Metal Phosphido Complexes

Activation of an electrophile by coordination to an electron-poor metal centre (a Lewis acid) and subsequent attack by an external nucleophile is extremely common in organic chemistry. The opposite scenario - the activation of the nucleophile by coordination to an electron-rich metallic centre and electrophilic attack - has been much less pursued. This chemistry can be studied with late transition metals phosphido complexes [LnM-PR2], known to have highly nucleophilic phosphorus atoms.  

The interplay between the rates of phosphorus inversion (which interconverts 2 and 2 ) and the rates of electrophilic attack to each diastereomer of 2 will dictate the enantiomeric excess of 3. 

The three reactions are likely to have phosphido complexes as intermediates. Pt-, Pd- and Ln-catalysed hydrophosphination of activated alkenes and Pd-catalysed phosphination of aryl halides (a cross-coupling reaction) have been known for some time whereas Pt and Ru-catalysed alkylation of secondary phosphines are more recent. 

Two metal-catalysed reactions not based on phosphido complexes are also discussed in this chapter Rh-catalysed desymmetiisation of prochiral dialkynyl-phosphine oxides and Ru- or Mo-catalysed metathesis. 

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Methanolysis of silylated metal phosphido complexes yields phosphido complexes, e.g. ... 

Nucleophilic attack of a metal-phosphido complex on an alk3me was also proposed in copper-catalyzed ann -hydrophosphination of 1-alkynylphosphines... 

Catalytic symmetric synthesis of P-stereogenic phosphines by cross-coupling of secondary phosphines with benzyl or other alkyl halides was promoted by chiral ft and Ru complexes [112-114]. The key step is believed to be the nucleophilic attack of a metal-phosphido complex on a free electrophile the background reaction of the unactivated nucleophilic substrate is much slower. The origin of enantioselec-tivity, as in the Pd-catalyzed asymmetric cross-couplings described above, is the interconversion of diastereomeric phosphido complexes, whose speciation and relative rates of nucleophilic attack determine the product ratio. In the case of ft ((R./ )-Me-DuPhos)(Ph)(PMels), as with the Pd analog in Scheme 43 above, the major product phosphine was formed from the major diastereomeric phosphido complex (Scheme 63) [112-113]. 

Primary and secondary phosphines can oxidatively add to low-valent transition metal complexes to form phosphido complexes. In contrast to phosphines, metal phosphido complexes are known to undergo fast pyramidal inversion, often on the NMR timescale. Inversion barriers for some platinum complexes, determined by NMR spectroscopy, range from 42 to 67kJmol. As a consequence, phosphido complexes containing other chiral ligands are mixtures of interconverting diastereomers. Reaction of these complexes with electrophiles yields tertiary phosphines and derivatives (Scheme 6.1). 

Transition-Metal-Phosphido Complexes (Written with Prof. Jack R. Norton)... 

As described in Chapter 2, phosphines have sufficiently large barriers to inversion that phosphines with three different substituents can be resolved. In contrast, the inversion of configuration at phosphorus in metal-phosphido complexes tends to occur near room temperature. In imsaturated transition metal complexes, a vacant acceptor orbital stabilizes the planar transition state. This effect is shown by the interconversion between pyramidal structures through a planar transition state, like the one shown in Equation 4.101 and formed during inversion of related compounds. - In saturated middle and late transition metal complexes, inductive effects explained below destabilize the groimd state and lead to lower barriers to inversion. The presence of an ancillary planar phosphide or amide ligand can contribute to a low barrier by accepting the lone pair from a p)Tamidal phosphide, as in Equation 4.101. 

The addition of reagents containing X-H bonds in which X is more electronegative than H typically lead to addition across the M-C bond in the direction opposite to the addition of silane or borane to the early metal catalysts. Polymerization of etiiylene with lanthanide catalysts in the presence of phosphines generates phosphine-terminated polymers (Scheme 22.12) - by a mechanism in which the alkyl chain is protonated, and a metal-phosphido complex is generated. This phosphido complex then inserts olefin to start the growth of a phosphine-functionalized polyolefin. Marks subsequently showed that a similar process can be conducted witii amines. In this case, the bulky dicyclohexylamine was needed to sufficiently retard the rate of protonation to allow chain growth. The steric bulk also makes the olefin insertion more favorable thermodynamically. 

The phosphido complex, Th(PPP)4 [143329-04-0], where PPP = P(CH2CH2P(CH2)2)2) has been prepared and fully characterized (35) and represents the first actinide complex containing exclusively metal—phosphoms bonds. The x-ray stmctural analysis indicated 3-3-electron donor phosphides and 1-1-electron phosphide, suggesting that the complex is formally 22-electron. Similar to the amido system, this phosphido compound is also reactive toward insertion reactions, especially with CO, which undergoes a double insertion (35,36). 

Metallacycle fomiation has also been observed in bis-Cp complexes. Heating Cp 2UR[P(Si(CH2)2)2] (R = Cl [146840-37-17, CH [146840-39-3]) results in the metaHation of the phosphido ligand. These complexes are stmcturaHy similar to the group 4 and 6 transition-metal metallacycle complexes, but show a dramatically reduced reactivity. 

Good examples of cyclic (mostly polycyclic) stractures are provided by metal alkoxides. Some spectacular rings are the gold(I)-phosphido complexes of the type [AuPR2] (n = 3, 4 or... 

Relatively few phosphido complexes of the transition metals are known that have an alkyl or aryl group (R) in addition to hydrogen attached to phosphorus [i.e., R(H)P"]. The reactivity of the P—unit has been studied in only a few cases. The syntheses described here are for dinuclear Rh( +1) or Ni( +1) complexes that have two t-BuP(H)" bridges. In both complexes the metals have pseudotetrahedral geometries. Of added interest for the rhodium complex is the presence of a Rh=Rh double bond. ... 

Attempts to extend the reaction (phosphido-complex formation) to other transition metal salts have not yet been successful although Rh shows some promise. 

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