Phosphines, alkylation from metal catalyzed

In 1970 the transition metal catalyzed formation of alkyl formates from CO2, H2, and alcohols was first described. Phosphine complexes of Group 8 to Group 10 transition metals and carbonyl metallates of Groups 6 and 8 show catalytic activity (TON 6-60) and in most cases a positive effect by addition of amines or other basic additives [26 a, 54-58]. A more effective catalytic system has been found when carrying out the reaction in the supercritical phase (TON 3500) [54 a]. Similarly to the synthesis of formic acid, the synthesis of methyl formate in SCCO2 is successful in the presence of methanol and ruthenium(II) catalyst systems [54 b]. 

Chiral phosphines are widely used as auxiliaries for various metal-catalyzed asymmetric reactions and can be prepared from stable phosphine-borane complexes. Secondary P-chiral phos-phine-boranes can be prepared by reductive lithiation of the corresponding tertiary phosphine-borane using LN (eq Likewise, P-chiral tertiary phosphine ligands can be produced by the reductive lithiation of phosphinite-boranes followed by alkylation, both proceeding with retention of configuration (eq 18). ... 

Several other mechanistically distinct metal-catalyzed dearomatization procedures have been reported, and almost all involve phenol or naphthol derivatives undergoing dearomatization via intramolecular transformations. Intramolecular Pd- and Rh-catalyzed C4-arylation and alkylation of /)ara-substituted phenols has been used to construct compounds of general structure 82 (Fig. 15.1) [86]. These reactions rely on generation of electrophilic aryl or alkyl o-metal complex intermediates that participate in tandem C4 metalation-reductive elimination with an attached phenol. Ruthenium- and Pt-catalyzed reactions of naphthalenes and alkynes deliver spirocyclic products such as 83 [87, 88]. An asymmetric intramolecular naphthalene dearomatization catalyzed by Pd(0)-phosphine complexes has been used to prepare carbazole derivatives 84 in good enantiomeric excess from l-(AI-2-bromophenyl)aminonaphthalene precursors [89]. 

One of the first mechanistic proposals for the hydrocarboxylation of alkenes catalyzed by nickel-carbonyl complexes came from Heck in 1963 and is shown in Scheme 24. An alternate possibility suggested by Heck was that HX could add to the alkene, producing an alkyl halide that would then undergo an oxidative addition to the metal center, analogous to the acetic acid mechanism (Scheme 19). Studies of Rh- and Ir-catalyzed hydrocarboxylation reactions have demonstrated that for these metals, the HX addition mechanism, shown in Scheme 24, dominates with ethylene or other short-chain alkene substrates. Once again, HI is the best promoter for this catalytic reaction as long as there are not any other ligands present that are susceptible to acid attack (e g. phosphines). 

Olefin isomerization has been widely studied, mainly because it is a convenient tool for unravelling basic mechanisms involved in the interaction of olefins with metal atoms (10). The reaction is catalyzed by cobalt hydrocarbonyl, iron pentacarbonyl, rhodium chloride, palladium chloride, the platinum-tin complex, and by several phosphine complexes a review of this field has recently been published (12). Two types of mechanism have been visualized for this reaction. The first involves the preformation of a metal-hydrogen bond into which the olefin (probably already coordinated) inserts itself with the formation of a (j-bonded alkyl radical. On abstraction of a hydrogen atom from a diflFerent carbon atom, an isomerized olefin results. 

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]. 

The first indication that metal phosphine complexes were active for catalytic reactions of alkanes came from Shilov s group finding that CoH3(PPh3)3 catalyzes H/D exchange between and CH4. These systems probably involve oxidative addition (equation 1), but they were not followed up at the time because of the greater interest in the Pt system discussed above. Since 1980, a variety of Rh, Ir, Re and Ru complexes have shown activity for alkane dehydrogenation and carbonylation. The intermediate alkyl... 

Simple triarylphosphines linked covalently to PS can be prepared from bro-mopolystyrene via lithiation and subsequent reaction with ClPhAr2 (Scheme 2.41) [144]. A serious side reaction may be the attack of alkyl Uthium on the resin, which causes eventually poor swelling properties or undesired contamination in the product. This problem can be overcome by reaction of metal phosphides with chloromethylated resins. Both lithiated and brominated PS can be prepared from PS. Phosphine ligands of this type have been tested in rhodium-catalyzed hydro-formylation [145]. 

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