How the Chalcogen Atom Size Dictates the Hydrogen‐Bond Donor Capability of Carboxamides, Thioamides, and Selenoamides

Photoinduced dual bond rotation of a nitrogen-containing system realized by chalcogen substitution

Photoinduced concerted multiple-bond rotation has been proposed in some biological systems. However, the observation of such phenomena in synthetic systems, in other words, the synthesis of molecules that undergo photoinduced multiple-bond rotation upon photoirradiation, has been a challenge in the photochemistry field. Here we describe a chalcogen-substituted benzamide system that exhibits photoinduced dual bond rotation in heteroatom-containing bonds. Introduction of the chalcogen substituent into a sterically hindered benzamide system provides sufficient kinetic stability and photosensitivity to enable the photoinduced concerted rotation. The presence of two different substituents on the phenyl ring in the thioamide derivative enables the generation of a pair of enantiomers and E/Z isomers. Using these four stereoisomers as indicators of which bonds are rotated, we monitor the photoinduced C-N/C-C concerted bond rotation in the thioamide derivative depending on external stimuli such as temperature and photoirradiation. Theoretical calculations provide insight on the mechanism of this selective photoinduced C-N/C-C concerted rotation.

Acidity of Isomorphic Substituted Zeolites with B, Al and Ga Revisited

Isomorphic substitution of zeolites with B, Al and Ga is a widely used approach in catalysis. The experimentally reported trend of their acidities decreases in the order: Al>Ga>B. However, a consistent explanation is still lacking in the literature. To bring more understanding of this trend, density functional theory computations were conducted on several model systems. First, the acidity of small clusters with two (2T) and five (5T) tetrahedral sites was analyzed. These systems were then projected onto three large void structures: H-[A]-BEA (52T), H-[A]-FAU (84T) and H-[A]-MOR (112T) with A=B, Al, Ga. Our electron density and Interacting Quantum Atom analyses show that the acidity of Al zeolites originates from the much stronger O-Al bond, which is dominated by the electrostatic attraction. The bridging hydroxyl therefore donates more charge density to the metal, the proton becomes more positive and consequently more acidic. Ga zeolites are more acidic than B zeolites due to the greater covalent nature on the O-Ga bond. The resulting acidity, as seen by ammonia, depends on both the acidic oxygen and the charge distribution of the surrounding oxygens exerted by the substituents.

Chalcogen Atom Size: A Key Parameter in Modulating Carbonyl Compound Properties

Exchanging oxygen in the functional group C=O (i. e., carbonyl) for the less electronegative Group 16 elements, sulfur or selenium, unexpectedly enhances the electronegativity of the C=X group in π-conjugated molecules and reduces the molecular π HOMO-LUMO energy gap. Quantum-chemical analyses revealed that the steric size of the chalcogen atom X is at the origin of this seemingly counterintuitive behavior. This tuning of the chemical properties of carbonyl compounds by varying the chalcogen atom size in the C=X bond can be applied in many fields of chemistry. This concept article delineates several useful applications in the fields of organocatalysis, supramolecular chemistry, and photo(electro)chemistry.

Multiple hydrogen-bonded dimers: are only the frontier atoms relevant?

Non-frontier atom exchanges in hydrogen-bonded aromatic dimers can induce significant interaction energy changes (up to 6.5 kcal mol-1). Our quantum-chemical analyses reveal that the relative hydrogen-bond strengths of N-edited guanine-cytosine base pair isosteres, which cannot be explained from the frontier atoms, follow from the charge accumulation in the monomers.

Thioimidate Solutions to Thioamide Problems during Thionopeptide Deprotection

Thioamides have structural and chemical similarity to peptide bonds, offering valuable insights when probing peptide backbone interactions, but are prone to side reactions during solid-phase peptide synthesis (SPPS). Thioimidates have been demonstrated to be effective protecting groups for thioamides during peptide elongation. We further demonstrate how thioimidates can assist thioamides through the most yield-crippling step of thionopeptide deprotection, allowing for the first isolation of an important benchmark α-helical peptide that had previously eluded synthesis and isolation.

Amide C-N bonds activation by A new variant of bifunctional N-heterocyclic carbene

We report an organocatalyst that combines a triazolium N-heterocyclic carbene (NHC) with a squaramide as a hydrogen-bonding donor (HBD), which can effectively catalyze the atroposelective ring-opening of biaryl lactams via a unique amide C-N bond cleavage mode. The free carbene species attacks the amide carbonyl, forming an axially chiral acyl-azolium intermediate. Various axially chiral biaryl amines can be accessed by this methodology with up to 99% ee and 99% yield. By using mercaptan as a catalyst turnover agent, the resulting thioester synthon can be transformed into several interesting atropisomers. Both control experiments and theoretical calculations reveal the crucial role of the hybrid NHC-HBD skeleton, which activates the amide via H-bonding and brings it spatially close to the carbene centre. This discovery illustrates the potential of the NHC-HBD chimera and demonstrates a complementary strategy for amide bond activation and manipulation.

More Electropositive is More Electronegative: Atom Size Determines C=X Group Electronegativity

Opposite to what one might expect, we find that the C=X group can become effectively more, not less, electronegative when the Pauling electronegativity of atom X decreases down Groups 16, 15, and 14 of the Periodic Table. Our quantum-chemical analyses, show that, and why, this phenomenon is a direct consequence of the increasing size of atom X down a group. These findings can be applied to tuning and improving the hydrogen-bond donor strength of amides H2 NC(=X)R by increasingly withdrawing density from the NH2 group. A striking example is that H2 NC(=SiR2 )R is a stronger hydrogen-bond donor than H2 NC(=CR2 )R.

Strengthened cooperativity of DNA-based cyclic hydrogen-bonded rosettes by subtle functionalization

Cooperative effects cause extra stabilization of hydrogen-bonded supramolecular systems. In this work we have designed hydrogen-bonded rosettes derived from a guanine-cytosine Janus-type motif with the aim of finding a monomer that enhances the synergy of supramolecular systems. For this, relativistic dispersion-corrected density functional theory computations have been performed. Our proposal involves a monomer with three hydrogen-bonds pointing in the same direction, which translates into shorter bonds, stronger donor-acceptor interactions, and more attractive electrostatic interactions, thus giving rise to rosettes with strengthened cooperativity. This newly designed rosette has triple the cooperativity found for the naturally occurring guanine quadruplex.