Arylboronic commercial availability

Very recently, well-defined complexes with general formula [PdCl(T -Cp) (NHC)] were synthesised and tested for the homocoupling of non-electrodeficient arylboronic acids at room temperature with good results (Scheme 7.7) [51]- This new class of catalysts were synthesised from commercially available NHC palladium(II) chloride dimers and are air-stable. 

There are several different routes to carboxamides. In most of these reactions, a carboxylic acid is converted to a more reactive intermediate, e.g. the acid chloride, which is then allowed to react with an amine. For practical reasons, it is preferable to form the reactive intermediate in situ. Arylboronic acids with electron-withdrawing groups such as (3,4,5-trifluorophenyl)boronic acid act as highly efficient catalysts in the amidation between carboxylic acids and amines. (3-Nitrophenyl)boronic acid and [3,5-bis(trifluoromethyl)phenyl]boronic acid are also effective eimidation catalysts and commercially available. 

Complementary to the use of zinc reagents for the introduction of (functionalized) alkyl groups is the rhodium-catalyzed conjugate addition of aryl- and alkenylboron reagents. This method rapidly became popular, also because arylboron reagents are air and moisture stable and a large variety of them is commercially available . 

The boronate esters used for Suzuki reactions can be synthesized from commercially available alkyl-, vinyl-, and arylboronic acids. Alkyl and vinyl boronate esters are also synthesized by the hydroboration of double and triple bonds, similar to the hydrobora-tion of alkenes and alkynes in Chapters 8 and 9. Note that the boron atom generally adds to the less substituted end of a double or triple bond. Also, the B and H add to the same side of a triple bond (syn addition) to give a trans alkenylboronate ester. 

Bromo- and iodoimidazoles are useful intermediates for further functionalization. 4(5)-Aryl- I //-imidazoles 57 can be efficiently and selectively prepared by palladium-catalyzed Suzuki-Miyaura reaction of commercially available 4(5)-bromo-l//-imidazole 56 with arylboronic acids under phase-transfer conditions, which then underwent highly selective palladium-catalyzed and copper(I) iodide mediated direct C-2-arylation with a variety of aryl bromides and iodides under base-free and ligandless conditions to produce 2,4(5)-diaryl-l//-imidazoles 58 in modest to good yields <07JOC8543>. A new procedure for the synthesis of a series of substituted 2-phenylhistamines 60 utilizing a microwave-promoted Suzuki... 

Diimine ligands can be obtained by a simple condensation reaction of a, -dike-tones with suitable aniline derivatives [23]. While the diisopropylamino compound (2e) is commercially available, fhe terphenyl anilines (2 b, c) had to be synfhesized via a Suzuki cross-coupling protocol based on 2,6-dibromoaniline and substituted arylboronic acids (lb,c. Scheme 2.1) [26]. Acids lb,c are easily accessible by treat-... 

As seen with pyridazines, palladium catalyzed coupling reactions were also frequently applied in the phthalazine field. For example, commercially available 1,4-dichlorophthalazine 185 was aminated to give 186 in good yield by aromatic nucleophilic substitution with A -methylpiperazine <01S699>. Then, 186 was coupled with various substituted arylboronic acids to obtain 187 by Suzuki-type cross-coupling reactions. Best results were obtained with electron-donating substituents on the arylboronic acid. 

Palladium-catalyzed cross-coupling reactions of alkyl bromides with arylboron reagents represent another breakthrough for the development of a general method for arylation of alkyl halides. Fu found that the palladiumprimary alkyl bromides with various arylboronic acids (Equation 5.6) [10]. An air-stable commercially available preligand, [HP(t-Bu)2Me]BF4 5, also worked well with a variety of arylboronic acids and gave the same products in comparable yields, as shown in the parentheses in Equation 5.6. 

Very recently, Beller and colleagues described a novel carbonylative coupling of benzyl chlorides with aryl boronic acids [43]. This was the first report on carbonylative Suzuki couplings of benzyl chlorides with arylboronic acids (Scheme 4.25). The reaction was carried out using a commercially available... 

Reports by the groups of Chan, Evans, and Lam in 1998 revealed an alternative method to conduct copper-mediated couplings that form C(aryl)-0 and C(aryl)-N bonds. In this process, arylboronic acids react with compounds containing N-H or 0-H bonds in the presence of a Cu(II) reagent or catalyst. TTiese reactions were initially conducted with stoichiometric amounts of copper reagents. " Amines, anilines, amides, ureas, carbamates, and sulfonamides underwent N-arylation in moderate to excellent yields by this process (Equation 19.124). The commercial availability of boronic acids and the ability to conduct these arylations in air under mild conditions has caused this method to be adopted quickly for synthetic applications on a small scale. 

Collman and Zhong reported the first catalytic, oxidative C-N cross coupling between an arylboronic acid and a substrate containing an N-H bond. " In the presence of 10 mol % of commercially available [Cu(OH)TMEDA]jClj in dichloromethane, imidazole coupled with phenylboronic acid at room temperature. Lam and co-workers and Antilla and Buchwald expanded the scope of this type of catalytic coupling to encompass the reactions of amines. Lam and co-workers reported the first catalytic version of the reactions of aiylboronic acids with phenols to yield aryl ethers in good yields. The highest yields were obtained when Oj was used as co-oxidant. After these early discoveries, several reports on... 

To the best of our knowledge, there are very few reports on the use of other metal-based systems. However, in 2007, Sames and coworkers [42] reported the ruthenium-catalyzed decarboxylative ary-lation of cyclic 2-amino esters. This process provides a rapid avenue to a variety of 2-arylpyrrolidines and piperidines from commercially available proline, hydroxyproline, and pipecolinate esters (Scheme 3.25). Examination of the substrate scope also showed that many arylboronic acids and boronate esters serve as coupling partners. It was also demonstrated that the required amidine- or... 

Arylboronic acid is the most versatile organometalloid due to its efficiency and ready commercial availability. Other organometalloids that can also participate in this copper-promoted cross-coupling reaction are aryltrialkylsiloxanes [51b, 54], aryltrime-thylstannanes [55], triarylbismuths [5,56], arylleads [57], diaryliodonium salts [58a], diethylzinc [58b] and dialkylaluminum chlorides [58c]. 

Microwave acceleration of reactions is a valuable tool for organic synthesis [39], and various specialized instruments are now commercially available. Tye and co-workers have reported the microwave-assisted Petasis borono-Mannich reaction of arylboron-ic acids and primary or secondary amines with either glyoxylic acid or salicylaldehyde [40]. Optimized reaction conditions employed didiloromethane as solvent, and microwave assisted heating at 120 °C for 10 min. Products were obtained in generally modest yields (10-83%), in part due to incomplete reaction conversion imder the reported conditions. 

Another example is the Rh-catalyzed cascade approach to phlhalides 84 from commercially available phthalaldehyde 82 and arylboronic acids 83 (Scheme 5.54) [52]. The first step of this transformation is the aryl addition of phthalaldehyde 82 with arylboronic acids 83, which leads to the formation of 2-(hydroxy(phenyl) methyl)benzaldehyde A. The second step probably involves an aldehydic C—H activation/C—O coupling. 

The most commonly employed aryl sources are arylboronic acids. Due to their widespread application in the Suzuki-Miyaura reaction, organoboronic acids have found wide acceptance, and currently more than 450 different arylboronic acids are available commercially [47]. These compounds unite several beneficial properties such as high air and moisture stability and great functional group... 

The increasing importance of boronic acids as synthetic intermediates has justified the development of new, mild and efficient methods to provide access to a large pool. Of particular interest is the synthesis of arylboronic acids substituted with a wide range of other functional groups. As a consequence of their growing popularity and improvements in methods available for their preparation, many functionahzed boronic acids have become available from several commercial sources. Although several methods, like the oxidation or hydrolysis of trialkylboranes, have significant historical and fundamental relevance, this section is devoted mainly to modem methods of practical value to synthetic chemists. 

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