Allylic halides organometallic

When reacting with allylic halides, organometallics tend to undergo Sa 2 and S/v2 processes concomitantly. Allylic amines and allylic ethers or acetals generally follow the latter mode provided that metalation is precluded. ... 

A striking feature of the reactions of indium and allylic halides is that they can be carried out in aqueous solution.192 The aldehyde traps the organometallic intermediate as it is formed. 

Reduction of unsaturated halides 0-78 Reduction of allylic alcohols 0-82 Reductive cleavage of enamines 0-86 Coupling of vinylic halides 0-87 Coupling of unsaturated halides with organometallic reagents 0-88 Coupling of allylic halides, tosylates, or acetates... 

Another, mechanistically closely related reaction is the cross-coupling reaction of aryl, benzyl, vinyl, or allyl halides with organometallic coupling reactions, such as Grignard reagents or, preferably, less reducing organometallics such as tin compounds or boronic acids RB(OH)2 the latter allow the reaction to be conducted even in aqueous media.83 The principal reaction steps are ... 

Tetrafluoropyridyl organometallic reagents can be prepared by reaction of 4-bromotetra-fluoro- or tctrafluoro-4-iodopyridine with zinc or cadmium in dimethylformamide at room temperature.83 Metathesis of these reagents with copper(l) bromide affords the (2.3,5,6-tetra-fluoro-4-pyridyl)copper reagent. The latter species undergoes high-yield substitution reactions with allyl halides, vinyl iodides, aryl iodides, and alkanoyl halides. 

Most of the early mechanistic studies that focused on the S ,j2 versus 8 2 nucleophilic substitution by an organometallic reagent were made with allylic halides. In the late 1940s, Wilson [7] showed that phenylmagnesium bromide reacted with a- and y-methylallyl chlorides 1 and 2, respectively, to afford a nearly identical mixture of primary and secondary substituted products 3 and 4, respectively, in a 75 25 ratio [Eq. (1)]. 

Meerifield, J. H., Godschal, J. P., Stille, J. K. Synthesis of unsymmetrical diallyl ketones the palladium-catalyzed coupling of allyl halides with allyltin reagents in the presence of carbon monoxide. Organometallics 3, 1108-1112. 

Allylation and benzylation reactions usually take place more efficiently than alkylation reactions due to the higher reactivity of the former derivatives. Allylation and benzylation is however difficult with the hardest organometallics of the series such as seleno- or thio-alkyllithiums. Finally, alkyl halides are usually less reactive towards a-thio- and a-seleno-alkyllithiums than aldehydes or ketones (see Scheme 1 1)25,122 which seem to possess a reactivity closer to that of allyl halides (see Scheme 12). ... 

Coupling ofC(sp3) Organometallics with Allylic Halides and Related Compounds... 

Coupling reactions between C(5p )-organometallics, mostly Grignard reagents, and C(sp ) halides and related compounds are usually achieved with Cu catalysts. Primary alkyl halides react smoothly, secondary alkyls are applicable in only a few cases, and no successful coupling has been reported for tertiary alkyl halides. Allylic halides and related compounds are treated separately, since there are important regiochemical problems. 

In most cases, treatment of allylic halides containing one ASG with a nucleophile does not result in formation of electrophilic cyclopropanes (MIRC product) instead, other reaction pathways are followed, e.g. addition, substitution, rearrangement and elimination reactions.However, with certain alkenes or nucleophiles or under the appropriate conditions a conjugate addition-nucleophilic substitution pathway is favored, resulting in cyclopropanes substituted with one ASG. Representative examples are compiled in Tables 20 and 21 where organometallic compounds or active methylene compounds are used as the nucleophilic species in combination with allyl bromides containing an ester or a sulfone as ASG. 

Copper derivatives, in the form of cuprates (RCu(X)Li), are usually prepared in situ from lithium, magnesium or zinc compounds, by reaction with a Cu(I) sonrce snch as CnCN. They are often used to improve conversions using, for example, an acid chloride or an allylic halide as electrophile - a number of examples appear in other sections of this book. They are also nsefnl as the organometallic partners in some crosscoupling reactions (using cobalt catalysts). ... 

In a similar study, intramolecular carbohalogenation of alkenes with allylic halides led to different product mixtures when mediated by either the tin reagent or the palladium(O) complex30. Thus, an organometallic palladium-allyl mechanism is supported. 

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