Photodegradable transient bilayered poly(phthalaldehyde) with improved shelf life

Metastable poly(phthalaldehyde) (PPHA) can be triggered to depolymerize under visible light by incorporation of photosensitive compounds, such as a photoacid generator (PAG), which can generate a strong acid in situ. However, photosensitive compounds can be thermally unstable and have limited shelf life, causing inadvertent device triggering. It can also be difficult to fabricate components that are photosensitive because special lighting conditions are needed. In this paper, nonphotosensitive PPHA films were formed and made photosensitive at the point of use. This improved the material shelf life and manufacturability by adding a second, PAG‐containing layer to the original nonphotosensitive layer at an optimal point before use. The catalytic photoacid was generated rapidly by exposure of the PAG‐containing layer to radiation. Depolymerization of PPHA via the acid catalyst was followed by diffusion of the acid into the nonphotosensitive layer causing it to depolymerize. Diffusion of the photoacid into the nonphotosensitive medium was quantified at various temperatures. Photoacid diffusion in a liquid, moving‐front caused depolymerization of the nonphotosensitive PPHA layer. The fabricated bilayer structure allowed for better stability of the structural material using PPHA while still achieving transience.     

Direct Acyl Substitution of Carboxylic Acids: A Chemoselective O‐ to N‐Acyl Migration in the Traceless Staudinger Ligation

A chlorophosphite‐modified, Staudinger‐like acylation of azides involving a highly chemoselective, direct nucleophilic acyl substitution of carboxylic acids is described. The reaction provides the corresponding amides with analytical purity in 32–97 % yield after a simple aqueous workup without the need for a pre‐activation step. The use of chlorophosphites as dual carboxylic acid–azide activating agents enables the formation of acyl C? N bonds in the presence of a wide range of nucleophilic and electrophilic functional groups, including amines, alcohols, amides, aldehydes, and ketones. The coupling of carboxylic acids and azides for the formation of alkyl amides, sulfonyl amides, lactams, and dipeptides is described.     

General One‐Pot Reductive gem‐Bis‐Alkylation of Tertiary Lactams/Amides: Rapid Construction of 1‐Azaspirocycles and Formal Total Synthesis of (±)‐Cephalotaxine

Amides are a class of highly stable and readily available compounds. The amide functional group constitutes a class of powerful directing/activating and protecting group for C? C bond formation. Tertiary tert‐alkylamine, including 1‐azaspirocycle is a key structural feature found in many bioactive natural products and pharmaceuticals. The transformation of amides into tert‐alkylamines generally requires several steps. In this paper, we report the full details of the first general method for the direct transformation of tertiary lactams/amides into tert‐alkylamines. The method is based on in situ activation of amide with triflic anhydride/2,6‐di‐tert‐butyl‐4‐methylpyridine (DTBMP), followed by successive addition of two organometallic reagents of the same or different kinds to form two C? C bonds. Both alkyl and functionalized organometallic reagents and enolates can be used as the nucleophiles. The method displayed excellent 1,2‐ and good 1,3‐asymmetric induction. Construction of 1‐azaspirocycles from lactams required only two steps or even one‐step by direct spiroannelation of lactams. The power of the method was demonstrated by a concise formal total synthesis of racemic cephalotaxine.     

Nickel‐Catalyzed N‐Arylation of Primary Amides and Lactams with Activated (Hetero)aryl Electrophiles

The first nickel‐catalyzed N‐arylation of amides with (hetero)aryl (pseudo)halides is reported, enabled by use of the air‐stable pre‐catalyst (PAd‐DalPhos)Ni(o‐tolyl)Cl ( C1 ). A range of structurally diverse primary amides and lactams were cross‐coupled successfully with activated (hetero)aryl chloride, bromide, triflate, tosylate, mesylate, and sulfamate electrophiles.     

Reactive intermediates in the initiation step of the photo‐oxidation of MDMO‐PPV

Because oxygen cannot be fully eliminated from organic solar cells, the occurrence of oxidative photo‐degradation of the device in operating conditions has to be considered. Polyphenylene‐vinylene‐based photovoltaic devices have a short lifetime that currently limits their applications. In this article, we focus on various transient species that are potentially involved in the initiation step of the photo‐oxidation of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylenevinylene] (MDMO‐PPV). The role of the transient species was investigated by a combination of quenching and sensitization experiments. Complementary information was obtained by transient absorption spectroscopy. Results evidenced the fact that ¹O₂ was not the principal reactive intermediate involved in the photo‐oxidation of MDMO‐PPV. This result was in contradiction with previous reports. It was shown that the MDMO‐PPV^?+ radical cation was generated after excitation. The presence of oxygen and the photo‐aging favored the formation of the radical cation, suggesting that oxygen and photoproducts act as electron acceptors. The charged radicals formed are likely to evolve and give radicals that initiate the oxidation of the polymer by abstraction of the labile hydrogen in α position of the ether function and by addition on the double bonds. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6044–6052, 2009     

Zinc‐Catalyzed Depolymerization of End‐of‐Life Polysiloxanes

Polymers occupy an important role in our current society. Besides their great success, an issue is the accumulation of huge amounts of end‐of‐life polymers. Currently, the waste management is based primarily on landfills, thermal recycling, and downcycling. Notably, only a small portion of end‐of‐life materials is recycled by depolymerization, which refers to the creation of synthetic precursors that can be polymerized to new polymers to close the cycle. Widely used polymers in modern times are silicones (polysiloxanes), the intrinsic properties of which make their depolymerization demanding; only a few high‐temperature or less environmentally friendly processes have been reported. In this regard, we have established an efficient low‐temperature protocol for the depolymerization of silicones with benzoyl fluoride in the presence of cheap zinc salts as precatalysts to yield defined products. Notably, the products can be useful synthetic precursors for the preparation of new polymers, so that an overall recycling process is feasible.     

The Photoredox‐Catalyzed Meerwein Addition Reaction: Intermolecular Amino‐Arylation of Alkenes

A variety of amides are efficiently accessible under mild conditions by intermolecular amino‐arylation using a photo Meerwein addition with visible light. The reaction has a broad substrate scope, tolerates a large range of functional groups, and was applied to the synthesis of a 3‐aryl‐3,4‐dihydroisoquinoline.     

Transition‐Metal‐Free Reductive Functionalization of Tertiary Carboxamides and Lactams for α‐Branched Amine Synthesis

A new method for the synthesis of α‐branched amines by reductive functionalization of tertiary carboxamides and lactams is described. The process relies on the efficient and controlled reduction of tertiary amides by a sodium hydride/sodium iodide composite, in situ treatment of the resulting anionic hemiaminal with trimethylsilyl chloride and subsequent coupling with nucleophilic reagents including Grignard reagents and tetrabutylammonium cyanide. The new method exhibits broad functional‐group compatibility, operates under transition‐metal‐free reaction conditions, and is suitable for various synthetic applications on both sub‐millimole and on multigram scales.     

Synthesis and characterization of novel photosensitive polyimide based on 5‐(2,5‐dioxotetrahydrofuryl)‐3‐methyl‐3‐cyclohexene‐1,2‐dicarboxylic anhydride

A negative type photosensitive polyimide with alicyclic moiety (NPI) was synthesized from 5‐(2,5‐dioxotetrahydrofuryl)‐3‐methyl‐3‐cyclohexene‐1,2‐dicarboxylic anhydride and 4,4‐diaminobenzophenone by one‐step polymerization in m‐cresol. Properties of the polyimides were characterized and a photo‐crosslinking mechanism was investigated using DEPT ¹³C‐NMR and FT‐IR spectroscopy. The negative polyimide showed good photosensitivity on exposure to UV light from a mercury xenon lamp. The polyimide showed remarkable solubility difference after photo‐ irradiation with an exposure dose of 500 mJ/cm². The resulting negative pattern of the photo‐cured NPI exhibited 10 μm resolution. Glass transition temperature of the photo‐crosslinked polyimide was about 307°C, which increased by 10°C compared to that of the polyimide before UV exposure. Transmittance of NPI after photo‐irradiation was about 87% at 500 nm. Copyright © 2004 John Wiley & Sons, Ltd.     

Copper Catalyzed C−H Activation

Activation of C?H bonds and their application in cross coupling chemistry has received a wider interest in recent years. The conventional strategy in cross coupling reaction involves the pre‐functionalization step of coupling reactants such as organic halides, pseudo‐halides and organometallic reagents. The C?H activation facilitates a simple and straight forward approach devoid of pre‐functionalization step. This approach also addresses the environmental and economical issues involved in several chemical reactions. In this account, we have reported C?H bond activation of small organic molecules, for example, formamide C?H bond can be activated and coupled with β‐dicarbonyl or 2‐carbonyl substituted phenols under oxidative conditions to yield carbamates using inexpensive copper catalysts. Phenyl carbamates were successfully synthesized in moderate to good yields by cross dehydrogenative coupling (CDC) of phenols with formamides using copper catalysts in presence of a ligand. We have also prepared unsymmetrical urea derivatives by oxidative cross coupling of formamides with amines using copper catalysts. Synthesis of N,N‐dimethyl substituted amides, 5‐substituted‐γ‐lactams and α‐acyloxy ethers was carried out from carboxylic acids using recyclable CuO nanoparticles. Copper nanoparticles afforded N‐aryl‐γ‐amino‐γ‐lactams by oxidative coupling of aromatic amines with 2‐pyrrolidinone. Reusable transition metal HT‐derived oxide catalyst was used for the synthesis of N,N‐dimethyl substituted amides by the oxidative cross‐coupling of carboxylic acids and substituted benzaldehydes. Overview of our work in this area is summarized.     

Carbamoyl Radical‐Mediated Synthesis and Semipinacol Rearrangement of β‐Lactam Diols

In an approach to the biologically important 6‐azabicyclo[3.2.1]octane ring system, the scope of the tandem 4‐exo‐trig carbamoyl radical cyclization—dithiocarbamate group transfer reaction to ring‐fused β‐lactams is evaluated. β‐Lactams fused to five‐, six‐, and seven‐membered rings are prepared in good to excellent yield, and with moderate to complete control at the newly formed dithiocarbamate stereocentre. No cyclization is observed with an additional methyl substituent on the terminus of the double bond. Elimination of the dithiocarbamate group gives α,β‐ or β,γ‐unsaturated lactams depending on both the methodology employed (base‐mediated or thermal) and the nature of the carbocycle fused to the β‐lactam. Fused β‐lactam diols, obtained from catalytic OsO₄‐mediated dihydroxylation of α,β‐unsaturated β‐lactams, undergo semipinacol rearrangement via the corresponding cyclic sulfite or phosphorane to give keto‐bridged bicyclic amides by exclusive N‐acyl group migration. A monocyclic β‐lactam diol undergoes Appel reaction at a primary alcohol in preference to semipinacol rearrangement. Preliminary investigations into the chemo‐ and stereoselective manipulation of the two carbonyl groups present in a representative 7,8‐dioxo‐6‐azabicyclo[3.2.1]octane rearrangement product are also reported.     

Factors Influencing the Course of the Macrocyclization of α,ω‐Diamines with Esters of α,ω‐Dicarboxylic Acids

The efficient synthesis of eight new macrocyclic amides (lactams) via reaction of diesters with diamines under normal dilution conditions is described. The role of intermolecular H‐bond formation and steric hindrance is discussed based on ¹H‐ and ¹⁵N‐NMR studies of appropriate model compounds. Principles for the optimal choice of esters that can be efficiently transformed into diamides have been developed.     

Dimeric Manganese‐Catalyzed Hydroarylation and Hydroalkenylation of Unsaturated Amides

An unprecedented Mn(I)‐catalyzed selective hydroarylation and hydroalkenylation of unsaturated amides with commercially available organic boronic acids is reported. Alkenyl boronic acids have been successfully employed for the first time in Mn(I)‐catalyzed carbon–carbon bond formation. A wide array of β‐alkenylated amide products can be obtained in moderate to good yields, which offers practical access to five‐ and six‐membered lactams. This protocol has predictable regio‐ and chemoselectivity, excellent functional group compatibility and ease of operation in air, representing a significant step‐forward towards manganese‐catalyzed C?C coupling.     

Stimuli‐Responsive Circularly Polarized Organic Ultralong Room Temperature Phosphorescence

Purely organic materials showing room temperature phosphorescence (RTP) and ultralong RTP (OURTP) have recently attracted much attention. However, it is challenging to integrate circularly polarized luminescence (CPL) into RTP/OURTP. Here, we show a strategy to realize CPL‐active OURTP (CP‐OURTP) by binding an achiral phosphor group directly to the chiral center of an ester chain. Engineering of this flexible chiral chain enables efficient chirality transfer to carbazole aggregates, resulting in strong CP‐OURTP with a lifetime of over 0.6 s and dissymmetry factor of 2.3×10^?3 after the conformation regulation upon photo‐activation. The realized CP‐OURTP is thus stable at room temperature but can be deactivated quickly at 50 °C to CP‐RTP with high CPL stability during the photo‐activation/thermal‐deactivation cycles. Based on this extraordinary photo/thermal‐responsive and highly reversible CP‐OURTP/RTP, a CPL‐featured lifetime‐encrypted combinational logic device has been successfully established.     

A novel photo‐acid generator bound molecular glass resist with a single protecting group

A novel photo‐acid generator (PAG) bound molecular glass photoresist with a single protecting group has been developed as a promising resist material for use in microelectronics. This single component molecular resist was prepared in four steps starting from 9,9‐bis(4‐hydroxyphenyl)fluorene. The single component molecular resist exhibited good thermal properties, such as a 10% weight loss temperature of 200 °C and a glass transition temperature of 91 °C. This resist showed a good sensitivity of 60 μC/cm² with e‐beam exposure (50 keV). On the other hand, the fine pattern with a half‐pitch of 50 nm in the presence of 4 wt % quencher, trioctylamine, was obtained using electron‐beam (100 keV) lithography. The LER value was 8.2 nm (3σ, 60 nm half‐pitch patterns). © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Enantioselective Hydroamidation of Enals by Trapping of a Transient Acyl Species

An enantioselective synthesis of β‐chiral amides through asymmetric and redox‐neutral hydroamidation of enals is reported. In this reaction, a chiral N‐heterocyclic carbene (NHC) catalyst reacts with enals to generate the homoenolate intermediate. Upon highly enantioselective β‐protonation through proton‐shuttle catalysis, the resulting azolium intermediate reacts with imidazole to yield the key β‐chiral acyl species. This transient intermediate provides access to diversified β‐chiral carbonyl derivatives, such as amides, hydrazides, acids, esters, and thioesters. In particular, β‐chiral amides can be prepared in excellent yield and ee (40 chiral amides, up to 95 % yield and 99 % ee). This modular strategy overcomes the challenge of disruption of the highly selective proton‐shuttling process by basic amines.     

Stereospecific 1,3‐Aminobromination of Donor–Acceptor Cyclopropanes

Sn(OTf)₂‐catalyzed 1,3‐aminobromination of donor–acceptor cyclopropanes with various sulfonyl amides or electron‐poor anilines and N ‐bromosuccinimide is reported. These experimentally straightforward reactions occurred with complete regio‐ and stereospecificity (for anilines) to give γ‐aminated α‐brominated malonic diesters in good to excellent yields (up to 98 %). These compounds served as valuable substrates for subsequent reactions to provide substituted azetidines and γ‐lactams in high yields.     

Direct Palladium‐Catalyzed β‐Arylation of Lactams

A direct and catalytic method is reported here for β‐arylation of N‐protected lactams with simple aryl iodides. The transformation is enabled by merging soft enolization of lactams, palladium‐catalyzed desaturation, Ar?X bond activation, and aryl conjugate addition. The reaction is operated under mild reaction conditions, is scalable, and is chemoselective. Application of this method to concise syntheses of pharmaceutically relevant compounds is demonstrated.     

Copper‐Catalyzed Carbonylative Coupling of Cycloalkanes and Amides

Carbonylation reactions are a most powerful method for the synthesis of carbonyl‐containing compounds. However, most known carbonylation procedures still require noble‐metal catalysts and the use of activated compounds and good nucleophiles as substrates. Herein, we developed a copper‐catalyzed carbonylative transformation of cycloalkanes and amides. Imides were prepared in good yields by carbonylation of a C(sp³)?H bond of the cycloalkane with the amides acting as weak nucleophiles. Notably, this is the first report of copper‐catalyzed carbonylative C?H activation.     

Protonation‐Assisted Conjugate Addition of Axially Chiral Enolates: Asymmetric Synthesis of Multisubstituted β‐Lactams from α‐Amino Acids

β‐Lactams with contiguous tetra‐ and trisubstituted carbon centers were prepared in a highly enantioselective manner through 4‐exo‐trig cyclization of axially chiral enolates generated from readily available α‐amino acids. Use of a weak base (metal carbonate) in a protic solvent (EtOH) is the key to the smooth production of β‐lactams. Use of the weak base is expected to generate the axially chiral enolates in a very low concentration, which undergo intramolecular conjugate addition without suffering intermolecular side reactions. Highly strained β‐lactam enolates thus formed through reversible intramolecular conjugate addition (4‐exo‐trig cyclization) of axially chiral enolates undergo prompt protonation by EtOH in the reaction media (not during the work‐up procedure) to give β‐lactams in up to 97 % ee.