Lewis acids molecules

Depending on the specific reaction conditions, complex 4 as well as acylium ion 5 have been identified as intermediates with a sterically demanding substituent R, and in polar solvents the acylium ion species 5 is formed preferentially. The electrophilic agent 5 reacts with the aromatic substrate, e.g. benzene 1, to give an intermediate cr-complex—the cyclohexadienyl cation 6. By loss of a proton from intermediate 6 the aromatic system is restored, and an arylketone is formed that is coordinated with the carbonyl oxygen to the Lewis acid. Since a Lewis-acid molecule that is coordinated to a product molecule is no longer available to catalyze the acylation reaction, the catalyst has to be employed in equimolar quantity. The product-Lewis acid complex 7 has to be cleaved by a hydrolytic workup in order to isolate the pure aryl ketone 3. 

The possibility of ion formation during the interaction between two Lewis acid molecules as shown in the scheme above is important for the initiation of cationic polymerizations in the absence of cation forming additives (e.g. HX or RX)1). When aluminum-halides A1X3 (X = Cl, Br) are concerned, the ion formation in solution could be experimentally proven163). The formation of ionic species in pure SbCl5/ SbFj system has already been pointed out. 

The first-formed intermediate (with an impaired electron) in combination with a second Lewis acid molecule has even greater electron affinity, and is reduced at a more positive potential to give a voltammogram that appears to be the result of an irreversible two-electron reduction process. In most cases it is an ECEC process in which each electron transfer (E part of the ECEC mechanism) to the Lewis acid (H30+) is reversible to give a product (H-) that forms a covalent bond with the substrate (H—Eh) (the C part of the mechanism). 

The Lewis acid-base definition focuses on the donation or acceptance of an electron pair to form a new covalent bond in an adduct, the product of an acid-base reaction. Lewis bases donate the electron pair, and Lewis acids accept it. Thus, many species that do not contain El are Lewis acids. Molecules with polar double bonds act as Lewis acids, as do those with electron-deficient atoms. Metal ions act as Lewis acids when they dissolve in water, which acts as a Lewis base, to form an adduct, a hydrated cation. Many metal ions function as Lewis acids in biomolecules. 

The embedding of the fert-butyldimethylsilyltrifluoromethanesulfonate Lewis acid molecules in the silica matrix led to a very specific pathway in the reaction of the methyl ester of the 1-cyclopentenylacefic acid [96]. On these catalysts, the main reaction product was 3-methylbenzaldehyde. Since the reaction involves a hydrogen transfer step, this transformation was found to be dependent on the solvent nature. 3-Methylbenzaldehyde was obtained only in cyclohexene and ortho- and para-xyl n (Scheme 8.18) while in the other solvents the main reaction product was isomerization to the exo-isomer. The best yields of 3-methylbenzaldehyde were obtained in para-xylem. No alkylation product was identified for these catalysts in the presence of the indicated solvents [96]. 

Scheme 8.18 Sp>ecific Pathway in the Reaction the Methyl Ester of the 1 -Cyclop>entenylacetic Acid Over tert-ButyIdimethylsilyltrifluoromethanesulfonate Lewis Acid Molecules in the Silica Matrix.

Lewis acid = molecule with an atom that is electron-deficient (a.k.a. electrophile). 

We have seen that a base can be defined as combining with a proton and, therefore, requires at least one lone pair of electrons. A more general definition of acids and bases, due to G. N. Lewis, describes a base as any species (atom, ion or molecule) which can donate an electron pair, and an acid as any species which can accept an electron pair— more simply, a base is an electron-pair donor, an acid an electron-pair acceptor. Some examples of Lewis acids and bases are ... 

Donor strengths, taken from ref. 207b, based upon the solvent effect on the symmetric stretching frequency of the soft Lewis acid HgBr2. Gutmann s donor number taken from ref 207b, based upon AHr for the process of coordination of an isolated solvent molecule to the moderately hard SbCL molecule in dichioroethane. ° Bulk donor number calculated as described in ref 209 from the solvent effect on the adsorption spectrum of VO(acac)2. Taken from ref 58, based on the NMR chemical shift of triethylphosphine oxide in the respective pure solvent. Taken from ref 61, based on the solvatochromic shift of a pyridinium-A-phenoxide betaine dye. 

Finally, the solvent also interacts with sites of the Lewis acid and the Lewis base that are not directly involved in mutual coordination, thereby altering the electronic properties of the complex. For example, delocalisation of charges into the surrounding solvent molecules causes ions in solution to be softer than in the gas phase . Again, water is particularly effective since it can act as an efficient electron pair acceptor as well as a donor. 

A combination of the promoting effects of Lewis acids and water is a logical next step. However, to say the least, water has not been a very popular medium for Lewis-acid catalysed Diels-Alder reactions, which is not surprising since water molecules interact strongly with Lewis-acidic and the Lewis-basic atoms of the reacting system. In 1994, when the research described in this thesis was initiated, only one example of Lewis-acid catalysis of a Diels-Alder reaction in water was published Lubineau and co-workers employed lanthanide triflates as a catalyst for the Diels-Alder reaction of glyoxylate to a relatively unreactive diene . No comparison was made between the process in water and in organic solvents. 

The thiazolium ring, as most heterocycloammoniums, is a Lewis acid conferring to the carbon atom in the 2-position the carbocationic property of adding the free pair of a base either organic or mineral that may be the molecule of solvent as ROH (Scheme 11). For many nuclei of suitable acidity, these equilibria can be observed in dilute solution by means of absorption spectra when species A and C possess different characteristics (24). For example, benzothiazolium and benzoxazolium in methanol and ethanol give at 10 mole liter 8 and 54% of the alkoxy derivatives for the former and 29 and 90% for the latter respectively. 

An unshared pair of electrons from the Lewis base is used to form a covalent bond between the Lewis acid and the Lewis base The Lewis acid and the Lewis base are shown as ions m the equation but they need not be If both are neutral molecules the analogous equation becomes... 

Uses. Boron triduoride is an excellent Lewis acid catalyst for numerous types of organic reactions. Its advantages are ease of handling as a gas and the absence of undesirable tarry by-products. As an electrophilic molecule, it is an excellent catalyst for Friedel-Crafts and many other types of reactions (63-65) (see Friedel-craftsreactions). 

The polymerization of ethyleneimine (16,354—357) is started by a catalyticaHy active reagent (H or a Lewis acid), which converts the ethyleneimine into a highly electrophilic initiator molecule. The initiator then reacts with nitrogen nucleophiles, such as the ethyleneimine monomer and the subsequendy formed oligomers, to produce a branched polymer, which contains primary, secondary, and tertiary nitrogen atoms in random ratios. Termination takes place by intramolecular macrocycle formation. 

Electrophile Addition Reactions. The addition of electrophilic (acidic) reagents HZ to propylene involves two steps. The first is the slow transfer of the hydrogen ion (proton) from one base to another, ie, from Z to the propylene double bond, to form a carbocation. The second is a rapid combination of the carbocation with the base, Z . The electrophile is not necessarily limited to a Lowry-Briiinsted acid, which has a proton to transfer, but can be any electron-deficient molecule (Lewis acid). 

Aromatic compounds may be chlorinated with chlorine in the presence of a catalyst such as iron, ferric chloride, or other Lewis acids. The halogenation reaction involves electrophilic displacement of the aromatic hydrogen by halogen. Introduction of a second chlorine atom into the monochloro aromatic stmcture leads to ortho and para substitution. The presence of a Lewis acid favors polarization of the chlorine molecule, thereby increasing its electrophilic character. Because the polarization does not lead to complete ionization, the reaction should be represented as shown in equation 26. 

Protonation or Lewis acid complexation of a heteroatom invites nucleophilic attack, including nucleophilic attack by a parent molecule. Oligomerization and polymerization are thus often the results of bringing heterocycles into an acid environment without making sure that all of the potentially nucleophilic sites are protonated. 

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