Carbonyl compounds Lewis acid-base interactions

Several Lewis acid-base interactions between alkali metal cations and heteroatom-containing molecules are indispensable in the promotion of reactions involved in critically important and fundamental transformations—deprotonation with lithium amides at the a-hydrogens of carbonyl or imino compounds and the addition of organolithium compounds to such electrophilic substrates. Because it is impossible to cover the multitude of these and other closely related subjects, this chapter describes only briefly general aspects of current interest. 

Ths Houk Modsl [CL4, CL5, HP3, HW1, PR1 ] The intervention of a favored conformation through which the reduction occurs can depend on the stereo-electronic interactions in a Lewis acid-base complex that is formed between a tricoordinated reducing agent (boranes, DIBAH) and a carbonyl compound. The complex associates in a way that minimizes the different repulsive interactions, and the transfer of the hydride takes place in second stage. Accordingly, the conforma-... 

The first important characteristic is the presence of functional groups within a compound that can interact specifically with CO2 in interactions of the Lewis acid/base type. These functional groups may be electron donating, such as carbonyls, to interact with CO2, which would be the electron acceptor. 

Lewis add-base interactions are very common in chemistry and are often rather subtle. You are about to meet, in the next chapter, an important way of making C-C bonds by adding organometallics to carbonyl compounds, and in many of these reactions there is an interaction at some point between a Lewis acidic metal cation and a Lewis basic carbonyl group. 

Fig. 2.15 Intermolecular interaction of boric ester (a) with hthium salt and (b) with carbonyl compound based on Lewis acid-base neutralization...

A regioselective aldol condensation described by Biichi succeeds for sterical reasons (G. Biichi, 1968). If one treats the diaidehyde given below with acid, both possible enols are probably formed in a reversible reaaion. Only compound A, however, is found as a product, since in B the interaction between the enol and ester groups which are in the same plane hinders the cyclization. BOchi used acid catalysis instead of the usual base catalysis. This is often advisable, when sterical hindrance may be important. It works, because the addition of a proton or a Lewis acid to a carbonyl oxygen acidifies the neighbouring CH-bonds. 

Basicity in the gas phase is measured by the proton affinity (PA) of the electron donor and in solution by the pAj,. A solution basicity scale for aldehydes and ketones based on hydrogen bond acceptor ability has also been established [186]. Nucleophilicity could be measured in a similar manner, in the gas phase by the affinity for a particular Lewis acid (e.g., BF3) and in solution by the equilibrium constant for the complexation reaction. In Table 8.1 are collected the available data for a number of oxygen systems. It is clear from the data in Table 8.1 that the basicities of ethers and carbonyl compounds, as measured by PA and p , are similar. However, the nucleophilicity, as measured by the BF3 affinity, of ethers is greater than that of carbonyl compounds, the latter values being depressed by steric interactions. 

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