Herrmann-Beller

The influence of high pressure on the Heck reactions of selected alkenyl and aryl halides, respectively, i.e., 1-iodocyclohex-l-ene, iodobenzene, bromobenzene, with methyl acrylate has been investigated and the activation parameters of these reactions determined [142], Two different catalyst cocktails were used in this study, the classical system (Pd(OAc)2, NEtg, PPhg) and the one reported by Herrmann, Beller and others [16] (la). The temperature-dependent and the pressure-dependent rate coefficients both follow the order PhI/Pd(OAc)2 > 1-iodocyclohexene/Pd(OAc)2 > Phl/la > PhBr/la and the activation enthalpies as well as the activation entropies exhibit the trend 1-iodocyclohexene/Pd(OA)2 < Phl/Pd(OAc)2 < Phl/la < PhBr/la. The absolute values of the activation volumes, which were ascertained from the pressure-dependent rate coefficients, increase as follows l-iodocyclohexene/Pd(OAc)2 < PhI/Pd(OAc)2 Phl/la < PhBr/la. Under high pressure, the lifetime of the active palladium catalyst and thereby the turnover numbers are greatly enhanced [88]. 

An elegant method for linking terminal alkynes with aromatic compounds and olefins is the Sonogashira reaction [15]. The palladium-catalyzed reaction enables the simultaneous introduction of two or even more alkyne units and thereby makes it possible to synthesize acetylene derivatives, for example hexaalkynyl-benzenes [16], (eq. (7)), which can be obtained only with difficulty by other methods. It has been shown by Herrmann, Beller, and co-workers that the copper reagent is not necessary as a co-catalyst for the coupling of terminal alkynes with sp -carbon halides. By using phosphapalladacyclic catalysts 1 the... 

Milstein and co-workers were the first to introduce catalysts capable of activating various aryl chlorides in 1992 [175]. By using palladium complexes of highly basic and sterically demanding chelating bisphosphines, for example, dippb [l,4-bis(di-isopropylphosphinyl) butane], even chlorobenzene was coupled with alkenes in high yields (Table 1, 70-95% yield TON = 70-95) [175]. However, these catalysts are extremely sensitive to air. Herrmann, Beller, and co-workers introduced more robust palladacycles [cyclopalladated... 

Herrmann-Beller palladacycle Fig. 10.12 Palladacycles and pincers used as pre-catalysts in the Heck and Suzuki reactions... 

On the other hand, even the recently prepared Herrmann-Beller catalystf still requires higher temperatures for efficient coupling rates of the Heck reaction. Interestingly, the complexation of chloroarenes with the Cr(CO)3 fragment activates the arene-chlorine bond considerably toward the oxidative addition. Thus, Cr(CO)3 complexed chloroarenes react about 15 times faster than iodoarenes in Pd-catalyzed cross-couphng reactions under mild conditions, in particular in Pd/Cu-catalyzed cross-couplings with terminal acetylenes in refluxing THF and/or tertiary amines (Scheme 36). ... 

Palladacycles are apparently the best studied subclass of the SRPCs. Moreover, the development of the concept itself originates from the invention and investigation of palladacycles, particularly the classical Herrmann-Beller palladacycle 52 (Figure 2.20, see below). 

As is now evident, all Mizoroki-Heck chemistry of palladacyclic precatalysts can be deduced from the behaviour of the first published representative of this class, namely the Herrmann-Beller palladacycle 52 (Scheme 2.20) [107,226], This precatalyst is capable of driving the type 1 reactions with activated aryl bromides, giving TON values up to 200000 in common solvents (0.0005 mol% 52, NaOAc, DMA, 135 °C) [107], which can be boosted to 1000000 in ionic liquids [227], With aryl iodides, the reactions can be run at as low... 

Figure 2.20 Palladacycles 172-174 including Herrmann-Beller palladacycle 52.

The Herrmann-Beller palladacycle 52 is the only representative of this class of precatalysts, which has found real application in organic synthesis as a convenient stable source of palladium for various syntheses involving inter- and intra-molecular Mizoroki-Heck reactions [229],... 

An important step forward was achieved when the idea became accepted that the most valuable virtue of a palladacyclic precatalyst is not any sophisticated structure, but rather their ready availability and low cost in conjunction with simple handling and storage. In fact, the Herrmann-Beller palladacycle 52 already satisfies all these criteria par excellence the only real or imaginary drawback is the relation of the latter with phosphines being regarded as toxic and expensive. 

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). 

The Herrmann-Beller phosphapalladacycle has been shown to catalyse the addition of terminal alkynes to unsymmetrical gem-disubstituted cyclopropenes, which affords alkynylcyclopropanes with high diastereoselectivity. ... 

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.

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