The photochemistry of various para-substituted tetrafluorophenyl azides in acidic media and the formation of nitrenium ions

Article abstract of DOI:10.1021/jp961100n

Laser flash photolysis of polyfluorinated aryl azides in acetonitrile at ambient temperature produces singlet nitrenes which have lifetimes of tens to hundreds of nanoseconds. The lifetimes are controlled by ring expansion and intersystem crossing to the

Full text of DOI:10.1021/jp961100n

14028
J. Phys. Chem. 1996, 100, 14028-14036
The Photochemistry of Various Para-Substituted Tetrafluorophenyl Azides in Acidic Media
and the Formation of Nitrenium Ions
Jacek Michalak,† Hong Bin Zhai, and Matthew S. Platz*
Department of Chemistry, The Ohio State UniVersity, 120 W. 18th AVenue, Columbus, Ohio 43210
ReceiVed: April 12, 1996X
Laser flash photolysis of polyfluorinated aryl azides in acetonitrile at ambient temperature produces singlet
nitrenes which have lifetimes of tens to hundreds of nanoseconds. The lifetimes are controlled by ring
expansion and intersystem crossing to the lower energy triplet state. In the presence of sulfuric acid the
singlet nitrene can be protonated to form a new species whose transient absorption is attributed to a nitrenium
ion. This assignment is based in part on analogy to other nitrenium ion spectra and studies reported recently
by Falvey (J. Am. Chem. Soc. 1993, 115, 7254) and McClelland and Novak (J. Am. Chem. Soc. 1994, 116,
4513) and to the protonation of p-biphenylnitrene reported by Davidse et al. (J. Am. Chem. Soc. 1994, 116,
4513). The polyfluorinated nitrenium ions likely have singlet ground states, consistent with recent calculations
of the parent structure.
I. Introduction
SCHEME 1
The photochemistry of aryl azides has attracted interest over
the last 30 years, and a number of comprehensive reviews of
this subject have been published.1 Understanding the mecha-
nisms of photochemical transformations of aryl azides has
progressed rapidly in the last 10 years. The photochemistry of
azides is of concern to a wide variety of chemists with specific
interests in lithography, biological macromolecules, and reactive
intermediates.1
Studies of phenyl azide have demonstrated that photolysis
produces a singlet arylnitrene which cannot be intercepted by
external olefinic trapping reagents1 as per arylcarbenes. At
ambient temperatures singlet phenylnitrene ring expands rapidly
(10-100 ps) to form a ketenimine (dehydroazepine), which is
the only detectable intermediate produced on photolysis of
phenyl azide. The ketenimine species can be trapped by
nucleophiles.2
More recently, photochemical transformations of polyfluori-
nated phenyl azides have received attention. It has been
demonstrated that diortho fluorination of phenyl azide has a
dramatic effect on the photochemistry of these species.3-10 The
major difference between the photochemistry of fluorinated and
nonfluorinated phenyl azides is that ring expansion of singlet
(pentafluorophenyl)nitrene is much slower (Ea ) 7-8 kcal/
mol)4b-d than that of singlet phenylnitrene (Ea ) 2-3 kcal/
mol).4a For this reason singlet (pentafluorophenyl)nitrene can
be readily trapped with nucleophilic reagents such as pyridine,
diethylamine, tetramethylethylene, and even unactivated alkanes
to form adducts in significant yield. Details of the mechanistic
photochemistry of pentafluorophenyl azide are summarized in
Scheme 1.
photoaffinity labeling experiments by the groups of Keana,10
Watt,11 Katzenellenbogen,12 Reed,13 Goeldner,14 Pandurangi,15
and Crouch.16
The chemistry of nitrenium ions is also of fundamental
interest. The pioneering work of Gassman and his co-workers17
indicates that nitrenium ions may be involved in a variety of
reactions in solution. Recently nitrenium ions have received
increasing attention because of their possible role in carcino-
genesis.18
Simple protonation or alkylation of a nitrene will produce a
nitrenium ion, a reaction recently demonstrated by McClelland
and co-workers using laser flash photolysis methods.19
Indeed water is a sufficiently strong acid to protonate this singlet
arylnitrene prior to its ring expansion. The singlet nitrenium
ion absorbs broadly with an absorption maxima near 460 nm.
The nitrenium ion reacts with water and/or hydroxide ion to
eventually produce a cyclohexadienone. The latter reaction was
first reported by Novak et al.23
It has been recently demonstrated that only partial fluorination
of the phenyl ring (2,6-difluoro-,5,7 2,4-difluoro-,5 2,4,6-trifluoro-,6,7
and certain para-substituted tetrafluorophenyl-6,8 azides) pro-
duces many of the same effects observed with pentafluorophenyl
azide.
It is also worth mentioning that polyfluorinated aryl azides
have been successfully applied, and with some success, in
† On leave from Institute of Applied Radiation Chemistry, Technical
University, Zwirki 36, 90-924 Lodz, Poland.
Nitrenium ions can have either spin-paired or unpaired
electrons. Thus nitrenium ions can, in principle, exist as either
X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)01100-8 CCC: $12.00 © 1996 American Chemical Society

Photochemistry of Tetrafluorophenyl Azides
J. Phys. Chem., Vol. 100, No. 33, 1996 14029
XeCl excimer laser (17 ns, 150 mJ). The concentration of azides
1-10 was kept constant with an optical density at 308 nm of
about 1.0. Solutions were degassed prior to photolysis by
purging with argon for 5 min. Because of the large photo-
chemical conversion of azide to product in each laser pulse,
the samples were changed after every laser shot.
Product Studies. Solutions studied by LFP were subjected
to exhaustive photolysis (300 nm, Rayonet Reactor) and
analyzed by capillary gas chromatography using a Perkin Elmer
Model 8500 chromatograph.
Materials. Acetonitrile, methanol, benzene, trifluoroethanol,
diethyl ether, pentane, ethanol, pyridine, and diethylamine were
dried by standard procedures prior to use. Sulfuric acid was
purchased from Mallinckrodt and used as received.
The synthesis of pentafluorophenyl azide (1),29 methyl
4-azidotetrafluorobenzoate (2),9,10b 4-azidotetrafluorobenzonitrile
(4),30 4-azidotetrafluorobenzyl alcohol (5),10b 4′-azido-2′,3′,5′,6′-
tetrafluoroacetophenone (7),10b and 4-azidotetrafluorobenzalde-
hyde (8)10b have been described elsewhere.
singlet or triplet state species. Theory predicts that the
arylnitrenium ions will be ground-state singlet species21 in
contrast to parent nitrenium ion NH2+ and alkylnitrenium ions
which are predicted to have ground triplet states.22
Despite the importance of monoarylnitrenium ions the first
direct observations of these species in solution was reported
only recently.23 Anderson and Falvey23a observed and charac-
terized tert-butyl(4-bromo-2-acetylphenyl)- and tert-butyl(4-
chloro-2-acetylphenyl)nitrenium ions in acetonitrile. They also
reported values of second-order rate constants of these nitrenium
ions with water and methanol to be in the range 106-108 M-1
4-Azido-2,3,5,6-tetrafluorobenzyl Bromide (3). To a solution
of 4-azido-2,3,5,6-tetrafluorobenzyl alcohol10b (228 mg, 1.03
mmol) in dry dichloromethane (12 mL) were added hydrobromic
acid (4 drops) and then phosphorus tribromide (1.5 mL, 16
mmol) at 10-15 °C. The mixture was refluxed for 2 days and
treated with water at 0 °C. After the solvent was evaporated,
the residue was dissolved in ether (30 mL), washed three times
with brine, dried over MgSO4, and then concentrated. The crude
product was purified by passing it through a column with silica
gel (EM Science Si 60, 15 g) with hexanes as eluent. The
fractions containing 4-azido-2,3,5,6-tetrafluorobenzyl bromide
were pooled, and the solvent was evaporated, leaving 32 mg
(11%) of the product as a white solid, mp 56-57 °C. 1H NMR
(200 MHz, CDCl3): δ 4.49 (t, 2 H, CH2, JHF ) 1.5). 13C NMR
(300 MHz, CDCl3): δ 16.65, 112.53, 121.00, 140.90, 145.10.
19F NMR (250 MHz, CDCl3): δ -152.48 (m, 2 F, 2 CF),
-143.78 (m, 2 F, 2 CF). IR (CH3Cl): 3019, 2122, 1653, 1499,
1243, 1216, and 993 cm-1. MS (relative intensity): 285, 283
(11, 12, M+), 257, 255 (17, 17, M - N2), 204 (100, M - Br),
176 (66, M - N2 - Br), 157 (14, M - N2 - Br - F). High-
resolution MS calcd for C7H2BrF4N3 282.9368, 284.9348, found
282.9369, 284.9343.
N,N-Dimethyl-4′-azido-2′,3′,5′,6′-tetrafluorobenzylamine (6).
To a stirred solution of 40% aqueous dimethylamine solution
(74 mL, 590 mmol) in acetonitrile (300 mL) was added
2′,3′,4′,5′,6′-pentafluorobenzyl bromide (16.07 g, 9.30 mL, 61.6
mmol). The reaction was complete in minutes. Aqueous
sodium hydroxide solution (5%) was added to neutralize the
hydrobromic acid produced, and the solvent was evaporated.
The residue was extracted with ether. The organic layer was
washed with saturated aqueous NaHCO3 solution and brine,
dried over Na2SO4, and then evaporated to leave 10.20 g (74%)
of N,N-dimethyl-2′,3′,4′,5′,6′-pentafluorobenzylamine as a liquid.
1H NMR (200 MHz, CDCl3): δ 2.24 (s, 6 H, 2 CH3), 3.60 (m,
2 H, CH2). 13C NMR (200 MHz, CDCl3): δ 44.53, 49.45,
110.99, 134.79, 137.99, 139.70, 143.07, 147.99. 19F NMR (250
MHz, CDCl3): δ -163.85 (m, 2 F, 2 CF), -156.64 (m, 1 F,
CF), -143.30 (m, 2 F, 2 CF).
s-1 23
.
McClelland, Novak, and co-workers23c observed and mea-
sured the lifetimes of 2-fluorenyl and 4-biphenyl-N-acetylni-
trenium ions in aqueous solution. They found that the rate
constants for these two nitrenium ions with water are 7.7 ×
104 M-1 s-1 and 5.9 × 106 M-1 s-1, respectively. Others studies
of Fishbein and McClelland24 and of Novak’s group20,25 have
led to estimated values of the lifetimes of arylnitrenium ions
by application of the “azide-clock” method. Electrogenerated
diarylnitrenium ions have been observed and characterized
previously.20 Reactions of a triplet arylnitrenium ion have
recently been reported.27
The photochemistry of polyfluorophenyl azides has been
widely investigated in neutral media.4-9 However, extensive
studies of the photochemistry of various para-substituted tet-
rafluorophenyl azides have not yet been reported. Herein we
report results of our LFP studies of several para-substituted
tetrafluorophenyl azides 1-10 in neutral solution and in acidic
media.
We will show that flash photolysis of azides 1-9 in
acetonitrile in the presence of mineral acid leads to the facile
formation of the corresponding nitrenium ions. These species
have been detected and characterized for the first time and have
surprisingly long lifetimes which are in the millisecond time
regime. The photochemistry of 10 in acid differs from that of
1-9 due to prior protonation of the diethylamino group.
To a stirred solution of N,N-dimethyl-2′,3′,4′,5′,6′-pentafluo-
robenzylamine (10.20 g, 45.3 mmol) in ether (50 mL) in an
ice-water bath was slowly introduced gaseous HCl. The solvent
was evaporated, and the solid was recrystallized in ethanol to
afford 11.85 g (100%) of N,N-dimethyl-2′,3′,4′,5′,6′-pentafluoro-
benzylamine‚HCl salt as a white solid, mp 196-197 °C. 1H
NMR (200 MHz, DMSO-d6): δ 2.76 (s, 6 H, 2 CH3), 4.43 (m,
2 H, CH2), 10.90 (s, 1 H, NH). 19F NMR (250 MHz, DMSO-
II. Experimental Section
Laser Flash Photolysis. The laser flash photolysis (LFP)
system in use at The Ohio State University and the method of
preparation of samples have been reported elsewhere.4b,28
Briefly, a solution of azide was investigated in a quartz cuvette
and photolyzed with a 308 nm laser pulse from a Lambda Physik

14030 J. Phys. Chem., Vol. 100, No. 33, 1996
Michalak et al.
d6): δ -162.59 (m, 2 F, 2 CF), -152.63 (m, 1 F, CF), -137.34
(relative intensity): 237 (6, M+), 209 (100, M - N2), 194 (98,
M - N2 - CH3), 190 (3, M - SCH3), 162 (45, M - N2 -
SCH3). High-resolution MS calcd for C7H3F4N3S, 236.9984,
found 236.9991.
(m, 2 F, 2 CF).
A mixture of 2.020 g (7.721 mmol) of N,N-dimethyl-
2′,3′,4′,5′,6′-pentafluorobenzylamine‚HCl salt and 517 mg (7.95
mmol) of NaN3 in acetone (13 mL) and water (5 mL) was
refluxed under N2 in the dark for 2 days. The mixture was
basified with aqueous NaOH solution (5%), and the solvent was
evaporated. The residue was extracted with ether. The organic
layer was washed with saturated aqueous NaHCO3 solution and
brine, dried over Na2SO4, and then concentrated. The crude
product was purified on a column (silica gel, with ether-
petroleum ether (1/30) as eluent) to give 348 mg (18%) of N,N-
dimethyl-4′-azido-2′,3′,5′,6′-tetrafluorobenzylamine as a yellow
liquid. 1H NMR (200 MHz, CDCl3): δ 2.20 (s, 6 H, 2 CH3),
3.55 (m, 2 H, CH2). 13C NMR (200 MHz, CDCl3): δ 44.36,
49.39, 111.71, 118.98, 137.77, 142.88, 143.24, 147.90. 19F
NMR (250 MHz, CDCl3): δ -153.73 (m, 2 F, 2 CF), -143.65
(m, 2 F, 2 CF). IR (neat): 2948, 2826, 2158, 1650, 1498, 1368,
1235, 1032 cm-1. MS (relative intensity): 248 (18, M+), 220
(6, M - N2), 58 (100, Me2NCH2+). High-resolution MS calcd
for C9H8F4N4 248.0685, found 248.0693.
4-(N,N-Diethylamino)-2,3,5,6-tetrafluorophenyl Azide (10).
To a solution of pentafluoronitrobenzene (8.27 g, 38.8 mmol)
and potassium carbonate (6.17 g, 44.6 mmol) in THF (40 mL)
was added a solution of diethylamine (2.98 g, 40.8 mmol) in
THF (30 mL) at -12 to -10 °C. The mixture was stirred at
-10 °C for 30 min and then warmed gradually to room
temperature. After the solvent was evaporated a minimal
amount of water was added to the residue. The mixture was
extracted with benzene and ethyl acetate (1:1), washed with
brine, and dried over MgSO4. Evaporation of the solvent gave
8.31 g (80%) of N,N-diethyl-4-nitro-2,3,5,6-tetrafluoroaniline
as a yellow solid, mp 28-29 °C. 1H NMR (200 MHz,
CDCl3): δ 1.19 (t, 3 H, CH3, J ) 7.1), 3.33-3.45 (m, 2 H,
CH2). 13C NMR (200 MHz, CDCl3): δ 13.50, 46.79, 136.97,
138.08, 142.31, 142.94. 19F NMR (250 MHz, CDCl3): δ
-148.76 (m, 2 F, 2 CF), -151.65 (m, 2 F, 2 CF). IR (CH3Cl):
2980, 1630, 1534, 1485, 1340, 1208, 1196, 1105, 1001, and
758 cm-1. MS (relative intensity): 266 (35, M+), 251 (100,
M - CH3), 223 (32, M - CH3 - C2H4), 220 (M - NO2), 205
(62, M - CH3 - NO2), 191 (10, M - NO2 - C2H5), 177 (46,
M - CH3 - C2H4 - NO2). High-resolution MS calcd for
C10H10F4N2O2 266.0679, found 266.0680.
4-Methylthio-2,3,5,6-tetrafluorophenyl Azide (9). To a flame-
dried 250 mL one-necked round-bottomed flask under argon
were introduced 2,3,5,6-tetrafluoroaniline (5.28 g, 32.0 mmol)
and dry THF (50 mL). After the mixture was cooled to -78
°C, a solution of n-butyllithium in hexanes (1.3 M, 85 mL, 110
mmol) was added dropwise through an addition funnel at -78
°C during a period of 1-2 h and the mixture was stirred at that
temperature for 3 h. Methyl disulfide (10.8 mL, 120 mmol)
was added during a period of 30 min, and the temperature was
raised slowly to ambient temperature. Water was added at 0
°C, and the solvent was evaporated. The residue was extracted
with ether, washed with brine, and dried (Na2SO4). The solvent
was removed under reduced pressure using a rotary evaporator,
leaving 6.38 g (94%) of 4-methylthio-2,3,5,6-tetrafluoroaniline
as a purple solid. Additional purification was achieved by
recrystallization from hexanes containing a few drops of
benzene, mp 81-82 °C. 1H NMR (200 MHz, CDCl3): δ 2.37
(s, 3 H, CH3), 4.09 (s, 2 H, NH2). 19F NMR (250 MHz,
CDCl3): δ -162.41 (m, 2 F, 2 CF), -138.02 (m, 2 F, 2 CF).
IR (CHCl3): 3490, 3389, 3187, 3014, 2936, 2614, 1655, 1490,
1378, 1278, 1217, 1170, 1116, 986, 924, and 863 cm-1. MS
(relative intensity): 211 (76, M+), 196 (100, M - CH3), 152
(23, M - CSCH3), 47 (1, CH3S+). High-resolution MS calcd
for C7H5F4NS, 211.0080, found 211.0063.
N,N-Diethyl-4-nitro-2,3,5,6-tetrafluoroaniline (2.27 g, 8.52
mmol), PtO2 (30 mg), and ethanol (30 mL) were combined in
a Parr bottle and hydrogenated at 50 psi for 80 min. The
mixture was filtered through celite, and the filtrate was
evaporated. The residue was stirred in a solution of concentrated
hydrochloric acid (8 mL) and water (100 mL). The mixture
was cooled to 0 °C, and a solution of sodium nitrite (1.24 g,
17.9 mmol) in water (10 mL) was added dropwise with stirring
to keep the temperature at 0-5 °C. After 10 min of further
stirring at 0-5 °C, a solution of sodium azide (1.66 g, 25.6
mmol) in water (10 mL) was added at such a rate that the
temperature of the reaction mixture did not exceed 5 °C toward
the end of the addition. The water layer was extracted with
ether. The combined organic layer was washed with saturated
sodium bicarbonate solution (two times) and brine (three times)
and dried over MgSO4. The solvent was removed under reduced
pressure using a rotary evaporator, and the residue was purified
by column chromatography (silica gel, with petroleum ether as
eluent) to give 1.92 g (86%) of 4-(N,N-diethylamino)-2,3,5,6-
tetrafluorophenyl azide as a brown oil. 1H NMR (200 MHz,
CDCl3): δ 1.04 (t, 3 H, CH3, J ) 7.1), 3.09-3.22 (m, 2 H,
CH2). 19F NMR (250 MHz, CDCl3): δ -149.73 (m, 2 F, 2
CF), -155.03 (m, 2 F, 2 CF). IR (neat): 2976, 2116, 1643,
4-Methylthio-2,3,5,6-tetrafluoroaniline (960 mg, 4.55 mmol)
was stirred in CF3COOH (20 mL). The mixture was cooled to
-5 °C, and solid sodium nitrite (656 mg, 9.51 mmol) was added
with stirring, in portions, to maintain the temperature at 0-5
°C. After 30 min of further stirring at 0-5 °C, solid sodium
azide (736 mg, 11.3 mmol) was added slowly so that the
temperature of the reaction mixture did not exceed 5 °C toward
the end of the addition. After the reaction was complete,
aqueous KOH solution was added at 0 °C to neutralize the
unreacted acid. The water layer was extracted with ether. The
combined organic layer was washed with brine and dried over
MgSO4. The solvent was removed under reduced pressure using
a rotary evaporator, and the residue was purified on a column
of neutral alumina with hexanes as eluent to give 560 mg (52%)
of 4-methylthio-2,3,5,6-tetrafluorophenyl azide as a yellow
liquid. 1H NMR (200 MHz, CDCl3): δ 2.47 (s, 3 H, CH3).
13C NMR (200 MHz, CDCl3): δ 17.58, 111.28, 119.36, 140.46,
146.83. 19F NMR (250 MHz, CDCl3): δ -152.75 (m, 2 F, 2
CF), -135.68 (m, 2 F, 2 CF). IR (neat): 2934, 2234, 2125,
2076, 1634, 1477, 1304, 1225, 1010, 973, and 854 cm-1. MS
1587, 1494, 1452, 1382, 1302, 1197, 1091, 991, and 943 cm-1
.
MS (relative intensity): 262 (18, M+), 234 (100, M - N2), 219
(19, M - N2 - CH3), 205 (19, M - N3 - CH3), 191 (30, M
- N3 - C2H5), 177 (70, M - N3 - CH3 - C2H4), 162 (35, M
- N2 - NEt2). High-resolution MS calcd for C10H10F4N4
262.0843, found 262.0846.
III. Results and Discussion
Laser flash photolysis (XeCl excimer laser, 308 nm, 17 ns,
150 mJ) of azides 1-8 at ambient temperature in acetonitrile
produces transient spectra whose main features can be confi-
dently1,4 attributed to the corresponding triplet nitrenes (three
characteristic bands at 330, 380-400, and 500-550 nm). The
transient spectrum of Figure 1 obtained upon LFP of azide 2 in
acetonitrile is representative of the azides studied. It is likely
that in acetonitrile the transient spectrum observed upon LFP

Photochemistry of Tetrafluorophenyl Azides
J. Phys. Chem., Vol. 100, No. 33, 1996 14031
Figure 3. The transient spectrum of a nitrene-pyridine ylide produced
by LFP of 1 in acetonitrile containing pyridine. The spectrum was
recorded over a 400 ns window immediately after the laser flash.
Figure 1. The overlapping transient spectrum of a triplet nitrene (350
and 550 nm) and ketenimine (380 nm) produced by LFP of 2 in
acetonitrile at ambient temperature. The spectrum was recorded over
a 400 ns window immediately after the laser flash.
Figure 2. The transient spectrum of a triplet nitrene produced by LFP
of 5 in methanol at ambient temperature. The spectrum was recorded
over a 400 ns window immediately after the laser flash. Note the
reduction in transient absorption at 380 nm relative to Figure 1.
of azides 1-8 is the superposition of the transient spectrum of
a triplet nitrene (main intermediate) and a ketenimine, absorbing
broadly, around 380 nm. In the case of azides 4, 7, and 8 the
decay of the triplet nitrene leads to the formation of a band
around 370 nm, which can be attributed to the corresponding
azobenzene. Capillary GC/MS analysis of the photoproducts
formed upon photolysis of azides 4, 7, and 8 in acetonitrile
confirmed the formation of aniline and azobenzene type products
upon LFP.
LFP of azides 1-8 in methanol cleanly gives the transient
spectra of triplet nitrenes (methanol catalyzes intersystem
crossing of the singlet to the ground triplet state of polyfluori-
nated arylnitrenes).4 The transient spectrum of the triplet nitrene
produced by LFP of azide 5 in methanol is presented in Figure
2, and this spectrum is representative of several of the other
(1-8) azides studied. Note the reduced transient absorption
around 380 nm, characteristic of a ketenimine intermediate,
relative to the transient absorption spectrum of Figure 1 where
this species is formed in low yield.
LFP of azides 1-8 in acetonitrile in the presence of pyridine
produces intense transient spectra with maxima around 400 nm,
which are attributed to pyridine ylides (e.g., Figure 3). The
formation of the pyridine ylide of (pentafluorophenyl)nitrene4
and other (tetrafluorophenylnitrenes8 has been the subject of
previous studies. The ylide (1c) can be isolated, and its crystal
structure has been obtained (Scheme 1).
Figure 4. The transient spectra produced by LFP of azide 10 in
acetonitrile. The spectra was recorded over a 400 ns window (a)
immediately after the laser flash and (b) 5 µs, (c) 10 µs, (d) 100 µs,
and (e) 800 µs after the laser flash.
The photochemistry of the thiomethyl and diethylamino
parasubstituted tetrafluorophenyl azides 9 and 10 differs from
that of azides 1-8. LFP of azides 9 and 10 in acetonitrile or
benzene immediately produces an intense transient absorption
(Figure 4a) which represents a mixture of triplet nitrene and
ketenimine. The transient absorption of the triplet nitrene decays
appreciably on the time scale of the LFP experiment. The
lifetime of the triplet nitrene is ≈25 µs. At long times after
the laser flash, the spectrum of the corresponding azobenzene
is seen. Azobenzene is formed as a result of triplet nitrene
dimerization. It has been shown by Li and Schuster et al.31

14032 J. Phys. Chem., Vol. 100, No. 33, 1996
Michalak et al.
TABLE 1: Transient Absorption Maxima of the Nitrenium
Ions Generated upon LFP of Azides 1-6 in Acetonitrile or
Methanol in the Presence of Sulfuric Acid
λmax (nm)
λmax (nm)
azide
azide
precursor CH3CN CH3OH precursor CH3CN CH3OH
1
2
3
375
375
380
365
370
365
4
5
6
365
380
370
365
375
370
transient spectra can be attributed to the corresponding nitrenium
ions by analogy to the work of McClelland and Wirz.19
A
referee has suggested that the transients may be due to nitrenium
ion-solvent complexes as shown below.
Figure 5. The transient spectrum of a nitrenium ion produced by LFP
of azide 1 in acetonitrile containing 0.31 M H2SO4. The spectrum was
recorded over a 400 ns window immediately after the laser pulse.
We cannot rule out this possibility. However, the transient
spectra observed in acetonitrile and methanol are identical,
which argues against this possibility. The transient absorptions
of the different para-substituted (tetrafluorophenylnitrenium ions
are relatively stable (lifetimes in the millisecond time regime).
The transient spectra of nitrenium ions were not produced upon
LFP of 1-8 in THF or CH2Cl2 in the presence of sulfuric acid.
The transient spectra of the para-substituted (tetrafluorophen-
yl)nitrenium ions are shifted to shorter wavelengths than the
maxima of nonfluorinated arylnitrenium ions. Arylnitrenium
ions observed by Anderson and Falvey,23 McClelland and Novak
et al.,19,24 and Wirz19 have absorption maxima closer to 470
nm. The maxima of the (tetrafluorophenyl)nitrenium ion
transient absorption are listed in Table 1.
Figure 6. The transient spectrum of a nitrenium ion produced by LFP
of azide 2 in methanol containing 0.47 M H2SO4. The spectrum was
recorded over a 400 ns window immediately after the laser pulse.
that the yields of the appropriate substituted azobenzenes are
greater for electron-donating para-substituted phenyl azides than
they are for phenyl azide itself. Thus our observations are in
good agreement with the results of preparative photolysis of
nonfluorinated azides. There is no evidence for the formation
of nitrene-nitrile or nitrene-alcohol ylides.
The blue shift observed upon ring fluorination of aryl
nitrenium ions can be explained by simple resonance pictures.
High-level calculations indicate that arylnitrenium ions are best
thought of as iminyl-substituted cyclohexandienyl cations.21
Laser Flash Photolysis Studies in the Presence of Sulfuric
Acid. LFP of azides 1-6 in acetonitrile or methanol in the
presence of sulfuric acid produces strong new transient spectra
with maxima around 370 nm (Figures 5 and 6).
With azides 7 and 8, there was no clear evidence of a new
transient produced upon LFP in the presence of sulfuric acid.
Any new species formed in the presence of acid is only formed
in low yield.
It is important to note that with these compounds (1-6), the
UV spectrum of the azide solution showed no change upon the
addition of acid. At the acid concentration used in this work
there was no evidence of a dark reaction with azides 1-8.
Additional experiments were performed to study the effect
of nucleophiles on the nitrenium ion lifetime. However, our
attempts to probe the reactivity of nitrenium ions failed. The
addition of halide ions (tetraalkylammonium halides were used)
to the azide-sulfuric acid mixture initiates a “dark” reaction
which consumes the azide. Other nucleophiles such as NaCN
or NaN3 were not sufficiently soluble in acetonitrile or methanol
to perform the required experiments.
It is interesting to note that flash photolysis of nonfluorinated
monoaryl azides in acetonitrile or methanol in the presence of
sulfuric acid, in our hands, fails to produce long lived nitrenium
ions under the conditions of this work. This observation
demonstrates that para-substituted (tetrafluorophenyl)nitrenium
ion formation is a consequence of the fluorine effect. The
retardation of ring expansion extends the lifetime of these singlet
nitrenes sufficiently to allow protonation. We believe that these
Although theory has yet to assign the observed electronic
absorption band, we think it may represent a transition from an
iminylcyclohexadiene to a nitrenium ion structure. The fluorine
substituent is likely to stablize the ground state by π back-
bonding and destablize the excited state by the σ inductive effect
of fluorine which will place an electron-deficient aryl ring next
to a positively charged nitrogen atom. This leads to the
observed blue shift.
In the presence of dilute (0.005 M) sulfuric acid, in aceto-
nitrile, where ISC is much less rapid (kISC(CH3CN) ) 107 s-1
,
kISC(CH3OH) ) 109 s-1 4,7,8
than in methanol, only nitrenium
)
ion intermediates were observed upon LFP of azides 1-3, 5,
and 6. In these cases the formation of the nitrenium ions was
faster than the time resolution of the apparatus (Figure 7, 20
ns).

Photochemistry of Tetrafluorophenyl Azides
J. Phys. Chem., Vol. 100, No. 33, 1996 14033
SCHEME 2a
Figure 7. The rise of the transient absorption of a nitrenium ion
produced by LFP of azide 1 in acetonitrile at ambient temperature in
the presence of 0.2 M H2SO4.
a R ) F (1), CO2CH3 (2), CH2Br (3), CN (4), CH2OH (5), CH2NMe2
(6), COCH3 (7), CHO (8), SCH3 (9).
Figure 8. Biphasic growth of transient absorption following LFP of 4
in acetonitrile in the absence (a) and presence (b) of H2SO4 (0.165 M).
Upon LFP of azides 4, 7, and 8 in acetonitrile the slow
formation of the corresponding azobenzene was also observed
(Figure 8), in addition to the prompt formation of nitrenium
ions. In the case of azides 4, 7, and 8 (Figure 8) there is an
increase in absorption immediately after the laser pulse which
corresponds to the formation of the nitrenium ion. With these
azides, and under these conditions, the initially formed singlet
nitrene partitions between reaction with a proton donor and
relaxation to the lower energy triplet nitrene. The triplet nitrene
will dimerize slowly to form an azobenzene whose absorption
spectrum overlaps that of the nitrenium ion. Alternatively, one
may argue that the triplet nitrene may re-form the singlet nitrene
(Scheme 2) which is then protonated. The latter explanation
seems unlikely because experiment and theory have demon-
strated that the singlet-triplet gap in parent phenylnitrene is
quite large (≈17 kcal/mol).32 We propose that the electron-
withdrawing para substituents of 4, 7, and 8 (CN, COCH3, and
CHO) likely reduce the absolute rate constant of protonation
of the nitrene (kH, Scheme 2) which allows some triplet nitrene
formation in the presence of moderate concentrations of sulfuric
acid.
Figure 9. The optical yield of nitrenium ion (A375) produced by LFP
of 1 in acetonitrile as a function of [H2SO4].
through product analysis were unsuccessful. Azides 1-8 were
exhaustively photolyzed in acidic acetonitrile or methanol
solutions and analyzed by gas chromatography. Only traces of
aniline and azo compounds were detected. The major products
were nonvolatile and probably polymeric.
Deduction of Singlet Nitrene Lifetimes. As mentioned
previously, it was not possible to resolve the growth of nitrenium
ions generated from azides 1-8 by nanosecond spectroscopy
even at very low concentrations of sulfuric acid. However, it
was possible to measure the optical yield of a nitrenium ion
(A375) produced in a laser flash (Figures 7 and 8) as a function
of sulfuric acid concentration. The value of A375 is not equal
to zero in the absence of sulfuric acid because the corresponding
triplet nitrene also absorbs at this wavelength. However, this
absorption is relatively small compared to that of the nitrenium
ion and will be neglected. Of course the value of the absorption
of the nitrenium ion, A375, increases as the concentration of
sulfuric acid increases (Figure 9). Above 0.1M H2SO4 the yield
of nitrenium ion is saturated (A∞375). Under these conditions
every singlet nitrene produced in a laser pulse is protonated to
form a nitrenium ion.
The nitrenium ion lifetimes were not shortened by the
presence of oxygen. This is further evidence that the nitrenium
ions of this work have singlet ground states.
p-Biphenylnitrenium ion reacts with water to produce a
characteristic cyclohexadienone product.19,20 Our attempts to
support the attribution of the transient spectra to nitrenium ions

14034 J. Phys. Chem., Vol. 100, No. 33, 1996
Michalak et al.
carbonyl and cyano substituents, and the π back-bonding
properties of fluorine substituents (Vide infra).
In methanol, at low concentration of sulfuric acid, the transient
spectrum produced by LFP of 1-8 represents a mixture of triplet
nitrene (due to catalyzed intersystem crossing and, consequently,
short singlet nitrene lifetime) and nitrenium ion. Thus it was
more difficult to precisely obtain values of kHτ in methanol than
in acetonitrile.
The higher value of kHτ obtained in alcohol relative to that
determined in acetonitrile is in good agreement with our earlier
observation4 that methanol shortens singlet nitrene lifetimes by
catalyzing the intersystem crossing rate of singlet (pentafluo-
rophenyl)nitrene. The fact that a higher concentration of acid
is necessary to produce nitrenium ions in methanol relative to
acetonitrile indicates that the nitrenium ions are formed by
protonation of singlet rather than triplet para-substituted (tet-
rafluorophenyl)nitrenes. It is also of note that the nitrenium
ion spectra we observe are quite different from the spectra of
the corresponding triplet nitrene which gives additional support
for the conclusion that the nitrenium ion produced by protonation
is a singlet state species.
The lifetime of para-substituted (tetrafluorophenyl)nitrenium
ion in acetonitrile or in methanol is also unusually long (on the
millisecond time scale or longer) in contrast to the lifetimes of
other arylnitrenium ions observed by Anderson and Falvey18
and by McClelland’s and Novak’s groups.19 This is another
consequence of the fluorine effect. We believe that the (σ2)
orbital of the singlet nitrene is protonated to form a nitrenium
ion which is a π cation.
Figure 10. A double-reciprocal treatment of the data of Figure 9.
TABLE 2: Values of kHτ for the Singlet Nitrenes Derived
from Azides 1, 2, 4, and 6 in Acetonitrile and in Methanol
compound CH3CN kHτa CH3OH kHτ CH3CN kDτ CH3CN kD/kH
1
192
167
125
93
5
1
3
204
213
1.06
1.28
2a
4
6
169
1.36
a The experimental value of τ is 200 ns. This indicates that kH for
this nitrene is probably 10 times lower (109 vs 1010 M-1 s-1) for nitrene
2a than for nitrene 1a. The electron-withdrawing para substituents
+
present in azides 2 and 4 also likely reduce kH in these singlet nitrenes
relative to that of singlet (pentafluorophenyl)nitrene.
A double-reciprocal plot of 1/A375 versus 1/[H2SO4] is
predicted and found to be linear (Figure 10) with a ratio of
intercept/slope equal to kHτ. This follows from eq 1,
kobs
A∞375Φ
1
A375
1
1
)
+
(1)
A∞375Φ
k [H SO ]
H
2
4
where A375 is the optical yield of nitrenium ion, A∞ is the
375
optical yield of nitrenium ion formation in the saturation region,
τ is the lifetime of the singlet nitrene in the absence of sulfuric
acid, and φ is the quantum yield of nitrene formation. In
acetonitrile and methanol solutions, the value of kSH can be
neglected (small contribution), and the singlet nitrene lifetime
is equal to 1/(kEXP + kISC) (Table 2).
If one assumes that kH is diffusion controlled, (1 × 1010 M-1
s-1), for singlet (pentafluorophenyl)nitrene, one can deduce
singlet nitrene lifetimes of 20 ns in acetonitrile and 0.5 ns in
methanol. These values are in excellent agreement with lifetime
values of this nitrene deduced from pyridine ylide trapping
experiments in methylene chloride.1,4 However, this type of
analysis incorrectly predicts the singlet nitrene lifetime of 2a
to be 17 ns, whereas the lifetime of this nitrene is known to be
200 ns in methylene chloride.8
This indicates that kH must be smaller for singlet nitrene 2a
(≈ 109 M-1 s-1) than for singlet nitrene 1a where a value of kH
) 1010 M-1 s-1 is indicated. For azides 4, 7, and 8 where there
is incomplete protonation we expect that kH e 109 M-1 s-1 for
singlet nitrenes 4a, 7a, and 8a (Schemes 1 and 2). This is
consistent with the biphasic growth of transient absorption
discussed previously, the electron-withdrawing properties of the
We postulate that π back-bonding from the fluorine substit-
uents greatly stabilizes the empty p-π orbital of the nitrenium
ion which results in a long lifetime for this species. The
situation is completely analogous to that of difluorocarbene,
another electron-deficient intermediate which is stabilized by
fluorine π back-bonding. Thus, both CF2 and para-substituted
(tetrafluorophenyl)nitrenium ion have exceptionally long life-
times and singlet ground states.33
InVerse Isotope Effects. The nitrenium ion formation experi-
ments were repeated with perdeuteriosulfuric acid in acetonitrile.
In the case of azides 1, 2, and 4, small inverse isotope effects
were observed (Table 2). Deuterated acids are generally
stronger acids than their protonated counterparts. This will often

Photochemistry of Tetrafluorophenyl Azides
J. Phys. Chem., Vol. 100, No. 33, 1996 14035
presence of sulfuric acid a new transient absorption is observed
which is presented in Figure 11. As we can see from Figure
11 in the presence of sulfuric acid (with a concentration of acid
up to 0.15 M) a new absorption band with a maximum at 380
nm was observed. This absorption represents the corresponding
nitrenium ion 9e formed from azide 9a upon LFP.
At greater acid concentrations (Figure 11e) a new transient
absorption with a maximum at 315 nm was observed. This
absorption is assigned to aniline derivatives formed from azide
9 upon photolysis. We postulate that the variable photochem-
istry of 9 as a function of sulfuric acid concentration is due to
protonation of the sulfur atom in the p-thiomethyl group of azide
9 in the presence of the higher concentrations of acid.
A
decrease in absorption at 308 nm of the azide was observed
after the addition of relatively large concentrations of sulfuric
acid to a solution of azide 9 in acetonitrile. LFP of azide 9 in
the absence of acid, in contrast to azides to 4, 7, and 8, gives
substantial yields of triplet nitrene and azobenzene whose
absorption overlaps with that of the nitrenium ion. For these
reasons it was not possible to obtain accurate values of kHτ.
LFP of azide 10 in dilute acid fails to produce a nitrenium
ion. This azide is rapidly protonated at the para position, and
its photochemistry is very complex.
IV. Conclusions
Laser flash photolysis of polyfluorinated aryl azides releases
singlet arylnitrenes which can be protonated by sulfuric acid to
form nitrenium ions. The polyfluorinated arylnitrenium ions
are extremely long lived (milliseconds) in acetonitrile and
methanol. These intermediates have singlet ground states.
Acknowledgment. This work was supported by The Na-
tional Institutes of Health (Grant GM34823). The authors are
indebted to Professors Wirz, McClelland, Novak, and Falvey
for sharing the results of their work prior to publication and for
valuable discussions.
Figure 11. The transient spectrum obtained by LFP of azide 9 in
acetonitrile in (a) the absence of H2SO4 and (b) in the presence of 0.003
M, (c) 0.05 M, (d) 0.03 M, (e) 0.14 M, and (f) 0.24 M H2SO4.
References and Notes
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A weak transient spectrum of a nitrenium ion (2e) was
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phosphoric acid. This nitrenium ion could not be detected in
acetonitrile containing hydrochloric acid.
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