Herrmann-Beller catalyst

Tietze et al. [83] were interested in the synthesis of medium-ring-sized estradiol derivatives such as 214 (Scheme 5.37). Mizoroki-Heck reaction of 213a in the presence of the Herrmann-Beller catalyst proceeded well and provided 214a without 8-c do-trig side-product in high yield (80%) and a 4 1 ratio of exocyclic to endocyclic (not shown)... 

A field of great interest at the moment is the synthesis of overcrowded tetrasubstituted alkenes which can act as switches and molecular motors [19]. They are also an interesting class of compounds for the development of optical data storage. Tietze and coworkers [Ic] have prepared a wide variety of these types of compounds, such as 16, by domino Mizoroki-Heck processes in high yield and complete control of the configuration of the double bond formed using aryl bromides 14a-f as substrates, which contain a triple bond and an allylsilane moiety. The best results were obtained using the Herrmann-Beller catalyst (15) [20]. It can be assumed that the palladium species 18a-c are intermediates,... 

Grigg et al. [41] first described a cyclization-cyclopropanation process which was later on developed further by de Meijere s group. It is a nice example of a domino process with four C—C bonds being formed in a single transformation [42]. Thus, reaction of 64 with Herrmann-Beller catalyst (15) furnished 66 as the only product. It can be assumed that the palladium compound 65 is an intermediate (Scheme 8.14). 

All C-H activation procedures for polymers reported thus far have been carried out under dry inert atmosphere using sealed vessels (Schlenk glassware). Similar to small-molecule procedures, polar (DMF, DMAc) and nonpolar (toluene, THE) aprotic solvents have been used and are degassed prior to use. Most thiophene-based monomers are not commercially available and must be synthesized according to literature procedures. It is very important that these monomers be extremely pure and free of all aryl impurities since other aryl bonds may undergo C-H activation and be incorporated into the polymer. All examples have employed palladium (II) acetate or the Herrmann-Beller catalyst. The latter can be prepared from Pd(OAc)2 and tris-(t)-tolyl)phosphine. All phosphine ligands, anhydrous bases and pivalic acid are commercially available and are stored under inert atmosphere. 

Scheme 19.10 Preparation of 15 using the Herrmann-Beller catalyst and pivalic acid.

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... 

The Leclerc group synthesis used DHAP to prepare copolymers containing bithiophene and terthiophene with TPD and FPD monomers." " The polymers are prepared by charging a microwave vial with the dibrominated thiophene, 5-alkylthieno-[3,4-c]pyrrole-4,6-dione (TPD) or 5-alkylfuro[3,4-c]pyrrole-4,6-dione (FPD), Herrmann-Beller catalyst (4mol%), tri(o-anisole)-phosphine (8mol%), CS2CO3 (2.3 equiv), and pivalic acid (0.3 equiv.) (Scheme 19.11). Toluene (0.2 M) is added and the reaction mixture heated to 120 °C under pressure for 24 36 h. The workup for the reaction is the same as that outlined in Example 2. Polymers 36-39 are prepared in moderate to high yields (38-94%). ... 

Scheme 19.13 Preparation of poly-3-hexylthiophene from 2-bromo-3-hexylthio-phene using Herrmann-Beller catalyst.

Similar direct arylations have been accomplished using hypervalent iodine reagents and the Herrmann-Beller catalyst 4 (Scheme 24.11) [ 15]. This method is applicable toward the phenylation of electron-neutral and electron-rich arene substrates and provides the products in modest to good yields. Increasing steric hindrance on the arene moiety leads to decreased product yields. An isomeric mixture of products is obtained, and the selectivity for phenylation of xylenes is similar to the transformations in Scheme 24.9. 

Rawal and co-workers reported that the reaction of phenolic substrates could be facilitated by generating the corresponding phenolate. The Herrmann-Beller catalyst I vide supra) was particularly effective. Upon treatment with CS2CO3 (3 equiv) and I (5 mol %) in DMA at 80 °C, the aryl iodide (Scheme 6) underwent smooth cyclization, thereby giving predominantly the ort/jo-substituted product in 97% yield. ... 

In most polymerization examples, Pd(OAc)2 was used as the palladium catalyst. Some research groups have exploited the highly stable Herrmann-Beller catalyst. Catalytic addition of pivalic acid can aid the C-H activation. Some groups have found that polymerization in the absence of phosphine ligand can give high molecular weight materials. 

For example, biatyls can be prepared from aryl iodides in the presence of the Herrmann-Beller catalyst (see 1.4, p. 791) and hydroquinone as a reducing reagent. ... 

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