass spectrometry and single crystal X-ray diffraction studies.24 Based on
spectroscopic evidence and single crystal XRD studies, we established the formation of cross
[2+2] photoproduct 2a under both direct and sensitized irradiation conditions. Inspection of Table
1 shows that broad band irradiation under a Pyrex cutoff in acetone (serving as both the solvent
and sensitizer) resulted in 56% isolated yield of the photoproduct 2a with a diastereomeric ratio
(dr) of 10:1 (Table 1; Entry 1). Visible light illumination with thioxanthone serving as the sensitizer
resulted in 70% isolated yield of the photoproduct 2a with a dr of 9:1 (Table 1; Entry 2). To
investigate the effect of solvents, we examined the [2+2] photocycloaddition of 1a in various polar
and non-polar solvents. In general, polar aprotic solvents (acetonitrile, dichloromethane and
chloroform) were found to facilitate the reaction efficiently and cleanly than non-polar solvent such
as methylcyclohexane in which decomposition was mainly observed. Additionally, good control
over diastereomeric ratio was observed in polar aprotic solvents. Acetonitrile (MeCN) was the
best choice among all the solvents that were screened. Other polar protic solvents like
tetrahydrofuran (THF) and methanol (MeOH) gave low yields and poor mass balance. We
expanded the scope of the study by evaluating oxazolidinone based hydrazides 1b and 1c
towards photocycloaddition under sensitized irradiation conditions. Utilizing acetone as a solvent
and sensitizer for photocycloaddition of 1b resulted in photoproduct 2b with 75% isolated yield
and a dr of 7:3 (Table 1; entry 3). Irradiation of 1b with 10 mol% of acetophenone as a sensitizer
under broadband illumination in a medium pressure mercury lamp led to cross [2+2]
3

photocycloaddition product with 88% isolated yield and a dr of 10:1 (Table 1; entry 4). Similarly,
irradiation of 1c with 30 mol% loading of thioxanthone as a sensitizer under visible light irradiation
in a Rayonet reactor led to the corresponding photocycloaddition product 2c with 75% isolated
yield and a dr of 3:1 (Table 1; entry 5). The above results clearly showcase the use of
oxazolidinones based hydrazide to influence excited state reactivity. To further highlight the scope
of employing oxazolidinone based templates as a ‘photo-auxiliary’ to access the cross-
cyclobutanes under mild conditions, we removed the chiral oxazolidinone motif under reductive
conditions using hydrazine hydrate in ethanol.19 This enabled us to convert photoproduct 2 to a
primary amine 3 that was characterized by 1H and 13C NMR spectroscopy.24
Scheme 2: Synthesis of oxazolidinone based hydrazides 1a-c
4

Table 1: Reaction conditions for photoreactivity of hydrazides 1a-c
Entry
Compound
Irradiation conditions a
% Isolated
yield
Diastereomeric
ratio c
1
1a
bb/Pyrex cutoff, Acetone, 3 h
420 nm, 30 mol % TX, MeCN, 4 h
bb/Pyrex cutoff, Acetone, 3 h
56
70
10 : 1
9 : 1
7 : 3
2
3
1a
1b
75 b
88 b
4
5
1b
1c
bb/Pyrex cutoff, 10 mol% AP, MeCN
420 nm, 30 mol % TX, MeCN, 8 h
10 : 1
3 : 1
75
a [1] ≈ 3.2 mM. Irradiations performed at room temperature under N2 atmosphere unless otherwise
noted. TX = Thioxanthone. AP=acetophenone. bb/Pyrex cutoff = broadband irradiation performed
using a medium pressure 450 W mercury lamp with a Pyrex cutoff (<295 nm cutoff) and ~420 nm
irradiations were carried out in a Rayonet reactor. b Straight addition product was observed as a
c
1
minor product along with cross addition product. Values are based on H NMR spectroscopy
(error ±5%). Restricted bond rotation around N-N bond in hydrazides leads to atropisomers
(Reference 19).
Table 2: Solvent effects on intramolecular [2+2] photocycloaddition of hydrazide 1aa
Entry
Solvent
% Conversion b
2a, % Yield b
Diastereomeric ratiob
1
2
3
MeCN
CH2Cl2
CHCl3
>99
>99
>99
56
30
34
19 : 1
19 : 1
49 : 1
a [1a] ≈ 3.2 mM Irradiation performed in a Rayonet reactor equipped with 300 nm tubes for 24 h
at room temperature under N2 atmosphere unless otherwise noted.b Values are based on 1H NMR
spectroscopy (error ±5%). Restricted bond rotation around N-N bond in hydrazides leads to
atropisomers (Reference 19).
Scheme 3: Optimized condition for the removal of photo-auxiliary unit.
5

Table 3: Free energy for photo-induced electron transfer (PET), GoET from excited thioxanthone
to hydrazides 1a-c.a
E°(D
+
/D) (eV)
G° ET (kcal/mol)
-0.38
Entry Substrate
E°(A/A-) (eV)
-0.97
E00 (eV)
2.8
1
2
3
1a
1b
1c
1.39
1.39
1.39
-1.19
2.8
-0.16
-0.73
2.8
-0.62
a G° ET calculated by Rehm Weller equation.29 G° ET = E°(D+/D) – E°(A/A-) – E0,0 + w; Go ET = free
energy of electron transfer (kcal/mol); Eox or E°(D+/D) = oxidation potential of electron donor (eV
vs Ag/Ag+); Ered or E°(A/A-) = reduction potential of electron acceptor (eV vs Ag/Ag+); E00
=
excitation energy (eV); w= Coulomb term (estimated to be 0.06 eV in MeCN);30 For TX, ground
state Eox or E°(D+/D) = 1.69 eV (vs SCE) or 1.39 eV (vs Ag/AgNO3). E° (vs Ag/AgNO3) = E° (vs
SCE) – 0.30.31,32
As our approach led to intramolecular [2+2] cross-photocycloaddition, we were interested
in deciphering the reaction pathway leading to excellent control of reactivity. To ascertain the
reaction pathway, photophysical studies were carried out using oxazolidinone 1b as
representative example. Detailed photophysical investigation of 1b was performed in 2-
methyltetrohydrofuran (2-MeTHF) as solvent. At room temperature, a structureless fluorescence
emission was observed with max at 365 nm (Figure 1). At 77 K in 2-MeTHF glass, a structured
phosphorescence emission was observed with two distinct triplet lifetimes of ~ 0.85 ms and ~
5.17 ms with a triplet energy (ET) of ~63 kcal/mol above the ground state (Figure 2). These
measurements revealed that the lowest triplet excited state features an n* character. The triplet
energy (ET) of 1b (~63 kcal/mol provided an important insight into photochemical reactivity where
irradiation of 1b led to cross [2+2] photoproduct in the presence of acetophenone (ET ~ 73
kcal/mol) and acetone (ET ~ 79 kcal/mol) (Table 1, entry 3 and 4) and thioxanthone (ET ~ 63
kcal/mol).33 As the triplet energy of evaluated sensitizers, acetophenone (ET ~ 73 kcal/mol),
acetone (ET ~ 79 kcal/mol) and thioxanthone (ET ~ 63 kcal/mol) are greater than or comparable
to that of oxazolidinone 1b (~63 kcal/mol), energy transfer from sensitizer to oxazolidinone 1b is
feasible. To evaluate the feasibility of electron transfer from excited thioxanthone, we calculated
the free energy of electron transfer using the Rehm Weller equation (Table 3).29 All the hydrazide
based substrates showed exergonic electron transfer indicating that electron transfer pathway is
also feasible from excited thioxanthone. Based on the above observations, we propose a
mechanism (Scheme 4) in which an encounter complex19,34 bifurcates to an energy transfer or
electron transfer pathway depending on the reaction conditions. If the reaction condition is
6

amenable to triplet energy transfer, the excited triplet state of 1 ([1]*3) that undergoes cyclization
to form triplet 1,4-biradical tBR-1. The triplet biradical intersystem crosses to form singlet 1,4-
biradical (sBR-1) that subsequently cyclizes to form photoproduct 2.35 On the other hand, if the
system favors an electron transfer pathway with favorable redox potentials, we conjecture that
electron transfer from excited sensitizer (e.g. thioxanthone) to the substrate results in the radical
anion of 1. Cyclization of radical anion followed by back electron transfer (beT) to thioxanthone
radical cation results in the intermediate 1,4-biradical that subsequently cyclizes to the
photoproduct. Our results with oxazolidinone derived hydrazides is similar to the one we observed
in phthalimides based systems that undergoes photoreaction with a combination of energy
transfer and electron transfer pathways.19 Based on the photophysical investigation and
photochemical reactivity, the intramolecular cross [2+2]-photocycloaddition of oxazolidinone
based hydrazide is postulated to occur based on a combination of electron transfer or energy
transfer pathways depending on the type of triplet sensitizer that is employed.35 When triplet
sensitizer such as acetone is employed the reaction likely proceeds via an energy transfer
pathway. On the other hand, with sensitizer such as thioxanthone, the reaction likely proceeds
with a combination of energy transfer and electron transfer pathways (Table 3).
TX
hn
Visible
beT
R1
CH2
light
O
R1
O
N
[TX]
O
O
3 [TX]*
O
O
N
N
O
N
CH2
R2
*
*
R1
Ph
Electron
transfer
O
Ph
R2
O
Encounter
complex
O
O
1
R2
N
N
*
Energy
transfer
beT
O
Ph
*3
R1
R1
R1
R1
O
O
O
O
O
O
N
O
N
O
R2
O
R2
O
O
O
ISC
N
N
N
N
N
N
*
*
*
*
R2
R2
O
O
O
Ph
Ph
Ph
O
Ph
2
1
tBR-1
sBR-1
Scheme 4: Proposed mechanism for cross photocycloaddition of oxazolidinone based hydrazide
templates 1.
7

Figure 1: Excitation (blue), fluorescence (black), phosphorescence (green) spectra of
oxazolidinone based hydrazide 1b in 2-methyl tetrahydrofuran (2-MeTHF) (c ~ 0.48 mM). UV-Vis
absorption, excitation and fluorescence spectra were recorded at room temperature.
Phosphorescence spectra was recorded at 77 K in 2-MeTHF glass.
Figure 2: Phosphorescence lifetime decay of 1b at 77 K in 2-MeTHF (c ~ 0.48 mM). Biexponential
decay was observed with a lifetime of 1 = 0.87 ms and 2 = 5.17 ms. 2value of 1.37 was
observed.
3.0 Conclusion
The present study demonstrates the use of oxazolidinone based hydrazides to control excited
state reactivity leading to [2+2] photocycloaddition. Spectroscopic evidences corroborate the
8

likely formation of an encounter complex that bifurcate to an energy and/or electron transfer
pathway based on the employed reaction conditions leading to the photoproduct. Post synthetic
modification of the photocycloaddition product to afford the primary amine derivative provides a
handle for constructing motifs that are synthetically useful targets featuring cyclobutane
functionality.
4.0 Experimental Details.
4.1 General procedure for photochemical reaction
Oxazolidinone based hydrazide 1 was dissolved in an appropriate solvent with concentration of 1
mg/mL in a Pyrex test tube. Appropriate amount of the sensitizer was added according to the
reaction mixture before irradiation. The reaction mixture was sealed and was degassed with
continuous flow of N2 for about 10 minutes. Broadband irradiations were performed with 450 W
medium pressure mercury lamp with a Pyrex cutoff (<295 nm cutoff) filter. Visible light irradiations
were performed at 420 nm using RPR-200 photochemical reactor equipped with 16 x 14 W (420
nm) lamps. The reaction was monitored by thin layer chromatography at regular time intervals.
After the compete consumption of starting material, the solvent was evaporated under reduced
pressure and product was purified by column chromatography. The purified product was
characterized by various analytical techniques.24
4.2 Photophysical methods
Spectrophotometric grade 2-methyl tetrahydrofuran was used for all photophysical
experiments unless mentioned otherwise. UV-Vis absorbance spectra was recorded with Agilent
Technologies Cary 60 UV-Vis spectrometer. Steady state emission, excitation and life time
experiments were performed using Edinburgh instruments FLS 1000. Continuous Xenon lamp
source was used for recording fluorescence and excitation spectra. Microsecond pulsed Xenon
lamp source was used to record phosphorescence spectra and phosphorescence lifetimes.
For photophysical studies of compound 1b, a solution of 0.48 mM concentration was
prepared in 2-MeTHF. Excitation was performed at a wavelength (ex) 307 nm with a
corresponding optical density of 0.02 with a slit width of 5 nm for both excitation and emission
arms respectively. Excitation spectra was monitored at the fluorescence maxima (em = 375 nm)
at room temperature with a slit width of 5 nm for both excitation and emission arms respectively.
Excitation spectra were recorded with an average of 4 scans. Phosphorescence spectra recorded
at 77 K. Phosphorescence decay lifetime was recorded using microsecond pulsed Xenon lamp
source of 5 Hz with detector gating delay of 0.2 ms and gate width of 100 ms.
9

4.3 Synthesis of oxazolidinone based hydrazides 1a-c36,37
The hydrazide based oxazolidinone 6 was synthesized using established literature procedures.36
Hydroxylamine hydrochloride (NH2OH.HCl) was converted to N-hydroxyl tert -butyl carbamate 4
using K2CO3 as a base in 80% yield.24,37 Carbamate 4 was utilized to perform amination of p-nitro
benzoyl chloride to afford the aminating agent O-(p-nitrobenzoyl)hydroxylamine (NbzONH2) 5.24
NbzONH2 5 was then treated with S-(+)-4-phenyl-2- oxazolidinone in a suspension of sodium
hydride (NaH) in dioxane at room temperature for 24 h to afford N-amino S-(+)-4-phenyl-2-
oxazolidinone 6. 24 Oxazolidinone based hydrazides 1a-c were synthesized by acylation of N-
amino S-(+)-4-phenyl-2- oxazolidinone 6 in 88 %, 62 % and 23 % yields respectively. For detailed
experimental procedures are provided in the supporting information.
4.4 Protocol for the removal of photo- auxiliary unit in 2b-c
Following the literature procedure19, photoproduct 2 (0.06 mmol) and hydrazine hydrate
(11 equiv) was dissolved in ~8 mL of absolute ethanol. The solution was refluxed until complete
consumption of starting material (1-2 hours). The progress of the reaction was monitored by thin
layer chromatography (TLC). Upon complete consumption of the reactant, the excess of solvent
was removed by rotary evaporation followed by a preparative TLC to afford the primary amine 3.
5.0 Acknowledgment
The authors thank the National Science Foundation for generous support for their
research (CHE-1465075 and CHE-1811795). We thank Dr. Angel Ugrinov for solving the single
crystal X-ray structures. Part of the work was carried out during author’s affiliation to North Dakota
State University, Fargo, ND.
6.0 Supporting information
Electronic Supplementary Information (ESI) available, experimental procedures,
characterization data, analytical conditions, and photophysical studies (PDF).
10

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