A Chiral Titanocene Complex as Regiodivergent Photoredox Catalyst: Synthetic Scope and Mechanism of Catalyst Generation

We describe a combined synthetic, spectroscopic, and computational study of a chiral titanocene complex as a regiodivergent photoredox catalyst (PRC). With Kagan's complex catCl₂ either monoprotected 1,3-diols or 1,4-diols can be obtained in high selectivity from a common epoxide substrate in a regiodivergent epoxide opening depending on which enantiomer of the catalyst is employed. Due to the catalyst-controlled regioselectivity of ring opening and the broader substrate scope, the PRC with catCl₂ is also a highly attractive branching point for diversity-oriented synthesis. The photochemical processes of cat(NCS)₂, a suitable model for catCl₂, were probed by time-correlated single-photon counting. The photoexcited complex displays a thermally activated delayed fluorescence as a result of a singlet-triplet equilibration, S₁ ⇄ T₁, via intersystem crossing and recrossing. Its triplet state is quenched by electron transfer to the T1 state. Computational and cyclic voltammetry studies highlight the importance of our sulfonamide additive. By bonding to sulfonamide additives, chloride abstraction from [catCl₂]^- is facilitated, and catalyst deactivation by coordination of the sulfonamide group is circumvented.