Cesium pivalate

The final paragraph in this section details carbazole syntheses that involve the formation of C-C bonds. Intramolecular Diels-Alder cycloaddition of ynamides provided a new route to [b -fused carbazoles <05OL2213>. An electrocyclization of 2,3-divinylindole intermediates produced functionalized carbazoles <05TL4045>. A domino alkynylation/palladium migration/C-H activation approach to 4-vinylcarbazoles was reported <050L701>. For example, treatment of A-arylaniline 134 with diphenylacetylene in the presence of palladium acetate, cesium pivalate (CsPv), and bis(diphenylphosphino)methane (dppm) gave carbazole 136 via post-palladium migration intermediate 135. 

Larock found that the reaetion of 2-iodo -methylbiphenyl (34) with acrylate provided two products 35 and 36 in equal amounts using cesium pivalate as a base. Of course, 35 is an expected produet [27]. Also the reaction of 37 afforded the mixture of 35 and 36. Gallagher also diseovered a similar migration using 3-bromo-4-phenylpyridine (38) and acrylate to afford 39 and 40 [28]. Although the mechanism of the migration process to form the crossover produets is not clear, certainly a reversible 1,4-Pd shift of arylpalladium intermediates 41 and 43 via the palladacycle 42 is occurring. 

A good synthetic method for substituted fluoren-9-ones is the carbonylation of o-halobiaryls [47]. Carbonylation of 2-bromobiphenyl (107) in DMF in the presence of cesium pivalate gave rise to fluoren-9-one (109) in quantitative yield via the palladacycle 108. Use of PCyg as a ligand is important. 

Pd(CF3CO)2 catalyses the decarboxylation of electron-rich aromatic acids (e.g. with OMe groups) in DMSO/DMF at 70-90° (1 to 24 hours) in high yields and is not affected by steric hinderance [Dickstein et al. Org Lett 9 2441 2007], and (in the presence of Cu(OAc)2 with Cesium pivalate + 3-nitropyridine as additives at 110-140° in a microwave) it catalysed direct cross-coupling between unactivated arenes and A-acetylindoles with coupling mostly at C3 of indoles but with no homo-coupling [Stuart Fangou Science 316 1172 2007]. 

Both products are presumably generated via a common intermediate, the seven-membered oxapalladacycle 136, from the domino Pd-catalyzed/norbornene-mediated reaction sequence. In the presence of a benzyhc C-H, an intramolecular redox process takes place by hydrogen migration from the benzyhc position to the aromatic position, aHbrding the aldehyde products 134. When using tertiary benzyl alcohols, C-O couphng is the predominant pathway leading to the dibenzopyran products 135. It was found that cesium pivalate was the base of choice in the former process and potassium carbonate was suitable for the latter. 

Leclerc and coworkers copolymerize monomer 44 with brominated monomers to prepare polymers 15 and 16 (Chart 19.4). The reactions are carried out in a sealed microwave vial, which is charged with 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2/f,5//)-dione 45, 2-octyl-thieno[3,4-t( -thiazole 44, Herrmann-Beller catalyst (2mol%), pivalic acid (0.3 equiv.), tri(o-anisole)phosphine (4mol%) and cesium carbonate (3 equiv.) (Scheme 19.10). THF [0.1 M] is added, and the reaction mixture heated to 120 °C under pressure for 24 h. The mixture is cooled to room temperature and... 

To date, only one report has referred to the direct allqmylation of indoles under aerobic conditions. In 2010, Li and co-workers described a palladium-catalyzed oxidative coupling of 1,3-dimethylindoles with phenylacetylenes." Using a buffer system composed of 20 mol% cesium carbonate and 2 equiv. pivalic acid at 80 °C, the desired cross-coupling products were obtained in moderate-to-good yields (Scheme 9.21). 

---