Fragmentation of 2,2,2-triphenylethoxychlorocarbene: Evidence for ultrafast fragmentation-rearrangement in excited diazirines

Article abstract of DOI:10.1021/ol061863l

(Chemical Equation Presented) Photolysis of 3-(2,2,2-triphenylethoxy)-3- chlorodiazirine gives 2,2,2-triphenylethoxychlorocarbene which fragments with 1,2-phenyl migration and loss of CO and Cl^- to yield the 1,1,2-triphenylethyl cation and thence 1,1,2-triphenylethene by proton loss. However, ps and fs laser flash photolysis provides evidence that up to 25% of the alkene product stems from carbocation that arises directly from excited diazirine rather than from the carbene.

Full text of DOI:10.1021/ol061863l

ORGANIC
LETTERS
2
006
Vol. 8, No. 21
807-4809
Fragmentation of
,2,2-Triphenylethoxychlorocarbene:
Evidence for Ultrafast
Fragmentation Rearrangement in
Excited Diazirines
4
2
−
Xiaolin Fu, Robert A. Moss,* Piotr Piotrowiak,*,‡ Mykhaylo Myahkostupov, and
†
,†
‡
,
§
Ian R. Gould*
Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of
New Jersey, New Brunswick, New Jersey 08903, Department of Chemistry, Rutgers,
The State UniVersity of New Jersey, Newark, New Jersey 07102, and Department of
Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287
moss@rutchem.rutgers.edu
Received July 27, 2006
ABSTRACT
Photolysis of 3-(2,2,2-triphenylethoxy)-3-chlorodiazirine gives 2,2,2-triphenylethoxychlorocarbene which fragments with 1,2-phenyl migration
-
and loss of CO and Cl to yield the 1,1,2-triphenylethyl cation and thence 1,1,2-triphenylethene by proton loss. However, ps and fs laser flash
photolysis provides evidence that up to 25% of the alkene product stems from carbocation that arises directly from excited diazirine rather
than from the carbene.
2
3
4
When alkylchlorocarbenes are generated photochemically
from alkylchlorodiazirines, rearrangements occurring in
conjunction with the loss of nitrogen from the diazirines’
excited states parallel rearrangements of the daughter car-
benes, so that the rearranged products stem from two
Triphenylethanol was converted to isouronium salt 1,
5
and thence, by oxidation with NaOCl, to 3-(2,2,2-triph-
enylethoxy)-3-chlorodiazirine 2. Photolysis of 2 at 350 nm
in CDCl gave dichloride 3 and triphenylethene 4 in yields
3
4
of 9% and 91%, respectively. Dichloride 3 is an HCl trapping
product of triphenylethoxychlorocarbene 5. In the presence
of pyridine, which scavenges HCl, the dichloride was
suppressed, and alkene 4 formed in >95% yield. The latter
1
sources. Here, we present ultrafast kinetic evidence implying
that similar complications attend the generation and frag-
mentation of 2,2,2-triphenylethoxychlorocarbene from a
diazirine precursor. This is the first indication that excited
alkoxyhalodiazirines can undergo rearrangement in concert
1
13
was identified by H and C NMR comparisons to an
authentic (commercial) sample, and verified by GC spiking
experiments. NMR gave no evidence for the presence of
2
with the loss of N and CO.
6
either (unrearranged) 1,1,1-triphenyl-2-chloroethane or (re-
†
Rutgers University, New Brunswick.
Rutgers University, Newark.
Arizona State University.
‡
§
(2) Davis, F. A.; Reddy, R. E.; Szewczyk, J. M.; Reddy, G. V.;
Portonovo, P. S.; Zhang, H.; Fanelli, D.; Reddy, R. T.; Zhou, P.; Carroll,
P. J. J. Org. Chem. 1997, 62, 2555.
(3) Moss, R. A.; Kaczmarczyk, G. M.; Johnson, L. A. Synth. Commun.
2000, 30, 3233.
(4) See the Supporting Information for experimental details.
(5) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
(6) Eisch, J. J.; Kovacs, C. A.; Chobe, P. J. Org. Chem. 1989, 54, 1275.
(1) (a) Nigam, M.; Platz, M. S.; Showalter, B. M.; Toscano, J. P.;
Johnson, R.; Abbot, S. C.; Kirchoff, M. M. J. Am. Chem. Soc. 1998, 120,
055 and references therein. (b) Review: Platz, M. S. In AdVances in
Carbene Chemistry; Brinker, U. H., Ed.; JAI Press: Stamford, CT,
998;Vol. 2, pp 133f. (c) Review: Merrer, D. C.; Moss, R. A. AdVances in
Carbene Chemistry; Brinker, U. H., Ed.; Elsevier: Amsterdam, 2001; Vol.
, pp 53f.
8
1
3
1
0.1021/ol061863l CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/13/2006

7
arranged) 1,1,2-triphenyl-1-chloroethane. Attempts to pre-
ratio vs the concentration of methanol in CDCl
(Figure 1); above ∼0.4 M MeOH, the product ratio levels
3
is curVed
7
8
pare the latter chloride from 1,1,2-triphenylethanol and
SOCl gave only triphenylethene.
2
The triphenylethene formed upon photolysis of diazirine
in CDCl mostly arises by proton loss from triphenylethyl
2
3
cation 6, which derives from the fragmentation of carbene 5
coupled with 1,2-phenyl migration. This process parallels
the formation of the t-amyl cation by the fragmentation of
neopentyloxychlorocarbene, coupled with 1,2-methyl migra-
9
tion. However, we now find that the origin of cation 6 is
more complicated.
Figure 1. Product ratios of Ph
for the photolysis of diazirine 2 in MeOH/CDCl
3
CCH
2
OH/Ph
2
CdCHPh vs [MeOH]
. The product ratios
3
were determined by calibrated capillary GC.
Absolute rate constants for the fragmentations of alkoxy-
5
6
-1 9,10
halocarbenes are generally ∼10 -10 s
and can be
off at ∼7.5:1. This behavior, which parallels that of alkyl-
determined by ns laser flash photolysis (LFP) using the
1
chlorocarbenes photogenerated from diazirines, implies that
11
pyridine ylide method. However, no pyridine ylide formed
from carbene 5 when diazirine 2 was decomposed by ns LFP
at 351 nm in pentane containing 4 M pyridine, in 1,2-
dichloroethane (DCE) containing 0.8-4.0 M pyridine, or in
methanol with 0.8-4.0 M pyridine. Either the fragmentation
of carbene 5 is .10 s and/or the rate of ylide formation
is reduced relative to fragmentation, perhaps because of steric
hindrance originating at the 3 â-phenyl groups of 5. Can
the carbene be efficiently trapped?
there are 2 sources of alkene 4, only one of which (carbene
5
) can be intercepted by methanol.
We considered the possibility that the 15-25% of triph-
enylethene that persists even when carbene 4 is generated
in neat methanol arose directly by a phenyl shift rearrange-
ment-fragmentation of excited diazirine 2 to cation 6,
followed by proton loss. The cation is known to absorb at
6
-1
14
4
30 nm (in sulfuric acid), but ns LFP of 2 in DCE, MeCN,
or MeOH gave no UV evidence for 6. However, ps LFP of
Photolysis of diazirine 2 in neat methanol gives 15-25%
of triphenylethene 4 and 75-85% of 2,2,2-triphenylethanol.
1
5
2
in MeOH at 355 nm revealed a transient at 440 nm that
was quenched upon addition of pyridine, and which we
assign to cation 6; cf. Figure 2. Decay of the cation was
observed with a lifetime of ∼85 ps in methanol, representing
proton loss with the formation of triphenylethene. Cation 6
was not formed upon ps LFP of triphenylethene in MeOH,
although the alkene’s excited state (420 and 670 nm) and
radical cation (490 nm) did appear.
(Both products are stable to photolysis in methanol at 350
nm.) The triphenylethanol stems from methanolic trapping
1
2,13
of the carbene.
Although methanol efficiently captures
carbene 5, even neat methanol cannot prevent the formation
of a significant quantity of rearranged alkene 4. Moreover,
On the ultrafast time scale, fs LFP revealed an even
shorter-lived transient, formed instantaneously upon n to π*
excitation of the diazirine at 350 nm. The transient exhibited
an absorption maximum at 450 nm and decayed with
complex, multiexponential kinetics. Within the initial 5 ps
time window, the decay was biphasic: a fast component of
a correlation of the triphenylethanol/triphenylethene product
(7) Klages, A.; Heilmann, S. Chem. Ber. 1904, 37, 1447.
(8) Carter, P. R.; Hey, D. H. J. Am. Chem. Soc. 1948, 150.
(9) Moss, R. A.; Ge, C.-S.; Maksimovic, L. J. Am. Chem. Soc. 1996,
1
1
10 fs (56% of the total decay) and a slower component of
.28 fs (27% of the decay); cf. Figure 3.
1
18, 9792.
We tentatively assign this very short-lived 450 nm transient
to zwitterionic species 7, formed by C-N bond cleavage
(10) Moss, R. A. Acc. Chem. Res. 1999, 32, 969.
11) Jackson, J. E.; Soundararajan, N.; Platz, M. S.; Liu, M. T. H. J.
(
Am. Chem. Soc. 1988, 110, 5595.
12) Smith, N. P.; Stevens, I. D. R. J. Chem. Soc., Perkin Trans. 2 1979,
298.
13) Moss, R. A.; Zheng, F.; Fed e´ , J.-M.; Ma, Y. Sauers, R. R.; Toscano,
J. P.; Showalter, B. M. J. Am. Chem. Soc. 2002, 124, 5258.
(
(14) Toone, T. W.; Lee-Ruff, E.; Khazanie, P. G.; Hopkinson, A. C. J.
Chem. Soc., Perkin Trans. 2 1975, 607.
(15) See the Supporting Information for details of the ps LFP instru-
mentation.
1
(
4808
Org. Lett., Vol. 8, No. 21, 2006

substituents on the carbon atom. Evolution to a singlet
biradical or zwitterion is a reasonable fate for the S state of
a diazirine. There are two major decay channels available
1
1
7c
to 7: loss of N
by 1,2-phenyl shift coupled with the loss of the OdCClNdN
anion, which itself fragments to CO, N , and Cl . The
2
2
to form carbene 5 or formation of cation 6
-
-
18
zwitterion can exist in a range of rotomeric forms which
could partition differently between these decay channels. The
4
50 nm signal reports the overall decay of 7 and does not
distinguish individual rotamers. However, we suggest that
the 110 fs component is the signature of “hot” zwitterions
which undergo chemistry before being fully vibrationally
cooled and solvated, while the slower 1.28 ps component
originates from fully thermalized and solvated 7. It is likely
that the hot and thermalized zwitterions yield different ratios
of the carbene and carbocation.
Figure 2. Transient absorption spectrum observed 60 ps after
excitation of diazirine 2 in MeOH at 25 °C at low laser power,
showing absorption due to cation 6 at 440 nm and negligible
absorption at 490 nm due to the radical cation of alkene 4.16
The ultrafast LFP results thus support the contention that
up to 25% of the triphenylethene formed upon photolysis of
2
stems from cation 6 that arises directly from the excited
1
9
diazirine rather than from carbene 5, cf. Scheme 1. Here,
from the “vertical” Franck-Condon S
ultrafast formation of 7 (<50 fs) is aided by release of strain
state of 2.17 The
1
Scheme 1
2
excited 2* (or 7) can lose N , affording carbene 5 and thence
cation 6 by carbene fragmentation concerted with phenyl
-
migration and the loss of CO and Cl . Competitively, 2*
(
7) can give cation 6 directly by phenyl migration cojoint
-
with the loss of N
2
, CO, and Cl . Subsequent loss of a proton
from 6 to yield 4 is rapid (<100 ps), so that cation 6, visible
at 440 nm when generated from 2* on the ps time scale, is
not detected by ns LFP when it arises from carbene 5. This
is the first implication of an excited diazirine carbene-
mimetic pathway in the photolytic fragmentation of an
alkoxychlorodiazirine. It seems quite possible that analogous
phenomena may be involved in the photolysis of other
alkoxychlorodiazirines.
Figure 3. Ultrafast decay of the excited state of diazirine 2 in
MeOH, τ ) 110 fs, A ) 0.56; τ ) 1.28 ps, A ) 0.27; A
.17. The sample was excited at 350 nm (2.5 µJ, 30 fs fwhm) and
probed at 450 nm (1.0 µJ, 45 fs fwhm).
Acknowledgment. The authors at all three institutions
are grateful to the National Science Foundation for financial
support.
1
1
2
2
∞
)
0
Supporting Information Available: Preparations of 1
and 2 and descriptions of the ps LFP facilities. This material
is available free of charge via the Internet at http://pubs.acs.org.
in the diazirine ring and by the likely charge-transfer
character of the S state, buttressed by the electronegative
1
OL061863L
(16) The diazirine contains small amounts of alkene 4 as an impurity.
LFP also gives the singlet excited state and the radical cation of 4. The
former has a lifetime of ∼10 ps; the latter is formed in a 2-photon process
and can be minimized at low laser power.
(18) Zwitterion 7 could also “relax” to the diazo isomer of diazirine 2.
However, we see no evidence for diazoalkane formation by ns LFP-UV.
Although a diazoalkane with two lone-pair substituents would be quite
unstable, it should survive on the µs-ns time scale.
(17) On the photophysics of diazirines, see: (a) Arenas, J. F.; Lopez-
Tocon, I.; Otero, J. C.; Soto, J. J. Am. Chem. Soc. 2002, 124, 1728. (b)
Muller-Remmers, P. L.; Jug, K. J. Am. Chem. Soc. 1985, 107, 7275. (c)
Platz, M. S. In ref 1b. (d) See also: Diau, E. W.-G.; Abou-Zied, O. K.;
Scala, A. A.; Zewail, A. H. J. Am. Chem. Soc. 1998, 120, 3245.
1
2
(19) It is highly unlikely that carbene 5 fragments to cation 6 at ∼10
s- , i.e., 5-6 orders of magnitude faster than the fragmentations of other
1
ROCCl.
Org. Lett., Vol. 8, No. 21, 2006
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