Heck-Mizoroki reaction catalysts

Alonso DA, Najera C, Pacheco C (2002) Oxime-derived palladium complexes as very efficient catalysts for the Heck-Mizoroki reaction. Adv Synth Catal 344 172-183... 

Progress of Hpase-catalyzed ester synthesis in ionic liquid 12CJ01186. Revisiting Heck—Mizoroki reactions in ionic Hquids 13RCA19219. Role of ionic Hquids as supports of catalysts 12H(85)281. 

Having seen steps such as oxidative addition, insertion, and reductive elimination in the context of transition metal-catalyzed hydrogenation using Wilkinsons catalyst, we can now see how these same types of mechanistic steps are involved in a mechanism proposed for the Heck-Mizoroki reaction. Aspects of the Heck-Mizoroki mechanism are similar to steps proposed for other cross-coupling reactions as well, although there are variations and certain steps that are specific to each, and not all of the steps below are involved or serve the same purpose in other cross-coupling reactions. 

All of these reactions involve transition metals such as palladium, copper, and ruthenium, usually in complex with certain types of ligands. After we see the practical applications of these reactions for carbon—carbon bond formation, we shall consider some general aspects of transition metal complex structure and representative steps in the mechanisms of transition metal—catalyzed reactions. We shall consider as specific examples the mechanism for a transition metal—catalyzed hydrogenation using a rhodium complex called Wilkinsons catalyst, and the mechanism for the Heck—Mizoroki reaction. 

Much of the chemistry of organic transition metal compounds becomes more understandable if we are able to follow the mechanisms of the reactions that occur. These mechanisms, in most cases, amount to nothing more than a sequence of reactions, each of which represents a fundamental reaction type that is characteristic of a transition metal complex. Let us examine three of the fundamental reaction types now. In each instance we shall use steps that occur when an alkene is hydrogenated using a catalyst called Wilkinsons catalyst. In Section G.7 we shall examine the entire hydrogenation mechanism. In Section G.8 we shall see how similar types of steps are involved in the Heck—Mizoroki reaction. 

Other irmovative approaches have been used to promote aromatization, once the RCM reaction involving the two alkenes has been accomplished. These two approaches are shown in Scheme 17.4. In the first one, triene 24 underwent an RCM reaction to afford the exocycUc double bond-containing compound 25 [18]. Use of the rhodium catalyst [RhCl(cod)]j then facihtated formation of the desired phenol 26. Another neat way to accomplish formation of the aromatic benzene ring was to allow the intermediate cyclic Michael acceptor 28 (formed by RCM of 27) to undergo a Heck-Mizoroki reaction with p-methoxybenzenediazonium tetrafluoroborate 30 to afford the second phenol 29 [18]. 

Given the expensive nature of Pd, other metals have been used as a replacement for this metal. In 2005, Li etal. [69] showed that copper(l) could be used as the catalyst for the Heck-Mizoroki reaction. They used 10 mol% Cul and 20 mol% l,4-diazabicyclo[2.2.2]octane (DABCO) for the coupling of a number of aryl iodides and an aryl bromide giving the corresponding internal olefins in moderate to good yields. The reaction conditions are shown in Scheme 1.25. The reaction mechanism that was proposed by these authors involved a four-centered transition state that was originally proposed by Castro and Stephens in 1963 (Scheme 1.26) [23]. 

In 2010, Chatani and coworkers reported a new variant of the Heck-Mizoroki reaction, in which an aryl-metal species is generated from aryl cyanides using a rhodium (I) catalyst [70]. The reaction conditions and reaction results are shown in Figure 1.17. 

Figure 1.20 Use of a polymer supported Pd catalyst for the Heck-Mizoroki reaction reported by Patel et al. [75],...

In 2008, Portnoy s group [76] reported the use of bidentate phosphine ligands immobilized to Wang polystyrene beads with polyether dendron spacers, which formed active catalysts for the Heck-Mizoroki reaction when treated with Pd(dba)2 (Figure 1.21). Two series of immobilized palladium catalysts were investigated, one series termed Gn-I was prepared by incubating the... 

In 2011, Jean Le Bras s team [87] reported a very mild Pd-catalyzed dehydrogenative Heck - Mizoroki reaction of furans and thiophenes with styrenes at room temperature (Scheme 1.32) Pd(OAc)2 was used as the catalyst with benzoquinone (BQ) as the oxidizing agent. 

Mermecke, K. and Kirschning, A. (2009) Polyionic polymers - heterogeneous media for metal nanoparticles as catalyst in Suzuki-Miyaura and Heck-Mizoroki reactions imder flow conditions. Beilstein J. Org. Chem., 5, 21. 

L Lavastre, O., and Vioux, A. (2009) Encapsulation of Pd(OAc)2 catalyst in an ionic liquid phase confined in silica gels application to Heck-Mizoroki reaction. 

As mentioned earlier, in many coupling reactions the choice of the correct base is very important. For example in a specific Heck-Mizoroki reaction, when Cy NMe and Cs COj are tested as the base, much higher conversions are obtained with the former. This is because the resting state of the catalyst is 7.48 for Cy NMe, but 7.51 for CSjCOj. In other words, for Cs COj, the sluggish conversion of 7.51 to 7.48 lowers the conversion. 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

Other successful examples of catalysts containing NHC ligands are found in palladium- and nickel-catalyzed carbon-carbon bond formations. The catalyst development with these metals has focused in particular on Heck-type reactions, especially the Mizoroki-Heck reaction itself [Eq. (42)] and various cross coupling reactions [Eq. (43)], e.g., the Suzuki-Miyaura reaction ([M] = and the Kumada-Corriu reaction ([M] = MgBr). " Related reactions like the Sonogashira coupling [Eq. (44)]326-329 Buchwald-... 

Recently, the present authors have achieved a facile recycling method for both catalyst and reachon medium using F-626 in a Mizoroki-Heck arylation reaction of acrylic acids [11]. The procedure employed a fluorous carbene complex, prepared in situ from a fluorous imidazolium salt, palladium acetate as the catalyst and F-626 as a single reaction medium. When acrylic acid was used as a substrate, separation of the product from the reaction mixture was performed simply by filtration with a small amount of FC-72. The FC-72 solution containing the fluorous Pd-catalyst and F-626 was evaporated and the residue containing the catalyst and F-626 (96% recovery) can be recycled for the next run (Scheme 3.5-6). They tried to reuse the catalyst, and observed no loss of catalytic activity in five re-use cycles. 

Pd-complexes have also been impregnated on an amorphous silica support vnth the aid of a solution containing [BM IMjlPFej dissolved in tetrahydrofuran and these systems were applied as highly efficient catalysts for promoting Mizoroki-Heck coupling reactions between various aryl halides and cyclohexyl acrylate in alkanes without the presence of additional ligand (Scheme 5.6-8) [105]. 

Scheme 5.6-8 Pd-catalyzed Mizoroki-Heck coupling reactions between aryl halides and cyclohexyl ac7late in alkanes using silica/[BMIM][PF6] supported ionic liquid phase catalysts [105].

An easy recycling method involving both catalyst and reaction medium was achieved in a Mizoroki-Heck arylation reaction of acrylic acid, using a fluorous carbene complex (prepared in situ fl om a fluorous ionic liquid and palladium acetate) as the catalyst and a fluorous ether solvent (F-626) as the reaction medium. Because of the very low solubility of arylated carboxylic acids in F-626, the products precipitated during the course of the reaction. After separation of the products and amine salts by filtration, the filtrate, which contained the fluorous Pd catalyst, could be recycled for several runs (Scheme 13). The Mizoroki-Heck reaction was effectively promoted by a fluorous SCS pincer palladium, which is discussed in Section 3.4.5. 

In the Heck arylation reaction (Scheme 1), invented independently by Mizoroki and Heck in 1970, a bond is formed between an olefin and an aromatic ring [3-8]. The catalyst needs to be a form of Pd(0) but can also be administered as a Pd(ll) compound since Pd(ll) is usually rapidly reduced to Pd(0) the electrons for this... 

Accordingly, catalytic and stoichiometric amounts of cuprous salts were employed for Mizoroki-Heck-type reactions of various conjugated alkenes [ 19]. Intermolecular catalytic arylations of methyl acrylate (1, not shown) and styrene (2) were accomplished under ligand-free conditions using CuBr (3) or Cul (4) as catalyst in A-methyl-2-pyrrolidinone (NMP) as solvent various aryl iodides could be employed (Scheme 10.2). On the contrary, aryl bromides and chlorides, as well as aliphatic halides, were found to be unsuitable substrates. The reactions employing an alkenyl bromide, methylmethacrolein or methyl methacrylate required stoichiometric amounts of copper salts. 

Scheme 10.12 Nickel(0) phosphite complex37as catalyst for Mizoroki-Heck-type reactions.

A heterogeneous cobalt catalyst was employed for arylations of styrene (2) and two acrylates with aryl iodides. Generally, isolated yields were significantly lower than those observed for heterogeneous nickel catalysts [24]. Further, a silica-supported poly-y-aminopropylsilane cobalt(II) complex was reported as a highly active and stereoselective catalyst for Mizoroki-Heck-type reactions of styrene (2) and acrylic acid (16) using aryl iodides [23,25]. 

Recently, cobalt hollow nanospheres were successfully used in stereoselective Mizoroki-Heck-type reactions of acrylates with aryl bromides and iodides [49]. The recyclable catalyst was most efficient when using NMP and K2CO3 as solvent and base respectively (Scheme 10.23). 

An early example for cobalt-catalysed Mizoroki-Heck-type reactions with aliphatic halides by Branchaud and Detlefsen showed that an intermolecular substitution of styrene (2) could be achieved with [Co(dmgH)2py] (70) (dmgH = dimethylglyoxime monoanion) as catalyst in the presence of visible light. This radical reaction led selechvely to the substitution products when using stoichiometric amounts of Zn (27) and pyridine (31) as additives (Scheme 10.24) [52]. 

In a related intramolecular rhodium-catalysed Mizoroki-Heck-type reaction of an alkene with an aryl iodide, Wilkinson s catalyst (84) gave significant amounts of side-products due to isomerization of the resulting double bond. In contrast to the corresponding palladium-catalysed transformation, the presence of Et4NCl had no beneficial influence either on the reactivity or on the selectivity [57]. 

Scheme 10.30 Rhodium-catalysed intermolecular Mizoroki-Heck-type reaction with Wilkinson s catalyst (84).

Studies on the use of complexes immobilized on quinoline-carboimine-functionalized FSM-16 mesoporous silica for Mizoroki-Heck-type reactions between methyl acrylate (1) and various aryl iodides highlighted the superior catalytic activity of a mthenium(III) catalyst compared with the corresponding platinum(IV) complex [44]. 

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