The Suzuki coupling couples boronic acids to halides  

The Suzuki coupling of a boronic acid or ester with a vinyl or aryl halide or triflate is probably the most commonly used of all cross-coupling reactions. The original version, first reported in 1979, involved hydroboration of an alkyne with catecholborane, followed by palladium (0) catalysed coupling of the resulting vinyl boronate with an aromatic iodide or bromide. The hydroboration is generally regioselective for the less hindered position and addition of boron and hydrogen occurs cis stereospecifically.

As in the Stille coupling, the geometry of both unsaturated components is preserved during the coupling so this is an excellent method for the stereoselective synthesis of dienes. Hydroboration of octyne followed by hydrolysis of the boronate gave exclusively the E-vinyl boronic acid. Coupling with the Z-vinyl bromide in toluene with palladium (0) catalysis with potassium hydroxide as the base gave the E,Z-diene in good yield. These dienes are very useful in the Diels–Alder reaction.

This sort of reaction has been used in the synthesis of the unsaturated units of a range of natural products, including trisporol B. The key step is the stereocontrolled synthesis of an E,Z-diene. The geometry of both double bonds comes stereospecifically with retention of configuration from single geometrical isomers of the starting materials.

The mechanism of the Suzuki coupling is very similar to that of the Stille coupling. Oxidative addition of the vinylic or aromatic halide to the palladium (0) complex generates a palladium (II) intermediate. This then undergoes a transmetallation with the alkenyl boronate, from which the product is expelled by reductive elimination, regenerating the palladium (0) catalyst. The important difference is the transmetallation step, which explains the need for an additional base, usually sodium or potassium ethoxide or hydroxide, in the Suzuki coupling. The base accelerates the transmetallation step, leading to the borate directly, presumably via a more nucleophilic ‘ate’ complex.

Sterically demanding substrates are tolerated well and Suzuki coupling is often used for aryl–aryl cross-couplings. This example has three ortho substituents around the newly formed bond (marked in black) and still goes in excellent yield.

Aromatic heterocycles also couple well. The 2-position of a pyridine is very electrophilic and not at all nucleophilic but couplings at this position are fine with either the halide or the boronic acid in that position. Clearly, it is a mistake to see either of these sub stituents as contributing a ‘nucleophilic carbon’. It is better to see the reaction as a coupling of two equal partners with the two substituents (the halide and the boronic acid) as control elements to ensure cross-coupling and prevent dimerization. 

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مواضيع ذات صلة

Hydropalladation–dehydropalladation

Palladium is the most widely used metal in homogeneous catalysis

Insertion reactions are reversible

Carbon monoxide incorporation extends the carbon chain

Migratory insertion builds ligand structure

Reductive elimination removes metal atoms and forms new single bonds

Oxidative addition inserts metal atoms into single bonds

Bonding and reactions in transition metal complexes

Ligands can be attached in many different ways

The 18-electron rule

Transition metals extend the range of organic reactions

The Ritter reaction and the Beckmann fragmentation