Ag-Catalyzed Oxidative Cyclization Reaction of 1,6-Enynes and Sodium Sulfinate: Access to Sulfonylated Benzofurans.

Aromaticity, one of the most fundamental concepts in chemistry, has attracted considerable attention from both theoreticians and experimentalists. Much effort on aromaticity in organometallics has been devoted to metallabenzene and derivatives. In comparison, aromaticity in other organometallics is less developed. This Account describes how our group has performed quantum chemical calculations to examine aromaticity in recently synthesized novel organometallic complexes. By collaborations with experimentalists, we have extended several aromaticity concepts into organometallics to highlight the power of transition metals. In general, the transition metal could participate in delocalization either out of rings or in the rings. We examined the former by probing the possibility of transition metal substituents in hyperconjugative aromaticity, where the metal is out of the rings. Calculations on tetraaurated heteroaryl complexes reveal that incorporation of the aurated substituents at the nitrogen atom can convert nonaromaticity in the parent indolium into aromaticity in the aurated one due to hyperconjugation, thus extending the concept of hyperconjugative aromaticity to heterocycles with transition metal substituents. More importantly, further analysis indicates that the aurated substituents can perform better than traditional main-group substituents. Recently, we also probed the strongest aromatic cyclopentadiene and pyrrolium rings by hyperconjugation of transition metal substituents. Moreover, theoretical calculations suggest that one electropositive substituent is able to induce aromaticity; whereas one electronegative substituent prompts nonaromaticity rather than antiaromaticity. We also probed the possibility of Craig-type Mobius aromaticity in organometallic chemistry, where the position of the transition metals is in the rings. According to the electron count and topology, aromaticity can be classified as Huckel-type and Mobius-type. In comparison with numerous Huckel aromatics containing 4 n+2 π-electrons, Mobius aromatics with 4 n π-electrons, especially the Craig-type species, are particularly limited. We first examined aromaticity in osmapentalynes. Theoretical calculations reveal that incorporation of the osmium center not only reduces the ring strain of the parent pentalyne, but also converts Huckel antiaromaticity in the parent pentalyne into Craig-type Mobius aromaticity in metallapentalynes. Further studies show that the transition metal fragments can also make both 16e and 18e osmapentalenes aromatic, indicating that the Craig-type Mobius aromaticity in osmapentalyne is rooted in osmapentalenes. In addition, Mobius aromaticity is also possible in dimetalla[10]annulenes, where the lithium atoms are not spectator cations but play an important role due to their bonding interaction with the diene moieties. We then examined the possibility of σ-aromaticity in an unsaturated ring. Traditional π-aromaticity is used to describe the π-conjugation in fully unsaturated rings; whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σ-electron conjugation. We found that the unsaturated three-membered ring in cyclopropaosmapentalene is σ-aromatic. Very recently, we extended σ-aromaticity into in a fully unsaturated ring. The concepts and examples presented here show the importance of interplay and union between experiment and theory in developing novel aromatic systems and, especially, the indispensable role of computational study in rationalization of unconventional aromaticity. All these findings highlight the strong power of transition metals originating from participation of d orbitals in aromaticity, opening an avenue to the design of unique metalla-aromatics.

Unconventional Aromaticity in Organometallics: The Power of Transition Metals.

One-pot access to sulfonylmethyl arylpyrroles via the domino aerobic Wacker-type aminocyclization/1,4-sulfonyl migration.

Synthesis of sulfonamide and their synthetic and therapeutic applications: Recent advances

A simple and convenient molecular iodine-catalyzed direct sulfonylation of pyrazolones with sodium sulfinates has been developed in the presence of TBHP at room temperature. This methodology can allow for the synthesis of a series of valuable sulfonated pyrazoles in good to excellent yields simply using readily-available starting materials without requiring any metal or cryogenics.

Metal-free molecular iodine-catalyzed direct sulfonylation of pyrazolones with sodium sulfinates leading to sulfonated pyrazoles at room temperature

Aromaticity, one of the most important concepts in organic chemistry, has attracted considerable interest from both experimentalists and theoreticians. It remains unclear which NICS index is best to evaluate the triplet-state aromaticity. Here, we carry out thorough density functional theory (DFT) calculations to examine this issue. Our results indicate that among the various computationally available NICS indices, NICS(1)zz is the best for the triplet state. The correlations can be improved from 0.840 to 0.938 when only neutral species are considered, demonstrating the significant effect of the charge on the triplet-state aromaticity. In addition, calculations suggest that five-membered cyclic species with "hyperconjugative" aromaticity (and antiaromaticity) in the S0 state will become antiaromatic (and aromatic) in the T1 state, indicating an important role of hyperconjugation. Finally, a moderate correlation (r(2) =0.708) is identified between the NICS(1)zz values and spin distributions.

Triplet State Aromaticity: NICS Criterion, Hyperconjugation, and Charge Effects.

Lewis base catalyzed synthesis of multisubstituted 4-sulfonyl-1H-pyrazole involving a novel 1,3-sulfonyl shift.

An efficient Cs2CO3-promoted synthesis of α-amino ketones using hydrazines, aldehydes, and α-haloketones as starting materials through a cascade condensation/nucleophilic substitution/N-N bond cleavage route is developed. The carbonyl group plays a key role in this novel N-N bond cleavage process.

Base-Catalyzed N ? N Bond Cleavage of Hydrazones: Synthesis of α-Amino Ketones

A simple one-pot procedure allows the conversion of N-propargylic sulfonylhydrazones into the title pyrazole derivatives via allenic sulfonamide formation and 1,3-sulfonyl shift followed by an intramolecular Michael addition.

Lewis Base Catalyzed Synthesis of Multisubstituted 4‐Sulfonyl‐1H‐pyrazole Involving a Novel 1,3‐Sulfonyl Shift.

α-Amino ketones are synthesized starting from aldehydes, hydrazines, and α-haloketones via a novel, efficient, Cs2CO3-promoted cascade condensation/nucleophilic substitution/ N-N bond cleavage sequence in one-pot.

Hong-Chao Sun

Papers

Triplet State Aromaticity: NICS Criterion, Hyperconjugation, and Charge Effects.

Lewis base catalyzed synthesis of multisubstituted 4-sulfonyl-1H-pyrazole involving a novel 1,3-sulfonyl shift.

Base-Catalyzed N ? N Bond Cleavage of Hydrazones: Synthesis of α-Amino Ketones

Lewis Base Catalyzed Synthesis of Multisubstituted 4‐Sulfonyl‐1H‐pyrazole Involving a Novel 1,3‐Sulfonyl Shift.

Base-Catalyzed N—N Bond Cleavage of Hydrazones: Synthesis of α-Amino Ketones.