A Strain Induced Change of Mechanism from a [2 + 2 + 2] to a [2 + 1 + 2 + 1] Cycloaddition Reaction

While investigating
the [2 + 2 + 2] cycloaddition as a tool to
build up strained oligophenyl systems with a diyne-ethylene glycol
macrocyle, a surprising change of mechanism was observed. Instead
of the expected [2 + 2 + 2] para-terphenyl, the ortho-terphenyl product explained by a formal [2 + 1 + 2
+ 1] cycloaddition was formed. An η4-coordinated
metal-cyclobutadiene is proposed as the key structure in the catalytic
cycle, which is formed to release the induced strain. The optical
properties of the ortho-terphenyl products have been
measured as well as the coordination ability of Na+ and
K+.