Cobaltocene-Induced Low-Temperature Radical Coupling Reactions in a Cobalt–Alkyne Series

A novel method for the low-temperature generation of
Co2(CO)6-complexed propargyl radicals is developed.
It consists
of an in situ preparation of the respective cationic species (−50
to −10 °C) and their rapid reduction with cobaltocene,
Cp2Co, at −50 °C. The optimized experimental
protocol is applied to both inter- and intramolecular reactions, affording
topologically diverse α-aryl and α-napthyl, d,l- and meso-1,5-hexadiynes and
1,5-cyclodecadiynes. The d,l configuration
is the most preferable steric arrangement in intermolecular radical
C–C bond-forming reactions (d,l 69–92%), while a reversal of stereoselectivity is observed
in intramolecular cyclizations (meso 79%). Under
oxidizing conditions (Ce4+), decomplexation affords d,l-3,4-diaryl- and d,l-3,4-(1-/2-naphthyl)-1,5-hexadiynes in good to excellent
yields (47–98%). An enhanced functional tolerance is showcased by introducing peripheral acid-sensitive functionalities,
such as benzyloxy and methylenedioxy groups, and carrying out a five-step
conversion schemefrom commercial aromatic aldehydes to radical
dimersunder nonacidic conditions.