Ultrafast study of p-biphenylyldiazoethane. The chemistry of the diazo excited state and the relaxed carbene

Article abstract of DOI:10.1021/ja067213u

Ultrafast photolysis of p-blphenylyldiazoethane (BDE) produces an excited state of the diazo compound in acetonitrile, cyclohexane, and methanol with λ_max = 490 nm and lifetimes of less than 300 fs. The decay of the diazo excited state correlates with the growth of singlet carbene absorption at 360 nm. The optical yields of diazo excited states produced by photolysis of p-biphenylyldiazomethane (BDM) and BDE are the same; however, the optical yield of singlet p-biphenylylmethylcarbene (¹BpCMe) is 30-40% less than that of p-biphenylylcarbene (¹BpCH) in all three solvents. The results are explained by rearrangement in the excited state (RIES) of BDE to form p-vinylbiphenyl (VB) in parallel with extrusion of nitrogen to form ¹BpCMe in reduced yield. This interpretation is consistent with product studies (ethanol-OD in cyclohexane) which indicate that there is an ～25% yield of VB that is formed by a mechanism that bypasses the relaxed singlet carbene. The decay of ¹BpCMe is biexponential, and that of ¹BpCH is monoexponential. This is attributed either to efficient relaxation of vibrationally excited ¹BpCMe by 1,2 migration of hydrogen to form VB (minor) or to the increased number of low-frequency vibrational modes provided by the methyl group (major). A methyl group retards the rate of intersystem crossing (ISC), relative to a hydrogen atom, and ISC is more rapid in nonpolar solvents. Reaction of ¹BpCMe with methanol is much faster than spin equilibration. Both the lifetime of ¹BpCMe and ¹BpCH are the same in cyclohexane and in cyclohexane-d₁₂. This demonstrates that spin equilibration is faster than reaction of either carbene with the solvent. The lifetimes of ¹BpCMe and ¹BpCMe-d₃ are the same in cyclohexane. This indicates that 1,2 hydrogen migration of ¹BpCMe to form VB is slower than spin equilibration in cyclohexane. In acetonitrile, however, the lifetime of ¹BpCMe-d₃ is 1.5 times longer than that of ¹BpCMe in the same solvent. Thus, in acetonitrile, where ISC is slow, the rate of 1,2 hydrogen shift of ¹BpCMe is competitive with ISC. In cyclohexene, the lifetime of ¹BpCH is shortened relative to that in cyclohexane. The lifetime of ¹BpCMe is the same in cyclohexene and cyclohexane. The data indicate that spin relaxation is slow relative to reaction of ¹BpCH with neat alkene but that spin relaxation is fast for ¹BpCMe relative to reaction with neat cyclohexene.

Full text of DOI:10.1021/ja067213u

Published on Web 02/10/2007
Ultrafast Study of p-Biphenylyldiazoethane. The Chemistry of
the Diazo Excited State and the Relaxed Carbene
Jin Wang, Gotard Burdzinski, Terry L. Gustafson, and Matthew S. Platz*
Contribution from the Department of Chemistry, The Ohio State UniVersity, 100 West 18th
AVenue, Columbus, Ohio 43210 and Quantum Electronics Laboratory, Faculty of Physics,
Adam Mickiewicz UniVersity, 85 Umultowska, Poznan 61-614, Poland
Received October 8, 2006; E-mail: platz.1@osu.edu
Abstract: Ultrafast photolysis of p-biphenylyldiazoethane (BDE) produces an excited state of the diazo
compound in acetonitrile, cyclohexane, and methanol with λmax ) 490 nm and lifetimes of less than 300 fs.
The decay of the diazo excited state correlates with the growth of singlet carbene absorption at 360 nm.
The optical yields of diazo excited states produced by photolysis of p-biphenylyldiazomethane (BDM) and
1
BDE are the same; however, the optical yield of singlet p-biphenylylmethylcarbene ( BpCMe) is 30-40%
1
less than that of p-biphenylylcarbene ( BpCH) in all three solvents. The results are explained by
rearrangement in the excited state (RIES) of BDE to form p-vinylbiphenyl (VB) in parallel with extrusion of
1
nitrogen to form BpCMe in reduced yield. This interpretation is consistent with product studies (ethanol-
OD in cyclohexane) which indicate that there is an ∼25% yield of VB that is formed by a mechanism that
bypasses the relaxed singlet carbene. The decay of 1BpCMe is biexponential, and that of BpCH is
1
1
monoexponential. This is attributed either to efficient relaxation of vibrationally excited BpCMe by 1,2
migration of hydrogen to form VB (minor) or to the increased number of low-frequency vibrational modes
provided by the methyl group (major). A methyl group retards the rate of intersystem crossing (ISC), relative
1
to a hydrogen atom, and ISC is more rapid in nonpolar solvents. Reaction of BpCMe with methanol is
1
1
much faster than spin equilibration. Both the lifetime of BpCMe and BpCH are the same in cyclohexane
and in cyclohexane-d12. This demonstrates that spin equilibration is faster than reaction of either carbene
1
1
with the solvent. The lifetimes of BpCMe and BpCMe-d
3
are the same in cyclohexane. This indicates that
1
1
,2 hydrogen migration of BpCMe to form VB is slower than spin equilibration in cyclohexane. In acetonitrile,
however, the lifetime of BpCMe-d
acetonitrile, where ISC is slow, the rate of 1,2 hydrogen shift of BpCMe is competitive with ISC. In
cyclohexene, the lifetime of BpCH is shortened relative to that in cyclohexane. The lifetime of BpCMe is
the same in cyclohexene and cyclohexane. The data indicate that spin relaxation is slow relative to reaction
of BpCH with neat alkene but that spin relaxation is fast for 1BpCMe relative to reaction with neat
cyclohexene.
1
1
3
is 1.5 times longer than that of BpCMe in the same solvent. Thus, in
1
1
1
1
I. Introduction
Scheme 1
Fifty years of study of carbenes by chemical, physical, and
computational methods have produced a sturdy structural and
mechanistic framework to organize the data and understand this
class of reactive intermediates.1-3 As shown in Scheme 1, singlet
carbenes can be generated by activation of a precursor (typically
a diazirine or diazo compound). The nascent singlet carbene
can either rearrange (as in a 1,2 hydrogen shift to form an
alkene), react with an external trap (such as methanol to form
an ether, E), or, in the case of aryl- and alkylarylcarbenes, relax
(
(
(
1) (a) Bucher, G.; Scaiano, J. C.; Platz, M. S. Landolt-Bornsrtein, Group II,
Vol 18, SubVolume, E2; Springer: Berlin, Germany, 1998; p 141. (b) Platz,
M. S.; Maloney, V. M. In Kinetics & Spectroscopy of Carbenes &
Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; p 239.
2) (a) Closs, G. L. In Carbenes Vol. II; Moss, R. A., Jones, M. J., Eds.;
Wiley: New York, 1973; p 159. (b) Trozzolo, A. M.; Wasserman, E. In
Carbenes Vol. II; Moss, R. A., Jones, M. J., Eds.; Wiley, New York, 1973;
p 185.
to the lower energy triplet carbene, which undergoes distinctly
1
-3
different reactions than the singlet carbene.
This simple
mechanistic scheme accommodates much of the known chem-
istry of carbenes, but exceptions are known. Alkylcarbenes
undergo numerous rearrangements to form a spectrum of
3) (a) Kirmse, W. Carbene Chemistry; Academic Press: New York, 1964.
(b) Carbenes Vol. I; Moss, R. A., Jones, M. J., Eds.; Academic Press: New
3
York, 1973.
products. Scheme 1 predicts that the same product distribution
10.1021/ja067213u CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 2597-2606
9
2597

A R T I C L E S
Scheme 2
Wang et al.
Scheme 3
4
). One can explain the experimental observations by positing
will be observed when the same precursor is activated in
different ways (pyrolysis, photolysis, sensitized photolysis )
4
,5
that electronically excited singlet carbenes are formed upon
photolysis of precursor and can rearrange to form a different
mixture of products than do relaxed singlet carbenes, and they
may rearrange faster than they can be chemically intercepted.4
The Eisenthal group13 has demonstrated that photolysis of
diphenyldiazomethane with 266 nm light does produce diphe-
nylcarbene in an electronically excited triplet state, which was
detected by fluorescence spectroscopy. It was posited that the
excited triplet carbene was formed by relaxation of singlet
excited diphenylcarbene initially produced by mono-photonic
excitation of the diazo precursor. The electronically excited
singlet state of diphenylcarbene was not observed in this study.
or when different precursors (diazirine, diazo compound, other
non-nitrogenous precursors6-8) are activated in the same way.
8
Often these predictions are not realized, pointing to inadequa-
cies in the simple mechanistic framework. Scheme 1 predicts
that the ratio of intermolecular/intramolecular singlet carbene-
derived products will be a linear function of the concentration
of trapping agent. This test is also known to fail at times because,
even in the presence of neat trapping agent, “too much”
14
rearrangement product is produced.6
-8
These observations can be explained by positing that there
is a “non-trappable carbene” route to the formation of rear-
8
rangement products. It has been proposed that rearrangements
The CdN double bond energy in diazomethane slightly
10
in the excited state (RIES) of the precursor can compete with
carbene formation (Scheme 2).7 In this mechanism, the excited
state of the precursor partitions between the formation of
trappable carbene and suffers a carbene-mimetic 1,2 hydrogen
migration in concert with nitrogen extrusion. The latter process
cannot be suppressed even by the presence of a neat trap, and
too much intramolecular reaction product will form relative to
intermolecular product (according to Scheme 1). Furthermore,
the rearrangement in the excited state of the precursor will give
a different mixture of rearrangement products than the relaxed
singlet carbene and thereby explain the experimental results.
Modern theory has not provided support for the RIES
mechanism to date and, in fact, argues against this mechanism
exceeds 30.6 kcal/mol. The S state of diazomethane is 67.5-
71.5 kcal/mol above the ground state. Thus, when the S state
of diazomethane decomposes to form the lowest electronic
carbene singlet state and molecular nitrogen, these fragments
can be born with 37-41 kcal/mol of excess energy. Decomposi-
tion of the S state of diazomethane produces singlet methylene
with even more excess vibrational energy. Thus, one can explain
the product study data by positing that vibrationally excited
singlet carbenes (carbene ) are formed upon photolysis of
1
,9
12
1
2
#
precursor and rearrange to a unique mixture of products at a
rate competitive with relaxation to the thermally equilibrated
carbene (Scheme 5).
8
Recently, we have reported the study of p-biphenylyldiaz-
10-12
for simple diazirines,
but it does predict that the S1,
15
omethane (BDM) using ultrafast spectroscopic techniques.
electronically excited singlet state of diazomethane can fragment
to form methylene in its lowest energy singlet state (major) and,
in a minor process, decompose to form methylene in an
electronically singlet excited state (carbene*, Schemes 3 and
Ultrafast spectroscopy provides a tool for the study of diazo
excited states and of the formation and relaxation of electroni-
cally and vibrationally excited singlet carbenes. This technology
seems well suited for the study of the “non-trappable carbene”
route to reaction products, thereby motivating this study of the
photochemistry of p-biphenylyldiazoethane (BDE) and the
chemistry of p-biphenylylmethylcarbene (BpCMe). Evidence
will be presented in favor of the RIES mechanism. The question
(
4) Chang, K. T.; Shechter, H. J. Am. Chem. Soc. 1979, 101, 5082.
5) (a) Frey, H. M. AdV. Photochem. 1966, 4, 225. (b) Frey, H. M.; Stevens,
I. D. R. J. Chem. Soc. 1965, 3101. (c) Friedman, L.; Shechter, H. J. Am.
Chem. Soc. 1959, 81, 5512.
6) Huang, H.; Platz, M. S. J. Am. Chem. Soc. 1998, 120, 5990.
7) Nigam, M.; Platz, M. S.; Showalter, B. M.; Toscano, J. P.; Johnson, R.;
Abbot, S. C.; Kirchhoff, M. M. J. Am. Chem. Soc. 1998, 120, 8055.
8) Platz, M. S. AdV. Carbene Chem. 1998, 2, 133.
9) (a) Bonneau, R.; Liu, M. T. H.; Kim, K. C.; Goodman, J. L. J. Am. Chem.
Soc. 1996, 118, 3829. (b) Chidester, W.; Modarelli, D. A.; White, W. R.;
Whitt, D. E.; Platz, M. S. J. Phys. Org. Chem. 1994, 7, 24. (c) Motschiedler,
K.; Gudmundsdottir, A.; Toscano, J. P.; Platz, M.; Garcia-Garibay, M. A.
J. Org. Chem. 1999, 64, 5139. (d) Platz, M. S.; Huang, H. Y.; Ford, F.;
Toscano, J. Pure Appl. Chem. 1997, 69, 803.
(
(
(
#
of whether or not vibrationally excited carbene (BpCMe )
undergoes 1,2 hydrogen migration to form p-vinylbiphenyl (VB)
(
(
(13) (a) Dupuy, C.; Korenowski, G. M.; McAuliffe, M.; Hetherington, W. M.,
III; Eisenthal, K. B. Chem. Phys. Lett. 1981, 77, 272. (b) Langan, J. G.;
Sitzmann, E. V.; Eisenthal, K. B. Chem. Phys. Lett. 1986, 124, 59. (c)
Wang, Y.; Sitzmann, E. V.; Novak, F.; Dupuy, C.; Eisenthal, K. B. J. Am.
Chem. Soc. 1982, 104, 3238.
(14) (a) Trozzolo, A. M.; Gibbons, W. A. J. Am. Chem. Soc. 1967, 89, 239. (b)
Ono, Y.; Ware, W. R. J. Phys. Chem. 1983, 87, 4426. (c) Horn, K. A.;
Allison, B. D. Chem. Phys. Lett. 1985, 116, 114. (d) Johnston, L. J.; Scaiano,
J. C. Chem. Phys. Lett. 1985, 116, 109.
(15) Wang, J.; Burdzinski, G.; Gustafson, T. L.; Platz, M. S. J. Org. Chem.
2006, 71, 6221.
(
10) (a) Arenas, J. F.; Lopez-Tocon, I.; Otero, J. C.; Soto, J. J. Am. Chem. Soc.
2
002, 124, 1728. (b) Yamamoto, N.; Bernardi, F.; Bottoni, A.; Olivucci,
M.; Robb, M. A.; Wilsey, S. J. Am. Chem. Soc. 1994, 116, 2064.
(
11) Bernardi, F.; Olivucci, M.; Robb, M. A.; Vreven, T.; Soto, J. J. Org. Chem.
2
000, 65, 7847.
(
12) Bigot, B.; Ponec, R.; Sevin, A.; Devaquet, A. J. Am. Chem. Soc. 1978,
00, 6575.
1
2598 J. AM. CHEM. SOC.
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VOL. 129, NO. 9, 2007

Ultrafast Study of p-Biphenylyldiazoethane
A R T I C L E S
Figure 1. Transient spectra of p-biphenylyldiazomethane (left) and p-biphenylyldiazoethane (right) in acetonitrile. Spectra were generated by ultrafast LFP
311 nm) with a time window of 0.20-1.00 ps.
(
Scheme 4
Scheme 5
remains open. We have not detected an electronically excited
singlet state of the carbene ( BpCMe*) in this study.
with BDM. The excited state of p-biphenylyldioazoethane
(BDE*) has a broad transient spectrum centered at 480 nm, and
it decays with a time constant of less than 300 fs. As the excited
state of the diazo precursor decays, transient absorption grows
at 360 nm, which can be attributed to the singlet carbene,
1
II. Ultrafast Spectroscopic Results
Our prior ultrafast experiments with BDM15 were conducted
in acetonitrile, methanol, and cyclohexane. The same solvents
will be used in the study of BDE in addition to cyclohexene to
learn whether singlet carbenes react with alkene solvent at a
rate faster or slower than intersystem crossing to their lower
energy triplet spin isomer. Biphenyl compounds were used
instead of phenyldiazomethanes because the extra phenyl group
shifts the absorption spectrum of transient species to longer
1
1
BpCMe. The absorption spectrum of BpCMe is in excellent
agreement with the predictions of time-dependent density
functional theory (TD-DFT) calculations (Supporting Informa-
16
tion (SI), Table S1) and the experimental and predicted spectra
1
5 1
of singlet p-biphenylylcarbene ( BpCH).
There are several important differences, however, in the BDM
and BDE data. It is interesting to note that the optical yields of
diazo excited states are about the same (BDM* versus BDE*)
1
5
wavelengths and facilitates the detection of carbenes.
II.1. Acetonitrile. Ultrafast photolysis (311 nm) of BDE in
acetonitrile produces the transient spectra shown in Figure 1.
As expected, the spectral data greatly resemble those obtained
(
16) (a) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem.
Phys. 1998, 108, 4439. (b) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J.
J. Chem. Phys. 1998, 109, 8218.
J. AM. CHEM. SOC.
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VOL. 129, NO. 9, 2007 2599

A R T I C L E S
Wang et al.
1
1
1
Table 1. Optical Yields of Singlet Carbenes BpCH, BpCMe, and BpCMe-d3 in Acetonitrile, Cyclohexane, and Methanol, Produced with
Ultrafast LFP (311 nm)
a
solvent
acetonitrile
BpCMe
cyclohexane
BpCMe
methanol
BpCMe
0.013
BpCH
BpCMe-d
3
BpCH
BpCMe-d
3
BpCH
BpCMe-d
3
optical yieldb
0.023
0.015
0.012
0.025
0.014
0.012
0.017
0.010
a
All the samples were argon-degassed for 5 min prior to experiments. b Optical yields are the highest intensities of the singlet carbene bands, measured
1
ps after the laser pulse.
Figure 2. Ultrafast LFP (311 nm) of p-biphenylyldiazomethane (left), p-biphenylyldiazoethane (middle), and p-biphenylyldiazoethane-d3 (right) in acetonitrile.
The kinetic traces were probed at 360 nm and fitted to a mono- or biexponential function.
Figure 3. Ultrafast LFP of p-biphenylyldiazoethane in acetonitrile. The kinetic traces were excited at 271 (left), 311 (middle), and 328 nm (right), probed
at 360 nm (all), and fitted to a biexponential function, ∆A ) A1 exp(-t/τ1) + A2 exp(-t/τ2) + Offset.
1
1
1
but that the yield of BpCMe is ∼40% less than that of BpCH,
as measured 1 ps after the laser pulse (Figure 1, Table 1). These
experiments were performed on the same day, using the same
optical alignment of the spectrometer, and using solutions of
equal diazo absorbance at the wavelength of excitation (311
nm).
the case of BpCMe. There is a weak band observed at 420 nm
in the case of BDM and BDE (not shown). As discussed
previously, this band is attributed to secondary photolysis of a
reaction product, which transpires during signal averaging.15 The
data obtained by ultrafast photolysis of BDE and BDE-d3 differ
somewhat. There is a kinetic isotope effect (KIE) of 1.5 on the
1
There is also a significant difference in the time dependence
of the transient spectra of BpCH and BpCMe. As shown
slow component of the decay of BpCMe-d3 in acetonitrile
(Figure 2 and Table 2).
1
1
1
in Figure 2, BpCH decays monoexponentially when excited
II.2. Cyclohexane. Ultrafast photolysis (328 nm) of BDE in
cyclohexane gives results reminiscent of the acetonitrile data.
The diazo excited state is observed at 480 nm; it is formed
instantaneously and decays with a time constant of less than
300 fs. The decay of the diazo excited state is accompanied by
the growth of the singlet carbene absorption at 360 nm (SI,
Figure S1). The optical yield of 1
BpCMe is 40% less than that
of BpCH. Although BpCH decays in a monoexponential
process in cyclohexane, the disappearance of BpCMe is
1
at 311 nm, but BpCMe and its deuterated isotopomer
1
BpCMe-d3 both decay biexponentially when pumped at the
same wavelength. Furthermore, the ratio of fast to slow decay
of BpCMe is wavelength dependent (Figure 3). The ratio of
1
the fast to the slow component of alkylarylcarbene disappearance
increases as the excitation wavelength decreases (0.5 at 328 nm,
.8 at 311 nm, and 4.0 at 271 nm).17
1
1
0
1
1
The slow component of decay of BpCMe has a longer
lifetime in acetonitrile than (the single lifetime) of BpCH in
this solvent. But in the case of BpCH, the transient spectrum
of the carbene evolves into that of the carbene-nitrile ylide,
1
1
(17) The short component of 1BpCMe or 1BpCMe-d
vibrational cooling of the carbene. The long component is assigned to the
3
is attributed to the
1
1
intrinsic lifetime of BpCMe or BpCMe-d
of BpCMe or BpCMe-d refers to the long component.
3
. If not specified, the lifetime
1
1
whereas the corresponding ylide is formed in very low yield in
3
2600 J. AM. CHEM. SOC.
9
VOL. 129, NO. 9, 2007

Ultrafast Study of p-Biphenylyldiazoethane
A R T I C L E S
Table 2. Lifetimes and Normalized Amplitudes (λex ) 311 nm) of
reorganization is the rate-determining step in the protonation
process rather than the formation of a C-H bond. Since the
reaction is exothermic, it will have an early, asymmetric
transition state and consequently a small kinetic isotope effect.
Kohler and co-workers21 concluded that proton transfer is a
1
1
1
Singlet Carbenes BpCH, BpCMe, and BpCMe-d3 in Acetonitrile,
a
Cyclohexane, and Methanol
solvent
carbene
A
1
τ
1
/ps
A
2
2
τ /ps
2 1
A /A
1
1
1
acetonitrile
BpCH
BpCMe
BpCMe-d3 0.51 688 ( 50 0.49 11.0 ( 1.2 1.0
1.00 200 ( 14
0.54 453 ( 53 0.46
6.4 ( 0.6 0.8
minor (30%) component of the reaction of singlet diphenylcar-
bene with methanol. The predominant reaction pathway is
actually direct insertion into the OH bond, but this pathway
does not lead to a UV-vis-active reaction product as does
proton transfer. Because the extinction coefficient of benzhydryl
cation (which is persistent under certain conditions) was known,
it was possible for the Kohler group to quantitate the yield of
this ion from singlet diphenylcarbene. p-Biphenylylmethyl cation
is not a persistent species; as a result, its extinction coefficient
at the absorption maximum is not known, and we cannot deduce
1
cyclohexane BpCH
1.00
0.31 189 ( 20 0.69
BpCMe-d3 0.33 174 ( 29 0.67
77 ( 5
1
1
BpCMe
6.4 ( 1.3 2.2
5.7 ( 1.0 2.0
1
1
1
methanol
BpCH
BpCMe
BpCMe-d3 1.00 13.8 ( 2.3
1.00
7.9 ( 1.3
1.00 10.8 ( 1.1
a
All the samples were argon-degassed for 5 min prior to experiments.
All the kinetic traces were probed at 360 nm and fitted in either
monoexponential (A2 and τ2 are not applicable) or biexponential equations.
1
its yield from BpCMe in methanol using absorption spectros-
biexponential (SI, Figure S2). The time constant of the fast
component is 6.4 ps, and the time constant of the slow
21
copy. Following the Kohler group, we will assume that cation
1
formation is a minor pathway of the reaction of BpCMe with
1
component, 189 ps, is longer than that of the decay of BpCH
methanol.
(77 ps) in cyclohexane.
1
II.4. Cyclohexene. Alkenes react rapidly with carbenes, and
Isotopic substitution of the methyl group has no discernible
study of the photochemistry of aryl diazo compounds in neat
alkene solvents played a large role in the development of
1
influence on the data. BpCMe-d3 decays biexponentially with
1
the same time constant of its slow component as BpCMe. The
3
mechanistic carbene chemistry. Hence, we have studied the
1
amplitude ratio of fast to slow decay is the same for BpCMe
photochemistry of BDM and BDE in neat cyclohexene. Ultrafast
photolysis (315 nm) of BDM and BDE in neat cyclohexene
1
and BpCMe-d3 as well.
The fast and slow decay lifetimes of BpCMe are the same
1
1
1
produces the transient spectra of BpCH and BpCMe, respec-
tively (SI, Figures S5 and S6). Similar to the data in acetonitrile,
the carbene formation efficiency is higher for BDM than for
in cyclohexane and in cyclohexane-d12 within experimental error
1
(
SI, Figure S3). Similarly, BpCH decays with the same rate in
both cyclohexane and cyclohexane-d12 (SI, Figure S4).
II.3. Methanol. The results obtained in methanol (Figure 4),
an excellent trap of singlet carbenes, differ substantially from
those obtained in cyclohexane and acetonitrile. The diazo
excited-state lifetime is still less than 300 fs, but the singlet
carbene (λmax ) 360 nm) lifetime is reduced to only 10.8 ps in
1
BDE in cyclohexene. The lifetime of BpCH is shortened to 25
ps in cyclohexene, as compared to 77 ps in cyclohexane (Figure
1
6
a). However, the lifetime of BpCMe is 170 ( 25 ps in
cyclohexene, which is the same as that determined in cyclo-
hexane 189 ( 20 ps (Figure 6b).
1
the reactive solvent. As the BpCMe signal decays, a broad new
III. Discussion
band, centered at 445 nm, grows (10.8 ps) and decays (7.4 ps)
The first striking observation to explain is the ∼40% lower
(Figure 5). An isosbestic point is also observed at 385 nm.
optical yield of singlet carbene realized upon replacing a
The carrier of the 445 nm band is assigned to p-biphenylyl-
1
1
hydrogen atom ( BpCH) with a methyl group ( BpCMe), as
measured 1 ps after the laser pulse. Part of the 40% difference
methyl cation C (Scheme 6) on the basis of the precedents
1
8
19
established by Kirmse and Steenken, Chateauneuf, Dix and
1
Goodman,20 and Peon and Kohler and co-workers.21 The cation
is due to the fact that BpCMe has a fast decay component
τ ) 6 ps) and BpCH does not. The optical yield of BpCMe
is depressed relative to that of BpCH for this reason. Using a
1
1
(
is formed by abstraction of a proton of methanol by the singlet
carbene. To our knowledge, the transient spectrum of cation C
has not been reported previously. However, benzyl cation
absorbs at 363 nm;22 thus, absorption of quinoidal cation C at
1
40
1
standard kinetic analysis, we estimate that the yield of BpCMe
is depressed by 10% for this reason.
One can posit that a methyl group will depress the extinction
coefficient of the carbene. This hypothesis is not supported by
TD-DFT calculations (SI, Tables S1 and S4). The calculations
support the intuitive expectation that the two carbenes have
similar molar absorptivities.
Alternatively, one can posit that the replacement of hydrogen
by methyl increases the rate and quantum yield of a process
which competes against the formation of singlet carbene. The
competing process is not emission, as neither diazo compound
fluoresces to a measurable extent. Enhanced internal conversion
of the excited state to the diazo ground state by methyl
substitution could account for the experimental observation. To
test this possibility, solutions of BDM and BDE were prepared
in methanol. The initial optical densities of the two solutions
were the same. The solutions were then exposed to the same
number of pulses from a XeCl excimer laser (308 nm). As
4
45 nm does not seem unreasonable. Furthermore, TD-DFT
1
6
calculations of the cation predict a vertical transition at 428
nm (f ) 0.7229), in good agreement with experiment (SI, Table
S2). Thus, the assignment of this transient absorption band to
cation C seems secure. For the sake of comparison, a TD-DFT
calculation of parent benzyl cation is also included in the
Supporting Information (SI, Table S3, λmax ) 264 nm).
The ultrafast results obtained in methanol are very similar to
those reported for diphenylcarbene in the same solvent. The
data in methanol-OD do not show a large isotope effect. Peon
et al. posited that the results indicated that the solvent
2
1
2
1
(18) (a) Kirmse, W.; Kilian, J.; Steenken, S. J. Am. Chem. Soc. 1990, 112, 6399.
(b) Steenken, S. Pure Appl. Chem. 1998, 70, 2031.
(
(
(
(
19) Chateauneuf, J. E. Res. Chem. Intermed. 1994, 20, 249.
20) Dix, E. J.; Goodman, J. L. J. Phys. Chem. 1994, 98, 12609.
21) Peon, J.; Polshakov, D.; Kohler, B. J. Am. Chem. Soc. 2002, 124, 6428.
22) Jones, R. L.; Dorfman, L. M. J. Am. Chem. Soc. 1974, 96, 5715.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 9, 2007 2601

A R T I C L E S
Wang et al.
Figure 4. Transient spectra of p-biphenylyldiazoethane in methanol. Spectra were generated by ultrafast LFP (311 nm) with time windows of 0.20-0.80
left) and 1-10 ps (right).
(
Scheme 6
shown in Figure 7, the two diazo compounds are consumed at
the same rate upon exposure to 308 nm light. The quantum
yields for decomposition of BDM and BDE are the same; thus,
there is no evidence to suggest that internal conversion of the
diazo excited state to the ground state is more efficient in BDE
than in BDM.
concentration of ethanol-OD, increasing from 0% to 15% of
the yield of all VB isotopomers with increasing ethanol-OD
concentration under our experimental conditions. Even in neat
ethanol-OD, the yield of VB-d is only 15% of the yield of all
1
VB isotopomers. Thus, the cation is only a minor source of all
isotopomers of VB. Control experiments demonstrate that
photolysis of VB in cylohexane with 0.5 M ethanol-OD does
not lead to incorporation of deuterium and that both VB and
ether E are stable to the photolysis conditions used in this
experiment. Photolysis with times substantially longer than those
used to generate Figure 8 results in the conversion of VB to
ether E.
We propose that the lower yield of carbene, per laser pulse,
1
1
for BpCMe relative to BpCH is due to 1,2 migration of
7-9
hydrogen in the excited state of BDE, as shown in Scheme 2.
This process is possible in excited BDE, but it is not available
in excited BDM, hence the greater yield of carbene in the latter
precursor.
To test this possibility, we measured the ratio of “untrappable
carbene” product (p-vinylbiphenyl, VB) to relaxed carbene
product (E) as a function of ethanol-OD concentration in
cyclohexane. If Scheme 1 were operative, then a plot of E/VB
versus [ethanol-OD] would be linear. As shown in Figure 8,
however, the plot reaches a limiting value at ∼0.3 M ethanol-
OD when BDE in cyclohexane/ethanol-OD is exposed to
monochromatic 308 nm light. These data demonstrate that there
is a non-trappable carbene route to form VB.
Figure 8 depicts a plot of the ratio of [ether E-d1]/[VB-d0]
versus the concentration of ethanol-OD in cyclohexane, pro-
duced by 308 nm photolysis of BDE. This plot is corrected for
the formation of alkene formed via cation C. The limiting value
of this plot is 3.0. Thus, we conclude that, with 308 nm light,
∼
25% of the photochemistry of BDE proceeds to form VB by
rearrangement in the exited state of the diazo precursor, in
excellent agreement with the spectroscopic data after correcting
1
for the effect of the fast decay component of BpCMe on the
One can argue that photolysis of BDE in ethanol can lead to
the formation of cation C, and this in turn could produce VB.
In other words, the cation might be the source of the “untrap-
pable” carbene product. As shown in Scheme 6, however, the
reaction of carbene with ethanol-OD will produce cation C-d1
transient absorbance.
The second remarkable observation is that the decay of
BpCMe is biexponential, whereas the decay of BpCH can be
1
1
described by a single-exponential function. The fast component
1
1
and VB-d1. Both H NMR and GC-MS analysis of the VB
of the disappearance of BpCMe is on a time scale typical of
produced by photolysis of the precursor reveal that the yield of
VB-d1, the product of the cationic route, depends on the
vibrational cooling. Vibrational cooling is obvious when a
spectroscopic band of a species formed on the femtosecond time
2602 J. AM. CHEM. SOC.
9
VOL. 129, NO. 9, 2007

Ultrafast Study of p-Biphenylyldiazoethane
A R T I C L E S
1
1
Figure 5. Decay of singlet carbenes BpCMe (left) and BpCMe-d3 (right)
at 360 nm (solid circles) and the growth and decay of transient absorption
of p-phenylbenzyl methyl cation at 450 nm (open circles), probed in
1
methanol. The time constants τcbn (lifetime of BpCMe) and τcat (lifetime
of p-phenylbenzyl methyl cation) were obtained by globally fitting the decay
at 360 nm into the function ∆A ) A1 exp(-t/τcbn) + A2 and the growth and
decay at 450 nm into the function ∆A ) A3{τcat/(τcat - τcbn)}{exp(-t/τcat)
Figure 7. Decrease in absorbance of solutions of BDM and BDE in
methanol as a function of exposure to 308 nm light from a XeCl excimer
laser.
-
exp(-t/τcbn)} + A4.
Figure 6. Ultrafast LFP (315 nm) of (a) p-biphenylyldiazomethane and
b) p-biphenylyldiazoethane in cyclohexene. The kinetic traces were probed
at 350 nm and fitted to a mono- or biexponential function.
(
23
scale undergoes reshaping in ∼10-20 ps. The absence of
spectral reshaping, however, does not indicate the absence of
1
1
vibrational cooling. We posit that both BpCH and BpCMe
are formed with excess vibrational energy (see Schemes 3 and
Figure 8. Plot of the ratio of the yield between ether E and p-vinylbiphenyl,
formed by 1,2 H shift upon photolyzing BDE in cyclohexane, as a function
of EtOD concentration. The contribution of p-vinylbiphenyl formed by the
deprotonation mechanism has been eliminated.
4
) but that this does not lead to dramatic spectral reshaping.
Replacement of the hydrogen atom of BpCH with a methyl
group clearly leads to vibrational cooling with increased
efficiency. The methyl group provides an increased density
of low-frequency vibrational modes, relative to that found with
a hydrogen atom.25 Alternatively, the vibrationally excited
methyl-substituted singlet carbene may be isomerizing to VB
in 6 ps, as shown in Scheme 5. If this were the case, however,
1
isomerization of “hot” carbene to VB, this is not the major
relaxation pathway of the “hot” carbene produced by 308 nm
2
4
radiation.
It is also possible that 1BpCMe# undergoes intersystem
3
#
1
crossing to BpCMe much more rapidly than does BpCMe.
Sadly, we can provide no evidence for this hypothesis, because
1
then the yield of relaxed BpCMe (and ether E-d1 in chemical
3
#
BpCMe was not detected in this experiments. In a separate
trapping studies with 308 nm light) would be very small, ∼25%
27
study, we are able to detect the interconversion of singlet
fluorenylidene to its triplet (λmax ) 470 nm). In this case, it is
clear that there is no prompt formation of the triplet carbene,
in cyclohexane.26 This is not consistent with the data of Figure
8
, indicating that there is a ∼75% yield of relaxed and trappable
carbene. Thus, we conclude that, while there may be some
27
which casts doubt on this proposal.
Moss and Sheridan and co-workers28 studied a singlet carbene
(by matrix IR spectroscopy) in which different products were
formed by an RIES mechanism than from an excited carbene.
In the present study, unfortunately, RIES mechanisms and
mechanisms involving electronically or vibrationally excited
carbenes all produce the same compound VB. Thus, the nature
(
23) (a) Laermer, F.; Elsaesser, T.; Kaiser, W. Chem. Phys. Lett. 1989, 156,
81. (b) Elsaesser, T.; Kaiser, W. Annu. ReV. Phys. Chem. 1991, 42, 83.
c) Miyasaka, H.; Hagihara, M.; Okada, T.; Mataga, N. Chem. Phys. Lett.
3
(
1
992, 188, 259. (d) Schwarzer, D.; Troe, J.; Votsmeier, M.; Zerezke, M.
J. Chem. Phys. 1996, 105, 3121. (e) Burdzinski, G. T.; Gustafson, T. L.;
Hackett, J. C.; Hadad, C. M.; Platz, M. S. J. Am. Chem. Soc. 2005, 127,
1
3764.
1
(
24) An alternative explanation for the fast component of BpCMe biexponential
decay at 360 nm could be the vibrational cooling of the ground-state BDE.
This requires that the excited state of BDE regenerates its ground state
with excess vibrational energy via an ultrafast internal conversion. As shown
in Figure S8, the red edge of the BDE ground state UV spectrum is 360
nm; however, in the case of BDM, the red edge of its UV spectrum is
blue-shifted to 345 nm. This could account for the absence of biexponential
(25) (a) Ito, M. J. Phys. Chem. 1987, 91, 517. (b) Assmann, J.; von Benten, R.;
Charvat, A.; Abel, B. J. Phys. Chem. A 2003, 107, 1904.
(26) This value is deduced assuming that the 50% of the transient absorption
1
with a 6 ps decay component and the missing 20% of the BpCMe signal
1
1
decay feature in the decay of BpCH at 360 nm. However, usually the
(relative to BpCH after correction) are both due to formation of VB.
quantum yield of diazo compounds is close to unity. We conclude that the
explanation of hot ground-state BDE might account for the data to some
extent, but it is not the main reason.
(27) Wang, J.; Kubicki, J.; Platz, M. S., unpublished results.
(28) Moss, R. A.; Tian, J.; Sauers, R. R.; Sheridan, R. S.; Bhakta, A.; Zuev,
P. S. Org. Lett. 2005, 7, 4645.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 9, 2007 2603

A R T I C L E S
Wang et al.
of the products cannot be used to determine the relative yields
of these different pathways.
singlet carbenes relax faster to their lower energy triplet states
than by reaction with this solvent. We conclude, as before,3
that it is the equilibrium mixture of singlet and triplet carbene
which reacts with the cyclohexane solvent. The lifetimes of spin-
equilibrated phenyl- and naphthylcarbenes are tens of nano-
seconds in cyclohexane, and the equilibrium mixture decays
primarily by reactions of the singlet carbenes.
5,36
III.1. Solvent and Structural Effects on Intersystem
Crossing Rates. Eisenthal and co-workers13 have studied the
rates of intersystem crossing (ISC) of diphenylcarbene (DPC)
as a function of solvent. The lifetime of singlet DPC in
acetonitrile is 310 ps, and this lifetime shortens to 95 ps in
isooctane. A plot of the log of the first-order rate constant of
3
5
Cyclohexane and cyclohexene are both nonpolar, and for that
reason the rates of ISC of singlet arylcarbenes should be the
1
29
ISC of DPC versus the Dimroth ET(30) parameter was linear.
Normally, one expects that the rate of ISC will increase as the
energy separation between the singlet and triplet states de-
creases.30 Eisenthal et al. proposed that polar solvents prefer-
entially stabilize singlet relative to triplet arylcarbenes, a
1
same in these two solvents. The lifetime of BpCH in neat
cyclohexene is 25 ps, as opposed to 77 ps in cyclohexane. This
1
indicates that BpCH reacts with solvent alkene to form products
faster than it relaxes to form the lower energy triplet carbene.
31
1
postulate to later receive experimental and computational
support.32 They then advanced density-of-states arguments to
explain why ISC rates of DPC are largest in nonpolar solvents,
On the other hand, the lifetime of BpCMe is the same in
cyclohexane and in cyclohexene. This indicates that the alkyl-
arylcarbene achieves spin equilibration in cyclohexene; ISC is
fast relative to bimolecular reaction of the alkylarylcarbene with
1
whereas the singlet-triplet energy separation is largest in
1
solution.
the alkene solvent. Unsurprisingly, BpCMe is less reactive than
1
1
BpCH. This must be a consequence of both steric and electronic
Our results are consistent with the DPC data. Both BpCH
and 1BpCMe have longer lifetimes in acetonitrile than in
cyclohexane. We have also studied the influence of propionitrile
in cyclohexane on the ISC rates of BpCH (note that acetonitrile
and cyclohexane are not miscible). The lifetimes of BpCH in
factors, as methyl differentially stabilizes singlet carbenes
relative to triplet carbenes, as compared to hydrogen.3
Methanol is a polar solvent, and for that reason one predicts
3,34
1
1
1
1
that BpCMe and BpCH in methanol will have ISC rates slower
than those found in cyclohexane and comparable to those found
cyclohexane containing 0, 0.5, 1.0, and 2.0 M propionitrile are
all the same (62-72 ps) within experimental error (SI, Figure
S7). Thus, this is a bulk solvent effect on the ISC rate and not
an effect due to complexation of a small amount of nitrile with
1
1
in acetonitrile. The lifetimes of both BpCMe and BpCH are
quite short in methanol (τ < 10 ps) and are much shorter than
the lifetimes measured in cyclohexane or acetonitrile. Thus, it
1
1
1
BpCH.
is clear that BpCMe and BpCH react with methanol before
they relax to their lower energy triplet states.
1
In both cyclohexane and acetonitrile, the lifetimes of BpCMe
1
1
BpCMe forms very little ylide in acetonitrile; thus, reaction
are longer than those of BpCH in the same solvent. Theory
1
33
of BpCMe with this solvent is only a minor contributor to the
predicts that methylcarbene has a triplet ground state but
dimethylcarbene34 has a singlet ground state. Methyl substitution
consumption of this carbene in acetonitrile. In the next section,
we will conclude that intramolecular 1,2 hydrogen shift is an
important reaction pathway, consuming the alkylarylcarbene in
acetonitrile. It is a spin-equilibrated alkylarylcarbene mixture
which reacts with this solvent to form ylide in low yield.
for hydrogen stabilizes singlet carbenes relative to triplet
carbenes. Thus, one can easily explain our ISC rate data, in a
1
3,14
manner consistent with Eisenthal et al.,
by positing that the
1
singlet-triplet energy gap of BpCH is greater than that of
1
1
BpCMe.
The situation of BpCH in acetonitrile is less clear. The
1
lifetime of BpCH in acetonitrile is 200 ps, and nitrile ylide is
III.2. Spin Relaxation versus Intermolecular Chemical
Reactions of BpCH and 1BpCMe. Mechanistic carbene
1
clearly present 1 ns after the laser pulse. It is difficult to resolve
the growth of the ylide precisely because of significant spectral
chemists1-3 have long wondered about the timing of arylcarbene
1
overlap with BpCH. It is possible, perhaps even likely, that
ISC relative to the rate of intermolecular reactions. It is clear
1
1
BpCH decays in acetonitrile by both ylide formation and
from this study that BpCH has the same lifetime in cyclohexane
1
relaxation to the lower energy triplet state of the carbene. Thus,
the nitrile ylide may well have a biphasic growth: a picosecond
and in cyclohexane-d12. This is true as well for BpCMe. Singlet
carbenes will react with cyclohexane by insertion into a C-H
1
growth due to capture of nascent BpCH and a nanosecond
bond. A typical kinetic isotope effect (KIE) on this reaction is
3
5,36
1
growth due to reaction of the equilibrium mixture of singlet
and triplet carbene with acetonitrile.
∼
2.
As no solvent KIE on the lifetimes of BpCH and
1
BpCMe is observed in cyclohexane, we conclude that these
III.3. Spin Relaxation versus Intramolecular Chemical
1
1
(
29) Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Justus Liebigs
Ann. Chem. 1963, 661, 1.
Reactions of BpCH and BpCMe. It is interesting to note
1
1
that the lifetime of BpCMe-d3 is the same as that of BpCMe
in cyclohexane, a solvent in which ISC is relatively rapid
compared to that in acetonitrile. It is also longer than that of
BpCH in the same solvent. If there were very rapid 1,2
migration of hydrogen in BpCMe, then BpCMe would have
a shorter lifetime than BpCH, which cannot undergo this
reaction. Since this is not the case and there is no KIE, we
conclude that ISC of BpCMe proceeds more rapidly in
cyclohexane than isomerization to VB.
(
30) (a) Robinson, G. W.; Frosch, R. P. J. Chem. Phys. 1962, 37, 1962. (b)
Robinson, G. W.; Frosch, R. P. J. Chem. Phys. 1963, 38, 1187. (c) Jortner,
J.; Rice, S. A.; Hochstrasser, R. M. AdV. Photochem. 1969, 7, 149.
31) Geise, C. M.; Wang, Y.; Mykhaylova, O.; Frink, B. T.; Toscano, J. P.;
Hadad, C. M. J. Org. Chem. 2002, 67, 3079.
(
1
(
(
32) Geise, C. M.; Hadad, C. M. J. Org. Chem. 2000, 65, 8348.
33) Sherrill, C. D.; Leininger, M. L.; Van Huis, T. J.; Schaefer, H. F. J. Chem.
Phys. 1998, 108, 1040.
1
1
1
(
34) Ford, F.; Yuzawa, T.; Platz, M. S.; Matzinger, S.; Fulscher, M. J. Am.
Chem. Soc. 1998, 120, 4430.
(
35) (a) Hadel, L. M.; Platz, M. S.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106,
1
2
83. (b) Admasu, A.; Gudmundsdottir, A. D.; Platz, M. S. J. Phys. Chem.
A 1997, 101, 3832. (c) Admasu, A.; Platz, M. S.; Marcinek, A.; Michalak,
J.; Gudmundsdottir, A. D.; Gebicki, J. J. Phys. Org. Chem. 1997, 10, 207.
36) Buron, C.; Tippmann, E. M.; Platz, M. S. J. Phys. Chem. A 2004, 108,
In acetonitrile, ISC is slower than in cyclohexane. This
provides the opportunity for another reaction that consumes the
(
1
033.
2604 J. AM. CHEM. SOC.
9
VOL. 129, NO. 9, 2007

Ultrafast Study of p-Biphenylyldiazoethane
A R T I C L E S
singlet alkylarylcarbene, such as isomerization by 1,2 hydrogen
migration, to become competitive with spin relaxation. The
importance of this reaction is indicated by the KIE of 1.5
of ether three times. The organic layer was dried by sodium sulfate.
After filtration, the solvent was removed on a rotary evaporator to give
1
4
-acetylbiphenyl-R,R,R-d
3
. H NMR (500 MHz, CDCl
3
): δ 8.04 (d,
1
37
J ) 9.0 Hz, 2 H), 7.69 (d, J ) 8.5 Hz, 2 H), 7.63 (t, J ) 7.0 Hz, 2 H),
.48 (t, J ) 7.5 Hz, 2 H), 7.41 (t, J ) 7.5 Hz, 1 H), 2.61 (t, J ) 2.0
Hz, 0.15 H, 95% D atom). GC-MS: the purity of the d isotopomer is
7%, calculated from the relative intensities between m/z 198 and 199.
p-Toluenesulfonylhydrazide-N,N,N-d . p-Toluenesulfonylhydrazide
1.86 g, 10.0 mmol) was dissolved in a mixture of 10 mL of anhydrous
THF and 10 mL of D O. The solution was stirred overnight and then
observed with BpCMe-d3 in cyclohexane. Sugiyama et al.
7
have reported that polar solvents increase the rate of isomer-
ization of singlet methylphenylcarbene to styrene. Given that
polar solvents both increase the rate of 1,2 hydrogen shift and
decrease the rate of ISC, we propose that rearrangement of
3
9
3
(
1
BpCMe is at least competitive with ISC in acetonitrile and
2
possibly even faster. Furthermore, this explains the low yield
extracted by 10 mL of ether three times. The organic layer was dried
1
of nitrile ylide produced from BpCMe, relative to that produced
by sodium sulfate. After filtration, the solvent was removed on a rotary
1
1
1
from BpCH. In acetonitrile, BpCH achieves spin equilibrium
and reacts with solvent to form ylide. The singlet methylcarbene
analogue isomerizes in competition with relaxation to the triplet,
hence the low efficiency of its reaction with solvent and
corresponding low yield of ylide.
evaporator to give p-toluenesulfonylhydrazide-N,N,N-d
MHz, CDCl
D atom), 3.74 (t, J ) 6.5 Hz, 0.17 H, 83% D atom), 2.46 (s, 3 H).
p-Biphenylyldiazoethane-d (BDE-d ). p-Acetylbiphenyl-R,R,R-d
tosylhydrazone (1.82 g, 5.0 mmol, which is made by refluxing
p-acetylbiphenyl-R,R,R-d (1.96 g, 10 mmol) with p-toluenesulfonyl-
3
. H NMR (500
3
): δ 7.80 (m, 2 H), 7.36 (m, 2 H), 5.52 (s, 0.30 H, 85%
3
3
3
3
IV. Experimental Section
hydrazide-N,N,N-d (1.86 g, 10 mmol)) was dissolved in 50 mL of
3
anhydrous THF. Sodium (0.23 g, 10.0 mmol) was dissolved in 10 mL
of methanol-O-d. This sodium methoxide solution was added via
syringe, and the mixture solution was stirred for 5 h. The methanol
and THF were then removed by a rotary evaporator. The solid
tosylhydrazone salt was broken up with a spatula and transferred to a
sublimator. The sublimator was immersed in an oil bath, and the
IV.1. Calculations. DFT and TD-DFT calculations were performed
3
8
using the Gaussian 03 suite of programs at The Ohio Supercomputer
Center. Geometries were optimized at the B3LYP/6-31G* level of
theory, with single-point energies obtained at the B3LYP/6-311+G**//
B3LYP/6-31G* level of theory. Vibrational frequency analyses at the
B3LYP/6-31G* level were utilized to verify that stationary points
obtained corresponded to energy minima. The electronic spectra were
computed using time-dependent density functional theory in Gaussian
temperature was raised to 140 °C. At this temperature, red
1
p-biphenylyldiazoethane-d
400 MHz, CDCl
.32 (t, J ) 7.4 Hz, 1 H), 6.98 (m, 2 H), 2.17 (s, 0.3 H, 90% D atom).
3
was collected on the coldfinger. H NMR
(
3
): δ 7.57-7.60 (m, 4 H), 7.43 (t, J ) 6.3 Hz, 2 H),
0
3 at the B3LYP/6-311+G** level, and 20 allowed electronic transi-
tions were calculated.
7
13
C NMR (125 MHz, CDCl
3
): δ 140.6, 136.1, 131.4, 128.8, 127.6,
IV.2. Ultrafast Spectroscopy. Ultrafast UV-vis broadband absorp-
tion measurements were performed using the home-built spectrometer
1
27.2, 127.0, 126.7, 121.6, 9.7 (septet, J ) 10.0 Hz). FT-IR (neat):
-1
3
9
2026 cm . GC-MS: the purity of the d
3
isotopomer is 90%; this purity
described previously. Samples were prepared in 50 mL of solvent
with absorption 0.70-0.80 at the excitation wavelength with 1.0 mm
optical length. All the sample solutions were purged with argon prior
to the experiments for 5 min and during the experiments.
is indirectly determined by the thermal rearrangement product VB-d
3
and calculated from the relative intensities between m/z 182 and 183.
1
IV.4. H NMR Analysis of Photolysis Mixtures. Photolysis of
p-biphenylyldiazoethane (BDE) was performed in cyclohexane contain-
ing various concentrations of ethanol-O-d using a 10 Hz excimer laser
IV.3. Materials. All materials and solvents were purchased from
Aldrich. The solvents for ultrafast studies were spectrophotometric grade
from Aldrich and used as received.
p-Biphenylyldiazoethane (BDE). In a 25 mL, single-necked, round-
bottomed flask was placed 1.82 g of 4-acetylbiphenyl tosylhydrazone
(
XeCl, 308 nm, 17 ns, ∼0.5 J/pulse) for 5 min. Note: Photolysis for a
substantially longer time reduces the yield of p-vinylbiphenyl. BDE
stock solutions were prepared by dissolving a known quantity of BDE
in 300 mL of cyclohexane to achieve A ) 0.7 at 308 nm. In a quartz
flask, 23.00 g of the stock solution was weighed, and a known amount
of ethanol-O-d was added. The sample solution was degassed for 5
min by bubbling with argon before photolysis. During photolysis, the
sample solution was stirred magnetically. The completion of the
photolysis was monitored by UV-vis spectroscopy. After photolysis,
(
1
5.0 mmol, which is prepared by refluxing 4-acetylbiphenyl (1.96 g,
0 mmol) with p-toluenesulfonylhydrazide (1.86 g, 10 mmol) in 50
mL of ethanol). A 1.0 M solution (10 mL) of sodium methoxide in
methanol (10.0 mmol) was added via syringe, and the mixture was
swirled until dissolution was complete. The methanol was then removed
by a rotary evaporator. The solid tosylhydrazone salt was broken up
with a spatula and transferred to a sublimator. The sublimator was
immersed in an oil bath, and the temperature was raised to 140 °C. At
the solvent was removed under vacuum at 35 °C, and the product
1
mixture was dissolved in CDCl
3
and analyzed by H NMR (500 MHz).
To obtain the best possible integration, a 3 s relaxation time was used.
We assumed that the products derived from BDE will have an intact
biphenyl ring system. The aromatic hydrogens (δ 7.2-8.0) were
integrated and defined as the total yield (100%). The peaks in the
mixture NMR were identified by comparison with either spectra of
authentic samples or the literature NMR data of the same compound
or its monophenyl counterpart in the literature. Generally speaking,
changing from phenyl to biphenyl should only change the chemical
shifts of their nonconjugated substituents downfield within 0.1 ppm,
this temperature, red p-biphenylyldiazoethane was collected on the
1
coldfinger. H NMR (400 MHz, CDCl
3
): δ 7.57-7.60 (m, 4 H), 7.43
(
3
1
t, J ) 6.3 Hz, 2 H), 7.32 (t, J ) 7.4 Hz, 1 H), 6.98 (m, 2 H), 2.20 (s,
13
H). C NMR (125 MHz, CDCl
3
): δ 140.6, 136.1, 131.4, 128.8, 127.6,
-1
27.2, 127.0, 126.7, 121.6, 10.3. FT-IR (neat): 2031 cm . GC-MS:
the molecular ion peak of the thermal rearrangement product VB was
detected with m/z ) 180.
p-Acetylbiphenyl-r,r,r-d
was dissolved in a mixture of 10 mL of anhydrous THF and 10 mL of
O. A catalytic amount of sodium methoxide was added, and the
solution was stirred overnight. The solution was extracted by 10 mL
3
. 4-Acetylbiphenyl (1.96 g, 10.0 mmol)
such as toluene (CH
3
, δ 2.34) and p-phenyltoluene (CH
3
, δ 2.39), and
, δ 2.61).
D
2
acetophenone (CH , δ 2.59) and p-phenylacetophenone (CH
3
3
On the basis of this rule and the NMR literature data of the phenyl
counterparts, the chemical shifts of the biphenyl photoproducts can be
reasonably estimated.
(
(
(
(
37) Sugiyama, M. H.; Celebi, S.; Platz, M. S. J. Am. Chem. Soc. 1992, 114,
9
66.
38) Frisch, M. J.; et al. Gaussian 03, Revision B.05; Gaussian, Inc.: Pittsburgh,
PA, 2003.
39) Burdzinski, G.; Hackett, J. C.; Wang, J.; Gustafson, T. L.; Hadad, C. M.;
Platz, M. S. J. Am. Chem. Soc. 2006, 128, 13402.
40) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms; McGraw-
Hill: New York, 1981; p 66.
The NMR chemical shifts of the three vinyl hydrogens of p-
vinylbiphenyl are δ 6.76 (R H, dd, J ) 17.5 Hz, J ) 10.5 Hz), 5.80
(trans H, dd, J ) 17.5 Hz, J ) 0.5 Hz), and 5.28 (cis H, dd, J ) 10.5
Hz, J ) 0.5 Hz). The R vinyl hydrogen of p-vinylbiphenyl-d is
1
J. AM. CHEM. SOC.
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VOL. 129, NO. 9, 2007 2605

A R T I C L E S
Wang et al.
deuterated and does not have any H NMR signals. Therefore, the
hydrogen with chemical shift δ 6.76 has a contribution only from
compound. The ratio of the fast to the slow component of
alkylarylcarbene disappearance increases as the excitation
wavelength decreases (0.5 at 328 nm, 0.8 at 311 nm, and 4.0 at
271 nm). This is attributed either to efficient relaxation of
vibrationally hot carbene by 1,2 migration of hydrogen to form
VB or to the increased number of low-frequency vibrational
modes provided by the methyl group.
p-vinylbiphenyl-d
0
, which is formed by the 1,2 H shift mechanism.
V. Conclusions
Ultrafast photolysis of BDE produces an excited state of the
diazo compound in acetonitrile, cyclohexane, cylohexene, and
methanol with λmax ) 480 nm and a lifetime of 300 fs. As the
diazo excited states decay, growth of singlet carbene absorption
at 360 nm is observed. The optical yields of diazo excited state
produced by photolysis of BDM and BDE are the same;
Acknowledgment. Support of this work by the National
Science Foundation, The Ohio State University Center for
Chemical and Biophysical Dynamics, and The Ohio Supercom-
puter Center is gratefully acknowledged. We also thank an
anonymous reviewer for alerting us to the effect of the fast decay
1
however, the optical yield of BpCMe is ∼40% less than that
1
of BpCH in all four solvents. The results cannot be explained
by differences in the molar absorptivities of the two carbenes,
nor can they be explained by differences in the quantum yields
of decomposition of the two diazo compounds. Instead, we
propose that the methyl-substituted diazo excited state suffers
1
component of BpCMe on the optical yield of the carbene
absorption.
Supporting Information Available: Tables S1-S4, TD DFT
calculations; Figures S1-S12, ultrafast LFP spectra, kinetic
traces, and NMR spectra; and complete ref 38. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
,2 migration of hydrogen to form VB and extrusion of nitrogen
1
to form carbene in reduced yield. The decay of BpCMe is
biexponential, and that of BpCH is monoexponential. The ratio
of fast (6 ps) to slow (450 ps) decay of BpCMe in acetonitrile
1
1
strongly depends on the wavelength of excitation of the diazo
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