Boronic esters reaction with halogen

Preparation of some azulenylmagnesium species was achieved by the halogen-magnesium exchange reactions of iodoazulenes with lithium tributylmagnesate at low temperatures (equations 29-33) . The reactions offer access to a variety of functionalized azulenes including azulenylphosphine, -stannane and -boronic ester. 

The cross-coupling of alkynylzinc halides or fluorinated alkenylzinc halides with fluori-nated alkenyl iodides allows the preparation of a range of fluorinated dienes or enynes - Functionalized allylic boronic esters can be prepared by the cross-coupling of (dialkylbo-ryl)methylzinc iodide 428 with functionalized alkenyl iodides. The intramolecular reaction provides cyclized products, such as 429 (Scheme 109) ° °. In some cases, reduction reactions or halogen-zinc exchange reactions are observed. 

Apart from the preceding processes, the hydroxy group in hydroxyalkyl-phosphonic or -phosphinic esters has been acylated straightforwardly or by carboxylic acids in the presence of dicyclohexylcarbodiimide " , a procedure also particularly useful for the N-acylation of aminoalkylphosphonic acids for the purpose of enantiomer analysis, phos-phitylated ", phosphorylated and replaced by halogen (Chapter 3, Section II.C. 1). Carbamates have been prepared from isocyanates or isothiocyanates " and hemiacetals formed in reactions with trichloroacetaldehyde the acetal 200 was prepared from ben-zaldehyde and the bis(a-hydroxybenzyl)phosphinic acid. Cyclic boron diesters have also been prepared. ... 

No examples of such reactions have been disclosed. Displacement of halogens on the parent heterocycle through metal-catalyzed processes have surprisingly not been reported to our knowledge on the neutral heterocycle. Recently, Suzuki-Miyaura cross-coupling reactions of imidazolium bromide 113 with various boronic acids or esters were reported <2005T6207> to proceed in good yield, without deprotonation at the C-3 position (Scheme 35). 

Bombykol is a well-known pheromone, first isolated from Bombyx mori L. Bombykol the three related isomers were synthesized by cross-coupling reaction. Three alkenylboronates or boronic acids (35 and 36) required for the coupling are prepared by starting from two alkynes. The stereoselective syntheses of ( )- and (Z)-l-alkenylboronic acids or esters have been discussed in Section 2.2.1. Halogenation of alkenylboronic acid 34 with either iodine or bromine provides ( )- and (Z)-haloalkenes [71]. The palladium-and base-assisted coupling of the Cj-boronate 32 or 33 with the C,-halide 35 or 36 stereoselectively provides bombykol and its three geometrical isomers (Scheme 2-29) [72]. 

In the aromatic and heterocyclic series, no hydroboration is possible. In general, an organoalkali intermediate is prepared by metalation or halogen/metal permutation before being treated with a boric acid derivative such as trimethyl borate or, because of its cleaner reaction, triisopropyl borate. The resulting ate complex sets free the oxidizable boronate upon addition of water or a stoichiometric amount of diluted hydrochloric acid. The boronate can be accessed directly when fluorodimethoxyborane is used instead of a boric acid ester as the adduct eliminates lithium fluoride spontaneously. The oxidation relies on the same procedures and principles as outlined above. 

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