Formation of nitrile ylides in the addition of singlet carbene to nitrile compounds - Laser flash photolysis of (biphenyl-4-yl)-chlorodiazirine in the presence of nitrile compounds

Article abstract of DOI:10.1039/a804675a

Formation of a nitrile ylide (NY) by addition of a singlet carbene to a nitrile compound was studied by laser flash photolysis (LFP) of (biphenyl-4-yl)chlorodiazirine (BCD) in the presence of nitrile compounds. Although the generated (biphenyl-4-yl)chlorocarbene (BCC) forms a NY by addition to a nitrile compound in an equilibrium reaction, the absorption spectrum of the NY can be measured only in the LFP of BCD in the presence of 2,4,6-trimethoxybenzonitrile. The equilibrium constants for the NY formation in the addition of BCC to propiononitrile or pivalonitrile were determined to be 0.45 and 0.37 M^-1 at 295 K, respectively. The reactivity of NY towards olefins seems to be substantially lower than those of BCC. The reaction of BCC with 2,3-dimethylbut-2-ene was carried out in 2,2,4-trimethylpentane and propionitrile in the temperature range of ca. 170 to 300 K. In both solvents, the rate constants increased with decreasing temperature and the maximum value appeared at around 210 K. Using the difference in the rate constants, the free energy difference for the equilibrium constant was also estimated to be ca. 4.2 kJ mol^-1.

Full text of DOI:10.1039/a804675a

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Formation of nitrile ylides in the addition of singlet carbene to
nitrile compounds—laser flash photolysis of (biphenyl-4-yl)-
chlorodiazirine in the presence of nitrile compounds
Ikuo Naito,*a Akira Oku,b Yoshihisa Fijiwarac and Yoshifumi Tanimotoc
a Department of Photography, Kyushu Sangyo University, Higashi-ku, Fukuoka 813-8503,
Japan
b Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku,
Kyoto 606-0962, Japan
c Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama,
Higashihiroshima 723-0046, Japan
Received (in Cambridge) 19th June 1998, Accepted 16th February 1999
Formation of a nitrile ylide (NY) by addition of a singlet carbene to a nitrile compound was studied by laser flash
photolysis (LFP) of (biphenyl-4-yl)chlorodiazirine (BCD) in the presence of nitrile compounds. Although the
generated (biphenyl-4-yl)chlorocarbene (BCC) forms a NY by addition to a nitrile compound in an equilibrium
reaction, the absorption spectrum of the NY can be measured only in the LFP of BCD in the presence of 2,4,6-
trimethoxybenzonitrile. The equilibrium constants for the NY formation in the addition of BCC to propiononitrile
or pivalonitrile were determined to be 0.45 and 0.37 MϪ1 at 295 K, respectively. The reactivity of NY towards olefins
seems to be substantially lower than those of BCC. The reaction of BCC with 2,3-dimethylbut-2-ene was carried
out in 2,2,4-trimethylpentane and propionitrile in the temperature range of ca. 170 to 300 K. In both solvents, the
rate constants increased with decreasing temperature and the maximum value appeared at around 210 K. Using the
difference in the rate constants, the free energy difference for the equilibrium constant was also estimated to be ca. 4.2
kJ molϪ1.
R1
R1
R2
Introduction
C:
+
R3 - CN
C– - N+ C - R3
(1)
Reactions of methylene with electron-deficient olefins, such as
fumaronitrile, acrylonitrile, dimethyl acetylenedicarboxylate,
were reported in detail to yield pyrrole derivatives in aceto-
nitrile (MeCN).1 It is concluded that these reactions proceed via
a nitrile ylide (NY) intermediate formed by the addition of
methylene to the nitrile solvent, as shown in eqn. (1). The reac-
tions of 1-naphthylcarbene (NC),2 2-naphthylcarbene,3 fluoren-
9-ylidene (FC)4 and cyclopentadienylidene5 in MeCN were also
reported to proceed by way of a NY because the rate constants
determined in the reaction of carbene with electron-deficient
olefins in MeCN agreed with those of the reactions of NY
formed from 2H-azirine and these olefins. Moreover, NC was
reported to form the corresponding NY by addition to benzo-
nitrile (PhCN) or 2,2-dimethylpropiononitrile (t-BuCN).2b
These NYs formed from NC were also reported to react with
electron-rich olefins such as 2,3-dimethylbut-2-ene (TME).2b
The NY formed from 3-(biphenyl-4-yl)-2H-azirine (NYa),
however, is a relatively stable species (τ = 12 ms in cyclohexane)
and does not react with TME [eqn. (2)].6 Moreover, FC in
MeCN was reported to yield 2H-azirine (in the absence of
a scavenger), pyrrole (in the presence of dimethylmaleic
anhydride), and cyclopropane (in the presence of 2-methylbut-
2-ene).4a The former two products were obtained by way of the
NY formed by the addition of FC to MeCN. The last product
was also obtained by the direct addition of FC to the olefin
even in MeCN. Thus, it seems that there is an equilibrium reac-
tion between FC and MeCN, as shown in eqn. (3). These
carbenes are triplets in their ground state. On the other hand,
phenylchlorocarbene (PCC), a typical singlet carbene, was
reported not to form a NY,7 but a small solvent effect was
reported in the reaction of PCC with TME.8 NY was detected
spectrometrically only in the reaction of (4-nitrophenyl)chloro-
carbene with MeCN.9
R2
(carbene) (nitrile compound)
(nitrile ylide, NY)
CH3
CH3
CH3
C–=N+=CH2
+
C=C
CH3
(NYa)
(TME)
N
C
CH2
(2)
CH3 - C C - CH3
CH3
CH3
(pyrrole)
–
N+ C - CH3 (3)
:
+
CH3 - CN
(MeCN)
(FC)
(NY)
The photoreaction of (biphenyl-4-yl)chlorodiazirine (BCD)
yielded a long-lived singlet carbene, (biphenyl-4-yl)chloro-
carbene [BCC, λmax = 360 nm, τ = 24.8 µs in 2,2,4-trimethyl-
pentane (i-Oc)] as shown in eqn. (4).10 Recently, we measured
the IR spectrum assigned to the heterocumulene structure
J. Chem. Soc., Perkin Trans. 2, 1999, 1051–1056
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+
C = N = C–
CH3
Cl
Fig. 1 Heterocumulene structure of NY formed by the addition of
BCC to MeCN.
Fig. 3 Typical digitizer traces monitored in the LFP of BCD in the
presence of TMB. λmoni. = 390 (1) and 440 nm (2). [BCD] = ca. 6 × 10Ϫ4
mol dmϪ3, [TMB] = 6.7 × 10Ϫ1 mol dmϪ3 in MeCN.
Fig. 4 Time-resolved transient absorption spectrum measured in the
LFP of BCD in the presence of TMB. Monitored at 0.5 (1), 3.4 (2) and
37.1 µs (3), [BCD] = ca. 6 × 10Ϫ4 mol dmϪ3, [TMB] = 6.7 × 10Ϫ1 mol
dmϪ3 in MeCN.
Fig. 2 Transient absorption spectra of BCC measured in MeCN (1)
and i-Oc (2) ca. 80 ns after the flash. [BCD] = ca. 6 × 10Ϫ4 mol dmϪ3
.
observed only in the presence of TMB in the wavelength range
from 420 to 490 nm, which was formed in accordance with the
decay of the absorption at λ = 370 nm. The spectrum seems to
exhibit a maximum at around 440 nm. Figs. 3 and 4 show
typical digitizer traces monitored at 390 nm (decay of BCC)
and 440 nm (formation of the new band), and time-resolved
absorption spectra ([TMB] = 0.67 M in MeCN), respectively.
The formation rate of the absorption at 440 nm (3.5 × 105 sϪ1)
agrees with the decay rate of BCC (2.6 × 105 sϪ1). Thus, the new
band is assigned to the NY formed by the addition of BCC to
TMB, as shown in eqn. (5).
N
hν
C:
+ N2
C
(4)
N
Cl
Cl
(BCD)
(BCC)
OCH3
(>C᎐Nϩ᎐CϪ–) formed by the addition of BCC to MeCN in an
᎐
᎐
argon matrix at 10 K, as shown in Fig. 1.11 In this regard, we
have studied the formation of NYs from BCC and various
types of nitrile compounds by means of laser flash photolysis
(LFP) of BCD.
Biph
CN
C:
+
CH3O
Cl
OCH3
(BCC)
(TMB)
Results and discussion
Transient absorption spectrum
Laser flash photolysis (LFP) of (biphenyl-4-yl)chlorodiazirine
(BCD) was carried out in acetonitrile (MeCN) at 295 K to
detect the nitrile ylide (NY) directly. A transient spectrum
measured immediately after the flash exhibits only one absorp-
tion maximum at λ = 370 nm, as shown in Fig. 2. Fig. 2
also shows the absorption spectrum of (biphenyl-4-yl)chloro-
carbene (BCC) in 2,2,4-trimethylpentane (i-Oc). The shape of
the spectrum did not change during ca. 10 µs after the flash (the
delayed measuring time). Decay profiles of the absorption at
CH3O
C
Biph
C
–
N+
(5)
OCH3
Cl
CH3O
(BCC-TMB NY)
370 nm obeyed first-order kinetics. The decay rate (τϪ1
=
Effects of addition of nitrile compounds on rate constants
7.94 × 104 sϪ1) was independent of the measured wavelength. It
is unclear whether these transient spectra belong to BCC or its
NY yielded by the addition of BCC to MeCN.
In the reaction of carbenes with ethers, the formation of oxon-
ium ylides (OY) has been studied by measuring the rate con-
stants of the typical carbene reactions, because the absorption
spectra of OY cannot be measured.10 Because the absorption
spectrum of NY seems to overlap with that of BCC, the
formation and reactivity of NY were studied by analyzing the
LFP of BCD was also carried out in the presence of a variety
of nitrile compounds [benzonitrile (solvent), 4-methoxybenzo-
nitrile, 2,4,6-trimethoxybenzonitrile (TMB), 4-cyanobiphenyl,
and 1-naphthonitrile in MeCN]. A weak absorption was
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Table 1 Rate constants (dm3 molϪ1 sϪ1) of the reactions of BCC with TME, MF and pyridinea
TME
MF
pyridine
Solvent
k/107
kЈ/107 b
Ratioc
k
kЈ/106 b
Ratioc
k/108
kЈ/108 b
Ratioc
i-Oc
MeCN
EtCN
i-PrCN
t-BuCN
ClMeCN
18.3
6.75
5.55
6.15
6.30
2.47
—
1.00
0.26
0.27
0.34
0.46
0.14
1.50 × 107
2.20 × 106
2.32 × 106
2.53 × 106
3.05 × 106
9.50 × 105
—
1.00
0.11
0.14
0.17
0.27
0.07
8.24
2.95
2.75
2.65
2.41
1.11
—
1.00
0.26
0.30
0.33
0.39
0.14
4.8
5.0
6.3
8.4
2.6
1.6
2.1
2.6
4.1
1.0
2.1
2.5
2.7
3.2
1.2
a Measured at 295 K. b Value corrected by means of solvent viscosity. c kЈRCN/ki-Oc
.
typical reactions of a singlet carbene BCC in the presence of
Because NY absorbs light in the same wavelength range as
that of BCC, the measured rate constant depends on the con-
centration of free BCC. The ratio of rate constants (kRCN/ki-Oc
nitrile compounds (RCN) [reactions with 2,3-dimethylbut-2-
ene (TME, an electron-rich olefin), dimethyl fumarate (MF, an
electron-deficient olefin) and pyridine (Py)]. MeCN, prop-
iononitrile (EtCN), methylpropiononitrile (i-PrCN) and
dimethylpropiononitrile (t-BuCN) were used as the nitrile
solvents, because there are no big differences in their viscosities.
Chloroacetonitrile (ClMeCN), which has an electron-
withdrawing alkyl group, was also used as a solvent. The decay
profiles of the transient absorption obeyed pseudo-first order
kinetics. The measured decay rates were independent of the
monitored wavelengths. Rate constants (k values) of the reac-
tion were determined from the slope of the linear plots of the
decay rate versus scavenger concentration, and are listed in
Table 1.
Barcus et al. reported that the rate constant for the reaction
of NY formed from NC and MeCN with an electron-deficient
olefin is substantially larger than the k value of the reaction of
the same NY with TME.2b Griller et al. reported similar results
for the reaction of NY formed from FC and MeCN,4b and we
have also reported that NYa, yielded from 3-(biphenyl-4-yl)-
2H-azirine, reacts well with electron-deficient olefins (k ~ 106–
108 dm3 molϪ1 sϪ1) but not with electron-rich olefins such as
TME.12a The logarithmic rate constant (log k) increased linearly
with increasing ionization potential of the olefin12a or carbonyl
compound.12b If BCC forms a NY in a nitrile solvent, a larger k
value is expected in the reaction with the electron-deficient
olefin MF. The determined rate constants for the reaction in
i-Oc (ki-Oc), however, were the maximum even in the reaction
with MF.
)
is assumed to be a parameter of the fraction of BCC {f =
[BCC]/([BCC] ϩ [NY])}. Thus, the fraction of NY (1 Ϫ f )
increased as follows, t-BuCN < i-PrCN < EtCN < MeCN
< ClMeCN. There are two probable effects of the solvent alkyl
group on the determination of the NY ratio, i.e. the inductive
and steric effects. When we compare the order of the steric
parameters15 with that of the f value, it is easily concluded that
the steric effect is not a dominant factor determining the NY
ratio. On the other hand, the order of the f value agreed with
that of Taft’s σ* value for the solvent alkyl group.15 It is con-
cluded that the formation rate of NY increases with increasing
inductive effect of the solvent alkyl groups, because this effect
increases the electron density on the nitrogen atom. The frac-
tion of NY, however, decreased with the decreasing inductive
effect of the alkyl groups. IR bands of the BCC–MeCN NY in
an argon matrix at 10 K indicate that NY assumes the hetero-
cumulene structure shown in Fig. 1.10 With nitrile compounds
which contain more electron-withdrawing alkyl groups in the
nitrile compounds, there is greater stabilization of the negative
charge on NY thus favoring the heterocumulene structure.
Thus, it is concluded that the of NY increases with decreasing
electron-donating inductive effect of the alkyl group of the
solvent.
Jones et al. studied the pyridinum ylide (PY) formation reac-
tion of PCC in the presence of trichloroacetonitrile.14b The
formation rate of PY increased even with increasing trichloro-
acetonitrile concentration, reflecting the PCC decay rate. NY
hardly reacts with TME.16 When BCC is in equilibrium with
NY in the reaction of FC with MeCN, the formation rate and
the decay rate of NY must be accelerated by the addtion of
TME. The addition effects of TME on the formation and decay
rates of BCC–TMB NY were also studied in MeCN
([TMB] = 0.714 mol dmϪ3). The decay rate of BCC and the
formation rate of the NY increased with increasing TME con-
centration, which strongly suggests that the NY is in equi-
librium with BCC and TMB. Indeed, the rate constant deter-
mined by means of the NY formation rate (7.6 × 107 dm3 molϪ1
sϪ1, λmoni. = 440 nm) agreed roughly with the value determined
by means of the BCC decay rate (5.58 × 107 dm3 molϪ1 sϪ1,
λmoni. = 380 nm). Because the formation of NY depends on the
concentration of free BCC, the formation rate of NY must
reflect the concentration of TME. In MeCN, the photo-
reactions of BCD with TME yielded neither azirines nor
pyrroles which were products from NY.16 It is concluded that
there is an equilibrium reaction between BCC and NY, as
shown in eqn. (7).
Because it is well-known that the rate constant depends on
the diffusion constant, the k values in nitrile solvents (kRCN
)
were corrected for the difference in viscosity relative to the rate
constants measured in i-Oc by means of eqn. (6) (corrected value
kЈRCN = (ηRCN/ηi-Oc) kRCN
(6)
kЈRCN), where the symbols ηRCN and ηi-Oc are the viscosities of
the nitrile solvent and i-Oc13 (ηi-Oc = 5.04 × 10Ϫ1 g mϪ1 sϪ1). In
any reaction, the kЈRCN value in ClMeCN was the minimum,
and it increased in other solvents as follows; ClMeCN < MeCN
< EtCN < i-PrCN < t-BuCN. The ratios kЈRCN/k1-Oc are also
listed in Table 1 to show the effect of the alkyl groups. The ratio
for the reaction with MF is lower than that for the reaction with
TME in each nitrile solvent. The ratio for the reaction with
pyridine was similar to that for the reaction with TME in each
nitrile solvent. Jones et al. reported the effect of the solvent
relative permittivity on the rate constants for the reaction of
phenylchlorocarbene (PCC) with pyridine to be small14a and
with trichloroacetonitrile to be large.14b BCC produced NY
with MeCN in a 10 K argon matrix.11 They are convinced that
some solvents react with a singlet carbene, even if the transient
signals are undetectable. Thus, it is concluded that the NY
formed by the addition of BCC to a nitrile solvent, hardly
reacts with olefins and pyridine and that the ylide can be most
effectively formed in ClMeCN.
K
(7)
BCC ϩ R–CN
NY
In a constant viscosity solvent, the equilibrium constant (K)
of eqn. (7) depends on the dissociation rate constant of NY
because the formation rate constant of NY is controlled by the
solvent viscosity. In this equlibrium, the electron-withdrawing
J. Chem. Soc., Perkin Trans. 2, 1999, 1051–1056
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Fig. 5 Change in the ratio of the rate constants, ki-Oc/kЈ, with RCN
concentration. The LFP of BCD in the presence of TME was carried
out at 295 K with EtCN (᭺) or t-BuCN (᭹) in i-Oc solution (every 5
TME concentrations for each solvent mixture). [BCD] = ca. 6 × 10Ϫ4
Fig. 6 Arrhenius plots for the reaction of BCC with TME in i-Oc (1)
and in EtCN (2).
Moss et al. reported the Ea values of the reactions of phenyl-
halocarbenes with TME in i-Oc, e.g., for PCC: Ϫ7.1 kJ molϪ1
(263–310 K) and 3.3 kJ molϪ1 (200–250 K).19 Our determined
Ea values for BCC agree well with the reported values.
Turro et al. reported that the addition of a carbene to an
olefin proceeds by way of a carbene–olefin complex intermedi-
ate.8,19,20 They proposed that a part of the complex yields the
product and another part dissociates to carbene and olefin, as
shown in eqn. (9), where k1, kϪ1 and k2 are the rate constants for
mol dmϪ3
.
alkyl groups of the solvent will decrease the dissociation rate
constant of NY due to the stabilization of negative charge.
However, although the NY decay rate seemed to increase
slightly with the addition of TME, the rate constant could not
be determined.
Effects of addition of nitrile compounds on the reaction of BCC
with TME
k1
k2
(9)
Carbene ϩ Olefin
Complex
Product
The free BCC concentration depends on the concentration of
RCN. Using EtCN and t-BuCN as typical nitrile compounds,
the effect of addition of RCN on the reaction of BCC with
TME was studied in i-Oc.17 The rate constants were found to
decrease with increasing nitrile concentration. In the presence
of RCN, the measured rate constant (kЈ) must be proportional
to free BCC concentration, because the reaction of NY with
TME is negligible.16 The fraction of BCC [f = [BCC]/([BCC] ϩ
[NY])] can be obtained using the equilibrium constant K
obtained from f [= (1 ϩ K[RCN])Ϫ1]. Thus, the kЈ value can be
expressed using the k value in i-Oc (ki-Oc) and the K value, as
shown in eqn. (8).
kϪ1
complex formation, dissociation of the complex and product
formation, respectively. The overall rate constant k was defined
as shown below in eqn. (10).
k = k1 k2/(kϪ1 ϩ k2)
(10)
When the k1 value is equal to the diffusion constant, the ratio
of the rate constants, kϪ1/k2, was calculated [=(k1 Ϫ k)/k]. When
the logarithmic ratio of the rate constants [log (kϪ1/k2)] was
plotted against the reciprocal temperature (TϪ1), a good linear
relationship was obtained in the reaction in i-Oc, as shown
in Fig. 7. From the slope of the linear relation, the thermo-
dynamic parameters, ∆∆G‡ (=∆G‡product Ϫ ∆G‡dissociation),
∆∆H‡, and ∆∆S‡, were determined to be 15.9 kJ molϪ1, Ϫ12.6
kJ molϪ1 and Ϫ96 J KϪ1 molϪ1, respectively. These parameters
agreed well with the reported values for the reactions of PCC
with TME (∆∆G‡ = 9.2 kJ molϪ1, ∆∆H‡ = Ϫ18.4 kJ molϪ1, and
∆∆S‡ = Ϫ96 J KϪ1 molϪ1 in i-Oc; ∆∆G‡ = 10.9 kJ molϪ1,
∆∆H‡ = Ϫ14.2 kJ molϪ1, and ∆∆S‡ = Ϫ84 J KϪ1 molϪ1 in tolu-
ene, ∆∆G‡ = 11.7 kJ molϪ1, ∆∆H‡ = Ϫ6.7 kJ molϪ1, and
∆∆S‡ = Ϫ63 J KϪ1 molϪ1 in MeCN; temperature range: 240–
300 K).8
Turro et al. also pointed out that the parameters in MeCN
were rather different from the values in i-Oc and in toluene.8
If PCC forms NY in MeCN, the same treatments cannot be
applied to the reaction in MeCN. Indeed, the plots of log (kϪ1/
k2) vs. TϪ1 measured in EtCN were curved. This curving is
ascribed to the NY formation. Because the dissociation of the
complex and product formation are unimolecular reactions, a
linear relationship can be expected to exist for the plots deter-
mined in the nitrile solvent with the slope being the same as that
in i-Oc. Substituting eqn. (10) into eqn. (8) gives eqn. (11).
Ϫ1
kЈ = f ki-Oc = (1 ϩ K[RCN])
k
(8)
i-Oc
When the ratios of the rate constants in the absence and in
the presence of RCN (ki-Oc/kЈ) are plotted against RCN concen-
tration, good linear relationships were obtained, as shown in
Fig. 5. The K values were determined to be 0.45 MϪ1 (EtCN)
and 0.36 MϪ1 (t-BuCN) from the slopes of the plots. These K
values also support the belief that the fraction of NY increases
with the decreasing electron-donating inductive effect of the
alkyl group of RCN.
Effects of temperature on the reaction of carbene with TME
The reaction of BCC with TME is one of the most typical
reactions of a singlet carbene. The effect of temperature on the
NY formation was studied by comparison of the reaction rate
constants in EtCN with those in i-Oc. Fig. 6 shows Arrhenius
plots of the reaction of BCC with TME in both solvents. The
rate constants increased with decreasing temperature. The maxi-
mum rate constants were measured at around 210 K. Below 210
K, the rate constants decreased with decreasing temperature.
Arrhenius activation energies (Ea) in i-Oc were determined to be
ca. Ϫ6.3 kJ molϪ1 (ca. Ϫ5.9 kJ molϪ1 in EtCN) for the temper-
ature range from 210 to 297 K and 4.2 kJ molϪ1 for the range
from 179 to 210 K. Liu and Subramanian reported the Ea value
of the reaction of benzylbromocarbene with TME in i-Oc to be
Ϫ6.3 kJ molϪ1 in the temperature range from 275 to 374 K.18
kЈ = f k1 k2/(kϪ1 ϩ k2)
(11)
When kϪ1 ӷ k2, the logarithmic ratio of the rate constants, log
(kϪ1/k2), is the sum of log (k1/kЈ) and log f. Thus, the vertical
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salt (vide infra).10 White crystals: UV–VIS-data, λmax = 358 nm
(ε = 4.2 × 103 dm2 molϪ1), 376 nm (ε = 6.2 × 103 dm2 molϪ1) and
395 nm (ε = 5.2 × 103 dm2 molϪ1) in 2,2,4-trimethylpentane
(i-Oc).
Spectroscopic grade i-Oc, acetonitrile (MeCN) and pyrid-
ine were used without further purifications. Propionitrile
(EtCN), 2-methylpropionitrile (i-PrCN), 2,2-dimethylpropio-
nitrile (t-BuCN) and chloroacetonitrile (ClMeCN) were
purified by refluxing five times for 10 hours over diphosphorus
pentoxide. After purification, the solvents were distilled twice
under N2 stream. Benzonitrile (PhCN) was purified by distil-
lation under reduced pressure over diphosphorus pentoxide
three times. 2,3-Dimethylbut-2-ene (TME) was distilled twice
over calcium hydride. Dimethyl fumarate (MF) was purified by
recrystallization twice from ethanol. The viscosities of MeCN,
EtCN, i-PrCN, t-BuCN, PhCN and ClMeCN were measured at
293.1 K.
Laser flash photolysis
The laser flash photolysis (LFP) of BCD was carried out using
a Quanta-Ray GCR-11 (355 nm, 15 ns pulse, ca. 60 mJ). The
concentration of BCD was ca. 6 × 10Ϫ4 mol dmϪ3.
References
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Fig. 8 Plot of log K[EtCN] vs. TϪ1
.
axis in Fig. 8 must be the term of [log (kϪ1/k2) Ϫ log f] and the
difference in the two relationships is log f. The fraction of
BCC, f, changes with temperature. Because LFP in EtCN was
carried out at different temperatures from those in i-Oc, the log
f value was obtained by means of the difference between the
ratio of the rate constants kϪ1/k2 in EtCN and the ratio deter-
mined by means of the relation in i-Oc. The molar ratio, [NY]/
[BCC] (=K[EtCN]), was calculated from log f value at each
temperature. When the term, log (K[EtCN]), was plotted
against TϪ1, a good linear dependence was obtained as shown in
Fig. 8. Because the EtCN concentration was constant in the
present study, the K value must increase with decreasing tem-
perature. The ∆H value derived from the K value was estimated
to be ca. Ϫ1.0 kcal molϪ1 from the slope of the linear relation in
Fig. 8. Although the thermodynamic parameter seems to be
changed by the solvent properties, the determined energy
depends chiefly on the free energy of the equilibrium constant.
10 I. Naito, A. Oku, Y. Fujiwara and Y. Tanimoto, J. Chem. Soc.,
Perkin Trans. 2, 1996, 725.
11 IR spectra of BCC were measured in the absence and in the presence
of MeCN in a ca. 15 K argon matrix. Measured IR bands in the
presence of MeCN agree well with heterocumulene-type NY
bands calculated by means of the MOPAC 6G-31G* basis set.
I. Naito, K. Nakamura, T. Kumagai, K. Matsuda, H. Iwamura,
K. Hori and A. Oku, unpublished results.
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J. Photochem. Photobiol., A: Chemistry, 1995, 92, 73; (b) I. Naito,
Y. Fujiwara, Y. Tanimoto, A. Ishida and S. Takamuku, Bull. Chem.
Soc. Jpn., 1995, 68, 2905.
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Chemistry, vol. 2, Organic Solvents, Physical Properties and Methods
of Purification, 4th edn., John Wiley & Sons, New York, 1986;
pp. 119, 394.
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M. B. Jones, V. M. Maloney and M. S. Platz, J. Am. Chem. Soc.,
1992, 114, 2163.
Experimental
Materials
(Biphenyl-4-yl)chlorodiazirine (BCD) was prepared from 4-
cyanobiphenyl via the corresponding 4-phenylbenzoamidine
J. Chem. Soc., Perkin Trans. 2, 1999, 1051–1056
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View Article Online
15 R. W. Taft, Steric Effects in Organic Chemistry, ed. M. S. Newman,
John Wiley & Sons, New York, 1956, pp. 598, 619.
(C13H12NCl) and pyrrole (C21H24NCl), yielded via NY, were
undetectable.
17 ClMeCN, a typical nitrile compound, is not mixed with i-Oc.
18 M. T. H. Lui and R. Subramanian, J. Phys. Chem., 1986, 90, 75.
19 R. A. Moss, W. Lawrynowicz, N. J. Turro, I. R. Gould and Y. Cha,
J. Am. Chem. Soc., 1986, 108, 7028.
16 The photoreactions of BCD with TME (0.10 mol dmϪ3) were
carried out in MeCN by irradiation with 366 nm light for 3 hours
([BCD] = 1.02 × 10Ϫ4 mol, [TME] = 5.05 × 10Ϫ3 mol in 5.0 × 10Ϫ2
dm3 MeCN; [BCD] = 2.61 × 10Ϫ4 mol, [TME] = 2.58 × 10Ϫ3 mol in
2.5 × 10Ϫ2 dm3 MeCN). Five products were obtained by GC-MS
analyses in each run. They were two minor C19H20 compounds,
which seemed to be dehydrochlorination products of the BCC–
TME adduct, the BCC–TME adduct (major product, C19H21Cl) and
two trace amounts of carbene dimers (C26H18Cl2). Azirine
20 N. J. Turro, G. F. Lehr, J. A. Butcher, R. A. Moss and W. Guo,
J. Am. Chem. Soc., 1982, 104, 1754.
Paper 8/04675A
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J. Chem. Soc., Perkin Trans. 2, 1999, 1051–1056