Evaluating Nucleophile Byproduct Formation during Phosphine- and Amine-Promoted Thiol-Methyl Acrylate Reactions

Impact of Endcap Molecules on Temperature-Responsive Cholesteric Liquid Crystal Oligomers in Structural Color Stability and Hypsochromic Shift

Cholesteric liquid crystal oligomers are an interesting class of temperature responsive structurally colored materials. However, the role of endcap molecules in these oligomers is rather unexplored. In this work, we demonstrate the role of endcap molecules on structural color stability and hypsochromic shift in temperature-responsive cholesteric liquid crystal oligomers. First, new liquid crystal monoacrylate endcap molecules are synthesized, which are then used to synthesize various cholesteric liquid crystal oligomers. In addition, cholesteric oligomers using commercial monoacrylate endcap molecules are also prepared. It is found that the molecular weight and the polydispersity of the oligomers can be tuned by the endcapping molecules. The oligomers are used to produce reflective, structurally colored coatings. It was found that the coatings using the commercial monoacrylate lose their color and crystallize over time, most likely due to the presence of crystalline dimers. The coatings containing the newly synthesized monoacrylate endcap molecules did not exhibit this crystallization, resulting in structurally colored coatings that remained stable over time. These latter coatings possessed temperature responsive hypochromic behavior, which makes them interesting for advanced optical applications.

A Study on Thiol-Michael Addition to Semi-Synthetic Elastin-Hyaluronan Material for Electrospun Scaffolds

Thiol-Michael addition is a chemical reaction extensively used for conjugating peptides to polysaccharides with applications as biomaterials. In the present study, for designing a bioactive element in electrospun scaffolds as wound dressing material, a chemical strategy for the semi-synthesis of a hyaluronan-elastin conjugate containing an amide linker (ELAHA) was developed in the presence of tris(2-carboxyethyl)phosphine hydrochloride (TCEP ⋅ HCl). The bioconjugate was electrospun with poly-D,L-lactide (PDLLA), obtaining scaffolds with appealing characteristics in terms of morphology and cell viability of dermal fibroblast cells. For comprehending the factors influencing the efficiency of the bioconjugation reaction, thiolated amino acids were also investigated as nucleophiles toward hyaluronan decorated with Michael acceptors in the presence of TCEP ⋅ HCl through the evaluation of byproducts formation.

Click Nucleophilic Conjugate Additions to Activated Alkynes: Exploring Thiol-yne, Amino-yne, and Hydroxyl-yne Reactions from (Bio)Organic to Polymer Chemistry

The 1,4-conjugate addition reaction between activated alkynes or acetylenic Michael acceptors and nucleophiles (i.e., the nucleophilic Michael reaction) is a historically useful organic transformation. Despite its general utility, the efficiency and outcomes can vary widely and are often closely dependent upon specific reaction conditions. Nevertheless, with improvements in reaction design, including catalyst development and an expansion of the substrate scope to feature more electrophilic alkynes, many examples now present with features that are congruent with Click chemistry. Although several nucleophilic species can participate in these conjugate additions, ubiquitous nucleophiles such as thiols, amines, and alcohols are commonly employed and, consequently, among the most well developed. For many years, these conjugate additions were largely relegated to organic chemistry, but in the last few decades their use has expanded into other spheres such as bioorganic chemistry and polymer chemistry. Within these fields, they have been particularly useful for bioconjugation reactions and step-growth polymerizations, respectively, due to their excellent efficiency, orthogonality, and ambient reactivity. The reaction is expected to feature in increasingly divergent application settings as it continues to emerge as a Click reaction.

Byproducts formed During Thiol-Acrylate Reactions Promoted by Nucleophilic Aprotic Amines: Persistent or Reactive?

The nucleophile-initiated mechanism of thiol-Michael reactions naturally leads to the formation of undesired nucleophile byproducts. Three aza-Michael compounds representing nucleophile byproducts of thiol-acrylate reactions initiated by 4-dimethylaminopyridine (DMAP), 1-methylimidazole (MIM), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) have been synthesized and their reactivity in the presence of thiolate has been investigated. Spectroscopic analysis shows that each nucleophile byproduct reacts with thiolate to produce a desired thiol-acrylate product along with liberated aprotic amines DMAP, MIM, or DBU, thus demonstrating that these byproducts are reactive rather than persistent. Density functional theoretical computations support experimental observations and predict that a β-elimination mechanism is favored for converting each nucleophile byproduct into a desired thiol-acrylate product, though an S_N 2 process can be competitive (i. e. within <2.5 kcal/mol) in less polar solvents.

Decomplexation as a rate limitation in the thiol-Michael addition of N-acrylamides

The thiol-Michael addition is a popular, selective, high-yield "click" reaction utilized for applications ranging from small-molecule synthesis to polymer or surface modification. Here, we combined experimental and quantum mechanical modeling approaches using density functional theory (DFT) to examine the thiol-Michael reaction of N-allyl-N-acrylamide monomers used to prepare sequence-defined oligothioetheramides (oligoTEAs). Experimentally, the reaction was evaluated with two fluorous tagged thiols and several monomers at room temperature (22 °C and 40 °C). Using the Eyring equation, the activation energies (enthalpies) were calculated, observing a wide range of energy barriers ranging from 28 kJ mol-1 to 108 kJ mol-1 within the same alkene class. Computationally, DFT coupled with the Nudged Elastic Band method was used to calculate the entire reaction coordinate of each monomer reaction using the B97-D3 functional and a hybrid implicit-explicit methanol solvation approach. The thiol-Michael reaction is traditionally rate-limited by the propagation or chain-transfer steps. However, our test case with N-acrylamides and fluorous thiols revealed experimental and computational data produced satisfactory agreement only when we considered a previously unconsidered step that we termed "product decomplexation", which occurs as the product physically dissociates from other co-reactants after chain transfer. Five monomers were investigated to support this finding, capturing a range of functional groups varying in alkyl chain length (methyl to hexyl) and aromaticity (benzyl and ethylenephenyl). Increased substrate alkyl chain length increased activation energy, explained by the inductive effect. Aromatic ring-stacking configurations significantly impacted the activation energy and contributed to improved molecular packing density. Hydrogen-bonding between reactants increased the activation energy emphasizing the rate-limitation of the product decomplexation. Our findings begin to describe a new structure-kinetic relationship for thiol-Michael acceptors to enable further design of reactive monomers for synthetic polymers and biomaterials.