Carbon-heteroatom coupling halides

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

During the cross-couplings to form C—N, C—O, C—S, and C—P bonds, the arylpalladium halide complexes are converted to arylpalladium amide, alkoxide, thiolate, and phosphide complexes. Examples of each type of complex have now been isolated, and the reductive elimination of the organic products has been studied. Although the reductive elimination to form carbon-hydrogen and carbon-carbon bonds is common, reductive elimination to form carbon-heteroatom bonds has been studied only recently. This reductive elimination chemistry has been reviewed.23... 

Very few transition-metal catalyzed electroreductive carbon-heteroatom bond formations have been described. The electrochemical silylation of allylic acetates was carried out in the presence of Pd-PPha [131]. The electrosynthesis of arylthioethers from thiophenol and aryl halides [132] and the coupling of bromobenzene with dichlorophenylphosphine [133] were performed with Ni-bpy as catalyst. 

Ishiyama, T. and Hartwig, J.R (2000) A Heck-type reaction involving carbon-heteroatom double bonds. Rhodium(I)-catalyzed coupling of aryl halides with A-pyrazyl aldimines. J. Am. Chem. Soc., 122, 12043-4. 

Recent renaissance in Ulmann chemistry has opened new opportunities for successful implementation of novel cross-coupling approaches, especially useful for carbon-heteroatom bond formation under mild conditions with copper compounds. Numerous publications have appeared dealing with thiolation and selenation of aryl-and alkenyl halides using various Cu complexes as catalysts. A high reaction temperature of 200-300 °C [55] was significantly reduced, to 100 °C and less. Therefore, a simplified reaction technique and cheaper solvents may be utilized in synthetic procedures. Lower temperature was also of much importance to avoid side reactions. 

To present, silica-supported rhodium catalysts have been successfully used for hydrogenation, hydroformylation, and hydrosilylation reactions. Zhang and coworkers" developed a heterogeneous rhodium complexes 23 catalyzed carbon-heteroatom bond formation. The reaction couples disulfides 21 or diselenides with an alkyl or acyl halide to generate unsymmetrical sulfides (24) and selenides in good yields. The catalyst could be easily recovered and recycled by filtration of the reaction solution and re-used for five cycles without significant loss of activity (maintains over 90% yield)." ... 

Because of the radical mechanism for SET reactions, introduction of both a perfluoroalkyl group and a heteroatom moiety to the carbon-carbon double [17-20] and even triple [21] bonds is possible. The initially generated perfluoroalkyl radicals add first to olefins to form a new radical intermediate (23), which then couples with anions (22) to form new anion radicals (24). The formation of the product (25) and the chain propagation via electron transfer from anion radicals (24) to perfluoroalkyl halides constitutes a chain reaction as shown in Scheme 2.38. Sulfur [19], selenium [20], tellurium [21], and phosphorus [22] anions (22) have been employed for these reactions [23]. 

Some alkenes with heteroatoms can be used in cross-coupling reactions. Vinyl boranes are an important group, as the carbon-boron bond can subsequently be converted into so many other functional groups, such as halides (Scheme 8.97). ° An example of cross-metathesis of a vinyl borane, followed by Suzuki coupling, can be found in Scheme 11.40. An example of a vinyl silane metathesis can be found in Scheme 2.110. 

The reaction of certain palladium-heteroatom complexes to alkenes and alkynes is a versatile tool for the synthesis of alkanes and alkene having heteroatoms attached. In particular, the various B-B, B-Si, and B-Sn compounds can be used for palladium-catalyzed borylation of alkenes and alkynes (Scheme 5-2). Borostannylation takes place at ambient temperature, whereas silylboration " only proceeds at a temperature above 80 °C due to the slow oxidative addition of a B-Si bond to a palladium(O) catalyst. Both reactions selectively provide cz j-products via addition of silicone or tin to the internal carbon and boron. The reactions are compatible with various functional groups for both terminal and internal alkynes. Cross-coupling reaction of boranes with organic halides selectively occurs at the terminal C-B bonds to provide regiodefined and stereodefined alkenylboron, alkenylsilicon, and alkenyltin compounds. 

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