Stereoselective Direct Chlorination of Alkenyl MIDA Boronates: Divergent Synthesis of E and Z α-Chloroalkenyl Boronates

NHC-Au-Catalyzed Isomerization of Propargylic B(MIDA)s to Allenes and Double Isomerization of Alkynes to 1,3-Dienes

The synthesis of allenyl boronates is an important yet challenging topic in organic synthesis. Reported herein is an NHC-gold-catalyzed 1,3-H shift toward allenyl boronates synthesis from simple propargylic B(MIDA)s. Mechanistic studies suggest dual roles of the boryl moiety in the reaction: to activate the substrate for isomerization and at the same time, to prevent the allene product from further isomerization. These effects should be a result of α-anion stabilization and α-cation destabilization conferred by the B(MIDA) moiety, respectively. The NHC-Au catalyst, which is commercially available, is also found to be reactive in alkyne-to-1,3-diene isomerization reactions in an atom-economic and base-free manner.

Formyl MIDA Boronate: C1 Building Block Enables Straightforward Access to α-Functionalized Organoboron Derivatives

Formyl MIDA boronate has been known to be an elusive type of acylboronate that has not been obtained to date. In this work, an approach to the one-pot preparation and chemical transformations of formyl MIDA boronate were developed to provide new types of α-functionalized organoboron compounds. Among them are acylboronate reagents which present boron-substituted analogues of ynones and β-dicarbonyl compounds. The developed synthetic procedures, utilizing formyl MIDA boronate, are tolerant to diverse functional groups, making this reagent an advantageous C₁ building block for extending the scope of organoboron chemistry.

Synthesis of Acylboron Compounds

Acylboron compounds are emerging as versatile functional groups with applications in multiple research fields. Their synthesis, however, is still challenging and requires innovative methods. This Minireview provides an overview on the obstacles of acylboron synthesis and highlights notable advances within the last three years on new strategies to overcome the challenges posed by the formation of acyl-boron bonds.

Halohydroxylation of alkenyl MIDA boronates: switchable stereoselectivity induced by B(MIDA) substituent

A switchable stereoselectivity in the halohydroxylation of alkenyl MIDA boronates was found. C–B(MIDA) bond hyperconjugation plays the key role.

Geometric E→Z Isomerisation of Alkenyl Silanes by Selective Energy Transfer Catalysis: Stereodivergent Synthesis of Triarylethylenes via a Formal anti-Metallometallation

An efficient geometrical E→Z isomerisation of alkenyl silanes is disclosed via selective energy transfer using an inexpensive organic sensitiser. Characterised by operational simplicity, short reaction times (2 h), and broad substrate tolerance, the reaction displays high selectivity for trisubstituted systems (Z/E up to 95:5). In contrast to thermal activation, directionality results from deconjugation of the π-system in the Z-isomer due to A1,3 -strain thereby inhibiting re-activation. The structural importance of the β-substituent logically prompted an investigation of mixed bis-nucleophiles (Si, Sn, B). These versatile linchpins also undergo facile isomerisation, thereby enabling a formal anti-metallometallation. Mechanistic interrogation, supported by a theoretical investigation, is disclosed together with application of the products to the stereospecific synthesis of biologically relevant target structures.

Diversity-Oriented Synthesis of α-Functionalized Acylborons and Borylated Heteroarenes by Nucleophilic Ring Opening of α-Chloroepoxyboronates

The ring-opening reactions of N-methyliminodiacetyl (MIDA) α-chloroepoxyboronates with different nucleophiles allow the modular synthesis of a diverse array of organoboronates. These include seven types of α-functionalized acylboronates and seven types of borylated heteroarenes, some of which are difficult-to-access products using alternative methods. The common synthons, α-chloroepoxyboronates, could be viably synthesized by a two-step procedure from the corresponding alkenyl MIDA boronates. Mild reaction conditions, good functional-group tolerance, and generally good efficiency were observed. The utility of the products was also demonstrated.

Regio- and Diastereoselective Synthesis of Cyclohexadienylborons via an Intermolecular Diels-Alder Reaction of Alkenyl MIDA Boronates with 2-Pyrones

An efficient synthesis of highly functionalized cyclohexadienylborons via an inverse electron-demand Diels-Alder reaction/CO₂ extrusion of alkenyl MIDA boronates with 2-pyrones is outlined. By controlling the reaction temperature, the corresponding C(sp³ )-rich bicyclolactones could also be readily formed. The exo-selective reactions feature good functional-group tolerance, broad substrate scope, and excellent regio- and diastereoselectivity. Oxidation of the cyclohexadienylborons in a one-pot procedure led to the construction of aromatic boronates bearing valuable functional groups. Synthetic transformations of the C-B bond were demonstrated.