# Visible-Light-Induced Bond Homolysis in Titanacyclopentadienes for the Catalytic Cyclodimerization of Internal Alkynes

## TL;DR

Scientists used visible light to break titanium-carbon bonds in a specific complex, enabling a new way to create cyclobutene rings from alkynes.

## Contribution

A novel catalytic cyclodimerization of internal alkynes via visible-light-induced Ti–C bond homolysis is demonstrated.

## Key findings

- Visible-light-induced Ti–C bond homolysis in titanacyclopentadienes leads to selective skeletal rearrangement.
- A strained cyclobutene dimer was obtained quantitatively from 2-butyne using the new catalytic method.
- Mechanistic studies confirm Ti–C bond homolysis is the rate-determining step in the reaction.

## Abstract

Ligand-to-metal charge
transfer (LMCT) processes offer significant
potential in photochemical synthesis but remain comparatively underdeveloped,
relative to metal-to-ligand charge transfer (MLCT) pathways. Titanium
organyls are particularly promising in this context, enabling the
direct generation of carbon-centered radicals. However, reports on
their photochemistry have remained scarce. Herein, we present the
visible-light-induced homolytic cleavage of a Ti–C bond in
a titanacyclopentadiene complex supported by a bulky pyridine-diamido
ligand. The resulting biradicaloid undergoes a selective skeletal
rearrangement via a H atom shift to form a titanacyclopentene. This
stoichiometric transformation was translated into a novel catalytic
cyclodimerization of internal alkynes. With 2-butyne, the unusual
dimer 1,2,3-trimethyl-4-methylenecyclobutene, a strained and readily
functionalizable small ring, was obtained quantitatively. Mechanistic
investigations of this previously unreported catalytic transformation
including quantum chemical calculations, single-turnover experiments,
quantum yield determination by actinometry, and reaction kinetics
reveal key features of the underlying reactivity and indicate that
visible-light-induced Ti–C bond homolysis is the rate-determining
step. Overall, our findings demonstrate that the combination of a
sterically demanding ligand, which suppresses alkyne cyclotrimerization,
with visible-light-induced bond homolysis offers a new entry point
into catalytic alkyne oligomerization chemistry.