Phosphines Heck reactions

Carbohydrate-substituted phosphines Heck reaction Suzuki coupling [218t]... 

The use of ionic liquids as reaction media for the palladium-catalyzed Heck reaction was first described by Kaufmann et ak, in 1996 [85]. Treatment of bromoben-zene with butyl acrylate to provide butyl trans-cinnamate succeeded in high yield in molten tetraallcylammonium and tetraallcylphosphonium bromide salts, without addition of phosphine ligands (Scheme 5.2-16). 

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

From a practical point of view, it is worth noting that Heck reactions catalyzed by Pd/P( -Bu)3 do not typically require rigorously purified reagents or solvents. In addition, the palladium and phosphine sources, Pd[P(7-Bu)3]2 and Pd2(dba)3, are commercially available and can be handled in air. 

An interesting parallel was found while the microwave-enhanced Heck reaction was explored on the C-3 position of the pyrazinone system [29]. The additional problem here was caused by the capability of the alkene to undergo Diels-Alder reaction with the 2-azadiene system of the pyrazinone. An interesting competition between the Heck reaction and the Diels-Alder reaction has been noticed, while the outcome solely depended on the substrates and the catalyst system. Microwave irradiation of a mixture of pyrazinone (Re = H), ethyl acrylate (Y = COOEt) and Pd(dppf)Cl2 resulted in the formation of a mixture of the starting material together with the cycloaddition product in a 3 1 ratio (Scheme 15). On the contrary, when Pd(OAc)2 was used in combination with the bulky phosphine ligand 2-(di-t-butylphosphino)biphenyl [41-44], the Heck reaction product was obtained as the sole product. When a mixture of the pyrazinone (Re = Ar) with ethyl acrylate or styrene and Pd(dppf)Cl2 was irradiated at 150 °C for 15 min, both catalytic systems favored the Heck reaction product with no trace of Diels-Alder adduct. 

A novel route to indoles and quinolines has been developed by sequential Wiltig and Heck reactions <96CC2253>. Thus, treatment of o-bromo- or iodo-lV-lrifluoroaceiylanilines (86) with a stabilized phosphorane affords the corresponding enamines 87 as a mixture of isomers. Cyclization to 88 is effected by heating with palladium acetate, tri phenyl phosphine, and bu.se. 

Other classes of complexes that have been studied in depth in the Mizoroki-Heck reaction are those having a bidentate ligand containing both a NHC and a phosphine. The development of these structures was encouraged by early theoretical work from Rosch, who calculated that such ligands should be promising catalysts for this... 

Heck reaction of norbomene with phenyltriflate with (P-A/-sulfonylami-noalkyl)phosphine ligands. 

Heck reactions can be carried out in the absence of phosphine ligands.141 These conditions usually involve Pd(OAc)2 as a catalyst, along with a base and a phase transfer salt such as tetra-n-butylammonium bromide. These conditions were originally applied to stereospecific coupling of vinyl iodides with ethyl acrylate and methyl vinyl ketone. 

Phosphine ligands based on the ferrocene backbone are very efficient in many palladium-catalyzed reactions, e.g., cross-coupling reactions,248 Heck reaction,249 amination reaction,250 and enantioselective synthesis.251 A particularly interesting example of an unusual coordination mode of the l,l -bis(diphenylphosphino)ferrocene (dppf) ligand has been reported. Dicationic palladium(II) complexes, such as [(dppf)Pd(PPh3)]2+[BF4 ]2, were shown to contain a palladium-iron bond.252,253 Palladium-iron bonds occur also in monocationic methyl and acylpalladium(II) complexes.254 A palladium-iron interaction is favored by bulky alkyl substituents on phosphorus and a lower electron density at palladium. 

The Heck reaction has proven to be an extremely useful method for the formation of C-C bond at a vinyl carbon centre. There are numerous reported examples of enantioselctive Pd catalyzed C-C bond forming reactions.10"13 Surprisingly, reports of Heck transformations using amino acid based phosphine, phosphinite ligands are rare. Recently Gilbertson reported a proline derived phosphine-oxozoline ligand in a catalytic asymmetric Heck reaction.5 In this paper we present some novel amino acids derived ligands as part of a catalytic system for use in asymmetric Heck reactions. 

Polyethylene glycol) (PEG) was used as a soluble polymeric support in the efficient preparation of the 2-benzazepine 58 via a phosphine-free palladium-catalysed Heck reaction from 57 <06T10456>. 

A short five-step synthesis of a bifuran, namely ( )-2,2 -bis(diphenylphosphino)-3,3 -binaphtho[2,l-I>]furan (BINAPFu) from naphthofuranone via a low-valent titanium-mediated dimerization was reported. The newly developed resolution procedure for phosphines was utilized to provide the optically active bidentate phosphine ligands (BINAPFu), which consistently outperforms BINAP in the asymmetric Heck reaction between 2,3-dihydrofuran and phenyl triflate . Another way in which a benzofuranone can be converted into benzo[7 ]furan is by treatment of the former with 1-BU2AIH at -78°C followed by an acidic work up <00TL5803>. 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

Normally, the oxidative addition of an aryl chloride to Pd(0) is reluctant to take place. But such a process is greatly accelerated in the presence of sterically hindered, electron-rich phosphine ligands [e.g., P(/-Bu)3 or tricyclohexylphosphine]. In late 1990s, Reetz [76] and Fu [77] successfully conducted intermolecular Heck reactions using arylchlorides as substrates, as exemplified by the conversion of p-chloroanisole to adduct 77 [77], The applications of this discovery will surely be reflected on future Heck reactions of non-activated heteroaryl chlorides. 

Conversion of 172 to the tetracyclic benzo-annulated pyrrolizinone 173 was achieved by using the Heck reaction in DMF/H20 (Scheme 38) the best conversion and yield (80%) were observed in the absence of phosphine ligands <2005JOC268>. 

Intramolecular Heck reactions.6 Heck intramolecular coupling of alkenyl or aryl iodides substituted by 3-cycloalkenyl group is an attractive route to fused, spiro, and bridged polycyclic products. Coupling is achieved with a Pd-phosphine catalyst such as Pd[P(QH5),]4 in combination with a base, N(C2H5)3 or NaOAc. The coupling tends to produce a mixture of two isomeric alkenes, in which the newly formed bond is allylic or homoallylic to the ring juncture. 

Similar intramolecular Heck reactions with Pd(OAc)2 and a base (KOAc) or a phosphine have been reported by two other groups.78 These conditions also result in two isomeric polycyclic alkenes. 

Greater durability of the colloidal Pd/C catalysts was also observed in this case. The catalytic activity was found to have declined much less than a conventionally manufactured Pd/C catalyst after recycling both catalysts 25 times under similar conditions. Obviously, the lipophilic (Oct)4NCl surfactant layer prevents the colloid particles from coagulating and being poisoned in the alkaline aqueous reaction medium. Shape-selective hydrocarbon oxidation catalysts have been described, where active Pt colloid particles are present exclusively in the pores of ultramicroscopic tungsten heteropoly compounds [162], Phosphine-free Suzuki and Heck reactions involving iodo-, bromo-or activated chloroatoms were performed catalytically with ammonium salt- or poly(vinylpyrroli-done)-stabilized palladium or palladium nickel colloids (Equation 3.9) [162, 163],... 

Palladium-catalysed Heck reactions have been carried out in a mixture of acetonitrile and D-100 as shown in Scheme 10.3. Using a fluorinated phosphine allowed simple recycling of the catalyst by decantation [4],... 

A fluorous version of the chiral bis-phosphine BINAP has been developed for asymmetric Heck reactions [5], Several fluorous-derivatized binaphthols and BINAP derivatives have been reported, (Scheme 10.4) [6], The silane spacer group present in one of the ligands was used to maximize the percentage fluorine on the molecule. Even so, the partition coefficient1 between FC-72 (see Chapter 3) and benzene was only 2.85, and not surprisingly, the reuse of the catalyst was poor. 

Fluorous-derivatized phosphines have also been used for Heck reactions in scCC>2 together with palladium acetate [7]. The fluorous groups improve the solubility of the catalyst in the SCF compared to nonfluorous ligands. An example of a Heck reaction that uses a fluorous-derivatized phosphine to improve the solubility of a Pd(OAc)2 catalyst is shown in Scheme 10.5. 

Palladacycles, such as (80), derived from tri(l-naphthyl)phosphine, proved to be very active catalysts for Heck reactions to produce (81) (ArX = Phi, 4-MeCOC6H4Br,... 

Addition of diphenylphosphine or phenylphosphine to methyl 1-cyclohexanecarboxylate under base catalysis yielded 186 and 187 [164] (Figure 17). The Pd-catalyzed Heck reaction of substituted olefins with (p-bromophenyl)diphenylphosphine oxide in dimethylformamide afforded substituted phosphine oxides which could be reduced with trichlorosilane to yiled the corresponding carboxylatedphosphines - 188, too, was prepared this way [165],... 

The palladium complex of the dibenzofuran-based water-soluble tertiary phosphine 49 was found catalytically active for the internal Heck reaction of N-aUyl-o-iodoaniline in CH3CN/H2O l/l(Scheme 6.6) [21],... 

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