Size-induced rate accelerations in organocatalysis

Steric effects through sizeable moieties are commonly attributed to be mainly repulsive in organic chemistry. However, the size of molecules also significantly influences the strength of dispersion forces. Transition state theory implicates that reaction rates should be accelerated if the transition state is stabilized, e.g. through dispersion energy. Thus, the influence of large aromatic moieties on the reaction rates of several organocatalysed protecting group reactions was investigated. For the Lewis base-catalysed silylation of secondary alcohols as well as Lewis acid-catalysed hydrosilylation reactions of ketones size-dependent rate accelerations were found by competition experiments and computational studies. The size of these effects depends crucially on the solvent. The kinetic resolution of secondary alcohols by fluxionally chiral pyridine catalysts is increasingly selective with increasingly bulky substrates. Detailed kinetic and computational studies show that this is mainly provoked by a selective reaction rate acceleration for the major isomer through non-covalent interactions. More bulky catalysts further increase reaction rate AND selectivity. In a second part empirical studies on an online video library for the organic chemistry laboratory are reported. The online video library was used intensively by bachelor-level students, but preferences and utilization depends on individual student characteristics. The study indicates benefits of the online video library on students’ cognitive, affective, and psychomotor learning in a laboratory course

Size-dependent rate acceleration in the silylation of secondary alcohols: the bigger the faster

Relative rates for the reaction of secondary alcohols carrying large aromatic moieties with silyl chlorides carrying equally large substituents have been determined in organic solvents. Introducing thoroughly matching pairs of big dispersion energy donor (DED) groups enhanced rate constants up to four times, notably depending on the hydrogen bond donor ability of the solvent. A linear correlation between computed dispersion energy contributions to the stability of the silyl ether products and experimental relative rate constants was found. These results indicate a cooperation between solvophobic effects and DED-groups in the kinetic control of silylation reactions

Polychlorides─A Safe and Convenient Alternative for Elemental Chlorine in Demonstration Experiments

Demonstration experiments involving chlorine are often impressive and can help to understand the reactivity of halogens. However, practical and safety concerns render handling of chlorine difficult in educational settings, particularly in high schools. Recent chemical research has demonstrated the potential of trichloride salts as a much safer and more affordable alternative to chlorine gas. Triethylmethylammonium trichloride, [NEt3Me][Cl3], is a stable and safe ionic liquid that is easy to use and handle. Upon addition of water, chlorine gas is released in a controlled manner. This process presents a convenient method for conducting chlorine demonstration experiments. Experimental procedures are presented for the production of chlorine gas, the chlorine-initiated formation and subsequent luminescence of singlet oxygen, the synthesis of iron chloride and sodium chloride, and the electrophilic addition to lycopene

Mechanochemically Initiated Achmatowicz Rearrangement

The Achmatowicz rearrangement converts furfuryl alcohols, obtainable from renewable carbohydrates, into 6-hydroxy-2H-pyrane-3(6H)-ones, which are versatile intermediates for organic synthesis. We describe here the first examples of a solvent-free mechanochemical Achmatowicz rearrangement. Furfuryl alcohols were prepared from furfurals using mechanochemically initiated reductions and Reformatsky reactions. Mechanochemical reaction conditions for the Achmatowicz rearrangement of the obtained furfuryl alcohols were optimized and applied to a series of derivatives, yielding the corresponding rearrangement products in yields of 39 to 95%