Stille borate complex

In other cases, the borate complexes were decomposed prior to acetylation, either by distillation with acidified methanol, or with methanol after removal of cations. In a study of the former method, it was concluded that, when the excess of borohydride is decomposed with acetic acid, five evaporations with methanol are required for optimal results.39 The methanol must be added to the residue obtained on concentrating the aqueous solution to dryness, as, otherwise, trimethyl borate will not be formed. It has been stated that removal of borate is more efficient when hydrofluoric acid (instead of hydrochloric acid) is used to neutralize the excess of borohydride.113 Alternatively, borate complexes have been decomposed by removing cations with an ion-exchange resin, followed by distillation with methanol. Acetylation may then be conducted at 100° for 20 minutes, or at room temperature overnight91 or for shorter periods.26 It should, however, be noted that, when cations were removed and the product was acetylated without prior distillation with methanol, 4 hours of refluxing was still required, and irreproducible results were obtained when this time was lessened to 30 min to 1 hour.219... 

A very reactive Lewis acid is obtained when the complex [(EBTHI)Zr(Me)2] is converted in situ to a dicationic species by protonation with the acid H-BARF (BARF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) in the presence of the Diels—Alder substrate oxazolidi-none [88] (Scheme 8.48). The dicationic species is stabilized through coordination by the oxazolidinone and by diethyl ether (derived from the acid etherate employed). The catalyst loading in the Diels—Alder reaction could be lowered to 1 mol% (Zr) and the reaction still... 

The ability of the 1-hydroxyanthraquinone system to form chelate complexes is, however, still employed in the synthesis of certain anthraquinone dyestuffs since it permits selective reactions to be carried out. Thus, treatment of the borate ester (215) of 1,2-dihydroxyanthraquinone with acetic anhydride yields l-hydroxy-2-acetoxyanthraquinone, the 1-hydroxy group being protected against electrophilic attack as a result of its being involved in coordination to boron. 

Among the coordination compounds obtained on the basis of polypyrazolyl-borates, it is worth emphasizing the copper chelates 235 which are still the only biomimetic model of blue copper proteins, reproducing all their physical (UV-and EPR-spectral) properties [441,446-448], Compound 236 [449] is also an example of complexes of this kind of system ... 

X-ray analysis of solid compounds such as ((CH3)2C5H3)2ZrCH3+ + H3CB(C6F5)3 (90) showed that a coordination bond still partly exists between the zirconocene and the borate. The olefin is tt-complex bonded into this compound and then inserted into the zirconium-methyl bond. 

The mechanism is very similar to that of the Stille coupling. Oxidative addition of the vinylic or aromatic halide to the palladium(O) complex generates a palladium(II) intermediate. This then undergoes a transmetallation with the alkenyl boronate, from which the product is expelled by reductive elimination, regenerating the palladium(O) catalyst. The important difference is the transmetallation step, which explains the need for an additional base, usually sodium or potassium ethoxide or hydroxide, in the Suzuki coupling. The base accelerates the transmetallation step leading to the borate directly presumably via a more nucleophilic ate complex,... 

The vast majority of tris(pyrazolyl)borate ruthenium complexes prepared until now contains the parent Tp ligand. In contrast to other transition metals, ruthenium compounds which bear bulky Tp ligands remain still rare and are, thus far, limited to Tp, Tp Tp and Tp as shown in Chart 1. 

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