Alkenyl Grignard reagents

As described in Section II, the preparation of 1,1-dizincioalkane can be achieved not only by reduction of 1,1-dihaloalkane, but also by allylzincation of alkenyl metal compounds. The reaction of allylzinc with alkenyl Grignard reagent, which is followed by the addition of aldehydes, gives dienes as shown in Scheme 1511,41. 

Allylzinc bromide is able to add to a variety of alkenyl Grignard reagents in THF at 35 °C123. The structure of the hydrocarbons 184, obtained in moderate yields after hydrolysis (due to their volatility), suggested that 1,1-dimetallic species tentatively formulated as 185 were regioselectively formed (equation 89). Additional examples were subsequently reported but the scope of this intriguing reaction remained rather unexplored and the reactivity of 185 towards various electrophiles was not investigated124,125. 

Reaction of fluorinated alkenyl Grignard reagents or lithium reagents with organotin halides has been utilized for the preparation of fluorinated alkenyl tin reagents [111, 182-184]. Recently, a novel, general and convenient method has been developed for the preparation of a variety of fluorinated alkenylstannanes... 

Scheme 5.15 Cross-coupling of alkenyl-Grignard reagents with alkyl electrophiles.

To our knowledge there is no example in the literature of a stable alkene (or alkyne) complex with magnesium as central atom, n Complexes have, however, been postulated as intermediates in the rearrangement reactions of some alkenyl Grignard reagents (86). 

Their advantage over other types of dendrimers is their straightforward synthesis and, most importantly, their chemical and thermal stabilities. Two distinct steps characterize their synthesis a) an alkenylation reaction of a chlorosilane compound with an alkenyl Grignard reagent, and b) a Pt-cata-lyzed hydrosilylation reaction of a peripheral alkenyl moiety with an appropriate hydrosilane species. Scheme 2 shows the synthesis of catalysts Go-1 and Gi-1 via this methodology. In this case, the carbosilane synthesis was followed by the introduction of diamino-bromo-aryl groupings as the precursor for the arylnickel catalysts at the dendrimer periphery. The nickel centers of the so-called NCN-pincer nickel complexes were introduced by multiple oxidative addition reactions with Ni(PPh3)4. 

Li2CuCl4 in THF-NMP efficiently catalyzes the reaction of alkenyl Grignard reagents with primary alkyl bromides. Functional groups (ketone, ester, nitrile, sulfonate) are tolerated in this reaction (Scheme 34).91... 

Table 5 Generation and Reactions of Alkenyl Grignard Reagents from Acetylene... 

Scheme 5 Equilibration of acetylene and alkenyl Grignard reagent under hydromagnesation conditions...

A transition metal catalyzes a similar carbomagnesation reaction, which has broaden the synthetic usefulness of this method for synthesis of alkenyl Grignard reagents. Among several transition metal salts that were reported to catalyze carbomagnesation of acetylenes [7,51,82 84], copper salts are most popular [7,13-15]. A variety of acetylenes, including acetylene itself [85], as well as substituted ones participated in this reaction. Some representative reactions are summarized in Table 11. 

A decrease in stereochemical purity of a product when compared with a starting alkenyl Grignard reagent was also observed. Thus, care should be taken, since the cis/trans ratio of the alkenyl portion of Grignard reagents will not necessarily be reflected in the final products under certain reaction conditions [246]. Equation (127) illustrates such an example. 

Vinyl cyclopropanols have been prepared by the addition of alkenyl Grignard reagents to a variety of cyclopropanone equivalents. Upon treatment with acid, the vinyl cyclopropanols rearrange to a-substituted cyclobutanones. Alternatively, a variety of a-heteroatora-substituted cyclopropyllithiura reagents have been developed. These react with aldehydes and ketones to afford cyclopropylcarbinols which also rearrange to cyclobutanones under acid catalysis.Lastly, vinyl cyclopropanols and cyclopropylcarbinols have been prepared by the cyclopropanation of enol silyl... 

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