Strategien zur Erforschung neuer Reaktionen

Harnessing the Potential of N‐Heterocyclic Carbenes for the Rejuvenation of Group‐Transfer Polymerization of (Meth)Acrylics

“…Figure 1 a shows the size‐exclusion chromatography (SEC) traces of five different PMMA samples, including that of a sample with a molar mass as high as 100 000 g mol −1 (Table 1, entry 4) which has not been achieved using regular anionic catalysts for GTP 1. 2 These results indicate that NHC‐catalyzed GTP of MMA is living, and confirm that bare NHCs 1 and 2 activate the SiO bond, in an identical fashion to Lewis bases in catalyzed Mukaiyama aldol‐type reactions 3. 12…”

“…At the time of its disclosure in 1983 by Webster et al., group‐transfer polymerization (GTP) was greeted as a major breakthrough in polymer chemistry, for it describes the living polymerization of (meth)acrylic monomers at ambient temperature 1. GTP is initiated and propagates by the conjugate addition of silyl ketene acetals to incoming (meth)acrylic monomers and the concomitant transfer of the trimethylsilyl group to chain end (Scheme ),1, 2 through repeated Mukaiyama–Michael‐type reactions 3. A catalyst is required for GTP to proceed: anions such as HF 2 − , F − or CN − perform best for methacrylic monomers in polar solvents, whereas Lewis acids, such as AlR 2 Cl or ZnCl 2 , and apolar media are most suitable for acrylic monomers 1–4.…”

High‐Speed Living Polymerization of Polar Vinyl Monomers by Self‐Healing Silylium Catalysts

“…[5] However, it has been suggested [6] that several key lines of experimental evidence are more consistent with a dissociative mechanism, [7] which involves ester enolate anions as the propagating species and a rapid, reversible complexation (termination) of small concentrations of enolate anions with SKA or its polymer homologue (path b, Scheme 1). The elementary C À C bond coupling step in GTP is that of the Mukaiyama-Michael reaction, [8] but activation of the inactive SKA is the critical first step in either the associative or dissociative pathway described above. Such activation can be viewed as reductive activation of SKA for both pathways in GTP, namely, conversion of the inactive, neutral Si center to the active, anionic Si center.…”

Generation of Stereochemically Defined Tetrasubstituted Enolborinates by 1,4‐Hydroboration of α,β‐Unsaturated Morpholine Carboxamides with (Diisopinocampheyl)borane

“…Compounds bearing a α,α-dimethyl-β-hydroxy quaternary center are often generated by using Mukaiyama aldol methods. [15] 1,4-Hydroboration of 7 (1.1 equiv) with ( d Ipc) 2 BH (1 equiv) in Et 2 O at ambient temperature for 3 h followed by addition of an aldehyde (0.85 equiv) and heating the reaction mixture at 50 °C in sealed tube for 16 h gave aldols 9 (see SI for variables studied during the optimization of this reaction). Results of the reductive aldol reactions of 7 with a range of representative aldehydes are presented in Scheme 2.…”