Carbonyls carbanion addition

Crossed aldol condensations, where both aldehydes (or other suitable carbonyl compounds) have a-H atoms, are not normally of any preparative value as a mixture of four different products can result. Crossed aldol reactions can be of synthetic utility, where one aldehyde has no a-H, however, and can thus act only as a carbanion acceptor. An example is the Claisen-Schmidt condensation of aromatic aldehydes (98) with simple aliphatic aldehydes or (usually methyl) ketones in the presence of 10% aqueous KOH (dehydration always takes place subsequent to the initial carbanion addition under these conditions) ... 

This reaction of aromatic aldehydes, ArCHO, resembles the Cannizzaro reaction in that the initial attack [rapid and reversible—step (1)] is by an anion—this time eCN—on the carbonyl carbon atom of one molecule, the donor (125) but instead of hydride transfer (cf. Cannizzaro, p. 216) it is now carbanion addition by (127) to the carbonyl carbon atom of the second molecule of ArCHO, the acceptor (128), that occurs. This, in common with cyanohydrin formation (p. 212) was one of the earliest reactions to have its pathway established— correctly —in 1903. The rate law commonly observed is, as might be expected,... 

The ylid intermediate (50) can be trapped by carrying out the decarboxylation in the presence of carbonyl compounds, e.g. PhCOMe, to yield the carbanion addition product, e.g. (52) this process can indeed be used preparatively. The reason for the much easier decarboxylation of (51), than of its 3-, and 4-isomers, is the stabilisation that the N can effect on the adjacent carbanion carbon atom in the intermediate ylid (50). 

Carbonylation (continued) of secondary alcohols, 34 94-96 (HRhIjCCO)] as active catalyst, 34 95 inhibitory effect of CO pressure, 34 94 2-propanol, 34 94 deuterium studies, 34 95 Carbonyl groups, addition of carbanions, 35 390-392... 

OH—on the adjacent (/ -) carbon atom. The possibility of such an elimination may displace the equilibrium over to the right in a number of simple aldol additions, where it would otherwise lie far over to the left. It is important to remember, however, that the overall process aldol addition + dehydration is reversible, i.e. (88) 4= (96), and that a -unsaturaled carbonyl compounds are thus cleaved by base under suitable conditions. It is also pertinent that (96) is still an aldehyde and can undergo further carbanion addition, followed by dehydration, and so on. This is how low molecular weight polymers are produced on heating simple aliphatic aldehydes with aqueous NaOH to stop at the aldol, the best catalysts are basic ion-exchange resins. 

In the absence of a proton donor, the alkoxide ion generated by carbanion addition to the carbonyl function can interact with a carbon-halogen bond in the 8 2 displacement reaction. Reactions of this type have led to some novel carbon chain forming processes. Ketones are converted to homologated enones in good yield by... 

The D-ring of steroids has been cleaved frequently by a Beckmann fragmentation. Typical strategy uses enolate chemistry to introduce an oximino group at the a-carbonyl carbon. Reduction of the carbonyl (or addition of a carbanion) produces an a-hydroxy oxime which serves as good substrate for the fragmentation. Two examples of this strategy are shown in equations 213 and 214. 

The reactions to be discussed in this section involve carbanion addition to carbonyl centers with a potential leaving group. The tetrahedral intermediate formed in the addition step then reacts by expulsion of the leaving group. The overall transformation results in the... 

The second group of cyclic compounds which illustrate the intramolecular carbanion addition to a carbonyl group, include 2-ethoxycarbonylcyclo-pentanone (23) and indane-l,3-dione (24). 

The third group of cyclic compounds formed by intramolecular carbanion addition to the carbonyl group are the target molecules 5,5-dimethylcyclo-hexane-l,3-dione (dimedone, 30) and 3-methylcyclopentane-l,2,4-trione (31). 

A few examples of chemoselective additions of allyltitanium reagents to aliphatic and aromatic carbonyl compounds are reported in Table 7. Appreciable chemoselectivity toward the aldehydic function is achieved by the titanium ate conqtlex (23), whereas the reverse chemoselectivity toward ketones is realized using aminotitanium complex (104) and the analogous ate complexes (24) and (25), as is shown in Table 7. This is very interestii since it represents a rare case of chemoselectivity in favor of carbanion addition to ketones. A tentative explanation of this inverse chemoselection considers a fast transfer of the aminyl ligand onto the aldehyde function which becomes protected , as in (105), and thus unreactive in respect to the keto group. Ketones react also selectively compared with esters, as is shown by the reaction of ethyl levulinate (1) with the ate complex (23 equation 40). ... 

Carbanion additions occur frequently at carbonyl or nitrile groups and occasionally at carbon-carbon double bonds. The Grignard synthesis of alcohols is an example ... 

The effect of substituents in the benzene ring provides an interesting example of the dual nature of carbanion addition to carbonyl groups. When an electron-releasing group is conjugated with the aldehyde function, a resonance form such as (XIV) apparently lowers the usual positive charge on the carbonyl carbon atom to a point where the addition... 

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