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An Exception to the Bredt Rule in the Photoenolization of 4-Benzoyl[2.2]paracyclophane

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An Exception to the Bredt Rule in the Photoenolization of 4-Benzoyl(2.2]paracyclophane **
By ffenning ffopJ* Thomas Laue, and Maximilian Zander *
Dedicated to Professor Siedried Hiinig on the occasion
of his 70th birthday
The photoenolization of ortho-alkylated benzophenones 1
to ortho-quinodimethane derivatives 2 has been known since
the work of Yang et al. in the early sixties"] and has been
studied by several authors.['* 31
We posed the question whether this photoreaction can be
applied to 4-benzoyl-[2.2]paracyclophane (3 a),141which is
easily prepared by Friedel-Crafts benzoylation of [2.2]paracyclophane. The resulting ortho-quinoid system 4 is a possible intermediate for the construction of [1.2]phanes by
Diels-Alder addition. The [I .2]phanes form an unknown
group of phanes with extremely short bridges. According to
molecular models the C2-C3 double bond, which violates
Bredt's rule, is extremely twisted. Nevertheless we could not
exclude the formation of this bridgehead olefin during the
irradiation of 3a by a 1,Smigration of the endo hydrogen
atom R' to the keto function, first because we had already
observed that the ethano bridges in [2.2]paracyclophanes
tolerate drastic deformations (for example, the formation of
a triple bondt5]),and second because intramolecular chargetransfer stabilization in 4 by the benzene ring situated opposite was to be expected.
On irradiation of ethanolic solutions of 3a at 88 K (with
light of wavelength 320 nm or with the unfiltered light of a
mercury lamp), we observed a pronounced yellow coloration, which was characterized in the low-temperature UV
spectrum by a new broad band at 430 nm (half-height width:
5200 cm-'). On slow warming of the irradiated, yellow solid
reaction mixture, the color disappeared again and the absorption spectrum of the ketone 3a was observed. The processes were completely reversible and could be repeated several times on the same solution. To test for the enol 4, the
photoenolization was repeated at - 30°C in [DJmethanol
to which a catalytic amount of [D,]sodium methanolate had
been added. Already after a few seconds the photolysis mixture had assumed its characteristic yellow color, which again
paled slowly on melting. An analytically pure sample of the
ketone (84 YO)was recovered from the photolyzed solution
by chromatography on silica gel. According to the mass
spectrum, this ketone was 36% mono- and 8.5 % dideuterated. The sites of deuteration were determined by 400-MHz 'H
[*] Prof. Dr. H. Hopf, Dip].-Chem. T.Laue
Institut fur Organische Chemie der Universitat
Hagenring 30, W-3300 Braunschweig (FRG)
Prof. Dr. M. Zander
Rutgerswerke AG
Kekulestrasse 30, W-4620 Castrop-Rauxel (FRG)
[*'I Cyclophanes, Part 36. This work was supported by the Fonds der Chemischen Industrie. We thank MI. K . Bullik, Rutgerswerke AG, for the spectroscopic measurements. Part 35: C. J. Bodwell, L. Ernst, H. Hopf, P. G.
Jones, J. P. McNally, D. Schomburg, Chem. Ber. 123 (1990) 2381.
Verlagsgesellschaft mbH, W-6940 Weinheim, 1991
NMR spectroscopy. The signals of the protons R' (multiplet
shifted to lower field, 6 = 3.33) and R2 (multiplet shifted to
higher field, 6 = 2.85) lie apart from all other signals of the
bridge protons (which appear between 6 = 2.90 and 3.28).
The degree of deuteration was calculated from the reduced
signal intensities: ca. 36% for the position R' and 24% for
R2. These results may be understood if two H/D exchange
mechanisms are postulated, both of which begin with the
loss of the hydroxyl proton of 4. The resulting enolate 6 can
then react further with the large excess of deuterated solvent
to yield 5, which equilibrates with the endo-deuterated ketone 3 b (stereospecificexchange of R' = H with deuterium).
Alternatively, 6 can be deuterated at C2 by the solvent. For
steric reasons this occurs from the side opposite to the substituents and yields the exo-monodeuterated product 8.
Through both exchange processes, which occur with approximately equal probability as the NMR data show, the 2position is completely deuterated and the product is 7.In the
absence of light no H/D exchange in 3a takes place even at
60°C and in dimethylsulfoxide, a solvent known to favor this
type of exchange.f3I
3b, R'=D. R2=H
Bridged benzophenones like 3a are not only of interest
because of their reactivity, but may also be used as chiral
sensitizers for photoreactions. We have therefore also characterized 3 a by luminescence spectroscopy. The intensity
maximum of the broad phosphorescence band (ethanol,
77 K) lies at 19 880 cm- Even when the (0,O) transition is
taken into account, it should lie < 22 200 cm-' ((0,O) phosphorescence transition of benzophenone: 24 040 cm- '). The
average phosphorescence lifetime of 3a is 128 ms (benzophenone: 5 ms). From this relatively long lifetime, we conclude
that the lowest triplet level of 3a is a 3 ~ , 7 c * state. The phosphorescence emission spectrum of 3a corresponds to the UV
absorption spectrum. Compound 3a does not fluoresce. The
most likely explanation is that the 'n,x* and 3n,7t* levels, or
just the 3n,x* level, lie between the lowest singlet 7c,7c* and
the lowest triplet n,n* state.[61Characteristic of compounds
with such term diagrams are high quantum yields when the
lowest triplet state is occupied. It follows that the application
0570-0833/91/0404-0432$ 3 . 5 0 + .2Sj0
Angew. Chem. Int. Ed. Engl. 30 (1991) N o . 4
of 3a as triplet sensitizer in suitable photochemical systems
is theoretically p~ssible.~']
Received: November 15,1990 [Z 4285 IE]
Publication delayed at author's request.
German version: Angew. Chem. 103 (1991) 441
CAS Registry numbers:
3n, 10028-97-4; 4, 132259-72-4.
[I] N. C. Yang, C. Rivas, J Amer. Chem. SOC.83 (1961) 2213.
[2] For a review see D. C. Neckers. Mechanistic Organic Photochemisrry, Van
Nostrand-Reinhold, New York 1967, p. 174ff.
[3] R. Haag, J. Wirz, P. J. Wagner, Helv. Chim. Acta 60 (1977) 2595.
(41 A. Izuoka, S. Murata, T. Sugawara, H. Iwamura, J. Am. Chem. SOC.109
(1987) 2631.
[S] H. Hopf, M. Psiorz, Angew. Chem. 94 (1982) 639; Angew. Chem. Int. Ed.
Engl. 21 (1982) 623.
[6] For a review see M. Zander, Fluorimetrie, Springer, Heidelberg 1981,
p. 28 ff.
[7] N. J. Turro, Modern Molecular Photochemistry, Benjamin/Cummings,
Menlo Park 1978, p. 351 ff.
Triaurated Phosphonium Cations [RP(AuPPh),]
and the Electron-deficient, Hypercoordinated
Phosphonium Dication (P(AuPPh,),j2@ **
The complex 2 a (R = o-tolyl), obtained in 29% yield
from ortho-tolylphosphane, crystallizes in the form of colorless needles (m.p. 183- 186 "C,decomp) from a mixture of
tetrahydrofuran, diethyl ether, and dichloromethane, and is
readily identifiable by analytical data and NMR spectra.['*]
In the case of the analogue 2 b (R = Ph) the yield and thermal stability are significantly lower. The very similar spectroscopic data, however, reveal a close structural relationship.
The 31P-NMR spectra of 2 a at -70°C show the expected
doublet/quartet multiplicity of the signals (6 = 46.0(d),
- 19.0 (q, Jpp= 249 Hz)); at higher temperature, signal
broadening with disappearance of coupling occurs. This is
attributed to exchange processes in which presumably LAu@
and not L units are involved.
The X-ray structure analysis of 2ar1'] confirms the presence of organo(triaurio)phosphonium cations and tetrafluoroborate anions. The triclinic unit cell (space group Pf)contains two formular units, which are related to each other via
a center of symmetry. The most important features are the
small deviation of the Au-P-Au angle (average 106") from
ideal tetrahedral geometry and the associated long Au . . .Au
contacts of ca. 3.7 A (Fig. I), i.e. the dimensions of the
By Hubert Schmidbaur,* Gabriele Weidenhiller,
and Oliver Steigelmann
Complex compounds of monovalent gold tend to display
intra- and intermolecular Au . .. Au interactions.[" This is
especially noticeable in the aggregation of gold atoms
around heteroatoms to give novel centered gold clusters.
Thus, not only could species of the type [RC(AUL),],[~I
and [N(AuL),]@['I with classical fourcoordination of carbon or oxygen (L = tertiary phosphane)
be synthesized, but also the hypercoordinated cations
[C(AuL),]@['] and [C(AUL),]~@,[~'
[ N ( A U L ) ~ ] ~ @but
, [ ~ Inot [N(AUL),]~@.['~]
The results allowed to correct literature inconsistencies[' confirmed predictions based on MO considerations,['2] and could be
placed on a sounder theoretical footing by new calculations
taking into account relativistic effects.['3*14] Our studies
were now also extended to phosphorus-centered gold clusters.
After preliminary investigations on mono- and dinuclear
gold complexes of tertiary,"'
and finally primary phosphanes,['61 the latter were now also triply aurated.
Similarly as in the case of primary amines, this was achieved
especially easily by reaction with tris(tripheny1 phosphanegold(1))oxonium tetrafluoroborate 1"'' at room temperature in tetrahydrofuran. The best results were obtained with
arylphosphanes, but even there the yields were only moderate.
+ [(Ph,PAu),O]
[RP(AuPPh,),] BF,
+ H,O
[*] Prof. Dr. H. Schmidbaur, Dip1.-Chem. G. Weidenhiller,
DipLChem. 0. Steigelmann[+'
Anorganisch-chemisches Institut der Technischen Universitat Miinchen
Lichtenbergstrasse 4, W-8046 Garching (FRG)
X-ray structure analysis
This work was supported by the Deutsche Forschungsgemeinschaft (Leibniz Program), the Fonds der Chemischen Industrie, Hoechst AG, Degussa
AG, and Heraeus GmbH. The authors thank J. Riede and H . Beruda for
assistance with the structure determination and Dr. Kreissl for the mass
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 4
Fig. 1. Structure of the cation in 2a in the crystal (ORTEP, thermal ellipsoids
at the 50% probabiity level; C atoms with arbitrary radii, H atoms omitted; of
the two possible positions of the methyl group of the disorded o-tolyl moiety
only one is shown.pSelected bond lengths [A] and angles ["I: Aul-PI 2.315(8),
Au2-P2 2.288(8), Au3-P3 2.280(9), Aul-P4 2.277(7), Au2-P4 2.317(8), Au3-P4
2.321(8); Aul-Au2 3.709, Aul-Au3 3.696, Au2-Au3 3.639; Pl-Aul-P4
176.4(3),P2-Au2-P4 172.3(3), P3-Au3-P4 176.4(3);Aul-P4-A~2107.7(3), AulP4-Au3 107.0(3), A u ~ - P ~ - A103.3(3),
A~l-P4-C40114.7(7), Aul-P4-C40
113.7(9), A~3-P4-C40109.6(9).
cations [RP(AuL),]@ differ significantly from those of the
cations [RN(AuL),]@, where markedly smaller Au-N-Au
angles (ca. 102") and correspondingly short Au ...Au contacts (ca. 3.1 A) are f ~ u n d . [ ~Apparently,
the peripheral
gold-gold distances are so large, due to the much larger
radius of the phosphorus atom compared to that of nitrogen,
that no noteworthy energy contributions to the stabilization
of the cluster can result from direct metal-metal interactions.
At least they do not suffice to induce a drastic distortion of
the tetrahedral framework. The same also holds true in the
Only an increase in the
case of diauriophosphonium
number of the polyhedral corners, e.g. from four to five,
could lead to a convergence of the metal atoms.
All earlier attempts[201to carry out tri- or tetraauration of
PH, or its trimethylsilyl derivative P(SiMe,),[211 failed at
Verlagsgesellschaft mbH, W-6940 Weinheim, 1991
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paracyclophane, photoenolization, exception, rules, benzoyl, bred
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