УДК 541.49
PALLADIUM CATALYZED HECK-MIZOROKI AND SUZUKI-MIYAURA
COUPLING REACTIONS (REVIEW)
© 2014 M. N. Zafar*, M. A. Mohsin, M. Danish, M. F. Nazar, and S. Murtaza
Department of Chemistry, Institute of Chemical and Biological Sciences (ICBS), University of Gujrat, Gujrat, 50700 Pakistan
*E-mail: analyticalc@yahoo.com Received March 20, 2014
This article is about the progress of palladium compounds as a catalyst for Heck—Mizoroki and Suzuki Miyaura coupling reactions. Industrial catalysts with broad applicability need continuous catalyst development process through modification of ligand design, geometry and functionality. Recently catalysts have been synthesized through attachment of the activated palladium complexes on the surface of polymer support, particularly, insoluble in reaction medium. An appropriate mixture of palladium salt and ligand is also used as an important modification in some cases to get better results. We surveyed the important palladium compounds synthesized up to early 2014 for Heck—Mizoroki and Suzuki Miyaura coupling reactions and summarize their progress in terms of ligand modification and other associated parameters.
DOI: 10.7868/S0132344X14110103
INTRODUCTION
Palladium(II) is considered as soft metal (class B) which is depicted in its rich chemistry that is not limited to soft bases like phosphorous and sulfur but also for hard ligands e.g. nitrogen and oxygen [1]. The study of palladium(II) metal complexes was greatly enhanced when it was discovered that they have good catalytic activity. Transition metal complexes of Pd(I) found rarely and mostly used as precatalysts in organic synthesis [2—4]. The organometallic chemistry of Pd(III) found in its initial stages when compared with Pd(0), Pd(II) and Pd(IV). Limited complexes of Pd(III) are also well known [5]. Palladium can exist in a number of different oxidation states but useful organic methods are dominated by the use of Pd(0) and Pd(II) [6-34], although the utility of Pd(IV) [35-38] has been steadily emerging in its own right. The types of transformations carried out by palladium in 0, +2 and +3 oxidation states are given in Table 1. The remaining oxidation states have not, as of yet, found practical applications hence their observation remains rare [39-44].
Cross-coupling is an extremely powerful tool available to synthetic chemists in their quest to create or reproduce intricate organic scaffolds. This reaction has enabled chemists to join two organic fragments using relatively mild conditions allowing for the manipulation and creation of delicate, complex molecules. Cross-coupling plays an important role in the synthesis of many drugs, natural products, optical devices and industrially important starting materials [45-58]. Its importance was also recognized by the Nobel committee in 2010 by awarding Richard F. Heck, Ei-ichi Negishi and Akira Suzuki the Nobel prize in Chemistry for what it termed "artwork in a test-tube". The cross-coupling reactions of vinyl, aryl, benzyl, allyl halides, triflates and acetates with alkenes recognized as Heck reactions and that of organo-boron compounds with pseudohalides or organic halides is termed as Suzuki-Miyaura reaction. Development of these technologies lies inherently in the development of the specific methods used. In a world where natural resources are becoming more strained, the execution and development of clean and efficient chemistry has become a common goal for many chemists.
Table 1. Types of transformation catalyzed by palladium in various oxidation states
Pd(0) Pd(II) Pd(III)
Cross couplings Wacker process Oxidative C-H coupling reactions
Allylic alkylation Cycloisomerization Oxidative carbon-heteroatom
Bond-forming reactions
Hydrogenation Alcohol oxidation Kumada and Negishi coupling reactions
Hydrogenolysis Allylic oxidation O2 Insertion reactions
Carbonylation Allylic rearrangements C-H Acetoxylation
Table 2. Summary of the results from Heck reactions using palladium—pyridinium amidate (A) and palladium—pyri-dinium amide (B) catalyst
Stilbenes
CH
I
N
3
CH3
CU CV
:pd
CK Cl'
Pd
Entry Catalyst Time, h Stilbenes formed, %
1 A 06 61
2 A 24 94
3 B 06 49.5
4 B 24 84.5
Mizoroki—Heck coupling. In the early 1970s Mizo-roki and Heck first revealed the use of certain palladium catalysts in the mediation of carbon—carbon coupling reactions. The first of these catalysts were based on zerovalent palladium complexes with labile tertiary phos-phanes. Subsequently it was found that complexes of divalent palladium could be just as effective and far more convenient due to their oxidative stability [53]. Typical catalysts for the Heck—Mizorki reaction with the active species Pd0(PPh3)2 and [Pd0(PPh3)2(OAc)]- are given below:
Pd0(PPh3)4
Pd(OAc)2
+
2PPh3
Pd0(PPh3)4 ■
+
PPh3
-PPh3
Pd0(PPh3)2
[Pd0(PPh3)2(OAc)]-
There is scope within the C—C coupling mechanism for reduction ofpalladium(II) to palladium(O) by a number of species including the solvent, added amine, the accompanying ligand and the alkene. Therefore, generation of the active species in the reaction mixture often allows the use of oxidized catalyst at the outset. Similarly, Pd(II) and Pd(IV) species have been identified and proposed as the active species in a number of Heck reactions. However, in the case of doubly ligated NHCs, Pd(0) and Pd(II) are generally accepted as the operative metal oxidation states [59].
To enhance the productivity and catalytic activity in Heck reaction, initial efforts were made by spencer in early 1980s [60]. He set up the experimental basis for arylation of olefins with activated aryl bromides and concluded that a high TON process can be carried out only in polar aprotic solvents, such as DMF, HMPA, DMA, NMP, in the presence of sodium acetate as a base, and phosphine ligands if using low loads of palladium catalyst (0.1 mol % of Pd(OAc)2 and 0.4 mol % of P(o-Tol)3). Gradual progress in Heck reaction showed that activated aryl chlorides, nonactivated chlorobenzene itself, even deactivated ^-methoxychlorobenzene and o-chlorotolu-ene can be utilized for this type of reaction by using 1.5 mol % of Pd2(Dba)3 (Dba = dibenzylideneace-tone) and 4 mol % P(i-Bu)3 as a catalyst [61]. However, palladium acetate with ligands 1 and 2 lost its activity at lower temperatures. This proved that these bi-dentate phosphine containing catalystic systems are less effective than monodenate phosphine system [62]. Dimeric palladium complex such as 3 was a big step forward in palladium catalysed coupling reactions. This Herrmann's catalyst can be used conveniently in applied homogeneous catalysis [63—65].
The ligand design, geometry and its electron donor strength plays a vital role in predicting the stability of metal complex. For Heck coupling reaction, the metal complex as a catalyst is more stable if its metal is strongly bonded with the ligand. Greater stability can lead to the better chances of catalyst survival under more severe reaction conditions. Due to the special design and better electron donor property, P—C—P pincer ligands [66, 32, 67] already proved successful for palladium catalyzed coupling reactions. Phosphorous donor ligands are very rapidly replacing by N-hetrocyclic carbenes (NHCs). The main reason is stronger interaction of NHCs donor function with palladium than its analogous phosphine complex. The literature quite clearly indicates that the combination of palladium and NHCs is perfect for Heck coupling reaction [68—75]. The concept of donor-functionalized carbene ligands has been expanded to pincer-type complexes [76] resulting in replacing P—C—P ligands by NHCs pincer [77, 78]. Crabtree [77] (4) in 2001 and Hahn [78] (5) in 2005 prepared famous palladium NHCs pincer complexes for Heck reactions. Mixed ligands (with two functionality) like imidazolin-2-ylidenes bridged by a neutral pyridine [77, 79—81] or a carbanionic function [82, 83] with two metallacycles also resulted in very active and stable palladium coupling catalysts.
CHART 1
In our past study [84] palladium complexes of pyridinium amidate (PYa, A) and amide (PYE, B) both showed catalytic activity for both Heck coupling reactions (Table 2). However, PYA is more active than PYE catalyst. The modified Crabtree procedure [84] was
initially used to find out the catalytic ability of PYA complex of palladium and the results were compared with those obtained for PYE palladium complex. The results are also summarized in Table 2 and entries 1—4 were carried out in dimethylacetamide solvent. The base employed was sodium acetate (1.1 mole equivalent), one mol % of catalyst was used and the temperature was 140°C. The blank was run without catalyst but with base present, and showed zero catalytic activity. The results are each reported as the average of two runs. The results showed that both catalysts acted as pre-catalysts in the Heck Miroroki coupling reaction.
In 2011, C. Zhu et al. worked on oxidative Heck reaction. The regioselectivities was attained by the hy-droxyl group of a homoallylic alcohol that coordinates with palladium (the yield was 61—80%) [85]. A variety of tetrahydrofurans with functionalized scaffolds was achievable by using small amount of TFA that carry
out oxyarylation of penultimate Heck intermediate (Table 3). In the same year, highly selective (E)-styre-nyl products were obtained by Matthew S. Sigman in the absence of substrate bias with the Pd(0)-catalyzed Heck reaction [86]. The main advantage of this method is the use of low temperature, no use of any base or additional oxidant, and compatibility with greater number of functional groups (Table 4).
In 2012, Armido Studer and his coworkers suggested that the nature of oxidant play more important role than the protecting groups in oxidative Heck arylation for the stereoselective synthesis of tetrasubstituted olefins [87]. They were able to synthesize Z-Tamoxifen by using variety of palladium catalysts and nitroxide as oxidan
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.