Limits...
Torquoselective ring opening of fused cyclobutenamides: evidence for a cis,trans-cyclooctadienone intermediate.

Wang XN, Krenske EH, Johnston RC, Houk KN, Hsung RP - J. Am. Chem. Soc. (2014)

Bottom Line: Electrocyclic ring opening of 4,6-fused cyclobutenamides 1 under thermal conditions leads to cis,trans-cyclooctadienones 2-E,E as transient intermediates, en route to 5,5-bicyclic products 3.Theoretical calculations predict that 4,5-fused cyclobutenamides should likewise undergo thermal ring opening, giving cis,trans-cycloheptadienones, but in this case conversion to 5,4-bicyclic products is thermodynamically disfavored, and these cyclobutenamides instead rearrange to vinyl cyclopentenones.

View Article: PubMed Central - PubMed

Affiliation: Division of Pharmaceutical Sciences and Department of Chemistry, University of Wisconsin , Madison, Wisconsin 53705, United States.

ABSTRACT
Electrocyclic ring opening of 4,6-fused cyclobutenamides 1 under thermal conditions leads to cis,trans-cyclooctadienones 2-E,E as transient intermediates, en route to 5,5-bicyclic products 3. Theoretical calculations predict that 4,5-fused cyclobutenamides should likewise undergo thermal ring opening, giving cis,trans-cycloheptadienones, but in this case conversion to 5,4-bicyclic products is thermodynamically disfavored, and these cyclobutenamides instead rearrange to vinyl cyclopentenones.

Show MeSH
Free energy profiles for thermal rearrangements of 8 to (a) 10 and (b) 12 in toluene.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4353010&req=5

fig3: Free energy profiles for thermal rearrangements of 8 to (a) 10 and (b) 12 in toluene.

Mentions: Why does 8 rearrangeto 12, rather thanto 10? Theoretical calculations (Figure 3a) in fact predict that the cis,trans-cycloheptadienone 9-E,E, which would be the precursor of 10, can be accessedfrom 8 with a barrier of 33.1 kcal/mol (TS9). This value is only 0.8 kcal/mol higher than the barrier for formationof 2-E,E from 1. It is therefore likely that some 9-E,E is formed transiently during the courseof the reaction at 195 °C. The transient formation of 9-E,E is not reflected in the finalproduct distribution, however, because the transannular cyclizationproduct 10 is higher in energy than the starting cyclobutenamide(ΔG = 10.4 kcal/mol).18 Given the thermodynamic driving force against the formation of 10, the cis,trans-cycloheptadienone 9-E,E reverts to 8 which rearranges by the alternative pathway shown in Figure 3b to 12. Conversion of 8 to the α,β-unsaturated intermediate 11 islikely to be acid catalyzed.19 Formationof 12-cis and 12-trans from 8 has ΔG ≈−12 kcal/mol.


Torquoselective ring opening of fused cyclobutenamides: evidence for a cis,trans-cyclooctadienone intermediate.

Wang XN, Krenske EH, Johnston RC, Houk KN, Hsung RP - J. Am. Chem. Soc. (2014)

Free energy profiles for thermal rearrangements of 8 to (a) 10 and (b) 12 in toluene.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4353010&req=5

fig3: Free energy profiles for thermal rearrangements of 8 to (a) 10 and (b) 12 in toluene.
Mentions: Why does 8 rearrangeto 12, rather thanto 10? Theoretical calculations (Figure 3a) in fact predict that the cis,trans-cycloheptadienone 9-E,E, which would be the precursor of 10, can be accessedfrom 8 with a barrier of 33.1 kcal/mol (TS9). This value is only 0.8 kcal/mol higher than the barrier for formationof 2-E,E from 1. It is therefore likely that some 9-E,E is formed transiently during the courseof the reaction at 195 °C. The transient formation of 9-E,E is not reflected in the finalproduct distribution, however, because the transannular cyclizationproduct 10 is higher in energy than the starting cyclobutenamide(ΔG = 10.4 kcal/mol).18 Given the thermodynamic driving force against the formation of 10, the cis,trans-cycloheptadienone 9-E,E reverts to 8 which rearranges by the alternative pathway shown in Figure 3b to 12. Conversion of 8 to the α,β-unsaturated intermediate 11 islikely to be acid catalyzed.19 Formationof 12-cis and 12-trans from 8 has ΔG ≈−12 kcal/mol.

Bottom Line: Electrocyclic ring opening of 4,6-fused cyclobutenamides 1 under thermal conditions leads to cis,trans-cyclooctadienones 2-E,E as transient intermediates, en route to 5,5-bicyclic products 3.Theoretical calculations predict that 4,5-fused cyclobutenamides should likewise undergo thermal ring opening, giving cis,trans-cycloheptadienones, but in this case conversion to 5,4-bicyclic products is thermodynamically disfavored, and these cyclobutenamides instead rearrange to vinyl cyclopentenones.

View Article: PubMed Central - PubMed

Affiliation: Division of Pharmaceutical Sciences and Department of Chemistry, University of Wisconsin , Madison, Wisconsin 53705, United States.

ABSTRACT
Electrocyclic ring opening of 4,6-fused cyclobutenamides 1 under thermal conditions leads to cis,trans-cyclooctadienones 2-E,E as transient intermediates, en route to 5,5-bicyclic products 3. Theoretical calculations predict that 4,5-fused cyclobutenamides should likewise undergo thermal ring opening, giving cis,trans-cycloheptadienones, but in this case conversion to 5,4-bicyclic products is thermodynamically disfavored, and these cyclobutenamides instead rearrange to vinyl cyclopentenones.

Show MeSH