Arylboronic acids dimer

Salicylideneamino alcohols are easily prepared ligands that react readily with arylboronic acids to form the corresponding esters, which may be monomeric 45-49 or dimeric 50-59 (Fig. 16) [87-94]. 

Fig. 16. Salicylideneamino alcohols react with arylboronic acids to form monomeric (45-49) or dimeric complexes (50-59)...

Fig. 20. Reaction of 2,6-pyridinedimethanol with arylboronic acids gives the tetrameric macrocycles 74 and 75. Dimeric compounds are known with silicon 76 and 77 and sulfur 78...

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. 

Generally, monophosphine complexes can be generated by decomposition of suitable precursors, among which the most notable are palladacycles (Section 9.6.3.4.7). A spectacular example makes use of spontaneous disproportionation of a dimeric complex of Pd1 with very bulky ligands to give one of the most reactive catalytic systems known so far, which catalyzes the fast crosscoupling of arylboronic acids with aryl chlorides and hindered aryl bromides at room temperature (Equation (28)) 389... 

Previously Davidson and Triggs have reported that arylboronic acids react with sodium palladate to give the dimeric biaryls. The synthetic utility of this dimerization reaction is, however, limited owing to stoichiometric requirement of the palladium compound. On the other hand, the palladium-catalyzed crosscoupling reaction between arylboronic acids and haloarenes in the presence of bases provides a clean synthesis of biaryls (Eq. 106) In this case, sodium carbonate has been proven to be most effective base. 

The negatively charged base reacts with the arylpalladium(II) halide to give the arylpalladium hydroxide or alkoxide complex, which is able to form the dimeric palladium-boron complex XXIII what is crucial for the transmetallation process [2-6]. It is apparent that the metal cation (from the base) accelerates the formation of the latter, as clearly showed by Zhang and coworkers [15]. They have developed the SM coupling procedure for sterically bulky arylboronic acids when the clear influence of the anion basicity and the cation effect were discovered. The cationic radius is presumably an important parameter which influences the formation of dimeric... 

The by-products observed are the coupling products of phosphine-bound aryls and dimers of arylboronic acids. The oxidative addition of ArX to palladium(O) to afford 3, followed by a sequence of transmetal-lation-reductive elimination yields a cross-coupling product 4. Triaryl-phosphines are an excellent ligand for stabilizing the palladium(0) species. However, an undesirable side reaction of the aryl-exchange between palladium- and phosphine-bound aryls (3 —> 5) leads to the coupling product 6 of phosphine-bound aryls (Eq. 51). 

Many electrochemical approaches to biaryls, e.g., electroreductive dimerization of aryl halides [12, 13] and electrooxidative dimerization of arylboronic acids [14], have been investigated. Substituted or non-substituted benzoic acids are electrochemically reduced to the corresponding alcohols or aldehydes. For instance, electroreduction of /M-phenoxybenzoic add proceeds smoothly to afford m-phenoxybenzaldehyde, an intermediate for insecticide [2,15]. 

Boronic acid derivatives are easily available and useful substrates especially in the palladium-catalysed Suzuki-Miyaura coupling reaction. In copper-catalysed reactions, they are also versatile starting materials. Dimerisation of arylboronic acids was first reported with copper acetate as a catalyst under ojgrgen at 100 Later, it was found that CuCl(OH)(phen) complex was much more efficient as a catalyst to afford dimer in high yields at 28 °C (Scheme 15.17). ... 

Tedicyp 66/allylpalladmm(II) chloride dimer (Scheme 29, Table 17) [171, 172]. In contrast, the reaction of 2-thienylboronic acid 68 with aryl halides 69 needs to be performed with 1-5% of the same catalytic system. Alternatively, cross-coupling can utilize arylboronic acids with thiophene halides this yields better results in most cases. 

The synthesis of homoallylic alcohols was achieved via a Pd-catalyzed cross coupling of allyl(isopropoxy)dimethyl-silane. Similarly to the previous example (eq 84), the dimer (1) was in some cases more efficient than Pd(dba)2. The reaction of 2-methyliodobenzene was improved by using 1, and the coupling of 2-methoxyiodobenzene was only observed in the presence of 1 (eq 85). A one-pot process was optimized starting from allyl(isopropoxy)dimethylsilane and benzaldehyde. Another strategy for the preparation of homoallylic alcohols was described via a Pd-catalyzed three-component coupling of an arylboronic acid with allenes and aldehydes. 

Oxidative Dimerization of Arylboronic Acids. Cu(OAc)2 was shown to mediate dimerization of various arylboronic acids forming symmetric biaryls in good yields. The oxidative homocoupling proceeds smoothly at rather elevated temperatures with Cu(OAc)2 present in catalytic or stoichiometric amounts. In an earlier case air was employed as an oxidant. The mechanism presumably involves transmetallation of arylboronic acids by copper followed by dimerization of the organocopper intermediate, followed by reductive elimination to give the product. ... 

A microwave-assisted tandem Ir-catalyzed C-H boronylation/Pd-catalyzed Suzuki-Miyaura cross-coupling reaction has been reported (Harrisson et al., 2009). This includes the synthesis of arylboronic acids, direct boronylation of arenes and alkanes, which provides access to synthetically useful compounds without relying on the accessibility of aryl or alkyl halides. They developed an elegant methodology for the direct C-H boronylation using a pinacoldiborane dimer under microwave irradiation at 80°C for 5-60 min. 

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