Synthesis and time resolved infrared analysis of photochemical oxiranyl carbene and diphenyl ketene precursors

Article abstract of DOI:10.1016/j.tetlet.2016.08.039

Synthesis of a nonnitrogenous phenanthrene-based precursor of oxiranyl carbene yielded a photochemical source for diphenyl ketene after an in situ intramolecular rearrangement. The initial carbene precursor targeted rearranged from a putative spiro-oxiranyl intermediate to a cyclobutanone. Photolysis of the phenanthrene–cyclobutanone generated an infrared signal centered at 2100?cm^?1consistent with diphenyl ketene formation. Time-resolved infrared spectroscopy followed the reaction progress and measured bimolecular rate constants (k_obs?=?9.1?×?10⁶?M^?1?s^?1; butyl amine) consistent with diphenyl ketene formation. Quenching experiments combined with Stern–Volmer kinetics suggests a radical pathway is likely involved in the formation of diphenyl ketene from the phenanthrene-based precursor. The generality of the rearrangement is also discussed.

Full text of DOI:10.1016/j.tetlet.2016.08.039

Tetrahedron Letters 57 (2016) 4785–4787
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and time resolved infrared analysis of photochemical
oxiranyl carbene and diphenyl ketene precursors
⇑
Eric M. Tippmann , Ryan Curtis
Department of Chemistry, Indiana-Purdue University Fort Wayne, IN 46805, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of a nonnitrogenous phenanthrene-based precursor of oxiranyl carbene yielded a photochem-
ical source for diphenyl ketene after an in situ intramolecular rearrangement. The initial carbene precur-
sor targeted rearranged from a putative spiro-oxiranyl intermediate to a cyclobutanone. Photolysis of the
phenanthrene–cyclobutanone generated an infrared signal centered at 2100 cmÀ1 consistent with diphe-
nyl ketene formation. Time-resolved infrared spectroscopy followed the reaction progress and measured
bimolecular rate constants (kobs = 9.1 Â 106 MÀ1 sÀ1; butyl amine) consistent with diphenyl ketene for-
mation. Quenching experiments combined with Stern–Volmer kinetics suggests a radical pathway is
likely involved in the formation of diphenyl ketene from the phenanthrene-based precursor. The gener-
ality of the rearrangement is also discussed.
Received 22 June 2016
Revised 8 August 2016
Accepted 11 August 2016
Available online 26 August 2016
Keywords:
Reactive intermediate
Carbene
Ketene
Photochemistry
Ó 2016 Elsevier Ltd. All rights reserved.
Introduction
Results and discussion
Carbenes are defined by a neutral divalent carbon.1 The nature
of the two remaining substituents define the multiplicity at the
carbene center and therefore the extent to which the molecule will
exhibit reactivity more characteristic of a singlet, a triplet, or a
mixture of both spin states.2–4 Numerous combinations of sub-
stituents have been prepared and investigated, and though exam-
ples of surprising reactivity appear,5 the properties of a given
carbene can often be predicted de novo given the structural similar-
ity to a known compound. For example, a singlet carbene gener-
Cyclopropyl phenanthrene and indane substituted compounds
are non-nitrogenous and efficient photochemical sources of dihalo-
and other carbenes.12,13 Thus, 2 was targeted as a potential photo-
chemical source of oxiranyl carbene 1 (Scheme 1). The retrosyn-
thetic analysis of 2 was based on benzophenone trapping of
ated from
a-diazo carbonyl compounds are known to yield
ketenes via the Wolff rearrangement.6,7 Rarely is a mechanism,
such as the Wolff rearrangement ‘solved’ in a single paper; and
indeed, the carbene and nitrene literature is rich with examples
of unexpected reactions and elusive mechanisms which were deci-
phered over years with many groups contributing.8,9 Here we
attempted to synthesize precursors specifically designed to photo-
chemically generate oxiranyl carbenes. This work had little to go
on itself given the targets are largely only known in theoretical
reports or as the related anions.10,11 In the end, the initial target
molecule was observed to rearrange during the final synthetic
transformation and instead resulted in
a carbene-free, non-
nitrogenous, photochemical source of a ketene other than by the
Wolff rearrangement.
⇑
Corresponding author.
Scheme 1.
http://dx.doi.org/10.1016/j.tetlet.2016.08.039
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

4786
E. M. Tippmann, R. Curtis / Tetrahedron Letters 57 (2016) 4785–4787
It was reasoned that if cyclobutanone 5 resulted from passing
through stabilized carbocation 2a, as depicted in Scheme 3, then
it follows that antiaromatic20 intermediate 6a would prevent rear-
rangement.21–24 Indeed, trapping of halocarbenoid 3 with 9-fluo-
renone in place of benzophenone resulted in oxirane 6 and not a
cyclobutanone. Unfortunately, LFP and time resolved infrared anal-
ysis (TRIR) did not detect any intermediates from 6. Neither the
mechanism of rearrangement, nor the photochemistry, was
explored further apart from a brief test reaction involving a second
gem-dibromo compound 7 (Scheme 4). The same reaction condi-
tions again resulted in a cyclobutanone (9) without formation an
epoxide (8). (Cyclobutanone 9 was not tested further.)
Scheme 2.
Since carbenes are generated efficiently from phenanthrene
precursors, 5 was tested as a potential photochemical precursor
of diphenylketene. Following 266 nm photolysis of 5 in acetoni-
trile, TRIR resolved a new peak centered at 2100 cmÀ1 consistent
with diphenylketene (Fig. 1). The signal was observed to grow-in
near the limit of the resolution of the instrument (ꢀ200 ns). Fur-
ther evidence was observed by monitoring the decay of the
2100 cmÀ1 intermediate as a function of potential nucleophiles
butyl amine and butyl alcohol. The amine quenched the
2100 cmÀ1 signal, and the bimolecular rate constant (kobs = 9.1 -
 106 MÀ1 sÀ1; Fig. 2) was in excellent agreement with that previ-
ously reported (1.1 Â 107 MÀ1 sÀ1 25
)
for the same ketene–amine
reaction in acetonitrile-d3. We observed that even at 4 M, butyl
alcohol does not react rapidly enough to be measured with our
spectrometer, a result that is consistent with previous studies of
diphenylketene hydration and protonation.26,27
We also observed that the formation of the 2100 cmÀ1 interme-
diate is sensitive to HSnBu3, HSiEt3, and molecular oxygen. Photol-
ysis of 5 in the presence of HSnBu3 in CH2Cl2/acetonitrile mixtures
Scheme 3.
Scheme 4.
halocarbenoid 3, which is readily generated from gem-dibromo
4.14–17 The intended product of the reaction was isolated and puri-
fied, and instead yielded cyclobutanone 5 (Scheme 2). n-BuLi and
CuCl2 are known to cause rearrangements of gem-dibromo com-
pounds similar to 3,18 and it was proposed that 5 similarly arises
from a rearrangement of an intermediate, possibly transiently
formed 2a in a reaction reminiscent of a pinacol-type19 rearrange-
ment (Scheme 3).
Figure 2. Plot of the observed rate of decay at 2100 cmÀ1 following photolysis of 4
as a function of [butylamine]. The black markers are experimental values; the black
line represents the least-square fit of the data to a linear function. [Inset shows TRIR
spectrum monitored at 2100 cmÀ1 in the absence (no decay) and presence (decay)
of 0.013 M butyl amine; time scale 0–80 ls.]
Figure 1. (left) TRIR difference spectra taken in acetonitrile averaged over 0–4.5
argon-saturated acetonitrile (red curves are experimental data; the black curves are the calculated best fit to an exponential function).
l
s, and (right) kinetic trace observed at 2100 cmÀ1 following 266 nm laser photolysis of 5 in

E. M. Tippmann, R. Curtis / Tetrahedron Letters 57 (2016) 4785–4787
4787
Figure 3. (A) Increase in yield of transient observed at 2100 cmÀ1 following photolysis of 5 as a function of HSnBu3 (blue trace: 0.087 M; black: 0 M); (B) Stern–Volmer plot
determined from the absorbance change at 2100 cmÀ1 following photolysis of 5 in the presence of HSiEt3. [Inset shows a decrease in yield of transient observed at 2100 cmÀ1
(top to bottom: 0 M ? 0.035 M ? 0.077 M ? 0.154 M HSiEt3)].
(1:1) acted to slightly increase the yield of the transient at
2100 cmÀ1, with no apparent effect on the rate of appearance or
the lifetime of the transient (Fig. 3). From this result it is concluded
that this reagent, a known sensitizer,28 may act to sensitize the
reaction and/or quench other decay pathways of an excited 5.
Stern–Volmer analysis was performed on the disappearance of
References and notes
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The authors gratefully acknowledge helpful discussions with
Professor Matthew Platz.
Supplementary data
Supplementary data (1H and 13C NMR spectra of all compounds)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2016.08.039.