Carbon-magnesium bonds, reactivity

The reactivity of the 1,1 -diorganometallics (61) has been extensively investigated by Knochel and co-workers.9293 Since the reactivity of a carbon-lithium or a carbon-magnesium bond is considerably different from that of a carbon-zinc bond, a selective reaction of (61) with two different electrophiles is often possible. The general reaction pathways are summarized in Scheme 33. The addition of allylic zinc... 

The greater electronegativity of the earlier halides in the periodic table is expected to cause an increase in the polarity of the carbon magnesium bond and, hence, to increase its reactivity. 

The effect of solvation on the strength and reactivity of the carbon-magnesium bond. 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

A facile intramolecular carbomagnesiation becomes possible by inserting one more carbon between the reactive magnesium center and the double bond. For example, when a 6-chloro-l-heptene was refluxed with magnesium, 1,2-dimethylcyclopentane (cis/trans = ca 1/4) was obtained in 88% yield after hydrolysis (Scheme 47) °. The cyclization shows a 5-exo-trig selectivity and the product derived from 6-endo-trig cyclization (methylcyclohexane) is not observed. 

Why are organotitanium reagents more chemoselective than the lithium and magnesium counterparts It is clear from the above presentation that the rate of reaction of classical carbanions is considerably higher than those of the titanated species. Generally, more reactive species are less selective. However, this does not really answer the above question, since the phenomenon of different rates remains unclear. Apart from steric factors, we believe it has to do with the different polarity of the carbon-metal bond. Whereas C—Li bonds are highly polar (and ionic in certain cases) 57,86,87) anaj0gS appear to be considerably less so (Sect. B). 

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