Phosphorous-Functionalized Bis(acyl)phosphane Oxides for Surface Modification

Bis(acyl)phosphide - Ambidentate Ligands for the Synthesis of Group 14 and 15 Main Group Element Compounds

The reactivity of the bis(acyl)phosphide ion [P(COR)₂ ]^- (BAP^- , R=Ph, Mes) with silicon halides SiX₄ (X=Cl, Br) and pnictogen chlorides ECl₃ (E=As, Sb and Bi) was investigated. The reaction with SiX₄ leads to the hexacoordinate silanes SiX₂ (BAP)₂ in which BAP^- is coordinated in the chelating κ² -O,O' mode, analogously to acac^- . Unexpectedly, the coordination behaviour of BAP^- differs from the one of acac^- in the interpnictogen compounds E(BAP)₃ (E=As, Sb) in which the formation of E-P bonds is favoured over κ² -O,O' chelation via the oxygen centres. Finally, the reaction of BiCl₃ with three equivalents of Na(BAP) leads to the formation of red, crystalline Bi₂ (BAP)₄ , an air stable dibismuthine, as product of a redox reaction.

Organocatalyzed Phospha-Michael Addition: A Highly Efficient Synthesis of Customized Bis(acyl)phosphane Oxide Photoinitiators

Addition of the P-H bond in bis(mesitoyl)phosphine, HP(COMes)2 (BAPH), to a wide variety of activated carbon-carbon double bonds as acceptors was investigated. While this phospha-Michael addition does not proceed in the absence of an additive or catalyst, excellent results were obtained with stoichiometric basic potassium or caesium salts. Simple amine bases can be employed in catalytic amounts, and tetramethylguanidine (TMG) in particular is an outstanding catalyst that allows the preparation of bis(acyl)phosphines, R-P(COMes)2 , under very mild conditions in excellent yields after only a short time. All phosphines RP(COMes)2 can subsequently be oxidized to the corresponding bis(acyl)phosphane oxides, RPO(COMes)2 , a substance class belonging to the most potent photoinitiators for radical polymerizations known to date. Thus, a simple and highly atom economic method has been found that allows the preparation of a broad range of photoinitiators adapted to their specific field of application even on a large scale.

Hydrostannylation of Red Phosphorus: A Convenient Route to Monophosphines

The preparation of valuable and industrially relevant organophosphorus compounds currently depends on indirect multistep procedures involving difficult-to-handle white phosphorus as a common P atom source. Herein, we report a practical and versatile method for the synthesis of a variety of monophosphorus compounds directly from the bench-stable allotrope red phosphorus (Pred ). The relatively inert Pred was productively functionalised by using the cheap and readily available radical reagent tri-n-butyltin hydride, and subsequent treatment with electrophiles yields useful P1 compounds. Remarkably, these transformations require only modest inert-atmosphere techniques and use only reagents that are inexpensive and commercially available, making this a convenient and practical methodology accessible in most laboratory settings.

Surface-Immobilized Photoinitiators for Light Induced Polymerization and Coupling Reactions

Straightforward and versatile surface modification, functionalization and coating have become a significant topic in material sciences. While physical modification suffers from severe drawbacks, such as insufficient stability, chemical induced grafting processes efficiently modify organic and inorganic materials and surfaces due to covalent linkage. These processes include the "grafting from" method, where polymer chains are directly grown from the surface in terms of a surface-initiated polymerization and the "grafting to" method where a preformed (macro)-molecule is introduced to a preliminary treated surface via a coupling reaction. Both methods require an initiating species that is immobilized at the surface and can be triggered either by heat or light, whereas light induced processes have recently received increasing interest. Therefore, a major challenge is the ongoing search for suitable anchor moieties that provide covalent linkage to the surface and include initiators for surface-initiated polymerization and coupling reactions, respectively. This review containing 205 references provides an overview on photoinitiators which are covalently coupled to different surfaces, and are utilized for subsequent photopolymerizations and photocoupling reactions. An emphasis is placed on the coupling strategies for different surfaces, including oxides, metals, and cellulosic materials, with a focus on surface coupled free radical photoinitiators (type I and type II). Furthermore, the concept of surface initiation mediated by photoiniferters (PIMP) is reviewed. Regarding controlled radical polymerization from surfaces, a large section of the paper reviews surface-tethered co-initiators, ATRP initiators, and RAFT agents. In combination with photoinitiators or photoredox catalysts, these compounds are employed for surface initiated photopolymerizations. Moreover, examples for coupled photoacids and photoacid generators are presented. Another large section of the article reviews photocoupling and photoclick techniques. Here, the focus is set on light sensitive groups, such as organic azides, tetrazoles and diazirines, which have proven useful in biochemistry, composite technology and many other fields.

A Primary Acyl Phosphine Stabilized by a Phosphonium Ylide

Primary acyl-phosphines are scarce in the literature. Here we show that the reaction of Ph₃ GePCO with the ylide Ph₃ PCH₂ proceeds to give the species Ph₃ PCHC(O)PH(GePh₃ ) 1. Deprotonation of 1 with Na[N(SiMe₃ )₂ ] generates the salt [Na(THF)₂ ][Ph₃ PCHC(O)P(GePh₃ )] 2 which provides subsequent access to the bis-germanylated acylphosphine, Ph₃ PCHC(O)P(GePh₃ )₂ 3. Alternatively, treatment of 1 with HCl in dioxane affords the primary acylphosphine Ph₃ PCHC(O)PH₂ 4. Compound 4 is a rare example of an air stable primary acyl-phosphines and the first devoid of a stabilizing heteroatom adjacent to the carbonyl fragment.

Chlorosilane-Catalyzed Coupling of Hydrogen Phosphine Oxides with Acyl Chlorides Generating Acylphosphine Oxides

We report a new method for the synthesis of acylphosphine oxides by the direct coupling of hydrogen phosphine oxides and acyl chlorides mediated by chlorosilanes. This new protocol is greener and safer, because it precludes the generation of volatile haloalkanes and the use of oxidants employed in the conventional methods. Moreover, moisture-unstable acylphosphine oxides that are difficult to prepare via the conventional methods can be generated using this new method.

Unprecedented Bifunctional Chemistry of Bis(acyl)phosphane Oxides in Aqueous and Alcoholic Media

Tailor-made photoinitiators play an important role for efficient radical polymerisations in aqueous media, especially in hydrogel manufacturing. Bis(acyl)phosphane oxides (BAPOs) are among the most active initiators. Herein, we show that they display a remarkable photochemistry in aqueous and alcoholic media: Photolysis of BAPOs in the presence of water or alcohols provides a new delocalized π-radical, which does not participate in the polymerization. It either converts into a monoacylphosphane oxide acting as a secondary photoactive species or it works as a one-electron reducing agent. Upon the electron-transfer process, it again produces a dormant photoinitiator. We have established the structure and the chemistry of this π radical using steady-state and time-resolved (CIDEP) EPR together with ESI-MS, NMR spectroscopy, and DFT calculations. Our results show that bis(acyl)phosphane oxides act as bifunctional reagents when applied in aqueous and alcoholic media.

Acylation Reactions of Dibenzo-7-phosphanorbornadiene: DFT Mechanistic Insights

Extensive DFT calculations provide deep mechanistic insights into the acylation reactions of tert-butyl dibenzo-7-phosphanobornadiene with PhCOX (X=Cl, Br, I, OTf) in CH2Cl2 solution. Such reactions are initialized by the nucleophilic P⋅⋅⋅C attack to the carbonyl group to form the acylphosphonium intermediate A+ together with X- anion, followed either by nucleophilic X-⋅⋅⋅P attack (X=Cl, Br, and I) toward A+ to eliminate anthracene or by slow rearrangement or decomposition of A+ (X=OTf). In contrast to the first case (X=Cl) that is rate-limited by the initial P⋅⋅⋅C attack, other reactions are rate-limited by the second X-⋅⋅⋅P attack for X=Br and I and even thermodynamically prevented for X=OTf, leading to isolable phosphonium salts. The rearrangement of phosphonium A+ is initiated by a P-C bond cleavage, followed either by sequential proton-shifts to form anthracenyl acylphosphonium or by deprotonation with additional base Et3N to form neutral anthracenyl acylphosphine. Our DFT results strongly support the separated acylphosphonium A+ as the key reaction intermediate that may be useful for the transfer of acylphosphenium in general.

Tetraacylstannanes as Long-Wavelength Visible-Light Photoinitiators with Intriguing Low Toxicity

The first tetraacylstannanes Sn[(CO)R]₄ (R=2,4,6‐trimethylphenyl (1 a) and 2,6‐dimethylphenyl (1 b)), a class of highly efficient Sn‐based photoinitiators, were synthesized. The formation of these derivatives was confirmed by NMR spectroscopy, mass spectrometry, and X‐ray crystallography. The UV/Vis absorption spectra of 1 a, b reveal a significant redshift of the longest wavelength absorption compared to the corresponding germanium compounds. In contrast to the known toxicity of organotin derivatives, the AMES test and cytotoxicity studies reveal intriguing low toxicity. The excellent performance of 1 as photoinitiators is demonstrated by photobleaching (UV/Vis) and NMR/CIDNP investigations, as well as photo‐DSC studies.

Recent Advances in Germanium-Based Photoinitiator Chemistry

Acylgermanes provide an outstanding photoinduced reactivity at very useful absorption wavelengths. This encouraged multidisciplinary research groups to utilize them as highly effective and non-toxic photoinitiators particularly for medical applications. In this Minireview, we present the most recent breakthroughs to synthesize acylgermanes. We also outline mechanistic aspects of photoinduced reactions of several acylgermane derivatives based on fundamental spectroscopic insights. These studies may aid future developments for tailor-made photoinitiators.

All-in-One Cellulose Nanocrystals for 3D Printing of Nanocomposite Hydrogels

Cellulose nanocrystals (CNCs) with >2000 photoactive groups on each can act as highly efficient initiators for radical polymerizations, cross-linkers, as well as covalently embedded nanofillers for nanocomposite hydrogels. This is achieved by a simple and reliable method for surface modification of CNCs with a photoactive bis(acyl)phosphane oxide derivative. Shape-persistent and free-standing 3D structured objects were printed with a mono-functional methacrylate, showing a superior swelling capacity and improved mechanical properties.

Star-shaped Polymers through Simple Wavelength-Selective Free-Radical Photopolymerization

Star-shaped polymers represent highly desired materials in nanotechnology and life sciences, including biomedical applications (e.g., diagnostic imaging, tissue engineering, and targeted drug delivery). Herein, we report a straightforward synthesis of wavelength-selective multifunctional photoinitiators (PIs) that contain a bisacylphosphane oxide (BAPO) group and an α-hydroxy ketone moiety within one molecule. By using three different wavelengths, these photoactive groups can be selectively addressed and activated, thereby allowing the synthesis of ABC-type miktoarm star polymers through a simple, highly selective, and robust free-radical polymerization method. The photochemistry of these new initiators and the feasibility of this concept were investigated in unprecedented detail by using various spectroscopic techniques.

A Simple Preparation of Photoactive Glass Surfaces Allowing Coatings via the "Grafting-from" Method

A simple and straightforward synthesis was developed to prepare the siloxy-substituted bis(acylphosphane)oxide 4-(trimethoxysilyl)butyl-3-[bis(2,4,6-trimethylbenzoyl)phosphinoyl]-2-methyl-propionate TMESI(2)-BAPO. This new photoinitiator was successfully fixed to glass surfaces. Subsequent irradiation with UV light in the presence of either a partially fluorinated acrylate or a specifically synthesized polysiloxane containing polymerizable acrylate functions allowed the generation of polymer chains which grew from the surface in an efficient radical polymerization process ("grafting-from" procedure). Durable hydrophobic surfaces were prepared which have contact angles between 93° and 95°. The silanization process with the photoinitiator and the grafting process were followed and analyzed with various techniques including high-resolution X-ray photoelectron spectroscopy.