Borylated reagents for multicomponent reactions

Solid-State Synthesis of B←N Adducts by the Amine-Facilitated Trimerization of the Phenylboronic Acid

Stable boroxine-amine adducts comprising dative B←N bond(s) were prepared by mechanochemically-induced reactions of phenylboronic acid (PBA) and amines (pyridine, DMAP, 1H-pyrazole, piperidine, DABCO, hexamethylenetetramine, or 4,4'-bipyridine). In-situ Raman monitoring, ex-situ PXRD and DFT calculations were used for product identification. Stoichiometry of the product (3 : 1, 3 : 2 or 6 : 1 adduct) was controlled by the amine structure and the molar ratio of the reactants. The 1 : 2 H-bonded assembly of PBA and 4,4'-bipyridine (bpy) was confirmed as an intermediate in the adduct formation for bpy. Competitive binding experiments indicated that the exchange of the amines in the 3 : 1 adducts follows the computed adduct stabilities that increase with the amine basicity. Following the DFT prediction, the first adduct with two different amines, DMAP and pip, bound to one boroxine moiety was isolated and structurally characterized. Results show that calculations can be used to predict possible and preferred product(s) and their spectral characteristics.

1,2-Metallate Rearrangement Using Indole Boronate Species to Access 2,3-Diarylindoles and Indolines

A transition metal-free multicomponent reaction using lithiated indole, boronic ester, pyridine, and ethyl chloroformate was developed to access C2,C3 bis-arylated indoles, which are present in several marketed drugs and bioactive compounds. One-pot access to unsymmetrical C2,C3-diaryl indole from the parent indole remains a huge synthetic challenge. Our group was able to achieve this goal through a transition metal-free 1,2-metalate rearrangement of the indole boronate complex. The reaction of indole boronate species with activated pyridine allows 1,2-migration to access pyridyl-indoleboronate species, which will convert to the corresponding indole upon oxidation and indoline after deborylation. The reaction tolerates substituted pyridines, quinolone, isoquinoline, and more. Both aryl and alkyl boronic esters were accommodated under optimized reaction conditions. Apart from mechanistic studies using ¹¹B-NMR, this methodology has been applied to the gram-scale synthesis of several bioactive compounds.

Towards depeptidized aminoboronic acid derivatives through the use of borylated iminium ions

Herein, we use α-boryl iminium intermediates to access progressively depeptidized branched β-aminoboronic acids that are functionalized with biologically relevant heterocycles. We investigate the interaction of these novel compounds with carbohydrates under physiological conditions and demonstrate their potential as synthetic building blocks.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

Photoredox-catalyzed multicomponent Petasis reaction in batch and continuous flow with alkyl boronic acids

Multicomponent reactions (MCRs) are ideal platforms for the generation of a wide variety of organic scaffolds in a convergent and atom-economical manner. Many strategies for the generation of highly substituted and diverse structures have been developed and among these, the Petasis reaction represents a viable reaction manifold for the synthesis of substituted amines via coupling of an amine, an aldehyde and a boronic acid (BA). Despite its synthetic utility, the inherent drawbacks associated with the traditional two-electron Petasis reaction have stimulated continuous research towards more facile and tolerant methodologies. In this regard, we present the use of free alkyl BAs as effective radical precursors in this MCR through a single-electron transfer mechanism under mild reaction conditions. We have further demonstrated its applicability to photo-flow reactors, facilitating the reaction scale-up for the rapid assembly of complex molecular structures.

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Multicomponent photocatalyzed Petasis reaction

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Boronic acids as alkyl radical precursors under mild reaction conditions

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Photo-flow chemistry application for Petasis reaction

Merging Boron with Nitrogen-Oxygen Bonds: A Review on BON Heterocycles

Cyclic boronate esters play important roles in organic synthesis, pharmacology, supramolecular chemistry and materials science owing to their stability in air and versatile reactivity. Most of these compounds contain a B-O-C linkage with an alkoxy- or carboxylate group bound to the boron atom (e.g. boronate-diol esters, MIDA boronates). Boron chelates comprising a B-O-N motif (BON heterocycles) are much less explored, although first representatives of this class were prepared in the early 1960s. In recent years, there has been a growing interest in BON heterocycles as new chemotypes for drug design. The exocyclic B-O-N linkage, which is readily formed under mild conditions, shows surprising hydrolytic and thermal resistance. This allows the formation of BON heterocycles to be used as click-type reactions for the preparation of bioconjugates and functionally modified polymers. We believe that BON heterocycles are promising yet underrated organoboron derivatives. This review summarizes the scattered information about known types of BON heterocycles, including their synthesis, reactivity and structural data. Available applications of BON heterocycles in materials science and medicinal chemistry, along with their prospects, are also discussed. The bibliography contains 289 references.

Transition metal catalyzed asymmetric multicomponent reactions of unsaturated compounds using organoboron reagents

Asymmetric multicomponent reactions allow stitching several functional groups in an enantioselective and atom economical manner. The introduction of boron-based reagents as a multicomponent coupling partner has its own merits. In addition to being non-toxic and highly stable, organoboron compounds can be easily converted to other functional groups in a stereoselective manner. In the last decade several transition metal catalyzed asymmetric multicomponent strategies have been evolved using boron based reagents. This review will discuss the merits and scope of multicomponent strategies based on their difference in the reaction mechanism and transition metals involved.

Cycloadditions of Alkenylboronic Derivatives

The literature on cycloaddition reactions of boron-containing alkenes is surveyed with 132 references. The data are categorized according to the reaction type ([2+1], [2+2], [3+2], [4+2], and [4+3] cycloadditions). The cyclopropanation and the Diels–Alder reactions of alkenylboronic derivatives have been studied more or less comprehensively, and for some substrates, they can be considered as convenient methods for the rapid regio- and stereoselective construction of even complex cyclic systems. Other types of the cycloadditions, as well as mechanistic aspects of the processes, have been addressed less thoroughly in the previous works.

1 Introduction

2 [2+1] Cycloaddition

2.1 Cyclopropanation

2.1.1 With Methylene Synthetic Equivalents

2.1.2 With Substituted Carbenoids

2.2 Epoxidation

2.3 Aziridination

3 [2+2] Cycloaddition

4 [3+2] Cycloaddition

4.1 With Nitrile Oxides

4.2 With Diazoalkanes

4.3 With Nitrones

4.4 With Azomethine Ylides

5 [4+2] Cycloaddition

6 [4+3] Cycloaddition

7 Conclusions and Outlook

Transition metal-free synthesis of alkyl pinacol boronates

This review systematically outlined the research in the area of transition metal free synthesis of alkyl pinacol boronates, which are versatile and important scaffolds to construct diverse organic compounds.

Rapid approach to complex boronic acids

The compatibility of free boronic acid building blocks in multicomponent reactions to readily create large libraries of diverse and complex small molecules was investigated. Traditionally, boronic acid synthesis is sequential, synthetically demanding, and time-consuming, which leads to high target synthesis times and low coverage of the boronic acid chemical space. We have performed the synthesis of large libraries of boronic acid derivatives based on multiple chemistries and building blocks using acoustic dispensing technology. The synthesis was performed on a nanomole scale with high synthesis success rates. The discovery of a protease inhibitor underscores the usefulness of the approach. Our acoustic dispensing-enabled chemistry paves the way to highly accelerated synthesis and miniaturized reaction scouting, allowing access to unprecedented boronic acid libraries.