C–C Cross-Coupling Reactions of Trityl Radicals: Spin Density Delocalization, Exchange Coupling, and a Spin Label

Organic radicals
are usually highly reactive and short-lived species.
In contrast, tetrathiatriarylmethyl radicals, the so-called trityl-
or TAM-radicals, are stable and do survive over longer times even
under in-cell conditions. In addition, they show strong EPR signals,
have long phase memory times at room temperature, and are reporters
on local oxygen and proton concentrations. These properties facilitated
their use for magnetic resonance imaging, dynamic nuclear polarization,
and spin-labeling EPR under in-cell conditions. Thus, synthetic approaches
are required for functionalization of TAM radicals tailored to the
desired application. However, most TAM derivatives reported in the
literature are based on esterification of the Finland trityl, which
is prone to hydrolysis. Here, we report on an approach in which TAM
is site-selective iodinated and subsequently C–C cross-coupled
to various building blocks in a modular approach. This yields conjugated
trityl compounds such as a trityl attached to a porphyrin, an alkinyl
functionalized trityl radical, and a strongly exchange-coupled trityl
biradical. This synthesis approach thus has implications not only
for magnetic resonance spectroscopy but also for the design of molecular
magnets or quantum computing devices.