J . Org. Chem. 2001, 66, 5269-5271
5269
Kin etics a n d Activa tion P a r a m eter s for th e
Sch em e 1
Th er m a l 1,5-Hyd r ogen Sh ifts
In ter con ver tin g th e F ou r
Mon od eu ter io-cis,cis-1,3-cycloocta d ien es
J ohn E. Baldwin,*,† Phyllis A. Leber, and
‡
Tamara W. Lee†
In eqs 1-3, the letters M, A, and V stand for methyl-
enic, allylic, and vinylic proton absorption intensities and
x ) exp(-kt). The equations themselves were derived from
an undetailed kinetic treatment of the reversible first-
order reactions of Scheme 1. Values for the rate constants
k and the related activation parameters secured for
isomerizations run with neat liquid samples were found
Department of Chemistry, Syracuse University,
Syracuse, New York 13244, and Department of Chemistry,
Franklin & Marshall College,
Lancaster, Pennsylvania 17604
-
5
-1
Received April 16, 2001
to be 3.46(( 1.06) × 10
s
at 150.2 °C, 1.42(( 0.37) ×
-
5
-1
-6 -1
1
0
s
at 137.1 °C, and 3.64(( 1.14) × 10
s
at 125.3
In 1962, thermally activated 1,5-hydrogen shifts in
deuterium-labeled cycloheptatrienes were demonstrated
and followed kinetically by ter Borg, Kloosterziel, and
q
q
°
C; ∆H ) 29.3 kcal/mol and ∆S -10 eu; E
a
) 30.1 kcal/
mol and log A ) 11.1.
The relatively large error bars on values of rate
constants followed from the NMR instrumental limita-
tions at that time, the modest temperature range covered,
and relatively small dynamic changes for the variables
1
Van Meurs, and similar interconversions among deute-
rium-labeled cyclopentadienes were reported by Russian
workers.2 Winstein and his collaborators noted such
reactions in 1,3-cyclooctadienes and cyclooctatrienes in
y
1
, y
isomer 3, as reaction times increased from 0 to infinity
and exp(-kt) decreased from 1.00 to 0, y and y de-
creased from 1.33 to 1.00 while y increased from 1.75 to
.00. Unfortunately, the relatively large error bars on
2 3
, and y . In kinetic runs starting with 100 mol %
1
963.3 Thus, thermal [1,5] sigmatropic migrations of
hydrogen were widely recognized even before Woodward
and Hoffmann in 1965 provided a theoretical account of
such isomerizations.4
In 1966, Glass, Boikess, and Winstein recapitulated
these developments and reported a kinetic study of
thermal interconversions among the four isomeric mono-
1
2
3
2
estimations of rate constants led inexorably to relatively
large uncertainties in activation parameters.
The error limits in rate constants are consistent with
straight-line fits having significantly different slopes and
intercepts, correlations that would extrapolate to very
different rate constants at substantially higher or lower
5
deuterio-cis,cis-1,3-cyclooctadienes (Scheme 1).
This noteworthy experimental study provided kinetic
information and derived activation parameters for the
isomerizations in the face of daunting technical limita-
tions associated with the analyses of reaction mixtures
using proton NMR spectroscopy at 60 MHz, especially
since the resolution was unable to resolve C(1,4)-H and
C(2,3)-H absorptions. The three observable NMR ab-
sorption intensity ratios, functions of the time-dependent
concentrations of isomers 1-4, were expressed as shown
in eqs 1-3.
a
temperatures. Assuming that the uncertainty in E stems
only from uncertainties in the rate constantssthat errors
associated with temperature measurements or the rela-
tively small temperature spread of the kinetic work, 24.9
°
C, contributes nothing to the uncertainty in E
a
sthen
) k(150.2 °C)
) k(125.3 °C). According to this estimate, ∆E is
/2.303RT ) 3.1.
6
the relationship of eq 4 applies; here, k
and k
.9 kcal/mol. Similarly, ∆[log A] ) ∆E
a
b
a
6
5
a
4
6
2
2
2
1/2
y ) M/A ) (55 - 2x + 5x + 2x )/
1
∆E/E ) [(1/ln(k /k )) ((∆k /k ) + (∆k /k ) )]
(4)
a
b
a
a
b
b
4
6
(
55 - x - 5x - 4x ) (1)
A continuing interest in prototypical pericyclic reac-
6
4
6
y ) V/A ) (55 + 3x + 2x )/(55 - x - 5x - 4x ) (2)
tions of “simple” hydrocarbons prompted a return and
reconsideration of the interconversions shown in Scheme
1. More precise values for the activation parameters could
be of importance in assessments of computational efforts
toward understanding the role of conformational factors
impinging on the rates of 1,5-hydrogen shifts. They could
also contribute to a clearer understanding of just how
bicyclo[4.2.0]oct-7-ene is converted thermally to cis,cis-
2
6
y ) (M + A)/V ) (110 - 3x - 2x )/
3
6
(
55 + 3x + 2x ) (3)
†
Syracuse University.
‡
Franklin & Marshall College.
1) (a) ter Borg, A. P.; Kloosterziel, H.; Van Meurs, N. Proc. Chem.
(
Soc. 1962, 359. (b) ter Borg, A. P.; Kloosterziel, H.; Van Meurs, N.
Rec. Trav. Chim. Pays-Bas 1963, 82, 717-740.
1
,3-cyclooctadiene. This reaction has attracted serious
mechanistic attention for at least 25 years and still seems
(
2) (a) Mironov, V. A.; Sobolev, E. V.; Elizarova, A. N. Dokl. Akad.
to be inconclusively rationalized.7
SSSR 1962, 143, 1112-1115. (b) Mironov, V. A.; Sobolev, E. V.;
Elizarova, A. N. Izv. Akad. Nauk SSSR, Otd. Khim. Nauk 1962, 2077-
Kin etic Equ a tion s. The expressions of eqs 1-3 for
variables y , y , and y may seem to the casual reader to
1 2 3
come out of the blue, but they may be understood readily,
given the theoretical expressions for the time-dependent
2
078. (c) Mironov, V. A.; Sobolev, E. V.; Elizarova, A. N. Tetrahedron
963, 19, 1939-1958.
1
(
3) Glass, D. S.; Zirner, J .; Winstein, S. Proc. Chem. Soc. 1963, 276-
2
2
67.
(
4) (a) Woodward, R. B.; Hoffmann, R. J . Am. Chem. Soc. 1965, 87,
511-2513. (b) Woodward, R. B.; Hoffmann, R. The Conservation of
Orbital Symmetry; Verlag Chemie: Weinheim, 1970.
(6) Benson, S. W.; O’Neal, H. E. Kinetic Data on Gas-Phase Uni-
molecular Reactions; National Bureau of Standards: Washington, DC,
1970; p 9.
(
5) Glass, D. S.; Boikess, R. S.; Winstein, S. Tetrahedron Lett. 1966,
9
99-1008.
1
0.1021/jo010389o CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/28/2001