Laser flash photolysis study of phenylcarbene, o-tolylcarbene and mesitylcarbene

Article abstract of DOI:10.1002/(SICI)1099-1395(199704)10:4<207::AID-POC883>3.0.CO;2-8

Laser flash photolysis (LFP, XeCl, 308 nm, 20 ns) of phenyldiazomethane (PDM), o-tolydiazomethane (TDM) and mesitydiazomethane (MDM) produces phenylcarbene (PC), o-tolycarbene (TC) and mesitylcarbene (MSC), respectively. Transient spectra of PC and TC cou

Full text of DOI:10.1002/(SICI)1099-1395(199704)10:4<207::AID-POC883>3.0.CO;2-8

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 207–220 (1997)
LASER FLASH PHOTOLYSIS STUDY OF PHENYLCARBENE,
o-TOLYLCARBENE AND MESITYLCARBENE
ATNAF ADMASU,1 MATTHEW S. PLATZ,1* ANDRZEJ MARCINEK,1† JACEK MICHALAK1† ANNA DÓRA
GUDMUNDSDÓTTIR1 AND JERZY GEBICKI2
1 Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210, USA
2
|
Institute of Applied Radiation Chemistry, Technical University, Zwirki 36, 90-924 Lodz, Poland
Laser flash photolysis (LFP, XeCl, 308 nm, 20 ns) of phenyldiazomethane (PDM), o-tolydiazomethane (TDM) and
mesitydiazomethane (MDM) produces phenylcarbene (PC), o-tolycarbene (TC) and mesitylcarbene (MSC), respectively.
Transient spectra of PC and TC could not be detected in pentane at ambient temperature; however, these carbenes could
be trapped with pyridine to form UV–Vis-active ylides. The rate of formation of these ylides was resolved in CF2ClCFCl2
(Freon-113) and yields, after analysis, values of kpyr and ␶, where kpyr is the absolute rate constant of the reaction of the
spin equilibrated carbene with pyridine and ␶ is the carbene lifetime in Freon-113 in the absence of pyridine. LFP of
MDM produces the transient spectra of both triplet MSC and 3,5-dimethyl-1,2-benzoquinodimethane (3,5-BQM). The
lifetime of 3MSC in pentane is ca 500 ns. 3MSC reacts with pyridine with an apparent rate constant of
3
kpyr =1·35ϫ107 MϪ1 sϪ1. The decay of MSC in a variety of solvents does not lead to increased absorption of 3,5-BQM
in solution, nor does the presence of a carbene scavenger (methanol) lead to a reduction in yield of 3,5-dimethyl-
3
1,2-benzoquinodimethane upon LFP of precursor. Thus, the decay of MSC in solution does not lead to the formation
of quinodimethane. This species (3,5-BQM) is formed, instead, by a hydrogen atom transfer in the excited state of the
precursor. Product analysis reveals that the excited state rearrangement which forms xylylene is a minor process.
Photolysis of MDM in a nitrogen matrix at 10 K leads to the UV–Vis spectra of matrix-isolated triplet mesitylcarbene
and 3,5-BQM. The carbene rearranges slowly to 3,5-BQM in the cryogenic matrix, in contrast to its behavior in solution.
© 1997 by John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 10, 207–220 (1997) No. of Figures: 16 No. of Tables: 1 No. of References: 22
Keywords: laser flash photolysis; phenylcarbene; o-tolylcarbene; mesitylcarbene
Received 29 September 1994; revised 18 November 1996; accepted 6 December 1996
INTRODUCTION
not been reported. In this paper, we describe such studies
and their extension to o-tolylcarbene (TC) and mesityl-
carbene (MSC) (Scheme 1). The data reveal first that PC
and TC are very short-lived species and that MSC is, as
expected, a much longer lived carbene intermediate.
Second, the data demonstrate that intramolecular hydrogen
atom transfer reactions proceed in the excited state of
singlet mesityldiazomethane. This process is particularly
noticeable in methanol solvent but is still a relatively minor
process.
Phenylcarbene (PC, Scheme 1) is the prototypical aromatic
carbene. Matrix spectroscopy1 and ab initio theory2 have
convincingly demonstrated that the triplet state of PC is the
ground state. Nevertheless, most of the chemistry of PC
proceeds through the singlet state of the carbene.3 This has
usually been interpreted as a consequence of a small
(theory2; 4 kcal molϪ1; 1 kcal=4·184 kJ) free energy sepa-
ration (⌬GST ) between 1PC and 3PC, rapid spin equilibration
and much greater reactivity of the higher energy singlet
state.
RESULTS AND DISCUSSION
Despite extensive laser flash photolysis studies of aryl
and diarylcarbenes,4 kinetic studies of PC have heretofore
Laser flash photolysis (LFP) studies of PC
* Correspondence to: M. S. Platz.
† On leave from the Institute of Applied Radiation Chemistry,
Technical University, Zwirki 36, 90-924 Lodz, Poland.
Contract grant sponsor: US National Scheme Foundation; Con-
tract grant number: CHE-8814950.
LFP (308 nm) of phenyldiazomethane (PDM) in deoxygen-
ated pentane fails to produce a UV–Vis-active transient.
However, LFP of PDM under the same conditions in the
presence of pyridine produces an intense transient absorp-
tion (Figure 1) attributed to ylide 1 on the basis of its
similarity to other carbene pyridine ylides.5 It was possible
|
|
Contract grant sponsor: US–Polish M. Sklodowska-Curie Joint
Fund; Contract grant number: MEN/NSF-93-146.
© 1997 by John Wiley & Sons, Ltd.
CCC 0894–3230/97/040207–14 $17.50

208
LASER FLASH PHOTOLYSIS OF CARBENES
Scheme 1
to resolve the growth of ylide 1 in CF2ClCFCl2 (Freon-113).
The ylide is formed in an exponential process following the
laser pulse. The growth of the ylide was analyzed to yield an
observed pseudo-first-order rate constant, kobs , to ylide
formation. The size of kobs is linearly dependent on the
concentration of pyridine (Figure 2). The slope of this plot
was determined in three experiments and found to be
predicted by theory (4 kcal molϪ1 ) for PC in the gas
phase.2
The intercept of the plot of Figure 2 is 4·9ϫ106 sϪ1. The
reciprocal of this value is ␶ or 205 ns, the lifetime of spin
equilibrated PC in Freon-113.
It was also possible to resolve the growth of this ylide by
nanosecond spectroscopy in pentane in the presence of
1·9±0·3ϫ107
M
Ϫ1 sϪ1 and is the absolute rate constant of
dilute (0·1
pyridine is linear and yields after analysis
pyr =9·3ϫ106 Ϫ1 sϪ1 and ␶=74 ns.
The optical yield of ylide, Ay, was also measured at
M) pyridine. As before, a plot of kobs versus
the reaction of spin equilibrated phenylcarbene with pyr-
idine. If one assumes that only singlet phenylcarbene can
react with pyridine, then
k
M
460 nm as a function of pyridine concentration. In the
absence of pyridine Ay =0, but this quantity grows as the
concentration of pyridine increases (Figure 3) until [pyr-
k
pyr =1kpyr
where 1kpyr is the absolute rate constant of PC with pyridine.
A reasonable estimate4–6 of this quantity is 1ϫ109 Ϫ1 sϪ1
K
M
,
idine]≥1
M. Above 1
M
pyridine, in pentane, the ylide yield
which predicts that K, the singlet-triplet equilibrium con-
stant, is 0·019. Thus, one can calculate a free energy
separation of the singlet and triplet (⌬GST ) of 2·3 kcal
molϪ1 at 20 °C. This value is close to but smaller than that
is saturated (Ay ), implying that every molecule of phenyl-
carbene generated in the laser pulse is captured by pyridine
prior to reaction with solvent or that kpyr[pyr]ӷkRH [RH],
where 1kpyr and 1kRH are defined in Scheme 2.6
Figure 1. Transient spectrum of the phenylcarbene–pyridine ylide 1 in pentane at ambient temperature. The spectrum was recorded over a
period of 400 ns immediately following the laser pulse
© 1997 by John Wiley & Sons, Ltd.
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A. ADMASU ET AL.
The quantum yield of ylide formation is given by6
209
␾ckpyr[pyridine]K
(1kRH[RH]K+1kpyr[pyridine])K
␾y =
(1)
Figure 2. Plot of kobs of ylide 1 versus pyridine concentration in
Freon-113
Figure 3. Plot of Ay versus [pyridine] in pentane
Scheme 2
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210
LASER FLASH PHOTOLYSIS OF CARBENES
where ␾ is the quantum yield of carbene formation. The
c
optical yield of ylide is related to ␾ylide by the equation6
Ay =␾yAy
(2)
Combining and rearranging equations (1) and (2) leads
to6
1
1kRH[RH]K
Ay ␾cAy 1kpyr[pyridine]K ␾cAy
1
=
+
(3)
Thus, a plot of 1/Ay versus 1/[pyridine] is predicted and
found (Figure 4) to be linear. Division of the intercept by the
1
1
slope gives kpyrK/1kRH[RH]K=1·61. As kpyrK in pentane is
9·3ϫ106 Ϫ1 sϪ1, then ␶ in pentane can be estimated to be
M
173 ns for spin-equilibrated phenylcarbene or about twice
that obtained by direct analysis, which is the more accurate
procedure. This lifetime of spin-equilibrated PC is longer
than that determined for fluorenylidene7 in pentane but
comparable to that of 1- and 2-naphthylcarbene.8 As neat
Figure 5. Transient spectrum obtained by LFP of TDM in pentane
at ambient temperature. The spectrum was recorded over a window
of 400 ns, immediately after the laser flash
pentane is 8·7
M
Ϫ1 sϪ1
and K=0·019, then kRH can be estimated to
be 4·9ϫ108
M
.
o-Tolylcarbene
1
3
This treatment assumes that PC and PC are in rapid
equilibrium and can be treated as a single kinetic unit9 and
that all of the decay chemistry of PC under these conditions
proceeds through the low-lying singlet state.3 In pentane the
decay processes are expected to be CH insertion reactions
with the solvent.3, 9 This assumption is based on the many
reported product studies involving phenylcarbene and its
various simple derivatives.3, 9
o-Tolylcarbene was studied by LFP techniques similarly to
phenylcarbene. LFP of o-tolyldiazomethane (TDM) in
pentane fails to produce a UV–Vis-active transient (Figure
5). We find that there is no evidence for the formation of
1,2-benzoquinodimethane (BQM) upon laser flash photoly-
sis of TDM (Scheme 3).
The transient spectrum of ylide 2 is detected upon flash
photolysis of TDM in pentane in the presence of pyridine.
Analysis of ylide yield as a function of pyridine concentra-
tion yields, as before, the ratio kpyr /kRH[RH]=1·75. If kpyr of
o-tolylcarbene and phenylcarbene are similar then their
lifetimes and absolute rate constants of reaction with
pentane must also be comparable.
Mesitylcarbene
Isolation of mesitylcarbene in rare gas matrices
Short photolysis (six photolysis cycles, 10 s) of mesityl-
diazomethane (MDM) in rare gas matrices (argon or
nitrogen) at 11 K with the full arc of a 200 W high pressure
mercury lamp releases mesitylcarbene (MSC). These irra-
diation conditions result in efficient conversion of
mesityldiazomethane into triplet mesitylcarbene (major UV
bands at 256, 260, 301 and 315 nm). Photolysis of
mesityldiazomethane is evident by the rapid disappearance
of the characteristic N=N diazo band at 2060 cmϪ1. The
assignment of the spectrum to 3MSC is based on the
similarity of the shape and intensities of the UV–Vis
absorption bands observed here (Figure 6) with those
reported by McMahon and co-workers10 with triplet o-
tolylmethylene and of Tomioka et al.11 with didurylcarbene.
A small red shift of the mesitylcarbene bands relative to o-
tolylcarbene is observed in the spectrum. The strong band at
315 nm is significant because it may be monitored by time
resolved techniques at ambient temperature (see below). For
Figure 4. Double reciprocal treatment of the data of Figure 3;
slope=0·70, intercept=1·15
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A. ADMASU ET AL.
211
Scheme 3
aryl- and especially naphthylcarbenes it is difficult to detect
carbenes directly because their absorption maxima fall
underneath the absorption spectra of their precursors.4
Utilization of the full arc of the photolyzing light leads to
the formation of a dimethyl-substituted o-quinodimethane
(3, Scheme 4), which is seen clearly in the UV–Vis
spectrum in the region above 330 nm (␭max =370 nm). The
assignment of this broad absorption, which is composed of
several vibronic bands (336, 355, 367 and 385 nm), to a
methylated o-quinodimethane is supported by comparison
of this spectrum with that of o-xylylene described in the
literature.10
the initial matrix spectrum may be the result of secondary
photolysis of carbene. Photolysis of mesitylcarbene with a
bandpass interference filter (␭max =313 nm) leads to its
complete conversion to the o-quinodimethane. However,
photolysis of mesityldiazomethane with green light
(450–600 nm, ␭max =530 nm) is not very efficient (less than
5% of starting material was decomposed in 1 h), but it does
lead exclusively to the formation of carbene with no trace of
methyl-o-quinodimethane. The o-xylylene product
observed by McMahon and co-workers10 under similar
irradiation conditions with o-tolyldiazomethane is probably
a result of slow thermal rearrangements of the carbene
during long periods of irradiation of the precursor.
The presence of the absorption of an o-quinodimethane in
Figure 6. UV–Vis spectrum obtained after photolysis with unfiltered light of a high-pressure mercury lamp (200 W, six times 10 s) of
mesityldiazomethane in a nitrogen matrix
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212
LASER FLASH PHOTOLYSIS OF CARBENES
Scheme 4
A diagnostic test for the matrix isolation of carbenes is
trimethylcycloheptatetraene. McMahon and co-workers10
observed the formation of 1-methylcycloheptatetraene on
photolysis of o-tolydiazomethane, but only as a result of
secondary photolysis of o-tolylcarbene.
their trapping with carbon monoxide to form a ketene. The
ketene product is easily detected by IR spectroscopy and
provides chemical evidence for the presence of carbenes in
a matrix.10–12 As expected, photolysis of mesityldiazo-
methane in a CO-doped argon matrix does generate a
ketene.
Triplet mesitylcarbene is thermally unstable even at 11 K
and will isomerize to an o-quinodimethane. The thermal
conversion can be monitored by UV–Vis spectroscopy.
Figure 7 shows the decay of the carbene bands and the
In none of our experiments were we able to detect
Figure 7. UV–Vis difference spectrum recorded (A) 60 min after photolysis stopped and (B) 150 min after photolysis stopped subtracted from
one taken just after photolysis stopped showing the decrease in mesitylmethylene and concomitant increase in o-quinodimethane absorption
bands
© 1997 by John Wiley & Sons, Ltd.
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A. ADMASU ET AL.
simultaneous growth of an o-quinodimethane absorption
213
band. UV and IR spectroscopy demonstrate a clean
conversion of carbene to o-quinomethane. The kinetics of
this process are dispersive (dependent on time) and, if
analyzed in terms of a (time)1/2 dependence,13 they are in
agreement with data presented by McMahon and co-
workers10 (k=8ϫ10Ϫ6 sϪ1, nitrogen matrix). The thermal
isomerization of triplet mesitylcarbene to an o-quinodi-
methane at 11 K suggests there is a very small activation
barrier for this process or more likely a quantum mechanical
tunneling (QMT) mechanism of the hydrogen shift.14
LFP studies of MSC
LFP (308 nm) of a dilute solution of mesityldiazomethane
(Scheme 4) in deaerated pentane produces a complex
transient spectrum in a very narrow region (330–400 nm)
(see Figure 8). However, the triplet carbene absorption band
at 320 nm can be clearly discerned in this spectrum. The
position of this band agrees well with that observed in rare
gas matrices at cryogenic temperatures. The sharp strong
absorption at 320 nm decays with a lifetime of 580 ns in
pentane. This is in excellent agreement with the lifetime
value deduced from pyridine ylide 5 (Scheme 5) (see
below). Thus, the lifetime of spin-equilibrated MSC in
pentane is at least 10 times longer than that of spin-
equilibrated PC or TC under the same conditions. Steric
blockade of the carbene center retards, we presume, the rate
of reaction of the carbene with solvent in accord with the
design principles of Zimmerman and Paskovitch15 and of
Tomioka et al.11
3
The lifetime of MSC in hexafluorobenzene is independ-
ent of the concentration of the precursor used in our
experiments. Thus, the lifetime of 3MSC in hexafluoro-
benzene is not controlled by azine formation under the
conditions employed in LFP studies.
3
It is surprising that the lifetime of MSC is roughly the
same in pentane, Freon-113 and hexafluorobenzene. This
might lead one to conclude, despite the solvent isotope
effects, that ␶ of 3MSC is limited to some extent by reaction
of the carbene with an o-methyl group. However, the yield
of quinodimethane (A400 ) is the same in cyclohexane and
cyclohexane-d12 (Table 1). Thus in cyclohexane, MSC does
not partition between the reaction with solvent and the
formation of o-xylyene 3.
Thus, if spin-equilibrated MSC reacts with an o-methyl
group, the product must be a benzocyclobutene (8), which
will not absorb above 300 nm, rather than an o-xylylene.
However, GC–MS analysis provides evidence for the
The broad absorption bands observed between 320 and
400 nm are attibuted to o-xylylene derivative 3. The broad
spectrum is in good agreement with the matrix spectrum of
this intermediate and of parent 1,2-benzoquinodimethane
(BQM).
The lifetimes of 3MSC in Freon-113, hexafluorobenzene,
cyclohexane and cyclohexane-d12 are 480, 470, 253 and
550 ns, respectively. The kinetic isotope effect (KIE) on the
3
lifetime of MSC is 1·9 in cyclohexane–cyclohexane-d12 .
This KIE is very similar to that observed in hydrocarbon
solvents with the naphthylcarbenes,8 which do not decay in
solution by intramolecular processes, and indicates that the
3
decay of MSC proceeds in large measure by reaction with
the alkane solvent. Photolysis of MDM in cyclohexane
followed by GC–MS analysis and product isolation demon-
strates the clean formation of an adduct (6) with solvent in
53% isolated yield:
Small amounts of stilbene and amine-type products were
also observed as well. A product possibly resulting from
dimerization of quinodimethane 3 in cyclohexane was
observed by GC–MS analysis, but only in trace amounts.
3
The decay of MSC in alkane solvents is not accom-
Figure 8. Transient absorption spectrum produced upon LFP of
mesityldiazomethane in deaerated pentane at ambient temperature.
The spectrum was recorded immediately after the 308 nm laser
pulse over a window of 400 ns
panied by the formation of a benzylic-type radical 7
3
3
(Scheme 4). MSC, as per PC,3, 9 decays primarily through
singlet carbene reaction channels.
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214
LASER FLASH PHOTOLYSIS OF CARBENES
formation of 8 in cyclohexane in no more than trace
quantities. The lifetime data in Table 1 are consistent with
GC–MS analysis, which reveals that MSC forms adducts
with hexafluorobenzene and Freon-113, hence these sol-
vents are not inert to the carbene. In fact, the lifetimes of
spin-equilibrated phenylcarbene in Freon-113 and pentane
are not vastly different (205 and 74 ns, respectively), which
further indicates that the Freon is not an inert solvent. Hence
there are neither kinetic nor GC–MS data to indicate that
the lifetime of spin-equilibrated MSC is limited by its
reaction with an o-methyl group.
accompanied by any discernible increase in transient
absorption of quinodimethane 3 in any solvent tested
(alkanes, Freon-113, hexafluorobenzene). This is further
evidence that the spin-equilibrated mixture of mesitylcar-
bene is not the source of the 3,5-dimethyl-o-xylylene 3
produced in the laser pulse. The rate constant of rearrange-
ment must be 3kR1Ӷ2ϫ107 sϪ1 from 3MSC or
1kRK<2ϫ107 sϪ1 from MSC. Later we will deduce that
K 0·0135
and
⌬G2S9T8 ≈2·6 kcal molϪ1
,
hence
1kR <1·5ϫ109 sϪ1 (Scheme 5) at ambient temperature. If the
Arrhenius pre-exponential factor to hydrogen shift in the
carbene is у1012 sϪ1 then the activation barrier to re-
The yield of quinodimethane 3 is also the same in CH3CN
and CD3CN, demonstrating that the decay of MSC in this
solvent also is not accompanied by the formation of an o-
quinodimethane.
arrangement in the singlet state must exceed 3·9 kcal molϪ1
.
Thus, if the hydrogen shift proceeds through 1MSC, then the
barrier to rearrangement of 3MSC is at least 5–6 kcal molϪ1
.
3
In acetonitrile solvent, MSC decays by formation of ylide
9 (max=350 nm) in addition to C—H(D) insertion to give
10, as demonstrated by the solvent isotope effect on the
yield of 9. This behavior is analogous to that of the
naphthylcarbenes.8
The facile rearrangement of MSC at 11 K, despite these
barriers, is further evidence of a QMT mechanism of
hydrogen migration in the matrix.
Methanol is an excellent trap of singlet carbenes.4 Singlet
carbenes react rapidly with alcohols by formal O–H
insertion reactions to form ethers (e.g. 4, Scheme 4) at near
diffusion-controlled rates. Indeed, the anticipated ether
adduct is formed in 74% isolated yield along with small
amounts of stilbene and mesitylaldehyde.
The decay of carbene 3MSC (␭max =320 nm) is not
3
Table 1. The optical yield (A320 ) and lifetime of MSC, the optical
yield of quinodimethane (3, A400 ), and the optical yield of ylide 9
(A350 ) as a function of solvent
Upon LFP of MDM in methanol, the transient absorption
3
of MSC is not observed. However, the yield of o-xylylene
a
b
c
Solvent
A320
A400
A350
␶
320 (ns)c
3 actually increases (Figure 9, Table 1) in methanol, relative
to the less reactive hydrocarbon and Freon solvents. This is
our final piece of evidence that 3 is not produced from a
relaxed carbene intermediate.
We propose that 3 is produced by hydrogen migration in
the singlet excited state 1MDM* (Scheme 4) via inter-
mediate 11. Although 11 can be pictured as a biradical, it
can also be thought of as a zwitterion.
Freon-113
Hexafluorobenzene
Pentane
Cyclohexane
Cyclohexane-d12
Methanol
Acetonitrile
Acetonitrile-d3
0·3
0·06
0·02
0·03
0·05
480
470
550
253
550
0·06
0·21
0·26
0·30
0
0·04
d
0·072
0·045
0·044
—
0·12
0·20
The increased yield of 3 in methanol relative to pentane
1
1
indicates that the partitioning of MDM* to 3 and MSC
must be solvent dependent. This suggests that 11 may
indeed have zwitterionic character.
a
±0·005.
±0·005.
±20 ns.
b
c
d Too short to measure.
The migration of hydrogen probably does not proceed in
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A. ADMASU ET AL.
215
1
the triplet state of MDM (3MDM*) because the yield of 3 is
the excited state TDM* can exist in conformations where
hydrogen migration is inefficient, but that this is much less
likely to be true for 1MDM*.
not suppressed upon LFP of MDM in the presence of 1
isoprene.
M
The yield of quinodimethane, per laser pulse, from
mesityldiazomethane is much more than the statistical
factor of 2 relative to o-tolyldiazomethane. We presume that
GC–MS analyses of the products of photolysis of MDM
in methanol and in methanol containing norbornene, a trap
of putative o-xylylene 3, indicated that products derived
Scheme 5
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216
LASER FLASH PHOTOLYSIS OF CARBENES
from 3 are present in at most very low yields. This
demonstrates that even in methanol where hydrogen
migration is most prevalent, the absolute yield of this
process is still low.
The intercept of the plot of Figure 11 is 1·93ϫ106 sϪ1
,
which predicts that the lifetime of the ylide-forming
intermediate in pentane is ca 500 ns. This is further support
for the attribution of the sharp 320 nm band to 3MSC.
3
The reaction of MSC with pyridine to form a singlet-
state ylide is formally spin forbidden.
A classical
Trapping of MSC with pyridine
interpretation3 maintains that it is the low-lying singlet state
of the carbene which reacts with pyridine (Scheme 5).
In this case, kpyr =1kpyrK, where kpyr is the measured rate
constant and 1kpyr is the absolute rate constant of reaction of
LFP of mesityldiazomethane in the presence of pyridine
produces an intense transient absorption at 425 nm (Figure
10), which is attributed to carbene–pyridine ylide 5
(Scheme 4). This transient spectrum is similar to that of the
ylides formed from the reactions of phenylcarbene and o-
tolylcarbene with pyridine. The absolute rate of formation
of the pyridine is first order in the concentration of pyridine
1MSC with pyridine (Scheme 4). If 1kpyr =1ϫ109 Ϫ1 sϪ1, as
M
is typical of singlet carbenes,4 then K=0·0135 and
⌬G2S9T8 =2·6 kcal molϪ1. These values are very similar to
those derived previously for phenylcarbene and close to, but
smaller than, the prediction of theory (4 kcal molϪ1 )2 for the
latter species.
k
pyr =1·35ϫ107 Ϫ1 sϪ1 (Figure 11). This is very similar to
M
the value observed with phenylcarbene, which indicates that
pyridine is not effectively blockaded by the o-methyl
groups.
Griller et al.16 postulated that triplet carbenes can react
via surface-crossing mechanisms, effectively bypassing the
low-lying singlet state minimum. If this is indeed the
3
mechanism of reaction of MSC with pyridine then ⌬GST
must be ≥2·6 kcal molϪ1; the fact that the experimental
value of the singlet–triplet splitting of PC is smaller than the
theoretical estimate indicates that 3PC (and presumably
3MSC) can intersect the singlet plus pyridine surface at a
point lower in energy than the singlet minimum as
suggested by Griller et al.16
CONCLUSIONS
A thermal intramolecular 1,4-hydrogen shift leading to
rearrangement of triplet mesitylmethylene (3MSC) to singlet
o-quinodimethane occurs at 11 K probably by a tunneling
mechanism. The estimated activation barrier for this process
is fairly large, >3·9 kcal molϪ1 for a singlet carbene or
>6·5 kcal molϪ1 for the triplet carbene if a classical pre-
equilibrium mechanism is operative. These results are in
accord with previous studies by McMahon and co-work-
ers.10 Photolysis of mesityldiazomethane (MDM) in
solution at ambient temperature produces a singlet excited
Figure 9. Transient absorption spectrum produced upon LFP of
mesityldiazomethane in deaerated methanol at ambient tem-
perature. The spectrum was recorded immediately after the laser
pulse over a window of 400 ns
© 1997 by John Wiley & Sons, Ltd.
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A. ADMASU ET AL.
217
Hydrogen migration in the excited state of the diazo
compound is more significant in methanol than in hydro-
carbon solvents. Phenylcarbene and o-tolylcarbene have
much shorter lifetimes (␶=74 ns) in pentane than mesityl-
carbene, as expected from the work of Zimmerman and
Paskovitch15 and of Tomioka et al.11
EXPERIMENTAL
Diazo precursors were synthesized in the standard man-
ner9, 10 as illustrated for phenyldiazomethane and
mesityldiazomethane. Solvents and pyridine were purified
and dried by standard procedures.
The matrix isolation techniques used in this study have
been described elsewhere.17 The experiments were con-
ducted in two different matrices utilizing argon and
nitrogen. Thermal processes were monitored by UV–Vis
spectroscopy (Philips PU8710). The analyzing light was cut
off between scans to avoid any photochemical processes.
Laser flash photolysis experiments utilized a Lambda
Physik EMG 101 excimer laser emitting 20 ns pulses of
308 nm light (150 mJ per pulse) and have been described
previously.18 The absorbance of the samples were kept at
0·3–0·5 at 308 nm. An EG&G Princeton Applied Research
Model 1460 optical multichannel analyzer (OMA) recorded
all transient absorption spectra. When necessary, oxygen
was removed by bubbling argon through the sample for
3 min.
Figure 10. Transient absorption spectrum produced upon LFP of
MDM in deaerated pentane containing pyridine at ambient
temperature. The spectrum was recorded during a window of
400 ns immediately after the laser flash
state (1MDM*), which forms a biradical or zwitterionic
intermediate that partitions between nitrogen extrusion and
carbene formation, and 1,4-hydrogen migration to form
directly 3,5-dimethyl-1,2-benzoquinodimethane. The life-
time of 3MSC in pentane solution at ambient temperature is
ca 500 ns and is controlled by intermolecular process.
Benzaldehyde tosylhydrazone. A 250 ml round-bot-
tomed flask containing a magnetic stirring bar was charged
with benzaldehyde (8·0 g, 0·075 mol) and 100 ml of abso-
lute ethanol. To this solution was added 15·3 g (0·082 mol)
of p-toluensulfonhydrazide. The flask was fitted with a
condenser and the mixture was refluxed for 5 h under an
atmosphere of nitrogen. The solution was cooled in an ice-
bath and the white crystalline solid was filtered and washed
with cold absolute ethanol. The filtrate was further concen-
trated and cooled in an ice-bath to give a second crop of
crystals. The solid was filtered and washed with cold
absolute ethanol. The combined crude product was recrys-
tallized from hot absolute ethanol to give 18·5 g (90%) of
the white crystalline tosylhydrazone. The melting point of
the product was 129–131 °C. 1H NMR (DMSO-d6 ), ␦: 2·33
(s, 3H, CH3 ), 7·35–7·75 (m, 9H, aromatic-H), 7·81 (s, 1H,
—CH=N—), 11·95 (s, 1H, NH).
Sodium salt of benzaldehyde tosylhydrazone. In a
100 ml Erlenmeyer flask containing a magnetic stirrer was
dissolved 2·5 g (8·45 mol) of benzaldehyde tosylhydrazone
in 75 ml of freshly distilled diethyl ether. The solution was
cooled in ice-bath and 1·274 g (6·86 mol) of NaH (60% in
mineral oil) was added with stirring. The solution was
stirred at room temperature for 24 h under an atmosphere of
nitrogen. The solid product which was formed, was filtered
and washed with dry diethyl ether. The solid was dried
completely under vacuum at room temperature. The yield of
Figure 11. Observed pseudo-first-order rate constant kobs of forma-
tion of pyridine ylide 5 in pentane as a function of pyridine
concentration
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218
LASER FLASH PHOTOLYSIS OF CARBENES
the dry colorless sodium salt was 2·7 g (80%).
which contained the sodium salt of mesitaldehyde tosylhy-
drazone. A receiving flask which was placed in an isopropyl
alcohol–dry-ice bath was connected to the distillation
apparatus. The two-necked, round-bottomed flask was
heated to 110 °C under vacuum (0·01 Torr) and the sodium
salt added slowly under stirring. The red liquid that formed
was collected to yield mesityl diazomethane. IR (NaCl
Phenyldiazomethane. In a 25 ml round-bottomed flask
equipped with a magnetic stirring bar and a short-path
distillation condenser was placed 0·8 g (2·70 mol) of the
sodium salt of benzaldehyde tosylhydrazone. A receiving
flask was cooled in a dry ice–isopropyl alcohol bath. The
salt was heated with stirring under vacuum (ca 0·5 mmHg)
in an 80 °C oil-bath for 45 min. A colored gas was produced
and collected as an orange solid in the receiving flask. The
oil bath temperature was gradually raised to 120 °C to
ensure that the salt was completely decomposed. The
isolated product was quickly weighed (0·12 g, 62%) and
dissolved in 100 ml of pentane. The flask containing the
pentane solution was wrapped with aluminum foil to protect
1
plates, CHCl3 ); 2058 and 1610 cmϪ1. H NMR (CDCl3 ,
200 MHz), ␦: 6·83 (s, 2H, Ar—H), 4·75 (s, 1H, N2C—H),
2·24 (s, 3H, CH3 ), 2·22 (s, 6H, CH3 ) ppm.
Preparative photolysis of MDM in cyclohexane. MDM
(55 mg, 0·34 mmol) was dissolved in cyclohexane (4 ml)
and argon was bubbled through the solution, after which it
was sealed. This solution was irradiated in a standard
Rayonet apparatus equipped with 300 nm light bulbs at
4 °C. GC Analysis of the reaction mixture indicated
formation of one major compound, which was shown to be
the carbene–cyclohexane insertion product, cyclohexyl-
mesitylmethane. The solvent was removed under vacuum to
yield a colorless oil, which was subjected to column
chromatography on silica gel with hexane–ethyl acetate as
eluent. The first fraction contained cyclohexylmesityl-
methane (36 mg, 0·18 mmol, 53% yield). The second and
third fractions contained trans-2,2Ј, 4,4Ј, 6,6Ј-hexamethyl-
stilbene (14 mg, 0·053 mmol, 32% yield) and
2,4,6-trimethylbenzalazine (3 mg, 0·01 mmol, 6% yield),
respectively. These three fractions accounted for ca 90% of
the mass balance. The trans-2,2Ј,4,4Ј,6,6Ј-hexamethyl-
1
it from light and stored in a freezer. H NMR (CDCl3 ), ␦:
4·88 (s, 1H, —CH=N2 ), 7·41–7·8 (m, 5H, aromatic-H)
ppm.
Mesitaldehyde tosylhydrazone. A solution of mesitalde-
hyde (0·9 g, 6·1 mmol) (Aldrich) and p-tosylhydrazide
(1·13 g, 6·1 mmol) (Aldrich) in ethanol (25 ml) was
refluxed under argon for 1 h. The solution was cooled to
room temperature and the crystals that formed were filtered
and recrystallized from ethanol to give mesitaldehyde
tosylhydrazone in 90% yield (1·7 g, 5·5 mmol), m.p.
156–158 °C, IR (KBr): 3203, 1597, 1326 and 1164 cmϪ1
.
1H NMR (CDCl3 , 200 MHz), ␦: 8·02 (s, 0·4ϫ1H, NC—H),
7·82 (d, 8Hz, 0·4ϫ2H, tosyl Ar—H), 7·75 (d, 8Hz,
0·6ϫ2H, tosyl Ar—H), 7·76 (s, 0·4ϫ1H, N—H), 7·60 (s,
0·6ϫ1H, NC—H), 7·50 (s, 0·6ϫ1H, N—H), 7·24 (d, 8 Hz,
0·6ϫ2H, tosyl Ar—H), 7·23 (d, 8 Hz, 0·4ϫ2H, tosyl Ar—
H), 6·80 (s, 0·6ϫ2H, mesityl Ar—H) 6·75 (s, 0·4ϫ2H,
mesityl Ar—H), 2·40 (s, 0·6ϫ3H, CH3 ), 2·38 (s, 0·4ϫ3H,
CH3 ) 2·30 (s, 0·4ϫ3H+0·6ϫ6H, CH3 ), 2·28 (s, 0·4ϫ3H,
CH3 ), 2·05 (s, 0·6ϫ6H, CH3 ) ppm. MS: m/z 316 (M+ , 22),
161 (100), 132 (82), 91 (65).
1
stilbene was characterized by comparison of its H NMR
spectrum with that reported in the literature,19 whereas the
spectra of cyclohexylmesitylmethane and 2,4,6-trimethyl-
benzalazine were compared with those of the analogues
compound cyclohexylbenzenemethane.20
Cyclohexylmesityl methane:21 IR (neat): 2922, 2850
(C—H), 1613 (C=C) cmϪ1. 1H NMR (CDCl3 , 200 MHz), ␦:
6·84 (s, 2H, AR—H), 2·50 (d, 6Hz, 2H), 2·29 (s, 6H,
o-CH3 ), 2·26 (s, 3H, o-CH3 ), 1·7–1·6 (m, 6H), 1·2–1·1
(m, 5H) ppm. 13C NMR (CDCl3 , 75 MHz), ␦: 136·6, 134·6,
128·8, 39·1, 36·8, 33·7, 26·6, 26·5, 20·8 (C—H, p-CH3 ),
20·4 (C—H, o-CH3 ) ppm. GC—MS: m/z 216 (M+ ), 133.
trans-2,2Ј, 4,4Ј, 6,6Ј-Hexamethyl stilbene:20 1H NMR
(CDCl3 , 200 MHz), ␦: 6·77 (s, 4H, Ar—H), 6·66 (s, 2H,
vinyl H), 2·16 (s, 6H, p-CH3 ), 1·98 (s, 12H, o-CH3 ) ppm.
GC—MS: m/z 264 (M+ ), 250, 234.
Sodium salt of mesitaldehyde tosylhydrazone. To a
solution of mesitaldehyde tosylhydrazone in THF under
argon at Ϫ10 °C was added NaH (60% in mineral oil). The
resulting solution was warmed to room temperature and the
white precipitate that formed was filtered and dried under
vacuum to afforded sodium salt of mesitaldehyde tosylhy-
drazone in 80% yield (1·71 g, 5·1 mmol), m.p. 70 °C
(decomp.). IR (KBr): 1250, 1221, 1140, 1094 and
2,4,6-Trimethylbenzalazine:21 1H NMR (CDCl3 ,
200 MHz), ␦: 8·98 (s, 2H, N=C—H), 6·92 (s, 2H, Ar—H),
2·51 (s, 6H, o-CH3 ), 2·30 (s, 3H, p-CH3 ) ppm. GC–MS: m/z
292 (M+ ), 277, 260, 145, 131.
GC–MS analyses of reaction mixture photolyses of
MDM in cyclohexane showed trace amounts of masses 132
and 266 in addition to the three products isolated from the
reaction mixture. It can be speculated that the compound
with an m/z of 132 is benzocyclobutane 8, (2,4-dimethyl
bicyclo[4.2.0]octa-1,3,5-triene) formed from intramolecular
[2+2] dimerization of o-xylylene 3. The compound with an
m/z of 266 can be assigned to dimesitylethane, which can be
1049 cmϪ1 1H NMR (DMSO-d3 , 200 MHz), ␦: 7·94 (s,
.
0·7ϫ1H, NC—H), 7·65 (d, 8Hz, 0·7ϫ2H, tosyl Ar—H),
7·45 (d, 8Hz, 0·3ϫ2H, tosyl Ar—H), 7·22 (d, 8Hz, 2H,
tosyl Ar—H), 6·94 (s, 0·3ϫ2H, tosyl Ar—H), 6·81 (s,
0·7ϫ2H, NC—H), 5·42 (s, 0·3ϫ1H, NC—H), 2·37 (s, 3H,
CH3 ), 2·30 (s, 6H, CH3 ), 2·26 (s, 0·3ϫ3H, CH3 ) and 2·25
(s, 0·7ϫ3H, CH3 ) ppm.
Mesityldiazomethane. A two-necked round-bottomed
flask equipped with a magnetic stirrer was connected to a
short-path distillation apparatus and an additional flask
© 1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 207–220 (1997)

A. ADMASU ET AL.
219
formed from triplet MDM.
ACKNOWLEDGEMENTS
GC–MS analyses of photolyses mixture of MDM in
cyclohexane–2-norbornene mixture showed three addi-
tional peaks, all having mass 226. Two of these peaks had
identical fragmentation patterns whereas the third peak had
a different fragmentation pattern. It can be suggested that
one of these compounds originates from trapping of MSC
and the other from Diels–Alder reaction between o-
xylylene 3 and 2-norbornene. To confirm this idea further,
product studies of MDM in methanol were undertaken.
This work was supported through grants No. CHE-8814950
from the US National Science Foundation and No. MEN/
NSF-93-146 from the US–Polish M. Sklodowska-Curie
Joint Fund. Anna Dóra Gudmundsdóttir’s work was sup-
ported through a NATO postdoctoral fellowship.
|
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Preparative photolysis of MDM in methanol. MDM
(54 mg, mmol) was photolyzed in methanol (5 ml) under
the same conditions as described above. The solvent was
removed under vacuum to yield an oil. 1H NMR analysis of
the reaction mixture showed that four products were
formed: 74% of the methanol insertion product, methyl
methylmesitylene
ether,22
10%
of
trans-
2,2Ј,4,4Ј,6,6Ј-hexamethylstilbene,19 12% of mesitaldehyde
and 7% of cis-2,2Ј,4,4Ј,6,6Ј-hexamethylstilbene.19 The
products were characterized by comparison of the 1H NMR
spectra with those reported in the literature.
Methyl methylmesitylene ether:22 1H NMR (CDCl3 ,
200 MHz), ␦: 6·93 (s, 2H, Ar—H), 4·48 (s, 2H, Ar—CH2—
OMe), 3·41 (s, —OCH3 ), 2·38 (s, 6H, o-CH3 ), 2·28 (s, 3H,
p-CH3 ) ppm. GC–MS: m/z 292 (M+ ), 277, 260, 145, 131.
cis-2,2Ј,4,4Ј,6,6Ј-Hexamethylstilbene:19
1H
NMR
(CDCl3 , 200 MHz), ␦: 6·93 (s, 4H, Ar—H), 6·55 (s, 2H,
vinyl-H), 2·38 (s, 12H, o-CH3 ), 2·30 (s, 6H, p-CH3 ) ppm.
GC–MS:
m/z 264 (M+ ), 250, 234.
GX–MS analysis of the photolysis mixture of MDM in
methanol identified (in addition to the four products
analyzed from the NMR spectra) trace amounts of com-
pounds with m/z 132 and 292. These masses can be
assigned to benzocyclobutane
8
(2,4-dimethylbi-
2,4,6-trimethyl-
cyclo[4.2.0]octa-1,3,5-triene)
benzalazine, respectively.
and
GC–MS analysis of the photolysis mixture of MDM in
cyclohexane containing 2-norbornene indicated that only
one of the three mass 226 products observed in cyclohexane
was obtained in methanol and it can therefore be assigned to
the Diels–Alder product formed between o-xylylene 3 and
2-norbornene.
Conclusion. About 90% of the mass balance from
photolyses in cyclohexane comes from carbene reactivity.
1H NMR analysis of the photoreaction mixture from
methanol identifies only those products that come from the
carbene intermediate. It can therefore be concluded that less
than 10% of the reaction goes through the excited state
reaction.
Trapping with 2-norbornene gives one additional product
in methanol and three in cyclohexane, which indicates that
in methanol o-xylylene 3 has trapped with 2-norbornene
whereas in cyclohexane both o-xylylene 3 and MDM were
trapped with 2-norbornene.
16. D. Griller, A. S. Nazran and J. C. Scaiano, J. Am. Chem. Soc.
106, 198 (1984).
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LASER FLASH PHOTOLYSIS OF CARBENES
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JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 207–220 (1997)