Oxygen Lewis acid-base interactions

Transmetalation of the perrhenate/aluminosilsesquioxane cube model with SnMe4 is considerably more exothermic than for the perrhenate/silsesquioxane cube model. A similar grafted trimethyltin fragment is formed, as is MeReOs however, the latter is not liberated. It remains bound to the aluminosilsesquioxane cube via the Lewis acid-base interaction with the A1 center. The optimized structure also contains a Lewis acid-base interaction between Re and an adjacent framework oxygen... 

The hypothesized delocalization of lone pair electrons in the above silicon compounds is supported by the lowered basicity of the silyl compounds as compared to the corresponding carbon compounds. This reduced basicity is contrary to that expected on the basis of electronegativity effects operating through the a system since silicon is less electronegative than carbon. It is consistent with an internal Lewis acid-base interaction between the nitrogen and oxygen lone pairs and empty acceptor d orbitals on the silicon. Experimentally this reduced basicity is shown by the absence of disiioxane adducts with BF3 and BO ... 

Because of the rather localized negative charge at the phenoHc oxygen atom , the standard dye (44) is capable of specific HBD/HBA and Lewis acid/base interactions. Therefore, in addition to the nonspecific dye/solvent interactions, the betaine dye (44) predominately measures the specific HBD and Lewis acidity of organic solvents. On the other hand, the positive charge of the pyridinium moiety of (44) is delocalized. Therefore, the solvent Lewis basicity will not be registered by the probe molecule (44). If this solvent property is relevant for the system under study, other empirical measures of Lewis basicity should be used cf. Section 7.7. 

In copyrolytic reactions of the aminosilylenes with unsaturated ketones or imines (heterodienes) we mainly obtained isomeric mixtures. The chemo- and regioselectivity of main- and byproducts can be explained with multistep-cycloadditions. We assume a primary Lewis acid-base interaction between the lone electron pair of the heteroatom (oxygen or nitrogen) and the electron gap at silylene, which is followed by a [2+l]-cycloaddition and a radical ring-opening ring-closure reaction. 

Polar compounds (water, oxygen-, nitrogen-, and sulfur-containing compoimds). They react with the organoaluminum co-catalyst (water, alcohols, amines, thiols), complex the aluminum center in a Lewis acid/base interaction (ketones, ethers, thioefhers), or coordinate strongly to the nickel catalytic center (amines, sulfur compounds). They reduce the concentration of the active species and may be responsible of the formation of sludges in the reaction section. 

In the case of solvents with a low solvating power, in which protonated or deprotonated species are not free from counterion effects, spectroscopic and thermodynamic parameters associated with hydrogen bonding or Lewis acid-base interactions can be usefully related to PA or GB. Kamlet and coworkers reported linear relationships for five series of bases with sp - and sp -hybridized oxygen and sp-, sp - and sp -hybridized nitrogen. At the same time, Zeegers-Huyskens presented the relationships " reported in Figure 2. 

Most Other studies have led to considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium and n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and through reaction with dissociated alkyllithium. Evidence for the initial formation of a complex can be observed in the form of a shift in the carbonyl absorption band in the infrared spectrum. Complex formation presumably involves a Lewis acid-base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

Because metal ions are positively charged, fliey attract the unshared electron pairs of water molecules. It is primarily this interaction, referred to as hydration, that causes salts to dissolve in water. (Section 13.1) The process of hydration can be thought of as a Lewis acid-base interaction in which the metal ion acts as a Lewis acid and the water molecules as Lewis bases. When a water molecule interacts with the positively charged metal ion, electron density is drawn from the oxygen, as illustrated in Figure 16.15 This flow of electron density causes the O—H bond to become more polarized as a result, water molecules bound to the metal ion are more acidic than those in the bulk solvent. 

Figure 7.21 Hydrated cobalt ion, a coordination complex with an octahedral structure. The unshared pairs of electrons on the oxygen are attracted to the metal cation via a Lewis acid-base interaction.

At the active site of carbonic anhydrase a water molecule loses a proton to form a hydroxide (OH ) ion. This proton is removed by a part of carbonic anhydrase that acts as a base. Ordinarily the proton of a water molecule is not very acidic. However, the Lewis acid-base interaction between a zinc cation at the active site of carbonic anhydrase and the oxygen atom of a water molecule leads to... 

The bonding for oxygen atoms in heteratomic molecules is viewed as essentially covalent (see Covalent Bonds) (e.g. MeOH, Mc2C=0, MeCHO, and MeC(O)OOH) and similar to that for carbon, nitrogen, and chlorine atoms. In contrast, a Lewis acid base formahsm often is used for metal oxygen compounds with ionic interactions by dianionic 0x0 groups (e.g. [Ba2+ q2-], [Fe + (0 -) ] -, [Mn + (Q2-)4]-, and [(Cu+)20 ]). This results from the thermodynamic relations for ionic solution equihbria, and the inference that the combination of ions results in molecules and complexes held together by electrostatic interactions (equations 33 35). 

Fu and co-workers recently reported a new approach to restricting the rotational degree of freedom (see complex F), which focuses on the development of Lewis acids that bear an empty rr-symmetry and an empty 7t-symmetry orbital as illustrated in Fig. 1-8 [27]. These vacant orbitals can simultaneously accept electron from an oxygen lone pair and from the 7r-system of a carbonyl group. The distinguishing feature of this approach is the r-symmetry interaction, which at once organizes the Lewis acid-base complex and activates the carbonyl toward nucleophilic addition. 

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