The Azulene-to-Naphthalene Rearrangement Revisited:  a DFT Study of Intramolecular and Radical-Promoted Mechanisms

Abstract

Intramolecular and radical-promoted mechanisms for the rearrangement of azulene to naphthalene are assessed with the aid of density functional calculations. All intramolecular mechanisms have very high activation energies (>/=350 kJ mol(-1) from azulene) and so can only be competitive at temperatures above 1000 degrees C. Two radical-promoted mechanisms, the methylene walk and spiran pathways, dominate the reaction below this temperature. The activation energy for an orbital symmetry-allowed mechanism via a bicyclobutane intermediate is 382 kJ mol(-1). The norcaradiene-vinylidene mechanism that has been proposed in order to explain the formation of small amounts of 1-phenyl-1-buten-3-ynes from flash thermolysis of azulene has an activation energy of 360 kJ mol(-1); subtle features of the B3LYP/6-31G(d) energy surface for this mechanism are discussed. All intermediates and transition states on the spiran and methylene walk radical-promoted pathways have been located at the B3LYP/6-31G(d) level. Interconversion of all n-H-azulyl radicals via hydrogen shifts was also examined, and hydrogen shifts around the five-membered ring are competitive with the mechanisms leading to rearrangement to naphthalene, but those around the seven-membered ring are not. Conversion of a tricyclic radical to the 9-H-naphthyl radical is the rate-limiting transition state on the spiran pathway, and lies 164.0 kJ mol(-1) above that of the 1-H-azulyl radical. The transition state for the degenerate hydrogen shift between the 9-H-azulyl and 10-H-azulyl radicals is 7.4 kJ mol(-1) lower. Partial equilibration of the intermediates in the spiran pathway via this shift may therefore occur, and this can account for the surprising formation of 1-methylnaphthalene from 2-methylazulene. The rate-limiting transition state for the methylene walk pathway involves the concerted transfer of a methylene group from one ring to the other and lies 182.3 kJ mol(-1) above that of the 1-H-azulyl radical. It is shown that rearrangement via a combination of 31% methylene walk and 69% spiran pathways can account semiquantitatively for all the products from 1-(13)C-azulene, 9-(13)C-azulene, and 4,7-(13)C(2)-azulene, in addition to accounting for the products from methylazulenes, and the formation of naphthalene-d(0) and -d(2) from azulene-4-d. It is also pointed out that a small extension to the spiran pathway could provide an alternative explanation for the formation of 1-phenyl-1-buten-3-ynes.

Azulene‐to‐Naphthalene Rearrangement: The Car–Parrinello Metadynamics Method Explores Various Reaction Mechanisms

“…[6,3,5,20] H-addition, which is commonly assumed to be the radical source for these reactions, readily produces six different H-azulene radicals.…”

“…[20] The initial configuration for the spiran route is the 8aÀH-azulene structure. The selected CV are the transannular bond breaking and the naphthalene transannular bond formation (S C3aC8a and S C3aC8 , respectively).…”

“…[13,[17][18][19][20] MNDO and MINDO/3 calculations [17,18,19] predicted a monomolecular mechanism, in which the initial steps agree with the norcaradiene mechanism, but then the reaction proceeds via either a carbene or a biradical intermediate to naphthalene. AM1 and PM3 methods have been applied to describe the reaction intermediates when the azulene rearrangement is initiated by hydrogen transfer from anthracene.…”

“…AM1 and PM3 methods have been applied to describe the reaction intermediates when the azulene rearrangement is initiated by hydrogen transfer from anthracene. [13] Very recently, a density functional theory (DFT) study has been published, [20] where both the intramolecular and the radical-pro-…”

“…[7] and ref. [20] in that the ring opening and the H-transfer steps take place in a different order. In fact, our predicted mechanism does not feature the biradical intermediate, since the hydrogen transfer prevents its formation, and allows instead the formation of the diallene structure.…”