Enantioselective Hydrophosphination of Terminal Alkenyl Aza-Heteroarenes

This paper presents a Mn(I)-catalysed methodology for the enantioselective hydrophosphination of terminal alkenyl aza-heteroarenes. The catalyst operates through H-P bond activation, enabling successful hydrophosphination of a diverse range of alkenyl-heteroarenes with high enantioselectivity. The presented protocol addresses the inherently low reactivity and the commonly encountered suboptimal enantioselectivities of these challenging substrates. As an important application we show that this method facilitates the synthesis of a non-symmetric tridentate P,N,P-containing ligand like structure in just two synthetic steps using a single catalytic system.

Manganese(I)-Catalyzed Asymmetric Hydrophosphination of α,β-Unsaturated Carbonyl Derivatives

Here we report catalytic asymmetric hydrophosphination of α,β-unsaturated carbonyl derivatives using a chiral Mn(I) complex as a catalyst. Through H-P bond activation, various phosphine-containing chiral products can be accessed via hydrophosphination of various ketone-, ester-, and carboxamide-based Michael acceptors.

Enantio- and Z-selective synthesis of functionalized alkenes bearing tertiary allylic stereogenic center

Olefins are ubiquitous in biologically active molecules and frequently used as building blocks in chemical transformations. However, although many strategies exist for the synthesis of stereodefined E-olefines, their thermodynamically less stable Z counterparts are substantially more demanding, while access to those bearing an allylic stereocenter with an adjacent reactive functionality remains unsolved altogether. Even the classic Wittig reaction, arguably the most versatile and widely used approach to construct Z-alkenes, falls short for the synthesis of these particularly challenging yet highly useful structural motives. Here, we report a general methodology for Z-selective synthesis of functionalized chiral alkenes that establishes readily available alkene-derived phosphines as an alternative to alkylating reagents in Wittig olefination, thus offering previously unidentified retrosynthetic disconnections for the formation of functionalized disubstituted alkenes. We demonstrate the potential of this method by structural diversification of several bioactive molecules.

In situ EPR and Raman spectroscopy in the curing of bis-methacrylate-styrene resins

The curing of bis-methacrylate-styrene resins initiated by the cobalt catalyzed decomposition of cumyl hydroperoxide is monitored at ambient temperatures in situ by EPR and Raman spectroscopy. EPR spectroscopy shows the appearance of organic radicals after ca. 1 h from initiation with an increase in intensity from both polystyrene and methacrylate based radical species over a further ca. 2 h period to reach a maximum spin concentration of ca. 2-3 mM. Alkene conversion to polymer was monitored by Raman spectroscopy in real time in situ with EPR spectroscopy and reveals that the appearance of the radical signals is first observed only as the conversion approaches its maximum extent (70% at room temperature), i.e., the resin reaches a glass-like state. The radicals persist for several months on standing at room temperature. Flash frozen samples (77 K) did not show EPR signals within 1 h of initiation. The nature of the radicals responsible for the EPR spectra observed were explored by DFT methods and isotope labelling experiments (D₈-styrene) and correspond to radicals of both methacrylate and polystyrene. Combined temperature dependent EPR and Raman spectroscopy shows that conversion increases rapidly upon heating of a cured sample, reaching full conversion at 80 °C with initially little effect on the EPR spectrum. Over time (i.e. subsequent to reaching full conversion of alkene) there was a small but clear increase in the EPR signal due to the methacrylate based radicals and minor decrease in the signal due to the polystyrene based radicals. The appearance of the radical signals as the reaction reaches completion and their absence in samples flash frozen before polymerization has halted, indicate that the observed radicals are non-propagating. The formation of the radicals due to stress within the samples is excluded. Hence, the observed radicals are a representative of the steady state concentration of radicals present in the resin over the entire timespan of the polymerization. The data indicate that the lack of EPR signals is most likely due to experimental aspects, in particular spin saturation, rather than low steady state concentrations of propagating radicals during polymerization.

Manganese(I)-Catalyzed H-P Bond Activation via Metal-Ligand Cooperation

Here we report that chiral Mn(I) complexes are capable of H–P bond activation. This activation mode enables a general method for the hydrophosphination of internal and terminal α,β-unsaturated nitriles. Metal−ligand cooperation, a strategy previously not considered for catalytic H–P bond activation, is at the base of the mechanistic action of the Mn(I)-based catalyst. Our computational studies support a stepwise mechanism for the hydrophosphination and provide insight into the origin of the enantioselectivity.