Reactions with tetrahedral borates

Unfortunately, the evaluation of rate constants for the substitution reactions of tetrahedral borates with acidic ligands is complicated by reaction pathways, which are kinetically indistinguishable due to proton ambiguity. There is therefore a comparative paucity of data for complexation with the tetrahedral... 

The substituted tris(2-thioimidazolyl)borate ligands with l-R-2-thioimidazole groups where R = tBu and CgH4-/>-CH(CH3)2 yielded tetrahedral complexes on reaction with zinc salts. The... 

Imidate salts are 0-alkyl derivatives of tertiary amides. Being activated tertiary amides, they are extremely reactive towards nucleophiles. There is instantaneous reaction with hydroxide ion they also react rapidly at room temperature with water under acidic conditions. When an imidate fluoro-borate salt such as 43 reacts with sodium hydroxide, it gives sodium fluoro-borate and the tetrahedral intermediate 44 which breaks down in an irrevers-... 

Scheme 16 As the reaction kinetics of tetrahedral borates with most ligands cannot be followed due to the problems associated with kinetically indistinguishable pathways and proton ambiguity, studies have examined the biruling of boric acid with bidentate ligands in a 1 2 complex. By considering the change from a 1 1 to a 1 2 complex a tetrahedral structure is enforced at boron. This allows parallels to be made with the complexation of other tetrahedral boronate anions. Yoshimura s proposed transition state is depicted here, illustrating the complexation of boric acid with chromo tropic acid. ...

A reaction looked at earlier simulates borate inhibition of serine proteinases.33 Resorufin acetate (234) is proposed as an attractive substrate to use with chymotrypsin since the absorbance of the product is several times more intense than that formed when the more usual p-nitrophcnyl acetate is used as a substrate. The steady-state values are the same for the two substrates, which is expected if the slow deacylation step involves a common intermediate. Experiments show that the acetate can bind to chymotrypsin other than at the active site.210 Brownian dynamics simulations of the encounter kinetics between the active site of an acetylcholinesterase and a charged substrate together with ah initio quantum chemical calculations using the 3-21G set to probe the transformation of the Michaelis complex into a covalently bound tetrahedral intermediate have been carried out.211 The Glu 199 residue located near the enzyme active triad boosts acetylcholinesterase activity by increasing the encounter rate due to the favourable modification of the electric field inside the enzyme and by stabilization of the TS for the first chemical step of catalysis.211... 

The tris(mercaptophenylimidazolyl)borate iron and cobalt complexes [(Tmph)2M] (M = Fe, Co) have been synthesized by reaction of (Tmph)Tl with MI2.156 Structural characterization by X-ray diffraction demonstrates that the potentially tridentate Tmph binds through only two sulfur donors in these sandwich complexes and that the tetrahedral metal centers supplement the bonding by interactions with two BH groups. Comparison of the structures of [(Tmph)2M] with those of related tris(pyrazolyl)borate counterparts indicates that the Tm favors lower primary coordination number in divalent metal complexes. 

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