Phenylpalladium complexes

An analogue of the above transformations is the ring closure depicted in 3.33., which involves the formal addition of the phenylpalladium complex formed in the opening step of the catalytic cycle onto the carbon-nitrogen triple bond. In the above process o-methylamino-benzonitrile is also formed as by-product in 38% yield, indicating the presence of competing pathways.41... 

Innovations in the chemistry of aromatic compounds have occurred by recent development of many novel reactions of aryl halides or pseudohalides catalysed or promoted by transition metal complexes. Pd-catalysed reactions are the most important [2,29], The first reaction step is generation of the arylpalladium halide by oxidative addition of halide to Pd(0). Formation of phenylpalladium complex 1 as an intermediate from various benzene derivatives is summarized in Scheme 3.1. 

A chiral 3-methyl-1-phenylpalladium complex is isolated in enantiomeric excess from (S)-E-3-acetoxy-l-phenyl-1-butene. The result is in agreement with the oxidative addition proceeding with inversion on carbonThe r/ -benzylcompound XCII (cf. the corresponding Mo complex XV) is stable at RT, and is obtained from benzyl chloride and Pd atoms (from a metal vapor generator) ... 

A phenylpalladium complex also causes ring-opening rearrangement of methylenecyclobutane to a ji-allylpalladium complex, which arises from p-carbon elimination of an intermediate (cyclobutylmethyl)palladium complex [82]. 

Organic compounds M—R and hydrides M—H of main group metals such as Mg, Zn, B, Al, Sn, SI, and Hg react with A—Pd—X complexes formed by oxidative addition, and an organic group or hydride is transferred to Pd by exchange reaction of X with R or H. In other words, the alkylation of Pd takes place (eq. 9). A driving force of the reaction, which is called transmetallation, is ascribed to the difference in the electronegativities of two metals. A typical example is the phenylation of phenylpalladium iodide with phenyltributyltin to form diphenylpalladium (16). 

We do not know if the vinylic alcohol is actually an intermediate or whether a hydride-71 complex of it rearranges directly to the aldehyde as probably happens in the palladium-catalyzed oxidation of ethylene to acetaldehyde. The formation of 4% 2-methyl-2-phenylpropanal is unexpected. This product must arise from a reversed addition of the phenylpalladium group followed by a hydrogen transfer from the hydroxyl-bearing carbon to the palladium, followed by reductive elimination of a hydridopalladium group. An alkyoxypalladium intermediate has been proposed (39). 

As an example of carbometallation, the 1,4-carbosilylation product 218 is obtained by the reaction of dienes, disilanes and acid chlorides of aromatic and a,/i-unsaturatcd acids at 80 °C. The phenylpalladium 216 is formed by the oxidative addition of benzoyl chloride, followed by facile decarbonylation at 80 °C, and reacts with butadiene to generate the benzyl-7i-allylic complex 217. Then, transmetallation with the disilane and reductive elimination afford 4-silyl-2-butenylbenzene 218 [92], Regioselective carbomagnesation of isoprene with allylic magnesium bromide 219 catalysed by Cp2TiCl2 gives 220, which is useful for terpene synthesis [93,94],... 

Oxidative cross-coupling with alkenes is possible with Pd(OAc)2 [109], The reaction proceeds by the palladation of benzene to form phenylpalladium acetate (164), followed by alkene insertion and elimination of /1-hydrogen. Heteroaromatics such as furan and thiophene react more easily than benzene [109]. Stilbene (177) is formed by the reaction of benzene and styrene. The complex skeleton of paraberquamide 179 was obtained in 80% yield by the Pd(II)-promoted coupling of the indole ring with the double bond in 178, followed by reduction of the intermediate with NaBELt [110]. 

Sy 1 -phenylethyltrifluorosilane 439 1-phenylethylzinc chloride 27, 29 1 -phenyl-3-methyl-1,2-butadiene 475 (1Z,3E)-1-phenyl-1,3-octadiene, synthesis 84-5 phenylpalladium iodide complex 137 l-phenylpent-4-en-l-yne 196-7 (Z)-phenyl-substituted dendralene 117 ( -(-)-1 -phenyl-1 -trifluoroacetoxy-2-propyne 475 l-phenyl-4-triinethylsilyl but-2-ene 351... 

More recently, however, results with monooelfins indicate that trans addition can occur to both Pd(II) and Pt(II) n complexes. The first demonstration was the trans addition of amines to Pt(II) complexes 215), and more recently the trans attack of acetate on cyclohexene has been demonstrated (Section III, A, 2, a). However, cis attack can also occur such as the addition of phenylpalladium to cyclohexene (Section III, A, 4) or addition of PdCOOR to cyclic olefins (Section III, A, 3). Also, in the exchange studies (Section III, B, 1) the stereochemistry indicates that some nucleophiles can attack cis or trans. Thus chloride ion containing acetic acid can attack the Pd(II) olefin n complex from either inside or outside the coordination sphere. What are the factors involved The most important appears to be the ability of the nucleophile to coordinate to Pd(II). Thus, phenyl is covalently bonded to Pd(II) and is therefore always in the coordination sphere. Chloride is both inside the coordination sphere as well as outside the coordination sphere and can thus attack both cis and trans. Acetate is not complexed to Pd(II) in chloride-containing media, and thus can only attack trans. On the other hand, in chloride-free acetic acid, acetate is both inside and outside the coordination. Stereochemical results indicate that in this system acetate can attack in both a cis and trans fashion. 

The reaction can be explained by the transmetalation of the borate with PdCl2 giving phenylpalladium intermediate and the subsequent insertion of a C=C double bond of norbornadiene into the Pd-Ph bond. Cationic 7t-allyl-palladium(II) and -rhodium(I) complexes having a BPhJ counter anion induce phenyl group migration from B to the transition metals at elevated temperature to form phenyl complexes [54,55] (Eq. 5.17). 

Among many substrates used for Pd(0)-catalyzed reactions, organic halides are most widely used. In Grignard reactions, Mg(0) metal reacts with organic halides of sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand, Pd(0) complexes react more easily with halides attached to sp carbons, namely aryl and alkenyl halides. In addition, several pseudohalides are used as well. They undergo facile oxidative addition to Pd(0) to form Pd complexes which have cr-Pd-carbon bonds. Scheme 3.1 summarizes the oxidative addition of phenyl halides and pseudohalides to form phenylpalladium halides. Aryl iodides and bromides have been used widely. 

Such a sequence would readily bypass the trfl i-PhPd(Nu)L2 formation. As a proof of the validity of this mechanism involving five-coordinate anionic arylpalladium(II) species, the 2-thiophenyl anion is found to react 2.5 times slower with the trfl 5-PhPdI(PPh3)2 complex (Scheme 1) than with the anionic phenylpalladium(II) complex generated by the oxidative addition to Pd°(PPh3)2Cr (Scheme 25). 

The reaction of norbornene with iodobenzene or bromobenzene in the presence of [Pd(PPh3)4] as the catalyst leads to pentacycle 83 (Scheme 11.29) [6, 31, 90-94], This process is known as the Catellani reaction. The parent transformation involves an insertion of the phenylpalladium(II) complex into the double bond of norbornene to give (Ti -phenyl)norbornyl palladium(II) complex 84, which then undergoes intramolecular paUadation to form 85. Further reaction of this paUadacycle with iodobenzene leads to 83. Biphenyl, tetracycle 86, and more complex derivatives 87 and 88, have also been isolated in this reaction [90, 93c, 94], Benzocyclobutenes... 

Transfer of aryl groups to palladium(ii) centers with the formation of arylpalladium(iv) complexes has been established.The first identified transfer of Ph from diphenyliodonium(iii) triflate (Scheme 26) to the metallacyclic complex [Pd(C4H8)(bpy)] led to the formation of a new phenylpalladium(iv) bond, providing easy access to species such as [Pd(OTf)G4H8(Ph)(bpy)] 163. This elegant procedure features a model reaction for suggested reactions of C(i/) )-X bonds with Pd(ii) substrates. " "" ... 

Treatment of the Pd(PPF-f-Bu)[P(o-tol)3] 30 with phenyl tosylate in benzene at room temperature formed the resulting phenylpalladium(II) tosylate complex 32 in good yields via an initial dissociation of the P(o-tol)3 ligand creating the more reactive bisphoshine palladium(O) species. The palladium(II) complex, holding the phenyl group cis to the less sterical hindered phosphino group, was produced as the... 

Even though an interchanged phenylpalladium(II) tosylate complex was obtained, the 1,2-migration proved to be very fast even at room temperature complementing the 1,2-migration from tricoordinated T-shaped palladium(II) complexes observed in the Heck reaction (Chap. 2). 

---