Transition Metal Catalyst Free Synthesis of Olefins from Organoboron Derivatives**

Abstract

Stereoselective preparation of highly substituted olefins is still a severe challenge that requires well defined elimination precursors. Organoboron chemistry is particularly suited for the preparation of molecules with adjacent stereocenters. As organo boron substrates with leaving groups in β‐position can undergo stereospecific syn‐ or anti‐elimination, this chemistry harbors great potential for the synthesis of complex olefins. In recent years three main strategies emerged, which differ in their approach to the β‐functionalized organoboron elimination precursor. (i) Stereoselective preparation of such elimination precursor can be achieved by addition of a boron‐stabilized anion (d1) to an aldehyde or ketone (a1) or diastereoselective 1,3‐rearrangement of suitable boron‐ate‐complexes. Stereospecific methods rely either on (ii) diastereospecific 1,2‐metalate rearrangement of boron‐ate‐complexes that involve opening of appropriate heterocycles or (iii) addition of chiral carbenoids (d1*) to chiral boronates (a1*) with a leaving group in α‐position.

Differentially Protected Glycols from α‐Halo Boronic Esters and Lithiated Benzoates

“…Thus, combining chiral carbanions of type 2 , generated by direct lithiation, and α‐halo boronic esters of type 3 should deliver differentially protected threo ‐ or erythro ‐diols ( 1 a‐b) after oxidation of 4 . Unfortunately, boronates with a leaving group in β‐position (such as 4 ) are notoriously prone to elimination [8] . This has been exploited e. g. for boron‐Wittig type olefinations [9] .…”

“…Unfortunately, boronates with a leaving group in β-position (such as 4) are notoriously prone to elimination. [8] This has been exploited e. g. for boron-Wittig type olefinations. [9] Lithiated carbamates have been employed by Blakemore and coworkers for the stereospecific synthesis of alkenes.…”

A unified strategy for the synthesis of aldohexoses by boronate assisted assembly of CH2X2 derived C1-building blocks

“…Vinylation of 10a was achieved by Zweifel-olefination. 17 Although this reaction had not been described for the sterically hindered and thermodynamically stable pinanediol boronic esters, it proceeded reasonably well after some optimization (see ESI ‡ ) yielding 11a. The product contained some unidentified contaminations, which were removed after the next step, in which Sharpless bishydroxylation delivered the desired allitol-derivative 12a (32% yield, calculated over both steps).…”

“…Reaction of 26 with NaOMe and I 2 yielded vinyl iodide. Vinyllithium (the preferred reagent for Zweifel reactions) 17 was obtained by iodo–lithium exchange. Efficient reaction with α-bromoboronate 27, required transmetallation with MgBr 2 .…”