Synthesis of chrysenes and other a-fused phenanthrenes by a palladium(0) coupling—electrocyclic ring closure sequence
2-Bromo-1-vinyl-3,4-dihydronaphthalene (1a) has been coupled with several vinyl-, aryl-, and heteroaryl-zinc halides in the presence of tetrakis(triphenylphosphine)palladium(0) to give the corresponding 2-vinyl-, 2-aryl-, and 2-heteroaryl-naphthalenes in good yield. These compounds, which can be regarded as conjugated trienes, undergo electrocyclic ring closure when heated in solution. In the case of the cyclopentenone derivative (5a) this cyclisation is preceded by a [1,7] hydrogen shift. Some related reactions are described starting from the 6-methoxy derivative (1b) of compound (1a). The coupling reaction has also been carried out with 2-bromo-3,4-dihydronaphthalene-1-carbaldehyde (2a) and with the acetals (2c) and (2d). An alternative method of cyclisation has been explored which involves the bromine–lithium exchange reaction of compound (1a) and of the isomeric bromonaphthalene (3b), followed by reaction with ketones and acid-catalysed dehydration. The polycyclic hydrocarbons (11) and (14) have been prepared by this method.