Cope Rearrangement in Bicyclo[5.1.0]octa-2,5-diene and its Mono- and Di-Hetero Analogues: A DFT Study

摘要

The Cope rearrangements of bicyclo[5.1.0Jocta-2,5-diene and its 4-hetero-(aza/oxa/phospha) and 4,8-dihctcro analogues were investigated using density functional theory at the B3LYP/6-31+G~* level in gas phase. The rearrangements of bicyclo[5.1.0]octa-2,5-dicnc and its symmetrical 4,8-dihctcro analogues followed a conccrtcd mcchanism involving synchronous transition states. In other eases, although a conccrtcd mcchanism was observed, asynchronous transition states were involved. In the ease ofbicyclo[5.1.0]octa-2,5-diene, a degenerate Cope rearrangement was expected to occur at room temperature (25°C) due to a low free activation energy ( src='540UA001／IP_2017_70_6_$_01.jpg'>= 14.46 kcal mol~(-1). However, under similar conditions, Ihc rearrangement of 4,8-dioxabicyclo[5.1.0jocta-2,5-dicnc was much slower ( src='540UA001／IP_2017_70_6_$_01.jpg'>=23.85 kcal mol~(-1) ) and the 4,8-diaza- and diphospha analogues did not undergo Cope rearrangement. The Cope rearrangements of 4-phospha-, 8-aza-, 8-aza-4-oxa-, 8-aza-4-phospha-, and 8-oxa-4-phospha-bicyclo[5.1.0]octa-2,5-diencs were exergonic and were cxpcctcd to occur spontaneously to form the corresponding products. In contrast, rearrangement of 8-oxabicyclo[5.1.0] octa-2,5-dicnc, though cxcrgonic, was accompanicd by a decrease in entropy, due to which Cope rearrangement would occur much more slowly and a mixture of both valcncc isomers would be formed. The Cope rearrangements of 4-aza-, 4-oxa-, 4-aza-8-oxa-, 8-phospha-, 4-aza-8-phospha-, 4-oxa-8-phospha-, and 4,8-diphospha-bicyclo[5.1.0]octa-2,5-dicnes were cndcrgonic; consequently either no Cope rearrangement would take place or it would occur sluggishly.