Alkoxides, coupling reactions with

Another interesting tandem Michael initiated sequence was developed in our laboratory by combining the conjugate addition of unsaturated alkoxides to alkylidene malonates with a palladium-mediated coupling reaction with an organic halide. In this cydization reaction, an organopalladium species acts as the electrophilic partner of the cydization. This reaction results in the trans addition of the organopalladium species and of the nucleophile across the unsaturation, and therefore, in overall difunctionalization of the unsaturated substrates [66,67]. 

The cross-coupling reaction of organoboron compounds with organic halides or triflates proceeds selectively in the presence of a base, such as sodium or potassium carbonate, phosphate, hydroxide, and alkoxide [11, 45], The bases can be used in aqueous solution, or in suspension in dioxane or DMF. In contrast, the cross-coupling reaction with certain electrophiles, such as allylic acetates [45], 1,3-butadiene monoxide [49], and propargyl carbonates [50], occurs under neutral conditions without any assistance from a base. The transmetallation of organoboron compounds with palladium halides under basic or neutral conditions can be considered to involve three routes, 1, 2, and 3 (Schemes 2-18, 2-20, and 2-23, below). 

The proposed mechanism for this samarium-catalyzed transformation is illustrated in Fig. 3-4. The pathway parallels that depicted in Fig. 3-3 for the chromium-catalyzed pinacol coupling reaction, with the exception of the timing of the silylation step. Based on the fact that pinacol reactions with catalytic and stoichiometric Sml2 furnish the same diastereoselection, Endo believes that carbon-carbon bond formation precedes silylation of the alkoxide. 

Coupling reactions with alkoxides. Influence of solvation. 

We will now give a more detailed analysis of the available data, using the following four cases 1. Coupling reactions with phenoxides influence of the electronic character and the effect of ion-pair dissociation. 2. Coupling reactions with alkoxides influence of solvation. 3. Nucleophilic substitution at silicon by carbon nucleophiles. 4. Reduction reactions by alanes, AlH Y3 n. 

In contrast to the borylation of alkane C-H bonds, the coupling of aryl halides with amines was based on a literature precedent from another group published about a decade before our initial studies. Kosugi, Kameyama and Migita published the coupling of aryl halides with tin amides." Mechanistic studies we conducted on this process led us to the perhaps obvious realization that the reaction" could be conducted with amines and a silylamide base instead of tin amides (equation 4)." Surveys of bases with similar p a values led Janis Louie to conduct reactions with alkoxide bases. Similar studies were conducted at nearly the same time by Steve Buchwald and coworkers."... 

Scheme 28 Cross-coupling reaction of organobismuth alkoxides with aryl triflates...

Scheme 39 Cross-coupling reaction of phenylbismuth alkoxide with alkenyl triflates...

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