Photochemically Switching Diamidocarbene Spin States Leads to Reversible Büchner Ring Expansions

The discovery of
thermal and photochemical control by Woodward
and Hoffmann revolutionized how we understand chemical reactivity.
Similarly, we now describe the first example of a carbene that exhibits
differing thermal and photochemical reactivity. When a singlet ground-state N,N′-diamidocarbene 1 was photolyzed
at 380 nm, excitation to a triplet state was observed. The triplet-state
electronic structure was characteristic of the expected biradical
σ1pπ1 spin configuration
according to a combination of spectroscopic and computational methods.
Surprisingly, the triplet state of 1 was found to engage
a series of arenes in thermally reversible Büchner ring expansion
reactions, marking the first examples where both cyclopropanation
and ring expansion of arenes were rendered reversible. Not only are
these photochemical reactions different from the known thermal chemistry
of 1, but the reversibility enabled us to perform the
first examples of photochemically induced arene exchange/expansion
reactions at a single carbon center.