Palladium complexes ferrocene

Such bis(diphenylphosphino)ferrocene-palladium complexes commonly undergo a reversible one-electron oxidation, centered on the ferrocene moiety, and an irreversible one-electron reduction, centered on the palladium fragment. The relevant redox potentials are reported in Table 7-29, together with those of related complexes. It must be noted that the ferrocene-based one-electron oxidation leads to ferro-cenium-palladium complexes that are more stable than the free diphenylphosphino-ferrocenium ion. 

Table 7-29. Formal electrode potentials vs. SCE) for the redox processes exhibited by some bis (diphenylphosphino)ferrocene-palladium complexes...

X-ray characterized ferrocene-palladium complexes include the following [Fe-(// -C5H4S)2]Pd(PPh3) [169, 170] ... 

A catalytic asymmetric [4+2]-cydoaddition of a vinylallene with butadiene has been achieved successfully, in which a palladium complex modified by a ferrocene-derived chiral monophosphine ligand proved to be a superior catalyst transferring chirality to the product (Scheme 16.80) [90],... 

Bidentate ferrocene ligands containing a chiral oxazoline substituent possess both planar chiral and center chiral elements and have attracted much interest as asymmetric catalysts.However, until recently, preparation of such compounds had been limited to resolution. In 1995, four groups simultaneously communicated their results on the asymmetric synthesis of these structures using an oxazoline-directed diastereoselective lithiation (Scheme 8.141). " When a chiral oxazolinylferrocene 439 was metalated with butyllithium and the resulting aryllithium species trapped with an electrophile, diastereomer 442 was favored over 443. The structure of the major diastereomer 442 was confirmed, either by conversion to a compound of known stereochemistry or by X-ray crystallography of the product itself or of the corresponding palladium complex. ... 

A number of additional cyclizations involving alkynes have been reported. For instance, it has been shown that indoles may also be accessed from 2-bromo- or 2-chloroanilines, as illustrated by the regioselective preparation of the carbinol 373 in the presence of the ferrocene 374 (Equation 104) <20040L4129>, whereas a one-pot sequence featuring titanium catalyzed hydroamination of 2-chloroanilines with acetylenes, followed by intramolecular Heck cyclization in the presence of an imidazol-2-ylidene palladium complex, has also been reported <2004CC2824>. A set of aryl-2-indolyl carbinols have been prepared in high enantiomeric purity by palladium-catalyzed annulation of... 

In 1999 Trost and Schroder reported on the first asymmetric allylic alkylation of nonstabilized ketone enolates of 2-substituted cyclohexanone derivatives, e.g. 2-methyl-1-tetralone (45), by using a catalytic amount of a chiral palladium complex formed from TT-allylpaUadium chloride dimer and the chiral cyclohexyldiamine derivative 47 (equation 14). The addition of tin chloride helped to soften the lithium enolate by transmetala-tion and a slight increase in enantioselectivity and yield for the alkylated product 46 was observed. Besides allyl acetate also linearly substituted or 1,3-dialkyl substituted allylic carbonates functioned well as electrophiles. A variety of cyclohexanones or cyclopen-tanones could be employed as nucleophiles with comparable results . Hon, Dai and coworkers reported comparable results for 45, using ferrocene-modified chiral ligands similar to 47. Their results were comparable to those obtained by Trost. 

The X-ray study 170, 171) established a planar structure for the cyclobutadiene ring with C-—C distance equal to 1.46 A and angles of 90°. All the M—C distances are equivalent and close to those observed in ferrocene. The phenyl and methyl substituents are distorted from the ring plane and bent towards the metal atom. If one assumes that cyclobutadiene occupies two coordination sites then in the known tetraphenylcyclobutadiene-nickel and -palladium complexes the metal atom has a coordination number of 5. This suggests coordinative unsaturation for the metal and a priori one may expect an associative substitution for such complexes. 

In the case of all of the catenate complexes discussed above, the ligand imposes a more or less distorted tetrahedral geometry to the metal center. However, com-plexation of Pd with 5 follows a different route. Insertion of Pd + into 5 yields the monocation (Pd"(5-H) + via ort/jo-metallation of one of the phenyl rings to give planar coordination by an N3C donor set. The structure of Pd (5-H)l is a compromise between the stereochemical and conformational preferences of metal and ligand where the disparity between the requirements of square planar Pd" and tetrahedral 5 is resolved by ort/jo-metallation. Pd (5-H) + shows a reversible oxidation at +0.55 V versus Fc+/Fc (Fc = ferrocene) assigned to a Pd +Z couple. Two reversible reductions are observed for the palladium complex at extreme cathodic potentials, E1/2 = —1.95 and -2.23 V) versus Fc+/Fc. 

As illustrated in Fig. 7-52, in the related l-[l,l -bis(diphenylphosphino)ferrocene]-palladatetraborane complex, the palladium atom still possesses a pseudo-square-planar environment, two bonds being formed with the phosphorus atoms and the two remaining bonds with the triborane ligand. The cyclopentadienyl rings of the sandwich fragment are parallel and nearly staggered [156]. 

Substitution of the two diphenylphosphino substituents for thiolate groups affords a series of complexes, the structure of which is typically represented by that of dichloro[l,r-bis(ibutylsulfido)ferrocene]palladium(ii) shown in Fig. 7-53 [10]. The assembly is similar to that of the preceding bis (diphenylphosphino) complexes, except for the eclipsed and less tilted (1.9°) conformation of the cyclopentadienyl rings of the ferrocene group. 

Scheme 7-34 shows a series of palladium complexes of dimethylaminomethyl-ferrocene, the electrochemical behavior of which has been reported [162]. 

Whereas the palladium complexation of dimethylaminomethyl-ferrocene renders the ferrocenyl oxidation slightly more difficult, it seems that the presence of electron-donating substituents X, L plays the major role in determining the accessibility of the ferrocenyl oxidation. 

In CH2CI2 no appreciable difference exists between the redox potential of the ferrocene-centered one-electron oxidation of the iV-decyl substituted complex ( = -h0.20 V) and the AT-ethyl substituted complex ( ° = -1-0.21 V). A rather significant role is played by palladium complexation, in that the one-electron removal for both of the two complexes becomes more difficult by about 80 mV than those of the corresponding free diazadithiaferrocenophane molecules [163]. 

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