LongLarf

Both electron‐neutral and electron‐rich non‐activated aryl fluorides reacted with potassium diorganophosphinites through a nucleophilic aromatic substitution (S_NAr) reaction. Even substantially stabilized anionic P nucleophiles reacted to form the corresponding tertiary phosphine oxides. Quantum chemical calculations suggested a nucleophile‐dependent mechanism involving both concerted and stepwise S_NAr reaction pathways (see scheme).

Abstract

Non‐activated aryl fluorides reacted with potassium diorganophosphinites through a nucleophilic aromatic substitution (S_NAr) reaction. Remarkably, both electron‐neutral and electron‐rich aryl fluorides participated in the reaction with substantially stabilized anionic P nucleophiles to form the corresponding tertiary phosphine oxides. Quantum chemical calculations suggested a nucleophile‐dependent mechanism that involves both concerted and stepwise S_NAr reaction pathways.

The critical role of double hydrogen bonds was addressed for the aerobic α‐hydroxylation of ketones catalyzed by 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD), in absence of either metal catalyst or phosphine reductant. Experimental and theoretical investigations were carried out to study the mechanism. In addition to initiating the reaction by proton abstraction, a more important role of TBD was revealed to enhance the oxidizing ability of peroxide intermediates, allowing DMSO an efficient substitute of commonly used phosphine reductants. Further characterizations with the nuclear Overhauser effect spectroscopy (NOESY) confirmed the double hydrogen bonds between TBD and ketone, and kinetic studies suggested the attack of dioxygen on the TBD‐enol adduct to be the rate‐determining step. This work uncovered the enhancing role of TBD on the oxidizing ability of peroxides, and should encourage the application of TBD as a catalyst for oxidations.

Abstract

Herein, we report the manganese catalyzed coupling of alcohols with phosphorus ylides. The selectivity in the coupling of primary alcohols with phosphorus ylides to form carbon‐carbon single (C−C) and carbon‐carbon double (C=C) bonds can be controlled by the ligands. In the conversion of more challenging secondary alcohols with phosphorus ylides the selectivity towards the formation of C−C vs. C=C bonds can be controlled by the reaction conditions, namely the amount of base. The scope and limitations of the coupling reactions were thoroughly evaluated by the conversion of 21 alcohols and 15 ylides. Notably, compared to existing methods, which are based on precious metal complexes as catalysts, the present catalytic system is based on earth abundant manganese catalysts. The reaction can also be performed in a sequential one‐pot reaction generating the phosphorus ylide in situ followed manganese catalyzed C−C and C=C bond formation. Mechanistic studies suggest that the C−C bond was generated via a borrowing hydrogen pathway and the C=C bond formation followed an acceptorless dehydrogenative coupling pathway.

Nature, Published online: 26 November 2020; doi:10.1038/d41586-020-03358-2

A mild copper‐mediated protocol has been developed that allows for dichotomic borylation of alkynyl‐substituted carbonates, affording either 1,2‐diborylated 1,3‐dienes or α‐hydroxy allenes as the principal products, depending on the nature of the diboron(4) reagent. A mechanistic rationale is presented that reveals the crucial role of the relative kinetics of the second borylation step versus i‐PrOH‐assisted protodemetalation.

Abstract

A mild copper‐mediated protocol has been developed for borylation of alkynyl cyclic carbonates. Depending on the nature of the borylating reaction partner, either stereoselective diborylation of the propargylic surrogate takes place, providing convenient access to (E)‐1,2‐borylated 1,3‐dienes, or traceless monoborylation occurs, which leads to α‐hydroxyallenes as the principal product. The dichotomy in this borylation protocol has been scrutinized by several control experiments, illustrating that a relatively small change in the diboron(4) reagent allows for competitive alcohol‐assisted protodemetalation to forge an α‐hydroxyallene product under ambient conditions.

The advanced Pd/L11 system enables highly regioselective carbonylation of alkynols for synthesis of a wide range of 4‐membered α‐methylene‐β‐lactones.

Abstract

The first general and regioselective Pd‐catalyzed cyclocarbonylation to give α‐methylene‐β‐lactones is reported. Key to the success for this process is the use of a specific sterically demanding phosphine ligand based on N‐arylated imidazole (L11) in the presence of Pd(MeCN)₂Cl₂ as pre‐catalyst. A variety of easily available alkynols provide under additive‐free conditions the corresponding α‐methylene‐β‐lactones in moderate to good yields with excellent regio‐ and diastereoselectivity. The applicability of this novel methodology is showcased by the direct carbonylation of biologically active molecules including natural products.

Nature, Published online: 23 November 2020; doi:10.1038/d41586-020-03235-y

About chemical recycling: We herein demonstrate an efficient conversion of polycarbonates and carbonates into monomer/diol using well‐defined, high‐valent Cp*Co(III)‐catalytic system. Commercially available polymer as well as compact disc (CD) converted into monomer through hydrogenation using molecular hydrogen.

Abstract

We herein report the catalytic hydrogenation of carbonates and polycarbonates into their corresponding diols/alcohols using well‐defined, air‐stable, high‐valent cobalt complexes. Several novel Cp*Co(III) complexes bearing N,O‐chelation were isolated for the first time and structurally characterized by various spectroscopic techniques including single crystal X‐ray crystallography. These novel Co(III) complexes have shown excellent catalytic activity to produce value added diols/alcohols from carbonate and polycarbonates through hydrogenation using molecular hydrogen as sole reductant or ⁱ PrOH as transfer hydrogenation source. To demonstrate the developed methodology's practical applicability, we have recycled the bisphenol A monomer from compact disc (CD) through hydrogenation under the established reaction conditions using phosphine‐free, earth‐abundant, air‐ and moisture‐stable high‐valent cobalt catalysts.

Using the bimetallic combination sBu₂Mg⋅2 LiOR (R=2‐ethylhexyl) in toluene enables a very fast regioselective Br/Mg exchange of dibromo(hetero)arenes in toluene. The regioselectivity of the exchange can be finely tuned by the coordination preference of lithium, which can be switched, in some cases, by the addition of Lewis donors such as PMDTA.

Abstract

Using the bimetallic combination sBu₂Mg⋅2 LiOR (R=2‐ethylhexyl) in toluene enables efficient and regioselective Br/Mg exchanges with various dibromo‐arenes and ‐heteroarenes under mild reaction conditions and provides bromo‐substituted magnesium reagents. Assessing the role of Lewis donor additives in these reactions revealed that N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (PMDTA) finely tunes the regioselectivity of the Br/Mg exchange on dibromo‐pyridines and quinolines. Combining spectroscopic with X‐ray crystallographic studies, light has been shed on the mixed Li/Mg constitution of the organometallic intermediates accomplishing these transformations. These systems reacted effectively with a broad range of electrophiles, including allyl bromides, ketones, aldehydes, and Weinreb amides in good yields.

Only one very bulky substituent R is required to stabilize the first stable heteroindanediyls (see picture), which represent a group of resonance‐stabilized phosphorus‐centered biradicals. Different types of oligomers formed by self‐activation of the aromatic backbone were observed for smaller substituents.

Abstract

Conversion of 1,2‐bis(dichlorophosphino)benzene with sterically demanding primary amines led to the formation of 1,3‐dichloro‐2‐aza‐1,3‐diphosphaindanes of the type C₆H₄(μ‐PCl)₂N‐R. Reduction yielded the corresponding 2‐aza‐1,3‐diphosphaindane‐1,3‐diyls (1), which can be described as phosphorus‐centered singlet biradical(oid)s. Their stability depends on the size of the substituent R: While derivatives with R=Dmp (2,6‐dimethylphenyl) or Ter (2,6‐dimesitylphenyl) underwent oligomerization, the derivative with very bulky R=^tBuBhp (2,6‐bis(benzhydryl)‐4‐tert‐butylphenyl) was stable with respect to oligomerization in its monomeric form. Oligomerization involved activation of the fused benzene ring by a second equivalent of the monomeric biradical and can be regarded as formal [2+2] (poly)addition reaction. Calculations indicate that the biradical character in 1 is comparable with literature‐known P‐centered biradicals. Ring‐current calculations show aromaticity within the entire ring system of 1.

Nature Communications, Published online: 17 November 2020; doi:10.1038/s41467-020-19723-8

CO₂ fixation by halogen‐bonding catalysis: A halogen bond is introduced into the catalytic cycloaddition of CO₂ into epoxide (CCE) reactions. N‐iodopyridinium halide salts are used as cationic halogen‐bond donor catalyst to activate the epoxides under ambient pressure to produce quantitative yields. This methodology could be a supplementary approach to classical hydrogen bonds to promote CCE reactions.

Abstract

Halogen bonding, parallel to hydrogen bonding, was introduced into the catalytic cycloaddition of carbon dioxide into epoxide (CCE) reactions. A series of halogen‐bond donor (XBD) catalysts of N‐iodopyridinium halide featured with N−I bond were synthesized and evaluated in CCE reactions. The optimal XBD catalyst, 4‐(dimethylamino)‐N‐iodopyridinium bromide ([DMAPI]Br), under screened conditions at 100 °C, ambient pressure, and 1 mol % catalyst loading, realized 93 % conversion of styrene oxide into cyclic carbonate in 6 h. The substrate scope was successfully extended with excellent yields (mostly ≥93 %) and quantitative selectivity (more than 99 %). ¹H NMR spectroscopy of the catalyst [DMAPI]Br on substrate epoxide certified that the N−I bond directly coordinated with the epoxide oxygen. A plausible mechanism of halogen‐bonding catalysis was proposed, in which the DMAPI cation functioned as halogen‐bond donor to activate the epoxide, and the counter anion bromide attacked the methylene carbon to initiate the ring‐opening of the epoxide. CCE reactions promoted by N‐iodopyridinium halide, exemplify a first case of halogen‐bonding catalysis in epoxide activation and CO₂ transformation.