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Non-equilibrating Diradicals Stereochemistry of the Thermal Reorganization of Methyl-substituted 6-Methylenebicyclo[3.2.0]hept-2-enes

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Non-equilibrating Diradicals" I:
Stereochemistry of the Thermal Reorganization of
Methyl-substituted 6-Methylenebicyclo[3.2.0]hept-2enes
By Dieter Hasselmann"]
Sigmatropic carbon migrations in bisallylic systems are the
subject of intense mechanistic investigations['! Thermolysis
of specifically deuteriated 6-methylenebicyclo[3.2.0]hept-2enes leads, alongside a relatively slow automerization
(1 a)$(l b), to the 5-methylenebicyclo[2.2.1]hept-2-enes
(2a) and (2b), the product of a formal [1,3]-sigmatropic
rearrangement always preponderating slightly['! Automerization of the rearrangement products (2a)$(2b) occurs
only at appreciably higher temperature131.
The experiments thus far performed, however, d o not permit
any statement with regard to the stereochemistry of the migrat-
more slowly than the starting material that has the lowest
rearrangement rate. The isomerization rates of (3) to (6)
and the product compositions in the gas phase were independent of pressure in the range studied (0.5-6 torr). Added propene or allene exerted no influence on the reaction. At any
one temperature the proportions of the stable reaction products proved largely independent of time, although a change
in the temperature of thermolysis did effect a change in these
product ratios. These results are consistent with the assumption of a set of concurrent, uncatalyzed, unimolecular rearrangements.
Because of the interrelated rearrangements of the starting
materials ( 3 ) to (6) the thermolyses follow a complex kinetic
scheme of first-order reactions, which can, however, be solved
numerically. Starting in each case from pure ( 3 ) , ( 4 ) , ( 5 ) ,
or (6) and using low conversions, we determined approximate
values for all the rate constants according to the law for
irreversible first-order reactions. These values were improved
by extrapolation to zero time. By using these k values educt
and product concentrations were calculated with numerical
integration by the Runge-Kutta methodI6] and agreed well
with the concentrations found experimentally, so that even
a small variation in the experimental k values led to excellent
agreement. Table 1reproduces the rate constants after improvement in this way.
For interpretation of the results three mechanistic alternatives
in particular must be taken into account: 1. Competing concerted [3,3]-(Cope) and [1,3]-sigmatropic rearrangements; 2.
A two-step mechanism with participation of equilibrated diradicals as intermediates; 3. A course involving non-equilibrating
diradicals or diradical transition states of comparable energy
("continuous diradi~al"~']).
Table 1. Rate constants [a] of thermolysis of (3) to ( 6 ) (gas phase, 189.0"C).
_
(5)
(3)
(4)
(5)
(6)
[a] kxlO'[s-'];
535.7 [b]
16.5
6.1
0.50
18.3
188.9 [b]
4.6
0.72
At ca. 190°C the isomers (3) to ( 6 ) rearrange, both in solution
and in the gas phase[41, in each case into one another and
into the four methylated 5-rnethylenebicyclo[2.2.l]hept-2-enes
( 7 ) to
(7) to (10) can be regarded as stable under
the reaction conditions since they rearrange at least 65 times
Angen. Chrm. internut. Edit.
I
Vol. 14 ( 1 9 7 5 )
1 No. 4
59 7 [b]
-0
0.67
0.48
:0
45.1 [b]
_
_
_ __-_
~
(7)
(8)
(9)
(10)
44.7
334.0
16.9
4.R
0.14
120.0
133.0
37.2
:0
2.20
0.75
-0
43.5
11.1
6.4
0.20
[b] Toial decrease of the educl
ing carbon C7 (or C'). The report below concerns the thermal
rearrangement of the specifically methyl-substituted hydrocarbons (3) to (6).
[*] Dr. D. Hasselmann
Abteilung fur Chemie der Universitat
463 Bochum, Postfach 21 48 (Germany)
I5 8
10.2
(6)
Since stereospecificity cannot be considered as a sufficient
criterion"] for orbital symmetry control[91 of a reaction, an
energy gain by resonance stabilization of the aromatic transition statefLo1
must be demanded in addition as an essential
factor. The activation energies for the decrease in amounts
of ( 3 ) to (6) can be estimated from the k values (Table
1) with Ig A = 13.5-14 to 38-41
kcal/mol (exp. (1)- ( 2 ) :
Ig A = 13.7, E,= 39.6 k ~ a l / m o l ~ ~
For
] ) .a hypothetical diradical
process a comparable activation energy would be expected" I-!
Therefore, according to this model, orbital symmetry controlled energetic assistance can be to a large extent excluded
both in the framework of the Frontier Orbital Theory of
Woodward and Hofmanl'] and of the Berson-Salem concept" 41
of a "subjacent orbital" participation.
Even for thermolysis of (1) a significant participation of the
[3,3]-sigmatropic (Cope) rearrangement could be excluded
on geometrical grounds (C3-C' separation > 4 A)[']. This
prediction is confirmed in this work by the finding that ( 9 )
is formed at comparable rates from ( 3 ) and (4); only the
reaction ( 4 ) -+ ( 9 ) follows formally the stereochemical course
of a Cope rearrangement; and further it is precisely in the
case of ( 4 ) , in contrast to ( 3 ) , ( 5 ) and (6), that the endo-methyl
group should make the transition state for the Cope rearrangement more difficult to reach.
257
+
The sigmatropic [1,3]-migrations ( 3 ) + (7) ( S ) , and
( 4 ) -+ (7) + ( 8 ) , d o not occur stereospecifically. This too contraindicates worthwhile resonance stabilization of an aromatic
transition state in the [1,3]-rearrangement. The formally orbital symmetry forbidden product (8) is actually formed from
( 4 ) faster than is the allowed product (7)['?
[6] Cf.: R . Zurmiihl: Praktische Mathematik fur Ingenieure und Physiker
(Practical Mathematics for Engineers and Physicists), 4th Edit., p. 407. Springer, Berlin 1963. I wish to thank Prof. M . Saunders for making his computer
program available
171 W uon E. Doering and K . Sachdeu, J . Amer. Chem. SOC.96, 1168 (1974).
[8] Cf., Y . g. [2c].
[9] R. B. Woodward and R . Hofmann,Angew. Chem. 81, 797 (1969); Angew.
Chem. internat. Edit. 8, 781 (1969).
[lo] M . J. S. Dewar, Tetrahedron Suppl. 8, 75 (1966); Angew. Chem. 83,
859 (1971); Angew. Chem. internat. Edit. 10, 761 (1971).
[ I 1 1 An estimate can be made from 49.5 kcal/mol ( E , for the degenerate
methylenecyclobutane rearrangement [12]) minus 12-13 kcal/mol 1131 (for
the allyl resonance energy of C'C*C3).
The epimerization ( 3 ) + ( 4 ) requires, in one phase of the
reaction, an orthogonal non-bonding relation between C 1
and C 7 and can therefore be regarded as non-concerted. Formal cleavage of the C'-C7 bond in ( 3 ) to (6) leads to
the resonance-stabilized cis- and trans-substituted bisallyl diradicals ( 1 1 ) and (12). From the product ratio (7)/(8), as
an example, one can test whether (Ila)z$(llb)
and
(12a)+(12b)
are
equilibrated
intermediates:
(5)*(7)/(8) = 1.33; (6)-+(7)/(8) = 1.43. Then, independently of whatever the cisltrans ratio (11)/(12) may be, the
values for (7)/(8) from ( 3 ) and ( 4 ) should be found to
be between 1.33 and 1.43; however: ( 3 ) + (7)/(8) = 0.134;
( 4 ) -+ (7)/(8) = 0.657. Thediscrepancy between these product
ratios is in conflict with equilibrated diradicals as intermediates, in spite of the high energetic stabilization in ( 2 1 )
and (12).
Non-equilibrating diradicals offer an explanation for the experimental findings. Even in our more complex system such diradicals appear to behave as individual transition states in the
sense of the "continuous diradicals" postulated by Doering
and Suchdev['I. The hypothesis is supported by the individual
rate constants of the rearrangements of ( 3 ) to (6). The diradical character of the transition states is supported by extensive
utilization of the resonance stabilization of both allyl triads
and by the gradual change of the rate constants for the total
decrease of ( 3 ) to (6), which reflects only a substituent effect
of the methyl group (cf. Table 1). The methyl-free compound
( I ) , with k-,1,=100.8x ~ O - ' S - ~ fits
( ~ ]in the middle of the
series. An alternative that cannot be excluded is the existence
of intermediates in flat energy troughs, where product formation competes successfully with rotation around the C 5 - C 6
bond in ( 1 1 ) and (12). Since the stereochemical course of
the rearrangements of ( 3 ) to (6) appears to be severely
influenced by the methyl substituents, investigations are in
progress with substrates that a) exclude this steric factor and
b) increase the flexibility of the system by ring expansion.
Received: November 13, 1974 [Z 158 IE]
German version: Angew. Chem. 87, 252 (1975)
CAS Registry numbers:
( 3 ) , 54739-12-7; ( 4 ) , 54774-07-1 ; ( 5 ) . 54139-13-8; (6). 54739-14-9;
(71, 54739-15-0; (8). 54739-16-1 ; (Y), 28304-67-8; (ZO), 28304-66-7
[I] Part 2. This work was supported by the Deutsche Forschungsgemeinschaft. Part I : D . Hassehmnn, Tetrahedron Lett. 1973, 3739.
[2] a) J . A. Berson, 7: Miyashi, and G . Jones, J. Amer. Chem. SOC.96,
3468 (1974); b) J. J. Cajewski, L. K . Hoffman,
and C. N. Shih, ibid. 96,
3705 (1974): c) F.-C. Kliirner, Angew. Chem. 86, 270 (1974); Angew. Chem.
internat. Edit. 13, 268 (1974).
[3] D. Hasse/mann, Tetrahedron Lett. 1972, 3465.
[4] Thermolyses in solution were carried out in n-heptane, those in the
gas phase in a 20-1 Pyrex round-bottomed flask (temperature constancy
better than 0.1 "C) at ca. I torr.
[5] (3) to ( 1 0 ) were unambiguously characterized by synthesis, elemental
analysis and MS, IR, 'H-NMR and FT-13C-NMR spectroscopy.
258
[12] W uon E . Doering and J. C . Gilbert, Tetrahedron Suppl. 7 , 397 (1966).
1131 W van E. Doering and C . H. Beasley, Tetrahedron 29, 2231 (1973);
W R . Roth, C . Ruf, and P. W Ford, Chem. Ber. 107. 48 (1974).
[14] J . A. Bersoit and L. Salem, J. Amer. Chem. SOC.94, 8917 (1972).
115) The [1,3]-rearrangement of the analogous 6-acetate gives a similar
result: J . A. Berson and G. N. Nelson, J. Amer. Chem. SOC.92, 1096 (1970).
N2S30-The First Oxide of a Five-Membered SulfurNitrogen R i n g [ * * ]
By Herbert ui Roesky and Hartmut Wiezer"]
The recent synthesis of N4S404[11has raised the question
whether oxides of cyclic SN compounds containing less than
six ring members can be prepared.
(CH3)4Sn2N4S4 (1)['. 41, prepared from S4N4 and
[(CH3)3Sn]3N by reaction in a molar ratio of 1 : 1, reacts
with SOF2 with elimination of (CH3)2SnF2according to
H3d CH3
(1)
The reaction product having the composition N2S30 ( 2 )
is isolated as a red liquid which can be vacuum-distilled
without decomposition (b.p. 50°C/0.01 torr); it does not wet
glass. No signs of decomposition were observed, even after
storage for several weeks at room temperature. In contrast,
the previously known isomer formulated as (S=N&SO is
readily decomposable[4!
The structure of the new compound was elucidated by elemental analysis, IR spectrum, and mass spectrum.
The molecular ion is observed at m/e 140 with a relative
intensity of 11 %, accompanied by the following fragments
m/e 94 NS2O (473,92 N2S2 (4%), 80 S20 (lo%), 78 S2N
(28%), 76 NzSO (12%), 64 Sz(S0z) (50%), 60 NzS @%),
48 SO (46%), and 46 NS (100%).
In the IR spectrum (cm-') the band at 1125vs could be
assigned to the SO stretching mode, and those at 981 s, 910s,
and 734 s to the ring skeleton. Further absorptions were registered at 1181 m, 663 s, and 583 m.
Procedure:
An excess of SOF2 is passed into a suspension of (1) (9.9g)
in CH2C12 (600ml). The mixture is then stirred for 3 h, the
[*] Prof. Dr. H. W. Roesky and Dr. H. Wiezer
Anorganisch-chemisches Institut I der Universitat
6 Frankfurt am Main 50, Niederurseler Hang (Germany)
[**I This work was supported by the Fonds der Chemischen Industrie
and the Deutsche Forschungsgemeinschaft.
Angnv.
Chem. inrernor. Edir. 1 Vol. 14 ( 1 9 7 5 ) 1 No. 4
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methyl, diradical, reorganization, thermal, enes, equilibration, methylenebicyclo, hept, non, substituted, stereochemistry
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