Palladium metallacycles

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

Displacement of the chelate carbonyl from palladium by ethene has never been observed in model studies, which accounts for the virtual absence of double ethene insertions in actual copolymerisation reactions. Indeed, (5-chelate opening is actually brought about by CO to generate a six-membered metallacycle (y-chelate), while p-chelates of catalytically active systems generally react with CO to yield carbonyl acyl complexes, even at very low temperature. For the systems investigated by Bianchini [5e, f], the activation barriers for the conversion of the P-chelates... 

In metal peroxide chemistry, the heterolytic or homolytic nature of catalytic oxidation seems to be strongly dependent on the heterolytic or homolytic dissociation mode of the peroxide intermediate, for which the triangular coordination mode of the peroxide moiety of the metal appears to be a key feature. Heterolytic oxidations require attainable coordination sites on the metal, involve strained metallacyclic reaction intermediates, and are highly selective. In contrast, homolytic oxidations involve bimolecular radical processes with no metal-substrate interactions and are less selective. In the important field of palladium oxidation chemistry, hydroperoxo... 

Biich F1M, Binger P, Benn R, Kruger C, Rufinska A (1983) Phosphane-induced, stepwise rearrangement of an octadienediyl palladium complex to seven- and nine-membered metallacycles. Angew Chem Int Ed 22 774-775... 

Selected structural data for 3-metallacyclobutanone complexes of palladium and platinum 5, obtained from both experimental and computational determinations, are provided in Table 5. The ab initio structures calculated at the self-consistent field (SCF) and Hartree-Fock (F1F) levels show good agreement with experimental values for the bond lengths of the metallacycle, but these computations deliver somewhat longer Pd-P bonds and smaller P-Pd-P... 

Palladacycle 5 reacts in various ways depending on ligands and reaction conditions. In particular it readily undergoes oxidative addition of alkyl halides to form a palladium(IV) complex 6, which has been isolated and characterized with stabilizing nitrogen ligands such as phenanthroline. This palladium(IV) metallacycle... 

Scheme 2. Palladium(IV) metallacycle formation and subsequent reductive elimination. Isolated species with L—L=phen R = Me, CH2=CHCH2, 4-N02C6H4 X= Br, Cl.

Complex 5 (Scheme 1) follows a different pathway in reactions with aryl halides (R = aryl in RX). This time the aryl group reacts preferentially with the norbornyl site of the palladacycle rather than with the aryl site (probably via a palladium(IV) metallacycle of type 6). As a consequence, norbornene is no longer expelled and is incorporated into a condensed ring instead. The minor aryl portion that migrates on to the aryl site (complex 14) also retains norbornene and gives the same product 13. A C-H transformation process is thus operating in ring formation from 12 or 14 to 13 (Scheme 5) [2],... 

The reactivities of metallacycles obtained from the same substrate with different metals can vary dramatically for example, in contrast to nickel, the palladacyles obtained from butadiene mainly convert into open-chain dimers as a consequence of the greater tendency of palladium to undergo p-hydrogen elimination. 

Formation of the palladium(II) complexes can be achieved using standard protocols and resulting in square planar complexes with mer (pincer) chelate structure. The six-membered, very flexible metallacycles featuring alkyl linker chains display chiral puckering that would make the use of chiral analogues difficult in asymmetric catalytic applications. 

The final neutral palladium(II) complex with the amide functionalised carbene hgand features two six membered metallacycles, just as the cationic not yet deprotonated one does. The amine nitrogen atom has four substituents (H, Pd and two C). Since the two metallacycles can adopt two distinct conformations that slowly flip into each other for steric reasons, the nitrogen atom appears chiral due to atropisomerism despite its overall C -symmetry. The deprotonated complex lacks this feature, clearly visible in the respective NMR spectra. 

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