HIGHLIGHTS


Sakurai, T. Hikima, A. Kawamura, M. Kohri, J. Matsui, T. Yamao; Chem. Commun., 2017, 53, 10703). The images shown in this paper reveal this product has a beautiful non-iridescent lustrous dark green surface.


Using light to synchronise catalytic action A light switchable organocatalytic motif has been described, which puts the ever-popular E/Z photoisomerism of azobenzenes to new use (M. D. Pescher, L. J. G. W. van Wilderen, S. Grgtzner, C. Slavov, J. Wachtveitl, S. Hecht, J. Bredenbeck, Angew. Chem. Int. Ed., 2017, 56, 12092). The ingenious structure (Figure 3) uses a spirocyclic lactone to hold the six-membered ring of a piperidine in a specific chair conformation, so that the substituent at nitrogen takes up an equatorial orientation. Consequently the catalytically-


active nitrogen lone pair is axial and with the diazo link in the E configuration, it is blocked by the aromatic ring of the Ar-N=N- substructure. In this paper, the photoswitching


Figure 3 With the – N=N- diazo link in the E-configuration (left structure), the bulky triaryl substituent blocks access to the catalytically active nitrogen lone pair. When converted into the Z isomer (right structure), the blocking group is moved away allowing base-mediated catalysis to be turned on. The Box shows the use of the bulky t-butyl group to hold the nitrogen none pair in the axial orientation. This ensures that it is pointing towards the blocking group.


O O


Nitrogen lone pair


blocked N N N E link Z link Blocking group moved away


Equatorial tBu group


N axial lone pair


is demonstrated by the photoexpulsion of methanol from the binding site as a consequence of a 450nm Z-E isomerisation, but the importance of this conformational change is that forming the E stereoisomer can open up the


Advanced materials


ARNO KRAFT Heriot-Watt University, Edinburgh, UK


Thermoplastic polyethylene elastomer Polyethylene is by far the most important synthetic polymer with annual production figures close to 100m t. The radical polymerisation of ethylene at high pressure and high temperature leads to a branched polymer termed low-density polyethylene, which contains both long and short chain branches. Although the polymer is cheap, the downside is the cost of building a high-pressure plant. Alternatively, many transition- metal catalysts can polymerise ethylene at comparatively low pressure and low temperature. The classical Ziegler-Natta catalysts consist of early transition metals capable of producing a linear polyethylene. The resulting high- density polyethylene possesses both high crystallinity and high strength. Over the last decade, late


transition metal catalysts have been found that are equally active polymerisation catalysts but are prone to a chain-walking mechanism where repeated β-elimination, followed by re-insertion gives rise to a highly branched and almost dendritic polymer microstructure. A recent paper has described the use of such late-transition metal catalysts based on a sterically hindered nickel α-diamine complex, which results for the first time in a polyethylene-based thermoplastic elastomer (K. Lian et al; Macromolecules, 2017, 50, 6074). As the nickel centre in complex


1 adopts a distorted tetrahedral geometry, the bulky aryl substituents on the two imine nitrogens can exist in the form of two rotational isomers that convert slowly into each other (Scheme 1). One isomer has both bulky arms on the same side (meso isomer); in the other case, the two bulky arms are on opposite sides (rac isomer) of the central nickel


α-diamine plane. With methylaluminoxane as


co-catalyst, the nickel complex was highly active in polymerising ethylene and produced polyethylene with molecular weights of over 1,000,000g/ mol at room temperature. Most unusually, the polyethylene samples possessed strain-at-break values of 300% to 1800% as well as S-shaped stress-strain curves characteristic of elastomeric materials. The elastic properties were not perfect as cyclic tests revealed some unrecovered strain that could be reduced by careful optimisation of the bulk of the substituent. Such elastomeric properties have hitherto only been seen in multiblock polyolefins other than polyethylene. The exact mechanism is still under


investigation, but the authors suspect that the rac isomer of the catalyst probably generated polyethylene hard segments with a low degree of branching, whereas the meso isomer led to the formation of highly


active site and switch on catalysis in a sychronised fashion allowing spectroscopic and kinetic studies to probe the details catalytic action. The Z isomer can be switched back to the catalytically active E form by irradiation at 365nm.


N N


44 09 | 2017


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