The investigation on the NH reactivity of pentacoordinate spirophosphoranes by H/D exchange and NMR experiments

Different pentacoordinate spirophosphoranes were investigated by H/D exchange and NMR tracking experiments. The H/D exchanged site of spirophosphoranes was confirmed to be the NH bond instead of the PH bond according to the integration of the signal changes in proton-decoupled, proton-coupled ³¹P NMR and ¹H NMR tracking experiments. The reactivity of NH bond at the spiroring of spirophosphoranes was explored by the rate constant of H/D exchange of different spirophosphoranes. The results demonstrated that the reactivity of NH bond of spirophosphoranes was influenced by steric hindrance and the species of substituent at phosphorus atom, and the electronic effect of the substituent was the main effect factor on the reactivity of the NH bond. The strong electron-withdrawing group at phosphorus resulted in more reactive NH bond of spirophosphoranes. These results are beneficial to further understand and explore the characteristics of pentacoordinate spirophosphoranes.     

Site selective [bmIm]OH catalyzed CC bond functionalization under green conditions

Effective methodology that activates selectively either end of a carbon–carbon triple bond requires key challenge of differentiating between the multitude of CC bonds present in complex organic molecule. The synthetic strategy exploited the electronic biases within the substrate and successfully achieved site–selective [bmIm]OH catalyzed CC bond functionalization under mild reaction conditions. This resulted in C-2-selective addition of phenacyl bromide on p-substituted phenyl acetylene and C-1 selective addition on the o-substituted phenyl acetylene leading to CC bond formation. The reaction proceeded smoothly with excellent yield under ambient conditions. This report demonstrates the progress on the catalytic activity of recyclable [bmIm]OH for selective CC bond formation.     

Total synthesis of lithospermic acid using Fe-catalyzed Cross-Dehydrogenative-Coupling reaction and Pd-catalyzed ester-directed CH olefination

The total synthesis of lithospermic acid has been accomplished employing two CH activation reactions as key steps. Fe-catalyzed Cross-Dehydrogenative-Coupling reaction was used for the rapid construction of benzofuran framework. Pd-catalyzed ester-directed CH olefination allowed the efficient assembly of the dihydrobenzofuran ester and acrylate moieties.     

One-pot synthesis of [1,2,4]Triazolo[1,5-a]pyridines from azines and benzylidenemalononitriles via copper-catalyzed tandem cyclization

A simple and efficient copper-catalyzed tandem radical cyclization reaction has been discovered for the synthesis of triaryl [1,2,4]triazolo[1,5-a]pyridines from easily accessible azines and benzylidenmalononitriles. The new transformation involves multiple CH/CC bonds cleavage and CC/CN bonds formation, with extrusion of gaseous hydrogen and methane. A wide variety of substrates with different functional groups could be converted into the corresponding products in good yields. The fused heterocycles have strong blue fluorescence with large Strokes shifts and high quantum yields.     

Visible light mediated aerobic photocatalytic activation of CH bond by riboflavin tetraacetate and N-hydroxysuccinimide

A green and highly efficient cooperative photocatalytic system which consists of riboflavin tetraacetate (RFT), N-hydroxysuccinimide (NHS) and FeCl₃·6H₂O has been developed for CH bond oxygenation with visible light under mild reaction conditions. Compared to the previous procedures about CH bond activation by using flavins as photocatalyst, we use cheap ferric chloride and NHS instead of expensive scandium triflate or biomimetic non-heme iron complex, further expand the substrate scopes. In addition, a possible radical mechanism for the CH bond oxygenation in this photocatalytic system is proposed.     

Recent topics in annulation reaction via double CH bond cleavage and double CC bond formation

Cyclic compounds have been consistently key components in organo functional molecules. To construct cycles, a catalytic two CH bond-cleaving annulation is one of the most ideal and straightforward methods with atom and step economies. Recently, many patterns of such annulation reactions have been developed, which construct a variety of cyclic compounds consisted of from simple to complex frameworks. This Digest focuses a recent progress in two or more than three CH bond-cleaving annulation reactions and is outlined as follows: (1) intramolecular annulation, (2) intermolecular annulation via double CH bond cleavages in one molecule, and (3) intermolecular annulation via double CH bond cleavages in two molecules.     

Guanidine organocatalysis for enantioselective carbon-heteroatom bond-forming reactions

Guanidine-containing molecules have been employed as organocatalysts, and chiral guanidines have been widely explored as catalysts for mediating asymmetric reactions by constructing an asymmetric reaction environment. Currently, many guanidine-catalyzed asymmetric reactions are available, and some excellent reviews have appeared. In this Digest, we focused on recent progress in the guanidine-catalyzed asymmetric construction of CN, CO, CP, and CS bonds, especially developments since 2010.     

Copper-mediated Chan-Evans-Lam N-arylation of 5-methylene-4-aryl-1,5-dihydro-2H-pyrrol-2-one derivatives

A simple and efficient procedure for the synthesis of N-aryl 5-methylene-4-aryl-1,5-dihydro-2H-pyrrol-2-one derivatives has been developed through copper-mediated CN bond formation. The synthetic protocol allows for versatile and robust CN arylation with a range of readily available boronic acids under mild conditions.     

Recent topics on catalytic transformations of aromatic molecules via η6-arene transition metal complexes

The π-complexation of an arene to a transition metal center delivers many useful reactivities to the arene moiety. Although synthetic methodologies that take advantage of such π-coordination have mostly been developed as stoichiometric processes, there have been considerable recent advances in the catalytic transformations of aromatic molecules through their activation in the form of transition metal η⁶-arene complexes. These advances include the π-coordination catalyzed transformations of aryl-heteroatom and side-chain CH and CC bonds and the palladium-catalyzed CH functionalization in pre-formed transition metal η⁶-arene complexes. This digest paper aims to provide a concise view of these recent advances in the area of transition metal η⁶-arene complexes.     

A DFT study on N-heterocyclic carbene catalyzed [4+2] annulation reaction with in situ generated heterocyclic ortho-quinodimethane: Mechanism,origin of enantioselectivity and role of catalyst

Activation of inert sp³ CH bonds has attracted widespread attention and been developed with significant progress in recent years, but understanding the mechanism of this kind of reaction continues to be one of the most challenging topics in organic chemistry. In this paper, detailed mechanism of N-heterocyclic carbine (NHC) catalyzed [4+2] annulation reaction with in Situ generated heterocyclic ortho-quinodimethane have been investigated using density functional theory (DFT). The computational results show that the additive DBU plays an important role in NHC-catalyzed CH activation. The title reaction proceeds the processes of the introduction of the NHC catalyst to the carboxylic ester, DBU-assisted sp³ CH deprotonation, Michael addition, [4+2] cycloaddition, the dissociation of the catalyst and the product generation. The calculated results indicate that Michael addition is rate-determining and stereoselectivity-determining. With the use of NHC as the chiral catalyst, optically active products were obtained in good yields with excellent enantioselectivities. Furthermore, the theoretically predicted the main product is S configuration, which is in good agreement with the experimental result. The special role of NHC and origin of stereoselectivity were also identified by natural bond orbital (NBO), global reaction index (GRI) and frontier molecular orbital (FMO) analyses. This work might be helpful for understanding the significant roles of NHC catalyst and thus provide insights on the rational design of potential new catalysts for this kind of reactions and suggest enlightening clues on rational design of efficient organocatalysts for the C (sp³)-H activation.     

Transition-metal-free synthesis of aromatic amines via the reaction of benzynes with isocyanates

An unexpected reaction between benzynes and isocyanates to generate aromatic amines has been developed under transition-metal-free conditions. The in situ prepared anions formed through cleavage of the NC bond in isocyanates, reacted with aryne precursors to afford various aniline derivatives in moderate to excellent yield and tolerated various substituents on the o-silyl aryl triflate and the isocyanate.     

Chemoselective oxidation and deprotection of para-methoxybenzylic position with (diacetoxyiodo)benzene in acetic-trifluoroacetic acid

(Diacetoxyiodo)benzene in the presence of acetic–trifluoroacetic acid in THF has been developed for the chemoselective para-methoxybenzylic CH bond oxidation to provide aryl carbonyl compounds at room temperature. The reaction condition is also applicable for the chemoselective deprotection of para-methoxybenzyl (PMB) ether in the presence of benzyl ether.     

Recent advances on catalytic asymmetric difunctionalization of 1,3-dienes

1,3-Dienes and derivatives are either feedstock chemicals or easily available materials. Catalytic difunctionalization of 1,3-diene is one of the most powerful methods for carboncarbon bond formation with rapid increase of the molecular complexity and synthetic value in an atom economic way. By choosing proper metals and chiral ligands, a variety of catalytic asymmetric difunctionalization of conjugated dienes in a highly regioselective fashion have been reported. In this digest review, we will summarize recent advances on this topic based on different metals. We will also introduce unique phenomena that include reversal of regio- and diastereoselectivity.     

NH4I-catalyzed chalcogen(S/Se)-functionalization of 5-membered N-heteroaryls under metal-free conditions

Herein, we described the NH₄I-catalyzed CH bond chalcogenation of N-heteroaryls in the presence of a minimum amount of DMSO/H₂O/acetic acid as additives (2.5/2.5/1?M equiv., respectively), under metal-free conditions. Under optimized conditions, a wide variety of sulfenyl/selenyl imidazo[1,2-α]pyridines were prepared in very good yields. Moreover, the present approach was also highly efficient for the chalcogenation of different 5-membered N-heteroaryls, e.g., indole, imidazothiazole, indazole and imidazopyrimidine derivatives.     

A bio-inspired cascade and a late-stage directed sp3 CH lithiation enables a concise total synthesis of (−)-virosaine A

The asymmetric total synthesis of (?)-virosaine A was achieved in 9% overall yield from commercially/readily available starting materials. Inspired by an intriguing biosynthetic proposal, a novel cascade reaction sequence was developed to efficiently construct the caged polycyclic core of virosaine A. The pivotal cascade precursor was readily available in enantiopure form via a robust route that featured an enantioselective one-pot Diels-Alder cycloaddition/organolithium addition. Several contemporary methods of CH functionalization were applied to the cascade product and yielded a diverse set of novel complex polycycles. Ultimately, a combination of NMR and computational analyses laid the groundwork for a successful directed lithiation strategy to selectively functionalize the caged core and complete the total synthesis of virosaine A.     

Highly regioselective photodimerization of 1,4-dihydropyridines: An efficient synthesis of novel 3,6-diazatetraasteranes

Conventional photocycloaddition of 1,4-dihydropyridines does not afford novel head-to-head 3,6-diazatetraasteranes. Herein, we describe a highly regioselective method to synthesize 3,6-diazatetraasteranes via an intramolecular photodimerization of 1,4-dihydropyridines. First, the 1,4-dihydropyridines were tethered by phthaloyl to direct a proximate parallel arrangement in head-to-head orientation by the rotation of CC single bonds in solution. An intramolecular [2 + 2] photocycloaddition proceeded subsequently to give desired 3,6-diazatetraasteranes in high yield (92–97%) and excellent regioselectivity. Furthermore, two different 1,4-dihydropyridines can also be regiocontrolled by this strategy and produce polysubstituted 3,6-diazatetraasteranes via a cross-photodimerization in a concise and efficient way. In addition, this approach can provide direct access to other polysubstituted polyhedron scaffolds from 1,4-dihydropyridine analogues.     

Chan-Lam cross-coupling reaction based on the Cu2S/TMEDA system

A catalyst based on the readily available Cu₂S/TMEDA system using a stable copper(I) source was developed for the Chan-Lam cross-coupling reaction. The capability of the catalyst was demonstrated with 1H-benzo[d]imidazol-2(3H)-one, 1H-benzo[d]imidazole, and 1H-imidazole together with electron-deficient, electron-rich, and sterically demanding boronic acids at room temperature in the presence of atmospheric oxygen to give the cross-coupling products in moderate to excellent yields. In addition, the coupling reaction of 1H-benzo[d]imidazole with several pinacol or neopentylglycol boronates indicated further potential of the catalyst. The reaction conditions tolerate the hydroxyl and bromo functional groups. The catalytic system also enables to synthesize the mono-N-substituted anilines from primary aliphatic amines. However, the two model compounds for the secondary and aromatic amines, piperidine and aniline, do not react. Two sterically demanding products with the restricted CN bond rotation, synthesized by the N-arylation of 1H-benzo[d]imidazol-2(3H)-one with o-tolylboronic acid, enabled to confirm the atropisomers prepared by the Chan-Lam cross-coupling reaction. Furthermore, an example of one-pot Chan-Lam and Suzuki-Miyaura reaction has been reported.     

Phosphinate selective hosts and importance of CH hydrogen bonding for affinity modulation toward anion guests

Even though phosphinate and its analogs are very important guests in nature, the artificial receptors which are capable of selective recognition of phosphinate are rare. Here, we report a series of acetate and phosphinate selective hosts (1, 2 and 3) which utilize amide NH and aliphatic CH groups as hydrogen bonding donors. In this series of receptors, even though the amide NH hydrogen bonding element was found to be the most significant, by varying the polarity of CH group, the magnitude of recognition could be modulated considerably. The affinities of host 3 against all the tested anion guests showed significantly higher affinities compared with those of hosts 1 and 2, and this could be attributed to the difference of CH group polarities among the receptors 1, 2 and 3. C_α-H hydrogen in host 3 is the most highly polarized by the charged pyridinium group. Therefore, it is the strongest host in this series of hosts. From the experiments shown here, we demonstrated the importance of CH hydrogen bonding element as a decisive modulating moiety for anionic recognition.     

Rhodium-catalyzed NH-indole-directed ortho CH coupling of 2-arylindoles with diazo compounds via metal carbene migratory insertion

A high regioselective rhodium-catalyzed ortho CH coupling of 2-arylindoles with diazo compounds via NH-indole-directed CH bond activation has been developed. The catalytic reaction features mild reaction conditions, broad substrate scope and good functional group tolerability.     

Active species transfer-type artificial light harvesting system in the nanosheet – Dye complexes: Utilization of longer wavelength region of sunlight

A novel light harvesting system that captures long-wavelength light of sunlight by the use of hole transfer was newly investigated. Ga phthalocyanine(Ga^IIITMAPc⁴⁺(D)) and Ru porphyrin(Ru^IIDMPyMP²⁺(A)) co-adsorbed reaction system on the synthetic nanosheet was examined as an artificial light harvesting system. By irradiating 660?nm light, where Ga^IIITMAPc⁴⁺(D) absorbs, to the system under a presence of PtCl₆^2? as an electron acceptor, cation radical of Ru^IIDMPyMP²⁺(A) was produced despite that Ru porphyrin adsorbs only 413 and 533?nm light. The efficient hole transfer reaction from Ga^IIITMAPc⁴⁺(D)⁺, that is generated from its excited state, to Ru^IIDMPyMP²⁺(A) takes place on the nanosheet.