Enhancer intramolecular

For simple carbonyl compounds, the equilibrium between an aldehyde or a ketone and its corresponding enol is usually so shifted towards the keto form that the amount of enol at equilibrium can neither be measured nor detected by spectroscopy. Nevertheless, as recently emphasised by Hart (1979), this does not mean that the enol cannot exist free, not in equilibrium with ketones and aldehydes. Several examples of kinetically stable enols in the gas phase or in aprotic solvents have been reported. Broadly speaking, it appears that enols have relatively large life-times when they are prepared in proton-free media [e.g. the half-life of acetone enol was reported to be 14 s in acetonitrile (Laroff and Fischer, 1973 Blank et al., 1975) and 200 s in the gas phase (MacMillan et al., 1964)]. These life-times are related to an enhanced intramolecular rearrangement, indicated by the very high energies of activation (85 kcal mol-1 for acetaldehyde-vinyl alcohol tautomerization) which have been calculated (Bouma et al., 1977 Klopman and Andreozzi, 1979) It has therefore been possible to determine most of the spectroscopic properties of simple enols [ H nmr,l3C nmr (CIDNP technique), IR and microwave spectra of vinyl alcohol... 

The Lewis acid-catalyzed ene reaction is synthetically useful methodology for forming new carbon-carbon bonds [85]. Ene reactions utilizing reactive enophiles such as formaldehyde and chloral can be promoted by SnCU SnCU also enhances intramolecular ene reactions (Eq. 52) [86,87], Proton scavenging aluminum Lewis acids such as RAICU are most often used in ene reactions to eliminate proton-induced side reactions. 

The effects of divalent ions (Cu , Ni", Co" and Zn") on the hydrolysis of phthalate, succinate and acetate esters of 2-(hydroxymethyl)picolinic acid (54) have been studied to establish if strong bonding of the metal ion to the leaving group oxygen might enhance intramolecular carboxylate attack. With these esters, saturation conditions are observed at low metal ion concentrations... 

King SM, Hintschich SI, Dai D, Rothe C, Monkman AP (2007) Spiroconjugation-enhanced intramolecular charge-transfer state formation in a polyspirobifluorene homopolymer. J Phys Chem C 111 18759-18764... 

In MMA and MA no such hydrogen bonds are operative. The distinct difference in the pre-exponential for bulk polymerization of these two monomers (see entries 4 and 7 in Table 1), however also originates from effects on internal rotational mobihty. The lower value of A(fep MMA) is due to enhanced intramolecular friction induced by the a-methyl groups on the polymer backbone. 

The rate enhancements accompanying intramolecular reactions when compared to related intermolecular counterparts have withdrawn the attention of several investigators in recent years not only for the phenomenon per se but also because the factors involved have close connections with the mechanism of enzyme catalysis. Out of the possible factors involved in enhanced intramolecular reactivity, changes in solvation of the reacting species and/or transition state in going from the intermolecular over to the intramolecular process have been taken into account from time to time. 

For lactone synthesis there is no need for special ester linker requirements. It can be performed using an ester activating reagent like TEA that enhances intramolecular nucleophilic attack by an alkoxy fimctionality. The distance between the ester group and the alkoxy functionality determines the resulting ring-size and therefore whether y- or 5-lactone-derivatives are released [134]. 

Figure 10.5 Structural modifications enhancing intramolecular hydrogen bonds.

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