Abstract
The complexes CpCoL2 (Cp = C5H5; L = CO or CH2=CH2) mediate the cycloisomerizations of α,δ,ω-enynenes containing allylic ether linkages
to 3-(oxacyclopentyl or cycloalkyl)furans via the intermediacy of isolable CpCo-η
4-dienes. A suggested mechanism comprises initial complexation of the triple bond and
one of the double bonds, then oxidative coupling to a cobalt-2-cyclopentene, terminal
double bond insertion to assemble a cobalta-4-cycloheptene, β-hydride elimination,
and reductive elimination to furnish a CpCo-η
4-diene. When possible, the cascade continues through cobalt-mediated hydride shifts
and dissociation of the aromatic furan ring. The outcome of a deuterium labeling experiment
supports this hypothesis. The reaction exhibits variable stereoselectivity with a
preference for the trans-product (or, when arrested, its syn-Me CpCo-η
4-diene precursor), but is completely regioselective in cases in which the two alkyne
substituents are differentiated electronically by the presence or absence of an embedded
oxygen. Regioselectivity is also attained by steric discrimination or blocking one
of the two possible β-hydride elimination pathways. When furan formation is obviated
by such regiocontrol, the sequence terminates in a stable CpCo-η
4-diene complex. The conversion of the cyclohexane-fused substrate methylidene-2-[5-(2-propenyloxy)-3-pentynyl]cyclohexane
into mainly 1-[(1R*,3aS*,7aS*)-7a-methyloctahydroinden-1-yl]-1-ethanone demonstrates the potential utility of
the method in complex synthesis.
Key words
enynes - β-hydride elimination - cobalt - cycloisomerization - furans