Chiral cobaltI and nickel0 complexes in the synthesis of nonracemic helicenes through the enantioselective [2 + 2 + 2] cyclotrimerisation of alkynes

Abstract

The enantioselective intramolecular [2 + 2 + 2] cycloisomerisation of triynes under catalysis by chiral transition-metal complexes (CoI, Ni0) in order to receive nonracemic helicene derivatives was explored. The use of the chiral neomenthylindene CoI complex led to a moderate 25% ee of tetrahydro[6]helicene, which was the first example of such a reaction catalysed by the chiral CoI complex. The alternative Ni0 catalysis employing privileged axially chiral monophosphines such as (−)-(aS)-NAPHEP led to tetrahydro[6]helicene with 64% ee, which is among the highest enantiomeric excesses so far observed for this Ni0-catalysed reaction.