Carbanion method

This chapter will begin with a brief overview of the development of carbanion chemistry followed by a section devoted to the structure and stability of carbanions. Methods of measuring carbon acidity and systematic trends in carbanion stability will be key elements in this chapter. Next, processes in which carbanions appear as transient, reactive intermediates will be presented and typical carbanion mechanisms will be outlined. Finally, some new developments in the field will be described. Although the synthetic utility of carbanions will be alluded to many times in this chapter, specific uses of carbanion-like reagents in synthesis will not be explored. This topic is exceptionally broad and well beyond the scope of this chapter. 

Carbanion Method AE Proton Affinity calc. exp. Reference... 

Polyarylmethyl polyradicals were prepared using the carbanion method (Fig. 9), as described in the original communications in 1990 and the other references cited... 

Typical reaction vessels are shown in Figs 10 and 11. Detailed description of the procedures of the carbanion method and their development since late 1980s until 2004 is available on our research group web site.62... 

Fig. 11 Implementation of the carbanion method for insoluble (gel forming) polyethers. Steps 1-4 correspond to swelling of polyether with THF-d8, generation of carbopolyanion, oxidation with removal of a Teflon-coated magnetic stirbar, and flame sealing of the quartz sample vessel for SQUID magnetic studies, respectively. Reproduced with permission from Ref. 62. Copyright 2004 Dr. Suchada Rajca.

The imine-carbanion method was modified to accomplish the interesting ketone conjugated aldehyde transformation. The a-lithioimine (310) reacted with cyclohexanone to give 311. Hydrolysis at a carefully controlled pH of 4.5 led to a conjugated aldehyde 312 in 90% yield. 

In an extension of the methodology described for the preparation of a-methylenebutyrolactones," it has been found that O-silyl enolates of esters, lactones, and ketones can be smoothly monoalkylated by reactive alkyl halides [e.g. benzylic, allylic, MeOCHzCl, PhSCH(Cl)R] in the presence of small amounts of zinc bromide. This approach could find extensive use especially in cases where direct carbanion methods are unsatisfactory. 

The acetylated glycosyl chloride of neuraminic acid ethyl ester, with allyltributyltin, gave a- and -C-allyl derivatives which were epoxidized. - C-Hydrotymethylation at C-2 of a related neuraminic acid derivative by carbanionic methods gave the a- and -products in 3 1 ratio. Aminomethyl analogues were also made, but none of these products led to... 

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