Novel aspects of epimetalation in organic synthesis A. Homo- and hetero- coupling of pi-bonded organic substrates via epititanation. B. Ring contraction of diazocines by epimetalation with alkali metal reductants and ring re-expansion by SET oxidants

Titanium(II) chloride bis(tetrahydrofuran), first developed in the Eisch group, was further shown to participate in pinacol coupling reactions through an epititanation precursor. The diastereoselectivity (racemic vs. meso) in the homo-coupling products of benzaldehyde has been evaluated under varied conditions, which selectivity was reversed in the case of titanium(II) isopropoxide. Further studies focused on homo-coupling of aliphatic carbonyl compounds and hetero-coupling of two different carbonyl compounds. The formation of benzilic acid from benzophenone and CO2 gives substantive evidence for the epititanation intermediate. 6,11-Diphenyldibenzo[b,f][1,4]diazocine was found to be an intermediate on the way to 2-(2-aminophenyl)-1,3-diphenyl- isoindole, through heating with o-diaminobenzene at 200°C. Also the redox chemistry existing between this [1,4]diazocine and the isoindole was shown by using titanium(II) reagents as reductant and O2 as the oxidant. Furthermore, groups 1 and 2 organometallics were evaluated as potential electron-transfer agents, where SET processes mostly likely occur.