Sebastian Beil

Nature Reviews Chemistry, Published online: 03 June 2020; doi:10.1038/s41570-020-0191-2

Go with the glow : A two‐step process has been developed that transforms parent diketopyrrolopyrrole into structures with up to 10 fused rings. The absorptions are bathochromically shifted to the deep‐red region and strong red emission is seen, even in nonpolar solvents, despite the presence of NO₂ groups.

Abstract

Red‐emissive π‐expanded diketopyrrolopyrroles (DPPs) with fluorescence reaching λ=750 nm can be easily synthesized by a three‐step strategy involving the preparation of diketopyrrolopyrrole followed by N‐arylation and subsequent intramolecular palladium‐catalyzed direct arylation. Comprehensive spectroscopic assays combined with first‐principles calculations corroborated that both N‐arylated and fused DPPs reach a locally excited (S₁) state after excitation, followed by internal conversion to states with solvent and structural relaxation, before eventually undergoing intersystem crossing. Only the structurally relaxed state is fluorescent, with lifetimes in the range of several nanoseconds and tens of picoseconds in nonpolar and polar solvents, respectively. The lifetimes correlate with the fluorescence quantum yields, which range from 6 % to 88 % in nonpolar solvents and from 0.4 % and 3.2 % in polar solvents. A very inefficient (T₁) population is responsible for fluorescence quantum yields as high as 88 % for the fully fused DPP in polar solvents.

Co‐Operation : An allied cooperation between cobalt catalysis and electrochemical synthesis enabled the mild catalytic carboxylation of allylic chlorides with atmospheric CO₂. The resulting products are useful as versatile synthons of γ‐arylbutyrolactones.

Abstract

The chemical use of CO₂ as an inexpensive, nontoxic C1 synthon is of utmost topical interest in the context of carbon capture and utilization (CCU). We present the merger of cobalt catalysis and electrochemical synthesis for mild catalytic carboxylations of allylic chlorides with CO₂. Styrylacetic acid derivatives were obtained with moderate to good yields and good functional group tolerance. The thus‐obtained products are useful as versatile synthons of γ‐arylbutyrolactones. Cyclic voltammetry and in operando kinetic analysis were performed to provide mechanistic insights into the electrocatalytic carboxylation with CO₂.

Vicinal diamines are ubiquitous materials in organic and medicinal chemistry, and methods for their preparation are of relevance to a broad range of chemical applications. The direct coupling of olefin and amine building blocks would represent an ideal approach to construct these motifs. However, alkene diamination remains a long‐standing challenge in organic synthesis, especially when two different amine components need to be introduced. Here we report a general strategy for the direct and selective assembly of vicinal 1,2‐diamines using readily available olefin and amine building blocks. This mild and straightforward approach exploits the in situ formation and photoinduced activation of N ‐chloroamines leading to aminium radicals that enable efficient alkene aminochlorination. Owing to the ambiphilic nature of the b ‐chloroamines accessed, conversion into tetra‐alkyl aziridinium ions was possible, thus enabling diamination by regioselective ring‐opening with both primary and secondary amines. Overall, this strategy streamlines the preparation of vicinal diamines from current multi‐step sequences to a single chemical transformation. This operationally simple process has broad functional group compatibility and enables the use of advanced building blocks for the preparation of drug‐like small molecules.

Come full circle : A highly chemoselective cobaloxime‐catalyzed acceptorless dehydrogenative cyclization of o ‐teraryls was developed. Due to the acid‐ and oxidant‐free conditions, diverse substrates with various electronic properties and sensitive functional groups, including electron‐poor arenes and electron‐rich heterocycles, are tolerated.

Abstract

A cobaloxime‐catalyzed acceptorless dehydrogenative cyclization of o ‐teraryls was developed. In stark contrast to the established methods such as the Scholl or Mallory reactions, this method does not require any strong acids or oxidants, and shows high atom economy and a broad substrate scope. It operates at near room temperature with light as the source of energy. Acid‐ or oxidant‐sensitive functional groups, such as 4‐methoxyphenyl, unprotected benzyl alcohol, silyl ether, and thiophene groups are tolerated. Remarkably, aryls with electron‐withdrawing groups, and electron‐poor heteroarenes, such as pyridine and pyrimidine, can also react. Preliminary mechanistic study reveals that hydrogen gas is released during the reaction, and both light and the cobalt catalyst are important for the dehydrogenation step.

Best left for later: Catenanes with dissymmetric cages (CDCs) were prepared by first synthesizing imine‐based CSCs (catenanes with symmetric cages) and then selectively reducing the outer surface with the voluminous reductant NaBH(OAc)₃ (see picture). Experimental and computational analysis further elucidated the template‐facilitated interlocking mechanism for the construction of three‐dimensional catenanes composed of purely organic constituents.

Abstract

Considerable efforts have been made to increase the topological complexity of mechanically interlocked molecules over the years. Three‐dimensional catenated structures composed of two or several (usually symmetrical) cages are one representative example. However, owing to the lack of an efficient universal synthetic strategy, interlocked structures made up of dissymmetric cages are relatively rare. Since the space volume of the inner cavity of an interlocked structure is smaller than that outside it, we developed a novel synthetic approach with the voluminous reductant NaBH(OAc)₃ that discriminates this space difference, and therefore selectively reduces the outer surface of a catenated dimer composed of two symmetric cages, thus yielding the corresponding catenane with dissymmetric cages. Insight into the template effect that facilitates the catenation of cages was provided by computational and experimental techniques.

Nature Chemistry, Published online: 20 April 2020; doi:10.1038/s41557-020-0455-y

Forget about forbiddance: Spectroscopic experiments reveal that photoreactions of heavy‐atom‐containing molecules with light in an apparently non‐absorbing region proceed via a forbidden direct S₀→T _n transition. This phenomenon occurs in the photoreaction of aryl iodides (oxidation states I, III, and V), aryl bromides, and bismuth(III)‐containing molecules.

Abstract

According to the Grotthuss–Draper law, light must be absorbed by a substrate to initiate a photoreaction. There have been several reports, however, on the promotion of photoreactions using hypervalent iodine during irradiation with light from a non‐absorbing region. This contradiction gave rise to a mystery regarding photoreactions involving hypervalent iodine. We demonstrated that the photoactivation of hypervalent iodine with light from the apparently non‐absorbing region proceeds via a direct S₀→T _n transition, which has been considered a forbidden process. Spectroscopic, computational, and synthetic experimental results support this conclusion. Moreover, the photoactivation mode could be extended to monovalent iodine and bromine, as well as bismuth(III)‐containing molecules, providing new possibilities for studying photoreactions that involve heavy‐atom‐containing molecules.

Re‐Arrangement silicissimo ! Vinylcyclopropanes (VCPs) undergo a formal (5+1) cycloaddition when reacted with in situ generated silylium ions. The bond reorganization is connected to an aryl migration to eventually yield a silacyclohexane derivative which is regioisomeric to the expected product. Reaction mechanisms that rationalize the formation of the major and minor products are presented based on a series of control experiments and quantum‐chemical calculations.

Abstract

A transition‐metal‐free (5+1) cycloaddition of aryl‐substituted vinylcyclopropanes (VCPs) and hydrosilanes to afford silacyclohexanes is reported. Catalytic amounts of the trityl cation initiate the reaction by hydride abstraction from the hydrosilane, and further progress of the reaction is maintained by self‐regeneration of the silylium ions. The new reaction involves a [1,2] migration of an aryl group, eventually furnishing 4‐ rather than 3‐aryl‐substituted silacyclohexane derivatives as major products. Various control experiments and quantum‐chemical calculations support a mechanistic picture where a silylium ion intramolecularly stabilized by a cyclopropane ring can either undergo a kinetically favored concerted [1,2] aryl migration/ring expansion or engage in a cyclopropane‐to‐cyclopropane rearrangement.

A 2,6‐anthrylene‐linked bis(m‐terphenylcarboxylic acid) strand forms a one‐handed homo double‐helix induced by chiral amines, thereby producing the chiral anti‐photodimer with up to 98 % enantiomeric excess upon photoirradiation. The chirality of the anti‐photodimer can be readily controlled by the chirality of the chiral amines.

Abstract

A novel 2,6‐anthrylene‐linked bis(m‐terphenylcarboxylic acid) strand (1) self‐associates into a racemic double‐helix. In the presence of chiral mono‐ and diamines, either a right‐ or left‐handed double‐helix was predominantly induced by chiral amines sandwiched between the carboxylic acid strands with accompanying stacking of the two prochiral anthracene linker units in an enantiotopic face‐selective way, as revealed by circular dichroism and NMR spectral analyses. The photoirradiation of the optically active double helices complexed with chiral amines proceeded in a diastereo‐ (anti or syn) and enantiodifferentiating way to afford the chiral anti‐photodimer with up to 98 % enantiomeric excess when (R)‐phenylethylamine was used as a chiral double‐helix inducer. The resulting optically active anti‐photodimer can recognize the chirality of amines and diastereoselectively complex with chiral amines.

Transition metal–catalyzed coupling reactions have become one of the most important tools in modern synthesis. However, an inherent limitation to these reactions is the need to balance operations, because the factors that favor bond cleavage via oxidative addition ultimately inhibit bond formation via reductive elimination. Here, we describe an alternative strategy that exploits simple visible-light excitation of palladium to drive both oxidative addition and reductive elimination with low barriers. Palladium-catalyzed carbonylations can thereby proceed under ambient conditions, with challenging aryl or alkyl halides and difficult nucleophiles, and generate valuable carbonyl derivatives such as acid chlorides, esters, amides, or ketones in a now-versatile fashion. Mechanistic studies suggest that concurrent excitation of palladium(0) and palladium(II) intermediates is responsible for this activity.