Aryl boron compounds

Aryl boron compounds, Ar,B(OH)3 (n = 1-3), bearing electron-withdrawing aromatic groups such as triarylboranes,... 

CHEMICAL PROPERTIES thermally unstable water reactive hydrolyzes in water to hydrogen and boric acid reacts with ammonia to form diborane diammoniate reacts slowly with bromine to form boron bromides reacts with hydrocarbons or organoboron compounds to give alkyl- or aryl-boron compounds reacts with metal alkyls to form metal borohydrides reacts with strong electron pair donors to form borane addition compounds FP (-90°C, -130°F) LFL/UFL (0.9%, 98%) AT (40-50°C, 104-122°F) HF (35.6 kJ/mol gas at 25°C). 

Scheme 5.23 Palladium(ll)-catalyzed 1,4-addition of aryl boron compounds to a,p-unsaturated carbonyl compounds, as described by Miyaura and coworkers [73].

In many instances, the scope of the reactions listed above is limited as to the functional groups that are unaffected. Except for aryl-stannanes and arylfluorosilanes, carbonyl functionalities are often not compatible with the reactions. A number of cross-coupling reactions are known of aryl triflates [64 a] and arylfluorosulfonates [64b] with aryl stan-nanes catalysed by Pd(0)-complexes (Stille reaction) [65], aryl zinc chlorides [64b], and aryl boron compounds (boronic acids and esters, Suzuki reaction [66]). Aryl stannates are relatively stable to air and moisture and many funetional groups are tolerated. The coupling reaction using Ar-X/Ar-SnR3 can be accelerated by Cu(I) [67] or Ag(I) [68] catalysis. 

Dimethylsulfoxide Acyl and aryl halides, boron compounds, bromomethane, nitrogen dioxide, magnesium perchlorate, periodic acid, silver difluoride, sodium hydride, sulfur trioxide... 

The aryl tin compounds are better substrates for fluorination because they give high yields of fliiorinated aromatics and they may be fluorinated with acetyl hypofluorite, cesium fluoroxysulfate, or fluorine [52, 54 (equation 28). Aryl boronic esters react with cesium fluoroxysulfate to produce fluoroaromatics [55] (equation 29). 

High ee values have also been obtained with organometallics," including organotitanium compounds (methyl, aryl, allylic) in which an optically active ligand is coordinated to the titanium," allylic boron compounds, and organozinc compounds. 

The widespread use of aryl boronic acids or aryl boronates in various metal-catalysed C-C bond-forming reactions has created a substantial demand for these versatile nncleophiles. A general procedure for the preparation of these compounds, based on a NHC/Pd catalysed coupling, has been reported by Fiirtsner and co-workers nsing aryl chlorides and the tetraalkoxy diboron derivative 27 as conpling partners. Very good yields were obtained in several cases especially when electron poor aryls were employed [169]. Milder reaction conditions can be achieved when diazonium salts are used instead of chlorides [170] (Scheme 6.51). 

After these seminal studies, the use of NHC-Rh systems for the addition of aryl boronic acids to carbonyl compounds became a very fertile area and many groups have reported on variations of this catalytic system [22],... 

Recently, iodobenzoates anchored onto an ionic liquid support (6.4) were coupled to various aryl boronic acids (6.5) in aqueous media using Pd(OAc)2 as the catalyst at 80°C to give the coupled product 6.6 (Scheme 6.3). Compounds 6.6 were purified simply by washing the reaction mixture with ether, which removed the unreacted starting materials and the side product 6.7 without the need of chromatography. Compounds 6.6 were then cleaved from the ionic liquid support... 

Relevant complexes have been isolated and fully characterized (4-10, 13-lb). The resulting arylpalladium complex 8 is able to react with various compounds such as terminal alkenes, alkynes, aryl boronic acids or hydrogen-transfer agents to give an organic molecule and palladium(O) (3, 17, 18). 

The first examples of microwave-assisted cross-couplings with organozinc compounds were recently reported [47]. In addition, the first high-speed synthesis of aryl boronates (Suzuki coupling reactants) has been performed under the action of single-mode irradiation with an in-situ-generated palladium carbene catalyst [48],... 

Cross-coupling reactions 5-alkenylboron boron compounds, 9, 208 with alkenylpalladium(II) complexes, 8, 280 5-alkylboron boron, 9, 206 in alkyne C-H activations, 10, 157 5-alkynylboron compounds, 9, 212 5-allylboron compounds, 9, 212 allystannanes, 3, 840 for aryl and alkenyl ethers via copper catalysts, 10, 650 via palladium catalysts, 10, 654 5-arylboron boron compounds, 9, 208 with bis(alkoxide)titanium alkyne complexes, 4, 276 carbonyls and imines, 11, 66 in catalytic C-F activation, 1, 737, 1, 748 for C-C bond formation Cadiot-Chodkiewicz reaction, 11, 19 Hiyama reaction, 11, 23 Kumada-Tamao-Corriu reaction, 11, 20 via Migita-Kosugi-Stille reaction, 11, 12 Negishi coupling, 11, 27 overview, 11, 1-37 via Suzuki-Miyaura reaction, 11, 2 terminal alkyne reactions, 11, 15 for C-H activation, 10, 116-117 for C-N bonds via amination, 10, 706 diborons, 9, 167... 

In a subsequent study, the impact of replacing one or both of the amide bonds with either a diacylhydrazine or a 1,3,4-oxadiazole was explored [36]. (fi)-Bromoallyl ethers were employed in the PI and Pi positions and similarly subjected to Suzuki conditions using the two aryl boronic acids that gave the most potent inhibitors in the previous study. Potent plasmepsin inhibitors were produced however, no real improvements in activity compared to the parent compound were observed (Scheme 5). The use of the preligand, [(f-Bu3)PH]BF4 [37], in combination with barium hydroxide was found beneficial although the yields were very low in these reactions. 

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