Singlet vinylcarbenes: Spectroscopy and photochemistry

Article abstract of DOI:10.1021/ja046309q

The first direct spectroscopic characterization of singlet vinylcarbenes is reported. Several vinyl amidines were converted to the corresponding vinylchlorodiazirines through straightforward Graham oxidation. Irradiation of the diazirines at 8 K in N₂ matrices gave the corresponding singlet vinylchlorocarbenes, which could be characterized by IR, UV/vis, and calculational modeling. Hence, vinylchlorocarbene, 1-methylvinylchlorocarbene, and 1-cyclopentenylchlorocarbene were all generated and investigated. The spectra are consistent with localized carbene structures compared to the very delocalized triplet vinylcarbenes. In all cases, the carbenes readily cyclize to the corresponding cyclopropenes on visible irradiation, together with H-shift in the parent vinylchlorocarbene. In the cyclopentenyl system, cyclization leads to a highly strained bicyclo[3.1.0]hexene. Copyright

Full text of DOI:10.1021/ja046309q

Published on Web 09/08/2004
Singlet Vinylcarbenes: Spectroscopy and Photochemistry
Peter S. Zuev and Robert S. Sheridan*
Department of Chemistry, UniVersity of NeVada, Reno, NeVada 89557
Received June 22, 2004; E-mail: rss@chem.unr.edu
In contrast to arylcarbenes, there is scant spectroscopic informa-
tion available on their simpler delocalized cousins, vinylcarbenes.
1
Thus, although extensive experimental data exist for both singlet
2
and triplet arylcarbenes, direct characterization of vinylcarbenes
has been limited to a select few triplet species by EPR and IR
spectra. Spectroscopic information on singlet vinyl carbenes is
3
4
nonexistent. Ironically, vinylcarbenes, particularly in their singlet
states, play a central role in thermal and photochemical cyclopro-
5
pene isomerizations, as well as in a wide variety of useful synthetic
applications.6 We are pleased to report the first spectroscopic
characterization of several singlet vinylcarbenes, together with an
investigation of their photochemical transformations. Moreover, we
describe the cyclization of one of these species to a highly strained
bicyclo[3.1.0]hexene.
Halogen substitution is well-known to stabilize carbene singlet
states, enabling the spectroscopic investigation of even highly
7
reactive species. Although halodiazirines are the most convenient
8
precursors to a broad range of halocarbenes, we were concerned
9
that the usual Graham oxidative cyclization conditions might prove
too harsh for vinyl substituted systems. However, in his seminal
paper, Graham alluded to the generation of vinylchlorodiazirine 1
9
but reported no details of its chemistry. We found that the usual
hypochlorite oxidation of the corresponding amidine indeed pro-
duced 1 in acceptable yield as an unstable oil, which we character-
ized by NMR, UV/vis, and IR spectroscopy.10 Irradiation of N
2
matrix isolated 1 at 8 K (385 nm, 1.5 h) gave vinylcarbene 2, as
supported by comparison of its spectra to calculation (vide infra)
Figure 1. (a) IR difference spectra (arbitrary absorbance units) showing
bleaching of carbene 2 (“down bands”) on 1 h irradiation at 578 nm
(products 3 and 4 “up bands”). Bands labeled * assigned to more labile
s-cisoid conformer of 2 (see text), and those labeled A assigned to allene
as well as by its subsequent photochemistry.1
1,12
The carbene
exhibited its strongest bands in the IR at 1573, 1389, 1158, and
-1
9
65 cm , together with a broad absorption in the UV/vis spanning
4
. (b) B3LYP/6-31+G** calculated IR spectrum for carbene 2; s-E filled
5
50-850 nm (λmax 690 nm). The carbene was stable for several
bars, s-cisoid open bars; (c) B3LYP/6-31+G** calculated IR spectrum for
days in N
2
at 9-25 K in the dark, but subsequent irradiation of the
12
cyclopropene 3. All calculated frequencies are scaled by 0.97 factor.
matrix at 578 nm rapidly converted 2 to a mixture of cyclopropene
3
and allene 4, confirmed by IR spectroscopy (Figure 1).13
Propenyldiazirine 5 could be produced similarly by Graham
oxidation of the corresponding amidine.10 Irradiation of 5 at 8 K
2
in a N matrix (385 nm, 40 m) gave chlorocarbene 6, with strongest
-
1
IR bands at 1311 and 844 cm and a broad UV/vis absorption
centered at 680 nm. The IR and visible spectra fit those predicted
for 6 at the B3LYP/6-31+G** level. Irradiation of matrix isolated
6
at 578 nm produced mainly methylchlorocyclopropene 7, by
comparison to DFT calculated IR spectra, but small amounts of
methyl-shifted allenic product 8 cannot be ruled out (Figure S-2,
Supporting Information).10
B3LYP/6-31+G** calculations locate two energy minima for
, a planar s-E conformer shown, and an s-cisoid conformer twisted
2
somewhat (42° out of plane) due possibly to steric interactions (and
predicted to lie 4.2 kcal/mol higher in energy). The IR spectra
similarly display two sets of bands for 2 (Figure 1). One set of
absorptions, which fits the predicted IR spectrum for the s-cisoid
conformation, is particularly photolabile, converting to products
after only brief irradiation at 578 nm. The other set, consistent with
predicted spectra for s-E 2, disappears more slowly at this
wavelength. Comparison to calculated IR band intensities suggests
that, at most, 10% of the s-cisoid conformer is produced from 1.
It is interesting to note the significant structural differences that
are calculated between the chlorovinylcarbene 2 and the parent
vinylcarbene 9, which directly lead to major spectroscopic conse-
quences. Calculations indicate that vinylcarbene 9 is a ground-state
triplet carbene, with a predicted triplet-singlet (S-T) energy gap
of ca. 12 kcal/mol.1 The carbene is predicted to have the highly
delocalized allylic diradical structure shown below. Although both
s-E and s-Z planar conformations are found to be energy minima
for the triplet carbene, only s-E is shown. As with other halocar-
4,15
4
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J. AM. CHEM. SOC. 2004, 126, 12220-12221
10.1021/ja046309q CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
benes, vinylchlorocarbene 2 is predicted to be a ground-state singlet.
At the B3LYP/6-31+G** level, the triplet state of carbene 2 lies
S-3, Supporting Information). Interestingly, calculations indicate
that formation of 13 from 12 is only ca. 1 kcal/mol exothermic.
1.3 kcal/mol higher (s-E, vibrationally corrected). For reference,
at this level of theory, the S-T gap in phenylchlorocarbene is
calculated to be 4.6 kcal/mol, a bit lower than estimates at higher
levels (7.8 kcal/mol at the QCISD(T) level).1 In contrast to
vinylcarbene/triplet diradical 9, the calculated geometry for singlet
carbene 2 reflects only minor delocalization with typical short CdC
and long C-C bonds. Natural resonance theory (NRT)16 calcula-
tions based on the B3LYP electron densities indicate that resonance
structures 2a and 2b contribute ca. 82 and 12%, respectively, to
the electronic structure of 2.
In sum, we have shown for the first time that a variety of
chlorovinylcarbenes can be generated from the corresponding
diazirines. This synthetic route makes these singlet vinylcarbenes
accessible for spectroscopic interrogation and promises broader
18
application.
Acknowledgment. We thank the National Science Foundation
and the donors of the Petroleum Research Fund for generous
support.
Supporting Information Available: Synthetic, spectroscopic, and
calculational details. This material is available free of charge via the
Internet at http://pubs.acs.org.
4
Maier and co-workers have reported recently that UV irradiation
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(
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(
1
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(
A transition state was located for the cyclization of s-E 2 to 3 at
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tionally corrected, is predicted to be 12.3 kcal/mol. Although
rearrangement to give chloroallene (4) is calculated to be thermo-
dynamically more favorable (ca. exothermic by -40 vs -17 kcal/
mol), the carbenic p-orbital is improperly aligned for the H-shift,
and the energy barrier is predicted to be 19.7 kcal/mol.
The synthetic methodology employed in this study appears to
be quite general, and we have found that cyclic singlet chloro-
vinylcarbenes can also be generated and characterized in similar
fashion. For example, 385 nm irradiation of the cyclopentenyldi-
(11) For a general description of the matrix isolation instrumentation, see:
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and references therein.
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1
2
(
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2
1
5
phenylmethylene (4 kcal/mol), suggests that the actual T - S energy
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azirine 11 matrix isolated in N
carbene 12, signified by strong IR bands at 1549 and 741 cm
2
at 8 K produced the corresponding
-
1
,
together with a broad visible absorbance centered at 658 nm.
Calculated IR and Uv/vis spectra for the s-Z and s-E conformations
of the carbene are very similar, and although they fit the
experimental spectra closely, they do not allow us to distinguish
whether one or more conformations are in our experiments.
Irradiation of 12 at 600 nm rapidly converts the carbene to a new
(
16) (a) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO, v 5.0; Theoretical
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(
-
1
product with a strong IR band at 1751 cm . The calculated IR
spectra are most consistent with formation of the highly strained17
bicyclopropene 13 as the major product, although it is impossible
to rule out the presence of the ring-expanded allene, 14 (Figure
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J. AM. CHEM. SOC.
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