Abstract:Viele Diels-Alder-Reaktionen laufen unter hohem Druck erheblich schneller ab, wobei diese Beschleunigung oftmals auch fur Synthesezwecke genutzt wird. Die aus der Druckabhangigkeit der Reaktionsgeschwindigkeit bestimmbaren Aktivierungsvolumina sind in der Regel sehr klein (A Y' im Bereich von -25 bis -45 cm3mol-'), manchmal sogar kleiner (d. h. im Betrag groDer) als die Reaktionsvolumina (A V, im Bereich von -30 bis -40 cm3mol-'; AV*/AV, 2 I) [']. Demnach sind die Ubergangszustande['I in ihrem Volumen produkti… Show more
“…At 7.7 kbar and 150 °C the intramolecular Diels−Alder reaction is the favored process by a factor of 4.2. From the pressure dependence of the product ratio one can extrapolate that the difference in the activation volumes (ΔΔ V ⧧ = Δ V ⧧ ( Z - 1 → cis - 4 ) − Δ V ⧧ ( Z - 1 → 7 ) < [ RT /(7700−1)]·ln(1/4.2·2.2) = −10 cm 3 mol -1 ) is comparable to those observed for pericyclic rearrangements involving monocyclic and bicyclic transition states, respectively …”
Section: Introductionmentioning
confidence: 90%
“…The volume of activation of the degenerate Cope rearrangement of 1,5-hexadiene can be estimated to be approximately Δ V ⧧ ≈ −10 cm 3 mol -1 if the packing coefficient of cyclohexene 4,7b is used for the unknown packing coefficient of the transition state. In fact, negative activation volumes of the expected magnitude were found for several Cope rearrangements, related Claisen rearrangements, and the electrocyclization of the ( Z )-1,3,5-hexatriene to 1,3-cyclohexadiene involving the formation of a new six-membered ring in each transition state. , …”
Section: Introductionmentioning
confidence: 98%
“…Many pericyclic rearrangements such as Cope rearrangements, , electrocyclizations, and intramolecular Diels−Alder reactions , show a pressure-induced acceleration which is also characterized by negative volumes of activation. As before, the effect of pressure on the pericyclic rearrangements, which is usually smaller than that on the intermolecular cycloadditions, can be explained by assuming larger packing coefficients for the cyclic transition states than for the acyclic reactants . This may be illustrated by the degenerate Cope rearrangement of parent 1,5-hexadiene.…”
The volumes of reaction determined for the hypothetical
cyclization of 1-alkenes to cycloalkanes
decrease continuously from the formation of cyclopropane
(ΔV
R = −5.5 cm3
mol-1) up to the formation of
cyclodecane (ΔV
R = −32.3 cm3
mol-1) and seem to be constant for the larger
rings. The analysis of the
packing coefficients (η = V
W/V)
leads to the conclusion that this ring-size dependent decrease in
volume
results from the different packing of cyclic and acyclic compounds
rather than from the changes in their intrinsic
molecular volumes. The investigation of the intramolecular
Diels−Alder reactions of (E)-1,3,8-nonatriene
(E)-1 and (E)-1,3,9-decatriene
(E)-2 leading to the
bicyclo[4.3.0]nonenes cis- and
trans-4
(ΔV
⧧/ΔV
R
[cm3
mol-1] −24.8/−32.0 and −24.8/−28.5,
respectively) or bicyclo[4.4.0]decenes cis- and
trans-6 (−37.6/−45.4
and −35.0/−37.4, respectively) confirms the ring-size dependence of
the activation and reaction volumes.
The dependence of the effect of pressure from the number of newly
forming rings is illustrated with the
thermolysis of (Z)-1,3,8-nonatriene
(Z)-1 in which an intramolecular Diels−Alder
reaction leading to bicyclo[4.3.0]nonene cis-4 competes with a
sigmatropic [1,5] hydrogen shift leading to
(E,Z)-1,5,7-nonatriene 7. The
use of high pressure causes a reversal of selectivity.
“…At 7.7 kbar and 150 °C the intramolecular Diels−Alder reaction is the favored process by a factor of 4.2. From the pressure dependence of the product ratio one can extrapolate that the difference in the activation volumes (ΔΔ V ⧧ = Δ V ⧧ ( Z - 1 → cis - 4 ) − Δ V ⧧ ( Z - 1 → 7 ) < [ RT /(7700−1)]·ln(1/4.2·2.2) = −10 cm 3 mol -1 ) is comparable to those observed for pericyclic rearrangements involving monocyclic and bicyclic transition states, respectively …”
Section: Introductionmentioning
confidence: 90%
“…The volume of activation of the degenerate Cope rearrangement of 1,5-hexadiene can be estimated to be approximately Δ V ⧧ ≈ −10 cm 3 mol -1 if the packing coefficient of cyclohexene 4,7b is used for the unknown packing coefficient of the transition state. In fact, negative activation volumes of the expected magnitude were found for several Cope rearrangements, related Claisen rearrangements, and the electrocyclization of the ( Z )-1,3,5-hexatriene to 1,3-cyclohexadiene involving the formation of a new six-membered ring in each transition state. , …”
Section: Introductionmentioning
confidence: 98%
“…Many pericyclic rearrangements such as Cope rearrangements, , electrocyclizations, and intramolecular Diels−Alder reactions , show a pressure-induced acceleration which is also characterized by negative volumes of activation. As before, the effect of pressure on the pericyclic rearrangements, which is usually smaller than that on the intermolecular cycloadditions, can be explained by assuming larger packing coefficients for the cyclic transition states than for the acyclic reactants . This may be illustrated by the degenerate Cope rearrangement of parent 1,5-hexadiene.…”
The volumes of reaction determined for the hypothetical
cyclization of 1-alkenes to cycloalkanes
decrease continuously from the formation of cyclopropane
(ΔV
R = −5.5 cm3
mol-1) up to the formation of
cyclodecane (ΔV
R = −32.3 cm3
mol-1) and seem to be constant for the larger
rings. The analysis of the
packing coefficients (η = V
W/V)
leads to the conclusion that this ring-size dependent decrease in
volume
results from the different packing of cyclic and acyclic compounds
rather than from the changes in their intrinsic
molecular volumes. The investigation of the intramolecular
Diels−Alder reactions of (E)-1,3,8-nonatriene
(E)-1 and (E)-1,3,9-decatriene
(E)-2 leading to the
bicyclo[4.3.0]nonenes cis- and
trans-4
(ΔV
⧧/ΔV
R
[cm3
mol-1] −24.8/−32.0 and −24.8/−28.5,
respectively) or bicyclo[4.4.0]decenes cis- and
trans-6 (−37.6/−45.4
and −35.0/−37.4, respectively) confirms the ring-size dependence of
the activation and reaction volumes.
The dependence of the effect of pressure from the number of newly
forming rings is illustrated with the
thermolysis of (Z)-1,3,8-nonatriene
(Z)-1 in which an intramolecular Diels−Alder
reaction leading to bicyclo[4.3.0]nonene cis-4 competes with a
sigmatropic [1,5] hydrogen shift leading to
(E,Z)-1,5,7-nonatriene 7. The
use of high pressure causes a reversal of selectivity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
