Photoactivation of (p-methoxyphenyl)(trifluoromethyl)diazirine in the presence of phenolic reaction partners

Article abstract of DOI:10.1002/chem.201203479

Shine light on your chemistry! Irradiating 3-(4-methoxyphenyl)-3- (trifluoromethyl)-3H-diazirine in the presence of equimolar solutions of phenol and tyrosine derivatives leads to Friedel-Crafts alkylations (see scheme), which suggests a strategy for the development of "cleaner" diazirines for chemical biology. Copyright

Full text of DOI:10.1002/chem.201203479

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DOI: 10.1002/chem.201203479
Photoactivation of (p-Methoxyphen
ACHTUNGTRENNyUNG l)(trifluoromethyl)diazirine in the
Presence of Phenolic Reaction Partners
Bjçrn Raimer and Thomas Lindel*[a]
Diazirines are of key importance for photoaffinity label-
solvent led to formal insertion into the CH bond (50%).
The majority of the subsequent, chemistry-oriented studies
also used the solvent as a reaction partner. For instance,
benzylmethylethers were obtained on irradiation of PTD de-
rivatives in methanol by the Hatanaka,[5] Welzel,[6] and
Maier[7] groups. The PTD moiety was also used as a partial
structure of benzoxazinones[5] and benzoxazolinones[8] that
were photolyzed in MeOH to give the methylethers in mod-
erate yield.[9] Beyond choosing MeOH as the reaction part-
ner, Chee and co-workers reported an LCMS/MS analysis of
oxygenated products formed on irradiation of a p-carba-
ing (PAL), a technique used for covalently linking noncova-
lently binding molecules in a bioorthogonal manner by irra-
diation at about 360 nm.[1,2] After separation and identifica-
tion, the covalent adducts are to be analyzed by mass spec-
trometry. From a chemistꢀs perspective, the products formed
on photolabeling of biomolecules should be known as exact-
ly as possible. However, studies on the chemical reactivity
of photogenerated carbenes that fully characterize and
quantify the adducts have remained surprisingly limited,
many of them using the solvent as a reaction partner. Basic
research is still needed.
AHCTUNGTREGmNNNU oyl-PTD analogue of the anticancer drug etoposide in the
In this paper, we present a quantum-mechanical analysis
of the preferred multiplicities of the carbenes expected to
be formed from phenyl(trifluoromethyl)diazirine (2) and its
derivatives 1 and 3–5 (Figure 1) in the gas phase and differ-
ent solvents. We then address the photoreactivity of 3-(4-
methoxyphenyl)-3-(trifluoromethyl)-3H-diazirine (p-OMe-
PTD, 5) towards phenolic reaction partners.
presence of EtOAc and MeCN showing insertion into the a-
[10]
À
C H bonds.
Kanoh and co-workers published a comprehensive study
on the true variety of photoproducts formed on irradiation
of a p-carbamoyl-PTD derivative in several aliphatic alco-
hols.[11] Ether formation dominated in solution, whereas in
the solid state (À1968C) CH insertion products became
dominant, according to LCMS data. Workentin and co-
workers reacted a m-alkoxy-PTD in the presence of a 10–
15-fold excess of alcohols, carboxylic acids, and amines in
À
C6D6 and observed exclusive X H “insertions” with com-
plete conversion after several hours, but no reaction of any
of the C D bonds of the solvent.[12] Cyclopropanation occur-
À
red with methylacrylate and stryrene. In contrast to most
other studies, Workentin characterized all products also by
NMR spectroscopy. By MS/MS analysis, Lindhorst and co-
workers showed that tyrosine was the preferred amino acid
reacting when irradiating the octapeptide angiotensin II
with a mannose-based p-oxymethyl-PTD derivative.[13]
Important for the reactivity of diazirine-derived carbenes
would be the multiplicity of their ground state. Control of
chemoselectivity of triplet carbenes appeared to be more
difficult than of singlet carbenes. Therefore, we conducted a
quantum-mechanical analysis of a series of carbene candi-
dates, aiming at identifying a suitable PTD derivative with a
singlet ground state. We included Brunnerꢀs original carbene
2a and the p-carbamoyl, ethyl, and m-methoxy carbenes 1a,
3a, and 4a, analogues of which had served in chemical stud-
ies.[10,11,12,13] We also analyzed the p-methoxyphenyl carbene
5a, anticipating that the donating effect of the p-methoxy
group might stabilize a singlet ground state.
Figure 1. Phenyl(trifluoromethyl)diazirine (2, PTD) and derivatives.
Phenyl(trifluoromethyl)diazirines, discovered by Brunner
and co-workers,[3] have become the most popular groups
that can be photoactivated for PAL. On irradiation at
365 nm, a reactive carbene is formed by loss of nitrogen, di-
rectly and via the diazo isomer.[4] The CF3 group prevents
rearrangement of the carbene and makes the isomeric long-
lived diazo compound unreactive enough.[1c] Already in
their seminal paper, Brunner and co-workers reported the
formation of the corresponding methylbenzylether (95%)
on irradiation of phenyl(trifluoromethyl)diazirine (PTD) in
MeOH as the solvent.[3] Irradiation in cyclohexane as the
[a] B. Raimer, Prof. Dr. T. Lindel
TU Braunschweig, Institute of Organic Chemistry
Hagenring 30, 38106 Braunschweig (Germany)
E-mail: th.lindel@tu-braunschweig.de
Figure 2 shows the calculated singlet-triplet gaps for five
aryl(trifluormethyl)carbenes in the gas phase and in di-
chloromethane, MeOH, and cyclohexane, calculated on the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203479.
Chem. Eur. J. 2013, 19, 6551 – 6555
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state in the gas phase and,
more clearly, in solution, and
was selected for our experi-
ments. The singlet state is sta-
bilized by only 2.3 kcalmolÀ1 in
cyclohexane, but by 5.3 kcal
molÀ1 in MeOH. Halogenated
solvents also prolong the life-
time of singlet carbenes.[18]
We synthesized p-OMe-PTD
(5) in six steps (38% overall
yield) via the diaziridine,
which was oxidized to diazirine
5 employing NEt3/I2 (see the
Supporting
Information).[19]
Compound 5 is less volatile
than PTD and can be isolated
and handled without problems.
On standing at À208C for 150
days, no formation of the cor-
responding diazo compound
was observed. However, azine
6 was formed in a yield of 4%.
For the case of methylphenyl-
diazirine, Liu and Ramakrishn-
an favored dimerization of the
diazo isomer as the most prob-
able pathway of azine forma-
tion.[20] In our case, the diazo
isomer does not seem to play a
role, because it is stable even
at 408C (7, Scheme 1).
Figure 2. A) Optimized structures and B) ST-gaps of the phenylcarbenes 1a–5a in the gas phase, DCM, metha-
nol, and cyclohexane (B3LYP 6-311G (2d,2p), PCM theory).
Irradiations were carried out
in a Rayonet reactor (RPR-
B3LYP/6-311G (2d, 2p) level of theory.[14] All structures
were optimized with Gaussian (03 or 09).[15] Subsequent vi-
brational analysis of different conformers gave no negative
frequencies. For each carbene the conformer with the lowest
energy was selected. It is confirmed that the parent phenyl-
200) at 350 nm (lmax, RPR3500[21]) in the inert solvent
DCM[22] at 10 mm concentration for 2 h. All reaction prod-
ucts with the exception of diazo compound 7 have been iso-
lated in substance by reversed-phase chromatography and
fully characterized. On irradiation in the absence of a reac-
tion partner, diazirine 5 afforded the new azine 6 as a major
product (39%). Formation of small amounts of the diazo
isomer (up to 10%, dF =À57.9 ppm, CDCl3) was also ob-
served. Characteristically, UV maxima differ for diazirine 5
(lmax =374 nm, log e=2.63, CHCl3) and diazo isomer 7
(lmax =270 nm, log e=4.15, DCM). The unstable diazo com-
pound 7 was identified by 19F NMR spectroscopy by com-
parison with an authentic sample of 7 synthesized from p-
methoxy trifluoroacetylbenzene via the tosylhydrazone fol-
lowed by a-elimination on treatment with NaOMe (see the
Supporting Information).[23] In the presence of oxygen, p-
methoxy trifluoroacetylbenzene (2%) was also formed.
Irradiation of diazirine 5 in the presence of equimolar
amounts of phenol led to benzylation of the phenol oxygen
(8, 45%, by 19F NMR spectroscopic integration, dF =
À77.4 ppm, CHCl3) and of the p- (9, 21%, dF =À66.8) and
o-positions (10, 22%, dF =À66.2), all of which were isolated
and fully characterized. We also identified traces of byprod-
ACHTUNGTRENNUNG(trifluoromethyl)carbene (2a) exhibits a triplet ground state
in the gas phase, in cyclohexane, dichloromethane (DCM),
and MeOH, favored by 0.9–4.4 kcalmolÀ1, respectively.[16]
The picture changes upon introduction of a p-methoxy
group at the phenyl ring. For (p-methoxyphenyl)(trifluoro-
methyl)carbene (5a), the singlet is more stable than the trip-
let state by 0.5 kcalmolÀ1 in the gas phase and by 4.6 kcal
molÀ1 in DCM. These results are in agreement with results
published in 2011 by Sheridan and co-workers who carried
out a DFT study for the m- and the p-methoxy isomer (4a
and 5a) in the gas phase, which was corroborated by matrix
experiments.[17] The p-ethyl derivative 3a changes its ground
state from triplet in the gas phase and in cyclohexane to sin-
glet in DCM and in MeOH. (p-Carbamoylphenyl)(trifluoro-
methyl)carbene (1a) and (m-methoxyphenyl)(trifluoro
yl)carbene (4a) exhibit a triplet ground state under all
chosen conditions. Thus, (p-methoxyphenyl)(trifluoro-
methyl)carbene (5a) is the only entry with a singlet ground
ACHTUNGTNERmNUNG eth-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
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Photoactivation of (p-Methoxyphenyl)(trifluoromethyl)diazirine
COMMUNICATION
and also m-substituted phenols 15 (14%), 16 (12%), and 17
(6%). Importantly, in none of the C-benzylated products
did we find any deuteration of the carbinol carbon atom. In
a second experiment with C6H5OD (Table 1), we found a
Table 1. Irradiation of diazirine 5 with equimolar amounts of C6H5OD.[a]
Scheme 1. Products formed on irradiation of pOMe-PTD (5) in DCM
(10 mm, 2 h, RT) in the absence of a second reaction partner and in the
presence of phenol and p-cresol, respectively. Percentages are based on
19F NMR spectroscopic peak integrals; 19F NMR spectroscopic chemical
shifts (CHCl3) are given.
X
20
9.9
8.1
18.0
21
6.8
6.1
12.9
22
6
19
23
24
ucts formed without participation of phenol, such as azine 6,
trifluoroacetylbenzene, and phenyl(trifluoromethyl)carbinol
(Scheme 1). Irradiation of 5 in the presence of p-cresol led
to benzylation of the phenolic oxygen (11, 59%, by
H
D
S [%]
10.2
6.7
16.9
2.1
–
2.1
19.1
20.5
39.6
3.7
2.7
6.4
4.3
–
4.3
[a] Relative yields and the ratios between H and D are given for each
product.
19F NMR
spectroscopic
integration,
dF =À77.4 ppm,
Scheme 1) and of the o-position 12 (20%, dF =À66.1 ppm).
Importantly, the methyl group of p-cresol did not react. In
a control experiment, compound 11 was irradiated for
30 min in the presence of p-cresol, which did not lead to the
formation of any new product, such as compound 12. We
conclude that compounds 8–10 and 11, 12 are formed direct-
ly from the corresponding carbenium ion.
We then analyzed the behavior of C6D5OH (13,
Scheme 2) and were able to quantify and isolate all possible
monobenzylation products. Again, the O-benzylated product
14 was dominant (37% relative yield), followed by o-, p-,
mixture of deuterated and nondeuterated products with
always approximately a 1:1 H/D ratio at the carbinol C
atom regardless of which position of the phenol had reacted.
The diazo compound formed from PTD has been shown
to be inert even towards 0.1m AcOH in cyclohexane.[3] The
corresponding diazo compound 7 formed from p-OMe-PTD
(5) should also be stable against 0.01m phenol. The excited
state of the diazo compound should rapidly decay to the car-
bene, which is protonated, to the carbenium ion, becoming
responsible for the formation of the Friedel–Crafts products.
At the moment, it is difficult to say whether or not in the
case of p-OMe-PTD (5) the diazo compound is generally
the intermediate of the carbene formation. For the related
phenyldiazirine and phenyldiazomethane pair, Platz and co-
workers studied the S0 to S3 excited states at the TD-
B3LYP/6-31+G(d) and RI-CC2/TZVP levels of theory.
They found that, in contrast to the S0 state, conversion of
the S1 excited state of phenyldiazirine to the S1 excited state
of phenyldiazomethane proceeds by means of a low activa-
tion energy barrier (2.5 kcalmolÀ1). Excited phenyldiazo-
ACHUTNGERNmNUG ethHCATUNGTRENNaUGN ne can decay to the carbene with an activation energy
of 0.3–2.7 kcalmolÀ1.[24]
Chemistry, kinetics, and spectral evidence of carbene pro-
tonation have been discussed by Kirmse in an excellent arti-
cle.[25] Our isotope labeling experiments with C6D5OH and
C6H5OD show that “insertion” of the carbene generated by
Scheme 2. Irradiation of diazirine 5 with equimolar amounts of C6D5OH.
Relative yields are given; azine 6 (24%) and phenyl(trifluoromethyl)car-
binol (3%) were also formed.
Chem. Eur. J. 2013, 19, 6551 – 6555
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6553

T. Lindel and B. Raimer
irradiation of p-OMe-PTD (5) into the aromatic ring is only
formal and in reality takes place by a protonation mecha-
nism. This parallels the behavior of diarylcarbenes.[26] Prod-
ucts to be expected for radical intermediates were not
formed. In the case of C6D5OH, protonation of the carbene
by the phenolic proton will be the fastest reaction and out-
compete deuteration of the carbene by the dienone inter-
mediate formed by alkylation of the benzene ring. In the
case of C6H5OD, deuteration of the carbene by PhOD and
protonation of the carbene by the dienone intermediate
probably become competitive, leading to a mixture of proto-
nated and deuterated benzylphenylethers. For a scheme of
the proposed mechanism, see the Supporting Information.
We conclude that pOMe-PTD (5) is indeed a source of a
singlet carbene in DCM, as predicted by the B3LYP/6-311G
(2d, 2p) quantum-mechanical calculations.
reaction of 5 with the protected dipeptide 28, products 29
(O-benzylation, 39%) and 30 (C-benzylation, 19%) were
obtained as mixtures of diastereomers. To our knowledge,
products 29 and 30 are the first diazirine-photolabeled pepti-
des, the structures of which have been established by NMR
spectroscopy.
In summary, irradiating pOMe-PTD (5) in the presence of
equimolar, 10 mm, solutions of phenol and tyrosine deriva-
tives leads to Friedel–Crafts alkylations. This finding sug-
gests a strategy for the development of “cleaner” diazirines
for chemical biology, because the problem could be reduced
to the question of designing singlet carbenes being protonat-
ed to chemoselective carbenium ions.
Acknowledgements
After having established pOMe-PTD (5) as a carbenium
ion equivalent in protic solvents, we investigated the irradia-
tion of 5 with amino acid derivatives 25 and 28 (Scheme 3).
Being aware of solubility problems observed for the free
amino acids by Lindhorst et al.[27] we chose N,N-dimethylat-
ed tyrosine methyl ester 25 and dipeptide methyl ester 28 as
reaction partners in equimolar amounts in DCM solution
(10 mm). Irradiation of 5 with trimethylated tyrosine 25 led
to two major products, O-benzylated 26 (54%, by 19F NMR
spectroscopic integration) and C-benzylated 27 (20%). On
Financial support of this research by the Deutsche Forschungsgemein-
schaft (DFG, Li 597/6-1) is gratefully acknowledged. We also thank
Merck KGaA (Darmstadt, Germany) for the generous gift of chromatog-
raphy materials. BASF Group (Ludwigshafen, Germany) and Honeywell
Specialty Chemicals Seelze GmbH (Seelze, Germany) are thanked for
the donation of solvents. Dr. J. Grunenberg (Institute of Organic Chemis-
try, TU Braunschweig) is thanked for helpful discussions on computation-
al chemistry. We also thank a referee for helpful discussions.
Keywords: carbenes
· density functional calculation ·
diazirines · photoaffinity labeling · photochemistry
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Photoactivation of (p-Methoxyphenyl)(trifluoromethyl)diazirine
COMMUNICATION
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Received: September 28, 2012
Revised: February 25, 2013
19690.
Published online: March 28, 2013
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