Ketones, reaction with allylic Grignard reagents

Hydroxy-1-carboxamides (e.g. 54) can be cyclized to butyrolactones (55) by thermolysis or treatment with acid. Spiro-butyrolactones have also been prepared from cyclic ketones by reaction with allyl Grignard reagents followed by hydro-boration and oxidation with chromic acid. ... 

Allyltin compounds can be prepared by simple modifications of the usual reaction involving allyl Grignard reagents (139), by the 1,4-addition of trialkyltin hydrides to 1,3-dienes 140,141), or by the reaction of an aldehyde or ketone with the appropriate, tin-carrying, Wittig reagents (142). 

However, in a recently published, detailed study on the stereochemistry of the addition reaction of allylic Grignard reagents (mainly crotylmagnesium chloride) with a,/ -ethylenic ketones [15], addition to aryl-substituted enones occurred without any selectivity at all. The authors suggested that this was due to a single-electron transfer, since poor diastereoselectivity is observed when a carbon-carbon bond is formed by a radical- radical combination [64]. No further mechanistic details on this difference in reactivity were given. 

The reactions of 2-propenyltitanium reagents with chiral aldehydes (0) or ketones ( and ) usually exhibit enhanced induced diastereoselectivities compared to allyl Grignard reagents. This enhanced diastereoselection is mainly attributed to the greater bulk and lower reactivity of the reagent. Some examples are collected ( )52. 

Normally, the reaction of an ester with one equivalent of a Grignard reagent leads to a mixture of tertiary alcohol, ketone, and unreacted ester. However, when allylic Grignard reagents are used in the presence of one equivalent of LDA, good yields of ketones are obtained. What is the role of the LDA in this process ... 

Because in the case of a sterically crowded imidazolide the formation of a carbinol is more difficult, reaction with the titanium reagent or the corresponding Grignard compound produces the allyl ketone in about the same yield [96]... 

Since an excess of n-BuLi has been inevitable, iron-ate complexes were proposed as catalytic relevant species, which generate a vinyllithium intermediate by carbometallation. The latter was verified by a quench with aldehydes and ketones with generation of the corresponding allylic alcohol. It is important to note that these reactions took place in low yields with alkyl-Grignard reagents. 

Bemannelation of ketones.1 This reaction can be effected by reaction of an allylic Grignard reagent such as methallylmagnesium chloride with the silyl ether of a 2-hydroxymethylene ketone as shown by conversion of 1 to 2 in 86% overall yield. 

These reagents can be obtained by reaction of vinyl Grignard reagents with allylic zinc bromides in THF at - 35 to 0°. Reaction of 1 with ClSn(CH3)3 provides an a-trimethyltin-zinc reagent (2) that is oxidized by air to an aldehyde or ketone (3). ... 

In that same period, interest had grown in the structure of allylic Grignard reagents in solution and in the mechanism of their reactions. To account for the results of the reaction of ketones with the Grignard reagent prepared from crotyl bromide (1-bromo-... 

Diastereoselective reactions of oxime (198) and phenylsulfenimines such as (201) with allyl metal reagents have been described (Scheme 33). The reaction of (198) and allylboronate (144) provides (199) with modest selectivity, while excellent diastereoselectivity was realized in the reactions of (201) and its C-2 epimer with the allylzinc reagent." The corresponding ketone derivatives, however, gave 70 30 mixtures of (204) and (205) upon reaction with diallylzinc, while with allyl Grignard, (205) is almost the exclusive product." The latter result is suggestive of a chelated transition state. 

The coupling reactions with the benzoyl chloride and allyl iodide required the addition of 10% Cul as catalyst to yield the corresponding ketone and allyl derivatives. In the absence of copper iodide catalyst, the prementioned two electrophiles gave only low yields ofthe expected products. 1,2-Dibromoethane was used to remove the excess of magnesium when benzoyl chloride or ally iodide was used as electrophiles (entries 6 and 7), and it did not react with the Grignard reagents under the given reaction conditions. 

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