Oxygen Lewis bases

The ideas which led us to understand the formation of carbonylmetallates in the reactions of metal carbonyls with nitrogen and oxygen Lewis bases have been discussed above, and in addition I have given elsewhere (VII) an exhaustive summary of anionic carbon monoxide complexes. 

With its dw electronic configuration and its/orbitals available, silver(I) ion can act as a Lewis acid,34 and relativistic effects reinforce such behavior.35 Moreover, silver salts are able to act as either or both a o-Lewis acid or a jt-Lewis acid, with a slight preference for o coordination over n coordination, as revealed by calculations. This preference is higher for nitrogen than for oxygen Lewis bases (Scheme 10.19).36... 

The activation energy for proton transfer can be viewed as a lattice oxygen Lewis-base and proton Br0nsted-acid synergetic event [3]. One generally finds that activation energies of proton-activated reactions arc rather high between 100 and 200 kJ/mol for proton-activated elementary reaction steps in hydrocarbon conversion catalysis. ITiis is the main reason for the relatively low TOP per proton ( 102 s ) for this type of reaction. Similarly to enzymes [31], the weak van der Waals-type interaction determines the size- and shape-dependent behavior. 

Step 1 Oxygen (Lewis base) protonated by H+ (Lewis acid). 

Tikhonova, I.A. Dolgushin. F.M. Tugashov. K.I. Petrovskii, P.V. Furin. G.G. Shur, V.B. Coordination chemistry of polymercuramacrocycles. Complexation of cyclic trimeric perfluoro-o-phenylenemercury with neutral oxygeneous Lewis bases. J. Organomet. Chem. 2002. 6.54. 

Recall that the carbon atom of carbon dioxide bears a partial positive charge because of the electron attracting power of its attached oxygens When hydroxide ion (the Lewis base) bonds to this positively polarized carbon a pair of electrons in the carbon-oxygen double bond leaves carbon to become an unshared pair of oxygen... 

Their polar carbon-oxygen bonds and the presence of unshared electron pairs at oxygen contribute to the ability of ethers to form Lewis acid Lewis base complexes with metal ions... 

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

Most other studies have indicated considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium or n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and by 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 IR spectrum. Complex formation presumably involves a Lewis acid-Lewis base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

For the activation of a substrate such as 19a via coordination of the two carbonyl oxygen atoms to the metal, one should expect that a hard Lewis acid would be more suitable, since the carbonyl oxygens are hard Lewis bases. Nevertheless, Fu-rukawa et al. succeeded in applying the relative soft metal palladium as catalyst for the 1,3-dipolar cycloaddition reaction between 1 and 19a (Scheme 6.36) [79, 80]. They applied the dicationic Pd-BINAP 54 as the catalyst, and whereas this type of catalytic reactions is often carried out at rt or at 0°C, the reactions catalyzed by 54 required heating at 40 °C in order to proceed. In most cases mixtures of endo-21 and exo-21 were obtained, however, high enantioselectivity of up to 93% were obtained for reactions of some derivatives of 1. 

Active Figure 2.5 The reaction of boron trifluoride, a Lewis acid, with dimethyl ether, a Lewis base. The Lewis acid accepts a pair of electrons, and the Lewis base donates a pair of nonbonding electrons. Note how the movement of electrons from the Lewis base to the Lewis acid is indicated by a curved arrow. Note also how, in electrostatic potential maps, the boron becomes more negative (red) after reaction because it has gained electrons and the oxygen atom becomes more positive (blue) because it has donated electrons. Sign in atwww. thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

The Lewis definition of a base as a compound with a pair of nonbonding electrons that it can use to bond to a Lewis acid is similar to the Bronsted-Lowry definition. Thus, H20, with its two pairs of nonbonding electrons on oxygen, acts as a Lewis base by donating an electron pair to an H+ in forming the hydronium ion, H30+. 

Notice in the list of Lewis bases just given that some compounds, such as carboxylic acids, esters, and amides, have more than one atom ivith a lone pair of electrons and can therefore react at more than one site. Acetic acid, for example, can be protonated either on the doubly bonded oxygen atom or on the singly bonded oxygen atom. Reaction normally occurs only once in such instances, and the more stable of the two possible protonation products is formed. For acetic add, protonation by reaction with sulfuric acid occurs on... 

A comparison of these p/CHB values at constant pK of the bases reveals that the most important factor which determines the correlation between the structure and the hydrogen bonding ability depends on the electronegativity of the acceptor atom in the Lewis bases employed. Thus, the strength of the basicities of the oxygen compounds described above is... 

When a carbonyl group is bonded to a substituent group that can potentially depart as a Lewis base, addition of a nucleophile to the carbonyl carbon leads to elimination and the regeneration of a carbon-oxygen double bond. Esters undergo hydrolysis with alkali hydroxides to form alkali metal salts of carboxylic acids and alcohols. Amides undergo hydrolysis with mineral acids to form carboxylic acids and amine salts. Carbamates undergo alkaline hydrolysis to form amines, carbon dioxide, and alcohols. 

In this reaction, the C atom of C02, the Lewis acid, accepts an electron pair from the O atom of a water molecule, the Lewis base, and a proton migrates from an H20 oxygen atom to a C02 oxygen atom. The product, an H2C03 molecule, is a Bronsted acid. 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

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