THE ADDITION OF BENZOCYCLOBUTENYLIDENE TO BENZENE. THE FACILE REARRANGEMENT OF SPIRO(BENZOCYCLOBUTENE-1,7'-CYCLOHEPTA-1',3',5'-TRIENE) TO 9a,10-DIHYDROBENZ(a)AZULENE

Article abstract of DOI:10.1016/S0040-4020(01)97702-6

Thermal decomposition of the sodium salts of benzocyclobutenone tosylhydrazone and 2-methylbenzocyclobutenone tosylhydrazone in benzene affords 9a,10-dihydrobenz(a)azulene 4 and trans-10-methyl-9a,10-dihydrobenz(a)azulene 3, respectively.A mechanism involving initially the addition of the carbene benzocyclobutenylidene, or its 2-Me derivative, to the benzene ring is postulated.A proposed intermediate in the reaction, spiro(benzocyclobutene-1,7'-cyclohepta-1',3',5'-triene) 12 has been synthesised, and shown to give rise to 4 under the reaction conditions.The rate of rearrangement of 12 -> 4 has been measured, and the activation energy determined: Ea = 125.9 +/- 0.8 kJmol^-1 and A = 1.38*10¹⁴ sec^-1.The mechanism for the rearrangement must involve ring opening of the benzocyclobutene moiety of 12 to give an o-xylylene intermediate which is postulated to possess considerable diradical character.At 71.8 deg C this ring opening is 2.7*10⁶ times faster than the ring opening of the parent benzocyclobutene molecule.The decomposition of the sodium salt of 2-(7'-cyclohepta-1',3',5'-trienyl)benzaldehyde tosylhydrazone has also been investigated, and is shown to yield 4a,10-dihydrobenz(a)azulene, 9,10-dihydrobenz(a)azulene and 8,9-benzotricyclo(5.3.0.0^2.10)deca-3,5,8-triene.A mechanism involving intramolecular 1,3-dipolar addition of a diazo grouping to a cycloheptatriene Π-bond, followed by decomposition of the resulting pyrazoline intermediate, is proposed.