Palladium acetate alkenylation

There are reports of an increasing number of palladium-assisted reactions, in some of which the palladium has a catalytic function. Thus furan and thiophene undergo facile palladium-assisted alkenylation giving 2-substituted products. Benzo[6 Jfuran and TV- acetyl-indole yield cyclization products, dibenzofurans and carbazoles respectively, in addition to alkenylated products (8UOC851). The arylation of pyrroles can be effected by treatment with palladium acetate and an arene (Scheme 86) (81CC254). 

Alkenyl, Alkynyl, Aryl and Heteroaryl Acids. Treatment of readily accessible (E)- and (Z)-alkyl and aryl substituted vinyl boronates (196) with triethyl phosphite in the presence of lead diacetate results in their stereospecific transformation into (E)- and (Z)-vinylphosphonates (197) (Scheme 53). ° Palladium acetate catalysed Mizoroki-Heck reaction of arylboronic acids (198) with diethyl vinylphosphonates (199) is an effective synthetic approach to... 

Commercially available palladium compounds in the presence of various ligands have often been used as catalysts (Table 3-1). The first choice is often the air-stable and relatively inexpensive palladium acetate however, several of the other published variants can be preferable in certain applications. It is commonly assumed that the palladium(II) species is reduced in situ by the solvent, the alkene [11], the amine [12] or the added ligand (frequently a phosphane, which is oxidized to a phosphane oxide) [13]. In some cases, highly dispersed elemental palladium on charcoal can be applied. In the case of alkenyl or aryl bromides, phosphanes are necessary to avoid precipitation of palladium black (c.f., however. Section 3.2.4.), whereas iodides have been reported to be less reactive in the presence of phosphanes. Triflates have been found to be more reactive in the presence of chloride ions, as the chloride ligand is more easily removed from palladium than the tiiflate ion [14], However, this also has become questionable, because successful coupling reactions of alkenyl triflates have been performed in the absence of chloride ions [15]. 

Alkenylations can also be carried out the optimum conditions for 2-bromopropene with palladium acetate include the use of triphenylarsine with silver carbonate and triethylamine, but the advantages over more amenable conditions, using triphenylphosphine with potassium or cesium carbonate, are marginal. ... 

Palladium(II)-promoted alkenylation involving a-bromo sulfonamide has been utilized to construct the bridgehead bicyclic sultam 193 <04OL1313>. Treatment of 192 with palladium acetate in DMF containing K2CO3, tri-2-furylphosphine and 4A molecular sieves at 100 C furnishes 193. Subsequent bromination with NBS and elimination with DBU give rise to conjugated diene 194. When irradiated at 350 nM, 194 is isomerized via a two-photon process to the structurally novel spiro heterocycle 198. 

Enantiopure alkenyl iodide 35 reacts with the unprotected diol 34 in the presence of palladium acetate to provide cyclopentanone 36 plus a diastereomer in 15% yield. 

A highly efficient protocol for C2 selective alkenylation of thiazoles and benzothiazoles provides easy access to 2-alkenyl-thiazole derivatives 95 (140L1798). Interestingly, when 1-octene is used under standard conditions, the branched product 97 is obtained in 58% yield. A plausible Pd-catalyzed vinylation mechanism has been proposed. Thiazoles 93 undergoes C2 selective electrophilic C-H substitution of palladium acetate to generate the palladium(II) intermediate 98. Next, the resulting... 

Functionalized benzenes preferentially induced ortho-para substitution with electron-donating groups and meta substitution with electron-withdrawing groups (see above). Additionally, the order of reactivity found with aromatics was similar to that of electrophilic aromatic substitution. These observations implicated an electrophihc metalation of the arene as the key step. Hence, Fujiwara et al. [4b] believed that a solvated arylpalladium species is formed from a homogeneous solution of an arene and a palladium(ll) salt in a polar solvent via an electrophilic aromatic substitution reaction (Figure 9.2). The alkene then coordinates to the unstable arylpalladium species, followed by an insertion into the aryl-palladium bond. The arylethyl-palladium intermediate then rapidly undergoes )8-hydride elimination to form the alkenylated arene and a palladium hydride species, which then presumably decomposes into an acid and free palladium metal. Later on, the formation of the arylpalladium species proposed in this mechanism was confirmed by the isolation of diphenyltripalladium(ll) complexes obtained by the C-H activation reaction of benzene with palladium acetate dialkylsulfide systems [19]. 

Itahara, T., Ikeda, M. and Sakakibara, T. (1983) Alkenylation of 1-acyUndoles with olefins bearing electron-withdrawing substituents and palladium acetate. J. Chem. Soc., Perkin Trans. 

The palladium-catalyzed domino assembly of norbornene (65), the ds-alkenyl iodide 66, and a terminal alkyne or cyanide reported by Torii, Okumoto et al. [315] provides an example for a sequence of oxidative addition, intermolecular double bond insertion, and interception of a copper acetylide or potassium cyanide. These reactions with acetylenes have been performed in good yields in the presence of diethylamine, tetra-n-butylammonium chloride, and catalytic amounts of palladium acetate, triphenylphosphine, and copper] I) iodide. Remarkably, they are characterized by complete inversion of the cis configuration of the alkenyl iodide and a high degree of discrimination for the enantiotopic ends of the double bond in norbornene. To account for that, intermediate formation of a cyclopropylcarbinyl-palladium species by a 3-exo-trig cyclization in 67 and subsequent cycloreversion to a new homoallylpalladium intermediate as the direct precursor to 68 and 69 has been assumed. Thus, the products 68 and 69 are formed virtually with complete stereoselectivity (Scheme 8.17). 

The exchange reactions of aryl or alkenyl bromides or iodides with alkenes (Scheme 26) is catalysed by a palladium acetate-triphenylphosphine catalyst in the presence... 

Keywords 2-Azido-3-arylacrylates, terminal alkenes, dichloromethane, palladium acetate, tri-fluoroacetic acid (TEA), oxygen-atmosphere, room temperature, domino reaction, intramolecular cyclization followed by olefination, 3-alkenyl indoles... 

Direct Alkenylation. A C2-alkenylation of purines and heteroarenes including oxazole utilizing styryl bromide was reported. Palladium acetate and Cul function as co-catalysts ... 

Methyl glyoxylate adducts of iV-Boc-protected allylic amines cyclize in the presence of a catalytic amount of palladium acetate and excess copper(II) acetate to 5-(l-alkenyl)-2-(methoxycarbonyl)oxazolidines (eq 5). These heterocycles are easily converted to unsaturatedIV-Boc protected /3-amino alcohols through anodic oxidation and mild hydrolysis. 

Vinyl triflates undergo carbonylative coupling with terminal alkynes to yield alkenyl alkynyl ketones in a reaction catalyzed by palladium acetate and dppp in the presence of triethylamine. When applied to 2-hydroxyaryl iodides (eq 35), subsequent attack by the hydroxyl group on the alkyne yielded flavones and aurones. The cyclization result depends on the reaction conditions. l,8-diazabicyclo[5.4.0]undec-7-ene as base in DMF yields mainly the six-membered ring flavone, whereas the only product observed when ert iloying potassium acetate in anisole was the flve-membered ring aurone. ... 

In 2006, Gaunt and co-workers described a regioselective alkenylation of pyrroles under mild aerobic oxidative conditions. A catalytic amount of palladium acetate was able to selectively functionalize tert-butojycarbamate (BOC)-protected pyrrole at the C2-position while a C3-selectivity was achieved with triisopropylsilyl (TIPS)-protected pyrroles (Scheme 9.4). The aerobic conditions worked well for reactive alkenes such as acrylate derivatives, nevertheless the use of peroxide ( BuOOBz) appeared necessary in order to achieve good yields with more challenging substrates [i.e., methyl vinyl sul-fone). Interestingly, intermolecular alkenylation of pyrrole can also be effected with complete selectivity and good yields. 

The use of transition metal catalysts to effect alkylations, alkenylations, arylations, and acylations in pyridines has been reviewed, and there has been a computational study of the ortho-alkylation of pyridine by scandium-catalysed reactions with alkenes. DFT calculations indicate that the stability of the developing metal-aryl bonds may be important in determining regioselectivity in palladium-acetate-catalysed carbon-hydrogen substitution in heteroarenes. ... 

Vinyl acetate reacts with the alkenyl triflate 65 at the /3-carbon to give the 1-acetoxy-1,3-diene 66[68]. However, the reaction of vinyl acetate with 5-iodo-pyrimidine affords 5-vinylpyrimidine with elimination of the acetoxy group[69]. Also stilbene (67) was obtained by the reaction of an excess of vinyl acetate with iodobenzene when interlamellar montmorillonite ethylsilyl-diphenylphosphine (L) palladium chloride was used as an active catalyst[70]. Commonly used PdCl2(Ph3P)2 does not give stilbene. 

Reduction of iV-(3-bromopropyl) imines gives a bromo-amine in situ, which cyclizes to the aziridine. Five-membered ring amines (pyrrolidines) can be prepared from alkenyl amines via treatment with N-chlorosuccinimide (NCS) and then BusSnH. " Internal addition of amine to allylic acetates, catalyzed by Pd(PPh3)4, leads to cyclic products via a Sn2 reaction. Acyclic amines can be prepared by a closely related reaction using palladium catalysts. Three-membered cyclic amines (aziridines)... 

Tributylstannyl)-3-cyclobutene-1,2-diones and 4-methyl-3-(tributylstan-nyl)-3-cyclobutene-l,2-dione 2-ethylene acetals undergo the palladium/copper-catalyzed cross coupling with acyl halides, and palladium-catalyzed carbon-ylative cross coupling with aryl/heteroaryl iodides [45]. The coupling reaction of alkenyl (phenyl )iodonium triflates is also performed by a palladium/copper catalyst [46],... 

When the allyl group is part of a cyclic structure, a k-g reaction cannot take place to epimerise one of the chiral carbon atoms after having formed the k-g intermediate, the same Jt-face comes back-on to palladium. Thus, if we start from a racemic cyclic alkenyl acetate mixture, racemisation at the complex is blocked and the product will also be a racemate at 100% conversion. Kinetic resolution is still an option to obtain chiral product and starting material. Nevertheless high ee s were obtained when cyclic alkenyl acetates were used. 

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