Chelate phosphane

The use of Cp rings with pendant phosphines in CpCoL2 complexes has also been reviewed. Carbonyl complexes of this type (Cp Co(CO)2) lose CO at room temperature to afford pendant phosphane adducts (equation 46). The chelated phosphane can then be uncoordinated with ligands such as cod substitution of cod (Section 5.1.4) with alkynes allows the cobalt complex to participate in cychzation reactions (Scheme 26). 

Interestingly, when menthyldiphenylphosphane (R = PPh2) is employed as chiral auxiliary, inversion of the configuration at C3 of the menthane skeleton [i.e. (IR,35,45 ) instead of (1R,3R,45)] results in an inversion of the induction direction in the catalysis the enantiomeric excess, however, drops from 36% to 17%. Chelating phosphanes such as (5,5)-Chiraphos [(25,35)-2,3-bis(diphenylphosphano)butane] or (5,5)-Norphos [(25,35)-2,3-bis(diphenylphos-phano)bicyclo[2.2.1]hept-5-ene] lead to acceptable conversions (65%) and yields (60-97%) but only low enantiomeric excesses (16% and 22%, respectively). ... 

In contrast to unfunctionalized ketones, Wilkinson-type catalysts are quite effective in the hydrogenation of 2-oxo esters. With in situ catalysts consisting of [Rh(cod)Cl]2 2 and a proline derived chelate phosphane BPPM 3l4, quantitative hydrogenation of 2-oxo esters to (7 )-2-hydroxy esters was achieved. Dry benzene or dry tetrahydrofuran as solvent were superior to alcohols usually used in hydrogenation reactions with Wilkinson-type catalysts. While methyl 2-oxopropanoate was reduced to methyl (R)-2-hydroxypropanoate in only 66% eel5, propyl and 2-methylpropyl 2-oxopropanoate gave the (R)-alcohols with 76% and 71 % ee, respectively (Table 2)15,10. 

There is also a significant difference in Jpp coupling between the metal complexes of monodentate and chelate phosphanes. Monodentate phosphanes can only couple through the metal, whereas chelate phosphanes can couple through the metal, and additionally through the backbone. 

An important question in light of the ease of chelation in the synthesis of the carbonyl complexes is whether it is possible to decoordinate the phos-phane arm, possibly to create a vacant coordination site for further chemistry. The question was addressed by treatment of 327 with 1,5-cyclooctadiene under photochemical reaction conditions, using the diene as the solvent, and resulted in a 41% yield of nonchelated cyclooctadiene complex 336 (Scheme 61). Treatment of this complex with diphenylethyne under reaction conditions normally allowing alkyne di- or trimerization reactions gave tetraphenylcyclo-butadiene complex 337 in 64% yield, showing that chemistry at the cobalt atom is possible without inhibition by a chelating phosphane arm. ... 

An interesting transformation of complexes 98—99 is observed after exposure to air for 24 h in a CDCI3 solution an oxidized new species is formed under these conditions whose structure, based on analytical and spectroscopic data, was determined to be a chelated phosphane—phosphine oxide Rh(I) complexes (2011EJO5649). 

Slugovc et al. [31] described the activity of a cis-dichloro catalyst 48 bearing a chelating phosphane ligand (Figure 12.11). This catalyst was slightly air-sensitive... 

Numerous investigations of asymmetric hydrogenation have been published since the early disclosure of the commercial production of /-DOPA. Most complexes are based on complexes of rhodium(I) with asymmetric chelate phosphane ligands. However, reduction catalyzed by Ru(II) as exemplified by [RuH(PPh3)3Cl] has been extended by use of the ligand (4R,5R)-( - )-DIOP (16). The precursor... 

CO plus ethylene polymer chain j = Bidentate chelating phosphane... 

Chelating alkylphosphanes that are water soluble by virtue of having hydroxy end groups were reported recently. The catalytic formylation of H2PC6H4PH2 and of H2PCH2CH2PH2 in the presence of formaldehyde in aqueous media yields hydroxymethyl bis(phosphanes) in near-quantitative yields (54). Hydroxypropyl-substituted compounds were prepared by radical addition of allylic substrates to 1,2-diphosphinoethane in methanol (55). 

Finally, there are two examples of intramolecular C,P chelates, viz. LPd k2-C,P-2-CH2C6H4P(o-To1)2 (L = Tp 468, Bp 469), both prepared by cleavage of the halide bridged dimer with KL (Scheme 46). 43 These complexes were targeted to explore their potential in the guise of catalysts for organic synthesis, a role to which the cyclopalladated tris(o-tolyl) phosphane precursor had been previously applied. However, this preliminary study revealed these materials to be unreactive toward electrophiles (see also Section III.C.3.b), and no further investigations have been reported. 

The mononuclear system has been developed largely by using the Mo(di-phosphane)2 [90] core. This site has the advantages that it is inert to protic attack and that it causes the metal to have a high affinity to N2 furthermore, the phosphane chelates remain bound to the metal during the reaction, and they occupy four of the six available coordination sites on the metal, leaving only two free positions where binding and activation of substrates can occur. 

Mechanistically, the latter sequence rationalizes as follows (Scheme 47). The Pd-catalyzed arylamination with the bulky monodentate phosphane stops at the stage of the N-aryl allyl benzene 128. Upon addition of the chelating ligand dpephos, a subsequent cycle is opened that commences with the oxidative addition of the aryl bromide to the Pd-dpephos chelate. After the complexation of 128 to the Pd complex, the insertion of the alkene takes place to furnish an alkyl aryl Pd species that, due to the chelating dpephos, undergoes reductive elimination rather than /1-hydride elimination to furnish the product 127. 

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