Identification and Characterization of Three Chitinases with Potential in Direct Conversion of Crystalline Chitin into N,N'-diacetylchitobiose.

Chitooligosaccharides (COSs) have been widely used in agriculture, medicine, cosmetics, and foods, which are commonly prepared from chitin with chitinases. So far, while most COSs are prepared from colloidal chitin, chitinases used in preparing COSs directly from natural crystalline chitin are less reported. Here, we characterize three chitinases, which were identified from the marine bacterium Pseudoalteromonas flavipulchra DSM 14401T, with an ability to degrade crystalline chitin into (GlcNAc)2 (N,N'-diacetylchitobiose). Strain DSM 14401 can degrade the crystalline α-chitin in the medium to provide nutrients for growth. Genome and secretome analyses indicate that this strain secretes six chitinolytic enzymes, among which chitinases Chia4287, Chib0431, and Chib0434 have higher abundance than the others, suggesting their importance in crystalline α-chitin degradation. These three chitinases were heterologously expressed, purified, and characterized. They are all active on crystalline α-chitin, with temperature optima of 45-50 °C and pH optima of 7.0-7.5. They are all stable at 40 °C and in the pH range of 5.0-11.0. Moreover, they all have excellent salt tolerance, retaining more than 92% activity after incubation in 5 M NaCl for 10 h at 4 °C. When acting on crystalline α-chitin, the main products of the three chitinases are all (GlcNAc)2, which suggests that chitinases Chia4287, Chib0431, and Chib0434 likely have potential in direct conversion of crystalline chitin into (GlcNAc)2.

Exploiting Pincer Ligands to Perturb the Geometry at Boron.

Boranes are ubiquitous in synthesis and materials but advancements in their development have been primarily restricted to the geometric energetic minima, trigonal planar complexes. This report discloses a class of boranes with expanded bond angles achieved by taking advantage of the structural rigidity of tridentate pincer ligands. The bonding of these novel boranes is investigated by X-ray crystallography and computationally.

Ring expansion reactions of anti-aromatic boroles: a promising synthetic avenue to unsaturated boracycles.

Conjugated boron heterocycles have emerged as attractive synthetic targets due to their potential in medicinal chemistry and as electronic materials. However, the development of unsaturated boracycles has been hampered by difficulties in their preparation. Recently, a new synthetic avenue to access these species has been developed that takes advantage of the high reactivity of boroles. These five-membered anti-aromatic heterocycles can react with substrates to furnish ring expansion products via the insertion of one, two, or three atoms into the boracyclic ring. The ring expansion can occur via two pathways, the first exploits the activated diene moiety of the heterocycle in Diels-Alder chemistry with the resulting bicyclic species undergoing further rearrangements. The second reaction pathway is initiated by the coordination of the Lewis basic site of a substrate to the highly Lewis acidic boron center rendering the endocyclic B-C bond of the borole nucleophilic, inducing the formation of larger boracycles via attack at the electrophilic site of the substrate. This review summarizes the current state of this chemistry and details the mechanisms leading to the products. The methodologies described herein could very well be extended to other substrates, as well as applied to other anti-aromatic heterocycles.

Probing the reactivity of pentaphenylborole with N-H, O-H, P-H, and S-H bonds.

The reactions of molecules containing E-H functionalities (E = Group 15 or 16 element) and pentaphenylborole were investigated revealing diverse outcomes. For aniline and water, protodeborylation ring opening reactions occurred via the N-H or O-H bonds. Pentaphenylborole reacted with water in a 1 : 1 or 2 : 1 ratio to yield the corresponding boroxane and diboroxane, respectively, whereas aniline reacted strictly in a 1 : 1 ratio. Interestingly, 1-naphthalenethiol reacted to produce a 1-bora-cyclopent-3-ene heterocycle. The reaction with a primary phosphine generated an adduct which was resilient, even at elevated temperatures. DFT calculations provide support for the observed reaction products, and identify the initial adduct as a key intermediate in determining the final product. In particular, ring opening may be linked to the lability of the hydrogen in the initial adduct. Collectively, these reactions provide insight into new reaction pathways, the stability of boroles, as well as mechanistic insight into previously reported transformations.

Peculiar Reactivity of Isothiocyanates with Pentaphenylborole.

The reactions of isothiocyanates with the antiaromatic pentaphenylborole were investigated, revealing significantly different outcomes than the analogous reactions with isocyanates. The 1:1 stoichiometric reaction products isolated include a seven-membered BNC5 heterocycle and a fused bicyclic 4/5-ring system. Studies suggest that the seven-membered ring undergoes an intramolecular [2 + 2] electrocyclic ring closure to produce the bicyclic system. The only derivative for which stoichiometry influenced the reaction outcome was 4-methoxyphenylisothiocyanate. The reaction of borole with an excess of 4-methoxyphenylisothiocyanate resulted in the formation of a fused tetracyclic species with two equivalents of isothiocyanate incorporated into the product. Rational pathways for these unusual transformations are presented.

Reactions of imines, nitriles, and isocyanides with pentaphenylborole: coordination, ring expansion, C-H bond activation, and hydrogen migration reactions.

The reactions of pentaphenylborole with imines, isocyanides, and acetonitrile were investigated experimentally and theoretically. On the basis of literature precedent, we envisioned that the dipolar substrates would undergo facile ring expansion reactions to yield new BNC5 heterocycles. For acetonitrile and one particular imine, this ring expansion process was observed. However, in many cases, unexpected reactivity occurred. This included hydride migration of an imine ring expanded product and the ortho C-H bond activation of an aryl group of an imine if two phenyl groups were present on the α-carbon. A bulky group on the nitrogen atom of an imine prevented coordination to the boron center, and no reaction was observed, indicating that coordination to the borole is a critical step for any type of reaction to occur. Isocyanides made coordination complexes, but heating to induce further reactivity resulted in mixtures. The mechanisms were elucidated via DFT calculations, which complement the experimental findings.

Ring expansion reactions of pentaphenylborole with dipolar molecules as a route to seven-membered boron heterocycles.

Reactions of pentaphenylborole with isocyanates, benzophenone, and benzaldehyde produced new seven-membered heterocycles in high yields. For 1-adamantyl isocyanate, a BNC5 heterocycle was obtained from the insertion of the C-N moiety into the five-membered borole, whereas for 4-methoxyphenyl isocyanate, a BOC5 heterocycle was generated from the insertion of the C-O unit. These reactions are believed to occur via a mechanism wherein coordination of the nucleophile to the borole (1-adamantyl, N-coordination or O-coordination for 4-methoxyphenyl) is followed by ring expansion to afford the observed seven-membered heterocycles. The selectivity to form B-O- or B-N-containing heterocycles is based on the polarization of the isocyanate implying tunable reactivity for the system. Having observed that isocyanates react as 1,2-dipoles with pentaphenylborole, we examined benzophenone and benzaldehyde, which both reacted to insert C-O units into the ring. This represents a new efficient method for preparing rare seven-membered boracycles.

Enantioselective hydrogenation of α,ß-disubstituted nitroalkenes.

The first highly chemo- and enantioselective hydrogenation of α,ß-disubstituted nitroalkenes was accomplished with rhodium/JosiPhos-J2 as a catalyst, with the yield and enantioselectivity of up to 95% and 94%, respectively. The α-chiral nitroalkanes will provide an entry to valuable chiral amphetamines which are otherwise not so easily accessed.

Rhodium-catalyzed asymmetric hydrogenation of unprotected NH imines assisted by a thiourea.

Asymmetric hydrogenation of unprotected NH imines catalyzed by rhodium/bis(phosphine)-thiourea provided chiral amines with up to 97% yield and 95% ee. (1)H NMR studies, coupled with control experiments, implied that catalytic chloride-bound intermediates were involved in the mechanism through a dual hydrogen-bonding interaction. Deuteration experiments proved that the hydrogenation proceeded through a pathway consistent with an imine.

Direct catalytic asymmetric reductive amination of simple aromatic ketones.

A green method for chiral amine synthesis, the direct catalytic asymmetric reductive amination, was developed. Phenylhydrazide is an ideal nitrogen source for reductive amination. Molecular sieves play dual roles in this reaction. They help to remove H2O to form imine, as well as promote an imine reduction. f-Binaphane minimizes the inhibition effect from amines and helps the coordination of sterically demanding imines to the iridium center, thus leading to a smooth reaction.

A novel chiral bisphosphine-thiourea ligand for asymmetric hydrogenation of ß,ß-disubstituted nitroalkenes.

A novel chiral bisphosphine-thiourea ligand was developed and applied in the highly enantioselective hydrogenation of ß,ß-disubstituted nitroalkenes (up to 99% yield and 99% ee). With low catalytic loading (0.25 mol %), 98% ee and 98% conversion were obtained. The thiourea group takes on an important role in this catalytic system.

Rhodium-catalyzed highly regioselective hydroaminomethylation of styrenes with tetraphosphorus ligands.

The highly linear-selective hydroaminomethylation of styrenes is very challenging. Herein, an efficient, highly chemoselective, and linear-selective hydroaminomethylation (l/b up to >99:1) of styrenes using Rh(nbd)2SbF6 with a pyrrole-based 3,3',5,5'-substituted tetraphosphorus ligand is documented. This is in sharp contrast to other available processes leading to branched amines and provides a novel atom economic approach to 3-arylpropylamines.

Cascade synthesis of fenpiprane and related pharmaceuticals via rhodium-catalyzed hydroaminomethylation.

A novel rhodium catalytic system with Naphos as ligand was developed for an efficient hydroaminomethylation of 1,1-diphenylethene under relatively mild conditions. This will allow for an atom-economic and environmentally benign synthesis of fenpiprane and related pharmaceuticals.

Rhodium-catalyzed enantioselective hydrogenation of oxime acetates.

Rh-catalyzed enantioselective hydrogenation of oxime acetates was first reported, which afforded a new approach for chiral amine synthesis.

Highly enantioselective hydrogenation of ß,ß-disubstituted nitroalkenes.

Building the building blocks: A highly enantioselective hydrogenation of ß-aryl-ß-alkyl disubstituted nitroalkenes 1 has been developed. This method results in enantiomerically pure nitroalkanes 2, which are versatile precursors for chemical synthesis.

Highly regioselective isomerization-hydroaminomethylation of internal olefins catalyzed by Rh complex with Tetrabi-type phosphorus ligands.

A highly regioselective isomerization-hydroaminomethylation of internal olefins has been developed. A 95.3% amine selectivity and 36.2 n/i ratio were obtained for 2-octene with a Tetrabi ligand and Rh(acac)(CO)(2), and a TON of linear amine was achieved of 6837 with a 39.1 n/i ratio of amine. The m-CF(3)-Ph substituted ligand was the best of the applied Tetrabi-type phosphorus ligands for different internal olefins, as up to a 99.2% amine selectivity and 95.6 n/i ratio were obtained for 2-pentene.

Highly regioselective hydroaminomethylation of terminal olefins to linear amines using Rh complexes with a Tetrabi phosphorus ligand.

A highly regioselective hydroaminomethylation of terminal olefins catalyzed by Rh complexes with 2, 2', 6, 6'-tetrakis ((diphenylphosphino)methyl)-1, 1'-biphenyl (Tetrabi) ligand has been developed. Up to 99 % amine selectivity, 168 linear/branched amine product ratio (n/i), and 97.4 % linear amine yield has been obtained at a substrate/rhodium precursor ratio (S/Rh) of 1000 with this methodology. The turnover number was achieved 6930 at 10,000 S/Rh ratio, and the n/i can reach up to >525. Several different olefins and secondary amines have been applied successfully with high chemoselectivity (99 %), yield (>98 %), and regioselectivity (>120).

Enantioselective synthesis of optically pure ß-amino ketones and γ-aryl amines by Rh-catalyzed asymmetric hydrogenation.

A series of optically pure ß-amino ketones have been synthesized in high enantioselectivities (ee > 99%) by Rh-DuanPhos-catalyzed asymmetric hydrogenation of readily prepared ß-keto enamides. Further reduction of these ß-amino ketones with hydrogen and Pd/C leads to the formation of a variety of protected enantiomerically pure γ-aryl amines (ee > 99%), which are key building blocks in many bioactive molecules.

Design and synthesis of a novel three-hindered quadrant bisphosphine ligand and its application in asymmetric hydrogenation.

A novel three hindered quadrant bisphosphine ligand has been synthesized. The ligand shows excellent enantioselectivities and reactivities for rhodium-catalyzed hydrogenations of various functionalized olefins.