A Three‐Membered Cyclic Phosphasilene

Small unsaturated phosphacycles are versatile reagents owing to their strain and the added functionality of the double bond and the phosphorus lone pair. Herein we report the synthesis and isolation of the smallest possible cyclic phosphasilene as a stable adduct with an N‐heterocyclic carbene (NHC). First reactivity studies show a) that the PSi₂ ring is a competent ligand to the Fe(CO)₄ fragment via the phosphorus lone pair and b) that the abstraction of the NHC by BPh₃ results in the rapid head‐to‐head or head‐to‐tail dimerization of the PSi₂ unit. The relatively facile NHC cleavage indicates that the P=Si double bond is available for further manipulation.     

BINOL Phosphoric Acid‐Catalyzed Asymmetric Mannich Reaction of Cyclic N‐Acyl Ketimines with Cyclic Enones

A highly enantio‐ and diastereoselective Mannich reaction of cyclic N‐acyl ketimines generated in situ from 3‐hydroxyisoindolin‐1‐ones with cyclic enones has been accomplished using a chiral phosphoric acid catalyst to afford the chiral isoindalinone derivatives in high yields with excellent enantioselectivities (upto 97 % ee). This is the first report on the synthesis of chiral isoindolin‐1‐ones bearing adjacent quaternary and tertiary stereogenic centers.     

Cyclic Polymers by Ring‐Closure Strategies

The preparation of cyclic macromolecules has always represented a challenging task for polymer science, mainly because of difficulties in connecting chain extremities together. Initiated by the pioneering studies of Jacobson and Stockmayer, preparative pathways to cyclic polymers have been considerably improved within the last two decades thanks to the advent of both controlled polymerizations and efficient coupling reactions in organic chemistry. This Review aims to provide a critical up‐to‐date overview and illustrate the considerable efforts that have been made in the past few years to improve the availability of macrocycles for industrial and academic investigations through the use of the ring‐closure approach. Particular attention is paid to methods for the preparation of monocycles over more complex architectures, since the latter are usually derived from the former.     

Dual Nickel‐ and Photoredox‐Catalyzed Enantioselective Desymmetrization of Cyclic meso‐Anhydrides

The enantioselective desymmetrization of cyclic meso‐anhydrides with benzyl trifluoroborates under nickel‐photoredox catalysis is described. The reaction tolerates a variety of sterically and electronically different trifluoroborates, as well as structurally unique cyclic anhydrides. The trans isomer of the keto‐acid products is also observed at varying levels dependent on the trifluoroborate identity and relative catalyst loading. A mechanism involving decarbonylation and Ni−C bond homolysis of a Ni^II adduct is proposed. This feature allows access to a trans keto‐acid as the major product in high enantioselectivity from a cis meso anhydride.     

Photocycloaddition of Six‐Membered Cyclic Enones to Propen‐2‐yl Isocyanate

On irradiation in the presence of propen‐2‐yl isocyanate ( 4 ), six‐membered cyclic enones 3 are converted into regio‐ and stereoisomeric mixtures of [2+2] cycloadducts 5 – 10 ; the preferentially formed HT products, 5 – 8 , can be converted into the corresponding bicyclic amines by acid hydrolysis, whereas, under these conditions, the regioisomeric HH‐isocyanato derivatives undergo a retro‐Mannich reaction.     

Cyclic Hydroboration – The Structure of Perhydro‐9b‐boraphenalenes

Complete hydroboration of cyclododecatrienes was reported to give two isomers, depending on conditions. The assignment of their structure had been attempted without unequivocal proofs. We have now used NMR spectroscopy (¹¹B, ¹³C, ¹⁵N and ²³Na NMR) to study the sodium amides of these two polycyclic boranes. In addition, one of the isomeric borates could be crystallized, and the X‐ray analysis revealed a cis‐,cis‐,trans configuration of the six‐membered rings reversing the original structural assignment.     

Cyclic alkoxyamines for nitroxide‐mediated radical polymerization

A 5‐membered cyclic alkoxyamine and a 17‐membered cyclic alkoxyamine were synthesized and used in the polymerization of styrene. Polymerizations using the 5‐membered cyclic alkoxyamine resulted in polymers with uncontrolled molecular weights and high polydispersities. Polymerizations using the 17‐membered cyclic alkoxyamine produced oligomeric polymers in which multiple polymer chains are linked through NO‐C bonds. EPR homolysis experiments revealed that the 5‐membered cyclic alkoxyamine does not dissociate to form a nitroxide species, even at temperatures as high as 403 K. In contrast, the 17‐membered cyclic alkoxyamine does dissociate to form nitroxide, but the rate of dissociation is slower than that of parent acyclic alkoxyamine 2,2,5‐trimethyl‐3‐(1‐phenylethoxy)‐4‐phenyl‐3‐azahexane. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 8049–8069, 2008     

Versatile Reactivity of Cyclic 1,2‐Dimethylhydrazinodiphosphines

A versatile reactivity from the cage compound P(NMeNMe)₃P is presented. The Staudinger reaction with Me₃SiN₃ is carried out. The crystal structure of the compound issued from the reaction on both sides (Me₃SiN=P(NMeNMe)₃P=NSiMe₃) is reported. When the reaction occurs on only one side, the remaining free phosphorus atom is complexed with RuCl₂(p‐cymene). P(NMeNMe)₃P reacts with PCl₃, leading to the heterocyclic compound ClP(NMeNMe)₂PCl. This heterocycle also displays a versatile reactivity. Substitution reaction with HNiPr₂ leads to iPr₂NP(NMeNMe)₂PNiPr₂. Very complex ¹H and ¹³C NMR spectra suggest that the cis isomer is the largely major isomer of this compound. The cis structure is confirmed by X‐ray diffraction. Besides the reaction on the P–Cl functions, the reaction on the lone pair of ClP(NMeNMe)₂PCl is carried out, leading to the complex (p‐cymene)Cl₂RuPCl(NMeNMe)₂ClPRuCl₂(p‐cymene). Characterization of this compound by X‐ray diffraction displays a cis isomer for this compound also.     

A Cyclic Hydroxylaminato‐aminoxide Complex of Gallium

The reaction of HON(tBu)CH₂CH₂N(tBu)OH with tri‐tert‐butyl gallium affords a hydroxylaminato complex of the formula [tBu₂Ga{ON(tBu)CH₂CH₂N(H)(tBu)O}], which contains a monoanionic bishydroxylaminato ligand with one anionic and one neutral, but tautomeric aminoxide end, both linked to gallium by their oxygen atoms leading to a seven‐membered ring. The compound was characterised by elemental analysis, ¹H and ¹³C NMR and determination of its crystal structure.     

Cyclic peptide–polymer conjugates: Grafting‐to vs grafting‐from

A systematic comparison between the grafting‐to (convergent) and grafting‐from (divergent) synthetic routes leading to cyclic peptide–polymer conjugates is described. The reversible addition–fragmentation chain transfer (RAFT) process was used to control the polymerizations and the couplings between cyclic peptide and polymer or RAFT agent were performed using N‐hydroxysuccinimide (NHS) active ester ligation. The kinetics of polymerization and polymer conjugation to cyclic peptides were studied for both grafting‐to and grafting‐from synthetic routes, using N‐acryloyl morpholine as a model monomer. The cyclic peptide chain transfer agent was able to mediate polymerization as efficiently as a traditional RAFT agent, reaching high conversion in the same time scale while maintaining excellent control over the molecular weight distribution. The conjugation of polymers to cyclic peptides proceeded to high conversion, and the nature of the carbon at the α‐position to the NHS group was found to play a crucial role in the reaction kinetics. The study was extended to a wider range of monomers, including hydrophilic and temperature responsive acrylamides, hydrophilic and hydrophobic acrylates, and hydrophobic and pH responsive methacrylates. Both approaches lead to similar peptide–polymer conjugates in most cases, while some exceptions highlight the advantages of one or the other method, thereby demonstrating their complementarity. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1003–1011     

Photocyclodimers of ‘Made‐to‐Measure’ Seven‐ and Six‐Membered Cyclic Enones

On irradiation (350 nm) in benzene as solvent, dioxepinone 6 and benzoxepinone 7 afford quantitatively mixtures of two diastereoisomeric head‐to‐head dimers, respectively. In both cases, on contact with SiO₂ the minor dimer containing trans‐ring fusions undergoes spontaneous isomerization to the (major) cis‐transoid‐cis diastereoisomer. In contrast, thiopyranone 8 is converted selectively, but in very low yield, to dimer 13 .     

ortho‐Phenylene‐Bridged Cyclic Oligopyrroles: Conformational Flexibilities and Optical Properties

ortho‐Phenylene‐bridged cyclic trimeric oligopyrrole C3 and hexameric oligopyrrole C6 were synthesized by Suzuki–Miyaura coupling reactions. The twisted structures of C3 and C6 were unambiguously revealed by X‐ray diffraction analysis. The optical properties of these cyclic oligopyrroles were compared with linear oligopyrrole L3 and cyclic tetramer C4 . The cyclic oligopyrroles exhibited large Stokes shifts and blue fluorescence with high quantum yields in solution and in the solid state. In addition, selective N‐methylation and N‐tolylation of C3 were used to tune the optical and electrochemical properties by changing the molecular twists and conformational flexibilities. Throughout these studies, the structure–property relationship of these cyclic strained oligopyrroles has been illustrated as an interesting molecular motif for novel cyclic π‐conjugated systems.     

Water‐Soluble and pH‐Responsive Polymeric Nanotubes from Cyclic Peptide Templates

Water‐soluble organic nanotubes were prepared by convergently conjugating polymers of hydroxyethyl acrylate (HEA) and acrylic acid (AA) to self‐assembling cyclic octapeptides of alternating D and L chirality. The structure of the self‐assembled tubes was characterised in a number of polar solvents, and notably water, by using light scattering, TEM and small angle neutron scattering (SANS) techniques. In addition, the self‐assembly into tubes could be controlled by exploiting the pH responsiveness of acrylic acid polymers.     

Photoinduced Formation of an Azobenzene‐Based CD‐Active Supramolecular Cyclic Dimer

A series of new photo‐responsive amino acid‐derived azobenzenedicarboxylic acid derivatives (S)‐ 1 a – e were synthesized. Compound (S)‐ 1 a in the trans form exhibited no circular dichroism (CD) signal in DMF under ambient conditions, whereas intense Cotton effects were observed upon UV irradiation, indicating the formation of a chiral supramolecular structure in the cis form. The CD signals disappeared when trifluoroacetic acid (TFA) was added to the solution. The ester counterpart [(S)‐ 1 a′ ] showed no CD signal. Hydrogen bonding between the carboxy groups seemed necessary for constructing the supramolecular structure. The kinetic studies of cis to trans isomerization of (S)‐ 1 a demonstrated that the formation of a chiral supramolecule enhances the stability of the cis‐azobenzene structure. The ESI mass spectrum of stilbenedicarboxylic acid (S)‐ 4 , an analogue of (S)‐ 1 b , confirmed the formation of a dimer. A theoretical CD study revealed that (S)‐ 1 a in the cis form should be present as a cyclic chiral dimer.