LongLarf

A catalytic carbonylative cleavage of carbon–carbon triple bonds is achieved. This allows for a general and direct synthesis of α,β-unsaturated piperidones by palladium-catalyzed cascade carbonylation of alkynols.

Abstract

A direct and selective synthesis of α,β-unsaturated piperidones by a new palladium-catalyzed cascade carbonylation is described. In the presented protocol, easily available propargylic alcohols react with aliphatic amines to provide a broad variety of interesting heterocycles. Key to the success of this transformation is a remarkable catalytic cleavage of the present carbon–carbon triple bond by using a specific catalyst with 2-diphenylphosphinopyridine as ligand and appropriate reaction conditions. Mechanistic studies and control experiments revealed branched unsaturated acid 11 as crucial intermediate.

Deracemization is an attractive approach in asymmetric synthesis, but it is challenging owing to thermodynamic and kinetic barriers. Here we describe a method for deracemization of alcohols based on tandem photochemical dehydrogenation and thermal enantioselective hydrogenation, using light as the only driving force.

Abstract

Deracemization of racemic chiral compounds is an attractive approach in asymmetric synthesis, but its development has been hindered by energetic and kinetic challenges. Here we describe a catalytic deracemization method for secondary benzylic alcohols which are important synthetic intermediates and end products for many industries. Driven by visible light only, this method is based on sequential photochemical dehydrogenation followed by enantioselective thermal hydrogenation. The combination of a heterogeneous dehydrogenation photocatalyst and a chiral molecular hydrogenation catalyst is essential to ensure two distinct pathways for the forward and reverse reactions. These reactions convert a large number of racemic aryl alkyl alcohols into their enantiomerically enriched forms in good yields and enantioselectivities.

A palladium-catalyzed intramolecular difunctionalization of (aza-)alkenols with excellent regioselectivity is reported. A new generation of C-6-modified Pyox ligands together with the use of unusual hypervalent iodine reagents as oxidants enabled the generation of acyloxy-functionalized heterocycles in excellent enantio- and diastereoselectivity.

Abstract

An oxidative Pd-catalyzed intra-intermolecular dioxygenation of (aza-)alkenols has been reported, with total regioselectivity. To study the stereoselectivity, different chiral ligands as well as different hypervalent-iodine compounds have been compared. In particular, by using a C-6 modified pyridinyl-oxazoline (Pyox) ligand and hypervalent iodine bearing an aromatic ring, an excellent enantio- and diastereoselectivity has been achieved.

Identifying plastics capable of chemical recycling to monomer (CRM) is the foremost challenge in creating a sustainable circular plastic economy. Polyacetals are promising candidates for CRM but lack useful tensile strengths owing to the low molecular weights produced using current uncontrolled cationic ring-opening polymerization (CROP) methods. Here, we present reversible-deactivation CROP of cyclic acetals using a commercial halomethyl ether initiator and an indium(III) bromide catalyst. Using this method, we synthesize poly(1,3-dioxolane) (PDXL), which demonstrates tensile strength comparable to some commodity polyolefins. Depolymerization of PDXL using strong acid catalysts returns monomer in near-quantitative yield and even proceeds from a commodity plastic waste mixture. Our efficient polymerization method affords a tough thermoplastic that can undergo selective depolymerization to monomer.

Nature Catalysis, Published online: 09 August 2021; doi:10.1038/s41929-021-00660-8

Stereochemical control in the asymmetric dihalogenation of alkenes and alkynes is challenging. Now, an organocatalytic method is developed, whereby installing a urea-directing moiety on these substrates enables their stereo- and regioselective homo- and hetero-dihalogenation.

Nature Chemistry, Published online: 09 August 2021; doi:10.1038/s41557-021-00750-x

Although organocatalysis has emerged as a powerful catalysis platform, its application for the selective transformation of inactive arenes remains challenging. Now, an organocatalyst-controlled site-selective C–H functionalization of arenes has been developed, with wide substrate generality and applicability, offering a general strategy for arene transformation.

High-resolution paramagnetic NMR resonances of a ferrous complex with an intermediate spin ground state with chemical shifts beyond ten thousand ppm were detected, the observability of which are attributed to a large rhombic zero-field splitting resulting in an effective suppression of the dipolar relaxation mechanism.

Abstract

We report an experimental observation of ³¹P NMR resonances shifted by over 10 000 ppm (meaning percent range, and a new record for solutions), and similar ¹H chemical shifts, in an intermediate-spin square planar ferrous complex [ ^tBu(PNP)Fe-H], where PNP is a carbazole-based pincer ligand. Using a combination of electronic structure theory, nuclear magnetic resonance, magnetometry, and terahertz electron paramagnetic resonance, the influence of magnetic anisotropy and zero-field splitting on the paramagnetic shift and relaxation enhancement is investigated. Detailed spin dynamics simulations indicate that, even with relatively slow electron spin relaxation (T ₁ ≈10⁻¹¹ s), it remains possible to observe NMR signals of directly metal-bonded atoms because pronounced rhombicity in the electron zero-field splitting reduces nuclear paramagnetic relaxation enhancement.