Coupling reactions arylboronic acids

The pme enantiomers can then be used in the construction of a final chiral liquid crystal or dopant. The chiral acid 141b can simply be esterified (Scheme 35) onto a simple mesogenic-like core (146) to provide a final chiral material (147). The ethyl ester can be reduced to the alcohol (145a) which can then either be esterified onto a carboxylic acid mesogenic core or, as shown in Scheme 36, be employed in a Mitsunobu etherification (DEAD reaction) to give aryl bromide 148, which is then involved in a conventional arylboronic acid coupling reaction to provide a final chiral liquid crystal (150) which has a particularly wide S range. 

Under copper catalyzed conditions azoles (i.e. imidazoles) couple not only with aryl halides but also with arylboronic acids. The reaction, run in the presence of oxygen, follows a unique path (for details see Chapter 2.5.). From the synthetic point of view, the arylation of imidazole proceeds in good yield, although the regioselectivity in the arylation of 4-substituted imidazoles is only moderate (6.70.),102... 

Phenols 26 substitute chloride in 2-chloro-4,6-dimethoxy-l,3,5-tri-azine 27 to give a series of corresponding 2-aryloxy-4,6-dimethoxy-l, 3,5-triazines 28 in high yields. This quite trivial reaction deserves special attention because the aryl—O bond in 28 can be selectively cleaved with inexpensive, air-stable catalyst NiCl2(dppf) in the Suzuki-Miyaura coupling of 28 with arylboronic acids.The reaction proceeds more easily in comparison with other aryl ethers or esters yielding different diaryls 29 in 50—90% yields (13JOC5078). 

Microwave and fluorous technologies have been combined in the solution phase parallel synthesis of 3-aminoimidazo[l,2-a]pyridines and -pyrazines [63]. The three-component condensation of a perfluorooctane-sulfonyl (Rfs = CgFiy) substituted benzaldehyde by microwave irradiation in a single-mode instrument at 150 °C for 10 min in CH2CI2 - MeOH in the presence of Sc(OTf)3 gave the imidazo-annulated heterocycles that could be purified by fluorous solid phase extraction (Scheme 9). Subsequent Pd-catalyzed cross-coupling reactions of the fluorous sulfonates with arylboronic acids or thiols gave biaryls or aryl sulfides, respectively, albeit it in relatively low yields. 

Remarkably, one year later Leadbeater described that biaryls can be synthesized via a Suzuki-type coupling under transition-metal free conditions [51, 52]. The reaction conditions were almost identical to those reported for the ligand-free process, with the difference being that a larger amoimt of Na2C03 and arylboronic acid were used. Only one successful example of a heteroaryl haUde substrate is shown namely, the coupling of 2-bromopyridine with phenylboronic acid (Scheme 32). 3-Bromothiophene did not couple under the same reaction conditions. Unfortimately, attempts to use heteroarylboronic acids such as 3-pyridinylboronic acid, 3-thienylboronic acid, and lH-indol-5-ylboronic acid on 4-bromoacetophenone completely failed. 

A closely related reaction that is currently receiving much attention is the palladium-catalysed Suzuki coupling of arylboronic acids with aryl halides (Fu and Littke, 1998). For example, this technology has recently been applied by Clariant workers for the production of o-tolyl-benzonitrile (Eqn. (13)), an intermediate to a series of so-called angiotensin-II antagonists, a new class of antihypertensive drugs (Bernhagen, 1998). 

The scope of Suzuki-Miyaura reactions is extremely broad, covering practically all types of organic residues. The cross-coupling of arylboronic acids with aryl halides or triflates is the most... 

In cross-coupling with arylboronic acids, some Cu1 carboxylates were found to provide a way to realize a very mild, base-free procedure, which is helpful in cases when added base can induce side reactions (59).222... 

These ligands afford high yields of cross-coupling products in the highly sterically burdened reactions of aryl bromides with arylboronic acids when both contain substituents in the ortho- (or equivalent) positions, although drastic reaction conditions are required (110). 

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