Identification of a Boron-Containing Intermediate in the Boron Tribromide Mediated Aryl Propargyl Ether Cleavage Reaction

Cu-Catalyzed Tandem Oxidation-Intramolecular Cannizzaro Reaction of Biorenewables and Bioactive Molecules

A tandem Cu-catalyzed oxidation-intramolecular Cannizzaro reaction leading to bioactive α-hydroxyesters from α-hydroxyketones is reported. The process uses oxygen as a sole oxidant to achieve the formation of glyoxals, which are efficiently converted in situ to important α-hydroxyesters. The mechanistic insights are provided by isotopic labeling and supported by DFT calculations. The transformation proved a robust synthetic tool to achieve the synthesis of human metabolites and hydroxyl esters of various biologically active steroid derivatives.

Haloboration: scope, mechanism and utility

Haloboration, the addition of B–X (X = Cl, Br, I) across an unsaturated moiety e.g., C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Y or C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Y (Y = C, N, etc.), is dramatically less utilised than the ubiquitous hydroboration reaction. However, haloboration of alkynes in particular is a useful tool to access ambiphilic 1,2-disubstituted alkenes. The stereochemical outcome of the reaction is easily controlled and the resulting products have proven to be valuable building blocks in organic synthesis and materials chemistry. This review aims at providing the reader with a brief summary of the historic development and of the current mechanistic understanding of this transformation. Recent developments are discussed and select examples demonstrating the use of haloboration products are given with a focus on the major areas, specifically, natural product synthesis and the development of boron-doped polycyclic aromatic hydrocarbons (B-PAHs).

Haloboration is a mostly overlooked technique, yet it is a powerful way of transforming alkynes stereoselectively into difunctionalised ambiphilic alkenes, which readily undergo a plethora of highly useful subsequent reactions.

Divergent Elementoboration: 1,3-Haloboration versus 1,1-Carboboration of Propargyl Esters

This work showcases the 1,3-haloboration reaction of alkynes in which boron and chlorine add to propargyl systems in a proposed sequential oxazoliumborate formation with subsequent ring-opening and chloride migration. In addition, the functionalization of these propargyl esters with dimethyl groups in the propargylic position leads to stark differences in reactivity whereby a formal 1,1-carboboration prevails to give the 2,2-dichloro-3,4-dihydrodioxaborinine products as an intramolecular chelate. Density functional theory calculations are used to rationalize the distinct carboboration and haloboration pathways. Significantly, this method represents a metal-free route to highly functionalized compounds in a single step to give structurally complex products.

Crystal structure of 4-(prop-2-yn-1-yl-oxy)benzo-nitrile

In the title compound, C₁₀H₇NO, the dihedral angle between the aromatic ring and the prop-2-yn-1-yl­oxy grouping is 9.47 (10)°. The bond lengths indicate electronic conjugation between the cyano group, the benzene ring and the propyn­yloxy oxygen atom. In the crystal, a hydrogen bond between the acetyl­enic C—H atom and the cyano nitro­gen atom link the mol­ecules into wave-like [30-1] C(11) chains. These chains are connected by Csp²—H⋯π_ac (π_ac is the acetyl­inic C—C triple bond) close contacts [2.794 (1) Å], resulting in a rolling sheet structure parallel to the ac plane and aromatic π–π stacking inter­actions between the sheets [centroid–centroid distance = 3.593 (2) Å] generate a three-dimensional network.

Organocatalytic functionalisation through boron chemistry

Diboron reagents can be activated both, from a metal or free-metal context, and consequently the addition of boryl units to unsaturated substrates proceeds sometimes with complementary selectivity. We highlight here the power of boron chemistry to functionalize molecules and provide new routes towards challenging compounds.

Haloboration of internal alkynes with boronium and borenium cations as a route to tetrasubstituted alkenes

Hail boration! 2-Dimethylaminopyridine-ligated dihaloborocations [X2B(2-DMAP)](+) with a strained four-membered boracycle were used for the haloboration of terminal and dialkyl internal alkynes (see scheme). Esterification then provided vinyl boronate esters as useful precursors to tetrasubstituted alkenes. Following mechanistic studies, the scope of the haloboration was expanded simply by variation of the amine. Pin = 2,3-dimethyl-2,3-butanedioxy.

Preparation of (Z)-1,2-dichloroalkenes from terminal alkynes

(Z)-1,2-Dihaloalkenes are thermodynamically disfavoured of the two stereoisomers. This paper reports the synthesis of some (Z)-1,2-dichloroalkene analogues from mucochloric acid. A more versatile approach involved the chloroboration of terminal alkynes to yield corresponding (Z)-chloroboronic acid as a first step. Treatment of the organoboronic acid with potassium hydrogen difluoride followed by tetrabutylammonium trichloride gave (Z)-1,2-dichloroalkenes in moderate to good yields in a stereospecific manner.