Organomagnesium compounds steric hindrance

Organomagnesium compounds sometimes behave as single-electron donors. Such behaviour is most likely to be observed in highly polar, cation-solvating media and/or when steric hindrance inhibits alternative pathways. Ate complexes of magnesium can also act as single electron donors (see p. 97). It should be noted that the presence of traces of transition metals may lead to electron transfer processes [2]. 

The usual reactions of organomagnesium compounds with esters, amides and nitriles are those described in Sections 5.3, 6.2 and 6.3. However, factors such as steric hindrance and stability of leaving group may lead to reactions resulting in overall nucleophilic displacement. Most of them are of limited applicability (see Ref. [39] for examples involving carbamate), but two warrant coverage. 

Reactions of organomagnesium compounds with phosphorus(lil) halides and trialkyl phosphites are normally straightforward the main limitations are steric hindrance, and the instability of the products. Some examples are listed in Table 15.3, and others are tabulated in General Ref. [A]. 

Addition reactions. a,0-Unsaturated carbonyl compounds 1 and 3 undergo exclusive 1,4-addition reactions with organolithium (and organomagnesium) compounds owing to steric hindrance of the carbonyl group. The adducts 2 and 4 are converted into carboxylic acids by cleavage with potassium t-butoxide and H2O in the case of 2 see potassium r-hutoxidc, this volume) and by treatment with ethanolic... 

Among the most obvious properties of an organomagnesium compound that influence its reactivity are its steric requirements the bulkier the reagent the more hindrance there will be in the approach of the substrate, and this may cause a considerable change in its chemical behavior. Whereas methylmagnesium bromide reacts with benzophenone (Scheme 3) to yield, on hydrolysis, the expected tertiary alcohol, 1,1-diphenyl-1-ethanol, in 100% yield. 

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