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Intramolecular [4 + 2]-Cycloreversion of the Two Symmetrical Homobasketenes.

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Intramolecular [4 2]-Cycloreversion of the Two
Symmetrical Homobasketenes
By Wolfram Grimme,* Wolfgang Mauer, and
Christian Sarter
Dedicated to Professor Giinther Wilke on the occasion
of his 60th birthday
The reverse of the Diels-Alder reaction proceeds exothermally only in those cases when the fragments gain in
aromatic resonance o r when strain energy in the reactant is
relieved. The latter-mentioned driving force is responsible
for the intramolecular [4 21-cycloreversion of basketene
1 ['I to give the syn-tricyc10[]deca-3,7,9-triene
2 and
of the asymmetric homobasketene 312'to give the syn-tricyclo[]undeca-3,8,10-triene
4 ; both cage-openings
are followed by further rearrangements.
ratio 1 : 5 ) in dichloroethane is heated to boiling for 7.5 h ;
small amounts of the cycloadduct of cyclopentadiene are
formed at the same time.
The C,-homobasketene 10 undergoes cycloreversion at
150°C to give the tricycl0[4.4.I.O~~~]undeca-3,7,9-triene
The structure of 13 was established by independent synthesis from the dicarbonyliron complex 14,'" which is
formed by photoaddition of cycloheptatriene to tricarbonyl(cyc1obutadiene)iron. Oxidation with cerium ammonium nitrate in aqueous acetone leads to liberation of the
organic ligand 13 from 14 (Table 1).
The same mode of stabilization is open to the highly
strained homobasketenes 7 (Table 1) and 10; since we are
interested in the relationship between structure and reactivity in cycloreversions, we have investigated the thermal
behavior of the two cage compounds. The homobasketenes
were obtained, using a standard method, from the bishomocubanones 5I3l and 8,L41
respectively, in which the symmetry of the target compounds is already present: Ring expansion with diazomethane in methanol/ether in the presence of lithium bromide affords the homologous ketones
and 9I6l, respectively, which are converted with lithium
diisopropylamide and diethyl chlorophosphate in tetrahydrofuran into the enol phosphates. Reduction of the enol
esters with lithium in liquid arnmonia/tetrahydrofuran/
tert-butyl alcohol furnishes the homobasketenes 7 and
Table 1. Some physical data of the compounds 7 , 12, and 13. The 'H-NMR
spectra were recorded at 90 MHz in CDCI, (that of 7 at 0°C in CFCI,).
7 , 'H-NMR: 6=6.06 (AA'XX' half-spectrum, 2H), 3.32 (AA'XX' half-spectrum, 2H), 2.9 (m, 2H), 2.0 (m, 4H), 1.16 (t, 2 H , CHZ); MS: m/z 141 (Mi),
78 (CsH 2 , 36%), 66 (CSH b, lOO%)
12, m.p. 217-219°C
(decamp.); 'H-NMR: 6=7.5-7.0
(m, SH), 5.64 (dd,
(AB, J = 8 Hz, 2 H, CHZ)
13,m.p.86"C; 'H-NMR:6=6.10(~,2H),S.7(m,4H),3.45(d,J=8Hz,2H),
2.6 (m, 3H), 2.23 (AB half-spectrum, J = 1 1 Hz, 1 H, CH2); UV (hexane):
(€=3800) sh, 257 (4200), 268 (3900), 279 (2100); MS: m / z 144
( M + ) , 129 (MI-CH,, lOO%), 91 (C,H:, 36%)
In order to determine the kinetic parameters for the
cleavage of 7 in dodecane solution between 81 and 8 5 T ,
the increasing extinction at 264 nm (cyclopentadiene absorption) was monitored until a constant value was observed. The kinetics of the isomerization of 10 were determined in heptane solution between 150 and 164°C by the
ampule technique, the reactant/product ratio being measured by gas chromatography.
The activation parameters for the intramolecular [4 21cycloreversion of basketene 1 and its three homologues 3,
7, and 10 are listed in Table 2. Also listed is the strain re-
The C,,-homobasketene 7 decomposes to benzene and
cyclopentadiene already at 80 "C. This cleavage results
from two consecutive [4 21-cycloreversions, the first of
which converts the homobasketene into the endo-tricycl016.2.1 .02.']undeca-3,5,9-triene 11. This tricycle has already been obtained via a n alternative route; its cleavage
at 40 "C to give benzene and cyclopentadiene is known."]
The intermediate 11 can be trapped as the cycloadduct 12
(Table 1) if a mixture of 7 and N-phenylmaleimide (molar
Table 2. Activation parameters and strain release for the intramolecular
[4+ 2]-cycloreversion of basketene and its homologues.
[*] Dr. W. Grimme, Dr. W. Mauer, DipLChem. C. Sarter
Institut fur Organische Chemie der Universitat
Greinstrasse 4, D-5000 Koln 41 (FRG)
Angew. Chem. I n f . Ed. Engl. 24 (198s)No. 4
logA [a]
E , [a]
[kcal/mol] [a] [kcal/mol]
13.2 [ I ]
12.8k0.5 121
29.7 [I]
29.8 [b]
28.2e0.1 [b]
113 -(30+26 +5)
[a] The error limits are the standard deviation from the linearly adjusted Arrhenius equation. [b] Because of twofold degeneracy of the reaction, RTln2
was added.
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33 I
lease that occurs on opening of the cage compounds. Force
field calculations are already available for the strain energy of the saturated educts;I9]the additional strain energy
due to the double bond is almost the same in all four compounds and has been neglected. The strain in the products
is made u p additively from the contributions of the individual rings.""'
There is a linear relationship [Eq. ( l ) ] between the loss in
strain energy AE,,and the free enthalpy of activation AG'
at 130°C; the correlation coefficient is 0.9999, at a confidence level of more than 99.5%.
AG' =49.9-0.39AE7,
Equation (1) indicates that in the transition state for the
opening of each of the structurally related cage compounds 39% of the strain release has occurred and that this
alone determines the stability of the compounds. The extrapolation of Equation (1) to a [4 21-cycloreversion without any loss of strain, which is not realizable in cage compounds, leads to AG = 50 k 0.6 kcal/mol. For the cycloreversion of cyclohexene to ethene and butadiene, which
proceeds without loss of strain, a free enthalpy of activation of 57+ 1 kcal/mol at 130°C is obtained from the activation parameters ;["I the agreement is satisfactory, considering the extent of extrapolation.
l b , R = C Z H 5 and [Ta(q5-C5Me5)
(SCH,CH,S),] 2.
The 'H-NMR spectrum of l a measured at 36°C shows a
broad singlet at 6 = 7 . 5 for the SCH=CHS. At temperatures above 4 0 T , the broad peak begins to sharpen, while
at low temperature ( - 50°C) it appears as two sharp singlets (6=7.14, 7.84) of equal intensity. The spectral
changes were found to be reversible and independent of
concentration. On the other hand, the singlet of qs-CSMe5
at 6 = 2.25 remained unchanged in the temperature range
studied. These observations indicate the presence of two
identical conformers in equilibrium, each of which contains orientationally nonequivalent dithiolate ligands.
Received: December 12, 1984 [Z 1108 IE]
German version: Angew. Chem. 97 (1985) 354
[I] H. H. Westberg, E. N. Cain, S. Masamune, J. Am. Chem. Soc. 91 (1969)
121 W. Mauer, W. Grimme, Tetrahedron Lett. 1976, 1835.
[3] W. L. Dilling, C . E. Reineke, R. A. Plepys, 3. Org. Chem. 34 (1969)
[4] W. G. Dauben, D. L. Whalen, J. Am. Chem. SOC.93 (1971) 7244.
151 K. Hirao, Y. Kajiwaka, 0. Yonemitsu, Terrahedron Lett. 1977, 1791.
161 W. Grimme, W. Mauer, G. Reinhardt, Angew. Chem. 91 (1979) 254; Angew. Chem. In,. Ed. Engl. I S (1979) 224.
171 A. R. Rye, D. Wege, Aust. J. Chem. 27 (1974) 1943.
[S] J. S. Ward, R. Pettit, J. Am. Chem. Soc. 93 (1971) 262.
191 a) E. M. Engler, J. D. Andose, P. von R. Schleyer, J. Am. Chem. Soc. 95
(1973) 8005; b j G. J. Kent, S. A. Godleski, E. Osawa, P. von R. Schleyer,
J. Org. Chem. 42 (1977) 3852.
[lo] S. W. Benson: Thermochemicul Kinetics, Wiley, New York 1968.
[I I] a) L. Kiichler, Trans. Furuday Soc. 35 (1939) 874; b) M. Kraus, M. Vavrudka, V. Baiant, Collect. Czech. Chem. Commun. 22 (1957) 484.
A New Class of Tantalum(v) Dithiolate Complexes:
Synthesis and Characterization of
[Ta(q5-C5Me4R)(SCH=CHS),I(R= Me, Et) and
By Kazuyuki Tatsumi, Junko Takeda, Yoitsu Sekiguchi,
Masaki Kohsaka, and Akira Nakamura*
There is obvious fundamental and practical interest in
understanding the chemistry of transition metal thiolate
complexes because of their biological and catalytic implications.[" In contrast to the ubiquitous S-coordinated chelate complexes of group 6, 7, and 8 transition metals, such
complexes of group 5 metals, particularly of N b and Ta,
Herein we report the
characterization15b-d1of [Ta(q5-C5Me4R)(SCH=CHS),1 l a ,
[*] Prof. A. Nakamurd, Dr. K. Tatsumi, J. Takeda, Y. Sekiguchi,
M. Kohsaka
Department of Macromolecular Science, Faculty of Science
Osaka University
Toyonaka, Osaka 560 (Japan)
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Given the stoichiometry, Ta(q5-C5Me5)(SCH=CHS)z,
the spectral data described above are consistent with the
structure (I) in which two SCH=CHS ligands are folded
around the SS axis, one with the open side pointing toward
the q5-C5Me5group and the other with the opposite orientation. The conformation appears to be fluxional, and the
mutual interconversion ( I ) s ( I ' ) occurs at a detectable
rate (AG&= 14.9 kcal/mol,
= 14.7 kcal/mol,
AS = - 0.6 cal mol I K- I , T, = 34"C),['] probably by an inversion process of the five-membered TaS2C, chelate rings.
A similar temperature dependence of the SCH=CHS proton resonance was observed for l b , where the activation
parameters were practically the same as those of l a . Nonplanarity of the TaS& ring is not surprising, but is common to SCH=CHS complexes with d* transition metals, a
typical example of which is the well-established structure
of [Ti(q5-C5H5),(SCH=CHS)1.[7'The folding in the Ti complex was attributed to the bonding interaction occurring
between a vacant Ti d orbital and the occupied C = C T[ orbital of SCH=CHS.[81
The interesting structure (I) can be explained with the
aid of molecular orbital considerations. l a and l b contain
four basal Ta-S CT bonds, forming a so-called four-legged
piano stool skeleton. The d" "piano stool" molecule carries two low-lying vacant d
(x2-y2 and z2),
which act as potential acceptors of electrons from the C = C
TI-donor orbital of SCH=CHS (Fig. 1); stabilization of 1 is
maximal when both d orbitals are fully utilized in such a
way that one C = C TI orbital interacts with x2-yz at the lateral side and the other T[ orbital with z2 at the bottom. Conformation (I) fulfills this requirement.
x i _y:
Fig. 1. Stabilization of the structure of l a and l b
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Angew. Chem. I n r . Ed. Engl. 24 (1988) No. 4
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two, intramolecular, cycloreversionen, symmetrically, homobasketenes
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