Palladium O

Palladium(o).—The preparation of [M(PEt3)4] (M = Ni, Pd, or Pt) has been carried out by treatment of the metal chloride with PEts and potassium metal in THF. Vacuum thermolysis of [PdlPEtj) ] produces the pale orange pyrophoric [Pd-(PEt3)3]. It was found that the [M(PEt3)4] (M = Ni or Pd) species were protonated in EtOH to yield [HM(PEt3)3], isolable as their [BPh4] salts. N.m.r. spectra 

2-NMe3-l,2-MCBioHio] (M - Pd, L = BuNC M = Pt, L = PEt3) have been isolated from the reaction between the appropriate carbaborane and a suitable labile metal complex. The structures were discussed on the basis of B, H, and P n.m.r. spectra, and the X-ray structure (see Table 1) of [l,l-(BuNC)2-2Me3N-l,2-PdCBio-Hio] shows a weak M—C bond (Pd—C = 2.60 A) with the metal to carbaborane bonding approximating square-plane co-ordination. 

6—10 K have been examined by use of matrix isolation i.r. spectroscopy. The species formed, [(02)2M(N2)] and [(02)M(N2)2], were both shown to contain side-on bonded O2 and end-on bonded N2. Force constant calculations indicate an overall bonding picture of a weakly bonding N2 (a-donor) to a M(02) moiety, formulated as M (02 ). The reaction of Pd atoms with olefins has also been studied using low-temperature i.r. spectroscopy. The most stable compound [(bicyclo[2,2,1 ]hept-ene)3Pd] was also obtained in large quantities in methylcyclohexane solution at -120°C 

Mono- and tri-nuclear arylnitroso-complexes [Pd(p-ClC6H4NOXBu NC)2], [M(PhNOXPPh3)2] (M = Ni, Pd, or Pt), [Pd3(PhNO)3L3] (L = PBu 3 or PPhBu 2) have been prepared by treatment of a suitable metal complex with the arylnitroso in 

Electrochemical reduction of the bis-maleonitriledithioiato complexes of Pd and Pt in MeCN gives the M species, the existence of which was conhrmed by e.s.r. measurements which yielded for the Pd complex = 2.07 and = 28 Reversible reduction of similar diaminomaleonitrile complexes of Ni, Pd, and Pt was found to lead to stable mono- and di-anionic species. 

A series of substituted phosphine complexes of palladium have been synthesized by displacement of the allyl ligand from (2-methylallyl)PdCl2 with excess phosphine, and 13C n.m.r. studies have shown that the equilibrium (1) lies well to the left  

There have been several papers reporting new oxidative addition reactions of [Pd(PPh3)4], Alkoxalyl compounds (1), formed by addition of C1C0C02R (R = Me or Et), decarbonylate in chloroform or benzene at room temperature to yield trans-[PdCl(C02R)(PPh3)2].7 The X-ray structure (Table 1, p. 399) of the methyl derivative 

The most common type of oxidative addition to [Pd(PPh3)4] has been shown to follow equation (3) (X = Cl, e.g. CC14, Id, PhCOCl, SnCl2, SnCl4, C12CS X = Br, 

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In related work, 3-chloromethylcephems were coupled with tributyl(tnfluoro-vinyl)stannane catalyzed by tri(2-furyl)phosphine palladium(O) [7S, 19] (equation 13). 

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

The aforementioned reaction is an example where even quinolinyl chloride is a good substrate for the oxidative addition to palladium(O) if the chlorine atom is at the activated position (a or 5). 

Heck reaction, palladium-catalyzed cross-coupling reactions between organohalides or triflates with olefins (72JOC2320), can take place inter- or intra-molecularly. It is a powerful carbon-carbon bond forming reaction for the preparation of alkenyl- and aryl-substituted alkenes in which only a catalytic amount of a palladium(O) complex is required. 

This reaction is not a bona fide Heck reaction per se for two reasons (a) the starting material underwent a Hg Pd transmetallation first rather than the oxidative addition of an aryl halide or triflate to palladium(O) (b) instead of undergoing a elimination step to give an enone, transformation 134 136... 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

The discovery of palladium trimethylenemethane (TMM) cycloadditions by Trost and Chan over two decades ago constitutes one of the significant advancements in ring-construction methodology [1]. In their seminal work it was shown that in the presence of a palladium(O) catalyst, 2-[(trimethylsilyl)methyl]-2-propen-l-yl acetate (1) generates a TMM-Pd intermediate (2) that serves as the all-carbon 1,3-di-pole. It was further demonstrated that (2) could be efficiently trapped by an electron-deficient olefin to give a methylenecyclopentane via a [3-1-2] cycloaddition (Eq. 1). 

Reductive elimination—to yield the coupling product 3 and regeneration of the catalytically active palladium-(O) complex 5. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

The electrophilic character of the palladium atom in the complexes formed by oxidative addition of aryl halides and alkenyl halides to palladium(o) complexes can be exploited in useful ways. 

During the course of an elegant synthesis of the multifunctional FR-900482 molecule [( )-43, Scheme 9], the Danishefsky group accomplished the assembly of tetracycle 42 using an intramolecular Heck arylation as a key step.24 In the crucial C-C bond forming reaction, exposure of aryl iodide 41 to a catalytic amount of tetra-kis(triphenylphosphine)palladium(o) and triethylamine in acetonitrile at 80 °C effects the desired Heck arylation, affording 42 in an excellent yield of 93 %. The impressive success of this cyclization reaction is noteworthy in view of the potentially sensitive functionality contained within 41. 

Because the Sonogashira coupling process outlined in Scheme 18 is initiated by the in situ reduction of palladium(n) to palladium(o), it would be expected that palladium(o) catalysts could be utilized directly. Indeed, a catalytic amount of tetrakis(triphenylphosphine)-... 

The postulated steps that constitute the Suzuki coupling process are shown in Scheme 25. After oxidative addition of the organic halide to the palladium(o) catalyst, it is presumed that a metathetical displacement of the halide substituent in the palladium(ii) complex A by ethoxide ion (or hydroxide ion) takes place to give an alkoxo-palladium(ff) complex B. The latter complex then reacts with the alkenylborane, generating the diorganopalladium complex C. Finally, reductive elimination of C furnishes the cross-coupling product (D) and regenerates the palladium(o) catalyst. 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

An intramolecular palladium(o)-catalyzed cross-coupling of an aryl iodide with a trans vinylstannane is the penultimate maneuver in the Stille-Hegedus total synthesis of (S)-zearalenone (142) (see Scheme 38).59 In the event, exposure of compound 140 to Pd(PPh3)4 catalyst on a 20% cross-linked polystyrene support in refluxing toluene brings about the desired macrocyclization, affording the 14-membered macrolide 141 in 54% yield. Acid-induced hydrolysis of the two methoxyethoxymethyl (MEM) ethers completes the total synthesis of 142. 

Palladium(O)-catalyzed isomerization of 2-dienylaziridines 201 to 3-pyrrolines 202 was reported in 1985 by Oshima, Nozaki, and coworkers (Scheme 2.49) [78]. This isomerization is in striking contrast to Ibuka s palladium-catalyzed isomerization of 2,3-trans-2-vinylaziridines to the corresponding 2,3-cis isomers (see Section 2.4.6) [29]. 

A related palladium(O)-catalyzed epimerization of y-aziridinyl-a,P-enoates 244 was also reported by Ibuka, Ohno, Fujii, and coworkers (Scheme 2.60) [43]. Treatment of either isomer of 244 with a catalytic amount of Pd(PPh3)4 in THF yielded an equilibrated mixture in which the isomer 246 with the desired configuration predominated (246 other isomers = 85 15 to 94 6). In most cases the isomer 246 could be easily separated from the diastereomeric mixture by a simple recrystallization, and the organocopper-mediated ring-opening reaction of 246 directly afforded L,L-type (E)-alkene dipeptide isosteres 243. 

Yamamoto and coworkers developed cycloadditions between activated olefins and vinylaziridines 253 with the aid of a palladium(O) catalyst (Scheme 2.63) [94], based on their three-component aminoallylation reaction. The corresponding 4-vi-nylpyrrolidines 255 were obtained as mixtures of diastereomers (ds trans= 55 45 to 23 77). 

In 1991, Ohfune and coworkers reported palladium(O)-catalyzed carbonylation of vinylaziridines 262 with carbon monoxide (1 atm.) in benzene (Scheme 2.65) [31]. Interestingly, 3,4-trans-azetidinone 264 was exclusively obtained from a dia-stereomeric mixture of trans- and cis-vinylaziridines 262 (3 1). Tanner and Somfai synthesized (+)-PS-5 (267) by use of palladium(O)-catalyzed trons-selective (3-lactam formation in the presence of Pd(dba)3 CHC13 (15mol%) and excess PPh3 in toluene. 

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