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Intramolecular Interconversion of syn- and anti-Tricarbonyl(1-ethoxy-1 2-dihydrocyclobutabenzene)chromium.

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The relationship of the two structures is thereby directly
shown. Both structures contain layers of corner-sharing
YF,] octahedra in accordance with 2[VF,,2F,,l] with two
truns FQ ions; the layers are also stacked in the same way.
The crucial difference between the two structure types is that
the [VFJ octahedra within a layer are tilted toward one
another; the bond angle of the fluorine bridge V-F2-V is
150.7' (SnF,: 1 80').
autoclave, which was then placed in an oven preheated to 280°C and kept for
about ten weeks at this temperature. After stepwise cooling (25 "C per day), the
autoclave was opened under inert gas (Ar, glovebox) and the VF, thus obtained
was transferred to glass ampoules for further analysis.
Received: December 4. 1989;
supplemented: January 29, 1990 [Z 3665 IE]
German version: Angew. Chem. 102 (1990) 426
CAS Registry numbers:
V, 7440-62-2; F, 7782-41-4: VF,, 10049-16-8.
[I] A. F. Wright, B. E. F. Fender, N. Bartlett, K. Leorg, Inorg. Chem. 17(1978)
748.
[2] R. D. Burbank, F. N. Bensey, USA E.C. Rep. (1956) K-1280.
[3] H. SchBfer, H. G. von Schnering, K. I. Niehues. H. G. Nieder-Vahrenholz.
J Less-Common M et . 9 (1965) 95.
[4] B. G. Muller, M. Serafin, Z . Nafurforsch. 8 4 2 (1987) 1102.
[5] R. G. Cavell, H. C. Clark, J Chem. Soc. 1962, 2692.
[6] A. J. Edwards, P. Taylor, Chem. Commun. 1970, 1474.
/3 = 119.89(0), Z = 2.
[7] P2,lr; u = 5.340(3), b = 5.170(2), c = 5.381(3)
Phillips PW 1100 AgK. four-circle diffractometer; 528 symmetry-independent reflections, R = 0.060, R, = 0.045. Further details of the crystal structure investigation may be obtained from the Fachinformationszentrum
Karlsruhe, Gesellschaft fur wissenschafltich-technische Information mbH.
D-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository
number CSD-54336, the names of the authors, and the journal citation.
[S] R. Hoppe, 2. KrisfuNogr. 150 (1979) 23.
191 H. G. von Schnering, Festkiirperkolloquium Kiel. Mirz 1987;personal communication.
A.
Intramolecular Interconversion of
syn- and anti-Tricarbonyl(1-ethoxy-1,Zdihydrocyc1obutabenzene)chromium. Trapping of the
ortho-Quinodimethane Intermediate **
L
b
,
By Ernst Peter Kiindig,* Gerald Bernardinelli,
James Leresche, and Patrick Romanens
Fig. 1. Unit cell of SnF, (dashed line) and VF, (solid line) [7]
Comparison of the interatomic distances of the three isotype or structurally related compounds MF, (M = Sn, Nb,
V) reveals that, for VF,, the difference between the distances
d(M-terminal FQ) and d(M-bridging Fo) is more pronounced than for SnF, or NbFL9' (Table 2).
Table 2. Comparison of the interatomic distances in four compounds of the
type MF,.
M
V
Nb
Sn
Mn
The reason for this is unclear, since, even for MnF,-albeit with cis-oriented terminal Fo and d = 0.871-a comparable quotient is found.
Susceptibility measurements on several samples of VF,
crystals between 4.2 and 251 K (p,,,(251 K) = 1.43 B.M.)
provided evidence for the absence of oxygen (VOF, is diamagnetic) and confirmed the complicated collective magnetic interaction^;'^^ the determination of the magnetic structure by neutron diffraction is in progress.
1,2-Dihydrocyclobutabenzenesare convenient and widely
used precursors of the highly reactive ortho-quinodimethanes, which are synthetically important dienes in Diels-Alder
reactions.["'] Although syntheses of n-cyclobutabenzenetransition metal complexes have been reported and reactions
of the compiexed cyclobutabenzene (nucleophilic addition,
the
lithiation) have been the subject of several studies,[3
effect of coordination on the transformation cyclobutabenzenelortho-quinodimethane has thus far remained unexplored. We report here on the thermal ring opening of com1, and on a
plexed l-ethoxy-l,2-dihydrocyclobutabenzene
[4 + 21-cycloaddition of the postulated intermediary orthoquinodimethane complex.
We chose 1 in this study for the following reasons: firstly,
reversible electrocyciic ring-opening of 1 -alkoxy-substituted
1,2-dihydrocyclobutabenzenesoccurs at moderate temperatures (80- 110 'C)@I and, secondly, the Cr(CO), complexes
of 1 are readily accessible by exchange of the arene iigdnd of
tricarbonyl(naphtha1ene)chromium ; the 2: 3 mixture of diastereomers (syn-2 and anti-2) that is formedc7]can be readily
separated by flash chromatography. The benzyl protons in
benzocycloalkene complexes readily undergo stereoselective
[*] Prof. Dr. E. P. Kundig, J. Leresche, P. Romanens
Department de Chimie Organique, Universite de Geneve
30 Quai Ernest Ansermet, CH-1211 Geneve-4 (Switzerland)
Dr. G. Bernardinelli
Laboratoire de Crystallographie Universite de Geneve
24 Quai Ernest Ansermet, CH-1211 Geneve-4 (Switzerland)
Experimental Procedure
Vanadium powder (500 mg, Ventron) was placed in a Monel autoclave under
inert gas (Braun glovebox). At - 196 "C. 0.8 mL of F, was condensed into the
Angen,. Chem Inr. Ed. EngI. 29 (1990) No. 4
0 VCH
[**I
This work was supported by the Swiss National Science Foundation
VerlagsgeseNsrhuff mbH, 0-6940 Wernheim. 1990
C~S70-0~33190j0404-0407
$02.5010
407
base-catalyzed hydrogen/deuterium exchange:'] this enables assignment of the structures of syn-2 and anti-2. Upon
addition of a catalytic amount of tBuOK to syn-2 or to anti-2
in [D,]DMSO, the 'H-NMR signal of one of the two methylene protons disappeared, and monodeuteriated complexes
were formed. These signals were assigned to the anfi-H
atoms. The tertiary benzylic C-H bond remained unaffected
in both diastereomers. The vicinal coupling constant of the
remaining benzyl protons is 4 Hz in anti-2 (JCJ and 1.5 Hz
in syn-2 (J,,,,,),in accord with data on uncomplexed 1-substiThis assignment was
tuted 1,2-dihydro~yclobutabenes.[~~
confirmed by an X-ray analysis of syn-2 (Fig. 1).
Fig. 1. Structure of syn-tricarbonyl(1 -ethoxy-l,2-dihydrocyclobutabenzene)chromium svn-2 in the crystal; @ = Cr. Q = 0 [lo].
On heating in benzene, syn-2 and anti-2 interconvert. The
reaction can be conveniently followed by 'H NMR in a
sealed tube. Clean first-order kinetics are observed for the
reversible reaction in the temperature range of 130- 180 "C
(Fig. 2).
180T 170%
16OOC
Table 1. Rate constants for the reaction syn-l$onii-2. A first order reversible
reaction was assumed by using the equation In (Ao-Aeq)/(A-Acq)= ( k , + k , ) i.
The correlation was in the range 0.992 to 0.999 for all plots.
130
140
1SO
160
170
180
0.78 x 10-5
2.1 10-5
5.2 x 10-5
25
13
4
2
0.8
0.33
10.0 x 10-5
23.0~10-5
56.0~
1.0~10-5
2.8 10-5
6.8 x
13.3 x
30.3 x
74.4x 1 0 - 5
19
I
28
1.43
0.63
0.26
enthalpy to be the source of this rate difference. If we assume
an o-quinodimethane-Cr(CO), complex as intermediate, the
activation entropy difference may indicate a more ordered
transition state." An 18-electron configuration of Cro
would be achieved if the metal coordinates to the cyclic diene
as well as to one of the exocyclic double bonds of the o-quinodimethane. This presumably would increase the barrier of
rotation around the metal ligand bond and could be the
cause of the entropy difference (loss of a degree of freedom).
An intramolecular process for the interconversion of syn-2
and anti-2 also concurs with the observation that no arene
exchange with the solvent (C,D,) takes place in the temperature range investigated. This would have been expected if the
Cr(CO), group were coordinated only to the diene portion of
the o-quinodimethane or if the mechanism of interchange
involved metal-arene bond dissociation. Similar bonding
situations are known in an o-quinodimethane
with tricarboand a methylene/cyclohexadiene complex
nylchromium as well as in q'-fuIvene and q6-azulene complexes." 3
Experimental evidence for the proposed o-quinodimethane intermediate was obtained via a [4 + 2]-cycloaddition
reaction. Slow addition of several olefin dienophiles to the
equilibrating mixture of syn- and anti-2 resulted in decomposition of the complex, possibly due to a CO/olefin exchange.
We therefore turned to trans-I ,2-bis(trimethylsilyI)ethene:''] which we considered to be too bulky to interact readily with the metal. Thermolysis of syn-2 in this dienophile
(solvent and reactant) at 160 "C for 6 h resulted in a 56 %
conversion of 2 into a 17: 1 mixture of complexes 4 and 5.
-1
t 151
-
_I
anti - 2
Fig. 2. First order kinetic plots for the interconversion of syn-2 and unli-2, as
measured by growth and decay of the 1H-NMR signals of the methyl protons.
Me3SiP==JSiMe3
Attempts to detect an intermediate failed. The 'H-NMR
spectrum at 150 "C merely showed the resonances of syn-2
and anri-2. The activation parameters calculated from the
data listed in Table 1 are: AH* = 29.9 & 1.0 kcal mol-'
and AS* = - 8.5 f 3 cal mol-' K - ' .[ l ' ] For comparison,
the values calculated from the data obtained by Sammes
et al. for the racemization of I-methoxy-I ,2-dihydrocyclobutabenzene are AH* = 30.7 kcal mol-' and A S * = 3.0 cal
mol-l
K-1.161
Q VCH Verlugsgesellsc4wfi mbH, 0-6940 Weinheim. 1990
SiMeg
C p 3
Cr(C0)3
+
5 (3 %)
+
syn/anti- 2 (35%)
SiMe3
4 (53%)
At 130 "C, the reaction of 2 is about 100 times slower than
that of non-complexed I -methoxy-I ,2-dihydrocyclobutabenzene ( k = 1 .I4 x 10-3s-1).r61Comparison of the activation parameters show the activation entropy rather than the
408
16OoC l 6 h
35% of 2 was recovered from the reaction as a synlanti
mixture." Structural assignment of complex 4 was based
0570-08~3i90io404-o4ON0404-040~
$02.SOj0
Angew. Chem. Int. Ed. Engl. 29 (1990) No. 4
on N M R spectral analysis and an X-ray diffraction study
(Fig. 3). The X-ray structure of complex 4 reveals a boat
conformation of the saturated ring of the tetralin complex
with a cis arrangement of the C(l) ethoxy and the C(2) trimethylsilyl groups. The structure of 5, a stereoisomer of 4,
has not yet been assigned unequivocally.
C(4)), 2.55 (dd, l H , J = 1 3 , 15.6Hz; H,&(4)),
3.42-358 (m, 2H,
OCH,CH,), 4.44 (d, 1 H, J = 6.7 Hz; H-C(l)), 4.49-4.58 (m, 2H. H C(6.7)).
4.70(d, 1 H, J = 5.5 Hz; H-C(5)), 5.55(d, 1 H . J = 5.7 Hz; H-C(8)). ”CNMR
(50 MHz. C,D,). 6 = 234.20. 112.74, 110.14, 93.23.92.29. 90.75, 90.56, 77.71.
66.47, 28.16, 27.71, 18.65, 15.58. Correct elemental analysis.
Received: December 11, 1989 [Z 3677 IE]
German version: Angew. Chem. 102 (1990) 421
CAS Registry numbers:
1, 125685-98-5; anti-2, 99571-34-3; svn-2, 99537-80-1 : 4, 125685-99-6: 5.
125761-44-6; tricarbonyl(napthalene)chromium, 12110-37-1; [runs-bis(trimethylsily1)ethene. 18178-59- 1.
Fig.3. Structure of the adduct 4 in the crystal; @
=
Si [lo].
This first study of a transition metal bound 1.2-dihydrocyclobutabenzenelo-quinodimethanereaction has focused on
mechanistic aspects. We are presently extending these studies
to more synthetically oriented projects. As the metal completely shields one face of the o-quinodimethane intermediate, high stereoselectivity can be expected in Diels-Alder
reactions and this will be of particular import in reactions
with chirdl complexes. The present, severe limitation of useful dienophiles may be overcome by judicious choice of benzocyclobutene complexes with other organometallic fragments, by using milder ring opening reactions and/or
different routes to n-complexed ortho-quinodimethane complexes.
Experimental
The mixture ofdiastereomers syn-2 and anti-2 obtained in 70% yield from the
arene exchange in tricarbonyl(naphtha1ene)chromium was separated by flash
chromatography using hexaneiether 4/1 (syn-2: R, = 0.13; unti-2: R, = 0.19).
ryn-2: M.p.(hexane): 59-60°C. IR(hexane, v(C0)): i[cm-’J): 1984(s), 1918
( s ) . ’H NMR (360 MHz, C,D,): 6 = 1.18 (t. 3H, J = 7 Hz; -CH,), 2.60 (dd,
1 H. J = 5.5. 14 Hz;Ha,,;C(2)),2.80(dd, 1 H, J = 1.5. 14Hz: Hb&(2)), 3.183.46 (m, 2 H . OCH,CH,) 4.12 (t, 1 H. J = 6 Hz; H,,,,), 4.20 (dd. 1 H, J = 2,
5.5 Hz; H-C(1). 4.46 (t, 1 H, J = 6 Hz; H.,,,), 4.52 (d, 1 H, J = 6 Hz; H.co,,),
Correct elemental analysis.
4.83 (d. 1 H, J = 6 Hz; Harnm).
anti-2: M.p.(hexane): 71.5-72.5”C. IR (hexane, v(C0) [cm-’1): 1984 (s),
1918 ( S ) . ‘ H NMR (360 MHz, C,D,): b = 1.02 (t, 3H. J = 7 Hz; CH,), 2.59
(dd, I H. J = 1.7, 14 Hz, Han,;C(2)), 2.98 (dd, 1 H. J = 4, 14 Hz; H3&(2)),
3.08 - 3.24 (m, 2 H ; OCH,CH,), 4.16 (t, 1 H, J = 6 Hz; H,,,,), 4 33 (t. 1 H,
J = 6 HZ. H,,,, ), 4.60 (d. 1 H, J = 6 Hz; H,,,,,), 4.69 (dd, 1 H, J = 1.7, 4 Hz;
H-C(l)), 5.02 (d, 1 H, J = 6 Hz; Harm,).Correct elemental analysis.
4: A degassed solution ofcomplex syn-2 (0.200 g, 0.70 mmol) in Iruns-bis(trimethylsily1)ethene was stirred at 160°C for 6 h in a closed tube. After cooling
to 20 C. the excess olefin was recovered by short path distillation. the residue
was taken up in ether, filtered, and evaporated to dryness. Flash chromatography on silica (hexaneiether = 4/1) furnished 4 (170 mg, 53%. R, = 0.48) followed by 5 (10 mg, 3%. R, = 0.35) and syn/unti-2 (70 mg, 35%).
M.p.(EtOH/H,O): 78- 80°C. IR (hexane, v((C0):i [cm-’I). 1970 (s), 1900(s).
‘H NMR (200 MHz, C6D,): 6 = - 0.20 ( s , 9H, Si(CH,),), 0.00 (s, 9 H ,
Si(CH,),). 0.80--0.94 (m. 1 H, H-C(3)), 1.10 (t. 3H, J = 6.9 Hz; CH,CH,O),
1.55 (dd. 1 H, J = 4. 6.7 Hz. H-C(2)). 2.00 (dd, 1 H, J = 7.5, 15.6 Hz; Ha”,,Angew. Chem. Int. Ed. Engl. 29 (1990) NI).4
J. L. Charlton, M. J. Alauddin, Tetrahedron 40 (1987) 2873.
W. Oppolzer, Synthesis 1978, 793.
E. P. Kiindig, Pure Appl. Chem. 57 (1985) 1855.
C. Elschenbroich, J. Koch, S . Schneider, B. Sprangenberg, P. Schiess.
J. Organomel. Chem. 317 (1986) 41.
H. G. Wey, H. Butenschon, J. Orgunome!. Chem. 350 (1988) C8.
B. J. Arnold, P. G. Sammes. T. W. Wallace. J Chem. SOC.Perkm Truns 1
1974,409.
E. P. Kundig. C. Perret. S. Spichiger, G. Bernardinelli, J Organornet.
Chem. 286 (1985) 183. All reactions in this study were carried out with
racemic 1. The arene exchange reaction yields two diastereomeric pairs of
complexes. For the sake of brevity, only one svn- and one anti-enantiomer
are schown in the equation and in the figures.
W. S. Trahanovsky, R. J. Card, J. Am. Chem. Soc 94 (1972) 2897.
W. A. Bubb, S. Sternhell, Aust. J Chem. 29 (1976) 1685.
Crystal data for syn-2: A suitable crystal of average dimensions
0.25 x 0.25 x 0.30 mm was sealed in a Lindemann capillary under Ar.
The lattice parameters and intensities were measured at room temperature
on a Philips PW 1100 diffractometer with a graphite monochromated
Mo,, radiation. Orthorhombic, space group P 2,2,2,. u = 9.938(2).
b = 10.296(3). c = 12.330(2) V = 1261.6(5)
2 = 4. F,,,,, = 1.496 g
ern-,, p(MoKo)= 0.886mm-’ I % 295 K . F ( 0 0 0 ) = 584. 15x4 Friedel
pairs were recorded (0-28 scan) of which 2002 had lF,I > 3o(F,) and
IF,] > 4, The structure was solved by direct methods (Multan 80) and
refined by full matrix least squares methods (XRAY 76). The refinement
of the absolute structure parameter (x = 0.51(4))[18]clearly indicates that
the sample is an inversion twrn with 50% of each enantiomer The final R
factor, based on 2002 observed reflexions, was 0.032 R, = 0.035.Crystal data for 4: A suitable crystal of average dimensions 0.05 x
0.20 x 0.35 mm was sealed in a Lindemann capillary under Ar. The lattice
parameters and intensities were measured at room temperature on a
Philips PW 1100 diffractometer with a graphite monochromated Mo,,
radiation. Triclinic. space group Pi. a = 8.494(1), h = 11.331(1), c =
14.513(2) a = 78.57(2), = 74.40(2), y = 68.26(3)”. V = 1233.7(4)A’.
Z = 2, F,,,,, = 1.23 g c r K 3 , ~(Mo,,) = 0.568 mm-’. F(000) =
484.3014 independent reflections were recorded (w-28 scan), of which 1864
had IF,] z 4u(F0).The structure was solved by direct methods (Multan 87)
and refined by full matrix least squares methods (XTAL 2.4). The final R
factor, based on 1864 observed reflexions, was 0.072. R, = 0.044. Further
details of the crystal structure investigations are available on request from
the Director of the Cambridge Crystallographic Data Center, University
Chemical Laboratory, Lensfield Road. Cambridge CB2 IEW (UK).
The error limits for the activation parameters represent 90% confidence
limits in the rate constants.
While an o-quinodimethane intermediate is a plausible proposal in view of
the structural analogy of isolated complexes [13 151, other intermediates,
e.g. an q6-arene diradical complex, cannot be excluded as yet. Likewise.
the entropy effect could result from a transition state with more (zwitter).
ionic character.
M. Yalpani, R. Benn, R. Goddard, G. Wilke, J Orpanomet. Chem. 240
(1982) 49.
J. Blagg, S. G. Davies, G. L. Goodfellow, K. H. Sutton, J. Chem. Sor.
Chem. Commun. 1986, 1283.
a) F. Edelmann, S. T o k e , U Behrens, J. Orgunomet. Chem. 309 (1986) 87:
b) S. Toke. U. Behrens, Angen,. Chem. 99 (1987) 134; Angen. Chem. I n l .
Ed. Engl. 26 (1987) 147; c) J. Bandy. V. S. B. Mtetwa. K. Prout, J. C
Green, C. E. Davies, M. L. H. Green, M. J. Hazel, A. lzquierdo, J. J. Martin-Polo, J. Chem. SOC.Dalton Trans 1985 2037.
J. Dunogues, R. Calas, N. Ardoin, C. Biran, P. Ladouyade, J. Orgunomet.
Chem. 32 (1971) C31.
Although no kinetic study has been carried out, samples removed in the
course of the trapping reaction, and analyzed by HPLC, indicate the rate
of formation of 4 to be considerably slower than the interchange of s.vnand anti-2. The rate of the latter appears to be roughly the same in the
ethene 7 medium as in benzene.
G. Bernardinelli, H. D. Flack, Acra Cryrtullogr. S r r ~ A. 41 (1985) 500.
A.
A’.
~
A,
a VCH Verlug.s~e.self.~chufimbH, D-6940 Weinherm. 1990
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0570-0833/90/0404-0409 $OZ.SOjO
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