C6H4 valence bond isomers: A reactive bicyclopropenylidene

Article abstract of DOI:10.1021/ol048307g

(Chemical Equation Presented) The simple bicyclopropenylidene derivative 21b, stabilized by fusion into naphthalene, results from reaction of dimesitylcyclopropenone 20b with the 1-trimethylsilyl-1H-cyclopropa[d] naphthalenyl anion. Although unstable in air, the molecule survives ambient conditions long enough for separation and mass spectral characterization. Aerial oxidation of 21 b leads to 2,3-dimesitylanthracene-1,4-dione 22b whose X-ray crystal structure has been determined. While diphenylcyclopropenone 20a does not give identifiable products, the di-tert-butyl analogue 20c gives quinone 22c but in lower yield.

Full text of DOI:10.1021/ol048307g

ORGANIC
LETTERS
2004
Vol. 6, No. 22
4017-4020
C6H4 Valence Bond Isomers: A Reactive
Bicyclopropenylidene
Brian Halton,*,† Mark J. Cooney,† Carissa S. Jones,† Roland Boese,*,‡ and
Dieter Bla1
ser‡
School of Chemical & Physical Sciences, Victoria UniVersity of Wellington,
P.O. Box 600, Wellington, New Zealand, and Institute for Inorganic Chemistry,
Duisburg-Essen UniVersity, UniVersitaetsstrasse 3-5, 45117 Essen, Germany
brian.halton@Vuw.ac.nz; roland.boese@uni-essen.de
Received August 24, 2004
ABSTRACT
The simple bicyclopropenylidene derivative 21b, stabilized by fusion into naphthalene, results from reaction of dimesitylcyclopropenone 20b
with the 1-trimethylsilyl-1H-cyclopropa[b]naphthalenyl anion. Although unstable in air, the molecule survives ambient conditions long enough
for separation and mass spectral characterization. Aerial oxidation of 21b leads to 2,3-dimesitylanthracene-1,4-dione 22b whose X-ray crystal
structure has been determined. While diphenylcyclopropenone 20a does not give identifiable products, the di-tert-butyl analogue 20c gives
quinone 22c but in lower yield.
The C6H4 energy surface has been recognized for more than
a century1 with o-benzyne 1 and a number of its valence
bond isomers characterized by chemical trapping, spectros-
copy, and more recently, matrix isolation.2 Compared to
benzene, benzyne 1 is strained and reactive, but its ready
accessibility has provided a cornerstone in the development
of modern physical organic chemistry.3 There can be little
doubt that m-2 and p-3 benzyne exist,4 especially from the
isolation of the perfluoro derivatives of 1-3 by Sander and
his group.5 There is no report of benzvalyne 4, but diene
isomer 5 is involved in dehydrobenzvalene chemistry.6 More
recently, attention has been directed toward the remaining
unknown C6H4 valence bond isomer, bicyclopropenylidene
(triafulvalene) 6.7
† Victoria University of Wellington.
‡ Duisburg-Essen University.
(1) Stormer, R.; Kahlert, B. Ber. Dtsch. Chem. Ges. 1902, 35, 1633-
1640.
Two derivatives of the triafulvalene ring system have been
recorded by way of the heavily substituted, crystalline
bicycloproparenylidenes 7, prepared from dimerization of the
carbenoid that results from reaction of the relevant gem-
(2) (a) Wenk, H. H.; Winkler, M.; Sander, W. Angew. Chem., Int. Ed.
2003, 42, 502-527. (b) Halton, B. Chem. N.Z. 2001, 65, 28-32.
(3) Warmuth, R. Eur. J. Org. Chem. 2001, 423-437.
(4) (a) Luis, S. V.; Gavina, F.; Ferrer, P.; Safont, V. S.; Torres, M. C.;
Burguete, M. I. Tetrahedron 1989, 45, 6281-6296. (b) Lockhart, T. P.;
Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 4091-4096. (c) Lockhart,
T. P.; Commita, P. B.; Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 4082-
4090. (d) Washburn, W. N.; Zahler, R.; Chen, I. J. Am. Chem. Soc. 1978,
100, 5863-5874.
(5) (a) Wenk, H. H.; Balster, A.; Sander, W.; Hrovat, D. A.; Borden,
W. T. Angew. Chem., Int. Ed. 2001, 40, 2295-2298. (b) Wenk, H. H.;
Sander, W. Chem.sEur. J. 2001, 7, 1837-1844.
10.1021/ol048307g CCC: $27.50
© 2004 American Chemical Society
Published on Web 09/28/2004

Scheme 1
dichlorocyclopropabenzene with BuLi (Scheme 1).8 These
compounds illustrate that annulation of cyclopropene into
an aromatic core provides compounds significantly more
stable than their nonaromatic congeners. From our own
studies, we have noted (i) that the highly strained benzynes
8 and 99 can be intercepted at ambient temperature, (ii) that
the exocyclic alkenes 10 are exceptionally stable com-
pounds,10,11 and (iii) that epoxides 11 survive at 0 °C,12 while
parent 12 is stable only to -50 °C.13
Our approach to 17 and 18 follows the established route
to the alkylidenecycloproparenes that employs Peterson
olefination of the 1,1-bis(trimethylsilyl)cycloproparene.10
However, the exceedingly foul odor of cyclopropabenzene
has dictated that all work involve the anion derived from
cyclopropa[b]naphthalene derivative 19.16 Initial attempts to
effect reaction between disilane 19 and diphenylcyclopro-
penone 20a led to much decomposition and the formation
of a plethora of largely inseparable reaction products. By
employing the sterically more demanding 2,3-dimesitylcy-
clopropenone 20b less decomposition occurred, but the
products were still difficult to separate. Repeated radial
chromatography provided an orange crystalline solid identi-
fied as 2,3-dimesitylanthracene-1,4-dione 22b (8%) (Scheme
2) from its spectroscopic data and confirmed by single-crystal
X-ray diffraction analysis.17
Polar fulvalenes have been prepared in the cycloproparene
series as illustrated by 13-16. Calculations14 show the
polarity to be directed away from the cycloproparene and
toward the cross-conjugated moiety, even in 15 and 16; these
last compounds distort from planarity to prevent the forma-
tion of an 8π 7C antiaromatic ring.15 The only cycloproparene
hydrocarbons predicted to have polarity directed toward the
cycloproparene unit are the triafulvalenes 17 and 18 (µcalcd
2.6 and 4.4 D, respectively14); they are also expected to be
less stable than cyclopentadienyl and cycloheptatrienyl
analogues and the unknown parent methylidene compounds.14
Thus, kinetically stabilized derivatives of 17 and/or 18 would
be the simplest bicyclopropenylidenes so far recorded. We
now provide evidence to support the formation of the simple
bicyclopropenylidene derivatives 21 as easily oxidized reac-
tive molecules.
Scheme 2
1
The H NMR spectrum of 22b shows singlets for the
(6) Schlueter, A. D.; Belzner, J.; Heywang, U.; Szeimies, G. Tetrahedron
Lett. 1983, 24, 891-894.
(7) (a) Huwyler, V. R.; Al-Dulayymi, A.; Neuenschwander, M. HelV.
Chim. Acta 1999, 82, 2336-2347. (b) Lang, C.; Muhlebach, M.; Neuen-
schwander, M. HelV. Chim. Acta 1997, 80, 2124-2136.
(8) Neidlein, R.; Poignee, V.; Kramer, W.; Glueck, C. Angew. Chem.,
Int. Ed. Engl. 1986, 25, 731-734.
(9) (a) Apeloig, Y.; Arad, D.; Halton, B.; Randall, C. J. J. Am. Chem.
Soc. 1986, 108, 4932-4937. (b) Halton, B.; Randall, C. J. J. Am. Chem.
Soc. 1983, 105, 6310-6311.
(10) (a) Halton, B. Chem. ReV. 2003, 103, 1327-1370. (b) Halton, B.;
Stang, P. J. Synlett 1997, 145-158.
equivalent H9/H10 protons of the anthracene core and the
meta/meta′ hydrogens of the pendant mesityl groups. The
o/o′- and p/p′-methyl groups appear as 2:1 singlets, and H5-
H8 give rise to the expected AA′BB′ pattern; the protonated
aromatic carbon atoms resonate in the narrow 128.4-130.1
ppm range, and the absence of shielded methine carbons
signifies the loss of the cycloproparene core.10 Figure 1 is
an ORTEP plot of 22b at 50% probability and shows the
quinone to exist as two independent molecules with slightly
differing interplanar angles of the mesityl rings with respect
to the anthracene system [molecule A (lower): 76.8° and
(11) Dixon, G. M.; Halton, B. Eur. J. Org. Chem. 2004, 3707-3713.
(12) (a) Bickers, P. T.; Halton, B.; Kay, A. J.; Northcote, P. T. Aust. J.
Chem. 1999, 52, 647-652. (b) Halton, B.; Cooney, M. J.; Wong, H. J.
Am. Chem. Soc. 1994, 116, 11574-11575.
(13) Billups, W. E.; Litosh, V. A.; Saini, R. K.; Daniels, A. D. Org.
Lett. 1999, 1, 115-116.
(14) Apeloig, Y.; Boese, R.; Bla¨ser, D.; Halton, B.; Maulitz, A. H. J.
Am. Chem. Soc. 1998, 120, 10147-10153.
(16) Halton, B.; Cooney, M. J.; Davey, T. W.; Forman, G. S.; Lu, Q.;
Boese, R.; Bla¨ser, D.; Maulitz, A. H. J. Chem. Soc., Perkin Trans. 1 1995,
2819-2827.
(15) Halton, B.; Jones, C. S. Eur. J. Org. Chem. 2004, 138-146.
4018
Org. Lett., Vol. 6, No. 22, 2004

Scheme 3
Figure 1. ORTEP plot of quinone 22b at 50% probability. The
two independent molecules have differing interplanar angles of the
mesityl rings with respect to the anthracene system (see text).
anthraquinone moiety in agreement with those from 22b;
accurate mass measurement (electrospray) provided a pro-
tonated molecular ion for 22c in accord with expectation.18
99.2° toward and molecule B (upper): 78.4° and 114.8°
against]. In both cases, the anthracene core is not strictly
planar but slightly distorted at the 1,4-positions (molecule
A: C9A-C1-C2-C3, 7.0° and C2-C3-C4-C4A, 14.1°;
molecule B: C9C-C1B-C2B-C3B, 1.5° and C2B-C3B-
C4B-C4C, 10.1°). There is no obvious C-H‚‚‚‚O hydrogen
bond and the difference of the molecular geometries is a
consequence of close packing. Accordingly, we expect only
small differences in the molecular energies.
Analogous reaction of di-tert-butylcyclopropenone 20c
provides yellow anthraquinone 22c, but in only 6% yield.18
This compound displays proton singlets for H9/H10 and the
tert-butyl protons, and has its carbon resonances for the
The reaction products from 19 and 20b gave rise to a small
(∼2 mg) sample of pale yellow compound from careful
repeated chromatography; it had limited stability and slowly
transformed into quinone 22b. The electron-impact mass
spectrum of this yellow solid prior to its decomposition
provided a molecular ion M•+ as base peak at m/z 412 with
fragment ions at m/z 397 (11) [M - 15]+, 382 (24) [M -
30]•+ and 367 (30) [M - 45]+. A contaminant at m/z 502
with 11% relative abundance was also recorded (see below).
It is our view that the reactive solid is the sought after
annulated bicyclopropenylidene 21b (Mr 412) arising from
the expected Peterson olefination (Scheme 2). The aerial
oxidation of 21b into quinone 22b responsible for the color
change can be explained by capture of molecular oxygen
by the reactive exocyclic double bond (Scheme 3). In this
regard, the behavior of the bicyclopropenylidene is analogous
to the instability of cyclobutadienes in air;19,20 tetraphenyl-
cyclobutadiene adds oxygen to give a dioxetane intermediate
that opens to (Z)-tetraphenylbut-2-ene-1,4-dione,20 while the
catalyzed addition of oxygen leads to a furan.21
(17) 22b: orange-red crystalline solid (22 mg, 8%); mp 173-5 °C
(dichloromethane-light petroleum, 1:1); IR λmax/cm-1 (KBr) 3055, 2960,
2919, 1662, 1612, 1585, 1460, 1398, 1343, 1277, 1187, 1058, 1026; UV
λmax (acetonitrile)/nm 235 (4.66), 271 (3.77), 289 (3.78), 299 (3.78), 356
(3.23), 397 (log ꢀ 3.27); δH (80 MHz; CDCl3) 2.00 (s, 12H), 2.23 (s, 6H),
6.75 (s, 6H), 7.69-7.71 (AA′, 2H, H6/7), 8.08-8.11 (BB′, 2H, H5/8), 8.73
(s, 2H, H9/10); δC (20 MHz; CDCl3) 20.9 (C12/16-Me), 21.1(5) (C14-
Me), 128.4 (C13/15), 128.8(5) (C4a/9a), 129.1 (C9/10), 129.4 (C6/7), 130.1
(C5/8 + C11), 135.1 (C8a/10a), 136.0(5) (C12/16), 137.7 (C14), 150.1 (C2/
3), 184.1 (C1/4). Anal. Calcd: C, 86.4(5); H, 6.3(5). Found: C, 86.2; H,
6.5. Crystal structure determination of 22b: C32H28O2, Mr 444.54, mp
174-5 °C, red plate dimensions 0.25 × 0.13 × 0.11 mm measured on a
Siemens P4 four circle diffractometer with Mo KR radiation, T 293(2) K,
cell dimensions a ) 11.3439(17) Å, b ) 14.390(2) Å, c ) 16.389(3) Å, R
) 100.405(14)°, â ) 108.679(13)°, γ ) 95.758(13)°, V ) 2456.6(7) Å3,
triclinic crystal system, Z ) 4, dcalcd 1.202 g/cm3, µ ) 0.073 mm-1 space
Despite many variations in the reaction conditions, no
experiment has provided 21b in anything other than trace
quantities, but they have given a large number of essentially
group P-1, data collection of 5241 intensities, 5025 independent (Rint
)
0.0118, Θ 1.75-22.50°), 3285 observes [Fo g 2σ(F)]; structure solution
and refinement on F2 (Bruker AXS SHELXTL 5.10) 614 parameters, the
hydrogen atoms as Riding model on idealized geometries with the 1.2-fold
(1.5-fold for methyl groups) isotropic displacement parameters of the
equivalent Uij of the corresponding carbon atom. Crystallographic data
(excluding structure factors) for this structure have been deposited with
the Cambridge Crystallographic Data Centre as supplementary publication
no. CCDC 245990. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax +44-
(0)-1223-336033 or e-mail: deposit@ccdc.cam.ac.uk) or from www.ccd-
c.cam.ac.uk/conts/retrieving.html.
(18) 22c: yellow solid (6.5 mg, 6%); MS (electrospray) [M + H]+ m/z
321.18647 amu, C22H25O2+ requires 321.18491, ∆ 4.9 ppm; δH (300 MHz;
CDCl3) 1.58 (s, 9H), 7.64-7.68 (AA′, 2H, H6/7), 8.02-8.07 (BB′, 2H,
H5/8), 8.44 (s, 2H, H9/10); δC (75 MHz; CDCl3) 32.0(5) (Me), 39.1 (CMe3),
126.9(5) (C9/10), 128.8 (C6/7), 129.3 (C4a/9a), 129.8 (C5/8), 135.1 (C8a/
10a), 163.6 (C2/3), 188.7 (C1/4).
(19) Krebs, A.; Kemper, R.; Kimling, H.; Klaska, K. H.; Klaska, R.
Liebigs Ann. Chem. 1979, 473-483.
(20) Cava, M. P.; Mitchell, M. J. Cyclobutadiene and Related Com-
pounds; Academic Press: New York, 1967.
(21) Omerod, R. M.; Lanmbert, R. M. Catal. Lett. 1990, 6, 121-129.
Org. Lett., Vol. 6, No. 22, 2004
4019

inseparable products, not least that appearing as a contami-
nant at m/z 502 in the mass spectrum of 21b. Of these
compounds, cyclopropa[b]naphthalene and tert-butyl 2-naph-
thoate have been identified by comparison of their spectral
data with literature values. The former is an anticipated
product when unreacted silyl anion reacts with water,22 and
the presence of naphthoate is easily understood from ring
opening of naphthalenone 24 (Scheme 3).23 Ketone 24 is to
be expected from dioxetane metathesis following oxygen
addition across the central double bond of 21 in analogy to
the reaction of Ph4C4 with O2.20
with markedly better steric protection than that offered by
tert-butyl or mesityl groups. In this context, the use of the
sterically very demanding 2,6-diisopropylphenyl (Dip) or
2,4,6-tris-[bis(trimethylsilyl)methyl]phenyl (Tbt) so effec-
tively used by Tokitoh and co-workers is attractive.24 We
leave others to explore such possibilities.
The product with m/z 502, and an isomer of it also obtained
in trace amounts, is less easy to account for. Without
imposing undue speculation, we tentatively assign benzox-
epin structure 25 (m/z 502) to this compound as it can arise
from initial 1,2-addition to 20b with subsequent rearrange-
ment and protonation on workup. Furthermore, 1,4-conjugate
addition of the anion to cyclopropenone 20b accounts for
isomeric 25 in which the proton and the trimethylsilyl groups
are reversed.
Acknowledgment. We thank Professor Manfred Regitz
(Kaiserslautern University) for a generous sample of cyclo-
propenone 20c. Financial assistance from Victoria University
by way of a postdoctoral fellowship (to M.J.C.) and Curtis-
Gordon scholarships (to C.S.J.) and from the Deutsche
Forschungs-gemeinschaft and the Fonds der Chemischen
Industrie (in Germany) is gratefully acknowledged.
OL048307G
These experiments indicate that room temperature isolation
of a kinetically stabilized bicyclopropenylidene is likely only
(23) Buckland, S. J.; Halton, B.; Stang, P. J. Aust. J. Chem. 1988, 41,
845-854.
(24) (a) Tajima, T.; Sasaki, T.; Sasamori, T.; Takeda, N.; Tokitoh, N.;
N. Chem. Commun. 2004, 402-403. (b) Tajima, T.; Hatano, K.; Sasaki,
T.; Sasamori, T.; Takeda, N.; Tokitoh, N.; Takagi, N.; Nagase, S. J.
Organomet. Chem. 2003, 686, 118-126.
(22) Halton, B.; Jones, C. S. J. Chem. Soc., Perkin Trans. 2 1998 2505-
2508, 1999, 387.
4020
Org. Lett., Vol. 6, No. 22, 2004