General and Theoretical

The intrinsic basicities of cyclopentenone and cyclohexenone (59), and their lactone analogues (60), have been accessed via measurement of their gas-phase proton affinities, and compared with the saturated carbonyl compounds in both cases. The results 

Ab initio calculations identify the sources of these effects for example, in unsam-rated ketones the double bond participates fully in the change in charge distribution accompanying protonation, while in the unsaturated lactones, the ring oxygen impedes this shift of electron density. 

The hydrogen-bond basicities of a very extensive range of aldehydes and ketones have been measured, and are reported in terms of Taft s p/fuB scale.  

Ab initio calculations on the interaction of HF with a wide variety of carbonyl types show correlations between the energy of hydrogen-bond formation and both the H-F 

When a nucleophilic reagent, Nu X+ (or Nu—X), is reacted with a ketone, com-plexation of oxygen by X+ may precede attack at carbon. Geometric changes associated with such complexation have been calculated for a series of 4-substituted cyclohexanones. The results allow the facial selectivity of the subsequent nucleophilic attack to be predicted, and without the need to calculate the transition-state geometry. 

CH/7T hydrogen bonds in organic reactions have been reviewed, including major sections on diastereoface- and enantioface-discriminating reactions.223 g) 

The nucleophilic reactivities of silyl enol ethers (58, R1 = alkyl) and silyl ketene acetals (58, R1 = 0-alkyl) have been measured for the triphenylsilyl (R2 = H5) substrate, and its perfluoro analogue (R2 = F5), using benzhydrylium cations as reference electrophiles.224 The triphenyl compound is 10 times less reactive than its trimethyl equivalent, but the perfluorination causes the C=C nucleophilicity to drop by 3-4 orders of magnitude. The new compounds have been placed on scales of nucleophilicity taken from the literature. 

A computational study of hydride addition to a range of carbonyl compounds suggests that most of the negative charge resides on hydrogens, and not on the carbonyl oxygen.225 

Proton affinities of a variety of simple ketones, a-diones, and -keto esters and lactones have been calculated by a variety of methods and compared with experiment.226 

Recent developments in the asymmetric addition of aldehydes have been reviewed,206 as have asymmetric catalysis using metal complexes207 and nucleophile isotope effects 208 

Natural bond orbital analysis of early and late TSs has been carried out to explore the factors involved in tt-selectivity of nucleophilic addition to carbonyls.209 Cieplak s o —r o hyperconjugation hypothesis (where o is the incipient bond) is not supported by the results for early TSs, and evidence in favour of Felkin-Anh s o er hypothesis is weak. Late TSs are devoid of o 7r(t=() interactions here, the Cieplak model may be applicable. 

Alcohol (74) undergoes an unusual extrusion of its hydroxymethyl group in the presence of sodium hydride.210 That it is a reverse reaction of nucleophilic addition to formaldehyde was confirmed by trapping experiments for the latter. Relief of steric congestion is a likely cause, with the sulfinyl also helping to stabilize the incipient carbanion. The two factors combined help to reverse the equilibrium which normally favours attack on formaldehyde. 

The atmospheric chemical kinetics of linear perfluorinated aldehyde hydrates, Cx-F2x+iCH(OH)2, have been measured for x = 1,3, and 4, focusing on formation (from aldehyde, by hydration), dehydration, and chlorine atom- and hydroxyl radical-initiated oxidation.211 The latter reaction is implicated as a significant source of perfluorinated carboxylic acids in the environment. 

Placing two methyl groups ortho to the carbonyl of acetophenone should twist the phenyl out of the C=0 plane. The extent to which this affects gas- and solution-phase basicities of a series of / /a-substituted acetophenones is reported.95 

4-Substituted norsnoutanes (61) have been introduced as substrates with sterically unbiased re-faces, which allow electronic effects in re-facial selectivity of nucleophilic additions to be evaluated.96 Examples indicate how this system allows separation of long-range electronic effects into orbital and electrostatic contributions. 

An extensive study of reactions of a variety of non-cyclic esters, aldehydes, and ketones with a range of nucleophiles has been undertaken in an attempt to find reliable rales for predicting 1,3-stereochemistry in the products.97 Despite comparison of the 

Nucleophilic addition/ring-closure sequences—especially additions to aldehydes, ketones, and aldimines—have been reviewed in the context of heterocyclic synthesis.98 

A -Tiimcthylsilylbis(trifluoromcthancsulfonyl)imidc, Me3SiN(SC 2CF3)2, has been reported as a better carbonyl activator than trimethylsilyl triflate.  

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G. Berthier and J. Serre, "General and theoretical aspects of the carbonyl group" in The Chemistry of the Carbonyl Group. Interscience, 1966. 

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