Magnesium electrophiles reaction

Carbenoids have both a nucleophilic and an electrophilic nature. This is one of the most striking characteristics of carbenoids. Especially, electrophilic reaction of magnesium carbenoids with carbon and nitrogen nucleophiles has recently received much attention and various new interesting synthetic methods have appeared. 

The electrophilic reaction of magnesium carbenoids with Af-lithio aryiamines was found to give non-stabiUzed a-amino-substituted carbanions (equation 17). Treatment of magnesium carbenoid 38 with 3.5 eq of Af-lithio Af-methylaniline at —70 °C followed... 

The electrophilic reaction of magnesium cyclopropylidene (113) with Ai-lithioaryl-amines was reported (equation 31) . Thus, electrophilic reaction of magnesium cyclopropylidene (113) derived from 112 with iV-lithio iV-methyl p-anisidine resulted in the formation of a-amino-substituted cyclopropylmagnesium (119) in good yield. Methanol-ysis of the reaction mixture with CH3OD gave a-deuteriated Af-cyclopropyl-Af-methyl-p-anisidine (120) in 82% yield with 98% D-content. 

The electrophilic reaction of magnesium alkylidene carbenoids with other nucleophiles than the original Grignard reagent can also be carried out. For example, treatment of magnesium alkylidene carbenoid 157, derived from 147, with a-sulfonyl lithium carbanion afforded allenes 159 in moderated yields (equation 39/. ... 

Stoichiometric, intramolecular magnesium-ene reactions involving terminal or strained alkene eno-phile units have served extensively as a cornerstone in efficient syntheses of natural products. One of the reasons is the propensity of the cyclized Grignard intermediates to be trapped by a vast array of electrophiles. On the other hand, the use of Mg is incompatible with several functionalities such as certain heteroatoms in the acyclic precursor (zinc-ene cyclizations seem to surmount some of these limitations). 

Scheme 5.2 Preparation of C(sp )-organomagnesium reagents by halogen-magnesium exchange reactions and subsequent trapping with electrophiles [19-23].

Based upon the bis-nucleophilicity of 1,3-diene-magnesium intermediates, reactions of these intermediates with bis-electrophiles can lead to spiro or fused bicyclic molecules, depending upon the regioselectivity of the cyclization. It has been shown that treatment of magnesium complexes of l,2-bis(methylene) cyclohexane with l, -dibromoalkanes results in overall 1,2-cyclizations of the original dienes, affording spirocarbocycles in good to excellent yields [115]. 

A Grignard reaction begins with an acid-base complexation of Vfg2+ to the carbonyl oxygen atom of the aldehyde or ketone, thereby making the carbonyl group a better electrophile. Nucleophilic addition of R then produces a tetrahedral magnesium alkoxide intermediate, and protonation by addition of water... 

---