Carbanions reactions, summary

Summary of Carbanion Reactions. In concluding our discussion of carbanion processes, we might summarize the reactions of compounds which can furnish carbanions by considering the products formed when esters are treated with strong base.17 There are four positions available f6r attack ... 

Obviously the structures and yields of Birch reduction products are determined at the two protonation stages. The ring positions at which both protonations occur are determined kinetically the first protonation or 7t-complex collapse is rate determining and irreversible, and the second protonation normally is irreversible under the reaction conditions. In theory, the radical-anion could protonate at any one of the six carbon atoms of the ring and each of the possible cyclohexadienyl carbanions formed subsequently could protonate at any one of three positions. Undoubtedly the steric and electronic factors discussed above determine the kinetically favored positions of protonation, but at present it is difficult to evaluate the importance of each factor in specific cases. A brief summary of some empirical and theoretical data regarding the favored positions of protonation follows. 

A summary of the in situ use of the azobenzene probases is given in Table 2. Apart from the generation of ylid, referred to above, the main applications have been for N- and C-alkylation of weak nitrogen and carbon acids, for the promotion of condensation and substitution reactions involving carbanions such as the cyano-methyl anion, for an interesting carboxylation reaction (entries 4 and 17), and for base-promoted cyclisations (entries 5 and 6). 

We have seen in this chapter how carbenes can be formed from many other reactive intermediates such as carbanions and diazoalkanes and how they can react to give yet more reactive intermediates such as ylids. Here is a summary of the main relationships between carbenes and these other compounds. Note that not all the reactions are reversible. Diazoalkanes lose nitrogen to give carbenes but die addition of nitrogen to carbenes is not a serious reaction. 

Summary Reacting 2-neopentyl substituted silacyclobutanes la,b with MeLi/HMPA (hexamethylphosphoric triamide) anionic polymerization to give polymers 3a,b plays only a minor role for product formation. Instead, the head-to-head dimers 2a,b are isolated as main products. Their formation is explained by a complex reaction mechanism, in which various carbanionic, highly reactive intermediates are discussed. Obviously, the bis-a-silyl substituted carbanions 10a,b are remarkably stable, as can be concluded from Si NMR spectroscopic investigations at low temperature and from the products formed by trapping reactions with alcohols. 

Scheme 17.6 shows the reaction process of 23 and 27 in more detail using compound 30 as the summary structure (23 has R = alkynyl and 27 has R = H). First, pro-R deprotonation would take place through chelate 30a to form C7- R) lithiated carbanion 31. When the C3-R group is alkynyl, strain with the 4-a-equatorial group would favor conformational inversion to 33,... 

The various reactions of nucleophiles with sulphines continue to be explored Scheme 3 provides a summary. Functionalized carbanions, e.g. XCH2 (X = CN, SOMe, or S02CjH4Me-p) or CHRCONEtj, proceed by the more usual thiophilic... 

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