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A Pd/C catalyzed redox neutral C¢O bond cleavage of 2-aryloxy-1-arylethanols has been developed. The reactions are carried out at 80 °C, in air, using a green solvent system to yield the aryl ketones in near quantitative yields. Addition of catalytic amounts of a hydrogen source to the reaction mixture activates the catalyst to proceed through a low energy barrier pathway. Initial studies support a transfer hydrogenolysis reaction mechanism that proceeds through an initial dehydrogenation followed by an enol adsorption to Pd/C and a reductive C¢O bond cleavage.

A new qualitative model for estimating the properties of substituted cyclopentadienes and siloles in their lowest ππ* excited states is introduced and confirmed through quantum chemical calculations, and then applied to explain earlier reported experimental excitation energies. According to our model, which is based on excited-state aromaticity and antiaromaticity, siloles and cyclopentadienes are cross-hyperconjugated "aromatic chameleons" that adapt their electronic structures to conform to the various aromaticity rules in different electronic states (Hückel's rule in the π(2) electronic ground state (S0) and Baird's rule in the lowest ππ* excited singlet and triplet states (S1 and T1)). By using pen-and-paper arguments, one can explain polarity changes upon excitation of substituted cyclopentadienes and siloles, and one can tune their lowest excitation energies by combined considerations of ground- and excited-state aromaticity/antiaromaticity effects. Finally, the "aromatic chameleon" model can be extended to other monocyclic compound classes of potential use in organic electronics, thereby providing a unified view of the S0, T1, and S1 states of a range of different cyclic cross-π-conjugated and cross-hyperconjugated compound classes.

The hydroxyl group of enantioenriched benzyl, propargyl, allyl, and alkyl alcohols has been intramolecularly displaced by uncharged O-, N-, and S-centered nucleophiles to yield enantioenriched tetrahydrofuran, pyrrolidine, and tetrahydrothiophene derivatives with phosphinic acid catalysis. The five-membered heterocyclic products are generated in good to excellent yields, with high degree of chirality transfer, and water as the only side-product. Racemization experiments show that phosphinic acid does not promote SN1 reactivity. Density functional theory calculations corroborate a reaction pathway where the phosphinic acid operates as a bifunctional catalyst in the intramolecular substitution reaction. In this mechanism, the acidic proton of the phosphinic acid protonates the hydroxyl group, enhancing the leaving group ability. Simultaneously, the oxo group of phosphinic acid operates as a base abstracting the nucleophilic proton and thus enhancing the nucleophilicity. This reaction will open up new atom efficient techniques that enable alcohols to be used as nucleofuges in substitution reactions in the future.

Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed-shell singlet state (normally ground state, S 0 )a nd lowest pp*t riplet state (T 1 or T 0 ), as given by Hückel's and Baird's rules, respectively,fulvenes are influenced by their substituents in the opposite manner in the T 1 and S 0 states. This effect is causedb yareversal in the dipole moment when going from S 0 to T 1 as fulvenes adapt to the differencei ne lectron counts for aromaticity in variouss tates;t hey are aromaticc hameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S 0 reduces (enhances) aromaticity in T 1 ,a llowing for rationalizationso ft he triplet state energies (E T )o fs ubstitut-ed fulvenes. Through quantum chemical calculations, we now assessw hich substituents and which positions on the pentafulvene core are the most powerful for designing compounds with low or inverted E T .A sam eans to increase the p-electron withdrawing capacity of cyano groups, we found that protonation at the cyano Na toms of 6,6-dicyanopentafulvenes can be ar oute to on-demandf ormation of af ulvenium dication with at riplet ground state (T 0 ). The five-membered ring of this speciesi sm arkedly Baird-aromatic, althoughl ess than the cyclopentadienyl cation knownt oh ave aB aird-aromatic T 0 state. [a] Dr.