Azide Passivation of Black Phosphorus Nanosheets: Covalent Functionalization Affords Ambient Stability Enhancement

Two‐dimensional (2D) black phosphorus (BP) has a unique band structure, but it suffers from low ambient stability owing to its high reactivity to oxygen. Covalent functionalization has been demonstrated to passivate the reactive BP effectively, however the reported covalent functionalization methods are quite limited to aryl diazonium and nucleophilic additions affording P?C and P?O?C single bonds, for which the retaining of one unpaired electron in the Group 15 phosphorus atom hampers the passivation effect. Now, covalent azide functionalization of BP nanosheets (BPNSs) is reported, leading to significant enhancement of the ambient stability of BP as confirmed by UV/Vis spectroscopic studies. The most stable configuration of the azide functionalized BPNSs (f‐BPNSs) is predicted by theoretical calculations, featuring the grafting of benzoic acid moiety onto BPNSs via the unprecedented P=N double bonds formed through in situ nitrene as a reactive intermediate.     

Effect of phosphoryl and thiophosphoryl groups on CH acidities

It is shown that the CH acidities of phosphoryl and thiophosphoryl compounds depend on the substituents attached to both the carbon atom and the phosphorus atom and comply with Hammett dependences; the parameters of the correlation dependences are very close for the two types of compounds, and this makes it possible to propose a general equation for calculating the pK values of CH acids that contain a phosphoryl or thiophosphoryl group.A. N. Nesmeyanov Institute of Heteroorganic Compounds, Academy of Sciences of the USSR, Moscow. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 3, pp. 284–288, May–June, 1991. Original article submitted February 18, 1991.     

Complexes of imidophosphoric compounds with BF3

IR,³¹P NMR and¹⁹F NMR spectroscopy was used to study triphenylphosphinimines with substituants at the nitrogen atom and their complexes with boron trifluoride. In the case of aryl or benzyl substituants, BF₃ adds to the nitrogen atom, while the state of the phosphorus atom is close to phosphonium. An analogous structure of the intermediate complexes is proposed upon the catalysis of the imide-amide rearrangement by BF₃. The introduction of acyl groups such as phosphoryl, thiophosphoryl, and methanesulfonyl groups to the nitrogen atom of the triphenylphosphinimine alters the site of attachment of BF₃, which is found at the oxygen atoms of the P-O or SO₂ groups or to the sulfur atom of the P-S group.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 713–718, March, 1991.The authors express their deep gratitude to M. G. Galakhov for taking and interpreting the¹⁹F NMR spectra.     

Nucleophilic Reactivity of the Thiophosphoryl Group

The difference in the nucleophilic reactivities of the and groups can be satisfactorily explained on the basis of Pearson's acid-base concept^[21]. The thiophosphoryl sulfur is a typical “soft base”, and reacts preferentially with sub-group B metals, halogens, and sp³ hybridized carbon, whereas it is largely inert to “hard acids” such as protons, carbonyl carbon, and tetrahedral phosphorus. Since a nucleophilic attack by the thiophosphoryl sulfur leads to a decrease in the charge density of the phosphorus atom, the latter (or the α-carbon atom in O-alkyl esters) becomes more open to nucleophilic attack. This reaction principle forms the basis of sulfur exchange, dealkylation, isomerization, and the Pistschimuka and Michalski reactions.     

Can Cyclen Bind Alkali Metal Azides? A DFT Study as a Precursor to Synthesis

Can cyclen (1,4,7,10‐tetraazacyclododecane) bind alkali metal azides? This question is addressed by studying the geometric and electronic structures of the alkali metal azide‐cyclen [M(cyclen)N₃] complexes using density functional theory (DFT). The effects of adding a second cyclen ring to form the sandwich alkali metal azide‐cyclen [M(cyclen)₂N₃] complexes are also investigated. N₃^? is found to bind to a M⁺(cyclen) template to give both end‐on and side‐on structures. In the end‐on structures, the terminal nitrogen atom of the azide group (N1) bonds to the metal as well as to a hydrogen atom of the cyclen ring through a hydrogen bond in an end‐on configuration to the cyclen ring. In the side‐on structures, the N₃ unit is bonded (in a side‐on configuration to the cyclen ring) to the metal through the terminal nitrogen atom of the azide group (N1), and through the other terminal nitrogen atom (N3) of the azide group by a hydrogen bond to a hydrogen atom of the cyclen ring. For all the alkali metals, the N₃‐side‐on structure is lowest in energy. Addition of a second cyclen unit to [M(cyclen)N₃] to form the sandwich compounds [M(cyclen)₂N₃] causes the bond strength between the metal and the N₃ unit to decrease. It is hoped that this computational study will be a precursor to the synthesis and experimental study of these new macrocyclic compounds; structural parameters and infrared spectra were computed, which will assist future experimental work.     

Diastereoselective cycloalkylation of phosphoryl and thiophosphoryl acetonitriles by α,ψ‐dihalogenalkanes under phase transfer catalysis conditions

Cycloalkylation of phosphoryl and thiophosphoryl acetonitriles 2 by α,ψ‐dihalogenalkanes was found to proceed diastereoselectively under phase transfer catalytic conditions yielding 1‐(phosphoryl)‐2‐methyl‐cycloalkane carbonitriles as trans‐isomers with identical configuration of asymmetric cyclic carbon atoms (R_C^* R_C^*). In the case of additional asymmetric phosphorus atom in the starting substrate, trans‐isomers are formed as a mixture of diastereomers differing in the configuration of the phosphorus atom (S_P^* R_C^* R_C^* and R_P^* R_C^* R_C^*). © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:13–21, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20186     

Imide-amide rearrangement of oxazaphosphorimidates: studies towards the application to the synthesis of chiral Lewis bases

The Lewis acid catalysed imide-amide rearrangement of oxazaphosphorimides to diazaphoshoramides is reported for the first time. In spite of the similarity to the previously reported Lewis acid catalysed imide-amide rearrangement of dioxaphosphorimides to oxazaphosphoramides we show that this rearrangement proceeds by a different mechanism, not involving the formation of an oligomeric intermediate. The oxazaphosphorimides are prepared in situ by the Staudinger reaction of the appropriate trivalent phosphorus compound with an azide and after the addition of BF₃·OEt₂, undergo rearrangement to the corresponding diazaphosphoramides. We have found that the rearrangement occurs with retention of configuration at the phosphorus atom and inversion of configuration at the rearranged carbon atom. When starting from chiral 1,2-aminoalcohol, substituted at the carbon atom that undergoes rearrangement, a mixture of diastereomers is obtained, but the diastereomeric ratio, initially obtained in the formation of the trivalent phosphorus compounds is maintained during the whole transformation. This implies that if the rearrangement is to be used for the preparation of chiral phosphoramides with defined stereochemistry at the phosphorus atom, a high diastereoselectivity during the preparation of the trivalent phosphorus precursors should be obtained.     

End‐group functionalization and postpolymerization modification of helical poly(isocyanide)s

This contribution presents the synthesis of helical alkyne‐terminated polymers using a functionalized Nickel complex to initiate the polymerization of menthylphenyl isocyanides. The resulting polymers display low dispersities and controlled molecular weights. Copper‐catalyzed azide/alkyne cycloadditions (CuAAC) are performed to attach various azide‐containing compounds to the polymer termini. After azido‐phosphonate moiety attachment the polymer displays a signal at 25.4 ppm in the ³¹P NMR spectrum demonstrating successful end‐group functionalization. End‐group functionalization of a fluorescent dye allows to determine the functionalization yield as 89% (±8). Successful ligation of an azide‐functionalized peptide sequence (M_KLA = 1547 g/mol) increases the M_n from 5100 for the parent polymer to 6700 for the bioconjugate as visualized by GPC chromatography. Analysis by CD spectroscopy confirms that the helical conformation of the poly(isocyanide) block in the peptide–polymer conjugate is maintained after postpolymerization modification. These results demonstrate an easy, generalizable, and versatile strategy toward mono‐telechelic helical polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2766–2773     

Three-dimensional structures of phosphorus-containing heterocycles 50. 2-dimethylamino-2-oxo- and -2-thiono-1,3,2-oxathiaphosphorinanes

Conclusions A chair conformation with an axial orientation of the phosphoryl or thiophosphoryl group is preferable for 2-dimethylamino-2-oxo- and-2-thiono-1,3,2-oxathiaphosphorinanes. The cis isomer of 2-dimethylamino-2-oxo-4-methyl-1,3,2-oxathiaphosphorinane has a similar structure with an equatorial orientation of the 4-methyl group, while the corresponding sulfide is characterized by a trans configuration with a reoriented positioning of the substituents attached to the phosphorus atom.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1369–1374, June, 1989.The authors thank I. A. Litvinov and V. A. Naumov for carrying out the x-ray diffraction analysis.     

Bis[tris(cyclohexyl)tin] azide hydroxide, (Cy3Sn)2N3(OH): X‐ray structure determination and comparison with analogous compounds

The structure of bis[tris(cyclohexyl)tin] azide hydroxide, (Cy₃Sn)₂N₃(OH) ( 1 ), contains infinite chains of molecules linked by regularly alternating and µ₂ bridging azide and hydroxide groups that create trigonal bipyramidal tin centres. The bridges, with Sn? N 2.436(11) and 2.385(11) Å and Sn? O 2.199(8) and 2.197(8) Å, are relatively symmetrical. This structure is similar to that of catena bis(trimethyltin) azide hydroxide, (Me₃Sn)₂N₃(OH) ( 2 ). In the structure of 1 , each terminal nitrogen atom of the azide is bonded to a different tin atom (1,3 or α,γ bridge formation). In the structure of 2 , however, only one nitrogen atom of each azide is involved in bridging and bonds to two different tin atoms (1,1 or α,α bridge formation). In this case, the remaining terminal nitrogen atoms act as acceptors for O? H?N hydrogen bonds that link the chains to form infinite sheets. It appears then, from these two examples, that in such compounds the size of the organic species bonded to tin can affect the azide bridging mode and also the packing of the polymeric chains Copyright © 2005 John Wiley & Sons, Ltd.     

ASYMMETRIC SYNTHESIS OF THE DERIVATIVES OF (+)-1R,3R(3S), 5S-2,4,3-DIAZAPHOSPHABICYCLO [3.2.1] OCTANE

A new series of dextrorotatory derivatives of 1R,3RS,5S-1,8,8-trimethyl-2,4,3-diazaphosphabicyclo [3.2.1] octane with a newly formed chiral phosphorus atom has been prepared by an asymmetric reaction of (+)-cis-1R,3S-1,2,2-trimethyl-1,3-cyclopentadiamine with some thiophosphoryl dichlorides. The optically active products consisted of two unequal amounts of diastereomers,(1R, 3R, 5S) isomers and(1R, 3S, 5S) isomers. Two regioselective reactions took place when phenoxy thiophosphoryl dichloride and N,N-dialkylamino thiophosphoryl dichlorides were chosen as the reagents for the reaction. The content of (1R, 3R, 5S) isomers in the product was over 90% for the former and there were only (1R, 3S, 5S) isomers obtained for the latter.     

Isolation and Characterization of Four Phosphorus Cluster Anions P73–, P144–, P162– and P264– from the Nucleophilic Functionalization of White Phosphorus with 1,4‐Dilithio‐1,3‐butadienes

Studies on the aggregation degrees of negatively charged phosphides derived from the nucleophilic P₄ functionalization could help understand the pathway of phosphorus atoms degradation or aggregation. In this report, we have isolated and characterized four phosphorus cluster anions (P₇^3–, P₁₄^4–, P₁₆^2–, and P₂₆^4–) from the nucleophilic functionalization of P₄ with 1,4‐dilithio‐1,3‐butadienes. These phosphorus clusters could be rationalized as the P‐atom‐containing products besides the main phospholyl lithium. Their structural features and ³¹P NMR behaviors are discussed based on single crystal X‐ray diffraction analysis and ³¹P{¹H} COSY NMR analysis.     

Zinc‐Catalyzed Functionalization of SiH Bonds with 2‐Furyl Carbenoids through Three‐Component Coupling

A three‐component coupling of alk‐2‐ynals, 1,3‐dicarbonyls and silanes is reported. ZnCl₂ serves as an inexpensive and low‐toxic catalyst for the overall transformation, which involves Knoevenagel condensation, cyclization, and carbene Si?H bond insertion. The process takes place with high atom economy in the absence of organic solvents and shows a broad scope. This reaction is also applicable to the functionalization of oligomeric siloxanes.     

The kinetics of the chlorine isotopic exchange between chloride ion at O,O-diarylphosphorochloridates or O,O-diarylphosphorochloridothionates

The kinetics of the chlorine isotopic exchange reaction between tetraethylammonium chloride-³⁶Cl and O,O-diarylphosphorochloridates (p-RC₆H₄O)₂POCl or O,O-diarylphosphorochloridothionates (p-RC₆H₄O)₂PSCl has been studied in acetonitrile solution. Good Hammett's correlations of the rate constants with Taft's σ⁰ constants were obtained. The values of the reaction constants ρ were found identical for phosphoryl and thiophosphoryl compounds. In comparison with oxygen in the phosphoryl group, the sulfur atom exhibits an electron-donating effect (Δσ⁰ ～ 0.80). No correlation has been found for the enthalpy and entropy of activation. The effect of the substituents aryloxy groups, oxygen, or sulfur atoms in the phosphoryl group on the kinetics of the S_N2-P reaction is discussed. The reactivity of the investigated compounds is determined by the extent of the positive charge localized on the phosphorus atom. The positive charge is formed by the direct interactions of the substituents with the reaction center and the indirect–intramolecular interactions revealed in the structure of the compound.     

Embedding Hydrogen Atom Transfer Moieties in Covalent Organic Frameworks for Efficient Photocatalytic C−H Functionalization

Covalent organic frameworks (COFs) have emerged as efficient heterogeneous photocatalysts for a wide range of relatively simple organic reactions, whereas their application in complex organic transformations, like site-selective functionalization of unactivated C−H bonds, is underexplored, which can be mainly attributed to the lack of highly active organophotocatalytic cores. Herein through bonding oxygen atoms at the N-terminus of quinolines in nonsubstituted quinoline-linked COFs (NQ−COFs), we successfully realized the embedding of active hydrogen atom transfer (HAT) moieties into the skeleton of COFs. This novel designed COF (NQ−COF_E5−O), serving as both an excellent photosensitizer and HAT catalyst, exhibited much higher efficiency in C−H functionalization than the corresponding NQ−COF_E5. Specially, we evaluated the photocatalytic performance of NQ−COF_E5−O on ten different substrates, including quinolines, benzothiazole, and benzoxazole, all of which were transferred to desired products in moderate to high yields (up to 93 %). Furthermore, the as-synthesized NQ−COF_E5−O displayed excellent photostability and could be reused with negligible loss of activity for five catalytic cycles.     

Syntheses of α‐Amino Acids by Using CO2 as a C1 Source

The incorporation of CO₂ into organic compounds is currently one of the most active research topics in organic chemistry, because CO₂ is an abundant, inexpensive, nontoxic, and renewable C1 source. However, CO₂ is also a thermodynamically stable and kinetically inert gaseous compound, and as such, special strategies are required to activate CO₂ and incorporate it into organic compounds. In particular, because the carbon atom adjacent to the nitrogen atom of amine derivatives is positively charged, umpolung carboxylation, which is a difficult chemical process, should be considered for the production of α‐amino acids by using CO₂. In this Minireview, we summarize recent synthetic methods for α‐amino acids that use CO₂ as a carboxylic acid unit.     

Staudinger reaction at in-bridgehead positions of phosphorus macrobicyclic compounds

Reaction of in,in-phosphite 1 with thiophosphoryl azide 2 affords in,in-dithiophosphate 3, in,in-thiophosphate-imidophosphate 4, and in,in-phosphite-imidophosphate 5. Compounds 4 and 5 are the first examples of the modification of in-bridgehead positions in macrobicyclic compounds with groups larger than methyl. The benzaldehyde arms of the in-substituent in 4 and 5 jut out of the cage bars. In 4 they are trapped between the macrocyclic arms to give the NMR spectra of a Cs-symmetric solution-state structure. In contrast, in 5 the benzaldehyde arms can move between the gaps of the cage. This results in 1H and 13C NMR spectra which are consistent for a compound with C3v symmetry. In,out-diimidophosphate 7 is obtained in moderate yield by reaction of in,out-phosphite 6 with thiophosphoryl azide 2. Its in-benzaldehyde moieties are not fixed between the cage arms, but can freely move from one gap to the next as is indicated by NMR measurements.     

Chemodivergent Staudinger Reactions of Secondary Phosphine Oxides and Application to the Total Synthesis of LL–D05139β Potassium Salt

Unprecedented Staudinger reaction modes of secondary phosphine oxides (SPO) and organic azides are herein disclosed. By the application of various additives, selective nitrogen atom exclusion from the azide group has been achieved. Chlorotrimethylsilane mediates a stereoretentive Staudinger reaction with a 2-N exclusion which provides a valuable method for the synthesis of phosphinic amides and can be considered complementary to the stereoinvertive Atherton–Todd reaction. Alternatively, a 1-N exclusion pathway is promoted by acetic acid to provide the corresponding diazo compound. The effectiveness of this protocol has been further demonstrated by the total synthesis of the diazo-containing natural product LL-D05139β, which was prepared as a potassium salt for the first time in 6 steps and 26.5 % overall yield.     

Selective functionalization of crown ethers via arene chromium tricarbonyl complexes

The crown ethers dibenzo-16-crown-4 and dibenzo-18-crown-5 and a diaryl polyether were complexed by the chromium tricarbonyl group for the purpose of selective functionalization. This complexation did indeed permit exclusive functionalization of the complexed ring. CHO and CH₂OH functionalities were introduced ortho to the ether group. It was noted that the nature of the two ether chains had a strong influence on the regioselectivity of the functionalization, which occurred preferentially on the side with the polyether chain. Photochemical decomplexation produced functionalized organic crown ethers.     

Visible-Light-Mediated Click Chemistry for Highly Regioselective Azide–Alkyne Cycloaddition by a Photoredox Electron-Transfer Strategy

Click chemistry focuses on the development of highly selective reactions using simple precursors for the exquisite synthesis of molecules. Undisputedly, the Cu^I-catalyzed azide–alkyne cycloaddition (CuAAC) is one of the most valuable examples of click chemistry, but it suffers from some limitations as it requires additional reducing agents and ligands as well as cytotoxic copper. Here, we demonstrate a novel strategy for the azide–alkyne cycloaddition reaction that involves a photoredox electron-transfer radical mechanism instead of the traditional metal-catalyzed coordination process. This newly developed photocatalyzed azide–alkyne cycloaddition reaction can be performed under mild conditions at room temperature in the presence of air and visible light and shows good functional group tolerance, excellent atom economy, high yields of up to 99 %, and absolute regioselectivity, affording a variety of 1,4-disubstituted 1,2,3-triazole derivatives, including bioactive molecules and pharmaceuticals. The use of a recyclable photocatalyst, solar energy, and water as solvent makes this photocatalytic system sustainable and environmentally friendly. Moreover, the azide–alkyne cycloaddition reaction could be photocatalyzed in the presence of a metal-free catalyst with excellent regioselectivity, which represents an important development for click chemistry and should find versatile applications in organic synthesis, chemical biology, and materials science.