On the Organocatalytic Activity of N-Heterocyclic Carbenes: Role of Sulfur in Thiamine

The reaction energy profiles of the benzoin condensation from three aldehydes catalyzed by imidazol-2-ylidene, triazol-3-ylidene, and thiazol-2-ylidene have been investigated computationally. The barriers for all steps of all investigated reactions have been found to be low enough to indicate the viability of the mechanism proposed by Breslow in the 1950s. The most remarkable difference in the catalytic cycles has been the increased stability of the Breslow intermediate in case of thiazol-2-ylidene (by ca. 10 kcal/mol) compared to the other two carbenes, which results in lower energy for the coupling of the second aldehyde molecule, thus, increasing the reversibility of the reaction. Since the analogous transketolase reaction, being involved in the carbohydrate metabolism of many organisms, requires an initial decouplinga reverse benzoin condensationthis difference provides a reasonable explanation for the presence of a thiazolium ring in thiamine instead of the otherwise generally more available imidazole derivatives. The “resting intermediate” found by Berkessel and co-workers for a triazole-based catalyst was found more stable than the Breslow intermediate for all of the systems investigated. The (gas phase) proton affinities of several carbenes were compared, the relative trends being in agreement with the available (in aqueous solution) data. The hydrolytic ring-opening reaction of the thiazole-based carbene was shown to be different from that of imidazole-2-ylidenes

π‑Rich σ²P‑Heterocycles: Bent η¹‑P- and μ²‑P-Coordinated 1,3-Benzazaphosphole Copper(I) Halide Complexes

The reaction of 1-neopentyl-1,3-benzazaphosphole <b>1</b> with CuCl, CuBr, or Cu­(SMe₂)Br in THF at room temperature provides sparingly soluble [Cu₇(μ²-L₆)­(μ²-X₇)]⁺[CuX₂]⁻ cluster complexes <b>2a</b>,<b>b</b> (L indicates coordinated <b>1</b>, <b>a</b> X = Cl, <b>b</b> X = Br), with loosely bound THF, in high yields. The conversions proceed via transient THF-soluble labile [(L₂CuX)₂] complexes. Separation before complete conversion, combined with suitable conditions for crystallization, allowed these intermediates to be trapped. Depending on the reactant ratios, crystals of the clusters or of dimeric L₂CuX complexes were formed. The crystal structure analyses of <b>2a</b>·4THF and the dimers <b>3b</b> [{Cu­(η¹-L)₂­(μ²-Br)}₂], <b>4b</b> [{Cu­(μ²-L)­(η¹-L)­(κBr)}₂], <b>5a</b>·2MeOH, and <b>5b</b>·2MeOH [{Cu­(μ²-L)­(η¹-L)­(κX···HOMe)}₂] generally display μ²-P- and/or tilted η¹-P-coordination, contrasting with the preference for the η¹-P in-plane coordination mode of phosphinine ligands in their copper­(I) halide complexes. DFT studies of geometry-optimized monomers LCuBr, L­(CuBr)₂, L₂CuBr, and the dimers <b>3b</b> and <b>4b</b>, calculated at the ωB97xD/cc-PVDZ level, suggest that weak competing interactions with the solvent THF and the entropy factor of the dimerization result in lability and a subtle balance between the different complexes in solution, whereas the particular coordination observed in the crystals is attributable to conservation of the delocalized π-system in the ligand. The HOMO of <b>4b</b> is composed of Cu d orbitals and the π-type HOMO of the bridging ligand. Interestingly, despite the rather short Cu···Cu interatomic separation (2.726 Ǻ), no bond critical point could be located in <b>4b</b>, indicating the absence of weak cuprophilic interactions in this compound

π‑Rich σ²P‑Heterocycles: Bent η¹‑P- and μ²‑P-Coordinated 1,3-Benzazaphosphole Copper(I) Halide Complexes

The reaction of 1-neopentyl-1,3-benzazaphosphole <b>1</b> with CuCl, CuBr, or Cu­(SMe₂)Br in THF at room temperature provides sparingly soluble [Cu₇(μ²-L₆)­(μ²-X₇)]⁺[CuX₂]⁻ cluster complexes <b>2a</b>,<b>b</b> (L indicates coordinated <b>1</b>, <b>a</b> X = Cl, <b>b</b> X = Br), with loosely bound THF, in high yields. The conversions proceed via transient THF-soluble labile [(L₂CuX)₂] complexes. Separation before complete conversion, combined with suitable conditions for crystallization, allowed these intermediates to be trapped. Depending on the reactant ratios, crystals of the clusters or of dimeric L₂CuX complexes were formed. The crystal structure analyses of <b>2a</b>·4THF and the dimers <b>3b</b> [{Cu­(η¹-L)₂­(μ²-Br)}₂], <b>4b</b> [{Cu­(μ²-L)­(η¹-L)­(κBr)}₂], <b>5a</b>·2MeOH, and <b>5b</b>·2MeOH [{Cu­(μ²-L)­(η¹-L)­(κX···HOMe)}₂] generally display μ²-P- and/or tilted η¹-P-coordination, contrasting with the preference for the η¹-P in-plane coordination mode of phosphinine ligands in their copper­(I) halide complexes. DFT studies of geometry-optimized monomers LCuBr, L­(CuBr)₂, L₂CuBr, and the dimers <b>3b</b> and <b>4b</b>, calculated at the ωB97xD/cc-PVDZ level, suggest that weak competing interactions with the solvent THF and the entropy factor of the dimerization result in lability and a subtle balance between the different complexes in solution, whereas the particular coordination observed in the crystals is attributable to conservation of the delocalized π-system in the ligand. The HOMO of <b>4b</b> is composed of Cu d orbitals and the π-type HOMO of the bridging ligand. Interestingly, despite the rather short Cu···Cu interatomic separation (2.726 Ǻ), no bond critical point could be located in <b>4b</b>, indicating the absence of weak cuprophilic interactions in this compound

π‑Rich σ²P‑Heterocycles: Bent η¹‑P- and μ²‑P-Coordinated 1,3-Benzazaphosphole Copper(I) Halide Complexes

The reaction of 1-neopentyl-1,3-benzazaphosphole <b>1</b> with CuCl, CuBr, or Cu­(SMe₂)Br in THF at room temperature provides sparingly soluble [Cu₇(μ²-L₆)­(μ²-X₇)]⁺[CuX₂]⁻ cluster complexes <b>2a</b>,<b>b</b> (L indicates coordinated <b>1</b>, <b>a</b> X = Cl, <b>b</b> X = Br), with loosely bound THF, in high yields. The conversions proceed via transient THF-soluble labile [(L₂CuX)₂] complexes. Separation before complete conversion, combined with suitable conditions for crystallization, allowed these intermediates to be trapped. Depending on the reactant ratios, crystals of the clusters or of dimeric L₂CuX complexes were formed. The crystal structure analyses of <b>2a</b>·4THF and the dimers <b>3b</b> [{Cu­(η¹-L)₂­(μ²-Br)}₂], <b>4b</b> [{Cu­(μ²-L)­(η¹-L)­(κBr)}₂], <b>5a</b>·2MeOH, and <b>5b</b>·2MeOH [{Cu­(μ²-L)­(η¹-L)­(κX···HOMe)}₂] generally display μ²-P- and/or tilted η¹-P-coordination, contrasting with the preference for the η¹-P in-plane coordination mode of phosphinine ligands in their copper­(I) halide complexes. DFT studies of geometry-optimized monomers LCuBr, L­(CuBr)₂, L₂CuBr, and the dimers <b>3b</b> and <b>4b</b>, calculated at the ωB97xD/cc-PVDZ level, suggest that weak competing interactions with the solvent THF and the entropy factor of the dimerization result in lability and a subtle balance between the different complexes in solution, whereas the particular coordination observed in the crystals is attributable to conservation of the delocalized π-system in the ligand. The HOMO of <b>4b</b> is composed of Cu d orbitals and the π-type HOMO of the bridging ligand. Interestingly, despite the rather short Cu···Cu interatomic separation (2.726 Ǻ), no bond critical point could be located in <b>4b</b>, indicating the absence of weak cuprophilic interactions in this compound

Dibenzophosphapentaphenes: Exploiting P Chemistry for Gap Fine-Tuning and Coordination-Driven Assembly of Planar Polycyclic Aromatic Hydrocarbons

A synthetic route to planar P-modified polycylic aromatic hydrocarbons (PAHs) is described. The presence of a reactive σ³,λ³-P moiety within the sp²-carbon scaffold allows the preparation of a new family of PAHs displaying tunable optical and redox properties. Their frontier molecular orbitals (MOs) are derived from the corresponding phosphole MOs and show extended conjugation with the entire π framework. The coordination ability of the P center allows the coordination-driven assembly of two molecular PAHs onto a Au^I ion