Carbonyl displacement carbanions

Reaction of the carbanion of chloromethyl phenyl sulphoxide 409 with carbonyl compounds yields the corresponding 0-hydroxy adducts 410 in 68-79% yield. Each of these compounds appears to be a single isomer (equation 242). Treatment of adducts 410 with dilute potassium hydroxide in methanol at room temperature gives the epoxy sulphoxides 411 (equation 243). The ease of this intramolecular displacement of chloride ion contrasts with a great difficulty in displacing chloride ion from chloromethyl phenyl sulphoxide by external nucleophiles . When chloromethyl methyl sulphoxide 412 is reacted with unsymmetrical ketones in the presence of potassium tcrt-butoxide in tert-butanol oxiranes are directly formed as a mixture of diastereoisomers (equation 244). a-Sulphinyl epoxides 413 rearrange to a-sulphinyl aldehydes 414 or ketones, which can be transformed by elimination of sulphenic acid into a, 8-unsaturated aldehydes or ketones (equation 245). The lithium salts (410a) of a-chloro-/ -hydroxyalkyl... 

The initial carbon-carbon bond formation (— 84) is often reversible, and a subsequent step—such as dehydration—may be necessary to displace the equilibrium. The many different (often named) reactions really differ from each other only in the nature of the particular carbonyl compound (aldehyde, ketone, ester, etc.), and in the type of carbanion, employed. 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

The Pd(0)-catalyzed displacement of allylic acetates (297) with various nucleophiles via the allylic Pd(II) complex (298) is a well-established procedure (Scheme 114). Through attack of electrons (+2e ) in place of nucleophiles, (298) is expected to undergo a reductive cleavage providing allylic carbanions (299) and the acetate anion along with Pd(0) complexes. The latter can then be captured by various electrophiles (polarity inversion. Scheme 114) leading to (300) [434]. This procedure is useful for the deprotection of allyl esters under neutral conditions. Recently, a mechanistic study of the Pd-catalyzed reaction of allylic acetate (297), using carbonyl compounds as an electrophile, has been reported [435]. 

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. 

An interesting intramolecular displacement occurs in the Darzens reaction, in which carbanions derived from a-haloesters react with carbonyl compounds to yield a-epoxyesters ... 

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... 

Many other uses of a-sulfinyl carbanions are found in the literature, and in the recent past the trend has been to take advantage of the chirality of the sulfoxide group in asymmetric synthesis. Various ways of preparation of enantiopure sulfoxides have been devised (see Section 2.6.2) the carbanions derived from these compounds were added to carbonyl compounds, nitriles, imines or Michael acceptors to yield, ultimately, with high e.e. values, optically active alcohols, amines, ethers, epoxides, lactones, after elimination at an appropriate stage of the sulfoxide group. Such an elimination could be achieved by pyrolysis, Raney nickel or nickel boride desulfurization, reduction, or displacement of the C-S bond, as in the lactone synthesis reported by Casey [388]. 

Deviating from the route via nucleophilic attack of the carbanion at the carbon atom of a CO ligand and then reaction of the acylmetallate with an electrophile are those methods which involve (a) addition of the carbanion to the carbon atom of a carbyne ligand, (b) displacement of halides from transition-metal carbonyl halides by cyclohepta-trienyllithium, or derivatives thereof, followed by hydride abstraction or (c) substitution of a coordinated solvent from a metal-carbonyl complex (see also reaction of LiR with carbene complexes). 

Carbanions can displace azide ion from carbonyl azides . For displacements at guanyl azides see section I.B and at carbamoyl azides see section I.C. 

Ethoxide ion abstracts (step 1) a hydrogen ion from the a-carbon of the ester to form carbanion 1. The powerfully nucleophilic carbanion I attacks (step 2) the carbonyl carbon of a second molecule of ester to displace ethoxide ion and yield the keto ester. 

Anions of a-silyl phosphonates of type (153) also undergo additions to carbonyl compounds. The corresponding addition products, 3-silyl alkoxides, can react with ketones to yield the product of the Peterson alkenation or the Wittig reaction. In practice only the Peterson product (154) is obtained, indicating that loss of OSiMes is faster than elimination of C PPhs (Scheme 68). 72 If the a-silyl carbanion is adjacent to a chlorine atom (155), an internal displacement reaction follows the initial formation of the -silyl alkoxide, and epoxides (156) are formed (Scheme 69). 74... 

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