Formation of Nucleophilic Allylboranes from Molecular Hydrogen and Allenes Catalyzed by a Pyridonate Borane that Displays Frustrated Lewis Pair Reactivity

Hetero Diels-Alder reactions of isolable N-borylenamines

A new strategy for N-borylenamines by reaction of 2-alkynyl benzyl azides with B(C6F5)3 was developed. This novel 1,3-carboboration reaction proceeded via a 5-exo-dig cyclization/formal 1,1-carboboration/B(C6F5)2 shift reaction sequence. Additionally, N-borylenamines can undergo hetero Diels-Alder (HDA) reactions with a variety of dienophiles. Our results are an attractive complement to HDA reactions.

Hydrogenation of Olefins, Alkynes, Allenes, and Arenes by Borane-Based Frustrated Lewis Pairs

In recent years, borane-based frustrated Lewis pairs have proved to be efficient hydrogenation catalysts and they have become an alternative to transition-metal-based systems. The hydrogen activation by classic FLPs leads to a protonated Lewis base and a borohydride. Consequently, hydrogenations catalyzed by classic FLPs consist of stepwise hydride transfer reactions and protonations (or vice versa). More recently, systems that operate via an initial hydroboration have allowed the substrate scope for FLP-catalyzed hydrogenations to be extended. In this review, hydrogenations of organic substrates catalyzed by borane­-based frustrated Lewis pairs are discussed. Emphasis is given to the mechanistic aspects of these catalytic reactions.

1 Introduction

2 FLP-Catalyzed Hydrogenation of Polarized Double Bonds

2.1 Hydrogenation of Michael Acceptors by FLPs

2.2 Asymmetric Hydrogenation of Polarized Double Bonds

2.3 Hydrogenation of Arenes and N-Heterocycles

3 Hydrogenation of Unactivated Olefins and Alkynes

3.1 Hydrogenation of Olefins and Alkynes by an Initial Hydroboration

4 Summary and Outlook

N-Heterocyclic Carbene Stabilized 1-Bora-1,3-butadienes

Deprotonation of [(NHC)(Fmes)B-allyl]⁺ borenium cations (NHC, IMes (a) or IMe₂ (b); Fmes, 2,4,6-(CF₃)₃C₆H₂) provides an easy entry to the NHC-stabilized 1-bora-1,3-butadienes. They feature a planar s-trans-conformation just like 1,3-butadiene. The 1-borabutadiene 7a undergoes hydroboration reactions; the HB(C₆F₅)₂ hydroboration product is trapped with CO or an isonitrile to give the respective cyclic zwitterionic borenium-borate enolate or enamide products. 1-Borabutadiene 7b undergoes 1,4-chalcogenation with elemental sulfur or selenium, and it gives the six-membered heterocyclic 1,4-addition product with the S═O bond of sulfur dioxide. Compound 7b served as a precursor for the formation of a borylated η³-allyl ligand at Ru. 7b formed a Rh complex by reaction with [Rh(ethylene)₂Cl]₂. It subsequently underwent an intramolecular C-H activation reaction to a mixture of η³-methyl-boraallyl Rh complex isomers.

Indene formation upon borane-induced cyclization of arylallenes, 1,1-carboboration, and retro-hydroboration

We herein report the reaction of arylallenes with tris(pentafluorophenyl)borane that yields pentafluorophenyl substituted indenes. The tris(pentafluorophenyl)borane induces the cyclization of the allene and transfers a pentafluorophenyl ring in the course of this reaction. A Hammett plot analysis and DFT computations indicate a 1,1-carboboration to be the C-C bond-forming step.

Controlling the Lewis Acidity and Polymerizing Effectively Prevent Frustrated Lewis Pairs from Deactivation in the Hydrogenation of Terminal Alkynes

Two strategies were reported to prevent the deactivation of Frustrated Lewis pairs (FLPs) in the hydrogenation of terminal alkynes: reducing the Lewis acidity and polymerizing the Lewis acid. A polymeric Lewis acid (P-BPh₃) with high stability was designed and synthesized. Excellent conversion (up to 99%) and selectivity can be achieved in the hydrogenation of terminal alkynes catalyzed by P-BPh₃. This catalytic system works quite well for different substrates. In addition, the P-BPh₃ can be easily recycled.

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.

Formation of Nucleophilic Allylboranes from Molecular Hydrogen and Allenes Catalyzed by a Pyridonate Borane that Displays Frustrated Lewis Pair Reactivity

Here we report the in situ generation of nucleophilic allylboranes from H₂ and allenes mediated by a pyridonate borane that displays frustrated‐Lewis‐pair reactivity. Experimental and computational mechanistic investigations reveal that upon H₂ activation, the covalently bound pyridonate substituent becomes a datively bound pyridone ligand. Dissociation of the formed pyridone borane complex liberates Piers borane and enables a hydroboration of the allene. The allylboranes generated in this way are reactive towards nitriles. A catalytic protocol for the formation of allylboranes from H₂ and allenes and the allylation of nitriles has been devised. This catalytic reaction is a conceptually new way to use molecular H₂ in organic synthesis.