Additions and Coupling Reactions

Addition and coupling reactions of boron-substituted carbanions... 

Alkyl radicals stabilized with boron atom are easily generated and undergo addition and coupling reactions. Such cr-boryl radicals were generated in situ by treatment of a-chloroalkylboronic esters with CrCl2 (Equation (188))706 or with radical initiators such as AIBN (Equation (189)).707... 

Transmetallation to other Metals for Addition and Coupling Reactions... 

The covalent B-G bond of organoboron compounds shows little reactivity toward representative electrophiles, but the presence of an empty low-lying orbital facilitates transmetallation to other metals. The transmetallation to zinc709 and other metal reagents has been extensively studied for carbon-carbon bond formation via addition and coupling reactions of trialkylboranes and alkylboronic acids. 

Photolytically generated carbene, as mentioned above, undergoes a variety of undiscriminated addition and insertion reactions and is therefore of limited synthetic utility. The discovery (3) of the generation of carbenes by the zinc-copper couple, however, makes carbene addition to double bonds synthetically useful. The iodo-methylzinc iodide complex is believed to function by electrophilic addition to the double bond in a three-center transition state giving essentially cis addition. Use of the... 

The Stille reaction of 2-chloro-3,6-diisopropylpyrazine (7) and 2-chloro-3,6-diisopropylpyrazine 4-oxide (9) with tetra(p-methoxyphenyl)stannane (readily prepared in situ from the corresponding Grignard reagent and SnCU) led to the corresponding arylation products 8 and 10, respectively [9]. Additional Stille coupling reactions of chloropyrazines and their N-oxides have been carried out with tetraphenyltin [10] and aryl-, heteroaryl-, allyl- and alkylstannanes [11]. 

The coupling occurs at the carbon atoms (generally via a-a links) which are the most reactive to addition and substitution reactions ... 

Reports in the literature that isolate any of these processes are rare and often require unusual conditions. For example, in addition to the oxazoI-5(4//)-one studies described above, Kemp and Rebek[27l were able to use kinetic isotope effects to distinguish the enolization mechanism from oxazol-5(4//)-one formation in a simple peptide coupling experiment. a-2H-Labeled Bz-L-Leu-OH and Z-Gly-Phe-OH were prepared and coupling reactions to H-Gly-OEt were carried out. In cases where oxazol-5(4//)-one formation is rate-determining, such as with Bz- L-Leu-OH, the isotope effect kHlkD is equal to 1 because the a-proton is not removed until after this rate-determining step. In contrast, enolization requires the direct removal of the a-proton, and the isotope effect measured for this mechanism was as high as 2.9 with Z-Gly-Phe-OH. Therefore, a measurement of the isotope... 

Despite its apparent noble character, gold catalysts have been recently found to be active in many homogeneous and heterogeneous catalytic processes such as oxidation reactions, nucleophilic additions, cross-coupling reactions, and alkene and imine hydrogenations (69—71). Corma and co-workers showed that Au... 

When the oxygen deactivates the freeze dried living polymers without additives, the coupling reaction becomes predominant and can reach up to 60%. We explain this result by the association of the living ends preexisting In the solution, and kept In the solid state. The proximity of these carbanlons and consequently of the radical after the electronic transfer favours the coupling reactions. 

Reactions of powerful alkyllithiums with halo pyridines, quinolines, and diazines may lead to nucleophilic substitution (by addition-elimination or hetaryne mechanisms), ring opening, halogen-scrambling, and coupling reactions, which compete with the desired DoM process. 

The CH—CH coupling constants in the proton nmr spectra of 14 and 15 are about 13 Hz, Work out the favorable conformations and the likely configurations of 14 and 15 and the stereochemistry of the addition and cleavage reactions. (Review Section 9-1 OH.)... 

Preconcentration on Cl 8 cartridge column, elution with MeOH, H,0. Dilution. pH adjustment to 2.1-2.6 with H2S04. Na2S204 addition. pH adjustment to 8.5 with NaOH. NaCl addition, CHCI3 extraction, H2S04 addition, diazoti-zation and coupling reaction with disodium-2-naphthol-3,6-disulfonate. Dissolution in MeOH. 

NaOH addition to water solution, multiple-step extraction on Extrelut QE columns with CHC13, H2S04 addition, evaporation. H2S04 addition, dia-zotization, and coupling reaction with R-salt or pyrazolone for aniline. 

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