Browsing by Subject "visible light"

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    Base- and Visible Light-Promoted Activation of Aryl Halides under Transition-Metal-Free Conditions: Applications and Mechanistic Studies
    (2021-12) Pan, Lei; Laulhé, Sébastien; Long, Eric; O'Donnell, Martin J.; Dai, Mingji
    Aromatic rings are universal motifs in natural products, pharmaceuticals, agrochemicals, and wide variety of organic materials. Aromatic halides are widely used as synthetic precursors in all these applications. Therefore, tremendous effort has been devoted to activate aryl halides in the past decades. The common methods to activate aryl halides require the use of transition- metals either in the form of Grignard reagents or through the use of transition-metal catalysis. Over the past decade, photoredox catalysis has attracted significant attention as a cogent tool to develop greener synthetic processes and enable new molecular activation pathways under mild conditions. The most common of these approaches uses a photoredox/nickel dual catalytic cycle. While this technology has greatly expanded the toolbox of organic chemists, this method still requires expensive rare-metal-based catalyts. Herein, we present a series of visible light-induced methods that are transition-metal-free. These new base-promoted transformations and their mechanistic work will be discussed in the following order: We will first present our discovery that the dimsyl anion enables visible-light-promoted charge transfer in cross-coupling reactions of aryl halides. This work was applied to the synthesis of unsymmetrical diaryl chalcogenides. This method has a broad scope and functional group tolerance. An electron-donor-acceptor (EDA) complex between a dimsyl anion and the aryl halide is formed during the reaction and explains the observed aryl radical reactivity observed. Then, a visible-light-induced borylation and phosphorylation of aryl halides under mild conditions was developed. Inspired by the mechanistic breakthroughs observed in the previous work. The mechanism of this reaction also involves an aryl radical that is presumed to be formed also via an EDA complex. In other work, a photo-induced phosphonation of ArI using N,N- diisopropylethylamine (DIPEA) and trialkyl phosphites was developed. This method uses very mild conditions, which allowed the preparation a wide variety of functionalized aromatic phosphonates derivatives, including natural products and medicinal compounds. Finally, a photochemical amination of amides was developed via a C(sp 3 )–H bond functionalization process under visible light irradiation. This reaction showed good functional group compatibility without the use of external radical initiators, strong oxidants, or heat source. An EDA complex between N-bromophthalimide and LiOtBu is formed during the reaction.
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    Improving gelation efficiency and cytocompatibility of visible light polymerized thiol-norbornene hydrogels via addition of soluble tyrosine
    (RSC, 2017-03) Shih, Han; Liu, Hung-Yi; Lin, Chien-Chi; Biomedical Engineering, School of Engineering and Technology
    Hydrogels immobilized with biomimetic peptides have been used widely for tissue engineering and drug delivery applications. Photopolymerization has been among the most commonly used techniques to fabricate peptide-immobilized hydrogels as it offers rapid and robust peptide immobilization within a crosslinked hydrogel network. Both chain-growth and step-growth photopolymerizations can be used to immobilize peptides within covalently crosslinked hydrogels. A previously developed visible light mediated step-growth thiol-norbornene gelation scheme has demonstrated efficient crosslinking of hydrogels composed of an inert poly(ethylene glycol)-norbornene (PEGNB) macromer and a small molecular weight bis-thiol linker, such as dithiothreitol (DTT). Compared with conventional visible light mediated chain-polymerizations where multiple initiator components are required, step-growth photopolymerized thiol-norbornene hydrogels are more cytocompatible for the in situ encapsulation of radical sensitive cells (e.g., pancreatic β-cells). This contribution explored visible light based crosslinking of various bis-cysteine containing peptides with macromer 8-arm PEGNB to form biomimetic hydrogels suitable for in situ cell encapsulation. It was found that the addition of soluble tyrosine during polymerization not only significantly accelerated gelation, but also improved the crosslinking efficiency of PEG-peptide hydrogels as evidenced by a decreased gel point and enhanced gel modulus. In addition, soluble tyrosine drastically enhanced the cytocompatibility of the resulting PEG-peptide hydrogels, as demonstrated by in situ encapsulation and culture of pancreatic MIN6 β-cells. This visible light based thiol-norbornene crosslinking mechanism provides an attractive gelation method for preparing cytocompatible PEG-peptide hydrogels for tissue engineering applications.