Phosphines activating function

Ph3P)4Pd and certain Pd(II) complexes in the presence of an excess of a tertiary phosphine also function as active catalysts (128). This indicates that palladium species may have potential provided they are protected from destructive reduction by the choice of suitable ligands. A complex species [(Ph3P)2Pd]jf gradually forms in the PhjP—Pd metal mixture ( 28). 

Attempts to address the difficulty associated with the distance of the chirality in the Pd ligand to the site of C—C bond formation have been most fruitful. New chiral phosphine ligands containing an optically active functional group remote from the phosphines have been very effective, providing optical yields up to 52% in the case where the new chiral center is one carbon removed from the allyl ligand437 (c/. 10% optical parity obtained previously 434 equation 355). 

Substituted cobalt carbonyls of the type Co2(CO)eL2 have also been compared to binary cobalt carbonyls as hydroformylation catalysts for linear aldehydes. One study compared four Co2(CO)6L2 complexes (L = tertiary phosphines with functionalized alkyl groups) to the well-known complexes Co2(CO)6(P n-Bu 3)2 and Co2(CO)8. The bulky phosphine substituent P(CH2CH2CH20CH2CH2)3 showed lower activity than the others, but analogous selectivity. 

D-Threonine-L-tert-leucine-derived bifunctional phosphine catalyzes highly enan-tioselective [3+2] annulation of maleimides with allenes, allowing the synthesis of optically active, functionalized bicyclic cyclopentenes containing two tertiary stereogenic centers (Scheme 6.30) [34],... 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

A few functional dihalides have been prepared. Some of these monomers are tabulated in Table 6.2. The monomers could be nitrated and then reduced to amine-functionalized monomers (Scheme 6.26).203 This approach was used to nitrate DCDPS or bis-4-fluorophenyl phenyl phosphine oxide.204 These monomers were used to copolymerize with some other activated dihalides as the comonomers. 

Besides its protective function of the labile phosphine group, the BHj group activates the adjacent substituents such as methyl group or P-H bond to deprotonation with a strong base [78]. This methodology provides an efficient alternative to the difficult synthesis of a variety of optically active tertiary phosphine derivatives, as will be described in Sect. 3. 

Functionally active preformed primary phosphines (e.g.,H2N(CH2)3PH2 3 or Br(CH2)3PH2 17) will provide important building blocks to functionaUze sim-ple/complex molecules with primary phosphine functionaUties. The user friendl/ nature of the air stable primary bisphosphines (e.g., 1,10,16,18-20) will open up new realms of exploratory research that utilize primary phosphines. It is also conceivable that the high oxidative stability and the ease with which primary phosphines can be incorporated on chiral backbones or peptides provide new opportunities for their appHcations in catalysis and biomedicine. 

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