Importance of Singlet Oxygen in Photocatalytic Reactions of 2-Aryl-1,2,3,4-tetrahydroisoquinolines Using Chalcogenorosamine Photocatalysts

Article abstract of DOI:10.1021/acs.organomet.9b00126

Aerobic oxidation of 2-aryl-1,2,3,4-tetrahydroisoquinolines was achieved photocatalytically using chalcogenorosamine photocatalysts and LED irradiation. The photocatalytic aza-Henry reaction between these substrates and nitromethane was more efficient wit

Full text of DOI:10.1021/acs.organomet.9b00126

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Importance of Singlet Oxygen in Photocatalytic Reactions of 2‑Aryl-
1,2,3,4-tetrahydroisoquinolines Using Chalcogenorosamine
Photocatalysts
Jennifer L. Clark,† Jackie E. Hill,† Irving D. Rettig,‡ Joshua J. Beres,† Roman Ziniuk,§
Tymish Y. Ohulchanskyy,§ Theresa M. McCormick,*,‡ and Michael R. Detty*,†
†Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States
‡Department of Chemistry, Portland State University, Portland, Oregon 97207, United States
§Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of
Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, P.R. China
S
* Supporting Information
ABSTRACT: Aerobic oxidation of 2-aryl-1,2,3,4-tetrahydroisoquinolines
was achieved photocatalytically using chalcogenorosamine photocatalysts
and LED irradiation. The photocatalytic aza-Henry reaction between these
substrates and nitromethane was more efficient with selenorosamine and
tellurorosamine photocatalysts than with thiorosamine and rosamine
photocatalysts, corresponding to the propensity of the photocatalysts to
generate singlet oxygen (1O2). Appropriately, yields for the photocatalytic
aza-Henry reaction were greatly reduced when the reactions were conducted
under a nitrogen atmosphere. The 2-aryl-1,2,3,4-tetrahydroisoquinolines were oxidized to the corresponding 2-aryl-3,4-
dihydroisoquinolones 13a−13c with selenorosamine and tellurorosamine photocatalysts in 2% aqueous acetonitrile. Di-2-aryl-
1,2,3,4-tetrahydroisoquinolin-1-yl peroxides 14a and 14b were shown to be intermediates in this reaction. Thiorosamine
1
photocatalysts, which do generate O2 upon irradiation, did not give 2-aryl-3,4-dihydroisoquinolones. These results suggested
1
that the exciplex between O2 and the chalcogen atom of the chalcogenorosamines (the corresponding pertelluoxide,
perselenoxide, or persulfoxide) and/or the hydrated perchalcogenoxide [hydroxy (perhydroxy)tellurane, -selenane, or -thiane]
might be an active oxidant in the formation of 13a−13c. Computational methods were employed to provide support for the
observed photocatalytic reactivity of the tellurorhodamine and selenorhodamine chromophores compared to the thiorosamine
chromophores. ΔG values were determined for the oxidation and hydration of 10-Te, 10-Se, and 10-S for formation of
perchalcogenoxides and hydroxyl(perhydroxy)chalcogenanes, respectively. Calculations indicate formation of the pertelluroxide
perselenoxide, and persulfoxide exciplex intermediates are energetically favorable. Hydration of the exciplexes of 10-Te and 10-
Se have similarly small ΔG of −3.49 and 4.51 kcal/mol, respectively. However, a significantly higher ΔG value of +22.4 kcal/
mol is observed for the hydration of 10-S, which suggests that this reactive intermediate is not readily formed.
photocatalyst can utilize atmospheric oxygen as an oxidant;
however, it is only active in the UV region, which drastically
reduces light-absorbing efficiency under solar irradiation.13
Transition-metal-based Ru(II) and Ir(III) polypyridyl com-
plexes have been more extensively studied as photocatalysts
and have readily available photophysical and electrochemical
properties, although these transition-metal complexes are
relatively expensive.1,6,14
INTRODUCTION
■
Photocatalysis is a single-electron-transfer process that employs
organic dyes or transition-metal complexes along with visible
light irradiation.1 Organic dyes have been utilized to a limited
extent in photoredox catalysis, whereas ruthenium(II) and
iridium(III) complexes have been extensively studied for over
30 years.2−7 There are several properties of organic dyes that
make them advantageous over transition-metal complexes.
Organic dyes are potentially less toxic, more cost-effective, and
have improved photophysical properties (increased absorption
in the visible region) and high singlet oxygen quantum yields
[Φ(1O2)], making them a more practical option.5
In the past decade, photocatalysis has become of increasing
interest as a “green” mode of small molecule activation within
organic chemistry.8 General reactions for alkylations, oxida-
tions, and anti-Markovnikov additions to alkenes have all been
developed using photocatalysis.9−12 Photocatalysts employed
to date have various insufficiencies. Titanium dioxide as a
The structures of several cationic organic dyes that have
been used successfully as photocatalysts are shown in Chart 1.
The Scaiano group has examined methylene blue (1);4,15,16 the
2
̈
Konig group studied eosin Y (2), and various acridinium salts
including the 9-mesityl derivative (3) were reported by the
Nicewicz and Fukuzumi groups.11,12 Rhodamine 6G (4, Chart
1) and various oxazine, thiazine, and azine dyes have been
Received: February 25, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00126
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Chart 1. Structures of Methylene Blue (1), Eosin Y (2), 9-Mesityl-10-methylacridinium (3), and Rhodamine 6G (4)
recently compared as photocatalysts for the oxidation of 2-
phenyl-1,2,3,4-tetrahydroisoquinoline (5a) in the light-medi-
ated aza-Henry reaction with nitromethane to give 1-
nitromethyl-2-phenyl-1,2,3,4-tetrahydroisoquinoline (6a)
shown in Scheme 1.7 Mechanistically, the cationic organic
photocatalysts are thought to follow two different pathways
(Scheme 2). Path A: the excited photocatalyst can initiate
single-electron oxidations via electron transfer from a donor
atom of the substrate (N in the case of 5a) to the excited state
of the photocatalyst to give a radical of the photocatalyst and a
cation radical of the donor atom.5,7 Alternatively, in path B,
photocatalysts that produce high triplet yields can generate
singlet oxygen (1O2), which oxidizes a donor atom of the
substrate to give superoxide (O2•−) as well as the cation radical
of the donor atom.7 In the light-mediated aza-Henry reaction
of 5a with nitromethane,7 methylene blue (1) with Φ(1O2) of
0.5217 was a superior photocatalyst to rhodamine 6G (4) with
Φ(1O2) of 0.01.18
Chalcogenorosamine dyes 7-E−10-E (Chart 2) are similar
in structure to the organic photocatalysts of Chart 1.7 Whereas
the chalcogenorosamines and structurally related chalcogeno-
rhodamines have not been examined as photocatalysts in
synthetic transformations of organic molecules, these mole-
cules have been examined as photosensitizers for photoinduced
charge-transfer reactions in dye-sensitized solar cells
(DSSCs)19 and for the generation of hydrogen via the
photoreduction of protons.20 The ability of the seleno- and
tellurorosamine/rhodamine analogues to generate 1O2 has
been exploited in photodynamic therapy (PDT)21 and in
fluorescence imaging.22,23
Scheme 1. Photocatalyzed Aza-Henry Reaction of Amine
Substrates Using 1 mol % of Photocatalyst in 4:1 CH3CN/
CH3NO2
Whereas dye chromophores incorporating O, S, Se, or Te in
1
the heterocyclic structure generate O2, the relative values of
Scheme 2. Proposed Mechanism for the Aza-Henry
Reaction
a
Φ(1O2) are very much dependent on the presence of heavy
atom(s). The presence of heavy atoms promotes spin−orbit
coupling, which scales with nuclear charge, leading to increased
rates of singlet−triplet intersystem crossing. Organic dyes
incorporating the heavier chalcogen atoms Se or Te have
values of Φ(1O2) higher than those of their lighter chalcogen
analogues because of this effect.22,24−27 The Te analogues have
Φ(1O2) lower than that of the corresponding Se compounds
1
likely due to reaction of O2 with the Te, resulting in lower
22b
1
observed O2 phosphorescence.
Furthermore, the exciplex
1
from O2 addition to the Te atom can undergo subsequent
chemistry with water to produce H2O2 and the telluroxide
oxidation state (Te(IV)) of the tellurorosamine.10 The
telluroxide, in turn, can function as an oxidant for the
conversion of thiols to disulfides in a photocatalytic process.10
Similar chemistry has also been observed with selenoxides,
where addition of hydrogen peroxide generates the corre-
sponding hydroxy(perhydroxy)selenane.28
a
In path A, the radical cation of the isoquinoline is formed by electron
transfer from the excited photocatalyst (PC), and in path B, it is
formed from electron transfer from singlet oxygen.5,7
Chart 2. Structures of the Chalcogenorosamines 7-E (E = O, S, Se, Te), 8-E (E = Se, Te), 9-Te, and 10-E (E = O, S, Se, Te)
B
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Herein, we compare the chalcogenorosamines 7-E−10-E
(Chart 2) as photocatalysts for two reactions of 2-aryl-1,2,3,4-
tetrahydroisoquinolines: (1) the aza-Henry reaction with
nitromethane to give 1-nitromethyl-2-aryl-1,2,3,4-tetrahydroi-
soquinolines and (2) the oxidation to the corresponding 2-aryl-
3,4-dihydroisoquinolones. The latter reaction is a new
photocatalytic reaction, and the importance of the singlet
oxygen−photocatalyst exciplex (pertelluroxide, perselenoxide,
or persulfoxide) in this process was examined using density
functional theory (DFT) calculations. Isoquinolones are of
interest due their prevalence in alkaloidal natural products. We
have chosen the substrates to allow for direct comparison to
previous photocatalytic reactions.7
Photocatalyzed Aza-Henry Reaction of 5a. The
photocatalyzed aza-Henry reaction of 5 with nitromethane,
as shown in Scheme 1, was evaluated using 1 mol % of each of
the chalcogenorosamines of Chart 2 and compared to
methylene blue (1) and rhodamine 6G (4) as controls. A
4:1 CH3CN/CH3NO2 solution of 5a (0.06 M) and photo-
catalyst (0.0006 M) was irradiated for 3.0 h with a GE 14 W,
850 lm LED light. The % conversion of 5a to 6a was
1
determined by H NMR spectroscopy comparing the integral
of the 2-nitromethyl methylene protons (δ 4.81 and 4.53) and
the 1-methine proton (δ 5.51) of 6a with the 1-methylene
protons (δ 4.42) of 5a. Values of % conversion are compiled in
Table 1 and are the mean of triplicate runs.
Both tetramethyltellurorosamine 7-Te and 9-phenyl
bisjulolidyltellurorosamine 8-Te bleached rapidly and gave
<1% conversion of 5a to the aza-Henry product 6a. The three
xanthylium core photocatalysts 4 (rhodamine 6G), 7-O, and
10-O gave ⩽18% conversion of 5a to 6a and were not
significantly different from one another (probability value, p >
0.10, one-way ANOVA). Photocatalysts 4, 7-O, 7-Te, 8-Te,
and 10-O were all significantly poorer photocatalysts (p ⩽
0.0064, one-way ANOVA) than the remaining catalysts of
Table 1. Selenorosamines 7-Se, 8-Se, and 10-Se and
tellurorosamine 10-Te were the best photocatalysts for
conversion of 5a to 6a after 3 h, collectively, with ⩾94%
conversion of 5a to 6a.
RESULTS AND DISCUSSION
■
Chalcogenorosamine Photosensitizers. The
tetramethylchalcogenorosamine 7-E,24,25,29 bisjulolidylchalco-
genorosamine 8-E,22b and the respective 9-mesityl derivatives
9-Te22b and 10-E (E = Se, Te)10,22a were prepared as
previously described. The lighter chalcogen analogues 10-O
and 10-S and bisjulolidylselenorosamine 9-Se were prepared as
shown in Scheme 3.22b Values of absorption maxima (λmax
)
Scheme 3. Synthesis of 9-Mesitylrosamine 10-O, 9-
Mesitylthiorosamine 10-S, and 9-Mesitylselenorosamine 9-
Se
Conversions of 5a to 6a were also run for 1.0 h with the best
performing photocatalysts 7-Se, 8-Se, 10-Se, and 10-Te in
aerated solvent. Values of % conversion are compiled in Table
1 and are the mean of triplicate runs. The 60% conversion with
7-Se as photocatalyst and 56% conversion with 10-Te as
photocatalyst were not significantly different (p > 0.10, one-
way ANOVA) from one another. However, the 1 h conversion
with both of these catalysts was significantly greater (p ⩽ 0.03)
than the 1 h conversion with 8-Se (37%) and 10-Se (45%),
which were not significantly different from one another (p >
0.07). However, for all four of these photocatalysts, greater
conversions were observed with longer reaction times.
In order to assess the importance of oxygen in the
photocatalyzed reactions, the reactions were repeated with
several of the photocatalysts following deaeration with N2
bubbling to remove most of the oxygen from the reaction
vessel. Values of % conversion under a N2 atmosphere with
irradiation for 3 h are compiled in Table 1 and are the mean of
triplicate runs. In every example, product yields were reduced
under a N2 atmosphere. The non-negligible yields (10−14%)
did not increase over time and are attributed to residual oxygen
(Table 1).
Preparative Photocatalyzed Aza-Henry Reactions.
The survey of chalcogenorosamine photocatalysts suggested
that 7-Se and 10-Te were perhaps the two most robust and
efficient among the photocatalysts compared in Table 1 for the
aza-Henry reaction. The aza-Henry reaction was repeated on a
1.0 mmol scale with 1 mol % of photocatalyst (either 7-Se or
10-Te), nitromethane, and tetrahydroisoquinoline substrates
5a, 5b, and 5c (Scheme 1 and Table 2). Reactions were
followed by thin layer chromatography (TLC) until con-
sumption of starting amine was complete. Following irradiation
with the LED light, reaction mixtures were concentrated and
the product(s) were isolated via chromatography on SiO2
eluted with 5% EtOAc in hexanes.
and extinction coefficients (ε) for 9-Se, 10-O, and 10-S are
compiled in Table 1. In order to compare complete sets of
photophysical properties among the rosamine series 7-E and
10-E, steady-state fluorescence spectra for 10-O and 10-S were
acquired as were values of Φ(1O2) (Table 1). Emission
maxima (λFL) in MeOH were 572 nm for 10-O and 598 nm
for 10-S. Quantum yields for fluorescence (ΦFL) in MeOH
were 0.74 0.04 for 10-O and 0.34 0.2 for 10-S. Values of
Φ(1O2) were measured using time-resolved spectroscopy of
1O2 phosphorescence in air-saturated methanol with 7-Se as a
reference [Φ(1O2) = 0.87].25,30−32 Values of Φ(1O2) were 0.03
0.01 for 10-O and 0.61 0.02 for 10-S.
2-Aryl-1,2,3,4-tetrahydroisoquinolines. The 2-aryl-
1,2,3,4-tetrahydroisoquinolines used in this study were
prepared as shown in Scheme 4.5 1,2,3,4-Tetrahydroisoquino-
line was coupled to an aryliodide using CuI as catalyst. 2-
Phenyl-1,2,3,4-tetrahydroisoquinoline (5a) was isolated in 70%
yield, the 2-(4-methoxyphenyl) derivative 5b in 39% yield, and
the 2-(4-chlorophenyl) derivative 5c in 78% yield.
The photocatalyzed aza-Henry reaction of N-phenylpyrro-
lidine (5d) with nitromethane and 1 mol % of 7-Se or 10-Te
C
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Table 1. Comparison of Photocatalysts for the Photocatalyzed Aza-Henry Reaction of 5a with Nitromethane To Give 6a
a
% conversion
photocatalyst
λmax, nm
ε, M−1 cm−1
3.0 h
N2, 3.0 h
1.0 h
Φ(1O2)
b
b
c
1
4
7-O
7-S
664
524
90000
78000
42
18
18
69
94
<1
98
<1
60
3
3
5
8
2
<1
<1
<1
0.52
b
b
d
0.01
0.08
0.21
0.87
0.43
0.68
0.53
e
536
557
568
601
604
108000
94400
e
9
4
e
7-Se
7-Te
8-Se
8-Te
9-Se
9-Te
10-O
10-S
10-Se
10-Te
117000
14
2
60
1
4
f
f
g
h
h
81000
h
h
135000
1
6
10
2
37
h
h
617
144000
604
617
145000
165000
h
h
80 20
537
557
568
124000
124000
12
44
4
7
0.03 0.01
0.61 0.02
i
i
i
103000
99.5 0.5
95
16
14
2
1
45
56
4
3
0.85
g
g
g
600
86000
1
0.75
a
Reagents and photocatalyst were dissolved in a 4:1 mixture of MeCN/MeNO2 on a 0.3 mmol scale, and the resulting solution was then irradiated
1
with a GE 14 W, 850 lm LED light with stirring for the indicated time period. Percent conversions were determined by H NMR spectroscopy.
Values represent the mean of triplicate runs 1 SD. From ref 7. From ref 18. From ref 19. From ref 24. From ref 28. From ref 22a. From ref
22b. From ref 10.
b
c
d
e
f
g
h
i
13a−13c33 (Chart 3) as minor products in the photocatalyzed
aza-Henry reaction, as compiled in Table 2. These materials
were isolated via chromatography on SiO2 eluted with 5%
EtOAc in hexanes.
Scheme 4. Coupling of Aryliodides to 1,2,3,4-
Tetrahydroisoquinoline To Give 2-Aryl-1,2,3,4-
tetrahydroisoquinolines 5a, 5b, and 5c
Chart 3. Isoquinolone Products Formed during the
Photocatalyzed Aza-Henry Reaction
Table 2. Photochemical Aza-Henry Reaction of 1.0 mmol 2-
Aryl-1,2,3,4-tetrahydroisoquinolines 5a, 5b, and 5c and N-
Phenylpyrrolidine 5d in 4:1 CH3CN/CH3NO2 with 1 mol %
of Photocatalyst 7-Se or 10-Te and LED Irradiation
entry
photocatalyst
substrate
hν, h
product
isolated yield, %
a
1
2
3
4
5
6
7
8
7-Se
10-Te
7-Se
10-Te
7-Se
10-Te
7-Se
5a
5a
5b
5b
5c
5c
5d
5d
11
5
8.5
6.5
5
4
30
30
6a
6a
6b
6b
6c
6c
6d
6d
84
Oxidation of 1,2,3,4-Tetrahydroisoquinolines to 3,4-
Dihydroisoquinolones. There are relatively few methods for
oxidizing tetrahydroisoquinolines to the corresponding dihy-
droisoquinolones, and the most successful example using relay
aerobic oxidation required an oxygen atmosphere and Schlenk
line techniques and was limited to 0.25 mmol in scale.33 For
most examples of oxidation of benzylic amines to benzylic
amides, the amides were isolated as a minor product of the
reaction,34 which is what was observed in the preparative aza-
Henry reactions described above to give the dihydroisoquino-
lones as minor products. We sought conditions to form the
dihydroisoquinolones as the major product of photocatalysis.
Running the photo-oxidations in pure CH3CN or CH2Cl2 with
7-S, 7-Se, 10-S, 10-Se, and 10-Te as photocatalysts gave little
if any dihydroisoquinolone. However, addition of 2% water to
CH3CN gave photo-oxidation of the tetrahydroisoquinolines
to the corresponding dihydroisoquinolones as the major
product upon LED irradiation with 7-Se, 10-Se, and 10-Te,
indicating that water was a necessary component of the
reaction.
b
85
c
81
d
75
e
62
f
85
3
6
10-Te
a
b
Isoquinolone 13a isolated in 16% yield. Isoquinolone 13a formed
in trace (<2%) amounts. Isoquinolone 13b isolated in 2% yield.
Isoquinolone 13b formed in trace (<2%) amounts. Isoquinolone
13c isolated in 17% yield. Isoquinolone 13c formed in trace (<2%)
c
d
e
f
amounts.
was also examined. The reaction with 5d was only partially
complete with either photocatalyst after 48 h of irradiation
with the LED light and is consistent with previous results with
this substrate and other organo photocatalysts.5 Following
chromatography on SiO2 eluted with 5% EtOAc/hexanes, the
aza-Henry product 6d was isolated in roughly 3% yield with 7-
Se as the photocatalyst and in roughly 6% yield with 10-Te as
the photocatalyst (Table 2).
Photo-oxidation of 5a, 5b, and 5c was carried out on a 1.0
mmol preparative scale with 1 mol % of chalcogenorosamine
photocatalysts 7-S, 7-Se, 10-S, 10-Se, or 10-Te using a 4.9:0.1
solution of CH3CN/H2O as solvent under irradiation with an
2-Aryl-1,2,3,4-tetrahydroisoquinolines also gave various
amounts of the corresponding 2-aryl-3,4-dihydroisoquinolones
D
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850 lm LED light. The reactions were followed by TLC until
the consumption of the starting tetrahydroisoquinoline was
complete. Using 7-Se (entries 2, 3, 7, 11, and 12), 10-Se
(entries 4, 9, 13, and 14), or 10-Te (entries 5, 10, and 15) as
the photocatalyst, the known 2-aryl-3,4-dihydroisoquinolin-
1(2H)-ones 13a−c33 were isolated as the major products of
reaction, as shown in Table 3. Irradiating 1,2,3,4-tetrahy-
Chart 4. meso- and D,L-Diastereomers of Peroxodimers 14a
and 14b and the Structure of Iminium Ions 15a and 15b
Table 3. Photochemical Oxidation of 1.0 mmol 2-Aryl-
1,2,3,4-tetrahydroisoquinolines 5a, 5b, and 5c in 4.9:0.1
CH3CN/H2O with 1 mol % of Photocatalyst and LED
Irradiation
the 1H NMR spectrum of the mixture of diastereomers of 14a
indicated that the pairs of methylene protons at C3 and C4
were no longer equivalent and geminal coupling was observed.
Two singlets were observed in a ratio of 45:55 at δ 6.09 and δ
5.98 for a methine proton at C1, which suggested two
compounds were produced in not quite a 1:1 ratio.
Combustion analysis was consistent with a dimer of the
iminium core perhaps bridged by two oxygen atoms with
calculated element percentages for C30H26Cl2N2O2 of C,
69.64; H, 5.06; and N, 5.41. Experimentally determined
percentages were C, 69.98; H, 4.92; and N, 5.41.
entry photocatalyst substrate
hν, h
product isolated yield, %
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
7-S
7-Se
10-S
10-Se
10-Te
7-S
5a
5a
5a
5a
5a
5a
5b
5b
5b
5b
5c
5c
5c
5c
5c
12
8
12
8
13a
13a
13a
13a
13a
13a
13b
13b
13b
13b
13c
13c
13c
13c
13c
<5
58
<5
83
63
<5
63
<5
73
75
8
12
13
12
13
8
7.5
36
8
7-Se
10-S
10-Se
10-Te
7-Se
The 13C{1H} NMR spectrum of the mixture of diaster-
eomers of 14a was also consistent with a nearly 1:1 mixture of
the meso- and D,L-diastereomers. Each diastereomer would
have 30 carbons with two identical halves. The symmetry of
the 4-chlorophenyl substituent in each would reduce the
number of different carbon signals to 26 for each diastereomer,
with the mirror plane and C2 axis reducing the number of
unique signals to 13 for each diastereomer or 26 signals in
total. The experimental 13C{1H} NMR spectrum displayed 21
unique signals for the mixture with the “overlapped” signals
observed at δ 136.0, 128.2, 125.7, 115.7, and 42.2. The
remaining signals were “paired” with very similar chemical
shifts (Supporting Information).
a
78
89
70
85
59
7-Se
b
10-Se
10-Se
10-Te
36
3
a
b
Peroxodimer 14a isolated in 6% yield. Peroxodimer 14a isolated in
12% yield.
droisoquinoline 5a or 5b for 12 h using thiorosamine
photocatalysts 7-S (entries 1 and 6) and 10-S (entries 3 and
8) gave essentially unreacted 5a or 5b and no isolable amounts
of 13a or 13b or other products.
The white precipitate of 14a began forming when irradiation
began and the quantity of precipitate reached a maximum and
began declining as irradiation continued. When irradiation was
stopped after 4 h (rather than 7.5 h), the white precipitate of
14a was isolated in 13% yield. After 7.5 h, consumption of 5c
was complete and the precipitate was isolated in 6% yield. The
precipitate of 14a disappeared after 36 h of irradiation, and
isoquinolone 13c was isolated in 89% yield at this point. If the
precipitate were returned to a fresh 4.9:0.1 solution of
CH3CN/H2O (no photocatalyst or 13c) and LED irradiation
continued, isoquinolone 13c was formed along with other
products, which were not identified.
The importance of oxygen in the photocatalysis was
demonstrated by reducing the oxygen content of the reaction
mixture prior to irradiation. Irradiating 1,2,3,4-tetrahydroiso-
quinoline 5a and photocatalyst 7-Se or 10-Te in a 4.9:0.1
solution of CH3CN/H2O as solvent that had been deaerated
with a stream of N2 gas and then placed under a N2
atmosphere returned unreacted 5a. Isoquinolone 13a was
not observed. The photocatalytic conversion of the tetrahy-
droisoquinolines to the dihydroisoquinolones is summarized in
eq 1. Oxygen as it is found in air is the reagent, and water is the
byproduct of the photocatalytic oxidation reaction, providing
an environmentally benign approach to the synthesis.
Similar products were observed in the photo-oxidation of 5a
using 7-Se or 10-Te as photocatalyst with LED irradiation in
1
4.9:0.1 CH3CN/H2O. The H and 13C{1H} NMR spectra for
the mixture of the meso- and D,L-peroxodimer 14b were
strikingly similar to those for the mixture of the meso- and D,L-
peroxodimer 14a (Supporting Information). After 3 h of LED
irradiation with photocatalyst 10-Te, photo-oxidation of 2-
phenyl-1,2,3,4-tetrahydroisoquinoline 5a in 4.9:0.1 CH3CN/
H2O as solvent was complete5a was no longer observed in
the reaction mixture by TLC. At this point, a 9:1 mixture of the
meso- and D,L-peroxodimer 14b to the isoquinolone 13a was
formed (Figure S1, Supporting Information). Further LED
irradiation for an additional 5 h gave complete disappearance
of the meso- and D,L-peroxodimer 14b, and isoquinolone 13a
was isolated in 63% yield from the reaction mixture.
Oxidized Peroxo Intermediate. During the photo-
oxidation of 5c with photocatalyst 7-Se in a 4.9:0.1 solution
of CH3CN/H2O, a white precipitate formed and, when
consumption of 5c was complete, was isolated by filtration.
The solid was identified as a mixture of the meso- and D,L-
diastereomers of the peroxodimer 14a (Chart 4) primarily
through NMR spectral data (Supporting Information) and
elemental analysis. The mass spectrum of the white solid gave
m/z 242 as the parent ion with no higher mass peaks, which is
consistent with iminium ion 15a (C15H1335ClN+). However,
E
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Scheme 5. Formation of Hydroxy(perhydroxy)tellurane (E = Te) or Hydroxy(perhydroxy)selenane (E = Se) Intermediate
upon Addition of Water to the Chalcogenide−Singlet Oxygen Exciplex (Pertelluroxide or Perselenoxide Intermediate)
1
The reaction mixture obtained after 3 h of LED irradiation
was purified via chromatography on SiO2, and a band
containing mostly a mixture of meso- and D,L-14b was isolated
as a colorless oil. The mass spectrum of the colorless oil gave
m/z 208 as the parent ion with no higher mass peaks, which is
consistent with iminium ion 15b (C15H14N+). The NMR
spectral data for meso- and D,L-14b was strikingly similar to that
of meso- and D,L-14a (Supporting Information). The 1H NMR
spectrum of the mixture showed that the pairs of methylene
protons at C3 and C4 were no longer equivalent, and geminal
coupling was observed. Two singlets were observed in a ratio
of 45:55 at δ 6.09 and δ 5.98 for a methine proton at C1,
which suggested two compounds were produced in not quite a
1:1 ratio. The mixture of meso- and D,L-14b was not stable and
decomposed to many products upon standing overnight under
ambient conditions or at −20 °C under a nitrogen atmosphere.
Again, for the meso- and D,L-14a, a total of 26 unique signals
would be expected in the carbon NMR, and 21 of the expected
26 signals were observed with the “overlapped” carbons
appearing at δ 136.0, 128.2, 127.3, 115.7, and 42.2 (Supporting
Information).
specifically, O2 in the photo-oxidation of the tetrahydroiso-
quinolines to the dihydroisoquinolones.
b. Importance of Water and the Chalcogenorosami-
ne-1O2 Exciplex in the Photocatalytic Oxidation to the
Dihydroisoquinolones. In the absence of water, the oxidation
of tetrahydroisoquinolines to dihydroisoquinolones is not
observed with 1 mol % of photocatalyst, LED irradiation,
and pure CH2Cl2 or CH3CN as solvent. However, the addition
of 2% water to CH3CN gives oxidation of the tetrahydroiso-
quinolines to the dihydroisoquinolones with selenorosamine
and tellurorosamine photocatalysts as compiled in Table 3.
The addition of water to the exciplex between 1O2 and
diorganotelluides and diorganoselenides (a pertelluroxide or
perselenoxide intermediate) has a rich chemistry, as shown in
Scheme 5, to give the corresponding hydroxy(perhydroxy)-
tellurane10 or hydroxy(perhydroxy)selenane28 (intermediate A
in Scheme 5). Intermediates A can function as an oxidant or
they can eliminate H2O2 to generate the corresponding
selenoxide or telluroxide.10 The telluroixde can be reduced
back to the starting catalyst in a Swern-like oxidation,
transforming the 1,2,3,4-tetrahydroisoquinoline to 3,4-dihy-
droisoquinolone (Supporting Information).
Mechanistic Implications with Chalcogenorosamine
Photocatalysts. a. Importance of Singlet Oxygen. The
c. Enigma of the Thiorosamine Photocatalysts 7-S and
10-S. The other interesting feature of the photo-oxidation to
the dihydroisoquinolones is that the thiorosamine photo-
catalysts 7-S and 10-S do not give isolable amounts of
dihydroisoquinolones as a product of the reaction after 12 h of
irradiation in 4.9:0.1 CH3CN/H2O. Both of these photo-
1
presence of oxygen and, in particular, the generation of O2
appear to be important both for the photochemical aza-Henry
reaction and for the photo-oxidation of the tetrahydroisoqui-
nolines to the dihydroisoquinolones. The importance of 1O2 in
1
the aza-Henry reaction has been well-documented, with O2
1
accepting an electron from the donor atom (in this case, the N
catalysts generate O2 upon irradiation with values of Φ(1O2)
•−
1
of the tetrahydroisoquinoline) to generate superoxide (O2
)
of 0.21 for 7-S and 0.61 for 10-S (Table 1). If O2 alone were
as well as the cation radical of the donor atom.5,7 Alternatively,
the excited-state photosensitizer can induce single-electron
transfer from the donor atom.
responsible for oxidation to the tetrahydroisoquinoline to the
isoquinolone, one would reasonably expect some oxidation to
be observed. Because no oxidation was observed, one might
examine the differences in the chalcogenorosamine photo-
catalysts upon generating O2. The other key feature of the
photo-oxidation reactions is the necessity of having water as
part of the solvent system.
Photocatalysis with the 3-aryl-1,2,3,4-tetrahydroisoquino-
lines has been shown to proceed through single-electron
oxidation of the 3-aryl-1,2,3,4-tetrahydroisoquinoline and
subsequent loss of a hydrogen atom to give the corresponding
iminium compound (structures 15a and 15b in Chart 4, as
examples). Both 7-S and 10-S are capable of generating the
intermediate iminium compound as both of these photo-
catalysts form product in the aza-Henry reaction (Table 1).
However, the hydroxy(perhydroxy)thiane intermediate corre-
sponding to A in Scheme 5 has not been implicated in
reactions of sulfoxides with hydrogen peroxide.28
We repeated the photo-oxidation of 5 with thiorosamine 10-
S as the photocatalyst in 4.9:0.1 CH3CN/H2O, with 8.8 M
aqueous H2O2 replacing water to give a 4.9:0.1 CH3CN/H2O
mixture that was 0.18 M in H2O2. After 8 h of irradiation with
the LED, tetrahydroisoquinoline 5a had been consumed and a
1:9 mixture dihydroisoquinolone 13a and peroxodimers 14b
The results compiled in Table 1 show that photocatalysts
with higher values of Φ(1O2) give greater conversions of
1,2,3,4-tetrahydroisoquinoline 5a to aza-Henry product 6a.
The exceptions are tellurorosamine catalysts 7-Te and 8-Te,
with phenyl substituents in the 9-position that do not shield
the 9-position from nucleophilic addition.35 The selenoros-
amines 7-Se and 8-Se and Se- and Te-containing photo-
catalysts with a 9-mesityl substituent 9-Se, 9-Te, 10-Se, and
10-Te have much greater stability and gave higher conversions
of 5a to 6a than methylene blue (1), rhodamine 6G (4),
rosamines 7-O and 10-O, thiorosamines 7-S and 10-S, and
tellurorosamines 7-Te and 8-Te. These results also reflect
relative values of Φ(1O2).
1
When the reaction mixtures were deaerated with a stream of
N2 gas prior to irradiation in the aza-Henry reaction, overall
conversion of 5a to 6a was greatly reduced (Table 1),
1
indicating the importance of O2 in the photocatalyzed aza-
Henry reaction with this set of photocatalysts. The absence of
formation of isoquinolone 13a upon photo-oxidation of 5a
with photocatalyst 7-Se or 10-Te in deaerated 4.9:0.1
CH3CN/H2O also indicates the importance of oxygen and,
F
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was observed by 1H NMR spectroscopy in the reaction mixture
(Figure S2, Supporting Information). These results strongly
suggest that either H2O2 or the hydroxy(perhydroxy)-
chalcogenane intermediate A in Scheme 5 is responsible for
oxidation of the iminium ion intermediates in the photo-
oxidation of the tetrahydroisoquinolines to the dihydroisoqui-
nolones. As only 1 equiv of H2O2 can be produced from each
oxidation, the high yields implicate oxidation by intermediate
A that would regenerate the catalyst. We next examined the
energetics of formation of the 1O2-chalcogenorosamine
exciplex (the corresponding pertelluroxide, perselenoxide, or
persulfoxide intermediate) and its addition product with H2O.
DFT Calculations. Computational methods were em-
ployed to provide support for the observed photocatalytic
reactivity of the tellurorhodamine and selenorhodamine
chromophores compared to the thiorosamine chromophores.
All structures were optimized using the Gaussian09 software
package36 at the B3LYP level of theory.37−39 The geometries of
10-Se and 10-S were optimized using a 6-31+G(d) basis
set.40−42 A split basis set was used for 10-Te (Lanl2DZ43−45
for the tellurium atom and 6-31+G(d) for all other atoms).46
All geometries were optimized in CH3CN using the SMD
solvation model.47 Energy values were obtained from the
frequency calculations of the optimized geometries and used to
determine the Gibbs free energies of the reactions (ΔG). ΔG
values were determined for the oxidation and hydration of
diorganochalchogenides 10-Te, 10-Se, and 10-S, as shown in
Scheme 5. The balanced equations used to determine the ΔG
values are shown in Scheme 6. All calculated values for ΔG are
summarized in Table 4.
Table 4. Calculated ΔG Values for the Formation of G-E
and J-E (E = Te, Se, S)
compound
calculated ΔG of formation (kcal/mol)
G-Te
G-Se
G-S
−1.62
2.06
3.08
J-Te
J-Se
J-S
−3.49
4.51
22.35
−3.49 kcal/mol for G-Te and 4.51 kcal/mol for G-Se.
However, a significantly higher ΔG of +22.4 kcal/mol is
observed for the hydration of G-S, which suggests that the
reactive intermediate J-S is not readily formed. This is
consistent with both the lack of dihydroisoquinolone
formation with thiorosamine photocatalysts as well as the
requirement of water in the reaction. A summary of the
calculated ΔG values for each reaction can be visualized in the
reaction coordinate diagram in Figure 1.
Scheme 6. Balanced Equations for (a) Formation of the
Exciplex from 10-E via Reaction with 1O2 and (b) Hydration
of the Exciplexes To Form the
(Perhydroxy)hydroxychalcogen Species
Figure 1. Reaction coordinate diagram of oxidation and hydration of
chalchogenorhosamine photocatalysts in Scheme 6. Energies are
relative to 10-Te, 10-Se, and 10-S and follow from Scheme 5.
Seferos and co-workers proposed the Te(VI)dioxo inter-
mediate for tellurophene derivatives;49 however, under the
oxidation conditions of this study, this intermediate is unlikely
to be observed, although this intermediate can be minimized
computationally. Other oxidative addition products, such as a
telluradioxirane with a three-membered ring with O2 and the
chalcogen atom as proposed previously,27 were studied by
DFT calculations. However, for 10-Te, the three-membered
ring opened when optimized to the pertelluroxide intermediate
proposed in the paper.
CONCLUSIONS
■
The seleno- and tellurorosamines bring very high quantum
yields for the generation of singlet oxygen to photocatalysts
used in organo photocatalysis. In well-studied reactions, such
as the photochemical aza-Henry reaction with 2-aryl-1,2,3,4-
tetrahydroisoquinolines,5−7 the higher values of Φ(1O2) for 7-
Se, 10-Se, and 10-Te lead to higher photochemical
conversions to product. With other substrates, the transient
formation of the chalcogen−singlet oxygen exciplex (the
corresponding pertelluroxide or perselenoxide intermediate)
can lead to oxidation chemistry not observed with other
organic photocatalysts. The oxidation of 2-aryl-1,2,3,4-
tetrahydroisoquinolines to 2-aryl-3,4-dihydroisoquinolones
1
The energy of formation of the O2-bound exciplex was
explored first to investigate the reactivity of 10-Se and 10-Te
relative to that of 10-S as photocatalysts. Although 10-Te has
the lowest energy of formation of G-Te at −1.67 kcal/mol, the
calculated ΔG values for G-Se and G-S have small positive
values, 2.06 and 3.08 kcal/mol. Thus exciplex formation occurs
with minimal change in ΔG for 10-Te, 10-Se, and 10-S and
would not explain the difference in reactivity. Hydration of the
exciplexes of 10-Te and 10-Se have similarly small ΔG, with
G
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°C: 1H NMR (500 MHz, CH2Cl2) δ 7.34 (d, 2 H, J = 10.0 Hz), 7.11
(d, 2 H, J = 2.0 Hz), 7.10 (s, 2 H), 6.92 (dd, 2 H, J = 2.0, 10.0 Hz),
3.24 (s, 12 H), 2.43 (s, 3H), 1.84 (s, 6 H); 13C{1H} NMR (125.7
MHz, CD2Cl2) δ 161.6, 154.2, 144.8, 139.8, 136.0, 135.6, 132.0,
129.1, 119.1, 116.2, 105.9, 40.9, 21.3, 19.7; λmax (MeOH) 558 nm (ε
= 1.03 × 105 M−1 cm−1); λmax (CH2Cl2) 557 nm (ε = 1.24 × 105 M−1
cm−1); HRMS (ESI) m/z 401.2044 (calcd for C26H29N2S+:
401.2046); Anal. Calcd for C26H29N2S·PF6: C, 57.14; H, 5.35; N,
5.13. Found: C, 57.41; H, 5.15; N, 5.12.
proceeds via formation of a peroxo dimer, which is formed
from the oxidized photocatalyst.
EXPERIMENTAL SECTION
■
Preparation of 9-(2,4,6-Trimethylphenyl)bisjulolidyl-
selenorosamine Hexafluorophosphate (9-Se). 2-Bromomesity-
lene (0.663 g, 0.510 mL, 3.33 mmol) was added to a stirred
suspension of magnesium turnings (0.088 g, 3.7 mmol) in dry THF
(2 mL) under a dry N2 atmosphere. The mixture was stirred at
ambient temperature for 1 h, and the resulting solution of Grignard
reagent was transferred via cannula to a stirred suspension of
bisjulolidylselenoxanthone 1222b (0.100 g, 0.222 mmol) in dry THF
(7 mL). The resulting solution was heated at reflux for 16 h, cooled to
ambient temperature, and then poured into 25 mL of 10% (by
weight) aqueous HPF6. The aqueous mixture was extracted with
CH2Cl2 (3 × 25 mL), and the combined organic extracts were washed
with brine, dried over MgSO4, and concentrated. The residue was
recrystallized from 9:1 CH3CN/diethyl ether to give 0.101 g (65%) of
the desired product as a green solid, mp 240−241 °C: 1H NMR (500
MHz, CD2Cl2) δ 7.05 (s, 2 H), 6.95 (s, 2 H), 3.55−3.42 (m, 8 H),
2.86 (t, 4 H, J = 6.0 Hz), 2.63 (t, 4 H, J = 6.0 Hz), 2.43 (s, 3 H), 2.18
(quint, 4 H, J = 5.0 Hz), 1.94 (quint, 4 H, J = 5.0 Hz), 1.81 (s, 6 H);
13C{1H} NMR (75.5 MHz, CD2Cl2), 161.1, 150.1, 142.5, 140.6,
Photocatalyzed Aza-Henry Reaction of 2-Phenyl-1,2,3,4-
tetrahydroisoquinoline (5a). Comparison of Photocatalysts. To
a 4-dram (15 mL) vial were added 0.063 g (0.30 mmol) of
tetrahydroisoquinoline 5a (0.063 g, 0.30 mmol) and 0.003 mmol (1
mol %) of photocatalyst. The contents were dissolved in 5 mL of a 4:1
mixture of MeCN/MeNO2. The reaction mixture was then irradiated
with stirring for 3.0 h with a GE 14 W, 850 lm LED light placed 1 cm
from the 4-dram vial. The reaction mixture was concentrated in vacuo,
1
and the % conversion was determined via H NMR spectroscopy
comparing the integral of the 2-nitromethyl methylene protons (δ
4.81 and 4.53) and the 2-methine proton (δ 5.51) of 6a with the 2-
methylene protons (δ 4.42) of 5a. The reactions were run in triplicate,
and values reported in Table 1 represent the mean of the three runs
the standard deviation (SD) for the three runs.
Preparative Conversion of 2-Phenyl-1,2,3,4-tetrahydroiso-
quinoline (5a) to 1-Nitromethyl-2-phenyl-1,2,3,4-tetrahydroi-
soquinoline (6a). A. 7-Se as Photocatalyst. To a 4-dram vial were
added tetrahydroisoquinoline 5a (0.210 g, 1.00 mmol) and 7-Se (5.5
mg, 0.010 mmol, 1 mol %). The contents were dissolved in 15 mL of
a 4:1 mixture of MeCN/MeNO2. The reaction mixture was then
irradiated with stirring for 11.0 h with a GE 14 W, 850 lm LED light
placed 1 cm from the 4-dram vial. At the completion of irradiation,
the reaction mixture was concentrated in vacuo and the residue was
partitioned between a mixture of ether (20 mL) and water (20 mL).
The organic phase was separated, and the aqueous layer was further
extracted with ether (2 × 10 mL). The combined organic extracts
were dried over anhydrous MgSO4 and concentrated in vacuo. The
crude product was purified via chromatography on SiO2 eluted with
5% EtOAc in hexanes to yield 0.212 g (84%) of 6a5 and 0.037 g
(16%) of dihydroisoquinolone 13a.32
136.6, 136.0, 134.9, 130.4, 126.9, 120.3, 118.1, 53.1, 52.2, 29.6, 27.8,
22.9, 22.4, 22.2, 21.3; λmax (CH2Cl2) 590 nm (ε = 1.31 × 105 M−1
cm−1); λmax (CH3OH) 591 nm (ε = 1.55 × 105 M−1 cm−1); HRMS
(ESI) m/z 553.2125 (calcd for C34H37N280Se+: 553.2116). Anal.
Calcd for C34H37N2Se·PF6: C, 58.54; H, 5.35; N, 4.02. Found: C,
58.28; H, 5.54; N, 4.03.
Preparation of 3,6-Bis(dimethylamino)-9-(2,4,6-trimethyl-
phenyl)-9H-xanthen-9-ylium Hexafluorophosphate (10-O). 2-
Bromomesitylene (1.35 mL, 8.85 mmol) was added to a stirred
suspension of magnesium turnings (0.13 g, 5.5 mmol) in dry THF (3
mL) under a dry N2 atmosphere. The mixture was stirred at ambient
temperature for 1 h, and the resulting solution of Grignard reagent
was transferred via cannula to a stirred suspension of 3,6-
bis(dimethylamino)-9H-xanthen-9-one29 (11-O, 0.100 g, 0.354
mmol) in dry THF (7 mL). The resulting solution was heated at
reflux for 16 h, cooled to ambient temperature, and then poured into
25 mL of 10% (by weight) aqueous HPF6. After 12 h, the resulting
precipitate was collected via filtration and washed with water (5 mL)
and diethyl ether (15 mL). The residue was purified by
chromatography on SiO2 eluted with 20% EtOAc/hexanes to remove
starting xanthone and then 10% ether/CH2Cl2 to collect the product.
The product was recrystallized from 9:1 CH3CN/diethyl ether to give
0.032 g (18%) of the desired product as a green solid, mp 254−255
B. 10-Te as Photocatalyst. The reaction was repeated using
photocatalyst 10-Te (6.0 mg, 0.010 mmol, 1 mol %) with 5 h of
irradiation. Products 6a5 and 13a32 were purified via chromatography
on SiO2 eluted with 5% EtOAc in hexanes to yield 0.22 g (85%) of
6a5 and 0.005 g (2%) of dihydroisoquinolone 13a.32
5
1
For 6a: colorless oil; H NMR (300 MHz, CDCl3) δ 7.08−7.28
(m, 4 H), 6.95 (d, 2 H, J = 7.5 Hz), 6.82 (t, 2 H, J = 7 Hz), 5.52 (t, 1
H, J = 7 Hz), 4.82 (dd, 1 H, J = 7.5, 11.7 Hz), 4.54 (dd, 1 H, J = 6.5,
11.7 Hz), 3.60 (m, 2 H), 3.04 (m, 1 H), 2.75 (m, 1 H); 13C{1H}
NMR (75 MHz, CDCl3) δ 148.3, 135.2, 132.8, 129.5, 129.4, 129.3,
128.0, 126.9, 119.3, 115.0, 78.6, 58.1, 41.9, 26.3.
1
°C: H NMR (500 MHz, CD2Cl2) δ 7.17 (d, 2 H, J = 9.0 Hz), 7.10
(s, 2H), 6.90 (dd, 2 H, J = 2.5, 9.5 Hz), 6.84 (d, 2 H, J = 2.0 Hz), 3.30
(s, 12 H), 2.42 (s, 3 H), 1.90 (s, 6 H); 13C{1H} NMR (125.7 MHz,
CD2Cl2) δ 160.2, 158.3, 158.0, 140.2, 135.9, 131.3, 129.1, 128.6,
114.9, 114.0, 96.9, 41.2, 21.3, 19.8; λmax (MeOH) 537 nm (ε = 1.18 ×
105 M−1 cm−1); λmax (CH2Cl2) 537 nm (ε = 1.24 × 105 M−1 cm−1);
HRMS (ESI) m/z 385.2265 (calcd for C26H29N2O+: 385.2274). Anal.
Calcd for C26H29N2O·PF6: C, 58.87; H, 5.51; N, 5.28. Found: C,
58.94; H, 5.61; N, 5.28.
For 13a:32 white solid, mp 119−120 °C (lit.33 mp 119−123 °C);
1H NMR (500 MHz, CDCl3) δ 8.20 (d, 1 H, J = 7.5 Hz), 7.55−7.35
(m, 7 H), 7.27 (m, 1 H), 4.02 (t, 1 H, J = 6.5 Hz), 3.17 (t, 1 H, J = 6.5
Hz); 13C{1H} NMR (125.7 MHz, CDCl3) δ 164.5, 143.5, 132.4,
130.1, 129.2, 129.0, 127.5, 127.3, 126.5, 125.6, 49.7, 28.9; MS (ESI)
m/z 224 (calcd for C15H13NO+H+: 224).
Preparation of 3,6-Bis(dimethylamino)-9-(2,4,6-trimethyl-
phenyl)-9H-thioxanthen-9-ylium Hexafluorophosphate (10-
S). 2-Bromomesitylene (0.77 mL, 5.0 mmol) was dissolved in dry
THF (3 mL) under a dry N2 atmosphere. Magnesium turnings (130
mg, 5.5 mmol) were added, and the resulting mixture was stirred at
ambient temperature for 1 h. The resulting solution of Grignard
reagent was transferred via cannula to a stirred solution of 3,6-
bis(dimethylamino)-9H-thioxanthene-9-one30 (11-S, 100 mg, 0.335
mmol) in dry THF (7 mL). The resulting solution was heated at
reflux for 16 h, cooled to ambient temperature, and then poured into
25 mL of 10% (by weight) aqueous HPF6. After 12 h, the resulting
precipitate was collected via filtration and washed with water (5 mL)
and diethyl ether (15 mL). The CH3CN-soluble material was filtered
through Celite, concentrated, and recrystallized from 9:1 CH3CN/
diethyl ether to give 0.112 g (61%) of 10-S as a green solid, mp >260
Preparative Conversion of 2-(4-Methoxyphenyl)-1,2,3,4-
tetrahydroisoquinoline 5b to 1-Nitromethyl-2-(4-methoxy-
phenyl)-1,2,3,4-tetrahydroisoquinoline (6b).5 A. 7-Se as Photo-
catalyst. To a 4-dram vial were added tetrahydroisoquinoline 5b
(0.240 g, 1.00 mmol) and 7-Se (5.5 mg, 0.010 mmol, 1 mol %). The
contents were dissolved in 15 mL of a 4:1 mixture of MeCN/MeNO2.
The reaction mixture was then irradiated with stirring for 8.5 h with a
GE 14 W, 850 lm LED light placed 1 cm from the 4-dram vial. At the
completion of irradiation, the reaction mixture was concentrated in
vacuo and treated as described above for the reaction with 5a. The
crude product was purified via chromatography on SiO2 eluted with
5% EtOAc in hexanes to yield 0.241 g (81%) of 6b5 and 0.005 g (2%)
of dihydroisoquinolone 13b.2
H
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B. 10-Te as Photocatalyst. The reaction was repeated using
photocatalyst 10-Te (6.0 mg, 0.010 mmol, 1 mol %) with 6.5 h of
irradiation. Products 6a5 and 13a32 were purified via chromatography
on SiO2 eluted with 5% EtOAc in hexanes to yield 0.224 g (75%) of
6b5 and 0.003 g (1%) of dihydroisoquinolone 13b.32
For 6b: colorless oil; H NMR (300 MHz, CDCl3) δ 7.08−7.28
(m, 4 H), 6.90 (d, 2 H, J = 9 Hz), 6.80 (d, 2 H, J = 9 Hz), 5.38 (dd, 1
H, J = 6, 8.5 Hz), 4.80 (dd, 1 H, J = 8.5, 11.7 Hz), 4.54 (dd, 1 H, J =
6, 11.7 Hz), 3.73 (s, 3 H), 3.55 (m, 2 H), 3.00 (m, 1 H), 2.67 (m, 1
H); 13C{1H} NMR (75 MHz, CDCl3) δ 153.8, 142.9, 135.3, 132.7,
129.3, 127.7, 126.8, 126.4, 118.6, 114.5, 78.7, 58.7, 55.4, 42.8, 25.6.
For 13b:32 white solid, mp 116−119 °C (lit.32 mp 120−121 °C);
1H NMR (300 MHz, CDCl3) δ 8.14 (d, 1 H, J = 7.5 Hz), 7.45 (td, 1
H, J = 1.6, 7.5 Hz), 7.37 (t, 1 H, J = 7.5 Hz), 7.20−7.32 (m, 4 H),
3.94 (t, 1 H, J = 6.5 Hz), 3.82 (s, 3 H), 3.13 (t, 1 H, J = 6.5 Hz);
13C{1H} NMR (125.7 MHz, CDCl3) δ 164.4, 157.8, 138.3, 136.1,
product was purified via chromatography on SiO2 eluted with 20%
EtOAc/hexanes. Isoquinolone 13a was recrystallized from dichloro-
methane/diethyl ether to yield 0.129 g (58% isolated yield) of 13a.
B. 10-Te as Photocatalyst. The reaction was repeated using
photocatalyst 10-Te (6.0 mg, 0.010 mmol, 1 mol %) with irradiation
for 8 h. Isoquinolone 13a was purified via chromatography on SiO2
eluted with 5% EtOAc in hexanes to yield 0.141 g (63% isolated
yield) of 13a.
C. 7-S as Photocatalyst. The reaction was repeated using
photocatalyst 7-S (5.2 mg, 0.010 mmol, 1 mol %) with irradiation
for 12 h. Unreacted 5a (0.19 g, 90%) was recovered from the reaction
mixture.
5
1
D. 10-S as Photocatalyst. The reaction was repeated using
photocatalyst 10-S (5.3 mg, 0.010 mmol, 1 mol %) with irradiation
for 12 h. Unreacted 5a (0.20 g, 95%) was recovered from the reaction
mixture.
Preparation of 3,4-Dihydro-2-(4-methoxyphenyl)-1(2H)-iso-
quinolinone 13b.32 A. 7-Se as Photocatalyst. In a 4-dram vial
equipped with a stir bar, 5b (0.239 g, 1.00 mmol) and 7-Se (5.5 mg,
0.010 mmol, 1 mol %) were dissolved in 15 mL of a 4.9:0.1 mixture of
MeCN/H2O. The reaction mixture was irradiated with a GE 14 W,
850 lm LED light for 13 h. The crude product was purified via
chromatography on SiO2 eluted with 20% EtOAc/hexanes.
Isoquinolone 13b was recrystallized from dichloromethane/diethyl
ether to yield 0.098 g (41% isolated yield) of 13b.
B. 10-Te as Photocatalyst. The reaction was repeated using
photocatalyst 10-Te (6.0 mg, 0.010 mmol, 1 mol %) with irradiation
for 8 h. Isoquinolone 13b was purified via chromatography on SiO2
eluted with 5% EtOAc in hexanes to yield 0.18 g (75% isolated yield)
of 13b.
131.9, 129.8, 128.7, 127.1, 126.9, 126.7, 114.2, 55.5, 49.7, 28.7; MS
(ESI) m/z 254 (calcd for C16H15NO2+H+: 254).
Preparative Conversion of 2-(4-Chlorophenyl)-1,2,3,4-tetra-
hydroisoquinoline (5c) to 1-Nitromethyl-2-(4-chlorophenyl)-
1,2,3,4-tetrahydroisoquinoline (6c). A. 7-Se as Photocatalyst.
To a 4-dram vial were added tetrahydroisoquinoline 5c (0.245 g, 1.00
mmol) and 7-Se (5.5 mg, 0.010 mmol, 1 mol %). The contents were
dissolved in 15 mL of a 4:1 mixture of MeCN/MeNO2. The reaction
mixture was then irradiated with stirring for 5.0 h with a GE 14 W,
850 lm LED light placed 1 cm from the 4-dram vial. At the
completion of irradiation, the reaction mixture was concentrated in
vacuo and treated as described above for 5. The crude product was
purified via chromatography on SiO2 eluted with 5% EtOAc in
hexanes to yield 0.187 g (62%) of 6c5 and 0.043 g (17%) of
dihydroisoquinolone 13c.32
Preparation of 3,4-Dihydro-2-(4-chlorophenyl)-1(2H)-iso-
quinolinone 13c32 Using 7-Se as Photocatalyst. In a 4-dram
vial equipped with a stir bar, 5b (0.244 g, 1.00 mmol) and 7-Se (5.5
mg, 0.010 mmol, 1 mol %) were dissolved in 15 mL of a 4.9:0.1
mixture of MeCN/H2O. The reaction mixture was irradiated with a
GE 14 W, 850 lm LED light for 7.5 h. The white precipitate was
collected by filtration, and the filtrate was then concentrated in vacuo.
The crude product was purified via chromatography on SiO2 eluted
with 20% EtOAc/hexanes. Isoquinolone 13c was recrystallized from
dichloromethane/diethyl ether to yield 0.121 g (78% isolated yield)
of 13c. The white precipitate was isolated in 0.031 g (6%) yield and
was identified as a mixture of D,L- and meso-diastereomers of the
B. 10-Te as Photocatalyst. The reaction was repeated using
photocatalyst 10-Te (6.0 mg, 0.010 mmol, 1 mol %) with 6.5 h of
irradiation. The products 6c5 and 13c32 were purified via
chromatography on SiO2 eluted with 5% EtOAc in hexanes to yield
0.25 g (85%) of 6c5 and 0.005 g (2%) of dihydroisoquinolone 13c.32
5
1
For 6c: colorless oil; H NMR (300 MHz, CDCl3) δ 7.26−7.34
(m, 4 H), 6.89 (dd, J = 1.5, 6.3 Hz), 5.48 (m, 1 H), 4.84 (ddd, 1 H, J
= 1.2, 8.4, 12.1 Hz), 4.56 (ddd, 1 H, J = 1.5, 5.4, 12.1 Hz), 3.62 (m, 2
H), 3.06 (m, 1 H), 2.78 (m, 1 H).
For 13c:32 white solid, mp 150−152 °C (lit.33 mp 150−151.5 °C);
1H NMR (300 MHz, CDCl3) δ 8.14 (d, 1 H, J = 7.6 Hz), 7.48 (dt, 1
1
peroxodimer 14a: H NMR (300 MHz, CDCl3) δ 7.32−7.06 (m, 12
H, J = 1.2, 7.6 Hz), 7.40−7.32 (m, 5 H), 7.24 (d, 1 H, J = 9.6 Hz),
3.97 (t, 2 H, J = 6.4 Hz), 3.14 (t, 2 H, J = 6.4 Hz); 13C{1H} NMR (75
MHz, CDCl3) δ 164.2, 141.5, 138.2, 132.2, 131.5, 129.4, 128.9, 128.7,
127.2, 126.9, 126.5, 49.2, 28.5.
H), 6.82 (t, 4 H, J = 7.4 Hz), 6.09 (s, 1 H, one diastereomer), 5.98 (s,
1H, one diastereomer), 3.59−3.46 (m, 1H), 3.42−3.18 (m, 3H),
2.88−2.69 (m, 4H); 13C{1H} NMR (300 MHz, CDCl3) δ 146.8,
146.7, 136.0, 131.91, 131.88, 128.5, 128.4, 128.36, 128.33, 128.2,
127.4, 127.3, 125.7, 123.6, 123.5, 115.7, 90.2, 90.0, 42.2, 27.50, 27.46;
MS (ESI) m/z 242 (calcd for C15H1335ClN+ 242; calcd for
C30H2635Cl2N2O2+H+ 517). Anal. Calcd for C30H26Cl2N2O2: C,
69.64; H, 5.06; N, 5.41. Found: C, 69.98; H, 4.92; N, 5.41. Organic
peroxide species are potentially explosive and care should be taken
handing these compounds.
Preparation of 2-(Nitromethyl)-1-phenylpyrrolidine 6d.5 A.
7-Se as Photocatalyst. To a 4-dram vial equipped with a stir bar were
added 1-phenylpyrrolidine 5d (0.086 mL, 0.60 mmol) and 1 mol % of
photocatalyst 7-Se (5.5 mg, 0.010 mmol, 1 mol %). The contents
were dissolved in 5 mL of a 4:1 mixture of MeCN/MeNO2. The
reaction mixture was irradiated for 90 h with a GE 14 W, 850 lm LED
light. The reaction mixture was then concentrated in vacuo. Product
6d was purified via chromatography on SiO2 eluted with 5% EtOAc in
hexanes to yield 0.0038 g (3.1% yield) of 6d.
Photo-oxidation of 5a with Photocatalyst 10-S in 4.9:0.1
CH3CN/8.8 M H2O2. In a 4-dram vial equipped with a stir bar, 2-
phenyl-1,2,3,4-tetrahydroisoquinoline 5a (0.209 g, 1.00 mmol) and
10-S (5.3 mg, 0.010 mmol, 1 mol %) were dissolved in 15 mL of a
4.9:0.1 mixture of MeCN/8.8 M H2O2 (final concentration of H2O2
of 0.18 M). The reaction mixture was irradiated with a GE 14 W, 850
lm LED light for 8 h and the reaction mixture was concentrated in
vacuo.
Quantum Yields for Fluorescence and Singlet Oxygen
Generation. Optical absorption measurements were performed using
PerkinElmer Lambda 900 UV−vis-NIR spectrophotometer. A
Fluorolog-3 spectrofluorimeter equipped with iHR320 spectrometer
for infrared range (Horiba) was employed to obtain fluorescence
spectra. Absorption for all the measured samples was matched at the
wavelength of excitation (532 nm) before spectra acquisition.
To quantify singlet oxygen generation, singlet oxygen phosphor-
escence emission was detected by the thermoelectrically cooled NIR-
B. 10-Te as Photocatalyst. The reaction was repeated using
photocatalyst 10-Te (6.0 mg, 0.010 mmol, 1 mol %). The product 6d
was purified via chromatography on SiO2 eluted with 5% EtOAc in
hexanes to yield 0.0075 g (6% yield) of 6d.
For 6d:5 1H NMR (300 MHz, CDCl3) δ 7.34−7.24 (m, 2H), 6.79
(t, J = 7.4 Hz, 1H), 6.70 (d, J = 8.4 Hz, 2H), 4.68−4.59 (m, 1H),
4.48−4.38 (m, 1H), 4.19 (t, J = 10.5 Hz, 1H), 3.51−3.44 (m, 1H),
3.27−3.14 (m, 1H), 2.18−2.04 (m, 1H); 13C{1H} NMR (300 MHz,
CDCl3): 145.5, 129.7, 117.3, 111.9, 75.8, 57.4, 48.1, 29.3, 22.8.
Preparation of 3,4-Dihydro-2-phenyl-1(2H)-isoquinolinone
13a.32 A. 7-Se as Photocatalyst. In a 4-dram vial equipped with a
stir bar, 2-phenyl-1,2,3,4-tetrahydroisoquinoline 5a (0.209 g, 1.00
mmol) and 7-Se (5.5 mg, 0.010 mmol, 1 mol %) were dissolved in 15
mL of a 4.9:0.1 mixture of MeCN/H2O. The reaction mixture was
irradiated with a GE 14 W, 850 lm LED light for 8 h. The crude
I
DOI: 10.1021/acs.organomet.9b00126
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
́
́
PMT unit (Hamamatsu H10330B-75) attached to the output of the
iHR320 spectrometer that was set to 1270 nm. Singlet oxygen decay
under pulsed excitation was directly recorded on the Tektronix
oscilloscope (TDS 3034C Digital Phosphor oscilloscope) coupled to
the NIR-PMT unit. A second harmonic (532 nm) from nanosecond
pulsed Q-switched Nd:YAG laser (LS-2137 from Lotis TII) lasing at
the SH with repetition rate of 10 Hz was used as an excitation source.
Data Analysis and Statistics. Statistical significance was assessed
using one-way ANOVA and a posthoc multiple pairwise comparison
using the Tukey test with a significance level of 0.05. The Student’s t
test was used for pairwise comparisons of independent samples. All t
tests were two-sided, and p values less than 0.05 were considered
significant.
Computational Details. Calculations were done with Gaus-
sian0936 input files, and results were visualized using GaussView05.48
All structures were optimized using the B3LYP37−39 level of theory
with the 6-31+G(d)40−42 basis set for all light atoms and
LanL2DZ43−45 for Te. Energy values were obtained from the free
energy from the frequency calculations.
(5) Condie, A. G.; Gonzalez-Gomez, J. C.; Stephenson, C. R.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00126.
Text and figures giving general methods, experimental
data for the synthesis of 5a, 5b, and 5c, NMR spectral
data (1H and 13C{1H} NMR) for all noncommercial
1
compounds, and H NMR spectral data for the mixture
of products from irradiation of 5a and 10-Te in 4.9:0.1
CH3CN/H2O and from 5 and 10-S in 4.9.1 CH3CN/8.8
M H2O2 (PDF)
AUTHOR INFORMATION
(13) Lang, X.; Ma, W.; Chen, C.; Ji, H.; Zhao, J. Selective Aerobic
Oxidation Mediated by TiO2 Photocatalysis. Acc. Chem. Res. 2014, 47,
355−363.
(14) Schultz, D. M.; Yoon, T. P. Solar synthesis: prospects in visible
light photocatalysis. Science 2014, 343, 1239176.
■
Corresponding Authors
*E-mail: t.m.mccormick@pdx.edu.
*E-mail: mdetty@buffalo.edu.
(15) Pitre, S. P.; McTiernan, C. D.; Ismaili, H.; Scaiano, J. C.
Mechanistic Insights and Kinetic Analysis for the Oxidative
Hydroxylation of Arylboronic Acids by Visible Light Photoredox
Catalysis: A Metal-Free Alternative. J. Am. Chem. Soc. 2013, 135,
13286−13289.
(16) Pitre, S. P.; McTiernan, C. D.; Scaiano, J. C. Understanding the
Kinetics and Spectroscopy of Photoredox Catalysis and Transition-
Metal-Free Alternatives. Acc. Chem. Res. 2016, 49, 1320−1330.
(17) Redmond, R. W.; Gamlin, J. N. A compilation of singlet oxygen
yields from biologically relevant molecules. Photochem. Photobiol.
1999, 70, 391−475.
ORCID
Tymish Y. Ohulchanskyy: 0000-0002-7051-6534
Theresa M. McCormick: 0000-0003-1745-2911
Michael R. Detty: 0000-0002-8815-1481
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported in part by the National Science
Foundation (CHE-1800599 to T.M.M. and CHE-1800288 to
M.R.D.). T.M.M. also thanks Portland State University for
financial support.
(18) Stracke, F.; Heupel, M.; Thiel, E. Singlet molecular oxygen
photosensitized by Rhodamine dyes: correlation with photophysical
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