“…Here, the low-lying vacant orbital of the formyl group provides a stabilizing interaction with the filled orbital of the breaking bond upon inward rotation. Calculations at numerous levels of theory have now been performed for about 20 substituted systems, and experiments have shown that they provide excellent predictions of both reactivity and stereoselectivity 5 Selected experimental results predicted by theoretical calculations. …”
Section: The Electrocyclic Ring Opening Of Cyclobutenementioning
The computational study of pericyclic reactions, an important
general class of organic reactions, now provides
information about the transition structures of these processes with
chemical accuracy, as judged by comparisons
with experimental data, such as activation energies, substituent
effects on rates, and kinetic isotope effects.
This article introduces the methods used to study these reactions
and describes how computational results
have contributed to the understanding of transition states and
mechanisms of the electrocyclic ring openings
of cyclobutenes, Diels−Alder cycloaddition reactions, and
[3,3]-sigmatropic shifts such as the Cope
rearrangement.
“…Here, the low-lying vacant orbital of the formyl group provides a stabilizing interaction with the filled orbital of the breaking bond upon inward rotation. Calculations at numerous levels of theory have now been performed for about 20 substituted systems, and experiments have shown that they provide excellent predictions of both reactivity and stereoselectivity 5 Selected experimental results predicted by theoretical calculations. …”
Section: The Electrocyclic Ring Opening Of Cyclobutenementioning
The computational study of pericyclic reactions, an important
general class of organic reactions, now provides
information about the transition structures of these processes with
chemical accuracy, as judged by comparisons
with experimental data, such as activation energies, substituent
effects on rates, and kinetic isotope effects.
This article introduces the methods used to study these reactions
and describes how computational results
have contributed to the understanding of transition states and
mechanisms of the electrocyclic ring openings
of cyclobutenes, Diels−Alder cycloaddition reactions, and
[3,3]-sigmatropic shifts such as the Cope
rearrangement.
“…On the basis of theoretical calculation, de Lera et al suggested that introduction of a methyl group at the vinylic terminus of a vinylallene renders the ring-closed methylenecyclobutene the thermodynamically favored form 4b. We have described the remarkable effects that silyl substituents have on the electrocyclization of vinylallenes. , A silyl substituent at the vinylic terminus stabilizes the ring-closed product and, in particular, lowers the temperature required for the ring closure to 110 °C.…”
mentioning
confidence: 87%
“…Cyclobutene undergoes a thermal ring-opening reaction in gas phase at temperatures ranging from 130 to 175 °C, producing 1,3-butadiene, which is thermodynamically more stable than cyclobutene . On the other hand, 1,2,4-pentatriene(vinylallene) and methylenecyclobutene possess comparable thermodynamic stabilities such that thermal treatment produces an equilibrium mixture of them (eq 1). , …”
The unidirectional thermal ring-closing reaction of cis-4-phenyl-5-borylpenta-1,2,4-triene giving 4-boryl-3-methylene-1-phenylcyclobutene proceeded significantly faster than that of the trans-isomer. The large rate difference between the cis- and trans-stereoisomers is ascribed to electronic participation of the vacant boron p orbital in the transition state SHOMO.
“…They also indicated that the reaction of vinylallene is the same ring-closure process as that of bis(allene). For the cyclic reaction of vinylallene, the substituted effects on vinyl group have been studied experimentally − and theoretically . However, the role of two types π orbitals (vertical π and side π‘ orbitals) of the allene group for the cyclization of bis(allene) and/or vinylallene is not known well.…”
The reaction mechanisms of the electrocyclic ring closure of bis(allene) and vinylallene were studied by ab initio MO methods. The conrotatory and disrotatory pathways of the electrocyclic reactions from bis(allene) to bis(methylene)cyclobutene were determined by a CASSCF method. The transition state on the conrotatory pathway is 26.8 kcal/mol above bis(allene) and about 23 kcal/mol lower than that on the disrotatory pathway at a MRMP calculation level. The activation energy on the conrotatory pathway is lower by 23 kcal/mol than that of the electrocyclic reaction of butadiene. This lower energy barrier comes from the interactions of the "side pi orbitals" of the allene group. The interaction of the "vertical pi orbitals" of the allene group is predominant at the early stage of the reaction. The activation energy of the electrocyclic reaction of vinylallene is about 8.5 kcal/mol higher than that on the conrotatory pathway of bis(allene).
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