Transition-Metal-Free Borylation of Allylic and Propargylic Alcohols

Mechanistic Insights into the Metal-Free Deoxygenative Borylation of Ketones and Aldehydes with Bis(catecholato)diborane

The mechanisms of direct deoxygenative borylation of acetone and benzaldehyde with bis(catecholato)diborane (B₂ cat₂ ) in the solvent N,N-dimethylacetamide (DMA) are investigated through detailed density functional theory calculations. These calculations show that the isomer 1,2-B₂ cat₂ in situ generated from 1,1-B₂ cat₂ induced by DMA is the reactive boron intermediate for the reactions. The addition of the B-B bond of 1,2-B₂ cat₂ to the C=O bond of acetone or benzaldehyde via a concerted [2σ+2π]-cycloaddition-like transition state is the rate-limiting step for both the triboration reaction of acetone and the monoboration reaction of benzaldehyde. DMA not only acts as the solvent but also promotes the structural isomerization of B₂ cat₂ , the deoxygenation of acetone to form the vinyl boronate intermediate and subsequent diboration of vinyl boronate with 1,2-B₂ cat₂ , as well as the protodeboronation of the gem-diboronate intermediate in the deoxygenative borylation of benzaldehyde. The presented computational results can explain the observed experimental facts and provide insight into the roles of the isomeric 1,2-B₂ cat₂ and the solvent DMA in related reactions.

Biomimetic Total Synthesis and Investigation of the Non-Enzymatic Chemistry of Oxazinin A

We report the first total synthesis of an antimycobacterial natural product oxazinin A that takes advantage of a multi-component cascade reaction of anthranilic acid and a precursor polyketide containing an aldehyde. The route utilized for the synthesis of the pseudodimeric oxazinin A validates a previously proposed biosynthetic mechanism, invoking a non-enzymatic pathway to the complex molecule. We found a 76 : 10 : 9 : 5 ratio of oxazinin diastereomers from the synthetic cascade, which is an identical match to that found in the fermentation media from the fungus Eurotiomycetes 110162. Further investigation of the non-enzymatic formation of oxazinin A using 1 H-15 N HMBC NMR spectroscopy allowed for a plausible determination of the stepwise mechanism. The developed route is highly amenable for the synthesis of diverse sets of analogs around the oxazinin scaffold to study structure-activity relationships (SAR).

Copper-Mediated Dichotomic Borylation of Alkyne Carbonates: Stereoselective Access to (E)-1,2-Diborylated 1,3-Dienes versus Traceless Monoborylation Affording α-Hydroxyallenes

A mild copper-mediated protocol has been developed for borylation of alkynyl cyclic carbonates. Depending on the nature of the borylating reaction partner, either stereoselective diborylation of the propargylic surrogate takes place, providing convenient access to (E)-1,2-borylated 1,3-dienes, or traceless monoborylation occurs, which leads to α-hydroxyallenes as the principal product. The dichotomy in this borylation protocol has been scrutinized by several control experiments, illustrating that a relatively small change in the diboron(4) reagent allows for competitive alcohol-assisted protodemetalation to forge an α-hydroxyallene product under ambient conditions.

Hexahalodiborate Dianions: A New Family of Binary Boron Halides

The electron‐precise binary boron subhalide species [B₂X₆]²⁻ X=F, Br, I) were synthesized and their structures confirmed by X‐ray crystallography. The existence of the previously claimed [B₂Cl₆]²⁻, which had been questioned, was also confirmed by X‐ray crystallography. The dianions are isoelectronic to hexahaloethanes, are subhalide analogues of the well‐known tetrahaloborate anions (BX₄⁻), and are rare examples of molecular electron‐precise binary boron species beyond B₂X₄, BX₃, and [BX₄]⁻.

Synthesis and Reduction of Sterically Encumbered Mesoionic Carbene-Stabilized Aryldihaloboranes

Sterically hindered, in situ generated 1,3,4-substituted 1,2,3-triazol-5-ylidene mesoionic carbenes (MICs) were employed to stabilize a number of aryl- and heteroaryldihaloboranes, as well as the first MIC-supported diborane. Reduction of borane adducts of the 1-(2,6-diisopropylphenyl)-3-methyl-4-tert-butyl-1,2,3-triazol-5-ylidene ligand with KC₈ in non-coordinating solvents led to intramolecular C-H- and, C-C-activation at an isopropyl residue of the supporting ligand. DFT calculations showed that each of these activation reactions proceeds via a different isomer of a borylene intermediate.