Abstract:Symmetrieerlaubt ist die [3,5]‐sigmatrope Umlagerung des Esters 1. Die Barriere dieser Umlagerung ist nach Ab‐initio‐Rechnungen bemerkenswerterweise sogar niedriger als die der konkurrierenden [3,3]‐Umlagerung. Dieses überraschende Ergebnis ist die Folge einer ausbleibenden Orbitalüberlappung im pseudopericyclischen Übergangszustand.
“…The [3,5]-sigmatropic shift of a dienyl ester predicted to follow a pseudo-pericyclic mechanism. [15] transition structure. Both B3LYP and BPW91 predict that hS 2 i 1.03 for the diradical intermediate, which indicates that the diradical is nearly 50:50 singlet and triplet, and the singlet and triplet are nearly degenerate.…”
Section: Resultsmentioning
confidence: 99%
“…[14] Birney et al have found that formal [3,5]-sigmatropic shifts of several dienyl esters (Scheme 5) are actually pseudopericyclic processes, not involving a cyclic delocalized system and not subject to the Woodward ± Hoffmann rules. [15] Such processes are facilitated by the involvement of lone pairs in orbitals orthogonal to the p-system. These lone pairs are absent in the all-hydrocarbon process, and a pseudopericyclic process is not possible.…”
The mechanisms of [3s,5s]‐sigmatropic shifts of octa‐1,3,7‐triene and 7‐methylenenona‐1,3,8‐triene have been elaborated using B3LYP and BPW91 density functional theory and CASPT2 methods. These orbital symmetry forbidden rearrangements are stepwise, involving diradical intermediates. A comparison with several [3,3]‐sigmatropic shifts of substituted hexadienes and of [5,5]‐sigmatropic shifts that are allowed, but nevertheless follow stepwise paths, shows that the activation barrier for the disallowed [3,5] shift is significantly larger than that for the stepwise reactions that are orbital symmetry allowed. Cyclic diradicals that have an aromatic circuit of electrons including the two radical centers and conjugated π or σ bonds are stabilized as compared to cyclic diradicals with an antiaromatic circuit of electrons. This applies to the transition states leading to and from the diradicals and influences the activation energies of stepwise sigmatropic shifts. The magnitudes of these effects are small but will have a significant influence on the rates of competing processes. This series of calculations has been used to assess the relative capabilities of the two functionals. We find that BPW91 underestimates the endothermicity of diradical formation and the barrier to diradical formation whereas B3LYP overestimates these quantities.
“…The [3,5]-sigmatropic shift of a dienyl ester predicted to follow a pseudo-pericyclic mechanism. [15] transition structure. Both B3LYP and BPW91 predict that hS 2 i 1.03 for the diradical intermediate, which indicates that the diradical is nearly 50:50 singlet and triplet, and the singlet and triplet are nearly degenerate.…”
Section: Resultsmentioning
confidence: 99%
“…[14] Birney et al have found that formal [3,5]-sigmatropic shifts of several dienyl esters (Scheme 5) are actually pseudopericyclic processes, not involving a cyclic delocalized system and not subject to the Woodward ± Hoffmann rules. [15] Such processes are facilitated by the involvement of lone pairs in orbitals orthogonal to the p-system. These lone pairs are absent in the all-hydrocarbon process, and a pseudopericyclic process is not possible.…”
The mechanisms of [3s,5s]‐sigmatropic shifts of octa‐1,3,7‐triene and 7‐methylenenona‐1,3,8‐triene have been elaborated using B3LYP and BPW91 density functional theory and CASPT2 methods. These orbital symmetry forbidden rearrangements are stepwise, involving diradical intermediates. A comparison with several [3,3]‐sigmatropic shifts of substituted hexadienes and of [5,5]‐sigmatropic shifts that are allowed, but nevertheless follow stepwise paths, shows that the activation barrier for the disallowed [3,5] shift is significantly larger than that for the stepwise reactions that are orbital symmetry allowed. Cyclic diradicals that have an aromatic circuit of electrons including the two radical centers and conjugated π or σ bonds are stabilized as compared to cyclic diradicals with an antiaromatic circuit of electrons. This applies to the transition states leading to and from the diradicals and influences the activation energies of stepwise sigmatropic shifts. The magnitudes of these effects are small but will have a significant influence on the rates of competing processes. This series of calculations has been used to assess the relative capabilities of the two functionals. We find that BPW91 underestimates the endothermicity of diradical formation and the barrier to diradical formation whereas B3LYP overestimates these quantities.
“…First, these transition structures are neither aromatic nor antiaromatic, because they lack a loop or interacting orbitals. This means that no pseudopericyclic reaction can be forbidden, but they are always orbital symmetry allowed. − ,, Second, because these reactions can avoid the electron−electron repulsion inherent in a cyclic overlap of orbitals, they can often have very low or even nonexistent barriers in situations where the geometry, nucleophilicity, and exothermicity are favorable. − ,, …”
Ring openings of 2-furylcarbene (5a), 2-pyrrolylcarbene (5b), 1-cyclopenta-1,3-dienylcarbene (5c), and of 11, as well as of the 3-and five-membered ring vinylogs (7a, 7c, 9a, 9c) were investigated at the B3LYP/6-31G(d,p) level of theory. The reactions of 5a, 5c, and 11 were also studied at the G2(MP2) level. This work provides the first comparison of pseudopericyclic and coarctate orbital topologies in concerted reactions. The ring openings of 5a, 5b, 11 and their vinylogs have pseudopericyclic (but not coarctate) topologies, while the ring openings of 5c and vinylogs are pericyclic and can be described as coarctate. The transition structure geometry for the ring opening of 5c is not well described by MP2 theory. The G2(MP2) barrier for this ring opening is estimated to be 6.4 kcal/mol, significantly higher than that calculated by Sun and Wong. The ring opening is an eight-electron, Mo ¨bius, conrotatory process, not the six-electron, Huckel process described by Herges in his original work describing coarctate orbital topology. The vinylogous reactions of 7c and 9c are disrotatory. The triplet photochemistry of 3a and 3b, as studied by Nakatani et al., was modeled by 6a and 12, respectively. The difference between the two is attributed to the greater stability of the triplet carbene 3 11 as compared to 3 syn-5.
Pericyclisch oder pseudopericyclisch? Obwohl beide Mechanismen zum gleichen Produkt führen, sind sie grundverschieden. Das Ringstrommodell erweist sich als nützliches Hilfsmittel, um verschiedene Arten von Aromatizität zu definieren und um zwischen pericyclischen und pseudopericyclischen Reaktionen (siehe Schema) zu unterscheiden.
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