Evidence of hydrogen migration in an alkylphenyldiazirine excited state

Article abstract of DOI:10.1021/ol101127c

(Figure Presented) Ultrafast photolysis (350 nm) of alkylphenyldiazirines promotes the diazirine to the S₁ excited state. Solvent and substituent effects on the excited state lifetimes indicate that the S ₁ state is highly polarized and undergoes a [1,2]-H shift in concert with nitrogen extrusion in cyclohexane.

Full text of DOI:10.1021/ol101127c

ORGANIC
LETTERS
2010
Vol. 12, No. 14
3182-3184
Evidence of Hydrogen Migration in an
Alkylphenyldiazirine Excited State
Yunlong Zhang,† Jacek Kubicki,‡ and Matthew S. Platz*,†
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue,
Columbus, Ohio 43210, and Quantum Electronics Laboratory, Faculty of Physics,
Adam Mickiewicz UniVersity, Umultowska 85, 61-614 Poznan, Poland
platz.1@osu.edu
Received May 15, 2010
ABSTRACT
Ultrafast photolysis (350 nm) of alkylphenyldiazirines promotes the diazirine to the S1 excited state. Solvent and substituent effects on the
excited state lifetimes indicate that the S1 state is highly polarized and undergoes a [1,2]-H shift in concert with nitrogen extrusion in cyclohexane.
Alkylphenyldiazirines are convenient thermal and photo-
chemical precursors of carbenes.1 We have previously
proposed that carbene mimetic 1,2 hydrogen migrations can
proceed in diazirine excited states,2 and this process has been
named “rearrangement in the excited states” (RIES).3 Herein,
we report femtosecond (fs) time-resolved UV-vis data on
alkylaryldiazirines that support this mechanistic proposal.
Photolysis (350 nm) of 3-methyl-3-phenyl diazirine
(PhCN2CH3) produces a transient species with λmax ) 580
nm in acetonitrile (Figure 1). The carrier of the transient
spectrum is assigned to the S1 state of the diazirine as 350
nm light is absorbed by the longest wavelength absorption
band of the ground state.
The transient decay is biexponential (Figure 2). The long
component is assigned to the lifetime of the S1 state, and
the short component is assigned to intramolecular vibrational
relaxation of the Franck-Condon state.4
There is a pronounced solvent effect on the lifetime of
the S1 state of 3-methyl-3-phenyl diazirine, which is length-
ened in the polar solvent. These trends are also evident upon
excitation of parent phenyldiazirine and other alkylphenyl-
diazirines (Table 1). The data is consistent with a previous
report of Wang et al. regarding arylhalodiazirines.5
The lifetime of 3-methyl-3-phenyl diazirine (long com-
ponent) is about 5 times longer than that of the S1 state of
parent phenyldiazirine (PhCN2H, Table 1). This indicates that
positive charge has developed on the diazirine carbon and
is better stabilized, as expected, by a methyl group as
compared to a hydrogen atom.6
† The Ohio State University.
‡ Adam Mickiewicz University.
(1) Liu, M. T. H., Ed. Chemistry of Diazirines; CRC Press: Boca Raton,
FL, 1987; Vols. I and II.
(2) 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–
8059.
Following Wang et al.5 we conclude that this state is highly
polarized, although theory predicts that the diazirine ring
(3) (a) Bonneau, R.; Liu, M. T. H.; Kim, K. C.; Goodman, J. L. J. Am.
Chem. Soc. 1996, 118, 3829–3837. (b) Jackson, J. E.; Soundararajan, N.;
White, W.; Liu, M. T. H.; Bonneau, R.; Platz, M. S. J. Am. Chem. Soc.
1989, 111, 6874–6875. (c) White, W. R.; Platz, M. S. J. Org. Chem. 1992,
57, 2841–2846. (d) Modarelli, D. A.; Morgan, S.; Platz, M. S. J. Am. Chem.
Soc. 1992, 114, 7034–7041.
(4) (a) Rabinovitch, B. S.; Rynbrandt, J. D. J. Phys. Chem. 1971, 75,
2164–2171. (b) Charvat, A.; Abmann, J.; Abel, B.; Schwarzer, D.; Henning,
K.; Luther, K.; Troe, J. Phys. Chem. Chem. Phys. 2001, 3, 2230–2240.
(5) Wang, J.; Burdzinski, G.; Kubicki, J.; Platz, M. S.; Moss, R. A.;
Fu, X.; Piotrowiak, P.; Myahkostupov, M. J. Am. Chem. Soc. 2006, 128,
16446–16447.
10.1021/ol101127c  2010 American Chemical Society
Published on Web 06/24/2010

formation).1 Two points persuade us that the S1 lifetimes
(τ2) recorded for different alkyl substitution (CH3, CD3, Et,
i-Pr, and t-Bu) in cyclohexane are consistent with the
presence of an additional, competitive decay pathway from
the S1 state: the previously proposed 1,2 migration of
hydrogen that proceeds in concert with alkene formation and
nitrogen extrusion (RIES, Scheme 1).
Scheme 1
.
Rearrangement in the Excited State of
Alkylphenyldiazirines
First there is a kinetic isotope effect (KIE) on the lifetimes
of 3-trideuteromethyl-3-phenyl diazirine (PhCN2CD3,Table 1)
versus 3-methyl-3-phenyl diazirine (PhCN2CH3) in cyclohexane,
but there is no normal KIE, within experimental error, in
acetonitrile. The data reported in Table 1 were based on fits
perfomed at a few selected wavelengths over the entire spectral
range recorded (e.g., at the maximum of the transient absorption
band). To gain greater confidence in our conclusion a global
fit (simultaneous fitting in the 400-650 nm spectral range) of
the PhCN2CH3 and PhCN2CD3 data was also employed, to
deduce the decay rate constants using all of the recorded data.
The S1 lifetimes obtained using global fitting (5.2 ( 0.3 ps for
PhCN2CH3 and 6.7 ( 0.5 ps for PhCN2CD3; see Figure S6 in
Supporting Information) has lower error limits than single
wavelength analysis. These results are entirely consistent with
the data given in Table 1, but the lower error limits resulting
from global analysis convince us that the PhCN2CD3 lifetime
is definitely longer than that of PhCN2CH3. We believe that
this is strong evidence for the presence of a KIE on the excited
state decay. The DAS (decay associated spectra) show that the
spectra for the S1 states of both compounds are very similar
(Figure S6 in Supporting Information), consistent with the fact
that the same species, the S1 state, was observed for both
compounds.
Figure 1. Transient spectra produced by photolysis of 3-phenyl-
3-methyldiazirine (λex ) 350 nm) in (a) acetonitrile and (b)
cyclohexane at selected time delays.
Figure 2. Decay of transient absorption produced by ultrafast LFP
(350 nm) of 3-methyl-3-phenyl diazirine. The kinetic traces were
recorded in (a) acetonitrile and (b) cyclohexane and fitted to
biexponential functions.
Table 1. Lifetimes of Transient Absorptions, Measured at
Maximum Absorption, Produced by 350 nm Ultrafast LFP of
Alkylphenyldiazirines
Finally, the lifetime of the 3-tert-butyl-3-phenyl diazirine
(PhCN2-t-Bu) in cyclohexane (Table 1), where RIES of
hydrogen is not possible, is quite long relative to the other
alkylphenyldiazirines (the lifetimes of methyl∼ethyl∼isopropyl,
where RIES is possible), but this trend is not observed in
acetonitrile. Thus, we conclude that the polarity of the solvent
determines the partitioning of the excited state decay
mechanisms (carbene formation versus 1,2 hydrogen migra-
tion to form alkene in concert with nitrogen extrusion) which
control the decay of the S1 excited states of alkylphenyl-
diazirines. The lack of RIES in acetonitrile is attributed to
acetonitrile
τ1 (ps)a τ2 (ps)b
0.8 ( 0.2 5.9 ( 1.2 0.05 ( 0.03
cyclohexane
diazirines
PhCN2H
τ1 (ps)a
τ2 (ps)b
1.2 ( 0.1
4.8 ( 0.5
7.8 ( 1.6
5.3 ( 1.6
14.6 ( 3.1
7.5 ( 1.6
PhCN2CH3 2.1 ( 0.4 29.2 ( 5.3
0.3 ( 0.1
1.2 ( 0.2
1.3 ( 0.2
1.6 ( 0.6
1.0 ( 0.2
PhCN2Et
PhCN2-iPr
0.9 ( 0.2 20.9 ( 2.1
0.7 ( 0.1 13.0 ( 0.8
PhCN2-tBu 3.0 ( 0.4 21.4 ( 2.1
PhCN2CD3 1.7 ( 0.3 24.9 ( 4.0
a τ1 is the time constant of intramolecular vibrational relaxation. b τ2 is
the lifetime of the S1 state.
remains intact with equivalent C-N bonds in the S1 states
of aryldiazirines.6
Diazirine excited states decay by a variety of processes
(internal conversion to S0, isomerization to diazo, carbene
(6) (a) Zhang, Y.; Wang, L.; Moss, R. A.; Platz, M. S. J. Am. Chem.
Soc. 2009, 131, 16652–16653. (b) Zhang, Y.; Burdzinski, G.; Kubicki, J.;
Vyas, S; Hadad, C. M; Sliva, M; Buntinx, G; Platz, M. S. J. Am. Chem.
Soc. 2009, 131, 13784–13790. (c) Zhang, Y.; Vyas, S; Hadad, C. M; Platz,
M. S. J. Phys. Chem. A. 2010, 114, 5902–5912.
Org. Lett., Vol. 12, No. 14, 2010
3183

the enhanced solvent stabilization of the highly polarized S1
states in this solvent.
Supporting Information Available: Short description of
the spectrometer, kinetic traces, and global analysis. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This work was performed at The Ohio
State University Center for Chemical and Biophysical Dynam-
ics. Financial support by NSF is gratefully acknowledged.
OL101127C
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Org. Lett., Vol. 12, No. 14, 2010