Spontaneous resolution versus formation of racemic crystals of indenylpotassium complexes

• Concentration-dependent crystallisation of three phases of [K(ind)(pmdta)]n.
• Two of the phases undergo spontaneous resolution to form chiral crystals.
• Concomitant crystallisation of racemic and chiral crystals occurs.
• Chiral crystals react with N-chlorosuccinimide to give 1-chloroindene.

During a study of stereochemically labile organometallic reagents, we found that crystallisation of [K(ind)(pmdta)]n (ind = indenyl, pmdta = N,N,N',N'',N''-pentamethyldiethylenetriamine) from tetrahydrofuran (THF) gave rise to three different crystalline phases depending on the concentration. The more concentrated solutions gave racemic crystals of the previously known coordination polymer [K(ind)(pmdta)]n, α-1. More dilute solutions gave crystals of a new phase, β-1, which was found to undergo spontaneous resolution. From even more dilute solutions, we were able to isolate a chiral inclusion compound with THF, 2, which also undergoes spontaneous resolution. Crystals of β-1 and 2 were enantiomerically pure, and since β-1 is a reactive organometallic reagent, attempts were made to use the crystals in enantioselective synthesis. Reacting single crystals with N-chlorosuccinimide gave rise to chiral 1-chloroindene, but no enantiomeric excess was observed.

Crystallisation of [K(ind)(pmdta)]n (ind = indenyl, pmdta = N,N,N',N'',N''-pentamethyldiethylenetriamine) from tetrahydrofuran was found to give rise to three different crystalline phases, two of which underwent spontaneous resolution and formed chiral crystals.Figure optionsDownload as PowerPoint slide