Generation, reactions, and kinetics of di(naphthyl)carbenes: Effects of the methyl group

Article abstract of DOI:10.1021/jp026410m

A series of di(naphthyl)carbenes (DNCs) having methyl groups on aromatic rings were generated by photolysis of the corresponding diazo precursors and studied not only by product analysis, but also by spectroscopic means. "Parent" triplet α-DNC was shown to have a half-life of 70 ms, which is some 30 times larger than that of triplet diphenylcarbene (³DPC), whereas parent triplet β-DNC was 2 orders of magnitude shorter-lived than the α-isomer. The lifetimes of triplet DNCs were significantly increased by introducing methyl groups near the carbene center. Thus, ³α-DNC, which has four methyl groups at 2,2′,4,4′-positions, was shown to have a half-life of 100 ms, and the replacement of the two methyl groups at the 2,2′-positions of this carbene with tri(deuterio)methyl groups resulted in an increase of the lifetime by approximately 3 times by quenching the intramolecular H transfer from the methyl groups to the carbene center leading to o-quinoid compounds. The results are discussed in terms of the counteracting effects of electronic properties stabilizing the singlet state and steric factors favoring the triplet and compared with similar studies with ³DPCs.

Full text of DOI:10.1021/jp026410m

J. Phys. Chem. A 2002, 106, 10261-10274
10261
Generation, Reactions, and Kinetics of Di(naphthyl)carbenes: Effects of the Methyl Group
Tomoki Koshiyama, Katsuyuki Hirai, and Hideo Tomioka*
Chemistry Department for Materials, Faculty of Engineering, Mie UniVersity, Tsu, Mie 514-8507, Japan
ReceiVed: June 29, 2002
A series of di(naphthyl)carbenes (DNCs) having methyl groups on aromatic rings were generated by photolysis
of the corresponding diazo precursors and studied not only by product analysis, but also by spectroscopic
means. “Parent” triplet R-DNC was shown to have a half-life of 70 ms, which is some 30 times larger than
that of triplet diphenylcarbene (3DPC), whereas parent triplet â-DNC was 2 orders of magnitude shorter-
lived than the R-isomer. The lifetimes of triplet DNCs were significantly increased by introducing methyl
3
groups near the carbene center. Thus, R-DNC, which has four methyl groups at 2,2′,4,4′-positions, was
shown to have a half-life of 100 ms, and the replacement of the two methyl groups at the 2,2′-positions of
this carbene with tri(deuterio)methyl groups resulted in an increase of the lifetime by approximately 3 times
by quenching the intramolecular H transfer from the methyl groups to the carbene center leading to o-quinoid
compounds. The results are discussed in terms of the counteracting effects of electronic properties stabilizing
3
the singlet state and steric factors favoring the triplet and compared with similar studies with DPCs.
The past decade has witnessed a renaissance in the study of
the kinetics and spectroscopy of carbenes as laser flash
photolysis (LFP) became a widely used technique in this area.1
In particular, diarylcarbenes, which are receiving considerable
attention as the archetypal triplet ground-state carbene, have
been studied in great detail, as they usually exhibit strong
absorption bands in a more accessible spectral region than those
of other carbenes and are hence readily studied by LFP with
UV-vis detection. Thus, diphenylcarbenes and fluorenylidenes
bearing various substituents have been extensively studied, and
these studies have demonstrated a large dependence of carbene
reactivity on the structures.1-3
series of methyl groups not only by product analysis but also
by means of spectroscopic methods.
Results and Discussion
The diazomethanes that we prepared in this study are listed
in Scheme 1. They are di(R-naphthyl)diazomethanes (R-1)
having a series of methyl groups at the 2- and/or 4-positions
and di(â-naphthyl)diazomethane (â-1) having two methyl groups
at the 1- and 4-positions. Before going into the details of the
effect of methyl groups on the reactivity of di(naphthyl)carbene
(DNC), we will first reveal the reactivity difference between
“parent” R-(R-1) and â-DNC (â-1)systems.
For instance, such studies on cyclic aromatic carbenes bearing
a series of aryl substituents have demonstrated that electron-
donor groups stabilize the electrophilic singlet carbene more
than they do the radical like triplet, whereas electron-withdraw-
ing groups destabilize the singlet and lead to a greater singlet-
triplet splitting (∆GST).2 On the other hand, the study of
reactivity as a function of the carbene bond angle has revealed
that the triplet states are stabilized with respect to the singlet as
the angle is increased.4
Another basic structural effect on carbene reactivity that
should be revealed in a quantitative manner might be the
perturbation by the extension of the π-system, as in benzo-
annelated derivatives. Although several diarylcarbenes with
polynuclear aromatic rings such as naphthalene5-8 and an-
thracene9 have been generated, the chemistry of these carbenes
has not been systematically studied yet.
The zero-field splitting parameters, D/E values, of diphenyl-
carbene, di(1-naphthyl)carbene, and di(9-anthryl)carbene are
reported to be 0.4055/0.194, 0.3157/0.0109, and 0.113/0.0011,
respectively. This clearly suggests that the triplet states achieve
a less bent structure with more extensive delocalization as the
number of aromatic rings is increased.10 These profound
structural changes in the rigid matrix should be reflected in the
reactivities in the solution phase.
A. Di(r-naphthyl)carbene to Di(â-naphthyl)carbene: A
Comparison. The chemistry of mono-naphthylcarbenes (NC)
has been examined by product analysis5 and also by spectro-
scopic means.6,7 A comparison of the data available on the
reactions of R-NC and â-NC with hydrocarbons shows that
R-NC exhibits lower reactivity and a higher singlet character
and possesses a smaller singlet-triplet energy gap (∆GST). Two
key molecular structural differences are responsible for the
observed difference in the reactivity. First, the R-position of
the naphthyl moiety has the largest HOMO orbital coefficient
(Hu¨ckel, 0.425 vs 0.263). This will lead to greater stabilization
of the singlet carbene center by a π-p overlap, thus making ∆GST
smaller. A second factor that should be taken into account is
the interaction between the carbene center and the peri-
hydrogen, which is clear from matrix-isolation EPR experiments,
which show a preference for the syn conformation of R-NC.6a
It is also evident that similar 1,8-interactions influence the rate
of formation of naphthylmethyl anions and cations.8
To examine how these two factors influence the reactivity
of sterically more congested diaryl systems, we first generated
R- and â-isomers of DNC (2a) and examined their reactivity.
a. Product Analysis Studies. Irradiation (λ > 300 nm) of
1a in 2-propanol gave propyl ether 3a as a major product along
with a small amount of di(naphthylmethanes) 4, the ratio of
3/4 being increased on going from R- to â-2a (Scheme 2, Table
1). These products are apparently derived from photolytically
As a part of our project on persistent triplet carbenes,11-13
we studied the chemistry of di(naphthyl)carbenes bearing a
10.1021/jp026410m CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/26/2002

10262 J. Phys. Chem. A, Vol. 106, No. 43, 2002
Koshiyama et al.
SCHEME 1
SCHEME 3
SCHEME 4
SCHEME 2
TABLE 2: Product Distributions in Photolysis of
Di(naphthyl)diazomethane (1) in the Presence of
E-â-Deuterio-r-methylstyrene
diazomethanes
E-5
Z-5
R-1a
R-1b
R-1c
R-1d
â-1a
â-1b
49
50
0a
51
50
0a
TABLE 1: Product Distributions in Photolysis of
Di(naphthyl)diazomethane (1) in 2-Propanol
0a
0a
49
49
51
51
diazomethanes
3
4
R-1a
R-1b
R-1c
R-1d
â-1a
â-1b
88
77
79
49
93
84
12
18
15
47
7
a A large amount of intractable materials were formed.
concentration. The finding suggests that only one spin state of
the carbene is responsible for the cyclopropanation of the
styrene, and the total loss of stereochemistry implies that this
state is the triplet.
12
generated carbenes 2a, which must react with 2-propanol14
because the diazomethanes 1a were completely recovered
without change in the dark under otherwise identical conditions.
It is generally accepted that the O-H insertion is the charac-
teristic reaction of singlet diarylcarbenes, whereas the triplet
carbenes abstract hydrogen from the C-H bond of alcohols to
give the corresponding diarylmethyl radicals, which either
undergo dimerization to produce tetra(aryl)ethane and/or abstract
second hydrogen to form diarylmethane.14,15 The results indicate
that the reaction from the triplet becomes more appreciable in
the reaction of R-2a than that of â-2a.
Generation of 2a in R-methylstyrene, on the other hand,
afforded cyclopropane (5a) almost exclusively. Addition of a
carbene to an olefin to form cyclopropane is the classical way
to diagnose the multiplicity of the reacting carbene.15 The
stereochemistry of this reaction was probed by employing E-â-
deuterio-R-methylstyrene.16 Integration of the NMR absorption
of the cyclopropyl hydrogens shows that the two isomeric
cyclopropanes are formed in nearly equal amounts regardless
of the position of carbene center (Scheme 3, Table 2). The
stereochemical outcome of the cyclopropanation depends on
the concentration of the styrene if both singlet and triplet
states are involved.16 However, the two cyclopropanes were
formed in nearly equal amounts regardless of the initial olefin
Photolysis in benzene resulted in the formation of products
that were much more dramatically affected by the change in
the carbene structure. Thus, irradiation of â-1a in benzene
afforded di(2-naphthyl)phenylmethane (â-6a), formally an inser-
tion product of the carbene â-2a into the C-H bond of the
solvent, as the major product along with a small amount of
radical dimer (â-7a), whereas similar irradiation of R-1 resulted
in the formation of a small amount of the dimer (R-7a) and the
azine (R-8a) along with a large amount of intractable oil, no 6
being detected (Scheme 4, Table 3).
b. Spectroscopic Studies. Irradiation of di(1-naphthyl)diazo-
methane (R-1a) in a 2-methyltetrahydrofuran (2-MTHF) glass
at 77 K gave a paramagnetic species shown in Figure 1 readily
characterized from its ESR spectrum as triplet di(1-naphthyl)-
carbene (R-2a).10,17 Essentially, the same spectrum was observed
when di(2-naphthyl)diazomethane (â-1a) was irradiated under
the same conditions. Two sets of signals were observed, as in
the ESR spectra of other triplet naphthylcarbenes, presumably
due to the presence of rotational isomers.5a,8c-e Detailed analysis
of these spectra will be reported elsewhere. The signals were
analyzed in terms of D and E values and are reported in Table
4. Because E measures the difference of the magnetic dipole
interaction along the x and y axes, it allows one to estimate the
bond angle at the carbene center, especially when weighted by

Di(naphthyl)carbenes: Effects of the Methyl Group
J. Phys. Chem. A, Vol. 106, No. 43, 2002 10263
TABLE 3: Product Distributions in Photolysis of
Di(naphthyl)diazomethane (1) in Benzene
ns, 70-90 mJ, and 308-nm pulse from a XeCl laser produced
a transient species showing a strong absorption at 377 nm, which
appeared coincident with the pulse and disappeared within 10
ms (Figure 3). On the basis of the low-temperature spectrum,
we assigned the transient product to triplet R-2a. Support is
lent to this assignment by trapping experiments using oxygen
and 1,4-cyclohexadiene (vide infra).
The oscillogram trace of the transient absorption due to R-2a
monitored at 377 nm is shown in the inset of Figure 3. The
decay was found to be second-order, and 2kd/ꢀl was determined
to be 7.4 × 104 s-1, where 2kd is the dimerization rate constant,
ꢀ the extinction coefficient, and l the optical path. The rough
lifetime of R-2a is estimated in the form of half-life, t1/2, to be
70 µs (Table 4).
diazomethanes
6
7
8
R-1a
R-1b
R-1c
R-1d
â-1a
â-1b
<1
<1
19
20
74
<1
0a
0a
0a
0a
60b
0a
0a
<1
0a
0a
11
0a
a A large amount of intractable materials were formed. b GC-MS
analysis of â-6a obtained in the photolysis of â-1a in a 1:1.009 mixture
of C6H6:C6D6 showed peaks at m/z 344 (â-6a -h6) and 350 (â-6a-d6)
in the ratio of 1:0.98. MS peaks corresponding to â-6a -h5d and â-6a-
hd5 were negligibly small (<1%).
When flash photolysis measurements were carried out on a
nondegassed benzene solution of R-1a, the half-life of triplet
R-2a was decreased, and a broad absorption band with a
maximum at 440 nm appeared. The rate of increase in the
absorbance at 440 nm was practically the same as that of the
decay of the peak at 377 nm, showing that triplet R-2a was
quenched with oxygen to form a new species. Analysis of the
spent solution showed the presence of a large amount of di(1-
naphthyl)ketone. It is well documented that diarylcarbenes with
a triplet ground state are readily trapped by oxygen to give the
corresponding diaryl ketone oxides, which are observed directly
by either matrix-isolation techniques18 or flash photolysis.19
These carbonyl oxides usually show a rather broad absorption
band centered around 390-450 nm. Thus, the observation can
be interpreted as indicating that the triplet R-2a is trapped
with oxygen to generate carbonyl oxide (R-9a) (Scheme 5).
This confirms that the transient absorption quenched by oxy-
gen is due to 3R-2a. The decay of the carbonyl oxide was found
to be first-order, and the decay rate was estimated to be 2.1 ×
Figure 1. EPR spectrum obtained by irradiation of R-1a in 2-meth-
yltetrahydrofuran (2-MTHF) at 77 K.
102 s-1
.
The kinetics of carbonyl oxide formation was studied by
monitoring their formation for several concentrations of oxygen
in acetonitrile. The build-up of the signal followed pseudo-first-
D. Inspection of the data in the table indicates that E/D values
decrease on going from â- to R-isomer of DNC, suggesting that
the carbene becomes less bent due to increased steric interaction.
Optical spectroscopy in the frozen medium gave analogous
but more intriguing results. Irradiation of R-1a in 2-MTHF glass
at 77 K resulted in the appearance of new absorption bands at
the expense of the original absorption due to R-1a. As is shown
in Figure 2, the new spectrum consists of structured bands in
300-450 nm with a strong and sharp maximum at 375 nm.
These features, especially the strong and sharp absorption around
370-380 nm, are also reported for the spectra of R-NC (λmax
) 370 nm).7,8 The glassy solution did not exhibit any changes
for several hours when kept at 77 K. However, when it was
allowed to warm to room temperature and was then cooled again
to 77 K, the characteristic bands disappeared. On the basis of
these observations coupled with ESR results, the absorption
spectrum can be attributable to triplet di(1-naphthyl)carbene
(R-2a) generated by the photodissociation of R-1a.
Similar irradiation of the â-isomer (â-1a) also gave new
absorption bands with strong maxima at 325 and 398 nm,
ascribable to triplet carbene (â-2a) (Table 4). Careful monitoring
of the spectral changes as a function of the temperature revealed
a significant difference in thermal stability between the two
isomers. Thus, no appreciable changes were observed for the
absorption bands ascribable to R-2a in 2-MTHF up to 100 K,
whereas the absorption band observed for â-2a started to
disappear already at 80 K.
order kinetics (kobs), and the bimolecular rate constant, kO , was
2
determined from plots of kobs growth vs [O2] according to eq 1,
where k represents the rate of decay of R-2a in the absence of
oxygen and kO is the rate constant for the reaction of the carbene
2
with oxygen
kobs ) k + kO [O2]
(1)
2
A plot of kobs against [O2] in the range of 0.5-9 mM is
linear, and the slope of this plot yields the absolute rate constant
for reaction of R-2a with O2, kO ) 1.7 × 109 M-1 s-1 (Scheme
2
5, Table 5).
On the other hand, when a solution of R-1a in a degassed
benzene solution containing 1,4-cyclohexadiene (CHD) was
excited, a new species was formed, showing an absorption with
λmax ) 432 nm as the 377-nm signals of R-2a decayed. Figure
4 shows the transient spectrum obtained 50 and 100 µs after
the photoexcitation, and the inset shows the decay of R-2a at
377 nm and the formation of the new species at 432 nm,
indicating that the decay of R-2a is kinetically correlated with
the growth of the new species. It is now well documented that
triplet arylcarbenes generated in good hydrogen donor solvents
undergo H abstraction, leading to the corresponding radical.1
The excellent hydrogen donor properties of 1,4-cyclohexadiene
have been recognized in its reaction with triplet benzophenone.20
Moreover, the diarylmethyl radicals are usually red-shifted with
respect to the corresponding triplet carbenes.1,21 All of the
To get more quantitative information on the reactivity, flash
photolysis was carried out. Laser flash photolysis (LFP) of R-1a
in a degassed benzene solution at room temperature with a 10

10264 J. Phys. Chem. A, Vol. 106, No. 43, 2002
Koshiyama et al.
TABLE 4: Kinetic and Spectroscopic Data for Di(naphthyl)carbene (2)
λmax (nm)
carbenes
Da (cm-1
)
Ea
(E/D)
2-MTHFa
PhHb
377
Tdc (K)
t1/2b (µs)
2k/ꢀlb (s-1
7.4 × 104
(1.2 × 106)e
)
R-2a
0.3232
0.0105
(0.0325)
330, 350, 362,
375, 412
325, 360, 398
100
70
â-2a
0.3832
0.3971
0.0182
0.0158
(0.0475)
(0.0398)
390
80
(0.83)d
a In 2-MTHF at 77 K. b In degassed PhH at 20 °C. c Temperature at which UV absorption bands of 3DNC start to disappear in 2-MTHF matrixes.
d Lifetime. e Unimolecular decay.
SCHEME 5
Figure 2. Absorption spectrum obtained by photolysis of di(1-
naphthyl)diazomethane (R-1a) in 2-methyltetrahydrofuran (2-MTHF)
at 77 K. (a) UV-vis spectrum of R-1a in 2-MTHF at 77 K. (b) Same
transient decayed following a first-order rate law with a rate
sample after 5-min irradiation (λ > 300 nm). (c) Same sample after
constant (kobs) that depends linearly on the alcohol concentration.
warming to room temperature and recooling to 77 K.
A plot of kobs against the concentration of methanol is linear,
and the slope gives a bimolecular rate constant [kq(MeOH)] of
3.5 × 105 M-1 s-1 for the reaction of the carbene (R-2a) with
the alcohols (Table 5).
It is widely believed that singlet carbenes readily undergo
insertion into O-H bonds. The reaction of diphenylcarbene
(DPC) with methanol has been exhaustively studied by spec-
troscopic techniques.1,14 Upon addition of alcohols in the LFP
experiments, quenching of both singlet DPC (kS ) 1010 M-1
s-1) and triplet DPC kt ≈ 107 M-1 s-1 1,22,23
)
was observed.
Preequilibrium and surface-crossing mechanisms have been
proposed for the relatively slow reaction of triplet diarylcarbenes
with O-H bonds24 (vide infra).
The LFP of R-1a either in acetonitrile-water (1:1) or in 2,2,2-
trifluoroethanol-acetonitrile(1.6:1), on the other hand, gave rise
to a new transient product showing the maximum at 630 nm
(Figure 5). The decay of the transient species was found to be
first-order, and the rate constant decreased as solvent nucleo-
philicity was decreased. It has been demonstrated that most
diarylcarbenes undergo protonation in hydroxylic solvents to
generate the corresponding diarylcarbocation, which are detected
by LFP in the less nucleophilic solvents.14,25 Therefore, the
species showing the maxima at 630 nm was assigned to the
di(1-naphthyl)carbocation (R-11a).26 Identification as a cation
follows arguments similar to those employed previously, namely
the observation of exponential decay kinetics with quenching
by a good nucleophile, azide, and no effect of oxygen. In
addition, essentially the same transient product was observed
in the LFP of di(1-naphthyl)methyl bromide in the same solvent
(Scheme 5, Table 5).
Figure 3. Absorption spectra of the transient products formed by
pulsing R-1a in degassed benzene with a 308 nm excimer laser recorded
after 50 µs. Inset shows oscillogram traces monitored at 377 nm.
preceding facts suggest that the new signal was attributable to
the di(1-naphthyl)methyl radical (R-10a) formed as a result of
the H abstraction of R-2a from the diene (Scheme 5). The
apparent build-up rate constant (kobs) of the radical is again
expressed in eq 2, where kCHD is a quenching rate constant of
R-2a by the diene
kobs ) k + kCHD[CHD]
(2)
A plot of the observed pseudo-first-order rate constant of the
formation of the radical against [CHD] in the range 0.05-1.0
M is linear, and the slope of this plot yields kCHD ) 1.24 × 104
M-1 s-1 (Scheme 5, Table 5).
LFP of R-1a in degassed benzene containing methanol formed
the same transient species that we had detected in the absence
of alcohol. The lifetime of this species decreased as the
concentration of alcohol in the solution was increased. The
Similar measurements were done for the â-isomer of carbene
(â-2a), and values are summarized in Tables 4 and 5.
As will be discussed in the next section in more detail, â-2a
was found to be significantly more reactive than R-2a, and the
rate constant with very efficient reagents, e.g., O2, was too fast
to be determined by our systems.

Di(naphthyl)carbenes: Effects of the Methyl Group
TABLE 5: Kinetic and Spectroscopic Data for Reactions Involving Di(naphthyl)carbene (2)
ketone oxide (10)a radical (11)b
2k/ꢀl
J. Phys. Chem. A, Vol. 106, No. 43, 2002 10265
cation (12)c
a
b
kO
λ
k
kCHD
(M-1 s-1
λ
kq (MeOH)
(M-1 s-1
λ
(nm)
k
2
carbenes
(M-1 s-1
)
(nm)
(s-1
)
)
(nm)
(s-1
)
)
(s-1
)
R-2a
1.7 × 109
>1010
440
2.1 × 102
2.0 × 104d
1.2 × 104
5.0 × 106
432
7.9 × 104
6.6 × 103
3.1 × 105
>1010
630
9.8 × 104
(6.6 × 106)e
â-2a
425
406
580
(too fast)e
a In MeCN. b In degassed PhH. c In CF3CH2OH-MeCN (1.6:1). d Second-order decay in 2k/ꢀl. e In MeCN-H2O (1:1).
and/or cycloheptatriene.15 It may be then that 6a is produced
from the triplet state of 2 by an abstraction-recombination
mechanism. To check this possibility, the irradiation of â-1a
was carried out in a 1:1 mixture of C6H6 and C6D6, and the
deuterium distributions in the product â-6a were determined
by mass spectrometric analysis, which clearly showed that direct
insertion products, i.e., â-6a-h6 and -d6, were formed almost
exclusively (>99%), whereas the yield of scrambled products
detected was less than 1%. The kinetic isotope effect (kH/kD)
for the formation of 6a was determined to be 0.99 from the
product distributions (Scheme 4, Table 3). The results indicate
that the C-H insertion product is formed almost exclusively
from the singlet carbene â-2a.
Olah and co-workers have studied the mechanism of the C-H
insertion of methylene (CH2:) into benzene using kinetic
hydrogen isotope effects as well as theoretical calculations and
concluded that, while the insertion of the triplet methylene (kH/
kD ) 12.1 ( 1.0) proceeds through a stepwise mechanism which
may involve an intermediate, the mechanism for the singlet
methylene (kH/kD ) 1.34 ( 0.04) seems to follow a concerted
pathway, although initial formation of π-complexes cannot be
excluded.27 Direct attack of the singlet â-2a at the C-H bond
of benzene is, however, rather unlikely in the light of the
decreased electrophilicity as well as increased steric crowded-
ness. Presumably, as the carbenic center of â-2a approaches
the π-electrons on the aromatic ring so as to form norcaradiene,
it will experience severe steric hindrance due to the two aromatic
rings, and the carbene will thus be forced to follow the pathway
to generate a π-complex or a neutral zwitterionic species that
will ultimately cascade to 6. Alternatively, a norcaradiene may
be formed and readily undergo rearrangement to give the
insertion product owing to steric repulsion either as it is formed
or during the workup. However, all attempts to detect the
norcaradiene have been unsuccessful to date (vide infra).
Figure 4. Absorption spectra of the transient products formed during
the irradiation of R-1a in degassed benzene containing 1,4-cyclohexa-
diene, recorded after (a) 50 µs and (b) 100 µs. Inset is oscillogram
traces monitored at 377 and 432 nm.
The almost complete absence of the solvent adduct (e.g.,
R-6a) in the reaction of R-2a is in sharp contrast with that
observed for â-2a and can be also explained in terms of the
steric effect. The approach of the singlet carbene to the
π-electrons on the benzene ring so as to a species leading to 6
should experience a much more severe steric hindrance in this
case, and carbenes are hence forced to decay by several
competing reaction pathways available to form rather complex
product mixtures.
Figure 5. Absorption spectra of the transient products formed during
irradiation of R-1a in trifluoroethanol-MeCN (1.6:1) recorded after
10 µs.
c. Reactivity Difference between r- and â-DNC. A
comparison of the data available on the product distributions
in the reaction of R-2a and â-2a with the substrates shows some
interesting differences in the reactivity, which can be explained
mostly in terms of steric effects. Thus, in the reaction with
efficient trapping reagents for singlet carbene, i.e., alcohols, no
significant changes in product distributions were observed.
Similarly, essentially the same product ratios were obtained in
the reaction with R-methylstyrene, which is believed to react
efficiently with the triplet states.
A dramatic difference in the reaction patterns is observed in
the reaction with benzene. The formation of a formal insertion
product (â-6a) into the C-H bonds of benzene in the reaction
of â-2a is rather unusual because carbenes react with benzene
by the initial attack on the π electrons on the aromatic ring in
the singlet state to result in the formation of a norcaradiene
Inspection of the kinetic data (Table 5) leads to an analogous
but more quantitative conclusion. Unimolecular vs bimolecular
decay kinetics for â- and R-isomers in benzene, for instance,
must reflect the difference in the reaction patterns between the
two carbenes. Unimolecular decay of triplet â-2a can be
interpreted as indicating that the triplet states are trapped by
the solvent benzene. However, the product studies clearly
suggest that the singlet state is responsible for the final product
(6). The fact that the optical density of 3â-2a was not decreased
3
significantly compared to that of R-2a under identical condi-
tions, i.e., in degassed benzene at 20 °C, indicates that not all

10266 J. Phys. Chem. A, Vol. 106, No. 43, 2002
Koshiyama et al.
of 6 is produced from the nascent singlet. Thus, monitored
3â-2a also leads to the final product either by the preequilibrium
mechanism, where triplet to singlet intersystem crossing occurs
prior to reaction of the singlet carbene, or by surface crossing,
where the potential energy surfaces of the singlet and triplet
carbene reaction cross after they have begun to interact with
the substrate (benzene).
The second-order decay kinetics of 3R-2a, on the other hand,
suggests that the triplet states decay by undergoing dimerization.
The product analysis of the mixtures obtained in conventional
light irradiation, however, did not completely support the
spectroscopic observation, although the absence of the solvent
adducts implies that R-2a is unreactive toward benzene. It is
probable that intense light from the laser can generate a high
concentration of R-2a. Under such conditions, the carbenes
become the most efficient trapping reagent for themselves.
Alternatively, the expected dimer, i.e., tetra(1-naphthyl)ethylene,
is simply unstable under photolysis conditions.
and â-1b, Scheme 1) bearing methyl groups on aromatic rings
and investigated the effect of methyl groups on the reactivities
of di(naphthyl)carbenes (2) photolytically generated from them.
a. Product Analysis Studies. Irradiation of methylated 1 in
2-propanol also gave the propyl ether 3 along with dinaphth-
ylmethane 4. The product distributions are rather sensitive to
the substituents, especially in the case of R-isomers. Thus, as
methyl groups are introduced at the ortho positions of the
carbene center, the product from the triplet carbenes, i.e., 4,
increases rather significantly; in the case of di[1-(2,4-dimethyl)-
naphthyl]carbene (R-2d), the 3/4 ratio is close to 1 (Scheme 2,
Table 1).
More dramatic changes in the product distributions are
observed in the reactions with other substrates (Tables 2 and
3). Thus, the photolysis of R-1c and d in R-methylstyrene and
benzene resulted in a large amount of intractable oil, almost no
meaningful products obtained in the reaction with the “parent”
R-2 being isolated. A similar but less significant change of the
product distributions upon the methyl substitution is also
observed in the reaction of the â-isomers. Thus, in the reaction
of â-2b, significant product formation was not discontinued until
the least reactive substrate, i.e., benzene, was used, while
essentially the same products were produced in other reagents
although the ratio was slightly changed, probably reflecting
increased steric factors.
These results are consistent with those observed for analogous
reactions with polymethylated diphenylcarbenes12d,i and are
interpreted similarly in terms of the steric effect on the reaction.
Thus, as methyl groups are introduced at the ortho position of
di(1-naphthyl)carbene, which is already sterically congested, the
carbenic center should be protected more strictly. As a result,
the reactions from the singlet side, which are generally more
sensitive to the steric factors,15,28 become energetically less
favored, and the singlet state is forced to undergo intersystem
crossing to the triplet. The triplet states can react with typical
triplet quenchers, e.g., hydrogen donors, but become less reactive
toward less efficient triplet quenchers. Steric hindrance is less
prominent in the reaction of â-2 because essentially the same
products are obtained in the reaction of â-2b even with less
reactive substrates, i.e., styrene, as in the reaction of â-2a. This
confirms the importance of the role of peri-H in the R-isomer.
Comparison of the lifetime in the form of the half-life of the
two carbenes in benzene showed that R-2a is some 2 orders of
magnitude longer-lived than the â-isomer. Because the decay
processes are not the same for both carbenes, the stability cannot
be discussed simply based on the kinetic data. Nevertheless,
the stability order is consistent with that of steric congestion.
The kinetic data obtained in typical triplet quenchers should
be more informative in order to estimate differences in the
reactivity between the two carbenes in the triplet states. Thus,
the quenching rate constant of 3â-2a with oxygen is too high to
3
be monitored and significantly higher than that of R-2a. The
rate constants for hydrogen abstraction from CHD observed with
3â-2a are 2 orders of magnitude greater than that of 3R-2a. These
3
results suggest that R-2a is considerably less reactive than
3â-2a and can be interpreted in terms of better steric shielding
3
in R-2a than in the corresponding â-isomer.
The stability of intermediates formed from DNC, e.g., 9-11,
is also to be noted. Although these intermediates should not be
influenced by the same electronic factors, rough trends in
stability are more or less similar to that of the precursor DNC.
This can be again interpreted as indicating that steric factors
play more important roles than electronic effects in the reaction
of these polynuclear aromatic systems.
It should be noted here that, in the case of o-methylated
diphenylcarbenes, triplet carbenes decay mainly by undergoing
intramolecular hydrogen abstraction from the ortho-methyl
groups to produce eventually benzocyclobutene derivatives, as
more methyl groups are introduced.12d,i,29All attempts to isolate
expected naphthocyclobutenes were unsuccessful, although the
intramolecular H abstraction process is actually observed
spectroscopically (vide infra).
b. Spectroscopic Studies. Irradiation of methylated diazo-
methanes (1) in 2-MTHF glass at 77 K gave persistent ESR
signals with large D values characteristic of triplet DNC. The
zero-field splitting parameters, D and E, are reported in Table
6. Inspection of the data in the table indicates that E/D values
steadily decrease as one introduces more methyl groups on the
aromatic rings, suggesting that the carbene becomes less bent
due to increased steric interaction between the methyl groups.
Optical spectroscopy in the frozen medium gave essentially
the same spectral changes observed with the “parent” carbenes.
Irradiation of methylated 1 in 2-MTHF glass at 77 K gave
absorption bands ascribable to the corresponding triplet carbenes
(2). Although the absorption maxima are slightly shifted, the
overall shapes of the bands are similar. However, careful
monitoring of the spectral changes as a function of the
B. Effects of Methyl Groups on the Reactivities of Di-
(naphthyl)carbenes. The above study reveals that triplet di-
(naphthyl)carbenes are significantly longer-lived than triplet
diphenylcarbene, confirming the expectation from the structural
studies based on the ESR measurement in low-temperature
matrixes. Thus, triplet di(R-naphthyl)carbene is more than 2
orders of magnitude longer-lived than triplet diphenylcarbene
(t1/2 ≈ 2 µs). This suggests that di(naphthyl)carbene can serve
as a more promising prototype diarylcarbene for persistent triplet
carbene than DPC. Naturally, one would expect that the lifetime
of di(naphthyl)carbene could be elongated by introducing
appropriate substituents around the carbene centers. Because
carbenes react even with very poor sources of electrons, such
as C-H bonds, the tert-butyl group, which has been successfully
employed as one of the most effective protecting groups for
many reactive centers in organic chemistry, was found to be
ineffective in protecting triplet carbenes.12f,i In this respect,
methyl groups can serve as an exceptionally good protecting
group for triplet carbene in the case of diphenylcarbene;
didurylcarbene is shown to have a half-life of 410 ms in solution
at room temperature, some 5 orders of magnitude longer-lived
than diphenylcarbene under the same conditions.12d,i Therefore,
we prepared a series of di(naphthyl)diazomethanes (R-1b-d

Di(naphthyl)carbenes: Effects of the Methyl Group
J. Phys. Chem. A, Vol. 106, No. 43, 2002 10267
TABLE 6: Kinetic and Spectroscopic Data for Methylated Di(naphthyl)carbene (2)
λ
max (nm)
c
b
Da
Ea
Td
t1/2
(µs)
kib,d
carbenes
(cm-1
)
(cm-1
)
(E/D)
MTHFa
PhHb
(K)
2k/ꢀlb (s-1
)
(s-1
)
R-2b
R-2c
R-2d
R-2d-d6
â-2b
0.3197
0.3256
0.2490
0.3265
0.3466
0.3484
0.0108
0.0089
0.0053
0.0099
0.0173
0.0141
(0.0337)
(0.0273)
(0.0215)
(0.0303)
(0.0500)
(0.0405)
380
380
90
110
105
28
1.1 × 105
379 (435)e
384 (454)e
383
379 (436)e
382 (451)e
375
1400
100 000
265 000
58
1.0 × 104
10.5
<1
3.6 × 103
5.2 × 104
375, 398
375
95
a In 2-MTHF at 77 K. .b In degassed PhH at 20 °C. c Temperature at which UV absorption bands of 3DNC start to disappear in 2-MTHF matrixes.
d Intramolecluar H abstraction rate constant. e Absorption maximum for o-quinodimethane.
SCHEME 6
The LFP of the methylated 1 in a degassed benzene solution
Figure 6. Absorption spectrum obtained by photolysis of di[1-(2,4-
dimethyl)naphthyl]diazomethane (R-1d) in 2-MTHF at 77 K. (a)
UV-vis spectrum of R-1d in 2-MTHF at 77 K. (b) Same sample after
5-min irradiation. (c) Same sample after thawing to 110 K. (d) Same
sample thawing to room temperature and recooling to 77 K.
produced similar transient absorption bands with those observed
for “parent” 1, although their decay modes and kinetics are
sensitive to the substitution patterns (Tables 6 and 7). Com-
parison of the data in the tables confirms expectations for the
role of steric effects in stabilizing the triplet carbene center more
quantitatively. Judging from the half-life in benzene, 3R-2
becomes destabilized slightly by the para-methyl groups (t1/2
decreases slightly on going from R-2a to R-2b) but stabilized
significantly by the ortho-methyl groups (t1/2 increases signifi-
cantly on going from R-2a to R-2c). This can be interpreted in
terms of electronic effects, which cause the upper-lying singlet
state to be close to the triplet ground states, and steric effects,
which stabilize the triplet states kinetically. The introduction
of two more methyl groups at the para positions (R-2c) resulted
in an increase in the lifetime by some 2 orders of magnitude,
indicating that para methyl groups in this case stabilize the triplet
carbene. The reason for this rather puzzling effect is not clear
at present. Benzene is known to be one of the least reactive
solvents toward triplet carbenes. Therefore, carbenes usually
decay either by undergoing dimerization or by abstracting
hydrogen from the o-methyl groups. Product analysis showed
that large amounts of complex mixtures were formed, little
simple carbene dimers being virtually isolated.
It is to be noted that trityl radicals undergo either methyl-
para or para-para coupling depending on substitution patterns
and that this coupling reactions are suppressed by para substit-
uents.31 Similar decaying pathways are suggested in the reaction
of persistent triplet diarylcarbenes.12h Thus, as the carbene center
is more sterically congested, simple dimerization at the carbene
site must suffer from severe steric repulsion, and the carbene is
hence forced to react at the sites where a spin can be
delocalized.3 Increased stability in R-2d compared to R-2c can
then be ascribable to the suppression of this coupling pathway.
However, the reactivities of R-2 toward typical triplet
quenchers, i.e., oxygen and CHD, are also decreased in this
order. For example, kCHD decreases by some 2 orders of
magnitude on going from R-2c to R-2d. This suggests that the
increased stability of R-2d in benzene cannot be simply
temperature showed that the thermal stability and the decay
pathway are sensitive to the substitution patterns. First, in the
case of R-DNC, the temperature at which the carbene bands
start to decay (Td) increases by about 10 K upon ortho-methyl
substitution (compare Td between R-2a and R-2c in Table 6).
This is obviously due to increased stability by the protecting
o-methyl groups. On the other hand, this temperature drops to
90 K by para-methyl groups (see the Td difference between
R-2a and R-2b in Table 6). This is interpreted in terms of the
electronic effect. It is generally accepted that electron-donating
substituents stabilize the electrophilic singlet carbene more than
they do the radical-like triplet, thus making ∆GST smaller.2 The
triplet states then would decay more easily by way of the upper-
lying singlet state.
Second, upon thawing the matrix, a new broad absorption
appeared around 440-450 nm as carbene absorption bands
3
decreased in the case of the R-2 bearing the methyl group at
the ortho position (Figure 6), while all other carbene bands
decayed monotonically without showing secondary transient
absorption bands. The secondary formed species showed ap-
preciable thermal stability and decayed gradually upon thawing
the matrix to room temperature. What is this intermediate? In
the analogous spectral monitoring of the decay process of ortho-
polymethylated diphenylcarbenes as a function of temperature,
we also observed similar broad, secondary transient bands in
visible regions, which we assigned as o-quinodimethanes
generated by the 1,4 H shift from the o-methyl to the carbenic
center based on the spectral profiles, i.e., broad absorption bands
centered around 350-450 nm and appreciable thermal stabil-
ity.30 These features are also present in the spectrum obtained
in the photolysis of R-1c and d. In these circumstances, the
secondary transient is attributable to o-quinodimethanes (12)
(Scheme 6).

10268 J. Phys. Chem. A, Vol. 106, No. 43, 2002
Koshiyama et al.
TABLE 7: Kinetic and Spectroscopic Data for Reactions Involving Methylated Di(naphthyl)carbene (2)
ketone oxide (10)a
radical (11)b
2k/ꢀl
(s-1
cation (12)c
a
b
kO
λ
k
kCHD
(M-1 s-1
λ
(nm)
kMeOH/K
λ
k
2
carbenes
(M-1 s-1
)
(nm)
(s-1
)
)
)
(M-1 s-1
)
(nm)
(s-1
)
R-2b
R-2c
R-2d
â-2a
1.9 × 109
6.5 × 108
1.0 × 108
1.7 × 109
430
430
415
420
5.6 × 104
1.8 × 102
1.4 × 102
6.6 × 104
2.8 × 103
3.6 × 10
1.8 × 105
422
435
460-470
426
3.6 × 103
2.2 × 104
9.5
∼106
630
630
645
610
6.5 × 105
9.2 × 106
2.2 × 103
3.1 × 106
1.1 × 104
12 × 103
4.1 × 102
a In MeCN. b In degassed PhH. c In CF3CH2OH-MeCN (1.6:1).
gas-phase acidities of the acids and the basicities of their methyl
esters combined with heat formation by using the principle of
isodesmic reaction has revealed that this effect is not zero even
in 2,5-dimethyl derivative, suggesting that the interactions
between more distant groups cannot be neglected.35 This effect,
if any, should be more significant for the 4-methyl group of
naphthalene derivatives than for that in benzenes due to the
presence of peri-hydrogen in the former case. In this light, the
o-methyl groups in R-2d are brought closer to the carbenic
center than those in R-2c. Therefore, R-2d can decay by
abstracting hydrogen from the o-methyl group to generate
o-quinodimethane and, since the carbene center in R-2d is more
tightly blocked by the o-methyl group, it reacts with typical
triplet quenchers less efficiently. The fact that R-2c decayed
without showing any sign of formation of o-quinodimethane
12c but generated 12c in a low-temperature matrix upon thawing
the matrix also suggests that the o-methyl groups in R-2c are
less close to the carbenic center than those in R-2d.
Figure 7. Absorption spectra of the transient products formed during
the irradiation of R-1d in degassed benzene recorded 50 µs after
photoexcitation. Insets are oscillogram traces monitored at 380 and 450
nm.
Rather significant decrease in the reactivity toward methanol
on going from R-2a to R-2d (Tables 5 and 7) can be interpreted
in terms of the change in a singlet-triplet energy gap. Thus, as
more methyl groups are introduced at position 2 of R-DNC,
the bond angle is increased (as indicated in an decrease in E/D
values), and triplet state is more stabilized with respect to the
singlet (vide supra). Similar decrease in the rate as a function
of methyl groups is also observed in the reaction of triplet DPC
with methanol.4c
ascribable to the decrease in the reaction sites in its dimerization
reaction but to the inherent decrease of the reactivities of the
carbene center. It should be noted here that LFP of R-1d in
degassed benzene gave a secondary transient species exhibiting
3
a broad band around 451 nm as the 382-nm signal of R-2d
decayed (Figure 7). On the basis of the low-temperature
spectrum, the secondary formed transient species is assigned
to o-quinodimethane (12d). It should be noted that the decay
3
of R-2d was found not to fit the first-order kinetic and to be
Similar effects of the methyl groups are also observed for
the kinetics of the â-isomer of the carbene. Thus, on going from
â-2a to â-2b, the lifetime increased by 2 orders of magnitude,
and kCHD decreased by 1 order of magnitude. The kinetic data
for â-2b and R-2c, both having protecting o-methyl groups,
should be compared here. Thus, the significantly shorter lifetime
in benzene and the larger reactivities toward triplet quenchers
of â-2b as opposed to R-2c again suggest the important steric
role of peri-hydrogen in the reaction of DNCs.
slower by half than the growth of 12d. This suggests that not
3
all of R-2d decays by intramolecular H abstracting. Similar
spectroscopic observations were noted in an LFP study of
polymethylated diphenylcarbenes, e.g., didurylcarbene, which
decays mainly by abstracting hydrogen from the o-methyl group
to generate o-quinodimethane.12f The similarity of the absorption
band and stability supports the assignment.
In marked contrast, 2,2′-dimethylated carbene (R-2c) decays
rather monotonically without showing any sign of formation
of o-quinodimethane (12c). These observations suggest that,
although both R-2c and R-2d carry methyl groups at the ortho
c. Deuterated Tetramethyldi(1-naphthyl)carbenes (r-2d-
d6)). Deuterium Isotope Effects. The above study reveals that
DNCs are effectively stabilized by methyl groups, especially
when they are introduced near the carbene center. These ortho
effects are shown to be strengthened even by the distant para
methyl groups, and the o-methyl groups are more tightly fixed
near the carbene center. However, the carbene center comes to
be quenched more easily by the o-methyl groups in this case,
and the lifetime might then not be increased as could have been
expected. One of the best ways to hinder this process and to
realize more persistent carbene is to replace the hydrogens at
the o-methyl groups with deuterium. Thus, we prepared di[1-
(2-trideuteriomethyl-4-methyl)naphthyl]diazomethane (R-1d-d6)
bearing fully deuterated o-methyl groups and studied the kinetic
deuterium isotope effects (KDIEs) on the intramolecular H
transfer reactions.
3
positions, R-2d can decay by abstracting hydrogen from the
o-methyl groups, whereas this process is not involved in the
3
decay pathway for R-2c.
A simple explanation for this difference is to invoke the
distance between the carbene center and hydrogen on the
o-methyl group. It is well-known that, in 1,2-disubstituted
benzene derivatives, the introduction of substituents in the
3-position exerts a very large effect on the rate of appropriate
reactions, which are considered in light of the importance of
bond bending; the 3-substituents “buttress” the 2-substitu-
ents.33,34 This is a steric effect exerted on a functional group by
a more distant substituent through another immediately adjoining
substituent. Therefore, 4-substituents are usually expected to
exert little steric effect, especially without 3-substituents.
However, recent re-estimation of the buttressing effects in
methyl-substituted benzoic acid derivatives on the basis of the
Irradiation of R-1d-d6 in 2-MTHF glass at 77 K gave
essentially the same transient absorption bands with those

Di(naphthyl)carbenes: Effects of the Methyl Group
J. Phys. Chem. A, Vol. 106, No. 43, 2002 10269
observed in the photolysis of the protio analogue (R-1d)
ascribable to the triplet carbene (R-2d-d6). A significant
difference was seen in the decay profile upon thawing the
matrixes. Thus, the bands ascribable to R-2d-d6 decayed
monotonically without showing any sign of formation of
secondary transient species, i.e., o-quinodimethane (12). This
suggests that there should be rather large KDIEs.
On the other hand, the E/D values decrease in the order of
3DPC, 3â-DNC, and 3R-DNC, suggesting that the carbene bond
angle increases in that order. Therefore, the electronic and steric
factors exert their influence in opposite directions. It is not
possible to estimate these two counteracting factors quantita-
tively; hence, this will be examined rather qualitatively based
on experimental observations.
A comparison of the reactivity of 3DPC with that of 3â-DNC,
a sterically less hindered isomer of DNC, will provide an insight
into the effect of the extension of the π-system. Two important
features, which suggest decreased ∆GST in DNC, are noted in
The LFP of R-1d-d6 in degassed benzene also resulted in
essentially the same transient absorption bands that were
observed for the protio analogue (R-1d). Thus, a transient
showing a strong absorption at 375 nm due to deuterated triplet
carbene (R-2d-d6) appeared coincident with the XeCl pulse.
3
the reaction of â-DNC with alcohol and benzene forming the
singlet product as opposed to that of 3DPC. First, 3â-DNC reacts
considerably faster with methanol than does 3DPC. Similar
enhancement on the reactivity toward methanol has been
reported for triplet (2-naphthyl)phenylcarbene as opposed to
3DPC.8 The reaction of the triplet carbenes with the O-H bonds
of alcohols is explained either by preequilibrium24 or by surface-
crossing mechanisms.23 In the preequilibrium mechanism, triplet
to singlet interconversion occurs prior to O-H insertion of the
singlet carbene, whereas, in the surface-crossing mechanism,
triplet states can react with alcohols, with surface crossing
occurring after the carbene has begun to interact with the O-H
bond. In either of the two mechanisms, the decrease in ∆GST
should result in the increase in the bimolecular rate constants
for the reaction of triplet carbene with methanol. Second, the
reaction of 3â-DNC with benzene producing formal C-H
insertion product (6) is not the usual reaction for diarylcarbenes.
DPC, for instance, reacts with the precursor diazo function to
form diphenylketazine in benzene.15 This is interpreted as
indicating that DPC has little reactivity toward benzene in either
of the multiplicities and that the attack on the terminal nitrogen
of the diazo functional group, presumably in its singlet state,
thus comes to be the only available major pathway.
However, the transient absorption disappeared monotonically
in this case. The decay of 3R-2d-d6 followed second-order
3
kinetics, and the lifetime of R-2d-d6 in the form of half-life
was estimated to be 265 ms, which is some 3 times greater
than that of the protio analogue. No new absorption band
ascribable to o-quinodimethane was detected to the limit of our
LFP time scale and detection. This is partly because the
intramolecular H migration pathway is not the exclusive pathway
for the decay of the protio analogue (3R-2d) (vide supra).
3
Therefore, we could not determine ki for D transfer in R-2d-
d6, and we could only estimate the highest limit for ki as <1
s-1. The KDIE on the intramolecular H transfer reaction was
then roughly estimated to be ∼10 (Table 6). This value is to be
compared with 4.8 determined for the similar reaction in triplet
decamethyldiphenylcarbene under identical conditions.12f
C. Di(naphthyl)carbene to Diphenylcarbene: A Compari-
son. The present investigation provides some insights into the
effect of the benzo-annelation on the reactivity of diarylcarbene.
Thus, it is quite important to compare the reactivity difference
3
3
between DNC and DPC.
The reactivity of carbenes should be examined in terms of
the singlet-triplet energy gap (∆GST). The factors which
influence ∆GST can be analyzed in terms of electronic and steric
effects. It is generally accepted that the triplet state has a σ1p1
configuration, whereas σ2 is thought to be the lowest energy
configuration for the singlet. The magnitude of ∆GST is then
roughly equal to electron-electron repulsion terms, caused by
confining two electrons to one orbital, minus the energy required
to promote an electron from the σ- to the p-nonbonding orbital.
If carbene substituents have a p or π* orbital that interacts with
the 2p orbital of the carbene, the separation of the 2p and σ
(σpn) orbitals will decrease. This will result in a larger ∆GST
than that of an unperturbed, prototype carbene. If carbene
substituents have a doubly occupied orbital that will interact
with the carbene 2p orbital, the 2p-σ gap becomes smaller, thus
making ∆GST smaller.2 The steric effect, on the other hand,
influences the carbene carbon bond angle. The maximum value
of ∆GST occurs when the two carbenic orbitals become
degenerate, as they would be for linear methylene. Bending
methylene removes the orbital degeneracy and reduces ∆GST.
Thus, as the carbene-carbon bond angle is further contracted,
the σ-orbital picks up more s-character and consequently
becomes even lower in energy. The smaller the bond angle,
the more energy it takes to promote an electron from the σ- to
the p-orbital, and the smaller ∆GST becomes.3,4
These contrasting reactivity patterns are also interpretable in
terms of the difference in ∆GST. Thus, 3â-DNC can react with
benzene through the low-lying singlet state, which is thermally
easily accessible. Alternatively and energetically more favorably,
3â-DNC starts to interact with benzene, and the surface of the
triplet reaction crosses with the singlet surface at the lower
energy region; this is because, in the preequilibrium mechanism,
surface crossing occurs near the energy minimum of 3DNC and,
hence, requires more energy than anywhere else on the singlet
reaction surface. For 3DPC, on the other hand, the energy
required to lead to the final product should not be small enough
in order to allow it to react with benzene within the lifetime,
most probably because of the larger ∆GST.
The reaction of â-DNC with R-methylstyrene is shown to
occur in its triplet state almost exclusively. This may sound
somewhat conflicting with the argument mentioned above. If
the reaction in the singlet side were easily accessible for
3â-DNC, then it would have reacted through the singlet energy
3
surface. Presumably, the reaction of â-DNC with R-methyl-
styrene should proceed very smoothly with the low energy
barrier to form a 1,3-biradical intermediate stabilized by aryl
delocalization, and the spin inversion can take place afterward
of the process. Therefore, surface crossing is not required at
the rate-determining step. On the other hand, the concerted
approach of the singlet DNC to the double bond of R-methyl-
styrene will suffer from steric hindrance due to two canted
naphthyl groups and, therefore, becomes kinetically less favor-
able in this case.
One difference between DPC and DNC is the relative energies
of the occupied and unoccupied aromatic orbitals that mix with
the carbene p orbital. In the case of DNC, more than that of
DPC, the energy of the singlet is predicted to become lower
than that of the triplet by this mixing. This is more significantly
so for R-DNC than for the â-isomer, since the coefficient of
the occupied π-orbital is greater at the R- than at the â-position.
The reactivity of R-DNC is too complicated to be compared
with that of DPC. Electronic effects stabilizing the singlet state

10270 J. Phys. Chem. A, Vol. 106, No. 43, 2002
Koshiyama et al.
and steric factors favoring the triplet exert a larger influence
for R-DNC than for â-DNC, and it is not easy to estimate its
∆GST. It is likely that the reactivity of R-DNC is dictated largely
by kinetic factors. In this respect, it is interesting to examine
R-DNC as a prototype for persistent triplet carbene as opposed
to DPC.
benes, for instance, unlike that of trityls, does not lead to a final
stable product and gives rise to an intermediate open-shell
molecules whose subsequent reactions will be complicated by
possible ambient reactivity.
In this light, R-DNC cannot be employed as a promising
prototype carbene for constructing persistent triplet carbenes
unless substituents can be introduced not only around the
carbene centers but also at all possible positions where a spin
can be delocalized. This may require tedious synthetic proce-
dures.
“Parent” 3R-DNC has a lifetime of 70 µs, which is some 30
times larger than that of “parent” 3DPC. This is obviously due
to the effect of peri-H, which not only stabilizes the triplet state
with respect to the singlet but also acts as a kinetic protector
3
toward the carbene center. ESR data suggest that R-DNC is
significantly less bent than 3DPC. Despite the increased stability
Experimental Section
3
3
of the unsubstituted R-DNC compared to DPC, the effect of
the methyl groups as a kinetic protector on the stability of the
triplet state is, disappointingly, rather small. Because the basic
skeletons between DNC and DPC are not exactly the same, one
cannot find an exact counterpart between the two. However,
General Methods. IR spectra were measured on a JASCO
A-100 recording spectrometer and the mass spectra were
recorded on JEOL MS route JMS-600W.
1H and 13C NMR spectra were determined either JEOL JNM-
EX 270 or JNM-500 spectrometers. UV-vis spectra were
recorded on a JASCO CT-560 Grating Monochromator.
Thin-layer chromatography was done on Merck Kieselgel 60
PF 254. Column chromatography was carried out on silica gel
(ICN for dry column chromatography). HPLC and GPC were
undertaken with a JASCO 800 chromatograph equipped with a
UVIDEC-100-II UV-vis detector using a Fine pack C18-T5
column (4.6 × 25 cm) and a Shodex GPC H-2001 (20 mm ×
50 cm) column, respectively, and GLC was carried out with a
Yanagimoto G-80 gas chromatograph using an OV-17 on
Diasolid L (5.0 mm × 50 cm) column.
3
one can compare the reactivity of R-2d with that of triplet
dimesitylcarbene (DMC) assuming that the peri-hydrogens at
the 8 positions are acting as one of the hydrogens of the
3
o-methyl groups in DMC. Actually, this comparison is fairly
valid because the peri-Hs are expected to be fixed tightly in
the same plane with the carbene carbon, whereas hydrogens on
the o-methyl groups are rather flexible, even though the van
der Waals radius of methyl groups may be estimated to be larger
than that of the net C8 substituent. The E/D values clearly
support the idea. This value for 3R-2d (E/D ) 0.0215) is
significantly smaller than that for 3DMC (E/D ) 0.033),
Unless otherwise noted, all the reagents employed in this
study are commercial products and used after standard purifica-
tion.
3
3
suggesting that R-2d is less bent than DMC. Nevertheless,
3
the lifetime of R-2d (t1/2 ) 100 ms) is not larger than that of
3DMC (t1/2 ) 160 ms).
Materials. (a) Preparation of Di(1-naphthyl)diazomethanes
(r-1). All R-1 used in this study were prepared by coupling of
naphthylmagnesium bromide with naphthonitrile, followed by
hydrazonation of the resulting ketimine and oxidation of the
hydrazone with nickel peroxide according to the procedure by
Reimlinger and Franzen.36 Thus, to a solution of Grignard
reagent prepared from Mg (1.5 g, 60 mmol) and 1-bromonaph-
thalene (12.42 g, 60 mmol) in anhydrous Et2O (20 mL), was
added a solution of naphthonitrile (6.13 g, 40 mmol) in
anhydrous Et2O (20 mL), dropwise under vigorous stirring, and
the mixture was refluxed overnight. A saturated NH4Cl aq
solution (50 mL) was added carefully to the reaction mixture
cooled by ice-bath under vigorous stirring and the mixture was
filtered. The filtrate was extracted with Et2O, and the organic
layer was dried and evaporated to leave crude product, which
was purified by silica gel column chromatography eluted with
pet-ether and Et2O (2:1) to afford di(1-naphthyl)ketimine in
74% yield as viscous oil.
A solution of the ketimine (200 mg, 0.71 mmol) and
hydrazine hydrochloride (378 mg, 3.6 mmol) in absolute EtOH
(5 mL) was placed in a round-bottom flask, and the whole
system was deaerated by repeated cycles of evacuation and
purge with nitrogen. Anhydrous N2H4 (1.12 g, 36 mmol) was
added to the mixture and the resulting solution was refluxed
until a TLC monitoring showed that most of the ketimine was
consumed. After cooling, the solvent was evaporated, and the
mixture was added to Et2O. The ethereal layer was washed with
H2O, dried (Na2SO4), and evaporated to give di(1-naphthyl)-
ketone hydrazone in 97% yield as yellowish solid (mp 143-
147 °C), which was used without further purification for the
next step.
The reason for this rather puzzling difference is not clear at
present. One possible explanation is the larger spin delocaliza-
3
3
tion in R-DNC than in DPC, as evidenced by decreasing D
values on going from 3R-DNC to 3DPC. It has been shown that
the variation of D is due to the variation in the π-electron spin
density at the divalent carbon, since the dominant interaction
is the one-center terms. Consequently, the ratio of that spin
density to D should be approximately constant. Thus, the
3
π-electron spin in R-DNC systems is largely delocalized into
aromatic rings. Therefore, if the carbene center becomes less
reactive due to steric hindrance, then the carbene can more easily
react at the aromatic rings where spin can be delocalized.
In this respect, it is very interesting to compare the products
obtained in the reaction in less reactive solvents, e.g., benzene,
between the two carbene systems. In the case of sterically
congested 3DPC, tetra(aryl)ethylenes probably formed as a result
3
of DPC coupling are the most frequently observed product in
benzene.12f,29 This is explained by assuming that the hindered
triplet diphenylcarbene concentration builds up due to decreased
reactivities by kinetic protectors, especially in unreactive solvent,
and the most reactive counterparts under these conditions must
hence be the triplet carbene themselves. Taking into account
the increased lifetime, similar reactions are likely decay
pathways for sterically more shielded 3DNC. Nevertheless, we
isolated almost no simple carbene dimers, i.e., tetra(naphthyl)-
ethylene, but mostly intractable oil, from which no assignable
products were isolated.
Trityl radicals are known to undergo either methyl-para or
para-para couplings depending on the substituent patterns.
Thus, it is likely that 3DNCs also undergo similar coupling. The
complexity of the products observed in the reaction of DNC
must be partly due to the complexity associated with the
coupling reactions. The carbene-para coupling of triplet car-
To a cooled solution of the hydrazone (600 mg, 2.02 mmol)
in dry CH2Cl2 (3 mL) was added NiO2 (1.1 g, 12.1 mmol) at 0
°C under vigorous stirring in the dark over a period of 1 h.

Di(naphthyl)carbenes: Effects of the Methyl Group
J. Phys. Chem. A, Vol. 106, No. 43, 2002 10271
After filtration, the solvent was removed under reduced pressure
to afford crude diazo compound, which was chromatographed
on a deactivated alumina eluted with CH2Cl2-n-hexane (1:1) at
-10 °C. The diazomethane coming through immediately from
the column was fairly pure but was further purified by repeated
chromatography on a gel permeation column with CHCl3 to
afford pure di(1-naphthyl)diazomethane (R-1a) as a red solid
in 60% yield: mp 72 °C (dec); 1H NMR (CDCl3) δ 7.35-7.52
methyl)naphthyl]diazomethane (r-1c) in 17.8% yield as a red
viscous liquid after aluminum column chromatography followed
1
by GPC purification: red liquid; H NMR (CDCl3) δ 2.48 (s,
3 H), 2.55 (s, 3 H), 6.84-6.87 (m, 1 H), 7.19-7.95 (m, 11 H);
IR (KBr) 2040 cm-1
.
(d) Di[1-(2,4-dimethyl)naphthyl]diazomethane (r-1d). 1,3-
Dimethylnaphthalene was transformed to the desired diazo-
methane by way of the following compounds as described
above. 1-Bromo-2,4-dimethylnaphthalene: 92%; yellow liquid;
1H NMR (CDCl3) δ 2.57 (s, 3 H), 2.61 (s, 3 H), 7.19 (s, 1 H),
7.46-7.59 (m, 2 H), 7.92 (d, J ) 7.26 Hz, 1 H), 8.23 (d, J )
8.25 Hz, 1 H). 1-Cyano-2,4-dimethylnaphthalene: 90%; white
(m, 8 H), 7.78-7.92 (m, 6 H); IR (KBr) 2050 cm-1
.
(b) Di[1-(4-methyl)naphthyl]diazomethane (r-1b). A mix-
ture of 1-bromo-4-methylnaphthalene (4.5 g, 20.4 mmol) and
CuCN (2.0 g, 22.4 mmol) in anhydrous DMF (30 mol) was
refluxed overnight. The mixture was poured into ice-water (400
mL), and the resulting precipitate was filtered. The solid was
transferred to a beaker containing ethylenediamine (115 mL)
and H2O (30 mL), and the precipitate formed was extracted
with CH2Cl2. The organic layer was washed with a 10% NaCN
solution and water, dried and evaporated. The crude product
was Kugelrohr-distilled under pressure at 130 °C/3.0 mmHg to
give 1-cyano-4-methylnaphthalene in 87% yield as a white
1
solid; mp 109.7-111.2 °C; H NMR (CDCl3) δ 2.67 (s, 3 H),
2.68 (s, 3 H), 7.21 (s, 1 H), 7.52-7.66 (m, 2 H), 7.97 (d, J )
8.91 Hz, 1 H), 8.17 (d, J ) 8.25 Hz, 1 H). Di[1-(2,4-dimethyl)-
naphthyl]ketimine: 16.7%; yellowish semisolid; 1H NMR
(CDCl3) δ 2.13 (s, 6 H), 2.68 (s, 6 H), 7.10 (s, 2 H), 7.44-7.51
(m, 4 H), 8.02 (d, J ) 8.25 Hz, 2 H), 8.30 (bs, 2 H). Di[1-
(2,4-dimethyl)naphthyl]ketone Hydrazone: 15.3%; yellow
gummy solid; 1H NMR (CDCl3) δ 2.11 (s, 3 H), 2.14 (s, 3 H),
2.61 (s, 3 H), 2.65 (s, 3 H), 7.07 (s, 1 H), 7.15 (s, 1 H), 7.40-
7.48 (m, 4 H), 7.91-7.98 (m, 2 H), 8.28 (bs, 1 H), 8.52 (bs, 1
H). Di[1-(2,4-dimethyl)naphthyl]diazomethane (R-1d): 46.3%;
reddish orange solid; mp 130.0-131.2 °C (dec); 1H NMR
(CDCl3) δ 2.32 (s, 6 H), 2.69 (s, 6 H), 7.23 (s, 2 H), 7.35 (dd,
J ) 8.42, 6.93 Hz, 2 H), 7.44 (dd, J ) 7.92, 6.93 Hz, 2 H),
7.93 (d, J ) 8.25 Hz, 2 H), 7.98 (d, J ) 8.24 Hz, 2 H); IR
1
solid: mp 49.8-51.1 °C; H NMR (CDCl3) δ 2.78 (s, 3 H),
7.38 (d, J ) 7.58 Hz, 1 H), 7.62-7.73 (m, 2 H), 7.82 (d, J )
7.58 Hz, 1 H), 8.07-8.10 (m, 1 H), 8.24-8.28 (m, 1 H). Di-
[1-(4-methyl)naphthyl]ketimine was obtained by the reaction
of 4-methylnaphthylmagnesium bromide, prepared from the
corresponding bromide (4.0 g, 18.1 mmol) and Mg (440 mg,
18.1 mmol) in Et2O (20 mL), with 1-cyano-4-methylnaphthalene
(3.0 g, 18.1 mmol) in anhydrous Et2O (15 mL), followed by
column chromatography (SiO2, Et2O-n-hexane 1:1) as a yellow
gummy solid in 30% yield: 1H NMR (CDCl3) δ 2.73 (s, 6 H),
7.24 (d, J ) 7.58 Hz, 2 H), 7.34 (d, J ) 7.58 Hz, 2 H), 7.45-
7.58 (m, 4 H), 8.07 (d, J ) 7.92 Hz, 2 H), 8.42 (bs, 2 H). The
ketimine (50 mg, 0.16 mmol) was treated with N2H4‚HCl (85
mg, 0.91 mmol) and anhydrous N2H4 (0.26 mL, 81 mmol) in
absolute EtOH (10 mL) to afford di[1-(4-methyl)naphthyl]-
ketone hydrazone in 96% yield as a yellow gummy solid: 1H
NMR (CDCl3) δ 2.62 (s, 3 H), 2.72 (s, 3 H), 5.49 (bs, 2 H),
7.28 (d, J ) 7.26 Hz, 2 H), 7.35 (d, J ) 7.26 Hz, 2 H), 7.44-
7.57 (m, 4 H), 7.88 (d, J ) 7.92 Hz, 1 H), 7.98-8.02 (m, 1 H),
8.07 (d, J ) 8.25 Hz, 1 H). The hydrazone (50 mg, 0.15 mmol)
was oxidized with NiO2 (80 mg, 0.88 mmol) in CH2Cl2 (2 mL)
and the reaction was worked up as described above to give pure
di[1-(4-methyl)naphthyl]diazomethane (r-1b) as a red crystal
(KBr) 2051 cm-1
.
(e) Di[1-(2-trideuteriomethyl-4-methyl)naphthyl]diazo-
methane (r-1d-d6). 3-Bromo-1-methylnaphthalene (8.4 g, 36
mmol) was treated with CuCN (1.5 g, 17 mmol) in DMF (30
mL). The reaction was worked up as usual to provide 1-methyl-
3-naphthonitrile in 96% yield as a yellow solid: mp 57.2-
59.8 °C; 1H NMR (CDCl3) δ 2.71 (s, 3 H), 7.44 (s, 1 H), 7.48-
7.72 (m, 2 H), 7.89 (d, J ) 8.58 Hz, 1 H), 8.03 (d, J ) 8.58
Hz, 1 H), 8.10 (s, 1 H). The nitrile (6.1 g, 36 mmol) was
hydrolyzed by refluxing it in H2O (50 mL) and conc H2SO4
(40 mL) overnight. After usual workup, 1-methylnaphthoic
acid was obtained in 88% yield as a white solid: mp 197.6-
1
199.1 °C; H NMR (CDCl3) δ 2.76 (s, 3 H), 7.55-7.69 (m, 2
H), 7.97 (s, 1 H), 7.99 (d, J ) 8.57 Hz, 1 H), 8.05 (d, J ) 8.25
Hz, 1 H), 8.58 (s, 1 H). Methyl 1-methyl-3-naphthoate was
obtained by treating the acid with CH2N2 in 99% yield as a
yellowish liquid: 1H NMR (CDCl3) δ 2.73 (s, 3 H), 4.03 (s, 3
H), 7.57-7.67 (m, 2 H), 7.91 (s, 1 H), 7.96 (d, J ) 7.92 Hz,
1 H), 8.03 (d, J ) 8.91 Hz, 1 H), 8.47 (s, 1H). To a stirred
solution of the ester (2.75 g, 13.7 mmol) in anhydrous THF
(30 mL) was added LiAlD4 (Aldrich, 576 mg, 13.7 mmol)
slowly at 0 °C. After usual workup, 3-[hydroxydi(deuterio)-
methyl]-1-methylnaphthalene was obtained as a viscous liquid
in 98% yield: 1H NMR (CDCl3) δ 2.70 (s, 3 H), 3.48 (bs, 1
H), 7.34 (s, 1 H), 7.49-7.55 (m, 2 H), 7.67 (s, 1 H), 7.84 (d,
J ) 8.90 Hz, 1 H), 7.98 (d, J ) 7.58 Hz, 1 H). A solution of
the alcohol (2.5 g, 13.7 mmol) in dioxane (50 mL) was added
PBr3 (1.43 mL, 15 mmol) and the mixture was stirred for 2 h
at room temperature. After usual workup, 3-[bromodi(deute-
rio)methyl]-1-methylnaphthalene was obtained as a brownish
1
in 52% yield: mp 71.0-72.1 °C (dec); H NMR (CDCl3) δ
2.72 (s, 6 H), 7.43 (dd, J ) 7.92, 7.26 Hz, 2 H) 7.55 (dd, J )
6.93, 8.57 Hz, 2 H), 7.95 (d, J ) 8.58 Hz, 2 H); 8.07 (d, J )
8.25 Hz, 2 H); IR (KBr) 2050 cm-1
.
(c) Di[1-(2-methyl)naphthyl]diazomethane (r-1c). Di[1-(2-
methyl)naphthyl]ketimine was obtained by the reaction of
2-methylnaphthylmagnesium bromide, prepared from the cor-
responding bromide (3.45 g, 15.6 mL) and Mg (378 mg, 15.6
mmol) in anhydrous Et2O (15 mL), with 1-cyano-2-methyl-
naphthalene (2.0 g, 12.0 mmol) in anhydrous Et2O (10 mL),
followed by column chromatography (SiO2, pet. ether-Et2O
2:1) as a brownish gummy solid in 17.8% yield: 1H NMR
(CDCl3) δ 2.12 (s, 3 H), 2.40 (s, 3 H), 6.91-7.19 (m, 6 H),
7.46-7.93 (m, 5 H), 8.85 (s, 1 H). The ketimine (56 mg, 0.163
mmol) was treated with N2H4‚HCl (85.7 mg, 0.816 mmol) and
anhydrous N2H4 (261 mg, 8.51 mmol) in absolute EtOH (3 mL)
to give di[1-(2-methyl)naphthyl]ketone hydrazone in 93%
yield as a yellowish semisolid: 1H NMR (CDCl3) δ 2.36 (s, 3
H), 2.64 (s, 3 H), 5.51 (bs, 2 H), 7.34-8.35 (m, 12 H). The
hydrazone (148.6 mg, 0.46 mmol) was oxidized with NiO2 (240
mg, 2.6 mmol) in anhydrous CH2Cl2 (1 mL) to form di[1-(2-
1
solid in 94% yield: mp 48.1-51.7 °C; H NMR (CDCl3) δ
2.69 (s, 3 H), 7.34 (s, 1 H), 7.47-7.56 (m, 2 H), 7.70 (s, 1 H),
7.81 (d, J ) 8.91 Hz, 1 H), 7.96 (d, J ) 7.57 Hz, 1 H). The
bromide was reduced with LiAlD4 as described above to give
1-methyl-3-tri(deuterio)methylnaphthalene in 93% yield as
a colorless liquid: 1H NMR (CDCl3) δ 2.66 (s, 3 H), 7.16 (s,
1 H), 7.41-7.47 (m, 3 H), 7.92-7.95 (m, 1 H). The tri-

10272 J. Phys. Chem. A, Vol. 106, No. 43, 2002
Koshiyama et al.
(deuterio)methylated naphthalene was transformed to the desired
diazomethane by way of the following compounds as described
above. 1-Bromo-2-tri(deuterio)methyl-4-methylnaphthalene:
95%; yellow liquid; 1H NMR (CDCl3) δ 2.61 (s, 3 H), 7.19 (s,
1 H), 7.46-7.59 (m, 2 H), 7.92 (d, J ) 7.26 Hz, 1 H), 8.23 (d,
J ) 8.25 Hz, 1 H). 2-Tri(deuterio)methyl-4-methyl-1-naph-
thonitrile: 94% brownish solid; mp 110.1-111.4 °C; 1H NMR
(CDCl3) δ 2.68 (s, 3 H), 7.21 (s, 1 H), 7.52-7.66 (m, 2 H),
7.97 (d, J ) 8.91 Hz, 1 H), 8.17 (d, J ) 8.25 Hz, 1 H). Di[1-
(2-trideuteriomethyl-4-methyl)naphthyl]ketimine: 31%; yel-
lowish semisolid; 1H NMR (CDCl3) δ 2.68 (s, 6 H), 7.10 (s, 2
H), 7.44-7.51 (m, 4 H), 8.02 (d, J ) 8.25 Hz, 2 H), 8.30 (bs,
2 H). Di[1-(2-trideuteriomethyl-4-methyl)naphthyl]ketone
Hydrazone: 18%; pale yellow semisolid; 1H NMR (CDCl3) δ
2.14 (s, 3 H), 2.61 (s, 3 H), 2.65 (s, 3 H), 7.07 (s, 1 H), 7.15 (s,
1 H), 7.40-7.48 (m, 4 H), 7.91-7.98 (m, 2 H), 8.28 (bs, 1 H),
8.52 (bs, 1H). Di[1-(2-trideuteriomethyl-4-methyl)naphthyl]-
diazomethane (r-1d-d6): 3%; orange liquid; 1H NMR (CDCl3)
δ 2.69 (s, 6 H), 7.23 (s, 2 H), 7.35 (dd, J ) 8.42, 6.93 Hz, 2
H), 7.44 (dd, J ) 7.92, 6.93 Hz, 2 H), 7.93 (d, J ) 8.25 Hz, 2
was refluxed for 2 h. After the usual workup, di[2-(1,4-
dimethyl)naphthyl]methanol was obtained as a white crystal
in 37% yield: mp 159.0-161.5 °C; H NMR (CDCl3) δ 2.11
1
(d, J ) 4.29 Hz, 1 H), 2.62 (s, 12 H), 6.69 (d, J ) 4.29 Hz),
7.40 (s, 2 H), 7.49-7.55 (m, 4 H), 7.98-8.02 (m, 2 H), 8.08-
8.12 (m, 2 H). To a solution of the methanol (500 mg, 1.47
mmol) in acetone (20 mL) was added a mixture of CrO3 (222
mg) and concentrated Na2SO4 (0.12 mL) in H2O (1.44 mL) at
room temperature under stirring. The reaction was worked up
as usual to leave di[2-(1,4-dimethyl)naphthyl]ketone in 98%
yield as a gummy solid, which was used to the next step without
further purification: 1H NMR (CDCl3) δ 2.61 (s, 6 H), 2.75 (s,
6 H), 7.24 (s, 2 H), 7.62-7.64 (m, 4 H), 8.02-8.06 (m, 2 H),
8.20-8.24 (m, 2 H). The crude ketone (498 mg, 1.47 mmol)
was refluxed with N2H4‚HCl (500 mg, 4.76 mmol) and
anhydrous N2H4 (5.0 mL, 158 mmol) in absolute EtOH (10 mL)
for 2 days. After usual workup, the crude mixture was purified
by preparative TLC eluted with n-hexane-Et2O (2:1) to obtain
di[2-(1,4-dimethyl)naphthyl]ketone hydrazone in 50% yield
as a white gummy solid: 1H NMR (CDCl3) δ 2.56 (s, 3 H),
2.61 (s, 3 H), 2.68 (s, 3 H), 2.79 (s, 3 H), 5.53 (bs, 2 H), 6.99
(s, 1 H), 7.15 (s, 1 H), 7.49-7.55 (m, 2 H), 7.57-7.63 (m, 2
H), 7.91-7.98 (m, 1 H), 8.00-8.05 (m, 1 H), 8.10-8.19 (m, 2
H). The hydrazone (100 mg, 0.28 mmol) was oxidized with
NiO2 (200 mg, 2.2 mmol) in anhydrous CH2Cl2 (5 mL) at 0
°C. After usual workup, the crude product was purified by
column chromatography (Al2O3, CH2Cl2-n-hexane 1:15), fol-
lowed by GPC yield di[2-(1,4-dimethyl)naphthyl]diazo-
methane (â-2c) in 61% yield as a reddish purple solid: mp
126.8-127.7 °C (dec); 1H NMR (CDCl3) δ 2.55 (s, 6 H), 2.60
(s, 6 H), 7.06 (s, 2 H), 7.53-7.59 (m, 4 H), 7.99 (d, J ) 6.92
H), 7.98 (d, J ) 8.24 Hz, 2 H); IR (KBr) 2050 cm-1
.
(f) Di(2-naphthyl)diazomethane (â-2a). A solution of di-
(2-naphthyl)ketone (500 mg, 1.77 mmol), N2H4‚HCl (931 mg,
8.86 mmol) and anhydrous N2H4 (2.8 mL, 88.6 mmol) in
absolute EtOH (10 mL) was refluxed for 3 h. The reaction was
worked up as usual, followed by preparative TLC to give di-
(2-naphthyl) ketone hydrazone in 60% yield as a brownish
solid: mp 138.0-141.2 °C; 1H NMR (CDCl3) δ 5.56 (s, 2 H),
7.36-7.45 (m, 2 H), 7.52-7.62 (m, 4 H), 7.78-7.96 (m, 6 H),
8.02-8.07 (m, 2 H). The hydrazone (130 mg, 0.44 mmol) was
oxidized with NiO2 (200 mg, 2.2 mmol) in anhydrous CH2Cl2
(10 mL). After usual workup, followed by GPC purification,
di(2-naphthyl)diazomethane (â-2a) was obtained as a reddish
purple solid in 80% yield: mp 125.8-127.3 °C (dec); 1H NMR
(CDCl3) δ 7.45-7.50 (m, 6 H), 7.72-7.88 (m, 8 H); IR (KBr)
Hz, 2 H), 8.09 (d, J ) 6.93 Hz, 2 H); IR (KBr) 2032 cm-1
.
Irradiation for Product Identification. In a typical run, a
solution of the diazo compounds (1, ca. 10 mg) in solvent was
placed in a Pyrex tube and irradiated with a high-pressure,
300-W mercury lamp at room temperature until all the diazo
compound was destroyed. The irradiation mixture was then
concentrated on a rotary evaporator below 20 °C. Individual
components were isolated by preparative TLC and/or GPC and
identified by NMR and MS. In this way, the following products
were isolated and characterized.
2016 cm-1
.
(g) Di[2-(1,4-dimethyl)naphthyl]diazomethane (r-2b). To
a solution of 1,4-dimethylnaphthalene (3.04 g, 19.5 mmol) in
CCl4 (30 mL) was added a solution of Br2 (3.12 g, 19.5 mmol)
in CCl4 (15 mL) under vigorous stirring slowly so that the
temperature did not raise over 5 °C. After stirring overnight at
room temperature, the mixture was washed with 10% NaOH-
H2O, and the organic layer was washed with H2O, dried
(Na2SO4) and evaporated to dryness to give a crude 2-bromo-
1,4-dimethylnaphthalene as yellowish oil which was used
without further purification: 1H NMR (CDCl3) δ 2.62 (s, 3 H),
2.76 (s, 3 H), 7.47 (s, 1 H), 7.51-7.56 (m, 2 H), 7.93-7.96
(m, 1 H). To a solution of the bromide (2.3 g, 9.8 mmol) in
ahydrous Et2O (20 mL) added n-BuLi (1.71 M in n-hexane,
5.93 mL, 9.8 mmol) dropwise. After stirring for 1 h, anhydrous
DMF (1 mL) was added under vigorous stirring and the mixture
was stirred for 2 h. A saturated NH4Cl aq solution was added
to the reaction mixture and the mixture was extracted with Et2O.
The ethereal phase was washed with H2O, dried (Na2SO4) and
evaporated to dryness to give a crude 1,4-dimethyl-2-naph-
thaldehyde as a yellowish liquid in 89% yield, which was used
to the next step without further purification: 1H NMR (CDCl3)
δ 2.67 (s, 3 H), 2.98 (s, 3 H), 7.46-7.68 (m, 2 H), 7.74 (s, 1
H), 8.02 (d, J ) 8.25 Hz, 1 H), 8.23 (d, J ) 8.91 Hz, 1 H),
10.62 (s, 1 H). To a solution of 2-(1,4-dimethylnaphthyl)lithium,
prepared by treating the corresponding bromide (2.18 g, 9.28
mmol) with n-BuLi (1.65 M in n-hexane, 5.6 mL, 9.28 mmol)
in absolute Et2O (15 mL) was added a solution the aldehyde
(1.6 g, 8.72 mmol) in anhydrous Et2O (2 mL) and the mixture
i
Products in Irradiation in PrOH. (a) Di(1-naphthyl)-
methyl Isopropyl Ether (r-3a). 1H NMR (CDCl3): δ 1.35 (d,
J ) 6.27 Hz, 6 H), 3.93 (sept, J ) 6.29 Hz, 1 H), 7.05 (s, 1 H),
7.38-7.49 (m, 8 H), 7.80 (d, J ) 7.92 Hz, 2 H), 7.87-7.90
(m, 2 H), 8.17-8.21 (m, 2 H). HRMS Calcd for C24H22O:
326.1671. Found: 326.1630.
(b) Di[1-(4-methyl)naphthyl]methyl Isopropyl Ether
(r-3b). 1H NMR (CDCl3): δ 1.34 (d, J ) 6.27 Hz, 6 H), 2.68
(s, 6 H), 3.91 (sept, J ) 6.27 Hz, 1 H), 7.01 (s, 1 H), 7.24 (d,
J ) 7.26 Hz, 2 H), 7.35 (d, J ) 7.24 Hz, 2 H), 7.44-7.54 (m,
4 H), 8.04 (d, J ) 7.92 Hz, 2 H), 8.21 (d, J ) 7.92 Hz, 2 H).
HRMS Calcd for C26H26O: 354.1984. Found: 354.1978.
(c) Di[1-(2-methyl)naphthyl]methyl Isopropyl Ether
(r-3c). 1H NMR (CDCl3): δ 1.48 (d, J ) 6.27 Hz, 6 H), 2.48
(s, 3 H), 2.61 (s, 3 H), 3.68 (sept, J ) 6.27 Hz, 1 H), 7.05 (s,
1 H), 7.33-7.45 (m, 6 H), 7.68-7.87 (m, 6 H). HRMS Calcd
for C26H26O: 354.1984. Found: 354.1718.
(d) Di[1-(2,4-dimethyl)naphthyhl]methyl Isopropyl Ether
(r-3d). 1H NMR (CDCl3): δ 1.21 (d, J ) 6.27 Hz, 6 H), 2.15
(s, 6 H), 2.65 (s, 6 H), 3.72 (sept, J ) 6.27 Hz, 1 H), 7.05 (s,
2 H), 7.12 (s, 1 H), 7.37-7.45 (m, 4 H), 7.98 (d, J ) 8.24 Hz,
2 H), 8.59 (d, J ) 8.24 Hz, 2 H). HRMS Calcd for C28H30O:
382.2295. Found: 382.2365.

Di(naphthyl)carbenes: Effects of the Methyl Group
J. Phys. Chem. A, Vol. 106, No. 43, 2002 10273
(e) Di(2-naphthyl)methyl Isopropyl Ether (â-3a). 1H NMR
(CDCl3): δ 1.28 (d, J ) 6.27 Hz, 6 H), 3.77 (sept, J ) 6.27
Hz, 1 H), 5.81 (s, 1 H), 7.43-7.51 (m, 6 H), 7.77-7.83 (m, 6
H), 7.87 (s, 2 H).
irradiated with a Wacom 500 W Xe lamp using a Pyrex filter.
ESR spectra were measured on a JEOL JES TE 200 spectrom-
eter (X-band microwave unit, 100 kHz field modulation). The
signal positions were read by the use of a gaussmeter.
Low-Temperature UV-Vis Spectra. Low-temperature spec-
tra at 77 K were obtained by using an Oxford variable-
temperature liquid-nitrogen cryostat (DN 1704) equipped with
a quartz outer window and a sapphire inner window. The sample
was dissolved in dry 2-MTHF, placed in a long-necked quartz
cuvette of 1-mm path length, and degassed thoroughly by
repeated freeze-degas-thaw cycles at a pressure near 10-5
Torr. The cuvette was flame-sealed, under reduced pressure,
placed in the cryostat, and cooled to 77 K. The sample was
irradiated for several minutes in the spectrometer with a Halos
300-W high-pressure mercury lamp using a Pyrex filter, and
the spectral changes were recorded at appropriate time intervals.
The spectral changes upon thawing were also monitored by
carefully controlling the matrix temperature with an Oxford
Instrument Intelligent Temperature Controller (ITC 4).
Flash Photolysis. All flash measurements were made on a
Unisoku TSP-601 flash spectrometer. Three excitation light
sources were used depending on the precursor absorption bands
and lifetime of the transient species. They were (i) a cylindrical
150-W Xe flash lamp (100 J/flash with 10-µs pulse duration),
(ii) a Quanta-Ray GCR-11 Nd:YAG laser (355 nm pulses of
up to 40 mJ/pulse and 5-6-ns duration; 266 nm pulses of up
to 30 mJ/pulse and 4-5-ns duration), and (iii) Lamda Physik
LEXTRA XeCl excimer laser (308 nm pulses of up to 200 mJ/
pulse and 17-ns duration). The beam shape and size were
controlled by a focal length cylindrical lens.
A Hamamatsu 150 W xenon short arc lamp (L 2195) was
used as the probe source, and the monitoring beam, guided using
an optical fiber scope, was arranged in an orientation perpen-
dicular to the excitation source. The probe beam was monitored
with a Hamamatsu R2949 photomultiplier tube through a
Hamamatsu S3701-512Q MOS linear image sensor (512 pho-
todiodes used). Timing of the excitation pulse, the probe beam,
and the detection system was acheived through an Iwatsu Model
DS-8631 digital synchro scope which was interfaced to a NEC
9801 RX2 computer. This allowed for rapid processing and
storage of the data and provided printed graphic capabilities.
Each trace was also displayed on a NEC CRT N5913U monitor.
A sample was placed in a long-necked Pyrex tube which had
a sidearm connected to a quartz fluorescence cuvette and
degassed using a minimum of four freeze-degas-thaw cycles
at a pressure near 10-5 Torr immediately prior to being flashed.
The sample system was flame-sealed under reduced pressure,
and the solution was transferred to the quartz cuvette which
was placed in the sample chamber of the flash spectrometer. A
cell holder block of the sample chamber was equipped with a
thermostat and allowed to come to thermal equilibrium. The
concentration of the sample was adjusted so that it absorbed a
significant portion of the excitation light.
(f) Di[2-(1,4-dimethyl)naphthyl]methyl Isopropyl Ether
(â-3b). 1H NMR (CDCl3): δ 1.29 (d, J ) 5.93 Hz, 6 H), 2.62
(s, 3 H), 3.75 (sept. J ) 5.93 Hz, 1 H), 6.39 (s, 1 H), 7.43 (s,
2 H), 7.47-7.54 (m, 2 H), 7.96-7.99 (m, 2 H), 8.08-8.11 (m,
2 H).
Products in Irradiation with r-Methylstyrene. (a) 1,1-Di-
(1-naphthyl)-2-phenyl-2-methylcyclopropane (r-5a). 1H NMR
(CDCl3): δ 1.47 (s, 3 H), 2.25 (bs, 1 H), 2.43 (bs, 1 H), 6.89-
7.24 (m, 2 H), 7.25-7.59 (m, 12 H), 7.65 (d, J ) 8.25 Hz, 1
H), 7.77 (d, J ) 7.92 Hz, 1 H), 8.15 (d, J ) 6.92 Hz, 1 H),
8.78 (d, J ) 8.25 Hz, 1 H), 8.95 (d, J ) 7.92 Hz, 1 H). HRMS
Calcd for C30H24: 384.1878. Found: 384.1817.
(b) 1,1-Di[1-(4-methyl)naphthyl]-2-phenyl-2-methylcyclo-
propane (r-5b). 1H NMR (CDCl3): δ 1.45 (s, 3 H), 2.25 (bs,
1 H), 2.42 (s, 3 H), 2.58 (s, 3 H), 2.70 (bs, 1 H), 6.73 (d, J )
7.59 Hz, 1 H), 6.95 (d, J ) 8.25 Hz, 1 H), 7.30-7.63 (m, 10
H), 7.85 (d, J ) 7.91 Hz, 1 H), 7.91 (d, J ) 8.25 Hz, 1 H),
8.01 (d, J ) 7.92 Hz, 1 H), 8.80 (d, J ) 7.92 Hz, 1 H), 8.99 (d,
J ) 7.59 Hz, 1 H). HRMS Calcd for C32H28: 412.2191.
Found: 412. 2192.
(c) 1,1-Di(2-naphthyl)-2-phenyl-2-methylcyclopropane
1
(â-5a). H NMR (CDCl3): δ 1.77 (d, J ) 5.28 Hz, 1 H), 2.39
(d, J ) 4.95 Hz, 1 H), 7.02-7.33 (m, 6 H), 7.39-7.67 (m, 6
H), 8.01 (s, 1 H).
(d) 1,1-Di[2-(1,4-dimethyl)naphthyl]-2-phenyl-2-methyl-
cyclopropane (â-5b). 1H NMR (CDCl3): δ 1.44 (bs, 3 H), 2.15
(bs, 3 H), 2.35 (s, 3 H), 2.47 (s, 1 H), 2.62 (s, 1 H), 2.72 (s, 3
H), 2.85 (bs, 3 H), 7.18-7.45 (m, 9 H), 7.77-7.98 (m, 4 H).
Products in Irradiation in Benzene. (a) 1,1,2,2-Tetra(1-
naphthyl)ethane (r-7a). 1H NMR (CDCl3): δ 6.99-7.05 (m,
4 H), 7.15 (s, 2 H), 7.26-7.45 (m, 12 H), 7.72 (d, J ) 6.60 Hz,
4 H), 7.82-7.87 (m, 8 H).
1
(b) Di(1-naphthyl)ketazne (r-8a). H NMR (CDCl3): δ
6.75-6.81 (m, 2 H), 7.09-7.17 (m, 4 H), 7.21-7.39 (m, 4 H),
7.45-7.51 (m, 2 H), 7.56-7.71 (m, 10 H), 7.92 (d, J ) 7.92
Hz, 2 H), 8.02-8.06 (m, 4 H).
(c) Di(2-naphthyl)phenylmethane (â-6a). 1H NMR
(CDCl3): δ 5.83 (s, 1 H), 7.44-7.51 (m, 7 H), 7.55 (d, J )
8.25 Hz, 2 H), 7.75-7.85 (m, 8 H), 7.93 (s, 2 H). HRMS Calcd
for C27H20: 344.4605. Found: 344.4603.
1
(d) Di(2-naphthyl)ketazine (â-7a). H NMR (CDCl3): δ
7.31-7.43 (m, 2 H), 7.46-7.67 (m, 4 H), 7.73-7.83 (m, 5 H),
7.88-8.00 (m, 3 H).
Irradiation for Analytical Purposes. All irradiations out-
lined in Tables 1-3 were carried out in a Pyrex tube of 5.0 mL
capacity. To avoid ambiguity in relative yields due to oxidation,
the solution was degassed by subjecting the sample to a
minimum of three freeze-degas-thaw cycles at a pressure near
10-5 Torr before irradiation, and the tube was flame-sealed
under reduced pressure. Irradiation was carried out with filtered
light from the mercury lamp through a Pyrex filter and generally
continued until all the diazo compound was consumed. Product
identities were established either by GC or by GC-MS
comparisons with “authentic”samples separated as described
above, and the product distributions were conveniently deter-
mined by GC and/or NMR using an internal standard.
ESR Measurements. The diazo compound was dissolved in
2-methyltetrahydrofuran (10-3 M) and the solution was degassed
in a quartz cell by three freeze-degas-thaw cycles. The sample
was cooled in an optical transmission ESR cavity at 77 K and
Acknowledgment. The authors are grateful to the Ministry
of Education, Science and Culture of Japan for support of this
work through a Grant-in-Aid for Scientific Research for
Specially Promoted Research (Grant No. 12002007). We also
wish to thank Professor A. J. Kresge of the University of
Toronto and Professor T. Takui of Osaka City University for
helpful discussions.
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pp 239-352.

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