Grignard reagents, bonding complex formation

When secondary Grignard reagents are used, the coupling product sometimes is derived from the corresponding primary alkyl group.169 This transformation can occur by reversible formation of a nickel-alkene complex from the cr-bonded alkyl group. Reformation of the cr-bonded structure will be preferred at the less hindered primary position. 

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. 

The reaction of aldehydes and ketones with Grignard reagents is a useful method of synthesising primary, secondary, and tertiary alcohols (Following fig.). Primary alcohols can be obtained from formaldehydes, secondary alcohols can be obtained from aldehydes, and tertiary alcohols can be obtained from ketones. The reaction involves the formation of a carbon-carbon bond and so this is an important way of building up complex organic structures from simple starting materials. 

In 1912, Straus [7] suggested that the first step in the reaction of a Grignard reagent with a carbonyl compound was the replacement of the ether in the complex (analogous to what had earlier been proposed by Kohler [8] see also Grignard s view on the first step of reactions with epoxyethane as represented in Fig. 2) followed by the formation of the carbon-carbon bond (Scheme 1) ... 

On the other hand, in a concerted mechanism, bond formation, which includes the shift of two electrons at a time for each bond to be formed, takes place simultaneously on different sites in the complexed reagents (see, as an example, the cyclic mechanism presented in Scheme 7). These two mechanisms are extremes in a complete picture of how bond formation takes place in reactions of Grignard reagents with whatever substrates is under discussion a reactivity spectrum then becomes evident. 

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