Grignard Reagents resonance forms

The required Grignard reagents are formed from the substituted guanidines and ethyl magnesium halide in ether [59), with simultaneous evolution of ethane. Their assigned structure (XLV) accounts satisfactorily for their reactions, although their true structure will obviously be essentially ionic and exhibit resonance effects. 

Similar to the formation of allylmagnesium chloride (25), the oxidative addition of allyl halides to transition metal complexes generates allylmetal complexes 26. However, in the latter case, a 7i-bond is formed by the donation of 7i-electrons of the double bond, and resonance of the n-allvl and 7i-allyl bonds in 26 generates the 7i-allyl complex 27 or (/ -allyl complex. The carbon-carbon bond in the 7i-allyl complexes has the same distance as that in benzene. Allyl Grignard reagent 25 is prepared by the reaction of allyl halide with Mg metal. However, the 7i-allyl complexes of transition metals are prepared by the oxidative addition of not only allylic halides, but also esters of allylic alcohols (carboxylates, carbonates, phosphates), allyl aryl ethers and allyl nitro compounds. Typically, the 7i-allylpalladium complex 28 is formed by the oxidative addition of allyl acetate to Pd(0) complex. 

In the next example the bromine, being more reactive than chlorine, selectively reacts to form the Grignard reagent. The product alcohol is especially prone to El elimination (the carbocation would be stabilized by resonance), so the weak acid, ammonium chloride, is used in the workup step. 

Additions occur more easily if a carbanion with resonance or inductive stabilization is formed in the addition. Thus, fulvenes are very reactive, vinylsilanes and highly fluorinated alkenes somewhat less so. Styrene, 1,3-dienes, and enynes are more reactive than isolated alkenes, and Grignard reagents may be used to initiate anionic polymerization of styrenes, dienes, and acryhc monomers. Strained alkenes such as norbomenes and cyclopropenes are also more reactive. Examples of additions facilitated by resonance or substitution are shown in Scheme 8. 

The configurational stability of primary Grignard reagents can be studied by an interpretation of exchange effects in the proton resonance spectra as a function of temperature (Whitesides et al., 1963). These changes in spectra are related to the high energy barrier in heavily substituted ethanes. The molecule studied is shown below in three rotamer forms of which only two are non-equivalent.)... 

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