Chiral cobalt(I) and nickel(0) complexes in the synthesis of nonracemic helicenes through the enantioselective [2+2+2] cyclotrimerisation of alkynes

The enantioselective intramolecular [2 + 2 + 2] cycloisomerisation of triynes under catalysis by chiral transition-metal complexes (Co-I, Ni-0) in order to receive nonracemic helicene derivatives was explored. The use of the chiral neomenthylindene Co-I complex led to a moderate 25% ee of tetrahydro[6] helicene, which was the first example of such a reaction catalysed by the chiral Co-I complex. The alternative Ni-0 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 Ni-0-catalysed reaction. (C) 2012 Elsevier B. V. All rights reserved.