Accelerated Polypeptide Synthesis via N-Carboxyanhydride Ring Opening Polymerization in Continuous Flow.

In nature, polypeptide-based materials are ubiquitous, yet their synthetic production is hampered by high cost, limited scalability, and often stringent reaction conditions. Herein an elegant approach is presented for N-carboxyanhydride ring opening polymerization (NCA ROP) of Nε-benzyloxycarbonyl-l-lysine (ZLL) and γ-benzyl-l-glutamate (BLG) NCA in continuous flow. The polymerization is initiated by primary amine initiators using N,N-dimethylformamide (DMF) as solvent. Carrying out the reaction in a silicon microflow reactor speeds up the rate of ROP (92% conversion in 40 min in flow as opposed to 6 h in batch) due to highly efficient permeation of CO2 through the reactor tubing. The polymerization strategy provides a facile, scale-up friendly alternative to traditional batch mode polymerization and has the capability of streamlining NCA ROP.

Molecular mapping of poly(methyl methacrylate) super-helix stereocomplexes.

In this study, by using X-ray powder diffraction profiles as blueprints, we successfully mapped the most probable molecular-level structural arrangement of the PMMA super-helix stereocomplexes through molecular dynamic simulations. Molecular-level resolution of the PMMA triple-helix supramolecule was not previously achievable by experimental methods. After constructing molecular models of stereo-regular complexes composed of linear it- and st-PMMAs, our all-atom molecular dynamics simulations identified the stereocomplex structure that best reproduces experimental diffraction profiles and thermodynamic properties as a double helix of isotactic (it-)PMMA with a helical pitch of 1.8 nm and 9 units per turn surrounded by a single helix of syndiotactic (st-)PMMA with an average helical pitch of 0.9 nm and 20 units per turn. The it-/st- complexing stoichiometry in the PMMA triple-helix is therefore 9 : 20. This presents the first all-atom model of the it-/st-PMMA triple-helix stereocomplex that accurately fits experimental X-ray diffraction profiles. In addition, the simulation results revealed the outer st-PMMA helix of the PMMA stereocomplex has a fiber diameter of at least 1.85 nm and adopts a non-ideal helical geometry. Furthermore, through dynamic simulations, surprising new sights into the effect of the structural configuration of the PMMA stereocomplex (i.e., helical pitch and direction, and tilt angle) on the physical properties of their crystal structures were obtained. Those crystal properties include X-ray diffraction profile, packing density, chain-chain spacing, chain width and cohesive energy density.

Stereospecific cyclic poly(methyl methacrylate) and its topology- guided hierarchically controlled supramolecular assemblies.

In this study, the stereocomplexation between a novel stereospecific cyclic vinyl polymer, that is, cyclic syndiotactic poly(methyl methacrylate) (st-PMMA), with the complementary linear isotactic (it-) PMMA was investigated. Surprising new insight into the effects of the topology (i.e., end groups), size, and tacticity of the assembling components on stereocomplex formation was obtained. Characterization of the stereocomplexes revealed that the self-assembly of cyclic st-PMMAs and linear it-PMMAs resulted in the formation of an unprecedented "polypseudorotaxane-type" supramolecular assembly. This stereocomplex exhibited remarkably different physical properties as compared to the conventional PMMA triple-helix stereocomplex as a result of the restricted topology imposed by the cyclic st-PMMA assembling component.

Nitrile imines: matrix isolation, IR spectra, structures, and rearrangement to carbodiimides.

The structures and reactivities of nitrile imines are subjects of continuing debate. Several nitrile imines were generated photochemically or thermally and investigated by IR spectroscopy in Ar matrices at cryogenic temperatures (Ph-CNN-H 6, Ph-CNN-CH(3)17, Ph-CNN-SiMe(3)23, Ph-CNN-Ph 29, Ph(3)C-CNN-CPh(3)34, and the boryl-CNN-boryl derivative 39). The effect of substituents on the structures and IR absorptions of nitrile imines was investigated computationally at the B3LYP/6-31G* level. IR spectra were analyzed in terms of calculated anharmonic vibrational spectra and were generally in very good agreement with the calculated spectra. Infrared spectra were found to reflect the structures of nitrile imines accurately. Nitrile imines with IR absorptions above 2200 cm(-1) have essentially propargylic structures, possessing a CN triple bond (typically PhCNNSiMe(3)23, PhCNNPh 29, and boryl-CNN-boryl 39). Nitrile imines with IR absorptions below ca. 2200 cm(-1) are more likely to be allenic (e.g., HCNNH 1, PhCNNH 6, HCNNPh 43, PhCNNCH(3)17, and Ph(3)C-CNN-CPh(3)34). All nitrile imines isomerize to the corresponding carbodiimides both thermally and photochemically. Monosubstituted carbodiimides isomerize thermally to the corresponding cyanamides (e.g., Ph-N═C═N-H 5 → Ph-NH-CN 8), which are therefore the thermal end products for nitrile imines of the types RCNNH and HCNNR. This tautomerization is reversible under flash vacuum thermolysis conditions.

Synthesis of N-Confused Porphyrin Analogues by beta-Azafulvenone Tetramerization.

beta-Azafulvenones 16 and 33 were generated from 14 or 18 and 31 or 37, respectively, and directly observed by IR spectroscopy. These heterocyclic ketenes tetramerize to the macrocycles 20 and 34.