Metal Free Azide-Alkyne Click Reaction: Role of Substituents and Heavy Atom Tunneling

Triple, Mutually Orthogonal Bioorthogonal Pairs through the Design of Electronically Activated Sulfamate-Containing Cycloalkynes

Interest in mutually exclusive pairs of bioorthogonal labeling reagents continues to drive the design of new compounds that are capable of fast and predictable reactions. The ability to easily modify S-, N-, and O-containing cyclooctynes (SNO-OCTs) enables electronic tuning of various SNO-OCTs to influence their cycloaddition rates with Type I-III dipoles. As opposed to optimizations based on just one specific dipole class, the electrophilicity of the alkynes in SNO-OCTs can be manipulated to achieve divergent reactivities and furnish mutually orthogonal dual ligation systems. Significant reaction rate enhancements of a difluorinated SNO-OCT derivative, as compared to the parent scaffold, were noted, with the second-order rate constant in cycloadditions with diazoacetamides exceeding 5.13 M-1 s-1. Computational and experimental studies were employed to inform the design of triple ligation systems that encompass three orthogonal reactivities. Finally, polar SNO-OCTs are rapidly internalized by mammalian cells and remain functional in the cytosol for live-cell labeling, highlighting their potential for diverse in vitro and in vivo applications.

Electrostatic Activation of Tetrazoles

Photoactivation of tetrazoles to form nitrile imines primed for 1,3-dipolar cycloaddition reactions is of widespread utility in chemistry. In contrast, the corresponding thermal reactions usually possess prohibitively high barriers and have garnered significantly less attention. Here, computational chemistry at the M06-2X/6-31+G(d,p) level of theory with SMD solvent corrections is used to show that these thermal activation barriers can be significantly reduced through the use of nonconjugated charged functional groups (CFGs). For 2,5-dimethyl-tetrazole, a positive CFG on the N-methyl (2-position) lowers the fragmentation barrier by around 80 kJ mol^-1 in the gas phase, while a negative charge has a smaller opposite effect. These CFG effects remain significant even in polar solvents, with barrier lowering on the order of 30 kJ mol^-1 in dimethyl sulfoxide and acetonitrile. In practical terms, the positive CFG decreases the fragmentation half-life of 2,5-dimethyl-tetrazole in refluxing o-xylene from 300,000 years to 1 week. While the resulting nitrile imine is stabilized, its subsequent 1,3-cycloaddition with N-methylmaleimide remains highly facile. Electrostatic effects on a range of 2-phenyl-5-methyltetrazoles, 2-methyl-5-phenyl-tetrazoles, and 2,5-diphenyl-tetrazoles follow similar trends and are explicable largely in terms of the stabilization of the developing dipole in the transition state.

Calculations Predict That Heavy-Atom Tunneling Dominates Möbius Bond Shifting in [12]- and [16]Annulene

The contribution of heavy-atom tunneling to reactions of [12]- and [16]annulene was probed using small-curvature tunneling rate calculations. At the CCSD(T)/cc-pVDZ//M06-2X/cc-pVDZ level, tunneling is predicted to account for more than 50% of the rate for Möbius bond shifting and ca. 35% of the rate for electrocyclization in [12]annulene, and over 80% of the rate for Möbius bond shifting in [16]annulene, at temperatures at which these reactions have been observed experimentally.

Tunneling by 16 Carbons: Planar Bond Shifting in [16]Annulene

Midsized annulenes are known to undergo rapid π-bond shifting. Given that heavy-atom tunneling plays a role in planar bond shifting of cyclobutadiene, we computationally explored the contribution of heavy-atom tunneling to planar π-bond shifting in the major (CTCTCTCT, 5a) and minor (CTCTTCTT, 6a) known isomers of [16]annulene. UM06-2X/cc-pVDZ calculations yield bond-shifting barriers of ca. 10 kcal/mol. The results also reveal extremely narrow barrier widths, suggesting a high probability of tunneling for these bond-shifting reactions. Rate constants were calculated using canonical variational transition state theory (CVT) as well as with small curvature tunneling (SCT) contributions, via direct dynamics. For the major isomer 5a, the computed SCT rate constant for bond shifting at 80 K is 0.16 s^-1, corresponding to a half-life of 4.3 s, and indicating that bond shifting is rapid at cryogenic temperatures despite a 10 kcal/mol barrier. This contrasts with the CVT rate constant of 8.0 × 10^-15 s^-1 at 80 K. The minor isomer 6a is predicted to undergo rapid bond shifting via tunneling even at 10 K. For both isomers, bond shifting is predicted to be much faster than competing conformation change despite lower barriers for the latter process. The preference for bond shifting represents cases of tunneling control in which the preferred reaction is dominated by heavy-atom motions. At all temperatures below -50 °C, tunneling is predicted to dominate the bond shifting process for both 5a and 6a. Thus, [16]annulene is predicted to be an example of tunneling by 16 carbons. Bond shifting in both isomers is predicted to be rapid at temperatures accessible by solution-phase NMR spectroscopy, and an experiment is proposed to verify these predictions.

Synthesis and self-seeding behavior of oligo(p-phenylene vinylene)-b-poly(N-(2-hydroxypropyl)methacrylamide)

This article reports the preparation of uniform fiber- and ribbon-like nanostructures via the self-seeding of OPV₅-b-PHPMA diblock copolymers.

In situ formation of interpenetrating polymer network using sequential thermal and click crosslinking for enhanced retention of transplanted cells

Injectable hydrogels, which are used as scaffolds in cell therapy, provide a minimally invasive strategy to enhance cell retention and survival at injection site. However, till now, slow in situ gelation, undesired mechanical properties, and weak cell adhesion characteristics of reported hydrogels, have led to improper results. Here, we developed an injectable fully-interpenetrated polymer network (f-IPN) by integration of Diels-Alder (DA) crosslinked network and thermosensitive injectable hydrogel. The proposed DA hydrogels were formed in a slow manner showing robust mechanical properties. Interpenetration of thermosensitive network into DA hydrogel accelerated in situ gel-formation and masked the slow reaction rate of DA crosslinking while keeping its unique features. Two networks were formed by simple syringe injection without the need of any initiator, catalyst, or double barrel syringe. The DA and f-IPN hydrogels showed comparable viscoelastic properties along with outstanding load-bearing and shape-recovery even under high levels of compression. The subcutaneous administration of cardiomyocytes-laden f-IPN hydrogel into nude mice revealed high cell retention and survival after two weeks. Additionally, the cardiomyocyte's identity of retained cells was confirmed by detection of human and cardiac-related markers. Our results indicate that the thermosensitive-covalent networks can open a new horizon within the injection-based cell therapy applications.

Experimental and computational kinetic investigations for the reactions of Cl atoms with unsaturated ketones in the gas phase

Cl atom initiated photo oxidation of unsaturated ketones.

External electric field control: driving the reactivity of metal-free azide–alkyne click reactions

An external electric field is demonstrated as an efficient catalyst to accelerate click reactions.