Mechanism of the photorelease of alcohols from the 9-phenyl-9-tritylone protecting group

Article abstract of DOI:10.1039/c9pp00183b

The 9-phenyl-9-tritylone photoremovable protecting group was examined using both photoproduct analysis and laser flash photolysis in order to determine the details of its mechanism of alcohol release. It is shown that formation of the tritylone anion radi

Full text of DOI:10.1039/c9pp00183b

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DOI: 10.1039/C9PP00183B
ARTICLE
Mechanism of the Photorelease of Alcohols from the 9-Phenyl-9-
tritylone Protecting Group
Received 00th January 20xx,
Accepted 00th January 20xx
*
Andrea N. Zeppuhar, Kevin Hill-Byrne, and Daniel E. Falvey
DOI: 10.1039/x0xx00000x
The 9-phenyl-9-tritylone photoremovable protecting group was examined using both photoproduct analysis and laser flash
photolysis in order to determine the details of its mechanism of alcohol release. It is shown that formation of the tritylone
anion radical is required for alcohol release. Attempts to trigger release via intramolecular photoinduced electron transfer
were unsuccessful due to rapid back electron transfer reactions of the triplet diradical anion.
Scheme 1. Previously proposed mechanism for the photorelease of alcohols under PET
conditions
1
. Introduction
Photoreleasable protecting groups (PPGs) provide means
for unmasking stable molecules under mild conditions and
1
with a high degree of spatiotemporal control. Successful
2
3,
application of PPG strategies in total synthesis, drug release,
4
5, 6
7
8
optogenetics, photolithography, photoimaging, and other
areas has inspired efforts at designing new PPGs with
expanded capabilities.
There have been many developments in the field of PPGs
when it comes to the photorelease of good leaving groups,
such as carboxylates and sulfonates. However, with less labile
leaving groups such as alcohols, there are fewer examples.
Some approaches to alcohol release include intramolecular
9
10
cyclization of silyl and cinnamate
lactonization employed with a quinone trimethyl lock,
protecting groups, standard photorelease conditions.
11
However, less clear was if
2
,
3
, or both, lead to substrate
could lead directly to substrate
release was intriguing as it forms readily in simple solvents
12
photoenolization, and the use of carbamate esters that rely release. The possibility that
3
13
on decarboxylation to effect alcohol release.
1
4
A recent report described the 9-phenyl-9-tritylone (PTO) with modestly reactive C–H bonds (e.g. methanol, 1,4-
group as a PPG capable of clean and efficient release of dioxane). Such a pathway could allow for the application of
primary and secondary alcohols through a photoinduced PTO release without an additional electron donor, and thus
electron transfer (PET) mechanism. PET is advantageous greatly expand its applicability. In fact, some early control
because it allows for decoupling of the light absorbing step experiments showed photoconversion of the PTO-ethers was
from release which allows for optimization of the occurring in the absence of strong electron donors.
chromophore without changing its mechanism. Excitation of
The work herein describes a series of mechanistic and
PTO ethers with UV light in the presence of strong ground spectroscopic studies designed to support further
state electron donors (e.g. triethylamine) could effect release development of the PTO group and similar PPGs. Specifically,
of alcohols in good yields, with polar solvents (methanol, the behavior of PTO-ethers will be probed under direct
acetonitrile) providing better yields than nonpolar solvents photolysis conditions, i.e. in the absence of an electron donor,
(
benzene). Alternatively, visible light photolysis of a strong in order to determine the details of the release mechanism.
excited state donor (e.g. Ir(ppy) ) in the presence of PTO These studies will show that (a) while forms readily, it is
3
2
3
ethers also caused release. In addition, preliminary laser flash that is necessary for the desired C—O bond scission; (b) the
photolysis (LFP) studies of these reactions confirmed that C—O bond scission is relatively slow, having a time constant of
photoinduced electron transfer from the donor to the PTO approximately 1 ms; (c) the PTO group can be modified in a
group was responsible for release (Scheme 1). However, the way to allow intramolecular electron transfer however, a more
latter experiments show that both the PTO anion radical
well as its conjugate acid, ketyl radical
, form under the electron transfer.
2
, as rapid bond scission step is needed to compete with back
3

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Journal Name
2
.5 Synthesis
2
. Experimental
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DOI: 10.1039/C9PP00183B
Synthetic procedures as well as characterization data for new
compounds can be found in the SI.
2
.1 General Procedures
UV-Vis spectra were collected on a Shimadzu UV-1800
spectrometer using UVProbe 2.43 software. Samples were
scanned using a fast scanning speed and a sampling interval of
3
. Results and Discussion
As outlined in Scheme 2, the PTO-ether chosen for these
1
.0 nm. Each sample was blanked with the solvent of choice
1
used in solvating the compound. All H NMR were obtained on
a Bruker 400 MHz instrument. Mass spectrometry experiments
were performed on a JEOL AccuTOF-CS-ESI-TOF. Kinetic growth
and decay curves were fitted using MATLAB software.
studies was benzyl ether
benzyl alcohol . This ether was chosen due to its ease of
synthesis as well as ease of product analysis.
5 which, upon photolysis, releases
6
Scheme 2. Direct photolysis of benzyl ether 5 to generate benzyl alcohol 6
2
.2 Gas Chromatography Analysis
Gas chromatography analysis was done using a Shimadzu GC-
7A, containing a RTX-5 stationary phase column (length = 15
1
m, inner diameter (i.d.) = 0.25 mm, film thickness = 0.25 μm),
equipped with a FID detection system, and using the following
method specifications: column temperature = 60°C, injection
temperature = 280°C, and detector temperature = 300°C, with
a temperature/pressure profile for injection of: 67 kPa, 3.0
minutes, 3.9 mL/minute, 98 kPa, 9.0 minutes, column of: 60°C,
In order to understand the role of solvent in the release
3
minutes, 30 mL/minute, 300°C, 9.0 minutes, injector
pressure of 60 kPa, total flow of 31 mL/min., column flow of
.45 mL/min., and a linear velocity of 39.1 cm/s. For yield
determination, a calibration curve was generated (see SI).
mechanism,
under direct photolysis conditions. The depletion of
, top) as well as formation of (Figure 1, bottom) was
monitored by gas chromatography.
5
was photolyzed at 350 nm in various solvents
5
(Figure
1
1
6
2
.3 Steady-State Photolysis
1
00%
0%
60%
0%
20%
Benzyl ether was photolyzed in an RPR-600 Rayonet reactor (8-
bulb, 36 watt, ~33°C operating temperature, 350 nm max
output) supplied by Southern New England Ultraviolet. 1 mL
of a 9 mM photolysis solution was placed in a quartz cuvette
and purged with nitrogen in both the solution (minimum of 15
minutes) and the headspace (minimum of 3 minutes). The
samples were irradiated at 350 nm for up to 3 hours. Upon
completion of photolysis time, 60 μL of the sample was
transferred into a GC vial and then acidified by adding 10 μL of
8
4
0
%
0
30
60
90
120
150
180
Time (min)
0
.1M acetic acid. 2 μL of this solution was injected into the GC
Benzene
MeCN
MeOH
for analysis.
1
00%
2
.4 Laser Flash Photolysis
8
6
4
0%
0%
0%
Laser flash photolysis studies were conducted using a Nd:YAG
laser (355 nm output) supplied by Continuum with pulses 4-6
ns in duration as the excitation source. The probe beam used
was a 350 W Xe arc lamp that passed through a
monochromator to a PMT detector. Solutions were purged
20%
with N , both in the solution (minimum of 15 minutes) and the
2
0
%
headspace (minimum of 3 minutes). Samples were prepared
such that the absorption at 355 nm was between 1.0 and 2.0.
When obtaining a full spectrum, a fresh supply of the reaction
mixture into the cuvette was attained by setting up a standard
N2 purged flow cell connecting the cuvette to the stock
solution via a double-headed needle. The photolysate was
then drained from the cuvette into a waste vessel. This setup
prevents accumulation of photoproducts and avoids the
depletion of the substrate during the experiment.
0
30
60
90
120
150
180
Time (min)
Benzene
MeCN
MeOH
Figure 1. Top: Monitoring conversion of 5 during direct photolysis in various solvents.
Bottom: Monitoring yield of 6 during direct photolysis in various solvents
In benzene and acetonitrile there was very little depletion
of
5 and no formation of 6. This is due to these solvents being
2
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poor hydrogen atom donors, which is supported by the bond resemblance to the triplet state of the structurally similar
View Article Online
16
DOI: 10.1039/C9PP00183B
dissociation energies (BDE) for hydrogen atom abstraction. The benzophenone chromophore. The band at 440 nm that
C—H bond BDEs for these solvents are 112.9 kcal/mol and persists beyond the triplet lifetime is attributed to the
1
5
9
7.0 kcal/mol, respectively. The BDE of benzene is too large tritylonyl radical
4
that would form as a result of homolytic
. Additionally, this band displays
and benzene is considered inert as a hydrogen atom donor. cleavage of the C—O bond of
5
While acetonitrile is more thermodynamically favored for an increase in decay rate when exposed to oxygen which is
hydrogen atom transfer, it is not kinetically favored due to the also consistent with the formation of a tritylonyl radical. It is
electron withdrawing cyano group. This electron deficient C— believed that this bond cleavage from the triplet state, while
H bond does not allow for a rapid reaction with the excited not very efficient, does explain the small amount of conversion
state triplet. Therefore, in these solvents, the excited state of
5 observed in benzene. However, in the absence of any
triplet is formed which returns to ground state without radical scavengers, the radicals that result from homolytic
releasing the alcohol. However, when a better hydrogen atom cleavage simply recombine. Figure 2 (bottom) displays the
donating solvent, such as methanol, is used, depletion of
observed. Methanol has a BDE of 96.1 kcal/mol and has a photolysis of
more electron rich C—H bond allowing for a more kinetically peak with λmax = 530 nm which has a half-life that exceeds 350
1
is transient absorption spectrum resulting from 355 nm pulsed
5
in methanol. This spectrum displays a sharp
14
favorable hydrogen atom abstraction. In this case, there is µs. Previous work has assigned this peak to the PTO ketyl
some formation of benzyl alcohol observed but in poor yields. radical . This is in agreement with the previous data that
The belief is that, when in the presence of a good hydrogen indicates is necessary for chemistry to occur as depletion of
atom donor, the excited state triplet can abstract a hydrogen was observed in methanol but not in benzene.
atom to form the ketyl radical . From , chemistry can begin These results led to the hypothesis that alcohol release
to occur. Since the yield of is poor, it is believed that release occurs through the anion radical intermediate . In order to
from is a minor pathway and depletion of the is due to confirm this, studies were conducted in methanol with the
other nonproductive pathways. addition of a base. In methanol would form and then would
Formation of these intermediates is further confirmed by be deprotonated by the base in order to form . It is from
3
3
5
3
3
5
2
3
5
3
2
laser flash photolysis (LFP), which is a technique used to here that C—O bond scission is expected to occur and release
identify short-lived intermediates. Figure 2 (top) displays the the alcohol. When using a strong base, such as hydroxide,
transient absorption spectrum resulting from 355 nm pulsed alcohol release was observed. As shown in Table 1, as the
photolysis of
5
in benzene. A short-lived peak with λma x1 = 540 concentration of hydroxide increases from 0-1 mM, the yield
4
nm forms immediately after the pulse. Previous studies have of alcohol also increases. However, for reasons that are not yet
assigned this peak to be the triplet state of PTO due to its clear, when the base concentration is greater than 1.5 mM the
yields start to drop. It is important to note that the yield of
0.16
0.12
0.08
0.04
0.00
alcohol under direct photolysis conditions is significantly less
14
than those reported under PET conditions indicating that,
from a practical standpoint, direct photolysis is not the most
beneficial route for alcohol deprotection.
Table 1. Yields of 6 in the 3-hour photolysis at 350 nm with varying concentrations of
sodium hydroxide. Starting ether concentration is 9 mM
3
90
490
590
690
a, b
[
NaOH] (mM)
% Yield Alcohol
Wavelength (nm)
0
0
0
.0
.1
.5
43.8 ± 0.96
68.4 ± 8.46
74.5 ± 6.51
77.5 ± 17.74
55.9 ± 11.73
46.7 ± 2.22
47.5 ± 2.81
0
.3 μs
0.5 μs
0.7 μs
1 μs
2 μs
0
.20
.15
.10
.05
.00
1.0
1
2
2
.5
.0
.5
0
0
0
0
a
Error bars are the result of triplicate experiments
Yields are corrected to reflect amount of starting material that converted
b
3
90
490
Wavelength (nm)
0.5 μs 0.7 μs
2 μs 5 μs
590
690
0
.3 μs
.5 μs
1 μs
10 μs
1
Figure 2. Transient absorption spectra from 355 nm pulsed photolysis of 5, Top: in
benzene, Bottom: in 3:1 methanol: 1,4-dioxane
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To test the hypothesis that complete deprotonation of
required for release, experiments were also performed using a
weaker base. For this, pyridine was chosen since it can act as a
base but it is a poor electron donor meaning it is not likely to
Journal Name
3
is
Scheme 3. Proposed mechanism for the photorelease of alcVoiheowlsArutincdleeOr ndliinreect
DOI: 10.1039/C9PP00183B
photolysis conditions
undergo PET. Under these conditions, depletion of
observed but there was no release of alcohol. Using previous
estimates on similar radicals, we estimate that ketyl radical
5 was
3
17
has a pKa ≈ 9 meaning pyridine is not a strong enough base
for deprotonation.
Further confirmation that
2 is formed when adding
hydroxide is provided by LFP experiments. Upon addition of
sodium hydroxide, a change in the spectrum is observed as
shown in Figure 3. In the 550-650 nm region, a new broad
14
band grows in. Previous work has assigned this band to be
anion radical . Formation of is due to deprotonation of by
sodium hydroxide. The isosbestic point at 530 nm is due to
2
2
3
simultaneous formation of
2
as
3
is decaying.
With these data, the mechanism of photorelease under
direct conditions can be proposed, as shown in Scheme 3.
Upon excitation with 350 nm light, the PTO-ether is excited to
its singlet state. Because benzophenone forms triplet rapidly
and with unit quantum yield , we assume the PTO-ethers
behave in a similar way and that the observed photochemistry
occurs exclusively from the triplet state. In the presence of a
good hydrogen atom donor, the triplet state abstracts a
0
0
0
0
0
0
.25
.20
.15
.10
.05
.00
18
hydrogen atom to form the ketyl radical
3. From 3, some
3
90
490
590
690
alcohol is released but in poor yields along with a complex
mixture of other by-products, the most likely being various
Wavelength (nm)
0
2
.3 μs
μs
0.5 μs
5 μs
0.7 μs
10 μs
1 μs
isomers formed from dimerization of
3. Similar products have
been observed in systems where the structurally similar
1
9
Figure 3. Transient absorption spectrum obtained from 355 nm pulsed photolysis
of 5 in 3:1 methanol: 1,4-dioxane with the addition of 1 mM sodium hydroxide
benzophenone ketyl radical is formed. When
in the presence of a base, it is deprotonated to form the anion
radical and it is from that C—O bond scission occurs to
cleanly release the alcohol. Previous studies
demonstrated that the alcohol is released as an anion and the
PTO chromophore is left behind as radical which abstracts a
hydrogen atom or combines with oxygen. The absorption of
3 is generated
2
2
Upon observing that the yields of alcohol were improved
with increasing concentrations of base, LFP studies were
performed to determine the effect of base concentration on
14
have
4
the formation of
of base is increased, the rate at which
2
. As shown in Figure 4, as the concentration
forms also increases.
2
2
can be fitted to first-order decay, and from that we derive a
lifetime of ca. 1-2 ms. This is near the upper limit of lifetimes
that can be accurately determined with our current LFP setup
as mixing of irradiated solution with dark areas is also
occurring on this timescale.
Using these data, a pseudo-first order analysis was done in
order to determine the second-order rate constant for proton
transfer between the
determined to be 1.84 × 10 M s .
3 and base. The rate constant was
9
-1 -1
0
0
0
0
.15
.10
.05
.00
Knowing that release occurs through the anion radical,
attempts were made to improve the PTO group. Modifications
were made in an effort to have intramolecular electron
transfer thus eliminating the need for an external electron
donor. As shown in Scheme 4, the phenyl group in
replaced with a 4-phenol group to make compound
1
was
7
. We
anticipated that, upon deprotonation, the phenoxide anion
would donate an electron to the excited tritylone moiety to
0
.0E+00
1.0E-05
2.0E-05
Time (s)
3.0E-05
4.0E-05
Scheme 4. Proposed reaction for intramolecular electron transfer for alcohol release
0
0
mM OH
.5 mM OH
0.1 mM OH
1 mM OH
Figure 4. Growth of anion radical 2 at 620 nm with changing concentration of
hydroxide. Starting ether concentration is 9 mM
4
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ARTICLE
form the anion diradical
alcohol.
9
, which would subsequently release leaving a longer-lived signal at 420 nm. The latter is
View Article Online
DOI: 10.1039/C9PP00183B
characteristic of an isolated phenoxy radical which has an
20
As shown in Figure 5, deprotonation of the pentyl ether absorption at ca. 400 nm. The 510 nm transient species is
derivative of leads to a new absorption band with a further assigned to the triplet diradical due its relatively long
7
maximum at 362 nm and tail extending out to nearly 500 nm. lifetime of 2.5 µs (in the absence of O ). This long lifetime is
2
This new high wavelength absorption is not present in either unusual for a covalently linked diradical species. Additional
the simple tritylone group or the isolated phenoxide anion characterization through computational modeling and ultrafast
(λmax at 290 nm, see SI) and is therefore attributed to charge spectroscopy will be undertaken in due course.
transfer absorption from the phenoxide group to the tritylone
moiety.
4
. Conclusions
4
3
2
1
0
.0
.0
.0
.0
.0
The above experiments provide further insight into the
mechanism of alcohol photorelease from PTO-ethers. LFP and
product analysis studies have confirmed that while the ketyl
radical intermediate
3 is formed during photolysis, it is from
the anion radical that C—O bond scission occurs. However,
2
under these conditions, alcohol release is relatively slow, on
the order of ca. 1 ms. In an effort to eliminate the need for an
external electron donor, modifications were made to the PTO-
ether in an effort to allow for intramolecular electron
donation. Unfortunately, even with extended photolysis, this
derivative did not demonstrate efficient alcohol release likely
due to back electron transfer being more rapid than alcohol
release.
2
00
300
400
Wavlength (nm)
pH 7 pH 14
500
600
700
Figure 5. UV-Vis spectrum of 7 in acetonitrile
Unfortunately, extended photolysis (20 hrs) in basic
acetonitrile of the pentyl ether derivative of at 350 nm does
not release 1-pentanol to any appreciable extent. Apparently,
back electron transfer to restore the ground state occurs more
rapidly than alcohol release. This can be further confirmed by
LFP. As displayed in Figure 6, LFP of
provides two transient absorption bands, one at 420 nm and
the other at 510 nm. These signals can be tentatively assigned
7
Conflicts of interest
There are no conflicts to declare.
7 in basic acetonitrile,
to the triplet anion diradical 9.
Acknowledgements
The authors would like to thank the National Science
Foundation.
0
0
0
0
.06
.04
.02
.00
Notes and references
1
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80
.1 μs
480
Wavelength (nm)
0.5 μs 1 μs
580
680
5 μs
0
2 μs
2
3
. Greene, T. W. and P. G. M. Wuts Protective groups in organic
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Figure 6. Transient absorption spectrum from 355 nm pulsed photolysis of 7 in basic
acetonitrile
This assignment is made on the basis of the following
considerations. First, prolonged photolysis of
or no decomposition. Thus, the transient species could not be
a radical or ionic fragment of Second, the transient
absorption bands from differ in both position and shape
from the simple tritylone-localized triplet state derived from
6 provides little
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4
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5
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to the corresponding spectra derived from (bottom of Figure
and Figure 3, respectively) Third, the observation of the
charge transfer absorption in the steady-state UV-Vis spectrum
Figure 5) indicates forms directly upon light absorption.
6
7
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(
9
Finally, we note that the signal at 510 nm is quenched by O2
This journal is © The Royal Society of Chemistry 20xx
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Photochemical & Photobiological Sciences
View Article Online
DOI: 10.1039/C9PP00183B
A mechanistic investigation of the 9-phenyl-9-tritylone photoremovable protecting group for alcohols
revealed that the anion radical is the key intermediate required for clean deprotection.
O
Ph
OR
h
e- donor
h
H• donor
OH
O
+
H+
H+
-
Ph
OR
Ph
OR
ROH
By-Products