Stereoselective polymerization of “carbenes” from diazo esters mediated by [(L-prolinate)RhI(Me2cod)] allows the formation of highly syndiotactic, fully functionalized carbon-chain polymers with the highest reported molecular weights in high yields, thus providing the best currently available catalyst for this new reaction.

Rh-mediated polymerization of carbenes gives access to new highly substituted and stereoregular polymers. While this reaction is of interest for the synthesis of syndiotactic polymers that are functionalized at every carbon atom of the polymer backbone, the catalyst activation, chain-initiation, and chain-termination processes were so far poorly understood. In this publication we present new information about these processes on the basis of detailed end-group analyses, dilutionkinetic studies, and a comparison of the activity of well-defined catalysts containing a preformed Rh−C bond. All data point toward complex catalyst activation processes under the applied reaction conditions. The use of well-defined Rh I (cod)-alkyl, aryl, and allyl complexes does not lead to better initiation efficiencies or higher polymer yields. MALDI-ToF MS of the oligomeric fractions indicates that during the incubation time of the reaction, the precatalysts are first transformed into oligomer forming species with a suppressed tendency toward β-hydrogen elimination, and accordingly a shift to saturated oligomeric chains that are terminated by protonolysis. Further catalyst modifications lead to a shift from atactic oligomerization to stereoregular high molecular weight polymerization activity. Dilution-kinetic studies reveal that under diluted conditions two different active species operate that differ largely in their chain-termination behavior. Analysis of the reaction products by MALDI-ToF MS also allows conclusions about chain-initiation and chain-termination. Chain-initiation can occur by insertion of a preformed carbene into a Rh-ligand or Rh-hydride bond or by (internal or external) nucleophilic attack of water and/or alcohol on a Rh-carbene moiety. Chain-termination takes place mainly by (nucleophilic) protonolysis involving water or alcohols, while β-H elimination plays only a minor role and is only observed for the shorter oligomers. The detection of ethoxy and hydroxyl end-groups demonstrates the importance of trace amounts of water and ethanol toward chain-initiation. Alcohols further function as a chain-transfer agent, and increasing the alcohol concentration accelerates the chain-transfer process (which remains however relatively slow compared to chain-propagation). On the basis of the chemical properties of the alcohols, we propose a chain-transfer mechanism involving nucleophilic attack of the alcohol (nucleophilic, σ-bond metathesis type, protonolysis). This further allows us to draw some (careful) new conclusions about the oxidation state of the actual polymerization species.

Breath‐taking activation: Stereoregular carbene polymerization proceeds via cationic [(allyl)RhIII–polymeryl]+ species. These are most efficiently generated by oxygenation of the [(diene)RhI] precatalysts, which involves an unusual rearrangement of 2‐rhodaoxetane intermediates. This discovery gives detailed insight in the reaction mechanism.

We show that the VCD signal intensities of amino acids and oligopeptides can be enhanced by up to 2 orders of magnitude by coupling them to a paramagnetic metal ion. If the redox state of the metal ion is changed from paramagnetic to diamagnetic the VCD amplification vanishes completely. From this observation and from complementary quantum-chemical calculations we conclude that the observed VCD amplification finds its origin in vibronic coupling with low-lying electronic states. We find that the enhancement factor is strongly mode dependent and that it is determined by the distance between the oscillator and the paramagnetic metal ion. This localized character of the VCD amplification provides a unique tool to specifically probe the local structure surrounding a paramagnetic ion and to zoom in on such local structure within larger biomolecular systems.

In this perspective we highlight the applicability of migratory carbene insertion reactions into TM-C bonds as a new tool for catalytic C-C bond formation. In Section 1 we introduce the reaction, wherein we also discuss the applicability of transition metal carbene formation from reactive carbene precursors. In Section 2 we summarise the available mechanistic information about this elementary step derived from stoichiometric model reactions. In Section 3 we review the available catalytic examples, with a focus on new developments in palladium mediated cross-coupling reactions (thus expanding the substrate scope with carbene precursors) and carbene polymerisation (allowing the synthesis of highly functionalised stereoregular polymers that are difficult to prepare otherwise). Recent developments in these fields in combination with the close analogy of carbene insertion reactions with CO (and alkene) insertions open up new possibilities for the development of interesting new reactions based on carbene insertions.