The electron as a protecting group. 3. Generation of acenaphthyne radical anion and the determination of the heat of formation of a strained cycloalkyne

Article abstract of DOI:10.1021/ja003069f

Acenaphthyne dicarboxylate (12) was transferred into the gas phase from solution via electrospray ionization and subsequently was sequentially fragmented in a Fourier transform mass spectrometer to afford acenaphthyne radical anion (9). Structural confirmation of 9 was achieved by converting it to acenaphthenone enolate (13) and demonstrating that this species is identical to the ion produced upon deprotonation of acenaphthenone (5). The reactivity of 9 was explored, and since an electron can serve as a protecting group, we were able to measure the heat of hydrogenation (98 ± 4 kcal mol^-1) and the heat of formation (160 ± 4 kcal mol^-l) of acenaphthyne (1) via the application of a thermodynamic cycle. Its strain energy (68 kcal mol^-1) and acenaphthylene's (10H) first and second C-H bond dissociation energies (117 ± 4 and 84 ± 2 kcal mol^-1) also were obtained. Ab initio and density functional theory calculations were carried out on the species of interest to explore their geometries and energetics. Our results were interpreted in comparison to cyclopentyne, and its predicted heat of formation (98 kcal mol^-l) and strain energy (59 kcal mol^-1) are reported.

Full text of DOI:10.1021/ja003069f

J. Am. Chem. Soc. 2001, 123, 4189-4196
4189
The Electron as a Protecting Group. 3. Generation of Acenaphthyne
Radical Anion and the Determination of the Heat of Formation of a
Strained Cycloalkyne
Katherine M. Broadus and Steven R. Kass*
Contribution from the Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455
ReceiVed August 17, 2000
Abstract: Acenaphthyne dicarboxylate (12) was transferred into the gas phase from solution via electrospray
ionization and subsequently was sequentially fragmented in a Fourier transform mass spectrometer to afford
acenaphthyne radical anion (9). Structural confirmation of 9 was achieved by converting it to acenaphthenone
enolate (13) and demonstrating that this species is identical to the ion produced upon deprotonation of
acenaphthenone (5). The reactivity of 9 was explored, and since an electron can serve as a protecting group,
-
1
we were able to measure the heat of hydrogenation (98 ( 4 kcal mol ) and the heat of formation (160 ( 4
-
1
kcal mol ) of acenaphthyne (1) via the application of a thermodynamic cycle. Its strain energy (68 kcal
-
1
mol ) and acenaphthylene’s (10H) first and second C-H bond dissociation energies (117 ( 4 and 84 ( 2
-
1
kcal mol ) also were obtained. Ab initio and density functional theory calculations were carried out on the
species of interest to explore their geometries and energetics. Our results were interpreted in comparison to
-1
-1
cyclopentyne, and its predicted heat of formation (98 kcal mol ) and strain energy (59 kcal mol ) are reported.
Introduction
Acenaphthyne (1) is one of a few cyclopentyne derivatives
that have been generated. Other examples include nor-
Cycloalkynes have been studied over the years in an attempt
to understand molecular strain and its ramifications. The
synthetic challenges, enhanced reactivity, distorted geometries,
and unique properties of this class of compounds have been
17,18
19
20
bornyne,
norbornenyne, benzocyclopentyne, and 2,2,5,5-
21
tetramethyl-1-thiacyclopentyne. Approximately a century after
the existence of 1 was postulated based on the recovery of trimer
22,23
2,
Chapman and co-workers succeeded in obtaining the first
1-3
4
the subject of several reviews. Cyclooctyne can be isolated
24
spectroscopic data (UV and IR) of 1. It was generated in an
argon matrix at 15 K upon irradiation of bis(diazo) ketone 3
and subsequent loss of carbon monoxide from 4 (eq 1). The
cyclopentyne moiety of 1 was found to react with water to give
acenaphthenone (5) plus a hydrated dimer and with oxygen to
afford acenaphthoquinone (6, eq 2). Decacyclene (2) was
recovered when the matrix was allowed to warm to room
temperature. The formation of 2 has served as indirect evidence
for the generation of acenaphthyne in other work. Specifically,
whereas the seven-, six-, and five-membered-ring analogues only
have been observed in low-temperature matrixes or their
existence as transient intermediates has been substantiated by
trapping and isotopic labeling studies.5-12 Despite extensive
1
3,14
efforts, cyclobutyne or a derivative of it remains elusive,
and singlet cyclopropyne is predicted not to be a minimum on
the potential energy surface.15,16
*
To whom correspondence should be addressed.
2
has been observed upon thermolysis of bis(tosylhydrazone)
(1) Meier, H. Cyclic Alkynes, Enynes and Dienynes. A Synthetic
Challenge. In AdVances in Strain in Organic Chemistry; JAI Press Ltd.:
salt 7, reaction of dibromosulfone 8 with base, and deoxygen-
ation of 6 with titanium(0) or phosphorus pentasulfide (Figure
Stanford, CT, 1991; Vol. 1, pp 25-272.
(2) Krebs, A.; Wilke, J. Top. Curr. Chem. 1983, 109, 189-223.
3) Nakagawa, M. Cyclic Acetylenes; Patai, S., Ed.; J. Wiley and Sons:
2
5-27
1
).
The last approach has received recent attention as 2
(
New York, 1978; pp 635.
4) Blomquist, A. T.; Liu, L. H. J. Am. Chem. Soc. 1953, 75, 2153-
154.
(
(15) Fitzgerald, G.; Saxe, P.; Schaefer, H. F., III J. Am. Chem. Soc. 1983,
2
1
9
105, 690-695.
(5) Gilbert, J. C.; Hou, D.-R.; Grimme, J. W. J. Org. Chem. 1999, 64,
(16) Fitzgerald, G.; Schaefer, H. F., III Isr. J. Chem. 1983, 23, 93.
(17) Gassman, P. G.; Gennick, I. J. Am. Chem. Soc. 1980, 102, 6863-
6867.
529-1534.
(6) Gilbert, J. C.; McKinley, E. G.; Hou, D.-R. Tetrahedron 1997, 53,
891-9902.
(18) Gassman, P. G.; Valcho, J. J. J. Am. Chem. Soc. 1975, 97, 4768-
4770.
(19) Kitamura, T.; Kotani, M.; Yokoyama, T.; Fujiwara, Y. J. Org. Chem.
1999, 64, 680-681.
(20) Wittig, G.; Heyn, H. Chem. Ber. 1964, 97, 1609-1618.
(21) Bolster, J. M.; Kellogg, R. M. J. Am. Chem. Soc. 1981, 103, 2868-
2869.
(22) Blumenthal, M. Ber. Dtsch. Chem. Ges. 1874, 7, 1092.
(23) Dziewonski, K. Ber. Dtsch. Chem. Ges. 1903, 36, 962.
(24) Chapman, O. L.; Gano, J.; West, P. R.; Regitz, M.; Maas, G. J.
Am. Chem. Soc. 1981, 103, 7033-7036.
(25) Nakayama, J.; Segiri, T.; Ohya, R.; Hoshino, M. J. Chem. Soc.,
Chem. Commun. 1980, 791-792.
(26) Nakayama, J.; Ohshima, E.; Ishii, A.; Hoshino, M. J. Org. Chem.
1983, 48, 60-65.
(
(
(
7) Gilbert, J. C.; Baze, M. E. J. Am. Chem. Soc. 1983, 105, 664-665.
8) Fitjier, L.; Modaressi, S. Tetrahedron Lett. 1983, 24, 5495-5498.
9) Fitjier, L.; Kliebisch, V.; Wehle, D.; Modaressi, S. Tetrahedron Lett.
1
1
1
8
7
4
982, 23, 1661-1664.
10) Wentrup, C.; Blanch, R.; Briehl, H.; Gross, G. J. Am. Chem. Soc.
988, 110, 1874-1880.
11) Bottini, A. T.; Frost, K. A., II; Anderson, B. R.; Dev, V. Tetrahedron
973, 29, 1975-1981.
12) Montegomery, L. K.; Applegate, L. E. J. Am. Chem. Soc. 1967,
9, 5305-5307.
13) Baumgart, K.-D.; Szeimies, G. Tetrahedron Lett. 1984, 25, 737-
40.
(
(
(
(
(14) Montegomery, L. K.; Roberts, J. D. J. Am. Chem. Soc. 1960, 82,
750-4751.
1
0.1021/ja003069f CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/12/2001

4190 J. Am. Chem. Soc., Vol. 123, No. 18, 2001
Broadus and Kass
affinity and electron binding energy of this ion provide a means
via a thermodynamic cycle to afford the desired thermochemical
data for 1. In the course of this work, we also explored the
thermochemistry of acenaphthylene (10H) which is an abundant
species in combustion processes as well as a known soil
3
3-35
contaminant.
High-level ab initio and density functional
theory calculations have been carried out to explore the
geometries and energetics of the species studied. Our results
are interpreted in comparison to cyclopentyne.
Figure 1. Additional precursors of 2 in which 1 is a proposed
intermediate.
has been shown to be a precursor for the synthesis of
buckminsterfullerene fragments.28 At present, successful trap-
ping of 1 with a diene has not been reported.2
Experimental Section
6,29
1
,2-Acenaphthylene dicarboxylic acid (11) was prepared in three
3
6,37
steps from acenaphthene according to literature procedures.
All
reagents were used as received except for chlorotrimethylsilane which
was distilled from quinoline. The acenaphthene impurity (20%) present
in commercial acenaphthylene was carried along the synthetic sequence
and ultimately separated from the trimethylsilyl derivative 15. NMR
spectra were collected on a Varian VXR-300 spectrometer and are
reported in ppm (δ). Infrared data were recorded on a Perkin-Elmer
-1
1
600 Series FTIR and the output is given in wavenumbers (cm ). High-
resolution mass spectra were obtained on a Finnigan 2001 FTMS or
MAT 95.
Acenaphthenone (5). Acenaphthenol (0.19 g, 1 mmol) was dissolved
in warm ether, the mixture was cooled to 0 °C, and 1.2 mL of an acidic
dichromate solution was added. The oxidizing solution was prepared
by dissolving 1.0 g of sodium dichromate in 4 mL of water and 0.8
mL of sulfuric acid. After being stirred for ∼15 min, the reaction
mixture was extracted twice with ether. The combined organic layer
was washed with 5% NaHCO and water, dried over MgSO , and
3 4
concentrated by rotary evaporation. The resulting ketone was purified
by column chromatography using 0-10% ethyl acetate in hexanes as
the eluting solvent to give 0.12 g (71%) of a white solid product, mp
1
)
119.5-120.5 °C. H NMR (300 MHz, CDCl
3
) δ 8.09 (d, J ) 8.1
Hz, 1H), 7.96 (dd, J ) 6.9 and 0.9 Hz, 1H), 7.82 (d, J ) 8.4 Hz, 1H),
7
7
.72 (dd, J ) 8.1 and 6.9 Hz, 1H), 7.60 (dd, J ) 8.4 and 6.9 Hz, 1H),
.47 (dd, J ) 6.9 and 0.9 Hz, 1H), 3.83 (s, 2H); 13C NMR (75 MHz,
CDCl
3
) δ 203.0, 143.0, 135.0, 134.7, 131.5, 131.0, 128.4, 128.0, 124.0,
1
8
1
21.5, 121.1, 42.0; IR (KBr) 1715, 1651, 1539, 1494, 1362, 1261, 1000,
-
1
+•
23, 775 cm ; HRMS-EI (M) calcd for C12
68.0580.
8
H O 168.0575, obsd
trans-1,2-Dibromoacenaphthene. Acenaphthylene (1.5 g, 9.8 mmol)
was dissolved in THF (40 mL) and the mixture cooled to -78 °C. A
solution of pyridinium tribromide (3.1 g, 11 mmol) in 15 mL of THF
was then added dropwise, and the mixture was allowed to slowly warm
to room-temperature overnight. The crude product was diluted with
hexanes (∼20 mL) and washed with aqueous sodium hydrosulfite (2×),
Several probes have been used to assess the strain energy in
cycloalkynes: heats of hydrogenation, competitive reactivity
studies, correlation of 13C NMR chemical shifts, frequencies
of the carbon-carbon triple bond stretch in IR spectroscopy,
water, and saturated NaCl. The organic layer was dried over MgSO
4
and energies of the lowest unoccupied molecular orbitals
and concentrated by rotary evaporation to afford 2.9 g (93%) of a light
1
2
brown solid sufficiently clean to be carried on. H NMR (300 MHz,
(
LUMO). We were interested in measuring the heat of
CDCl
3
) δ 7.84 (d, J ) 3 Hz, 2H), 7.82 (d, J ) 3 Hz, 2H), 7.62 (t, J )
formation and thus the heat of hydrogenation of acenaphthyne.
Previous work with 1 indicates that it would be difficult, if not
impossible, to obtain these data by conventional approaches.
Therefore, we proposed to use the electron as a protecting group
3
1
3
Hz, 2H), 6.05 (s, 2H); 13C NMR (75 MHz, CDCl
3
) δ 140.5, 134.9,
+
•
31.0, 128.9, 126.0, 122.7, 55.0; HRMS-EI (M) calcd for C12
09.8993, obsd 309.8993.
8 2
H Br
1-Bromoacenaphthylene. A mixture of trans-1,2-dibromoacenaph-
and generate acenaphthyne radical anion (9) in a Fourier
thene (0.5 g, 1.6 mmol), diazobicyclo[5.4.0]undec-7-ene (DBU) (0.3
g, 1.9 mmol), and 35 mL of DMF was heated at 85 °C for 3 h. The
reaction was allowed to cool and then poured into ice water. The product
was extracted (2×) with a mixture of ether and hexanes. The combined
transform mass spectrometer.3
0-32
Measurement of the proton
(
(
27) Zimmerman, K.; Haenel, M. W. Synlett 1997, 609-611.
28) Scott, L. T.; Bratcher, M. S.; Hagen, S. J. Am. Chem. Soc. 1996,
1
18, 8746-8744.
(
29) For earlier unsuccessful efforts directed toward the synthesis of 1
(31) Reed, D. R.; Hare, M. C.; Kass, S. R. J. Am. Chem. Soc. 2000,
122, 10689-10696.
see: (a) Rasheed, K. Tetrahedron, 1966, 22, 2957-2966. (b) Cadogan, J.
I. G.; Scott, R. J.; Wilson, N. H. J. Chem. Soc., Chem. Commun. 1974,
(32) Broadus, K. M.; Kass, S. R. J. Am. Chem. Soc. 2000, 122, 10697-
9
02-903. (c) Cadogan, J. I. G.; Rowley, A. G.; Wilson, N. H. Liebigs
Ann. Chem. 1978, 74-77.
30) Anions often are easier to generate and less prone to rearrangements
10703.
(33) Necula, A.; Scott, L. T. J. Am. Chem. Soc. 2000, 122, 1548-1549.
(34) Lafleur, A. L.; Howard, J. B.; Lummer, E.; Taghizadeh, K.; Necula,
A.; Scott, L. T.; Swallow, K. C. Polycycl. Aromat. Compd. 1998, 12, 223-
237.
(35) Mazzera, D.; Hayes, T.; Lowenthal, D.; Zielinska, B. Sci. Total
EnViron. 1999, 229, 65-71.
(36) Herold, D. A.; Rieke, R. D. J. Org. Chem. 1979, 44, 1359-1361.
(37) Trost, B. M.; Brittelli, D. R. J. Org. Chem. 1967, 32, 2620-2621.
(
and fragmentations than their neutral counterparts. They also can be used
to probe neutral molecules. Consequently, the electron can be viewed as a
protecting group. While we are not aware of this terminology being used
before, the concept of using anions to probe neutral molecules is not new
and many examples can be found in the literature. For instance, see refs 31
and 32 (parts 1 and 2 of this series), and references therein.

The Electron as a Protecting Group. 3
J. Am. Chem. Soc., Vol. 123, No. 18, 2001 4191
organic layer was washed with water, 1 M HCl, water, saturated
NaHCO , and saturated NaCl, and then dried over MgSO and
3 4
Scheme 1. Synthesis of Acenaphthyne Radical Anion (9)
concentrated by rotary evaporation. The crude product was purified
by flash chromatography with hexanes as the eluting solvent to afford
1
0
.31 g (81%) of a yellow oil. H NMR (300 MHz, CDCl
3
) δ 7.87 (d,
J ) 8.1 Hz, 1H), 7.79 (d, J ) 8.1 Hz, 1H), 7.70 (d, J ) 6.6 Hz, 1H),
.62 (d, J ) 6.9 Hz, 1H), 7.61 (dd, J ) 8.1 and 6.9 Hz, 1H), 7.51 (dd,
7
1
3
J ) 8.1 and 6.6 Hz, 1H), 7.16 (s, 1H); C NMR (75 MHz, CDCl
1
1
3
) δ
38.3, 138.0, 129.0, 128.6, 128.0, 127.7, 127.1, 123.7, 123.5, 122.3,
20.8, 119.2; IR (neat) 3039, 1427, 1262, 1029, 832, 765, 727 cm
-
1
;
+
HRMS-CI (4% ammonia in methane mixture) (M + H) calcd for
C
12
H
1
8
Br 230.9809, obsd 230.9802.
-Trimethylsilylacenaphthylene (15). 1-Bromoacenaphthylene (0.59
g, 2.6 mmol) was dissolved in 60 mL of anhydrous THF and cooled to
78 °C. n-Butyllithium (2.5 M in hexanes, 1.1 mL) was added and
-
the mixture was stirred for 1 h. Chlorotrimethylsilane (0.9 g, 8 mmol)
was then added and the reaction was allowed to slowly warm to room-
temperature overnight. The crude product was diluted with hexanes
and washed with 1 M HCl and water. The organic material was dried
over MgSO and concentrated by rotary evaporation. Purification by
4
column chromatography with hexanes afforded 0.49 g (85%) of the
desired silane as a yellow oil. For the gas-phase experiments, the
compound was further purified by medium-pressure liquid chroma-
1
tography again using hexanes as the eluent. H NMR (300 MHz, CDCl
3
)
δ 7.79 (d, J ) 8.1 Hz, 2H), 7.74 (d, J ) 6.9 Hz, 1H), 7.66 (d, J ) 6.6
Hz, 1H), 7.55 (dd, J ) 8.1 and 6.9 Hz, 1H), 7.53 (dd, J ) 8.1 and 6.6
1
3
Hz, 1H), 7.25 (s, 1H), 0.39 (s, 9H); C NMR (75 MHz, CDCl
3
) δ
nature of stationary points was investigated by a full vibrational analysis.
Zero-point-energy corrections were made in all cases, and for computing
acidities the energies were adjusted to 298 K. The structures of
acenaphthyne (1) and cyclopentyne (16) and their alkene analogues
1
1
9
43.4, 143.2, 140.5, 137.9, 130.4, 128.5, 127.83, 127.77, 127.3, 126.9,
24.8, 123.6, -0.15; IR (neat) 3037, 2954, 1482, 1427, 1249, 1150,
31, 838, 770, 755 cm-1; HRMS-EI (M) calcd for C15
+•
H16Si 224.1021,
obsd 224.1042.
(10H and 17) were also investigated at the CAS(4,4)/cc-pVDZ and
Gas-Phase Experiments. All work was carried out in a dual cell
Finnigan Model 2001 Fourier transform mass spectrometer (FTMS)
equipped with a 3 T superconducting magnet and interfaced with a
custom-built Analytica electrospray ionization (ESI) source. Acenaph-
thylene dicarboxylate was sprayed (3 µL/min) into the gas phase from
a 750 µM solution of the diacid in methanol-water (35:65 v/v) which
contained 2 equiv of cesium hydroxide (pH 8-9). Ions were ac-
cumulated for 0.5 s in the hexapole of the ESI source and then allowed
to travel to the FTMS cell during a 45 µs period (time-of-flight). The
following typical settings for the ESI source were found to give optimal
signal of acenaphthylene dicarboxylate: cylinder (needle housing) 2.6
kV, end plate 3.6 kV, capillary 6.0 kV (front) and -36 V (back),
skimmer cone -11 V, acceleration lenses 319 V, deceleration lenses
CAS(2,2)/cc-pVDZ levels of theory, respectively, and the energies were
corrected with zero-point energies from BPW91/cc-pVDZ optimized
42,43
geometries.
For these calculations, the active space was comprised
of the C1-C2 π bonding and antibonding orbitals. Dynamic electron
correlation was accounted for by carrying out CASPT2 calculations
44
on the CAS structures.
Results and Discussion
We have generated the radical anion of acenaphthyne (9) in
a Fourier transform mass spectrometer (FTMS) and explored
its reactivity and thermochemistry to gain insight into the
corresponding neutral cyclopentyne (i.e., 1). A methodology
1
9.0 V, and high voltage element in acceleration/deceleration lens stack
.35 kV. Fluoride ion was prepared by electron ionization of carbon
recently developed in our laboratory was used to prepare 9
1
31
(
Scheme 1). Specifically, dicarboxylate 12 (m/z 119) was
tetrafluoride at 6 eV and used to generate 10 and 13 upon reaction
with 1-trimethylsilylacenaphthylene and acenaphthenone, respectively.
In general, ions of interest were isolated by ejecting unwanted species
sprayed from a basic solution of diacid 11 into our FTMS where
it was isolated and then fragmented by sustained off-resonance
45
3
8
39
irradiation (SORI). This technique involves kinetically exciting
an ion by applying energy to its cyclotron frequency off-
resonance in the presence of an inert collision gas. In this case,
application of 1.3 eV (lab) of energy for 30 ms concurrent with
with a SWIFT waveform or a chirp broad band excitation for low
-
5
masses and were vibrationally cooled with pulses of argon (10 Torr).
Neutral reagents were introduced via slow leak or pulsed valves, and
all reactions were monitored as a function of time.
Computations. All calculations were performed with Gaussian9840
installed on IBM and SGI workstations. Geometries were optimized
by using the B3LYP functional and the 6-31+G(d) basis set,41 and the
-5
a pulse of argon (∼10 Torr) led to the loss of carbon dioxide
and an electron (m/z 194). Further excitation (1.1 eV (lab) for
1
0 ms) afforded the desired ion 9 (m/z 150, Scheme 1, and
Figure 2). On-resonance irradiation of 12 gives 9 directly, but
the efficiency of this process is considerably lower than that of
the stepwise route. In addition to the loss of an electron and
carbon dioxide, electron detachment of dicarboxylate dianion
(
38) Wang, T. C. L.; Ricca, T. L.; Marshall, A. G. Anal. Chem. 1986,
5
8, 2935-2938.
(
39) Marshall, A. G.; Roe, D. C. J. Chem. Phys. 1980, 73, 1581-1590.
(40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Oritz, J. V.; Baboul, A. G.; Stefanov, B. B.; Lui, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Ketih, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian
1
2 without fragmentation to yield the corresponding radical
anion at m/z 238 is a minor dissociation pathway.
(42) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100. (b) Perdew,
J. P.; Burke, K.; Wang, Y. Phys. ReV. B 1996, 54, 16533-16539.
(43) (a) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007-1023. (b)
Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys. 1992,
96, 6796-6806.
(44) Andersson, K.; Borowski, P.; Fowler, P. W.; Malmqvist, P.-Å.; Roos,
B. O.; Sadlej, A. J. Chem. Phys. Lett. 1992, 190, 367-373.
(45) Gauthier, J. W.; Trautman, T. R.; Jacobson, D. B. Anal. Chim. Acta
1991, 246, 211-225.
9
8, Revision A.7; Gaussian, Inc.; Pittsburgh, PA, 1998.
41) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(

4192 J. Am. Chem. Soc., Vol. 123, No. 18, 2001
Broadus and Kass
Table 1. Summary of Deuteron Transfer and
Hydrogen-Deuterium Exchange Results for Enolate 13 Generated
upon Reaction of 9 with tert-Butyl Alcohol (Derivative) and by
Deprotonation of 5 (Authentic)
derivative
D+
H/D
authentic
D+
H/D
acid
CH
∆H°acida
transfer exchangeb transfer exchange
CF
CH
3
2
OD 361.8 ( 2.5 no
no
no
no
no
no
no
(
3
)
3
CSD 352.5 ( 2.2 no
C
6
H
5
OD
349.2 ( 2.1 slow
348.6 ( 2.9 yes
yes, 1
no
slow
yes
yes, 1
no
3 2
CD CO D
a
Acidities are for the protio reagents and are in kcal mol-1. They
was prepared and
are taken from ref 51. b For these experiments, 13-d
1
reacted with the protio reference acid. This was done because the
experiment was carried out in one cell; tert-butyl alcohol-OD was pulsed
-
6
at pressures of 1 × 10 Torr and the reference acid was present at 4
-
8
×
10 Torr.
channel is observed with acetic acid-d4, but not with compounds
-
1
whose ∆H°acid is larger than 350 kcal mol , which leads to a
-
1
bracketed acidity of 349 ( 2 kcal mol for acenaphthenone.
Enolate 13 is rather robust and does not react with dimethyl
disulfide or neopentyl nitrite in the manner characteristic of more
4
7,48
basic enolates that possess an R hydrogen.
Moreover,
fragments of 13 are not observed under CID or SORI conditions,
and large energies (>25 eV (lab)) are required to produce ion
4
9
signal loss.
Figure 2. (a) Spectrum obtained upon electrospray ionization of the
dicesium salt of 1,2-acenaphthylene dicarboxylic acid. Dicarboxylate
1
1 is at m/z 119 and the protonated dianion is at m/z 239. (b) Isolation
of 11. (c) Spectrum obtained upon SORI of 11, m/z 238 and 194
correspond to loss of an electron, and carbon dioxide and an electron,
respectively. (d) SORI dissociation of m/z 194 and isolation of
acenaphthyne radical anion (9, m/z 150).
With confirmation of its structure, we began to explore the
thermochemistry of acenaphthyne radical anion. The proton
affinity of 9 or, equivalently, ∆H°acid of its conjugate acid (10r),
was measured by the bracketing method. This involves reacting
To verify the structure of acenaphthyne radical anion (9), it
was derivatized to acenaphthenone enolate (13) and compared
with an authentic sample. Ion 9 was found to readily afford 13
upon reaction with alcohols (eq 3); this reactivity is analogous
to the conversion of neutral acenaphthyne to acenaphthenone
upon reaction with water. A plausible mechanism for this
transformation is initial proton (or deuteron if ROD is employed)
transfer to afford intermediate 14a which may dissociate to give
9
with a series of reference acids and monitoring the occurrence
or nonoccurrence of proton transfer (Table 2). Acenaphthyne
radical anion was found to deprotonate ethanol and stronger
acids which allows us to assign ∆H°acid(10r) ) 379 ( 2 kcal
-1 50,51
mol .
This result can be compared with a 298 K computed
-1
value of 376.4 kcal mol at the B3LYP/6-31+G(d) level. In
accord with its proton affinity, 9 is able to undergo up to four
hydrogen-deuterium exchanges at less acidic sites with deu-
-
RO or combine to lead to adduct 14b. Subsequent fragmenta-
•
46
tion of R yields acenaphthenone enolate (13). The reactivity
of 13 formed in this way was found to be identical with that
observed for the ion generated upon deprotonation of acenaph-
thenone with fluoride ion. In both cases the enolate undergoes
one hydrogen-deuterium exchange with phenol-OD as well as
a small amount of deuteron transfer (Table 1). The latter reaction
52
terium oxide. It is also worth noting that 9 reacts with
methanol-OD primarily by a reaction channel that produces
(
47) King, G. K.; Maricq, M. M.; Bierbaum, V. M.; DePuy, C. H. J.
Am. Chem. Soc. 1981, 103, 7133-7140.
48) Grabowski, J. J.; Zhang, L. J. Am. Chem. Soc. 1989, 111, 1193-
1203.
49) This behavior contrasts with 1- and 2-naphthoxide which (1) do
(
(
-
1
(46) For a related mechanistic discussion addressing the reactivity of
not undergo H/D exchange, (2) are less basic (PA ) 344 ( 2 kcal mol ),
and (3) give fragment ions upon collision-induced dissociation. See ref 31
for additional details.
o-benzyne radical anion with alcohols, see: Guo, Y.; Grabowski, J. J. J.
Am. Chem. Soc. 1991, 113, 5923-5931.

The Electron as a Protecting Group. 3
J. Am. Chem. Soc., Vol. 123, No. 18, 2001 4193
Table 4. Results for the Electron Affinity Bracketing of 1
Table 2. Results for the Proton Affinity Bracketing of 9
∆H°acid (kcal mol-
1)a
proton transfer
no
compd
carbon disulfide
perfluoropyridine
o-trifluoromethylbenzonitrile
p,p′-difluorobenzophenone
perfluorotoluene
EA (eV)a
electron transfer
acid
ND
3
403.6 ( 0.8
392.9 ( 0.1
387.2 ( 2.5
383.5 ( 0.7
379.8 ( 2.5
378.3 ( 1.0
375.9 ( 1.2
374.6 ( 2.1
372.9 ( 2.1
371.2 ( 2.9
370.7 ( 2.3
361.8 ( 2.5
0.51 ( 0.10
0.68 ( 0.11
0.70 ( 0.10
0.78 ( 0.05
0.86 ( 0.11
1.08 ( 0.11
no
no
no
no
yes
yes
b
D
2
O
H
no
C
6
5
F
no
no
c
CH
CH
CH
3
3
3
OD
CH
CH
2
CH
OD
2
CtCH
no
2
yes
yes
yesd
yes
yese
yes
yes
perfluorobenzonitrile
(
(
CH
CH
3
)
)
2
CHOH
COH
a
Values taken from ref 51.
3
3
CD
3
CN
CH
CtCH
CH OH
FCH
C H
6 5
2
2
OH
The electron binding energy of 9, or equivalently, the electron
affinity (EA) of 1, was determined by the bracketing method
as well. Acenaphthyne radical anion was found to undergo
electron transfer with perfluorotoluene and perfluorobenzonitrile,
but not with other compounds whose radical anions have smaller
electron binding energies (Table 4). These results lead to a value
CF
3
2
a
b
Values taken from ref 51. Four hydrogen-deuterium exchanges
c
are observed. Reaction leads to loss of the ion signal, see text.
d
e
Cyanide ion is the major product (ratio 2:1). Ions at m/z 39 and 83
-
-
2 2
which are assigned as HF‚F and HF‚FCH CH O clusters also are
observed and both make up 7% of the relative product distribution.
-
1
56
of 0.82 ( 0.07 eV (18.9 ( 1.6 kcal mol ) for the EA of 1.
Although the B3LYP hybrid functional generally reproduces
5
7
Table 3. Product Distributions in the Reaction of 9 with Alcohols
experimental electron affinities, it does a poor job (44.4 kcal
-
8
a
at a Pressure of 3 × 10 Torr
proton
-1
mol ) in this instance. Given that the substrate is a ground-
%
% enolate
13 formation
BDE (RH),
state singlet with considerable biradical character, this failing
is not surprising because the Hartree-Fock part of the B3LYP
hybrid functional is a single determinant method that is
inadequate in this case. Similar errors are observed with
o-benzyne, EA ) 13.0 ( 0.2 (expt) and 20.5 kcal mol
B3LYP/6-31+G(d)), and related molecules.
-
1
alcohol (ROH)
transfer
kcal mol
C
2
H
5
3
3
OH
CHOH
COH
37
22
8
20
76
63
78
92
80
24
101.1 ( 0.4
97.8 ( 0.5
96.6 ( 0.3
(
(
CH
CH
)
)
2
3
-1
FCH
CF
2
CH
2
OH
58,59
(
One approach
3
CH OH
2
106.7 ( 1.1
to avoid this difficulty that has been successfully employed in
some cases is to compute the energy difference between the
radical anion and its corresponding neutral in its triplet electronic
state and then subtract the singlet-triplet (S-T) gap obtained
by some other means (i.e., experiment or a different level of
a
Bond energies are taken from ref 55.
virtually no ionic (detectable) products; a small trace of enolate
3 is observed. Based on similar reactivity studies with benzyne
and benzocyclobutadiene radical anions, the former pathway is
assumed to be net abstraction of a molecule of hydrogen from
1
31,60
theory).
Application of this approach to acenaphthyne gives
mixed results. In particular, using a B3LYP/aug-cc-pVDZ value
-
1
methanol to afford acenaphthylene (10H), formaldehyde, and
of 58.7 kcal mol for the energy difference between 9 and
an electron.3
2,46
Strained cycloalkynes also dehydrogenate
3 61
1 and the experimentally determined S-T gap for o-benzyne
alcohols in the condensed phase.5
reacts with the other reference alcohols to give enolate 13 (eq
). The product distributions were found to correlate empirically
3,54
As mentioned earlier, 9
-1 59
(37.5 ( 0.3 kcal mol ) as an estimate for the value of 1,
-
1
gives an EA of 21.2 kcal mol for 1 in good accord with
experiment. If, on the other hand, one uses computed BPW91/
cc-pVDZ or CASPT2(4,4)/cc-pVDZ//CAS(4,4)/cc-pVDZ
singlet-triplet gaps (13.9 and 22.1 kcal mol , respectively)
and corrects (adds) for the previously reported errors for
3
with the corresponding R-H bond dissociation energies (BDEs,
Table 3). For example, enolate formation accounts for 92% of
the products with tert-butyl alcohol (BDE((CH3)3C-H) ) 96.6
0.3 kcal mol ) whereas with 2,2,2-trifluoroethanol this value
is only 24% (BDE(CF3CH2O-H) ) 106.7 ( 1.1 kcal mol ).
-
1
-1
-1
62,63
(
o-benzyne (6.3 and 7.1 kcal mol , respectively),
one obtains
-1
55
-1
electron affinities of 38.5 and 29.5 kcal mol which are in poor
agreement with experiment. These results suggest that S-T gaps
obtained at the BPW91/cc-pVDZ and CASPT2/cc-pVDZ (with
small active spaces) levels may not be generally reliable despite
(
50) Isomerization of 9 to an o-aryne radical anion can be ruled out on
the basis of our chemical derivatization experiment. In addition, (1) o-aryne
radical anions derived from naphthalene do not undergo H/D exchange with
D2O, more acidic reagents are needed. (2) o-Aryne radical anions derived
from naphthalene react differently with a wide variety of reagents (e.g.,
ROH, SO2, CS2, MeSSMe, etc.). See ref 31 for further details. (3) The
6
4-68
previous successes.
-
10
(56) The reaction between 9 and perfluorotoluene is fast (g∼5 × 10
-
1
3
-1 -1
computed PA of 371.3 kcal mol for 6,7-dehydroacenaphthylene (using
cm molecule
s ) indicating that electron transfer is exothermic. The
-
1
the numbering scheme in Table 5) is 5.1 kcal mol smaller than that for
and is in poor agreement with experiment.
51) All acidities, unless otherwise noted, come from the following:
electron affinities and their uncertainties were assigned by using the
2
9
following relationships: yj ) (y1 + y2)/2 and ∆y ) [{(y1 - y2)/2} +
2 1/2 2 1/2
(
({(∆y12 + ∆y2 ) }/2) ] where, for example, y1 ( ∆y1 ) 0.86 ( 0.11
eV and y2 ( ∆y2 ) 0.78 ( 0.05 eV.
Bartmess, J. E. In Secondary NIST Chemistry WebBook; NIST Standard
Reference Database Number 69, Mallard, W. G., Linstrom, P. J., Eds.;
National Institute of Standards and Technology: Gaithersburg MD 20899
(57) Merrill, G. N.; Kass, S. R. J. Am. Chem. Soc. 1997, 119, 12322-
12337.
(
http://webbook.nist.gov).
(58) Reed, D. R.; Kass, S. R. J. Mass Spectrom. 2000, 35, 534-539.
(59) Wenthold, P. G.; Squires, R. R.; Lineberger, W. C. J. Am. Chem.
Soc. 1998, 120, 5279-5290.
(52) Following the numbering scheme in Table 5 and defining the radical
-
1
site to be C1, the acidity at C6 in 10r is computed to be 382.9 kcal mol
(
B3LYP/6-31+G(d)). This indicates that isomerization of 9 to a 1,n radical
(60) This approach for computing electron affinities was suggested to
us by Prof. W. T. Borden.
anion is endothermic. H/D exchange can take place, nevertheless, because
D2O is capable of exchanging less acidic sites in a molecule. For example,
see: Squires, R. R.; DePuy, C. H.; Bierbaum, V. M. J. Am. Chem. Soc.
(61) B3LYP/6-31+G(d) zero-point energies were used and the same
energy difference is obtained by using a larger triple-ú basis set (i.e., B3LYP/
aug-cc-pVTZ//B3LYP/aug-cc-pVDZ).
(62) The latter error was obtained at the CASPT2(8,8)/cc-pVDZ//CAS-
(8,8)/cc-pVDZ level, see ref 63.
(63) Squires, R. R.; Cramer, C. J. J. Phys. Chem. A 1998, 102, 9072-
9081.
1
981, 103, 4256-4258.
(53) Krebs, A.; Colberg, H. Chem. Ber. 1980, 113, 2007-2014.
(54) Spang, W.; Hanack, M. Chem. Ber. 1980, 113, 2025-2027.
(55) Second electronic edition: Lide, D. L. In CRC Handbook of
Chemistry and Physics, 80th ed.; CRC Press: Boca Raton, FL, 1999. See:
http://www.knovel.com/knovel/Databook/default.htm?WCIdBrowse
Book&WCE)34&WCU)1.
(64) Cramer, C. J.; Squires, R. R. J. Phys. Chem. A 1997, 101, 9191-
9194.

4
194 J. Am. Chem. Soc., Vol. 123, No. 18, 2001
Broadus and Kass
Table 6. Results for the Proton Affinity Bracketing of 10
acid
∆H°acid (kcal mol-1)a proton transfer H/D exchange
ND
Table 5. Calculated Acidities at 298 K for the Various Positions
in Acenaphthylene at the B3LYP/6-31+G(d) Level of Theory
403.6 ( 0.8b
no
3
3
7
3
b
(
D
H
C H F
6 5
CH
CH
3
)
2
ND
396.5 ( 0.7
no
slowc
slowd
yes
2
O
O
392.9 ( 0.1
390.7 ( 0.1
387.2 ( 2.5
383.5 ( 0.7
2
3
OD
yes
no
a
Acidities are taken from ref 51. b Values for the protio reagents
are given. c After a 4 s reaction period with D
16%), H/D exchange (73%), and an adduct (11%) are observed. After
a 8 s reaction period with H
or an isomerized) ion (76%), and an adduct (11%) are observed.
O (8 × 10 Torr), OD
-8
-
2
d
(
-8
-
2
O (4 × 10 Torr), OH (13%), the starting
(
Table 7. Results for the Electron Affinity Bracketing of 10r
compd
sulfur dioxide
perfluoronitrobenzene
perfluorobenzophenone
tetramethyl-p-benzoquinone
EA (eV)a
electron transfer
1.11 ( 0.01
1.45 ( 0.11
1.52 ( 0.11
1.62 ( 0.06
1.76 ( 0.06
1.85 ( 0.09
no
no
no
no
yes
yes
The reactivity of 9 was briefly explored. Analogous to the
chemistry observed with alcohols, 9 undergoes proton transfer
•
-
and S abstraction with 2-propanethiol to afford the corre-
sponding thioenolate at m/z 183. Upon reaction with sulfur
dioxide the ion signal for 9 is quenched and only a small amount
of atomic sulfur radical anion is detected. Presumably, acenaph-
thoquinone (6) which arises from abstraction of two oxygen
atoms from SO2 is the neutral product. Signal loss also is
observed with carbonyl sulfide and carbon disulfide, two
commonly used probe reagents. Products arising from the
reaction of 9 with dimethyl disulfide and neopentyl nitrite were
found to be plentiful and illustrate 9’s radical behavior. The
assigned structures of the products in the former reaction and
their relative amounts after a 4 s reaction period are given in
eq 4; the dominant ions correspond to net addition of SCH3
2
2
,5-dimethyl-p-benzoquinone
-methyl-p-benzoquinone
a
Values from ref 51.
ion at C1 by fluoride-induced desilylation of 1-trimethylsilyl-
acenaphthylene (15, eq 5).
(
m/z 197, 38%) and SH (m/z 183, 31%). With neopentyl nitrite,
The necessary thermochemistry for 10 was obtained by the
bracketing method. The proton affinity of 10 was assigned to
abstraction of NO is observed along with other products.
-
1
be 392 ( 3 kcal mol based on the following observations
Table 6). The ion deprotonates fluorobenzene and methanol-
(
OD as well as deuterium oxide and water to a small extent.
While an acid-catalyzed isomerization of the 1-anion to the 6
position (Table 5) may be taking place, the observation of three
hydrogen-deuterium exchanges with dimethylamine-d1 indi-
cates that these two positions must be very similar (0-2 kcal
-
1
mol ) in acidity. As is often the case, deuterium oxide
exchanges several different sites and all seven hydrogens can
be replaced with deuteriums. The electron binding energy of
-
1
1
0 was determined to be 1.69 ( 0.08 eV (39 ( 2 kcal mol )
based on observed electron transfer with 2,5-dimethyl-p-
benzoquinone and 2-methyl-p-benzoquinone and the absence
of this reaction channel with tetramethyl-p-benzoquinone (Table
5
6
7
). This result is in good accord with the predicted B3LYP/
To measure the heat of hydrogenation of acenaphthyne, we
needed the C1-H bond energy of acenaphthylene (10H). This
was obtained by carrying out measurements on acenaphthyl
anion 10. Since the computed acidities for the various positions
in 10H (Table 5) are similar and suggest that a mixture of ions
would be generated upon deprotonation, we prepared the desired
6-31+G(d) value of 1.65 eV.
Acenaphthyl anion 10 was also allowed to react with probe
reagents and its reactivity was found to be similar to that of
other aryl ions. It readily abstracts an oxygen atom from sulfur
dioxide and does the same with nitrous oxide, but less
efficiently. Sulfur atom transfer is observed upon reaction with
carbonyl sulfide and carbon disulfide as well as adduct forma-
tion. These processes are competitive with the former reagent
(∼1:1) while atom abstraction is favored in the latter instance
(
65) Cramer, C. J.; Nash, J. J.; Squires, R. R. Chem. Phys. Lett. 1997,
77, 311-320.
66) Worthington, S. E.; Cramer, C. J. J. Phys. Org. Chem. 1997, 10,
2
(
7
55-767.
(
∼28:1). Upon reaction with carbon dioxide, 10 forms an adduct,
(
(
67) Cramer, C. J.; Debbert, S. Chem. Phys. Lett. 1998, 287, 320-326.
68) We explored the basis set dependence of the computed S-T gap
- -
and with dimethyl disulfide, CH3S and CH3SCH2S are
observed in a 13:1 ratio.
for cyclopentyne and found that upon going from cc-pVDZ to cc-pVTZ
via single-point calculations) that the BPW91 and CASPT2(4,4) energies
increase by 2.9 and 3.7 kcal mol , respectively. Changes of this magnitude
would be insufficient to bring the computed EA into accord with experiment,
but in conjunction with a larger active space (e.g. 14,14), the latter method
might be satisfactory.
(
The proton affinity and the electron binding energy of 9 can
be combined with the analogous thermochemical quantities for
-1
1
0 along with the known ionization potential of hydrogen atom
and the bond dissociation energy of molecular hydrogen to

The Electron as a Protecting Group. 3
J. Am. Chem. Soc., Vol. 123, No. 18, 2001 4195
-
1
afford the heat of hydrogenation of 1 (98 ( 4 kcal mol , eq
). This result is in good agreement with the 0 K value (101.8
Table 8. Experimental Heats of Formation (∆H°
f
) and Computed
Strain Energies (SE)
6
compd
∆H° a
f
SEb
cyclopentane
-18.3 ( 0.2
8.1 ( 0.3
6.7
5.6
59
17
6
c
c
1
98
acenaphthene
37.3 ( 0.7
62.1 ( 1.1
160 ( 4d
-1.70 ( 0.2
34.82 ( 0.3
5.3
6.1
68
1
1
0H
e
cis-2-butene
-butyne
2
a
All values are in kcal mol-1 and come from ref 71 unless otherwise
noted. b Computed by using Benson’s group equivalents. See refs 76
and 77. Derived from eq 7, see text. d Value comes from this work.
Obtained using eq 8. Application of Benson’s strain-free equivalents
c
e
-
1
gives a value of 69 kcal mol . See text for additional details.
-
1
This quantity was estimated for 1 (69 kcal mol ) by the use of
7
6-78
strain-free (Benson’s) group equivalents.
(
A similar value
-
1
68 kcal mol ) is obtained by using an approach employed by
Johnson and Daoust for other cycloalkynes. Specifically, the
7
9
strain energy of 1 is taken to be the heat of reaction 8 plus the
-
1
SE of 10H (6.1 kcal mol , Table 8). We have also derived the
-
1
strain energy of cyclopentyne (59 kcal mol ) based on the SE
-
1
80
of 1 (68 kcal mol ) and the heat of reaction 7. The latter
-
1
quantity was adjusted to -8.7 kcal mol to account for the
difference in the strain energies of 17 and 10H. In contrast,
Johnson and Daoust computed a strain energy for cyclopentyne
of 74 kcal mol by carrying out an analogous comparison to
that in eq 8 using MP4(SDTQ)/6-31G(d)//MP2/6-31G(d) ener-
gies; apparently this estimate is too high as a consequence of
using a single determinant wave function and deficiencies in
-
1
-
1
kcal mol ) obtained from the CASPT2/cc-pVDZ//CAS/cc-
pVDZ energies for 1 and 10H and the exact energy for H2
including the zero-point energy (-1.164019 hartrees).69,70 Given
8
1
the basis set.
-
1
that the heat of formation of 10H (62.1 ( 1.1 kcal mol ) has
71
been determined by combustion calorimetry, we can derive
-
1
∆
H°f(1) ) 160 ( 4 kcal mol . The first and second C-H
-
1
bond energies of 10H (117 ( 4 and 84 ( 2 kcal mol ,
respectively) also were obtained by using the data in eq 6. These
quantities indicate that the second π bond in acenaphthyne is
-
1
weak (33 ( 4 kcal mol ) in comparison to that in acetylene
-
1 72
(
76 kcal mol ) but similar to that for o-benzyne (36 ( 3
-
1 73
kcal mol ) even though the heat of hydrogenation for 1 is
-
1
74
1
1 ( 5 kcal mol larger than that for o-benzyne.
Calculations (CASPT2(4,4)/cc-pVDZ//CAS(4,4)//cc-pVDZ)
The strain energy in acenaphthyne arises from its rigid
geometry which is unable to accommodate a triple bond. In 1,
the C1-C2-C3 bond angle (R) is 111.5° (CAS(4,4)/cc-pVDZ)
which is 68.5° smaller than the optimum value for an alkyne
(Table 9). Shortening of the C1-C2 bond (1.269 Å) in 1 is
accompanied by a decrease in the C2-C3-C4 bond angle (â,
102.8° in 1 versus 105.8° in 10H). This contributes to the strain
energy as well since it is a further deviation from the typical
were used to compare 1 with cyclopentyne (16). Equation 7
indicates that the triple bond is favored in the latter compound
-
1 75
by 8.2 kcal mol . The energetics of this equation also can
-
1
be used to estimate the ∆H°f of 16 (98 kcal mol ) by using
the known heats of formation of the other molecules (Table 8).
While several factors contribute to the heat of reaction 7, the
differences in strain energy (SE) undoubtedly play a major role.
2
value of 120° for an sp -hybridized center. Lengthening of the
C3-C4 and C4-C5 bonds (0.015 Å) in 1 relative to 10H also
is disfavored because it disrupts the delocalization (aromaticity)
in the naphthalene fragment. In contrast, cyclopentyne is
(
(
(
69) Herzberg, G.; Howe, L. L. Can. J. Phys. 1959, 37, 636-659.
70) Kolos, W.; Wolniewicz, L. Phys. ReV. Lett. 1968, 20, 243-244.
71) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. In Thermochemical Data
of Organic Compounds; Chapman and Hall: New York, 1986.
(
72) Nicolaides, A.; Borden, W. T. J. Am. Chem. Soc. 1991, 113, 6750-
(76) Cohen, N.; Benson, S. W. Chem. ReV. 1993, 93, 2419-2438.
(77) Benson, S. W. In Thermochemical Kinetics, 2nd ed.; John Wiley
and Sons: New York, 1976.
6
755.
(73) Wenthold, P. G.; Hu, J.; Squires, R. R. J. Am. Chem. Soc. 1996,
-
1
1
18, 11865-11871.
(78) A value of 24.6 kcal mol was used for the Ct-(CB) carbon. See
ref 76.
(79) Johnson, R. P.; Daoust, K. J. J. Am. Chem. Soc. 1995, 117, 362-
367.
(
74) Recent calculations by Cramer and Thompson indicate that the π
bond strength and heat of hydrogenation of 1,2-didehydroindene are
-
1
essentially the same ((1 kcal mol ) as for o-benzyne. These results in
combination with ours indicate that cyclopentyne and 1,2-didehydroindene
have essentially the same heats of hydrogenation. Cramer, C. J.; Thompson,
J. To be submitted for publication.
-
1
(80) If one uses ∆Hf°(16) ) 98 kcal mol and Benson’s group
-1
equivalents (ref 76) then a SE of 58 kcal mol is obtained for cyclopentyne.
(81) It is interesting to note that B3LYP/6-31+G(d) poorly predicts the
-
1
-1
(
75) B3LYP/6-31+G(d) predicts a value of -8.5 kcal mol for eq 7.
energy of eq 7 (-80.0 kcal mol ), and in the analogous comparison with
Apparently, the errors associated with computing strained cycloalkynes at
this level of theory cancel in this isodesmic equation.
cyclopentyne gives virtually the same result (∆H°rxn ) -71.5 vs -68.1
-
1
kcal mol ) as reported by Johnson and Daoust in ref 79.

4196 J. Am. Chem. Soc., Vol. 123, No. 18, 2001
Broadus and Kass
Table 9. Computed Geometries for Acenaphthylene (10H) and Cyclopentene (17) at the CAS(2,2)/cc-pVDZ Level of Theory and
Acenaphthyne (1) and Cyclopentyne (16) at the CAS(4,4)/PVDZ Levela
parameter
10H C2Vb
1 C2V
17 C
s
18 C
s
C1-C2
1.362 (1.366) [1.395]
1.486 (1.474) [1.466]
1.412 (1.418) [1.441]
1.362 (1.383) [1.382]
1.429 (1.427) [1.425]
1.368 (1.387) [1.383]
1.426 (1.428) [1.434]
1.382 (1.398) [1.386]
108.9 (109.0) [108.8]
105.8 (105.9) [106.9]
110.6 (110.2) [108.6]
1.269 (1.243)
1.485 (1.474)
1.427 (1.441)
1.357 (1.375)
1.431 (1.430)
1.367 (1.386)
1.428 (1.430)
1.380 (1.395)
111.5 (112.6)
102.8 (102.0)
111.4 (110.0)
1.342 (1.338)
1.513 (1.514)
1.543 (1.554)
1.253 (1.228)
1.510 (1.511)
1.569 (1.590)
C2-C3
C3-C4
C5-C6
C6-C7
C7-C8
C8-C9
C4-C9
C1-C2-C3 (R)
C2-C3-C4 (â)
C3-C4-C5 (γ)
111.6 (112.0)
102.9 (103.0)
105.5 (105.5)
114.7 (115.8)
99.6 (98.7)
106.6 (106.3)
a
Bond lengths and angles are in angstroms and degrees, respectively. Numbering schemes follow that shown in Table 5. Parenthetical numbers
b
were obtained at the B3LYP/6-31+G(d) level. The numbers in brackets correspond to electron diffraction data from: Wood, R. A.; Welberry, R.;
Rae, A. D. J. Chem. Soc., Perkin Trans. 2 1985, 451-456.
-
1
somewhat more flexible and is able to undergo slightly larger
structural distortions upon introduction of a second π bond (i.e.,
the C3-C4 bond lengthens by an additional 0.011 Å, R
increases, and â decreases by an additional 0.3-0.5° relative
to the corresponding alkenes). These differences in conjunction
with hyperconjugative interactions between the in-plane low-
lying π* orbital of the C1-C2 triple bond and the C3-H and
C3-C4 σ bonds presumably accounts for the greater stability
sample. The heat of hydrogenation (98 ( 4 kcal mol ) and
-
1
formation (160 ( 4 kcal mol ) of acenaphthyne (1) were
determined by the application of a thermodynamic cycle.
Acenaphthyne is estimated to have a strain energy of 68 kcal
mol and is computed (CASPT2(4,4)/cc-pVDZ//CAS(4,4)/cc-
pVDZ) to be destabilized relative to cyclopentyne by 9 kcal
mol . We have also briefly explored the reactivity of acenaph-
thyne radical anion with probe reagents and found it to exhibit
both anion and radical behavior.
-1
-1
74
of cyclopentyne (and 1,2-dehydroindene) relative to acenaph-
thyne.82
Acknowledgment. Support from the National Science
Foundation, the donors of the Petroleum Research Foundation,
as administered by the American Chemical Society, and the
Minnesota Supercomputer Institute are gratefully acknowledged.
Conclusion
Acenaphthyne radical anion (9) was generated in a FTMS
by dissociation of dicarboxylate dianion 12. We have provided
structural evidence for 9 by converting it to acenaphthenone
enolate and comparing the reactivity of this ion with an authentic
Supporting Information Available: Computed structures
and energies (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
(
82) We wish to thank one of the referees for suggesting hyperconjugation
as an explanation to account for the difference in energy between 1 and
6.
1
JA003069F