Boronic stoichiometric formation

Although there are several reports in the literature on boron-mediated amide formations, the boron reagents had to be used in stoichiometric amounts.1-4-5-6-7-8-9 Recently, Yamamoto et al. presented the first truly catalytic method allowing for a direct amide formation from free carboxylic acids and amines as the reaction partners.10-1112 Best results were obtained by using phenylboronic acids bearing electron withdrawing substituents in the meta- and/or para-positions such as 3,4,5-trifluorophenylboronic acid or 3,5-bis(trifluoromethyl)boronic acid as the catalysts. 

A significant change in the fluoro halide to methoxy halide ratio does not occur with large changes in the boron trifluoride to hypohalite ratio. For example, experiments with hex-1-ene (13) and methyl hypochlorite (Table 7) have shown that variation of the boron trifluoridc methyl hypochlorite ratio from 10 1 to 1 10 does not produce large changes in the proportion of fluoride incorporation. These experiments also have shown that boron trifluoride is consumed in stoichiometric amounts in the formation of fluoro halide from methyl hypochlorite and that all three of the fluorine atoms of boron trifluoride are about equally effective. 

Eoex 21) reports the successful formation of borides in a rotating batch melting furnace, fired by an RE induction plasma. An amount of 0.8 kg from a stoichiometric mixture of Zr02 and boron, previously compressed in the rotating vessel to a pressure of 40 kg/cm, is treated by the action of an argon plasma (1 m /hr) for 0.5 hr. Zirconium diboride is formed by the reaction,... 

Ester formation occurs with complete stoichiometric conversion at relatively low reaction rates (t./, 100-600 s). In the absence of water no back reaction takes place. There is a very slow ester-ester exchange reaction ty, k 5000 s). The boronic ester moieties can be readily cleaved in water/alcohol. This reaction is, in principle, fast and complete (t./, 100 s), so that the template can be released from an imprinted macroporous polymer in 85-95% yield. 

The fust important test of this methodology came in Hanessian s investigation of the spiroketal portion of avermectin Bu- This highly convergent approach incorporates all the oxidation levels and functionality required for carbons C(15)-C(28), except for the necessity of alkyne to alkene conversion. The lithium alkynide was prepared at -78 C and then mixed with boron trifluoride etherate under the conditions of Yamaguchi (Scheme 19). (Direct condensation of the lithium salt and lactone lead to substantial amounts of a, -unsaturated lactone.) Addition of the lactone in stoichiometric amounts to the solution of the modified alkynide led to the formation of the desired hemiketal in acceptable yield. Further improvements could be obtained by the recycling of starting material. ... 

Whereas the Friedel-Crafts alkylations require only catalytic quantities of the Lewis acidic AICI3 catalyst, Friedel-Crafts acylations of phenols require excess Lewis acids, due to the complex formation of the Lewis acids with the hydroxyl group . Boron trifluoride-phosphoryl chloride, in stoichiometric amounts, is used for the Fridel-Crafts reaction of phenol with /3,/S-dimethylacrylic acid to give the acrylophenone. ... 

In 2000, Guy reported the stoichiometric coupling of alkane thiols and arylboronic acids, which was initially thought to be mediated by Cu(ll) [71]. Liebeskind proposed that the reaction was more likely catalyzed by Cu(l), generated by oxidation of the alkane thiols into dialkyl disulfides. Based on this hypothesis, Liebeskind predicted that disulfides and disulfide equivalents should be effective reagents for thioether formation [34]. This process would constitute a modification of the Chan-Evans-Lam, which involves the coupling of arylboronic acids and amines or alcohols in the presence of tertiary amine bases, generating aryl amines and ethers, respectively. Indeed, the coupling of diphenyl disulfide with phenyl boronic acid would yield diphenyl sulfide. 

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