Benzene exhibits a rich photochemistry
which can provide access
to complex molecular scaffolds that are difficult to access with reactions
in the electronic ground state. While benzene is aromatic in its ground
state, it is antiaromatic in its lowest ππ* excited
states. Herein, we clarify to what extent relief of excited-state
antiaromaticity (ESAA) triggers a fundamental benzene photoreaction:
the photoinitiated nucleophilic addition of solvent to benzene in
acidic media leading to substituted bicyclo[3.1.0]hex-2-enes. The
reaction scope was probed experimentally, and it was found that silyl-substituted
benzenes provide the most rapid access to bicyclo[3.1.0]hexene derivatives,
formed as single isomers with three stereogenic centers in yields
up to 75% in one step. Two major mechanism hypotheses, both involving
ESAA relief, were explored through quantum chemical calculations and
experiments. The first mechanism involves protonation of excited-state
benzene and subsequent rearrangement to bicyclo[3.1.0]hexenium cation,
trapped by a nucleophile, while the second involves photorearrangement
of benzene to benzvalene followed by protonation and nucleophilic
addition. Our studies reveal that the second mechanism is operative.
We also clarify that similar ESAA relief leads to puckering of S
1
-state silabenzene and pyridinium ion, where the photorearrangement
of the latter is of established synthetic utility. Finally, we identified
causes for the limitations of the reaction, information that should
be valuable in explorations of similar photoreactions. Taken together,
we reveal how the ESAA in benzene and 6π-electron heterocycles
trigger photochemical distortions that provide access to complex three-dimensional
molecular scaffolds from simple reactants.
The increasing energy demand calls for the development of sustainable energy conversion processes. Here, the splitting of H2O to O2 and H2, or related fuels, constitutes an excellent example of solar-to-fuel conversion schemes. The critical component in such schemes has proven to be the catalyst responsible for mediating the four-electron oxidation of H2O to O2. Herein, we report on the unexpected formation of a single-site Ru complex from a ligand envisioned to accommodate two metal centers. Surprising N-N bond cleavage of the designed dinuclear ligand during metal complexation resulted in a single-site Ru complex carrying a carboxylate-amide motif. This ligand lowered the redox potential of the Ru complex sufficiently to permit H2O oxidation to be carried out by the mild one-electron oxidant [Ru(bpy)3](3+) (bpy = 2,2'-bipyridine). The work thus highlights that strongly electron-donating ligands are important elements in the design of novel, efficient H2O oxidation catalysts.
Several chiral sulfonyl compounds were prepared using the iridium catalyzed asymmetric hydrogenation reaction. Vinylic, allylic and homoallylic sulfone substitutions were investigated, and high enantioselectivity is maintained regardless of the location of the olefin with respect to the sulfone. Impressive stereoselectivity was obtained for dialkyl substitutions, which typically are challenging substrates in the hydrogenation. As expected, the more bulky Z-substrates were hydrogenated slower than the corresponding E isomers, and in slightly lower enantioselectivity.
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