Palladium complexes 5-coordinate

Recently, a palladium complex coordinated with an axially chiral, monoden-tate phosphine ligand, MeO-MOP (7a) or its analogs [21], has been reported to be highly effective for the enantioselective hydrosilylation of alkyl-substituted terminal olefins (Scheme 4) [22,23]. Simple terminal olefins 8 were transformed efficiently into the corresponding optically active 2-alkanols 11 with enantiose-lectivities ranging between 94% and 97% ee by the catalytic hydrosilylation-ox-... 

The asymmetric cross-coupling was successfully applied to the synthesis of optically active allylsilanes [50,51] (Scheme 10). The reactions of a-(trimethyl-silyl)benzylmagnesium bromide (49) with vinyl bromide (4b), (E)-bromopro-pene (( )-50), and (R)-bromostyrene E)-8) in the presence of 0.5 mol % of a palladium complex coordinated with chiral ferrocenylphosphine, (R)-(S)-PPFA (10a), gave the corresponding (R)-allylsilanes (51) with 95%, 85%, and 95% ee, respectively, which were substituted with phenyl group at the chiral carbon center bonded to the siHcon atom. These allylsilanes were used for the S. ... 

A different coordination mode is found for the neutral palladium complex coordinated with two 2,6-di-/i t/-butylphenolate groups, prepared as shown in Equation (18). The crystal stucture of the compound supports, in this case, 7 -allyl coordination of the ring, with the palladium flipping in solution between the two equivalent allyl positions in each ring. The C-O distance observed is 1.234 A, supporting a double bond, and the ring is not planar. 

Cantat T, Mezailles N, Ricard L, Jean Y, Le Floch P (2004) A bis(thiophosphinoyl) methanediide palladium complex coordinated dianion or nncleophilic carbene complex Angew Chem Int Ed 43 6382-6385. doi 10.1002/anie.200461392... 

The reversible reaction of tri-n-butylstannylfuran with the cyclometallated palladium complex 24 yields the ti C) coordinated 2-furyl complex 25 (98JA11016). 

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. 

Reaction of the diphosphines Ph2P(CH2) PPh2 (n = 1-3) with MCl2(PhCN)2 affords 1 1 m-complexes (Figure 3.46) [102]. (Note the use of the labile PhCN adducts if the MCl salts are used, Magnus type compounds M(P-P)2+MCl4- are formed.) Similar complexes are formed with other halides for the thiocyanates see section 3.8.6. The structures of the palladium complexes have been determined (Table 3.10) with square coordination only achieved for n = 3 with the formation of a six-membered metal-chelate ring. 

The reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

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