Treating the camphors with potassium in liquid ammonia leads to a double Horeau duplication

Article abstract of DOI:10.1021/ja00030a044

When potassium dissolves in solutions of the enantiomeric camphors R-1 and S-1, variously enantioenriched camphors (1), and racemic camphor RS-1 in liquid ammonia/THF at -77°C, potassium alcoholates of the borneols R-2 and S-2 and isoborrieols R-3 and S-3 plus equivalent amounts of the potassium enolates of R-1 and S-1 - enantiomeric, enantioenriched, and racemic - are produced by a transfer of a β-hydrogen from some 1-derived unit to another [a ketyl disproportionate (hydrogen atom transfer)?]; the exact mechanism is still unknown. Hydrolysis gives enantiomeric, racemic, and enantioenriched 1-3. The mole fractions and enantiomeric compositions (ec's) of 2 and 3 were determined and plotted against the ec's of the substrates 1. The extremes of the resulting three curves are defined by the enantiomers R-1 and S-1 leading to about 1/1 mixtures of R-2 and R-3 and S-2 and S-3, respectively, and the turning points by RS-1 leading to a 9/1 mixture of RS-2 and RS-3. The ec vs ec curve is close to linear in the case of 2 and strongly nonlinear in the case of 3: from enantioenriched substrates 1, one obtains isoborneols 3 with ec's that are strongly amplified with respect to the ec's of the substrates. Fitting the plots into a statistical kinetic model suggests (1) that 3 is formed via one homochiral process (involving units with the same chirality) and 2 via a combination of second homochiral process with a single heterochiral one (involving units with opposite chirality), (2) that the rate-determining steps in these processes are fourth order with respect to the substrates 1 (!), and (3) that all parallel steps have similar or identical rate constants. The homochiral process that leads to 3 amounts to a double Horeau duplication. Statistical oligomerization or condensation of enantioenriched monomers to short oligomers leads to homochiral oligomers with strongly amplified ec. (+)-Camphor R-1 (ec 99.6%) and (-)-camphor S-1 (ec 98.3%) from the chiral pool were not quite enantiopure.