Grignard reagents allylic shift

This facile type of sigmatropic of rearrangement appears general. Julia and his group [96-98] have prepared and characterised a number of unsaturated sulfines. Instead of an oxidation route to the starting sulfoxides, they performed the reaction of vinyl Grignard reagents with allyl sulfenates at -78 °C. In most cases the subsequent [3.3] shift is so rapid that the intermediate unsaturated sulfoxides were not even evidenced sulfines were isolated instead. 

A second group of complexes that have received detailed attention are the (i73-allyl)nickel(i75-cyclopentadienyl) complexes, which may be prepared by reacting nickelocene with an allyl-Grignard reagent (69). Data for typical examples are shown in Table IX. In this particular case, a sufficiently large number of complexes have been studied that a series of chemical shift increments can be extracted from the data (shown in Table X) and have been used with considerable success in assigning structures to ij3-allyl nickel complexes of unknown structure. 

Yet another development which is worth mentioning in this context is the 5n substitution of acetals of unsaturated carbonyl compounds. The phenomenon that the reaction of an allylic acetal with a Grignard reagent in the presence of CuBr may occur as a vinylogous substitution with double bond shift has been long known.The reaction can be utilized in an efficient synthesis of 3-substituted propionaldehydes using the acrolein acetal as a homoenolate cation equivalent (Scheme 38). ... 

Six-centre transition states for the reactions of a carbonyl compound with a Grignard reagent arc very efficient if they allow the formation of a Mg-O bond in concert with the shifts of two pairs of rr or or electrons. Five examples arc shown in Scheine 1.9 (a)-(e). Concerted reduction iind enolization are mentioned above. The allylic addition. Scheme 1.9 (b), is efficient because of the absence of steric hindrance. The carbonyl carbon of an aldehyde may attack ortho to the side chain in benzylmagnesium chloride forming a reactive dihydrobenzene compound, which may-add a second molecule of aldehyde. Scheme 1.9... 

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