Chelation involving organolithiums

Threo diastereoselectivity is consistent with a chelation-controlled (Cram cyclic model) organolithium addition (Figure 8a). Since five-membered chelation of lithium is tenuous, an alternative six-membered chelate involving the dimethylamino nitrogen atom of the thermodynamically less stable (Z)-hydrazone (in equilibrium with the ( )-isomer) cannot be discounted. The trityl ether (entry 4, Table 9) eliminates the chelation effect of the oxygen atom such that the erythro diastereomer predominates (via normal Felkin-Ahn addition) (Figure 8b). 

The addition reactions of alkyllithium-lithium bromide complexes to a-trimethylsilyl vinyl sulfones that have as a chiral auxiliary a y-mono-thioacetal moiety derived from ( + )-camphor are highly diastereoselective. A transition state that involves chelation of the organolithium reagent to the oxygen of the thioacetal moiety has been invoked. The adducts are readily converted via hydrolysis, to chiral a-substituted aldehydes22. 

A number of other examples have been reported which involve highly selective Grignard or organolithium additions to carbohydrates. Unfortunately, no general trends for these complex systems have been observed. Hie selectivities reported are often specific for one substrate under a particular set of reaction conditions. Reetz has reviewed the chelation and nonchelation control addition reactions (not confined to organolithium or organomagnesium reagents) of a- and 3-aUcoxycarbonyl compounds. ... 

In 1962 and 1963 the chelated organolithium complexes and their application in ethylene telomerization were discovered independently of each other by Eberhardt (12) at Sun Oil Co. This research led to two patents (13, 14) in which the claims involving chelated catalysts were... 

Metalation of Weak Acids. The ability to metalate very weak acids is a property which has been used widely for the direct synthesis of organolithium compounds, for new or improved organic syntheses, for polymer grafting, and for telomerizations involving chain transfer by transmetalation. These major topics are covered in seven chapters of this volume, indicating both the scope and utility of this property of N-chelated organoalkali metal compounds. 

All the above reactions involve hydrogen—lithium interconversion (metalation). There are a limited amount of data available indicating that N-chelated organolithium intermediates also undergo nucleophilic addition to carbon-carbon double and triple bonds much more readily than does the organolithium reagent alone. Indeed, this enhanced reactivity toward addition reactions is a key factor in the telomerization of ethylene onto aromatic hydrocarbons (6, 7,8,9). 

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