[reaction: see text] Reaction conditions are described for the oxidation of anilines furnishing nitrosoarenes and the synthesis of unsymmetrically substituted azobenzenes. In a comparative study, the catalytic oxidation of methyl 4-aminobenzoate by hydrogen peroxide was investigated, and SeO(2) proved to be superior or equal to methyl trioxorhenium (MTO) and Na(2)WO(4), respectively. Nevertheless, the application of the inexpensive, environmentally friendly, Oxone in a biphasic system proved to be more efficient, and a variety of useful nitrosoarenes for the synthesis of azo compounds were prepared in high yield and purity on a large scale.
We report on the electron induced intramolecular rotation of a single phenyl ring of an azobenzene derivative
adsorbed on a Au(111) surface using a low-temperature scanning tunneling microscope (STM). By proper
functionalization of each of the two azobenzene's phenyl rings with CN end groups, we are able to identify
two distinct isomers at the metal surface corresponding to two possible alignments of the functional groups
in the trans conformer. Tunneling electrons induce molecular motion and intramolecular conformational changes
both on isolated molecules and H-bonded molecular islands. Particular enhancement is observed for the electrons
resonantly tunneling through affinity levels, which is consistent with electronic molecular excitations as the
basic mechanism for this manipulation process. On the basis of quantum chemical calculations of a free
azobenzene molecule, we propose a dynamical model for the ring-rotation pathways, which includes the
electric field in the STM junction to effectively couple electronic excitation with intramolecular rotations.
The photochemical reaction dynamics of a set of photochromic compounds based on thioindigo and stilbene molecular parts (hemithioindigos, HTI) are presented. Photochemical Z/E isomerization around the central double bond occurs with time constants of 216 ps (Z --> E) and 10 ps (E --> Z) for a 5-methyl-hemithioindigo. Chemical substitution on the stilbene moiety causes unusually strong changes in the reaction rate. Electron-donating substituents in the position para to the central double bond (e.g., para-methoxy) strongly accelerate the reaction, while the reaction is drastically slowed by electron-withdrawing groups in this position (e.g., para-nitrile). We correlate the experimental data of seven HTI-compounds in a quantitative manner using the Hammett equation and present a qualitative explanation for the application of ground-state Hammett constants to describe the photoisomerization reaction.
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