A matrix isolation, laser flash photolysis, and computational study of adamantene

Article abstract of DOI:10.1021/jp003682l

Photolysis (350 nm) of 3-noradamantyldiazirine (5) in argon at 14 K produces adamantene (1) (λ_max = 320 nm; IR, v = 1478 cm^-1) and protoadamant-3-ene (8) (λ_max = 220 nm); IR, v -1580 cm1). Strained alkenes 1 and 8 can be interconverted photochemically in argon at 14 K. Laser flash photolysis (351 nm) of 5 in benzene solution produces 1. The decay of 1 is monitored at 325 nm. Absolute rate constants of reaction of 1 with methanol, 1,3-cyclohexadiene, tris(trimethylsilyl)silane, acetic acid, and oxygen are reported.

Full text of DOI:10.1021/jp003682l

J. Phys. Chem. A 2001, 105, 3803-3807
3803
A Matrix Isolation, Laser Flash Photolysis, and Computational Study of Adamantene
Eunju Lee Tae, Zhendong Zhu, and Matthew S. Platz*
Department of Chemistry, The Ohio State UniVersity, 100 W. 18th AVenue, Columbus, Ohio 43210
ReceiVed: October 6, 2000; In Final Form: January 24, 2001
Photolysis (350 nm) of 3-noradamantyldiazirine (5) in argon at 14 K produces adamantene (1) (λmax ) 320
nm; IR, ν ) 1478 cm-1) and protoadamant-3-ene (8) (λmax ) 220 nm); IR, ν ) 1580 cm-1). Strained alkenes
1 and 8 can be interconverted photochemically in argon at 14 K. Laser flash photolysis (351 nm) of 5 in
benzene solution produces 1. The decay of 1 is monitored at 325 nm. Absolute rate constants of reaction of
1 with methanol, 1,3-cyclohexadiene, tris(trimethylsilyl)silane, acetic acid, and oxygen are reported.
I. Introduction
SCHEME 1
Bredt was the first to recognize that nature avoids placing
double bonds at the bridgehead positions of bicyclic molecules.1
Adamantene (1) is a famous violation of Bredt’s rule. Calcula-
tions indicate that the heat of formation of 1 is 32.36 kcal/mol
and that the twist angle between the orbitals making up the π
portion of the double bond is 64°.2 Adamantene has a large
olefinic strain energy of 39.5 kcal/mol,3 and consequently it is
a transient species in solution and in the gas phase.4 It can be
trapped with butadiene to form a Diels-Alder adduct, for
example.4c,d In the absence of trapping agents it will dimerize.5
Several workers have generated bridgehead alkenes from the
rearrangement of bridgehead carbenes.6 Jones7 and Michl2 and
co-workers have generated adamantene from what is formally
noradamantylcarbene. Tomioka et al. has generated 1-phenyl-
adamantene (4) from a bridgehead diazo precursor (2).8 The
ring expansion of a bridgehead carbene has also been used to
prepare tricyclo[2.2.2]-1-octene,9 1-chlorohomoadamantene,10
homoadamantene,11 and homocubene.12,13 The lifetime of 4 in
SCHEME 2
benzene solution was found to be 5.6 ms at ambient temperature.
The long lifetime of 4 is not surprising considering that sterically
blocked homoadamantenes are remarkably persistent.14,15 The
strained alkene 4 reacts with methanol with an absolute rate
constant of 8.7 × 101 M-1 s-1. The phenyl-substituted bridge-
head alkene reacts with oxygen with absolute rate constant 7.0
× 106 M-1 s-1. Furthermore, the strained alkene reacts with
tri-n-butyltin hydride with rate constant 6.3 × 104 M-1 s-1 to
form a species with λmax ) 310-320 nm, attributed to a benzylic
radical.8 Bridgehead carbene 3 was not trappable with pyridine
and was not observed by flash photolysis.
This work stimulated us to synthesize the desphenyldiazirine
5 (Scheme 1). Herein, we are pleased to report the results of
this study and absolute rate constants of reaction of 1 with
typical quenchers.
II. Experimental Section
1
General Methods. H NMR spectra were obtained on a
Bruker DRX-500 (500 MHz) or Bruker AM-200 MHz spec-
10.1021/jp003682l CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/27/2001

3804 J. Phys. Chem. A, Vol. 105, No. 15, 2001
Tae et al.
trometer. IR spectra were recorded on a Perkin-Elmer 1710
Fourier transform spectrometer interfaced with a Perkin-Elmer
3700 data station with 2 cm-1 resolution. The GC-MS
spectrometer was an HP-6890 series GC system with an HP-1
methylsiloxane capillary column (40.0 m × 100 µm × 0.20
µm). The gas chromatograph was linked to an HP 5973 mass-
selective detector.
Benzene, toluene, and cyclohexane were purified by distil-
lation from sodium-benzophenone and stored under an argon
atmosphere. 1,3-Cyclohexadiene, 1,3-diphenylbenzofuran, acetic
acid, 1,3-butadiene, and tris(trimethylsilyl)silane were purchased
from Aldrich Chemical Co. Piperidine and methanol were dried
by distillation prior to use.
Laser Flash Photolysis (LFP) Studies. For LFP studies, a
stock solution of 5 in benzene was prepared to an optical density
of 0.5-1.0 and placed in each cuvette (3 mL per cuvette) with
various amounts of trapping agents. For Nd:YAG laser flash
studies, suprasil quartz fluorescence free static cells and a special
sample holder were used to avoid scattering of the laser beam.
The LFP apparatus utilized a Lumonics TE-861-4 excimer laser
(351 nm, 60 mJ, 7 ns, KRF excimer) and a Continuum PY62C-
10 Nd:YAG laser (355 nm, 30 mJ, 2 ns). Samples used in LFP
experiments were deoxygenated by passing a flow of dry argon
through the sample for 2 min. The analysis of the data was
carried out by the program Igor designed by Wavemetrics.
Transient absorption spectra were obtained on an EG&G PARC
1460 optical multichannel analyzer fitted with an EG&G PARC
1304 pulse amplifier, an EG&G PARC 1024 UV detecter, and
a Jarrell-Ash 1234 grating. The monochromator used was an
Oriel 77200. Signals were obtained with a photomultiplier tube
detector and were digitized by a Tektonix 5818A A/D transient
digitizer. The spectrometer has been described elsewhere in
some detail.16
Figure 1. UV-vis spectra obtained upon photolysis of 5 in an argon
matrix: (a) before photolysis; (b) after photolysis at 350 nm (33 min);
(c) after photolysis at 320 nm (35 min); (d) after photolysis at 254 nm
(25 min).
hypochlorite (0.113 mL, 1 mmol) was added to the mixture
(cooled in an ice bath), and the resulting mixture was stirred
for 1 h. The mixture was extracted with pentane and dried with
sodium sulfate. The solvent was evaporated, and then the
mixture was chromatographed using only pentane. The yield
of 3-noradamantyldiazirine was 67%. 1H NMR (500 MHz,
CDCl3, ppm) δ 2.24 (m, 1H), 2.17 (s, 2H), 1.65-1.41 (m, 10H),
0.96 (s, 1H); 13C NMR (126 MHz, CDCl3, ppm) δ 47.9, 46.0,
43.8, 41.3, 37.0, 34.9, 26.8; UV-vis λmax 340 nm; IR 1582
cm-1; MS (EI) m/z (rel intens) 134 (M+ - N2, 1), 119 (17),
Matrix Isolation Spectroscopy. The diazirine precursor 5
was placed in a glass U tube which was directly connected to
the closed cycle cryogenic system cooled by helium (Air
Products). Argon gas blew over the precursor and condensed
on the surface of a CsI window. The argon matrix formed was
maintained at 14 K during the entire experiment. UV-vis
spectra were measured with a Lambda 6 UV-vis spectropho-
tometer. Ray-O-Net lamps (254 and 350 nm) were used to
photolyze the sample, and IR and UV-vis spectra were recorded
sequentially in each step of the photolysis cycle.
105, 91 (100), 77 (67); HRMS (EI) calcd for C10H14 (M+
N2) 134.1096, found 134.1090.
-
GC-MS Product Study. 1,3-Butadiene was condensed into
a solution of 5 in benzene at 273 K. Photolysis (350 nm, Ray-
O-Net)) produced four adducts as determined by GC-MS. The
major adduct was identical (GC retention time, MS fragmenta-
tion pattern) with an authentic sample of 10 provided by
Professor Jones. An HP-6890 series GC system with an HP-
SMS phenylmethylsiloxane capillary column (300 nm × 250
µm × 0.25 µm) was employed. The GC was linked to an HP
5973 mass-selective detector.
Density Functional Calculations. Geometries of singlet
noradamantylcarbene, adamantene, and protoadamant-3-ene
were fully optimized at the B3LYP level using the 6-31G* basis
set, and harmonic frequencies were calculated at the same level.
All the calculations were carried out with the Guassian software
package.17
III. Results
Matrix Isolation Spectroscopy. Photolysis (350 nm, 33 min)
of 3-noradamantyldiazirine in an argon matrix (curve a, Figure
1) led to the disappearance of the diazirine and to the appearance
of new bands at 250 and 320 nm (curve b, Figure 1). The band
at 320 nm is assigned to adamantene on the basis of the work
of the Michl group.2 The band at 250 nm likely contains
contributions from protoadamant-3-ene (8) and diazo compound
72 as the IR spectrum of this matrix exhibits an intense band at
2057 cm-1 assigned to the latter compound (Scheme 1).
Subsequent photolysis of adamantene with 320 nm light (35
min) resulted in the disappearance of the 320 nm band (curve
c, Figure 1). The difference spectrum (c) - (b) in the UV-vis
region is shown in Figure 1, and that in the IR region is shown
in Figure 2.
Preparation of 3-Noradamantyldiazirine. 3-Noradamantyl
alcohol18 was prepared by reduction of 3-noradamantanecar-
boxylic acid (Aldrich) with lithium aluminum hydride (LAH).
3-Noradamantyl alcohol was isolated by column chromatogra-
phy (33% EtOAc in hexane). The yield was 99%. 3-Norada-
mantylcarboxaldehyde19 was prepared from 1 g (6.58 mmol)
of 3-noradamantyl alcohol by Swern oxidation.20 The yield of
aldehyde19 was 96%. The desired diazirine was synthesized from
the aldehyde. 3-Noradamantylcarboxaldehyde (0.15 g, 1 mmol)
was stirred with 2 mL of 1 M lithium bis(trimethylsilyl)amide
in 3 mL of THF cooled with an ice bath for 30 min. The mixture
was cooled with an acetone-dry ice bath, and then hydoxyl-
amine-O-sulfonic acid (0.12 g, 1 mmol) dissolved in 1 mL of
diglyme was added slowly over 30 min. The solution was
warmed with an ice bath and stirred for 1 h. tert-Butyl
The (c) - (b) curve shown in Figure 1 and the spectra of
curve a in Figure 2 are assigned to 8 (1580 cm-1) on the basis

LFP Study of Adamantene
J. Phys. Chem. A, Vol. 105, No. 15, 2001 3805
TABLE 1: Calculated and Experimental Data on the
Strained Alkenes Produced on Photolysis of
3-Noradamantyldiazirine
a In argon matrix and in the calculations, the top values are those of
Michl et al.2 and the lower values in parentheses are this work. b MNDO
value multiplied by 0.915 (a value chosen to optimize agreement for
CdC stretch in ethylene and tetramethylethylene). c B3LYP/6-31G*,
frequencies with a scaling factor of 0.97. d INDO/S.21 e INDO/S with
double excitations.21
Figure 2. Theoretical and observed IR spectrum of adamantene
obtained upon photolysis (350 nm) of 3-noradamantyldiazirine. The
spectrum of adamantene (curve b) corresponds to the conditions of (d)
- (c) of Figure 1. The IR spectrum of protoadamantene (curve a)
corresponds to the conditions of (c) - (b) of Figure 1. The spectrum
was calculated at the B3LYP/6-31G* level without scaling.
Figure 3. Transient spectrum of 1 produced by LFP of 5 in benzene
at ambient temperature. The spectrum was recorded over a window of
200 ns, 300 ns following the laser pulse.
protoadamant-3-ene and adamantene were calculated using the
INDO/S method with single and double excitations.21
The first excited state of adamantene and protoadamant-3-
ene consists of 99% electron promotion from orbital 2 to orbital
3 as shown in the Supporting Information. This is a pure π to
π* excitation. The differences of HOMO to LUMO excitation
energies in adamantene and protoadamant-3-ene are 6.8 and 8.2
eV, respectively.
of a B3LYP/6-31G* calculation (1670 cm-1). Our computational
result is in excellent agreement with that of the Michl group
(1587 cm-1)2 (Table 1). Continuous irradiation of the matrix
containing protoadamant-3-ene with 254 nm light (25 min)
regenerated the 320 nm band (curve d, Figure 1). The difference
spectrum (d) - (c) in UV-vis is shown in Figure 1, and the IR
spectra are shown in Figure 2 (curve b). The (d) - (c) curve in
Calculations of noradamantylcarbene and its rearrangement
to adamantene and protoadamant-3-ene will be reported else-
where.23
Figure 1 and curve b in Figure 2 were assigned to 1 (1478 cm-1
)
on the basis of a B3LYP/6-31G* calculation (1590 cm-1). The
reported value of 1481 cm-1 for adamantene by Michl2 is in
good agreement with our experimental and computational work.
Full details concerning the results of the calculations can be
found in the Supporting Information. This is the first observation
of photochemical interconversion between protoadamant-3-ene
and adamantene and is summarized in Scheme 1. We posit that
photolysis of alkenes 1 and 8 forms bridgehead carbene 6, which
is a transient species in argon at 10 K and which immediately
re-forms the olefins. The difference in absorption spectra of 1
and 8 allows one alkene to be pumped to the other isomer.
Photochemical transformation of alkenes to carbenes is well-
known.22
LFP Studies. LFP (351 nm, 60 mJ, 17 ns, KrF excimer) of
3-noradamantyldiazirine in benzene at ambient temperature
produces the transient absorption spectrum shown in Figure 3.
The maximum of the transient spectra is at 325 nm. The carrier
of this absorption band is assigned to adamantene on the basis
of our own and previously reported matrix experiments.2
2-Phenyladamantene has longer wavelength absorption than does
1, λmax ) 434 nm, due to the extended conjugation of the π
system.8
Pyridine was added to trap putative carbene intermediate 6.
There was no transient spectrum of an ylide formed in the visible
region upon LFP of diazirine 5. Either the lifetime of 3-no-
radamantylcarbene (6) is too short to permit reaction to form
ylide 9 or the yield of carbene 6 produced upon photolysis of
5 is too small to be detected. We suspect22 that some adamantene
is formed in a rearrangement of the diazirine excited state in a
manner reminiscent of the homocubyl13 carbene system. Thus,
the yield of carbene and of pyridine ylide 9 in solution is very
The absorption spectra of strained alkenes 1 and 8 was
adequately described by INDO/S calculations with single and
double excitations.21 The geometry optimizations were per-
formed at the B3LYP/6-31G* level with the Gaussian 94
software package. The stationary points were verified by
harmonic frequency calculations. The excitation energies of

3806 J. Phys. Chem. A, Vol. 105, No. 15, 2001
Tae et al.
TABLE 2: Lifetime and Decay Rate Constant of Adamantene with Representative Quenchers
quencher
kQ(1), M-1 s-1
kQ(48), M-1 s-1
8.7 × 101
quencher
kQ(1), M-1 s-1
kQ(48), M-1 s-1
methanol
1,3-cyclohexadiene
tris(trimethylsilyl)silane
8.4 × 104
1.8 × 105
6.3 × 105
tri-n-butyltin hydride
acetic acid
oxygen
6.3 × 104
7.0 × 106
1.4 × 107
5.4 × 107
low. Furthermore, it was not possible to trap carbene 6 with
piperidine to form an adduct in more than trace quantities.23
However, photolysis of 5 in the presence of 1,3-butadiene yields
a product with the same retention time (gas chromatography)
and mass spectral fragmentation pattern as an authentic sample
of 10 generously provided by Professor Jones.
methanol, 8.4 × 104 M-1 s-1; 1,3-cyclohexadiene, 1.8 × 105
M-1 s-1; acetic acid, 1.4 × 107 M-1 s-1; oxygen, 5.4 × 107
M-1 s-1. Adamantene is considerably more reactive than
2-phenyladamantene.
Acknowledgment. Support of this work by the NSF is
gratefully acknowledged (Grant CHE-9613861). We are in-
debted to Professors Berson and Wiberg of Yale University for
their generous donation of the matrix isolation equipment used
in this work. We are also indebted to Professor M. Jones, Jr.
for an authentic sample of the adamantene-1,3-butadiene
Diels-Alder adduct.
1 decays over many microseconds in a second-order process
to form a mixture of known dimers. The decay of 1 is
accelerated in the presence of a saturating concentration of
oxygen by a factor of roughly 3.6. From this it is possible to
estimate kO as 5.4 × 107 M-1 s-1 (Table 2).
2
The decay of adamantene is pseudo-first-order in the presence
of quenchers (methanol, 1,3-cyclohexadiene, tris(trimethylsilyl)
silane, tri-n-butyltin hydride, acetic acid) of the strained alkene.
The experimental decays are analyzed to yield time constants
Supporting Information Available: Energies and geom-
etries of the stationary points for 1, 6, and 8 and configurations
and states involved in the electronic transitions of 1 and 8. This
material is available free of charge via the Internet at http://
pubs.acs.org.
k
obs. Plots of kobs versus the concentration of quencher, Q, are
linear (Figure 4). The slopes of these plots represent values of
kQ, the absolute second-order rate constant of reaction of
adamantene with quencher (Table 2). Adamantene undergoes
reaction with a diene, a hydrogen atom and a proton donor at
easily measured rates as expected from its large strain energy
and known chemistry.2,4,7
Unsurprisingly, as seen in Table 2, parent adamantene is
significantly more reactive than sterically blocked derivative
4.8
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3-Noradamantyl diazirine 5 was deposited into an argon
matrix at 14 K. Exposure to 350 nm light consumed the diazirine
and led to the appearance of the known UV and IR spectra of
adamantene. Noradamantylcarbene was not observed in the
matrix. Photolysis (320 nm) of matrix-isolated adamantene led
to its conversion into protoadamant-3-ene. Photolysis (254 nm)
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produced the transient spectrum of adamantene (λmax ) 325
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LFP Study of Adamantene
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