Oxyallyl Exposed: An Open-Shell Singlet with Picosecond Lifetimes in Solution but Persistent in Crystals of a Cyclobutanedione Precursor

Article abstract of DOI:10.1021/ja109494b

Photoinduced decarbonylation of 2,4-bis(spirocyclohexyl)-1,3- cyclobutanedione 1 in the crystalline solid state resulted in formation of a deep blue transient with λ_max = 550 nm and a half-life of 42 min at 298 K, identified as kinetically stabilized oxyallyl. Support for an open-shell singlet species was obtained by spectroscopic analysis and (4/4) CASSCF calculations with the 6-31+G(d) basis set and multireference MP2 corrections. The electronic spectrum of the singlet biradical, confirmed by femtosecond pump-probe studies in solution, was matched by coupled cluster calculations with single and double corrections.

Full text of DOI:10.1021/ja109494b

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Oxyallyl Exposed: An Open-Shell Singlet with Picosecond Lifetimes in
Solution but Persistent in Crystals of a Cyclobutanedione Precursor
Gregory Kuzmanich,† Fabian Sp€anig,‡ Chao-Kuan Tsai,† Joann M. Um,† Ryan M. Hoekstra,† K. N. Houk,*,†
Dirk M. Guldi,*,‡ and Miguel A. Garcia-Garibay*,†
†Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1559, United States
‡Department of Chemistry and Pharmacy & Interdisciplinary Center of Molecular Materials, Friedrich-Alexander-Universit€at Erlangen-
N€urnberg, Egerlandstrasse 3, 91058-Erlangen, Germany
S Supporting Information
b
Scheme 1
ABSTRACT: Photoinduced decarbonylation of 2,4-bis-
(spirocyclohexyl)-1,3-cyclobutanedione 1 in the crystalline
solid state resulted in formation of a deep blue transient with
λmax = 550 nm and a half-life of 42 min at 298 K, identified as
kinetically stabilized oxyallyl. Support for an open-shell
singlet species was obtained by spectroscopic analysis and
(4/4) CASSCF calculations with the 6-31þG(d) basis set
and multireference MP2 corrections. The electronic spec-
trum of the singlet biradical, confirmed by femtosecond
pump-probe studies in solution, was matched by coupled
cluster calculations with single and double corrections.
xyallyl (OA) is the oxygen analogue of trimethylenemethane
that is formally obtained by cleavage of the 2,3-bond of
O
cyclopropanone. OA has been invoked in numerous reactions mech-
anisms, including the Nazarov cyclization,1 Favoroskii rearran-
gement,2 cycloaddition of cyclopropanones,3 photochemical reac-
tions of cross-conjugated dienones,4 and prostaglandin biosyn-
thesis5 (Scheme 1a). While many recent synthetic applications
rely on the formulation of OA,6 in contrast to nearly all other
reactive intermediates, until now, it has resisted all attempts at
direct detection and isolation.
advantage of our experience with the generation of biradicals by
photoinduced decarbonylation of crystalline ketones,13 we decid-
ed to explore the generation and potential stabilization of a tetra-
substituted OA. Among several possible precursors, we selected
2,4-bis(spirocyclohexyl)-1,3-cyclobutanedione (1) as an ideal
test system. Compound 1 is a relatively high melting solid (mp
= 159 °C), known to react photochemically in solution14 by a
singlet-state R-cleavage followed by fragmentation to cyclohexyl
ketene 4, or by loss of CO to generate a transient cyclopropanone
3 through an intermediate OA 2 (Scheme 1b). While dicyclo-
hexylidene 5 is the major product in inert solvents, reaction in
furan leads to formation of adduct 6, in support of a trappable
intermediate.
Irradiation of 1 with λ = 254 nm in the crystalline state led
exclusively to alkene 5 by photodecarbonylation of the singlet
state, followed by a secondary photoreaction of cyclopropanone
3. Attempts to isolate 3 were unsuccessful, consistent with FTIR
and 1H NMR observations indicating that it does not accumulate
under the reaction conditions. While quantum yields for the de-
composition of 1 (ΦSol) in methylene chloride and in benzene were
0.34 and 0.31, respectively,14,15 reaction of 1 (and formation of 5) in
Questions about OA abound: Is it an intermediate or a transition
state? A zwitterion or an open-shell biradical? If biradical, does it
have singlet or triplet multiplicity in its lowest energy state? Some
answers are available. Quantum mechanical calculations suggest that
the electronic nature of OA depends on the substituent patterns.7
While early quantum mechanical calculations suggested that
unsubstituted OA (Scheme 1a) could be a ground state triplet,8
subsequent refinements predicted a singlet biradical.9,10 Experi-
mental studies addressing the solvent polarity-dependence on
the rates of reactions thought to proceed via OA, also supported
by calculations,9 indicated that tetraalkyl substitution favors a
singlet biradical, which is 15-25 kcal/mol higher in energy than
the corresponding cyclopropanone.11 The singlet biradical nat-
ure of unsubstituted OA was recently confirmed by photoelec-
tron spectroscopy and by computational analysis with zero-point
energy corrections, both suggesting that closure occurs with no
barrier!
Knowing that species trapped in the solid state tend to have
Received: October 21, 2010
Published: February 7, 2011
longer lifetimes than their solution counterparts,12 and taking
r
2011 American Chemical Society
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Figure 1. Diffuse reflectance spectrum of cyclobutanedione 1 before
(black) and after 10, 20, and 30 min of UV irradiation (blue) at 298 K.
Inset: Powder before and after irradiation. The blue color faded out
after several hours, and its bleaching was accelerated by irradiation at
λ = 530 nm.
Figure 3. Picosecond decay measured at 600 nm of the OA transient
generated by excitation of cyclobutanedione 1 in dichloroethane. Inset:
Changes in the lifetime of OA as a function of the solvent polarity
parameter ET(30).
Figure 4. CASSCF/6-31þG(d) geometrical parameters, free valence,
and charges of the singlet biradical 1BR.
Figure 2. Transient absorption spectra upon excitation of cyclobuta-
nedione 1 in dichloroethane by excitation at 258 nm with 150 fs laser
pulses. No other transients were seen at delay times up to 3 ns.
a lifetime of 3.3 ps, with an apparent vibrational structure
displaying several maxima (at ca. 525, 555, 570, 590, 625, 650,
and 690 nm, Figure 2). A comparison of the transients observed
in solution and in the solid state shows that they are very similar,
with the solution spectrum displaying a bathochromic shift of ca.
40 nm and some vibrational resolution (Supporting Information
Figure S7). Measurements in ethers, nitriles, hydrocarbons, and
halocarbons showed identical transients with lifetimes between
1.0 and 8.0 ps, depending on the polarity of the solvent
(Supporting Information Figure S9). A plot of the lifetime vs
the ET(30) polarity parameter produced a straight line (Figure 3,
inset) with a modest polarity dependence. As analogous plots for
reactions with zwitterionic intermediates have lifetime changes of
ca. 102-104,21 our results are consistent with a neutral species.
The rapid formation of the transient indicates a reaction from the
excited singlet state, and a lifetime of ca. 5 ps in solution is con-
sistent with the need for high concentrations of trapping species,
even with reactions that should be diffusion controlled.22
To gain further evidence for or against our assignment, we carried
out complete active space self-consistent field (CASSCF)23-25
calculations with the 6-31þG(d) basis set. Multireference
Møller-Plesset second-order perturbation theory (MRMP2)26
corrections were also implemented to account for the effects of
dynamic correlation. The lowest energy singlet biradical struc-
ture shows a nearly coplanar geometry of the carbonyl and R-
substituents, with most of the spin density on the R-carbons
(Figure 4). The corresponding triplet biradical was calculated to
be 1.0 kcal/mol higher in energy. Equation of motion coupled
a nanocrystalline suspension using dicumyl ketone (Φ = 0.2)16 as
a chemical actinometer resulted in a value of only Φ ≈ 0.001. In a
key observation, we discovered that polycrystalline samples irra-
diated for ca. 30 min displayed an intense blue color that faded
slowly over many hours (Figure 1). The diffuse reflectance
UV-vis spectrum showed a relatively broad band with λmax
=
580 nm and a half-life of ca. 42 min at 298 K that was not affected
by Ar or O2 atmospheres.
Given the saturated nature of the dispirodiketone and its
known photoproducts, we realized that OA could be associated
with the blue color,17 and we set out to test our hypothesis. In
fact, while the color disappears upon dissolution in most solvents
and chemical analysis reveals the formation of 5, exposure to neat
furan resulted in the exclusive formation of small amounts of 6
(ca. 6%), consistent with an interfacial (surface-solution) cyclo-
addition between furan and OA 2 (Supporting Information
Figure S3).18 The blue intermediate showed no fluorescence in
the solid, and it was later shown to be EPR silent between 77 and
298 K, indicative of a singlet state.19
With guidance provided by the UV-vis diffuse reflectance
spectrum, we set out to look for the formation of a similar tran-
sient in solution. Consistent with reports by Neckers,3c,20 there
were no transients that could be assigned to OA on the
nanosecond time scale. Excitation at 258 nm with 150 fs pulses
in dichloroethane showed stimulated emission from 1 below
475 nm and only one transient in the region of 500-700 nm with
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120°, indicating that a ca. 60° bending of the two cyclohexane
mean planes is needed on going from reactant to product. As
shown in Figure 5, the packing structure consists of molecules
arranged in layers, with the cyclobutanedione planes aligned
parallel to the crystal (100) plane and the carbonyl groups of
alternating chains of molecules aligned in orthogonal directions.
Each molecule is surrounded by six close neighbors in the plane,
which presumably limit the displacement of the cyclohexyl rings.
However, calculations of singlet biradical energies and UV
spectra indicate that some torsional relaxation is required to
allow for partial conjugation of the CO π bond and the R-carbon
radical centers.25
In conclusion, we have generated and trapped an oxyallyl by
the photodecarbonylation of a crystalline spirocyclohexylcyclo-
butanedione and supported its assignment by a combination of
solid-state UV-vis absorption, EPR, and femtosecond
pump-probe spectroscopy and computational analysis. We have
also shown that crystals of cyclobutanedione can extend the
lifetime of OA by up to 14 orders of magnitude and, in agreement
with recent work by Borden and Lineberger, the electronic
structure of 2 is appropriately described as a singlet-state
biradical.10
Figure 5. Space-filling model of the molecular structure of 1 and its
packing arrangement within crystals in the space group Cmca. The
cyclohexane rings of the central molecule must displace close neighbors
from their equilibrium positions to form cyclopropanone 3.
cluster theory with single and double corrections27 was used to
predict the excitation energies. Comparison of the experimental
result with the predicted excitation energy of the singlet biradical
(1BR) showed a reasonable match, with a broad absorption
between 400 and 800 nm with λmax(1BR) = 530 nm or 1.89 eV.
Given the short lifetime in solution and the position of the
spectrum, we conclude that the transient is unlikely to arise from
a photoproduct. Tetraalkylcyclopropanone transients have life-
times longer than microseconds,28 and dialkyl ketenes as well, as
acyl radicals absorb, with λ < 450 nm.29 In addition, the long
lifetime of the blue transient in the crystal is not compatible with
an excited-state species. The ring-closure of 1BR to cyclopropa-
none 3 is calculated to be exothermic by 14.8 kcal/mol, with a
barrier of only 2.6 kcal/mol,30 which is consistent with a lifetime
of a few picoseconds in solution.
’ ASSOCIATED CONTENT
S
Supporting Information. Computational, synthetic, and
b
photochemical procedures; Raman, IR, H and 13C NMR, and
1
X-ray diffraction spectra; complete refs 24 and 30. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
mgg@chem.ucla.edu
The large size of the system under study precluded a vibrational
frequency analysis at the CASSCF level. However, vibrational
analysis of the analogous tetramethyloxyallyl gives a scaled CO
stretch of 1758 cm-1 for the singlet and 1555 cm-1 for the pure
triplet,31 which are similar to those recently reportedfor the parent
structure.32 Unfortunately, FTIR measurements in the solid state
using attenuated total reflection and grazing angle reflectance
spectroscopies showed insignificant changes in the region of the
triplet, while the starting material absorbs very strongly in the
region of the singlet, making the IR assignment of OA impossible
despite the deep blue color.33 Only a weak band assigned to
ketene 4 was observed at 2111 cm-1, with the carbonyl signal
of cyclopropanone 3 at ca. 1813-1850 cm-1 also absent.34
With the blue transient assigned as the singlet 1,3-biradical, it
is remarkable that its lifetime in crystals is extended by 14 orders
of magnitude with respect to the lifetime in solution! We
attribute this to the kinetic stabilization provided by the rigidity
of the crystal lattice. A single-crystal X-ray structure of 135 solved
in the orthorhombic space group Cmca supports this view. The
adjacent molecules severely hinder the ca. 0.33 Å displacement
between the two radical carbons and the concomitant bending of
the two cyclohexane mean planes, which are needed to convert
OA 2 to cyclopropanone 3. The molecular structure of 1 is
characterized by two cyclohexyl groups in chair conformations
related by a C2 axis that passes through the center of the four-
membered ring (Figure 5). The angle formed by the vectors
drawn from the outer cyclohexyl carbons to the center of the
cyclubutanedione is 180°, and the corresponding angle to the
midpoint of the calculated cyclopropanone C2-C3 bond is
’ ACKNOWLEDGMENT
We acknowledge support by NSF grants DMR0605688 and
CHE0844455 (M.A.G.-G.), IGERT MCTP DGE0114443
(G.K.), NIH grant GM 36700 (K.N.H.), and grant GU 517/4-2
and Cluster of Excellence EAM (D.M.G.).
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