Thermal Rearrangements of Alienes. Synthesis and Mechanism of Cycloaromatization of π and Heteroatom Bridged Diallenes

The synthesis and thermal rearrangement of several π and heteroatom bridged diallenes has been investigated. o-Diallenylbenzenes 5, 14, 24 and bis(Γ,Γ-dimethylallenyl) ether 8 were prepared by addition of dibromocarbene to the corresponding divinyl precursor, followed by treatment of the resulting dibromocyclopropane derivative with methyllithium. Bis(Γ,Γ-dimethylallenyl) sulfide 10 was generated by reaction of (Γ,Γ-dimethylallenyl)lithium with sulfur dichloride, while the corresponding selenides 12 and 16 were synthesized by an S_N2′ reaction of sodium selenide with α,α-dimethylpropargyl bromide. All diallenes prepared display a remarkable thermal reactivity and undergo a facile cycloaromatization. Gentle heating of diallenes 5,14,12, and 16 gave the naphthalene derivatives 6 and 15 and selenophene derivatives 13 and 17, respectively, in practically quantitative yields. Diallenes 8,10, and 24 underwent spontaneous cyclization during preparation yielding furan 9, thiophene 11, and naphtho[b]cyclobutane 25, respectively. A kinetic study of the rearrangement and measurement of the kinetic isotope effect using diallenes 5, 14, 12, and 16, revealed that the cyclization is a two-step process. The first and rate-determining step is an electrocyclic reaction yielding as intermediates ω-xylylenes 20 and 21, while the second and more rapid step entails a [1,5]hydrogen transfer.