Reactions of complex ligands 85: Chiral quinoid and hydroquinoid [2.2]metacyclophanes via chromium-mediated intramolecular benzannulation

Article abstract of DOI:10.1016/S0022-328X(98)01125-5

A meto-alkynyl (chromium alkenylcarbene) functionalized benzene 5 is synthesized in a six-step sequence starting from 3-bromomethyliodobenzene. Upon gentle warming in tetrahydrofuran methoxy[alkynylaryl(alkenyl)carbene complex 5 undergoes an intramolecula

Full text of DOI:10.1016/S0022-328X(98)01125-5

Journal of Organometallic Chemistry 578 (1999) 223–228
Reactions of complex ligands 85: chiral quinoid and hydroquinoid
[2.2]metacyclophanes via chromium-mediated intramolecular
benzannulationꢀ
K.H. Do¨tz *, A. Gerhardt
Kekule´-Institut fu¨r Organische Chemie und Biochemie der Rheinischen Friedrich-Wilhelms-Uni6ersita¨t Bonn, Gerhard-Domagk-Straße 1,
D-53121 Bonn, Germany
Received 1 August 1998
Abstract
A meta-alkynyl (chromium alkenylcarbene) functionalized benzene 5 is synthesized in a six-step sequence starting from
3-bromomethyliodobenzene. Upon gentle warming in tetrahydrofuran methoxy[alkynylaryl(alkenyl)carbene complex 5 undergoes
an intramolecular benzannulation to give [2.2]metacyclophane 6 bearing two chiral planes due both to the cyclophane skeleton
and the Cr(CO)3-coordinated methyl-substituted benzohydroquinone deck. In situ oxidation of the benzannulation product by
cerium(IV) ammonium nitrate directly affords chiral [2.2]metabenzoquinonophane 7. © 1999 Elsevier Science S.A. All rights
reserved.
Keywords: Benzannulation; Carbene complexes; Chromium complexes; Cyclophanes; Quinones
mediated assembly of a densely substituted distorted
arene deck, and we now report on our efforts to apply
the intramolecular version of the benzannulation of
alkenyl(alkoxy)carbene chromium complexes with alky-
nes [10] to the synthesis of chiral [2.2]metahydro-
quinono- and quinonophanes.
1. Introduction
The construction of strained carbocycles is an ongo-
ing challenge in organic synthesis [2]. Among these
compounds cyclophanes have received considerable in-
terest in organometallic chemistry [3]. The unsymmetri-
cal boatlike conformation imposed on the benzene
decks in [2.2]para- and meta-cyclophanes has stimu-
lated efforts to explore the ability of distorted arenes
for coordination to organometallic fragments [4] culmi-
nating in the metal vapor cocondensation synthesis of
the [2.2]paracyclophane chromium sandwich [5]. The
synthesis of cyclophanes is generally based on the dilu-
tion principle [6], the cesium template effect [7] and the
junction of the arene decks via a temporary assistance
by heteroatoms which are finally removed by oxidation
and reductive elimination [8,9]. We pursued a comple-
mentary strategy based on a transition metal template-
2. Synthesis of the alkynyl(alkenylcarbene) complex
precursor
Early studies directed towards pyranoquinone antibi-
otics demonstrated that the intramolecular chromium-
mediated benzannulation is favored by a flexible spacer
separating the alkyne and the aryl-(or vinyl-)carbene
moieties [11]. If, however, both p-systems are connected
by a rigid arene bridge, the alkyne insertion into the
chromium–carbene bond, which represents the first
carbon–carbon bond forming step in the benzannula-
tion, is blocked; instead, an intermolecular reaction
results in an unusual formal dimerization of the
ꢀ For part 84 see Ref. [1].
* Corresponding author.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01125-5

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K.H. Do¨tz, A. Gerhardt / Journal of Organometallic Chemistry 578 (1999) 223–228
by coordination to the CꢀC bond. Thus, we focused on
a propanal equivalent provided by (2-bromomethyl)-
1,3-dioxane which was attached to the arene after
iodine/lithium exchange. Deprotection of the acetal oc-
curred via acidic transacetalization by methanol. Olefi-
nation of aldehyde 3 was first achieved by a two-step
sequence via the geminal dibromide applying the te-
trabromomethane/triphenylphosphine protocol [17] fol-
lowed by debromination with lithium tri-n-butylzincate
[18] which resulted in the formation of bromoalkene 4
in a low overall yield of 27% as a 1.1/1 mixture of
E/Z-isomers. The yield could be improved by a one-
step ‘salt free’ Wittig protocol using triphenyl(bro-
momethyl)phosphonium bromide/potassium tert-buto-
xide [19] which afforded a 1/9 E/Z-mixture of isomers.
The synthesis of the alkene–alkyne carbene complex
5 was based on the Fischer route [20]. Low temperature
lithiation of bromoalkene 4 by tert-butyllithium and
addition of hexacarbonylchromium was followed by
alkylation of the resulting acylchromate with trimethyl-
oxonium tetrafluoroborate to give a 47% yield of 5.
NMR studies confirmed the exclusive formation of the
E-isomer (3JHH=15.1 Hz) irrespective of the ratio of
E/Z-isomers in the bromoalkene precursor 4 suggesting
a configurational lability of the lithioalkene intermedi-
ate under the reaction conditions.
Scheme 1. Intramolecular benzannulation and carbene dimerization
to chrysenes.
(alkynylaryl)carbene ligand to give a chrysene skeleton
[12] (Scheme 1).
The construction of the cyclophane hydroquinone
deck via intramolecular benzannulation requires a tai-
lored alkoxycarbene chromium complex 5 bearing a
vinyl–carbene substituent separated from the alkyne
moiety by a meta-diethylene benzene spacer. A suitable
starting material was 3-bromomethyl-1-iodobenzene 1
which allowed for the sequential elaboration of the
alkynyl and the chromiumalkenylcarbene side chain
(Scheme 2). Coupling of the benzyl bromide with al-
lenylmagnesium bromide followed by methylation with
methyl iodide gave 3-(3%-pentynyl)-1-iodobenzene 2 in
excellent yield [13,14].
3. Diastereoselective intramolecular benzannulation
Previous studies on metal carbene modified
[2.2]metacyclophanes [21] have demonstrated that the
boatlike deformation of the benzene ring does not
hamper further elaboration of the arene deck. Penta-
carbonyl[4-[2.2]metacyclophanyl(methoxy- or amino)-
carbene]chromium underwent clean intermolecular
diastereoselective benzannulation and cyclopentannula-
tion upon reaction with alkynes [22]. In order to inves-
tigate whether this type of reaction can be extended to
the synthesis of strained and distorted oxygenated are-
nes we aimed at an intramolecular benzannulation of
(alkynylaryl)alkenylcarbene complex 5.
Attempts to replace the iodo substituent for the
required formylethyl side chain via the Heck reaction
using allyl alcohol [15] were successful with the
iodobenzene model compound [16]. However, this ap-
proach failed with the alkynyliodoarene substrate 2
probably due to deactivation of the palladium catalyst
Warming a carefully deoxygenated 0.27 N solution of
carbene complex 5 in tetrahydrofuran to 65°C resulted
in the intramolecular [3+2+1]cycloaddition to give
the [2.2]metahydroquinonophane complex 6 which was
isolated as a yellow solid in 38% yield after chromato-
graphic workup. IR- and 13C-NMR spectra—which
reveal significantly shielded arene carbon atoms for the
substituted hydroquinone deck—indicate that the
Cr(CO)3 fragment is coordinated to the hydroquinoid
ring formed upon the benzannulation. At low tempera-
ture in solution [2.2]metacyclophane may adopt a syn-
conformation containing superimposed benzene rings;
above 0°C, however, it is known to rearrange to the
Scheme 2. Synthesis of alkynyl(alkenylcarbene) complex 5.

K.H. Do¨tz, A. Gerhardt / Journal of Organometallic Chemistry 578 (1999) 223–228
225
diastereomer bearing the metal on the more congested
‘endo’ face of the hydroquinone may be formed as a
minor product; however, attempts to induce a similar
haptotropic migration of the Cr(CO)3 moiety in this
isomer resulted in decomplexation [22]. Obviously, the
inner hydrogen atom of the other benzene deck does
not tolerate the coordination of the central arene ring
from the ‘endo’ face. On the basis of these results,
supported by the fact that a sharp singlet is observed
for the Cr(CO)3 group in the 13C-NMR spectrum indi-
cating an unhindered rotation along the arene–
chromium axis on the NMR time scale, we favor the
assignment of enantiomers B/B% for the intramolecular
benzannulation product 6.
Scheme 3. Possible anti-pairs of enantiomers A/A% and B/B%.
Achiral [2.2]metabenzoquinonophanes and -hy-
droquinonophanes have been previously synthesized
from two adequately substituted xylene-type precursors
via the classical disulfone route, and their electron
donor acceptor properties have been extensively studied
by Staab [24]. They form intramolecular quinhydrones
which reveal transannular charge transfer phenomena
for both the syn and the anti conformations. The
intramolecular benzannulation strategy also provides a
straightforward access to chiral [2.2]metaquinono-
phanes. In situ oxidation of the benzannulation product
obtained from carbene complex 5 by cerium(IV)
ammonium nitrate afforded a 40% yield of benzo-
quinonophane 7 bearing a single plane of chirality
(Scheme 4).
thermodynamically favored anti-conformation in which
both arene decks are arranged in a stepwise manner
[8,23]. The anti-conformation is generally observed for
metal-coordinated [2.2]metacyclophanes [4]. As a conse-
quence of two planes of chirality arising from the
non-symmetric substitution pattern of the hy-
droquinone ring and the coordination of the metal
from either the top or the bottom face of the hy-
droquinone two anti-pairs of enantiomers A/A% and
B/B% might be expected for the benzannulation product
6 (Schemes 3 and 4).
The intramolecular benzannulation occurred with
considerable diastereoselectivity; only
a
single
diastereomer of 6 could be detected and isolated from
the reaction mixture. Intermolecular benzannulation of
the anti-[2.2]metacyclophane skeleton generating a
naphthohydroquinoid deck has been shown to favor
the formation of the anti-diastereomer bearing the
Cr(CO)3 fragment on the remote arene ring opposite to
the other arene deck; at elevated temperature the metal
underwent a haptotropic migration along the ‘exo’ face
to the less substituted naphthalene ring. The other
4. Conclusion
Intramolecular
benzannulation
of
(alkynyl-
aryl)alkenylcarbene complexes of chromium provides a
novel straightforward synthetic access to chiral
Cr(CO)3-coordinated
[2.2]metahydroquinonophanes
bearing two planes of chirality. It is based on the
assembly of a densely substituted distorted arene deck
mediated by a chromium template, and thus comple-
ments the customary syntheses of cyclophanes which
rely on the heteroatom-assisted combination of two
differently functionalized meta-xylene precursors. The
benzannulation is anti-diastereoselective generating a
single pair of enantiomers. In situ oxidation affords a
chiral [2.2]metaquinonophane.
5. Experimental
All operations involving organometallics were carried
out in flame-dried glassware under an atmosphere of
argon. Di-n-butyl ether, dichloromethane and
petroleum ether were dried over calcium hydride, di-
ethyl ether and THF over Na/K. All solvents were
saturated with argon and stored over molecular sieves.
Scheme 4. Synthesis of [2.2]metahydroquinonophane complex 6 and
quinonophane 7 via intramolecular benzannulation.

226
K.H. Do¨tz, A. Gerhardt / Journal of Organometallic Chemistry 578 (1999) 223–228
Silica gel (Merck, 0.063–0.200 mm) was degassed under
vacuum and stored under argon. FT-IR spectra were
recorded on a Nicolet Magna 550 spectrometer, NMR
spectra on a Bruker DRX 500, AM 400 or AM 250
spectrometer. All chemical shifts are given in ppm
relative to TMS as external standard. HR-MS were
determined on a Kratos MS-50 spectrometer.
of n-BuLi dissolved in 40 ml diethyl ether. The mixture
was stirred for 30 min and then cooled to −50°C
before 50 mmol (9.75 g) of 2-(2%-bromoethyl)-1,3-diox-
ane were slowly added. The mixture was allowed to
warm to r.t. overnight, the reaction quenched with
water and the mixture extracted with diethyl ether. The
combined organic layers were dried over magnesium
sulfate and concentrated to dryness. The resulting oil
was purified by column chromatography (silica gel,
petroleum ether/ethyl acetate 10:1) to give 6.13 g (23.73
mmol, 56%) of {2-[1-(3%-pentynyl)-3-(2%%-phenylethyl)]}-
1,3-dioxane.
5.1. 3-(3%-Pentynyl)-1-iodobenzene 2
A 2 N solution of allenylmagnesium bromide (98
mmol, 49 ml) in diethyl ether was added at 0°C to
3-iodobenzyl bromide (89.8 mmol, 23.0 g) dissolved in
60 ml of tetrahydrofuran. The mixture was allowed to
warm to room temperature (r.t.), stirred for another 3
h, hydrolyzed at 0°C, extracted with diethyl ether and
dried over magnesium sulfate. Distillation gave 20.9 g
(81.7 mmol, 91%) of 3-(3%-butynyl)-1-iodobenzene.
HR-MS: Found: 255.9756. C10H9I Calc.: 255.9749.
MS (EI): m/z 256 (31%), 217 (58%), 129 (100%), 90
HR-MS: Found: 258.1628. C17H22O2 Calc. 258.1620.
MS (EI): m/z 258 (3%), 182 (22%), 129 (24%), 87
(100%). 1H-NMR (500 MHz, CDCl3): l=7.07–7.26
(m, 4H, aryl-CH), 4.54 (t, 1H, CH), 4.15 (ddd, 2H,
CH), 3.78 (ddd, 2H, CH), 2.80 (t, 2H, CH2), 2.73 (t,
2H, CH2), 2.43 (tq, 2H, CH2), 2.13 (dtt, 1H, CH), 1.94
(m, 2H, CH2), 1.81 (t, 3H, CH3), 1.37 ppm (dtt, 1H,
CH). 13C-NMR (125 MHz, CDCl3): l=142.16, 141.46
(2C, aryl-C), 129.08, 128.76, 126.72, 126.31 (4C, aryl-
CH), 79.09 (CꢀCCH3), 76.53 (CꢀCCH3), 67.31 (C-3,5),
1
(58%). H-NMR (400 MHz, CDCl3): l=7.58 (t, 1H,
4JHH=1.6 Hz, aryl-H), 7.55 (d, 1H, 3JHH=7.8 Hz,
aryl-H), 7.18 (d, 1H, 3JHH=7.7 Hz, aryl-H), 7.02
(pseudo-t, 1H, aryl-H), 2.75 (t, 2H, 3JHH=7.4 Hz,
37.11
(CH2CH),
35.95
(CH2CH2CꢀC),
30.49
3
4
(CH2CH2CH), 26.27 (C-4), 21.40 (CH2CꢀC), 3.92 ppm
(CH3).
aryl-CH2), 2.45 (td, 2H, JHH=7.4 Hz, JHH=2.6 Hz,
4
CH2–CꢀC), 1.98 ppm (t, 1H, JHH=2.6 Hz, CꢀCH).
13C-NMR (100 MHz, CDCl3): l=142.7, 137.4, 135.4,
129.9, 127.7, 94.4 (6C, aryl-C), 83.1 (CꢀCH), 69.3
(CꢀCH), 34.1 (aryl-CH2), 20.3 ppm (CH2–CꢀC).
The dioxane acetal (7.04 mmol, 1.82 g) was cleaved
by stirring in a mixture of 75 ml of methanol, 20 ml of
glacial acetic acid and 0.5 ml of conc. hydrochloric acid
for 3 days. This mixture was cautiously poured into a
saturated aqueous solution of sodium hydrogen car-
bonate, and the dimethyl acetal was extracted with
diethyl ether. The combined extracts were dried over
magnesium sulfate and evaporated. Then the residue
was dissolved in a mixture consisting of 50 ml of glacial
acetic acid, 5 ml of water and 0.5 ml of conc. hy-
drochloric acid and stirred for another 3 days. The
solution was slowly poured into saturated aqueous
sodium hydrogen carbonate. Additional sodium hydro-
gen carbonate was added until the solution became
basic. Extraction with diethyl ether, drying over magne-
sium sulfate, evaporation of the solvent and column
chromatography on silica gel using petroleum ether/
ethyl acetate (10:1) as eluent afforded 5.84 mmol (1.17
g, 83%) of aldehyde 3.
A solution of 86.4 mmol (8.74 g) diisopropyl amine
and 86.4 mmol of n-butyllithium in 40 ml of tetrahy-
drofuran was added to a solution of 73.7 mmol (19.9 g)
of 3-(3%-butynyl)-1-iodobenzene in 60 ml of tetrahydro-
furan at 0°C. After 30 min 110.57 mmol (15.69 g)
methyl iodide were added; then the mixture was al-
lowed to warm slowly to r.t., stirred for 3 h, the
reaction quenched with water at 0°C and the mixture
extracted with diethyl ether. The organic phase was
washed with water and dried over magnesium sulfate.
Distillation gave 16.33 g (60.44 mmol, 82%) of 2.
HR-MS: Found: 269.9908. C11H11I Calc.: 269.9906.
MS (EI): m/z 270 (50%), 217 (82%), 143 (100%), 128
(29%), 90 (46%). 1H-NMR (250 MHz, CDCl3): l=7.58
4
3
(t, 1H, JHH=1.6 Hz, aryl-H), 7.55 (d, 1H, JHH=7.8
3
Hz, aryl-H), 7.18 (d, 1H, JHH=7.7 Hz, aryl-H), 7.02
(pseudo-t, 1H, 3JHH=7.7 Hz, aryl-H), 2.72 (t, 2H,
HR-MS: Found: 200.1199. C14H16O Calc.: 200.1201.
3JHH=7.5 Hz, aryl-CH2), 2.38 (tq, 2H, JHH=7.5 Hz,
3
1
MS (EI): m/z 200 (1%), 147 (22%), 105 (100%). H-
5JHH=2.5 Hz, CH2–CꢀC), 1.75 ppm (t, 3H, JHH=2.5
5
3
NMR (500 MHz, CDCl3): l=9.81 (t, 1H, JHH=1.5
Hz, CH3). 13C-NMR (62.9 MHz, CDCl3): l=143.46,
137.70, 135.30, 130.18, 127.88, 94.50 (6C, aryl-C), 78.18
(CꢀCCH3), 76.70 (CꢀCCH3), 35.08 (aryl-CH2), 20.90
(CH2–CꢀC), 3.64 ppm (CH3).
Hz, CHO), 7.03–7.28 (m, 4H, aryl-CH), 2.91 (t, 2H,
CH2), 2.78 (t, 4H, CH2), 2.40 (tq, 2H, CH2),1.75 ppm
(t, 3H, CH3). 13C-NMR (125 MHz, CDCl3): l=202.06
(CHO), 141.78, 140.75 (2C, aryl-C), 128.76, 126.81,
126.73, 126.62 (4C, aryl-CH), 78.97 (CꢀCCH3), 76.66
(CꢀCCH3), 45.68 (CH2CHO), 35.95 (ArCH2), 28.51
(ArCH2CH2CHO), 21.38 (CH2CꢀCCH3), 3.92 ppm
(CH3).
5.2. 3-(3%-Pentynyl)-phenylpropanal 3
A solution of alkyne 2 (11.46 g, 42 mmol) in 20 ml of
diethyl ether was added slowly at −10°C to 45 mmol

K.H. Do¨tz, A. Gerhardt / Journal of Organometallic Chemistry 578 (1999) 223–228
227
(61%), 52 (54%). FT-IR (petroleum ether): w˜(CO) 2061
5.3. 1-Bromo-4-(3%-pentyn-3%%-yl-phenyl)-1-butene 4
(m), 1987 (w), 1962 (s), 1948 (vs.) cm−1
.
1H-NMR
3
(400 MHz, CDCl3): l=7.30 (dm, 1H, JHH=15.1 Hz,
CHꢁCHCH2), 7.21 (m, 1H, aryl-CH), 7.01–7.07 (m,
3H, aryl-CH.), 6.31 (dt, 1H, 3JHH=15.1, 7.1 Hz,
CH2CHꢁCH), 4.71 (s, 3H, OCH3), 2.76 (m, 4H, aryl-
A total of 5 mmol (0.56 g) of potassium tert-butox-
ide was added at −78°C to a suspension of 5 mmol
(2.18 g) of [Ph3PCH2Br]Br in 15 ml of tetrahydrofuran,
and the mixture was stirred for 30 min. Then a solu-
tion of 5 mmol (1.00 g) of aldehyde 3 in 5 ml of
tetrahydrofuran was added slowly at −78°C. After 30
min the mixture was allowed to warm to r.t. and
stirred for another 2 h. The mixture was hydrolyzed
with water and extracted with petroleum ether. Chro-
matography on silica gel using petroleum ether as elu-
ent gave 1.84 mmol (0.51 g, 37%) of bromoalkene 4 as
a 1/9 E/Z mixture of isomers.
3
CH2), 2.49 (td, 2H, JHH=7.4, 7.1 Hz, CH2CHꢁCH),
3
5
2.39 (tq, 2H, JHH=7.4 Hz, JHH=2.4 Hz, CH2CꢀC),
1.77 ppm (t, 3H, JHH=2.4 Hz, CH3). 13C-NMR (100
5
MHz, CDCl3): l=336.14 (CrꢁC), 223.98 (trans-CO),
216.70 (cis-CO), 144.47 (CHꢁCHCH2), 141.29, 140.61
(2C, aryl-C), 135.62 (CHꢁCHCH2), 128.60, 128.51,
126.34, 126.14 (4C, aryl-CH), 78.59 (CꢀCCH3),
76.18(CꢀCCH3),
66.40
(OCH3),
35.48
(aryl-
CH2CH2CꢀC), 34.55 (CH2CHꢁCH), 34.00 (aryl-
HR-MS: Found: 276.0505. C15H17Br Calc.: 276.0514.
MS (EI): m/z 276 (0.3%), 197 (52%), 143 (100%).
1H-NMR (500 MHz, CDCl3): Z-isomer: l=7.18–7.23
(m, 1H, aryl-H), 6.97–7.08 (m, 3H, aryl), 6.18* (m,
CH2CH2CHꢁCH), 21.00 (CH2CꢀC), 3.50 ppm (CH3).
5.5. Tricarbonyl[3,4,5,6,7,8-p6-(8-hydroxy-5-meth-
oxy-4-methyl[2.2]metacyclophane)]chromium 6
3
1H, CHꢁCHBr), 6.16 (dm, 1H, JHH=7.0 Hz, CHBr),
6.10 (pseudo-q, 1H,3JHH=6.9 Hz, CHꢁCHBr), 6.02*
3
(dm, 1H, JHH=13.5 Hz, CHBr), 2.64–2.84 (m, 4H, 2
A solution of 0.81 mmol (0.35 g) of carbene complex
5 in 30 ml of tetrahydrofuran was warmed to 65°C for
3 h while the reaction was monitored by FT-IR spec-
troscopy focusing on the decrease of the A11 w˜(CO)
absorption band of 5 and the increase of the E w˜(CO)
absorption band of the arene complex formed. Then
the solvent was removed in vacuo and the residue was
purified by column chromatography on silica gel using
petroleum ether/dichloromethane (3:1) as eluent to give
0.31 mmol (0.124 g, 38%) of cyclophane complex 6 as
a yellow solid.
CH2), 2.31–2.53 (m, 4H, 2 CH2), 1.77 ppm (t, 3H,
CH3). 13C-NMR (500 MHz, CDCl3): l=141.28 (2C,
aryl-C), 134.09 (CHꢁCHBr), 128.77, 128.55, 126.43,
126.31 (4C, aryl-CH), 108.53 (CHꢁCHBr), 78.82
(CꢀCCH3), 76.33 (CꢀCCH3), 35.68 (aryl-CH2CH2CꢀC),
34.34 (CH2CHꢁCHBr), 31.53 (aryl-CH2CH2CHꢁ
CHBr), 21.18 (CH2CꢀCCH3), 3.68 ppm (CH3).
The asterisk (*) denotes signals of the minor E-
isomer.
5.4. Pentacarbonyl{(E)-1-methoxy-5-[(3%-(pentyn-3%%-
yl)-phenyl]-2-pentenylidene}chromium 5
HR-MS: Found: 404.0715. C21H20O5Cr Calc.:
404.0716. MS (EI): m/z 404 (14%, M+), 376 (4%,
M
+ −CO), 348 (11%, M+ −2CO), 320 (79%, M+
−
−
A solution of 2.02 mmol (0.56 g) of bromoalkene 4
in 5 ml of diethyl ether was added at −78°C to a
solution of 4.16 mmol tert-BuLi in 10 ml diethyl ether.
The mixture was stirred for 2 h and then 2.04 mmol
(0.45 g) of hexacarbonyl chromium were added at
−78°C. After 30 min the mixture was allowed to
slowly reach r.t., and was stirred for another 3 h. Then
the solvent was evaporated and the brown residue was
dissolved in dichloromethane. The solution was cooled
to −20°C, 3.04 mmol (0.45 g) of trimethyloxonium
tetrafluoroborate was added and the mixture was al-
lowed to warm to r.t. overnight. After removal of the
solvent the residue was purified by column chromatog-
raphy on silica gel using petroleum ether/
dichloromethane (3:1) as eluent to give 0.95 mmol
(0.41 g, 47%) of carbene complex 5.
3CO), 268 (100%, M+ −3CO–Cr), 236 (78%, M+
Cr(CO)3–CH3OH). FT-IR (petroleum ether): w˜(CO)
1956 (vs.), 1878 (s) cm−1 13C-NMR (125 MHz, C6D6):
.
l=235.03 (Cr(CO)3), 136.29, 134.95 (2C, aryl-C),
129.25, 126.51, 126.48, 126.29 (4C, aryl-CH), 124.37,
119.57 (Cr(CO)3–arene–C), 112.80 (Cr(CO)3–arene–
CH), 92.23, 92.07, 91.12 (Cr(CO)3–arene–C), 68.27
(OCH3), 29.17, 29.13, 28.84, 25.26 (4C, benzyl-CH),
14.45 ppm (CH3).
5.6. 4-Methyl-[2.2]metacyclophane-5,8-dione 7
A solution of 0.35 mmol (0.15 g) of carbene complex
5 in 10 ml of di-n-butyl ether was warmed to 90°C for
2 h while the reaction was monitored by FT-IR spec-
troscopy. Then the solvent was removed, and the
residue was dissolved in 10 ml of diethyl ether. This
mixture was treated with 20 ml of a 0.5 N solution of
cerium(IV)ammonium nitrate overnight. Then water
was added to the mixture, and the product was ex-
HR-MS: Found: 432.0652. C22H20O6Cr Calc.:
432.0650. MS (EI): m/z 432 (0.2%, M+), 404 (0.1%,
M
+ −CO), 376 (1.2% M+ −2CO), 348 (2.3%, M+
−
3CO), 320 (2%, M+ −4CO), 292 (11%, M+ −5CO),
240 (7%, M+ −Cr(CO)5), 201 (26%), 159 (100%), 105

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tracted with diethyl ether. Chromatographic workup on
silica gel using petroleum ether/diethyl ether (2:1) gave
0.14 mmol (0.036 g, 40%) of quinone 7.
HR-MS: Found: 252.1147. C17H16O2 Calc.: 252.1150.
MS (EI): m/z 252 (100%, M+), 234 (77%), 206 (56%),
191 (22%), 104 (88%). FT-IR (petroleum ether): w˜(CO)
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1
1671 cm−1. H-NMR (250 MHz, CDCl3): l=7.35 (t,
3
1H, JHH=7.6 Hz, H-13), 7.01 (m, 2H, H-12,14), 6.79
4
(t, 1H, JHH=1.8 Hz, H-16), 6.40 (s, 1H, H-6), 2.92–
3.00 (m, 2H, benzyl-H), 2.60–2.78 (m, 5H, benzyl-H),
2.50 (m, 1H, benzyl-H), 2.03 ppm (s, 3H, CH3). 13C-
NMR (125 MHz, CDCl3): l=188.08 (C-5), 187.06
(C-8), 156.64 (C-3), 150.44 (C-7), 142.18, 141.49 (C-
4,11,15), 132.39 (C-16), 130.37 (C-6), 129.98 (C-13),
126.91, 126.68 (C-12,14), 38.79, 37.07, 35.59, 32.34 (4C,
benzyl-CH), 11.81 ppm (CH3).
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Acknowledgements
Support from the Deutsche Forschungsgemeinschaft
(SFB 334), the Graduiertenkolleg ‘Spektroskopie
isolierter und kondensierter Moleku¨le’ and the Fonds
der Chemischen Industrie is gratefully acknowledged.
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