Reactions with Organometallic Nucleophiles

Most main group organometallic compounds undergo nucleophilic reactions with carbonyl groups, whereas 1,4-conjugate addition to enones... 

Two available examples are nucleophilic reactions with organometallic cations and with activated bicyclobutanes. Sweigart and co-workers (9, 10) showed that a linear correlation is obtained between the log k values for the reactions of nucleophiles (Nu) with various organic cations and the transition metal complexes of the same cations. The latter reactions resemble SN2 reactions if the carbon-metal bond cleavage is considered as a departure of the leaving group ... 

For alkynyl transfer, no direct evidence has been presented to determine whether organometallic nucleophiles interact with the electrophilic iodine centre or the jS-carbon although, in the case of transfer to the nitrogen nucleophile of a coordinated cyano ligand, the S-interaction mechanism is clearly implicated in the formation of 68 (Scheme 38, related to the mechanism in Scheme 14). This example indicates, for the synthesis of alkynylmetal species, the attractiveness of RC=C(Ph)I reagents where the R group cannot participate in such reactivity, e.g. R = SiMcs, Ph. Alternatively, this type of reaction, where the metal centre may act as a nucleophile at the -carbon, is worthy of exploration as a mode of organometallic synthesis as also is electrophilic attack at the K-carbon of ylides (illustrated by the mechanism in Scheme 14). 

Reactions.—The chemical behaviour of thioketones has mainly been investigated in four directions reaction with organometallic compounds, reaction with nucleophiles, thermal cycloadditions, and photochemical reactions. 

Hard carbon nucleophiles of organometallic compounds react with 7r-allyl-palladium complexes. A steroidal side-chain is introduced regio- and stereo-selectively by the reaction of the steroidal 7T-allylpalladium complex 319 with the alkenylzirconium compound 320[283]. 

Mechanistically the reaction can be divided into two steps. Initially the alkyl halide 1 reacts with sodium to give an organometallic species 3, that can be isolated in many cases. In a second step the carbanionic R of the organometallic compound 3 acts as nucleophile in a substitution reaction with alkyl halide 1 to replace the halide ... 

Nitriles are similar in some respects to carboxylic acids and are prepared either by SN2 reaction of an alkyl halide with cyanide ion or by dehydration of an amide. Nitriles undergo nucleophilic addition to the polar C=N bond in the same way that carbonyl compounds do. The most important reactions of nitriles are their hydrolysis to carboxylic acids, reduction to primary amines, and reaction with organometallic reagents to yield ketones. 

The initial formation of an ate complex by attack of the nucleophile on the aluminum reagent, followed by reaction with the ketone, is unlikely since treatment of the ketone with a mixture of MAD and the organometallic reagent gave results comparable to those obtained with the organometallic reagent in the absence of MAD. 

Both overt carbanions and organometallic compounds, such as Grignard reagents, are powerful nucleophiles as we have seen in their addition reactions with C=0 (p. 221 et seq.) they tend therefore to promote an SN2 pathway in their displacement reactions. Particularly useful carbanions, in preparative terms, are those derived from CH2(C02Et)2, (3-ketoesters, l,3-( 3-)diketones, e.g. (55), a-cyanoesters, nitroalkanes, etc.—the so-called reactive methylenes ... 

Addition of C-nucleophiles to nitrile oxides is of special interest. There are examples of reactions with both carbanions and neutral carbon nucleophiles. To the former group belong reactions of nitrile oxides with organometallic... 

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