1,2-Carboboration of Arylallenes by In Situ Generated Alkenylboranes for the Synthesis of 1,4-Dienes

We herein report a novel method for the coupling of unactivated alkynes and arylallenes, which relies on an unprecedented and regioselective 1,2‐carboboration of the allene by an alkenylborane. The alkenylborane is conveniently prepared in situ by hydroboration of an alkyne with Piers’ borane, i. e., HB(C₆F₅)₂. The boryl‐substituted 1,4‐dienes that are formed by this carboboration are well‐suited for a subsequent Suzuki‐Miyaura coupling with aryl iodides. This allowed us to develop a three‐step, one‐pot protocol for the synthesis of aryl‐substituted 1,4‐dienes. The generality of the reaction was demonstrated by the synthesis of twenty dienes with modular variations of all three reaction partners. The mechanism of the new 1,2‐carboboration was investigated using dispersion corrected double‐hybrid DFT computations that allowed us to rationalize the chemo‐ and regioselectivity of this key step.

Organocatalytic insertion into C–B bonds by in situ generated carbene: mechanism, role of the catalyst, and origin of stereoselectivity

The calculated results showed that C–H⋯X (X = I, Br) hydrogen bond interactions should be the determining factors of stereoselectivity, and FMO overlap mode and ELF analyses along IRC results were performed to explore the nature of carbene insertion.

Piers' Borane-Induced Tetramerization of Arylacetylenes

We herein report that the reaction of Piers′ borane, i. e. HB(C₆F₅)₂, with an excess of arylacetylenes at room temperature leads to tetramerization of the acetylene and the diastereoselective formation of boryl‐substituted tetra‐aryl‐tetrahydropentalenes. The reaction mechanism was investigated by isotope labeling experiments and DFT computations. These investigations indicate that a series of 1,2‐carboboration reactions form an octatetraene that undergoes an electrocyclization. Two skeletal rearrangements then presumably lead to the formation of the tetrahydropentalene core. Overall, this intricate and unprecedented transformation comprises five carbon‐carbon bond formations in a single reaction.

Small Molecule Activation with Intramolecular "Inverse" Frustrated Lewis Pairs

The intramolecular "inverse" frustrated Lewis pairs (FLPs) of general formula 1-BR₂ -2-[(Me₂ N)₂ C=N]-C₆ H₄ (3-6) [BR₂ =BMes₂ (3), BC₁₂ H₈ , (4), BBN (5), BBNO (6)] were synthesized and structurally characterized by multinuclear NMR spectroscopy and X-ray analysis. These novel types of pre-organized FLPs, featuring strongly basic guanidino units rigidly linked to weakly Lewis acidic boryl moieties via an ortho-phenylene linker, are capable of activating H-H, C-H, N-H, O-H, Si-H, B-H and C=O bonds. 4 and 5 deprotonated terminal alkynes and acetylene to form the zwitterionic borates 1-(RC≡C-BR₂ )-2-[(Me₂ N)₂ C=NH]-C₆ H₄ (R=Ph, H) and reacted with ammonia, BnNH₂ and pyrrolidine, to generate the FLP adducts 1-(R₂ HN→BR₂ )-2-[(Me₂ N)₂ C=NH]-C₆ H₄ , where the N-H functionality is activated by intramolecular H-bond interactions. In addition, 5 was found to rapidly add across the double bond of H₂ CO, PhCHO and PhNCO to form cyclic zwitterionic guanidinium borates in excellent yields. Likewise, 5 is capable of cleaving H₂ , HBPin and PhSiH₃ to form various amino boranes. Collectively, the results demonstrate that these new types of intramolecular FLPs featuring weakly Lewis acidic boryl and strongly basic guanidino moieties are as potent as conventional intramolecular FLPs with strongly Lewis acidic units in activating small molecules.

Late 3d Metal-Catalyzed (Cross-) Dimerization of Terminal and Internal Alkynes

This review will outline the recent advances in chemo-, regio-, and stereoselective (cross-) dimerization of terminal alkynes to generate 1,3-enynes using different types of iron and cobalt catalysts with altering oxidation states of the active species. In general, the used ligands have a crucial effect on the stereoselectivity of the reaction; e.g., bidentate phosphine ligands in cobalt catalysts can generate the E-configured head-to-head dimerization product, while tridentate phosphine ligands can generate either the Z-configured head-to-head dimerization product or the branched head-to-tail isomer. Furthermore, the hydroalkynylation of silyl-substituted acetylenes as donors to internal alkynes as acceptors will be discussed using cobalt and nickel catalysts.

Boron-Ligand Cooperation: The Concept and Applications

The term boron-ligand cooperation was introduced to describe a specific mode of action by which certain metal-free systems activate chemical bonds. The main characteristic of this mode of action is that one covalently bound substituent at the boron is actively involved in the bond activation process and changes to a datively bound ligand in the course of the bond activation. Within this review, how the term boron-ligand cooperation evolved is reflected on and examples of bond activation by boron-ligand cooperation are discussed. It is furthermore shown that systems that operate via boron-ligand cooperation can complement the reactivity of classic intramolecular frustrated Lewis pairs and applications of this new concept for metal-free catalysis are summarized.

Synthesis of 6-Adamantyl-2-pyridone and Reversible Hydrogen Activation by the Corresponding Bis(perfluorophenyl)borane Complex

We herein describe the two-step synthesis of 6-adamantyl-2-pyridone from 1-acetyladamantane. The borane complex derived from 6-adamantyl-2-pyridone and the Piers borane liberates dihydrogen at 60 °C. The reverse reaction, hydrogen activation by the formed pyridonate borane is accomplished under mild conditions. The mechanism of the hydrogen activation is studied by DFT computations.

Semihydrogenation of Alkynes Catalyzed by a Pyridone Borane Complex: Frustrated Lewis Pair Reactivity and Boron-Ligand Cooperation in Concert

The metal‐free cis selective hydrogenation of alkynes catalyzed by a boroxypyridine is reported. A variety of internal alkynes are hydrogenated at 80 °C under 5 bar H₂ with good yields and stereoselectivity. Furthermore, the catalyst described herein enables the first metal‐free semihydrogenation of terminal alkynes. Mechanistic investigations, substantiated by DFT computations, reveal that the mode of action by which the boroxypyridine activates H₂ is reminiscent of the reactivity of an intramolecular frustrated Lewis pair. However, it is the change in the coordination mode of the boroxypyridine upon H₂ activation that allows the dissociation of the formed pyridone borane complex and subsequent hydroboration of an alkyne. This change in the coordination mode upon bond activation is described by the term boron‐ligand cooperation.

Reversible OH-bond activation and amphoterism by metal-ligand cooperativity of calix[4]pyrrolato aluminate

Most p-block metal amides irreversibly react with metal alkoxides when subjected to alcohols, making reversible transformations with OH-substrates a challenging task. Herein, we describe how the combination of a Lewis acidic square-planar-coordinated aluminum(iii) center with metal–ligand cooperativity leverages unconventional reactivity toward protic substrates. Calix[4]pyrrolato aluminate performs OH-bond activation of primary, secondary, and tertiary aliphatic and aromatic alcohols, which can be fully reversed under reduced pressure. The products exhibit a new form of metal–ligand cooperative amphoterism and undergo counterintuitive substitution reactions of a polar covalent Al–O bond by a dative Al–N bond. A comprehensive mechanistic picture of all processes is buttressed by isolation of intermediates, spectroscopy, and computation. This study delineates how structural constraints can invert thermodynamics for seemingly simple addition reactions and invert common trends in bond energies.

The combination of structural constraint and metal–ligand cooperativity in calix[4]pyrrolato aluminate inverts common trends of bond energies and enables reversible OH-bond activation.

Rhodium(I)-NHC Complexes Bearing Bidentate Bis-Heteroatomic Acidato Ligands as gem-Selective Catalysts for Alkyne Dimerization

A series of Rh(κ² -BHetA)(η² -coe)(IPr) complexes bearing 1,3-bis-hetereoatomic acidato ligands (BHetA) including carboxylato (O,O), thioacetato (O,S), amidato (O,N), thioamidato (N,S), and amidinato (N,N), have been prepared by reaction of the dinuclear precursor [Rh(μ-Cl)(IPr)(η² -coe)]₂ with the corresponding anionic BHetA species. The Rh^I -NHC-BHetA compounds catalyze the dimerization of aryl alkynes, showing excellent selectivity for the head-to-tail enynes. Among them, the acetanilidato-based catalyst has shown an outstanding catalytic performance reaching unprecedented TOF levels of 2500 h^-1 with complete selectivity for the gem-isomer. Investigation of the reaction mechanism supports a non-oxidative pathway in which the BHetA ligand behaves as proton shuttle through intermediate κ¹ -HBHetA species. However, in the presence of pyridine as additive, the identification of the common Rh^III H(C≡CPh)₂ (IPr)(py)₂ intermediate gives support for an alternative oxidative route.

Dimerization of terminal alkynes promoted by a heterobimetallic Zr/Co complex

Enynes are important synthetic intermediates that are also found in a variety of natural products and other biologically relevant molecules. The most atom economical synthetic route to enynes is via the direct coupling of readily available terminal alkyne precursors. Towards this goal, we demonstrate the formation of 1,3-enynes from terminal alkynes facilitated by a reduced ZrIV/Co-I heterobimetallic complex. An intermediate is trapped as a tBuNC adduct, revealing that bimetallic activation of the terminal C-H bond of the alkyne is an essential mechanistic step.