Polar reaction with Lewis acids

Studies on solvent effects on the endo-exo selectivity of Diels-Alder reactions have revealed the importance of hydrogen bonding interactions besides the already mentioned solvophobic interactions and polarity effects. Further evidence of the significance of the former interactions comes from computer simulations" and the analogy with Lewis-acid catalysis which is known to enhance dramatically the endo-exo selectivity (Section 1.2.4). 

The hexatriene is polarized by unsymmetrical substitution with the C=0 group, and further activated by coordination with Lewis acid. The catalyzed reaction is polar. The similarity between the catalyzed and the photochemical reactions can be understood if polar reactions belong to the pseudoexcitation band as has been proposed in Sect 1. 

Ethers are relatively unreactive substances, which is why diethyl ether and tetrahydrofuran are widely used as solvents for organic reactions. However, the lone pairs on the oxygen atom are a source of reactivity. The oxygen atom may be protonated, and it reacts with Lewis acids. The increased polarity of the C-O bond then makes the neighbouring carbon atoms sensitive to nucleophilic attack. 

In general, cyclopropane undergoes addition less readily than propylene chlorination, for example, requires a Lewis acid catalyst to polarize the chlorine molecule (compare Sec. 11.11). Yet the reaction with sulfuric acid and other aqueous protic acids takes place considerably faster for cyclopropane than for propylene. (Odder still, treatment with bromine and FeBrj yields a grand mixture of bromo propanes.)... 

In a review of Diels-Alder reactions. Sauer330 notes that no catalysis with Lewis acids has been observed in the case of dienophiles with no polar substituents (CO. CN). IR spectra show that the I. ewis acid forms a complex with the polar group. 

In the last year some attention has been paid to the electropolymerizations in which the electrodic depolarizer is a complex between the monomer and some Lewis acids (see Ref. 7, p. 650). These researches, pioneered by Funt, were oriented towards the formation of alternating copolymers, in reactions in which the Lewis acid gives a monomer pair charge transfer complex. This research is connected to the discovery that certain polar monomers, containing nitrile or carbonyl groups, (A), can complex with Lewis acids such as zinc halides. These complexes I can undergo a thermal homopolymerization by themselves, or can react with some electron donor monomers (D), giving rise to complexes like II,... 

Solvatation, solvolysis and ionic dissociation phenomena, in both aqueous and nonaqueous solutions are subsumed by the Lewis definitions. In addition to the previous discussion of the dual polarity character of Lewis acids and bases, it should be noted that many of them are amphoteric, by definition. Donor number, DN, was developed in order to correlate the behavior of a solute in a variety of donor solvents with a given basicity or donicity. A relative measurement of the basicity of a solvent D is given by the enthalpy of its reaction with an arbitrarily chosen reference acid (SbCls in the Gutmann s scale). Latter Mayer introduced an acceptor number, AN, as the relative P NMR shift induced by triethylphosphine, and relative to acidic strength (AN=0 for hexane and 100 for SbCls). In 1989, Riddle and Fowkes modify these AN numbers, to express them, AN ", in correct enthalpic unit (kcaLmol). Table 10.2.3 gathers electron acceptor number AN and AN " and electron donor number DN for amphoteric solvents. 

An activation energy of this magnitude would lead to an unobservably slow reaction at normal temperature. There is an abundance of evidence that carbocations can be intermediates in nucleophilic substitution reactions. Carbocation formation in solution is feasible because of the solvation of the ions that are produced. One of the earliest pieces of evidence for the existence of carbocation intermediates was the observation that triphenylmethyl chloride (trityl chloride) gave conducting solutions when dissolved in liquid sulfur dioxide, a polar non-nucleophilic solvent. Trityl chloride also reacted with Lewis acids, such as aluminum chloride, to give colored salt-like solids. 

It will be interesting to explore the reactions using Lewis acids such as ZnCla, SnCU, and TiCU) by means of physical methods centered on transition metal nuclei of these Lewis acids (NMR or Mossbauer spectrometry) to discover whether true pyrylium salts are obtained directly or whether crystalline or liquid complexes of 1,5-enediones are precursors that become converted into salts only in contact with polar solvents. X-ray diffractometry of crude products may also help solve some of these problems. 

Reactions such as these that involve polar molecules are best understood in terms of Highest Occupied Molecular Orbital—Lowest Unoccupied Molecular Orbital (HOMO-LUMO) orbital interactions. As we saw in Section 1.7, p. 41, when a filled occupied orbital overlaps an empty orbital, the two electrons are stabilized in the new, lower energy molecular orbital. The words Lewis bases react with Lewis acids are essentially equivalent to saying, The interaction of a filled and empty orbital is stabilizing. Indeed, this notion is one of the central unifying themes of organic reactivity, as essentially all reactions involving polar molecules can be understood this way. 

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. 

Gothelf presents in Chapter 6 a comprehensive review of metal-catalyzed 1,3-di-polar cycloaddition reactions, with the focus on the properties of different chiral Lewis-acid complexes. The general properties of a chiral aqua complex are presented in the next chapter by Kanamasa, who focuses on 1,3-dipolar cycloaddition reactions of nitrones, nitronates, and diazo compounds. The use of this complex as a highly efficient catalyst for carbo-Diels-Alder reactions and conjugate additions is also described. 

Shibasald et al. reported that lithium-containing, multifunctional, heterobimetallic catalysts such as LaLi3tris((l )-6,6 -dibromobinaphthoxide) 35, with moderate Lewis acidity in non-polar solvents, promote the asymmetric Diels-Alder reaction to give cycloadducts in high optical purity (86% ee) [53] (Scheme 1.67). The lithium... 

---