Published on Web 07/19/2006
Thermal Reactions of 7-d- and 8-d-Bicyclo[4.2.0]oct-2-enes
John E. Baldwin,*,‡ Phyllis A. Leber,§ and David C. Powers§
Departments of Chemistry, Syracuse UniVersity, Syracuse, New York 13244, and Franklin and Marshall College,
Lancaster, PennsylVania 17604
Received May 23, 2006; E-mail: jbaldwin@syr.edu
Detailed studies of the thermal chemistry of bicyclic vinylcy-
clobutanes have provided telling insights on the stereochemical
course of [1,3] carbon shifts and other kinetically competitive
reactions.1 This communication reports the first study of the thermal
conversions of bicyclo[4.2.0]oct-2-ene (1) to bicyclo[2.2.2]oct-2-
ene (2) and to the fragmentation products 1,3-cyclohexadiene and
ethylene. The chemistry exhibited by 1 stands in sharp contrast to
the pattern of transformations found for the extensively investigated
bicyclo[3.2.0]hept-2-ene reactant (3).2
Gas phase kinetic runs at 300 °C with cyclooctane as an internal
standard and N2 as the bath gas gave rate constants for decay of 1
(ko ) 1.39 × 10-5 s-1), formation of 2 from 1 (k13 ) 4.26 × 10-6
s-1), and of 1,3-cyclohexadiene and ethylene from 1 (kf ) 9.60 ×
10-6 s-1). No isomerization to 1,3(Z),7-octatriene was observed.
Samples of both 7-d-1 and 8-d-1 rich in the endo isomers were
heated at 300 °C for various times up to 30 h. Analyses of thermal
The [1,3] carbon shifts converting 3 to norbornene take place
with a noteworthy preference for inversion of configuration: work
with deuterium-labeled analogues has been used to determine ksi/
ksr ratios of ∼3 at 276 °C and ∼8 at 312 °C.2b,c One interpretation
of this stereochemical outcome has posited competition between a
favored orbital-symmetry-controlled sigmatropic shift with inversion
and a minor stereochemically randomizing diradical-mediated path.
Were this model correct, one would expect that [1,3] shifts from 1
would favor migration with inversion more dominantly, for the
larger homologue has more conformational flexibility than the
[3.2.0] system and could facilitate a concerted process with
inversion by providing access to a less geometrically demanding
transition structure.3
For a diradical-mediated [1,3] shift, however, the increased
distance between the migrating carbon and the migration terminus
might favor a diminished ksi/ksr ratio and more kinetically prominent
roles for stereomutation at C8 and fragmentation to 1,3-cyclohexa-
diene and ethylene.3
To prepare for securing the required kinetic and stereochemical
data, the well-known compounds 1 and 2 and the deuterium-labeled
analogues 7-n-d-1, 7-x-d-1, 8-n-d-1, 8-x-d-1, 5-n-d-2, and 5-x-d-2
were prepared and characterized.
Selective reductions of 7-d- and 8-d-bicyclo[4.2.0]octa-2,7-
dienes4 with diimide generated in situ by treatment of hydrazine
with 30% hydrogen peroxide at -20 °C2h,5 gave the 7-d- and 8-d-
bicyclo[4.2.0]oct-2-enes, which were isolated and purified by
preparative GC. The diastereomeric ratios determined by 2H NMR
at 92.124 MHz reflected a preferential reduction from the exo face
of the C7-C8 double bond: 7-n-d-1 (δ 1.77):7-x-d-1 (δ 1.91) )
4.8:1 and 8-n-d-1 (δ 1.63):8-x-d-1 (δ 2.23) ) 5.9:1. These chemical
shift assignments were based on NOE experiments with unlabeled
1.
2
product mixtures by H NMR provided measures of equilibration
between exo and endo d-labeled versions of 1 and ratios of 5-n-
d-2 and 5-x-d-2 products. Interconversions between 7-n-d-1 and
7x-d-1 were too slow to be detected: the 7-n-d-1 component in
the mixture of epimers remained 82.7 ( 2.5% over six kinetic runs,
while the 5-x-d-2 isomer in the mixture of product epimers was
83.9 ( 2.3%. As a percentage of the two epimers, after 29.5 h at
300 °C, there was 85.4% of 7-n-d-1 and 84.8% of 5-x-d-2 in the
thermal reaction mixture.
The absence of a structural isomerization of 1 to 1,3(Z),7-
octatriene and of a detectable two-centered ring-inverting stereo-
mutation interconverting 7-n-d-1 with 7-x-d-1 implies that C1-
C6 bond cleavage to form a diradical intermediate is not kinetically
competitive. That the thermal profile of 3 includes both an
isomerization to a linear triene and a C1-C5 two-centered
epimerization reaction is undoubtedly dependent on a common
diradical intermediate formed by C1-C5 bond cleavage. The
absence of both types of reaction from 1 suggests that it has more
to overcome thermodynamically to cleave its C1-C6 bond than 3
has to break its C1-C5 bond.
When 8-d-1 was followed kinetically, the endo-exo equilibration
proved kinetically dominant (Figure 1). The simple exponential
function for approach to 8-n-d-1:8-x-d-1 equilibrium characterized
by rate constant keq ) 6.11 × 10-5 s-1 ) 2k8e was some 14 times
larger than k13 ) (ksi + ksr) for the isomerization of 1 to 2. A very
modest kinetically controlled stereochemical preference for ksi over
ksr (ksi ) 2.46 × 10-6 s-1; ksr ) 1.80 × 10-6 s-1; ksi/ksr ≈ 1.4) was
2
deduced from H NMR spectral data obtained for thermal rear-
rangement samples and calculations based on integrated rate
expressions (Figure 2). The substantial amount of C8 epimerization
coupled with little kinetically controlled stereoselectivity for the
‡ Syracuse University.
§ Franklin and Marshall College.
9
10020
J. AM. CHEM. SOC. 2006, 128, 10020-10021
10.1021/ja0636035 CCC: $33.50 © 2006 American Chemical Society