Hydrogen carbon—phosphorus bonds

Recently it has become clear that tertiary phosphine-metal complexes are reactive and liable to undergo carbon-phosphorus bond scission. The reaction between the C— P " bond and the transition metal to which the tertiary phosphine is bound has profound implications on homogeneous catalysis, particularly on the mode of homogeneous catalyst deactivation in hydroformylation (Rh- and Co-catalyzed) and various other hydrogenation/dehydrogenation reactions, including asymmetric hydrogenation. 

It has been known for many years that transition metals catalyze reactions of coordinated phosphines (2). Known reactions of phosphines as ligands include carbon-hydrogen lx)nd cleavage (cyclometalation), as well as direct carbon-phosphorus bond cleavage. Such metal-catalyzed reactions of phosphines lead to formation of new metal complexes which can affect catalyst properties. A known example is the reaction of triphenylphosphine to propyldiphenylphosphine during the rhodium-catalyzed propylene hydrogenation or hydroformylation (5). 

The presence of electron-withdrawing groups positioned on an aromatic nucleus bonded directly to phosphorus can be a source of instability in the phosphorus-carbon bond. This situation is found particularly in (4-nitrophenyl)phosphonic acids. (2-Methoxy-4-nitrophenyl)phosphonic acid can be demethylated in 40% HBr, but cleavage of the carbon-phosphorus bond becomes more pronounced in a reaction with 48% HBr moreover, hydrogenation of the same acid over Raney nickel yields the expected (4-amino-2-methoxyphenyl)phosphonic acid, but a similar reduction of (2-hydroxy-4-nitrophenyl)-phosphonic ac id results in the separation of 3-aminophenol. These and other, similar, reactions have been surveyed by Freedman and Doak ". ... 

Carbon-phosphorus bonds can be formed by the addition of dialkyl hydrogen phosphites or alkyl hydrogen phosphinates to activated ethylenic compounds. ... 

Pauling, L. (1977) "The nature of bonds formed by the transition metals with hydrogen, carbon, and phosphorus," Acta Crystal-logr., in press. 

Pauling, L. Nature of the Bonds Formed by the Transition Metals with Hydrogen, Carbon, and Phosphorus Acta Crystallogr. 1978, B34, 746-754. 

Because organophosphorus compounds are important in the chemical industry and in biology, many methods have been developed for their synthesis [1]. This chapter reviews the formation of phosphorus-carbon (P-C) bonds by the metal-catalyzed addition of phosphorus-hydrogen (P-H) bonds to unsaturated substrates, such as alkenes, alkynes, aldehydes, and imines. Section 5.2 covers reactions of P(lll) substrates (hydrophosphination), and Section 5.3 describes P(V) chemistry (hydrophosphorylation, hydrophosphinylation, hydrophosphonylation). Scheme 5-1 shows some examples of these catalytic reactions. 

Ionic dissociation of carbon-carbon a-bonds in hydrocarbons and the formation of authentic hydrocarbon salts, 30, 173 Ionization potentials, 4, 31 Ion-pairing effects in carbanion reactions, 15, 153 Ions, organic, charge density-NMR chemical shift correlations, 11,125 Isomerization, permutational, of pentavalent phosphorus compounds, 9, 25 Isotope effects, hydrogen, in aromatic substitution reactions, 2,163... 

In intramolecular arylations, a new bond is created between two aromatic moieties of the same molecule or between an aromatic nucleus and an atom of a side-chain. Many intramolecular arylation reactions of homocyclic and heterocyclic aromatic halides have been studied mainly in view of their synthetic applications, and it is not always clear which mechanistic pathway is followed. The reaction may start with homolytic or heterolytic dissociation of the carbon-halogen bond and proceed by attack of the aryl radical or aryl cation on another part of the molecule. Electrocyclization followed by elimination of hydrogen halide is another possibility. Especially when heteroatoms such as nitrogen, sulphur or phosphorus are involved, the initial step may be a nucleophilic attack on the carbon atom bearing the halide atom. 

---