Carbon-phosphorus bond rearrangement

In this section we will survey the use of transition metal-catalyzed additions, at times accompanied by rearrangement processes, that lead to the generation of new carbon-phosphorus bonds. 

Instability in the supposedly stable phosphorus-carbon bond displays itself not only in unfortunate ways, leading as it does to side reactions and the formation, in synthesis, of unwanted by-products, but also in a constructive manner, forming the basis of reaction sequences of outstanding value in synthesis, as for example in alkene-forming reactions. Instability is an inherent property of (a-hydroxyalkyl)phosphonic acids which manifests itself in phosphorus-carbon bond cleavage as a result of the action of heat or of alkali, and which can lead either to dissociation into precursors or to rearrangement to phosphates (a-oxoalkyl)phosphonic derivatives are susceptible to attack by nucleophiles, a process which also results in carbon-phosphorus bond fission. 

NMR spectroscopy has shown that many compounds, previously described as (1-hydroxyalkylidene)bisphosphonic esters, are actually either totally rearranged compounds or mixtures of initial and rearranged compounds. Such was the case with the products from dialkyl hydrogenphosphonates and dialkyl acylphosphonates, for which, when R R the potential for confusion is obvious (Scheme 18). Provided that the compounds 104 are not heated above 80 °C, they may sometimes be isolated by crystallization, but others, e.g. 104 (R = Ph or aryl), rearrange very easily and cannot be isolated. (1-Hydroxyethylidene)bisphosphonic acid is stable in solution at pH 1.6 up to 125 °C, and in alkaline solutions at pH 8.5-11.5 up to 195 °C the compound then undergoes thermolysis with fission of the carbon-phosphorus bond to give, initially, acetylphosphonic and (1-hy dr oxyethy l)phosphonic acids ... 

This compilation embraces a wide variety of subjects, such as solid-phase and microwave stereoselective synthesis asymmetric phase-transfer asymmetric catalysis and application of chiral auxiliaries and microreactor technology stereoselective reduction and oxidation methods stereoselective additions cyclizations metatheses and different types of rearrangements asymmetric transition-metal-catalyzed, organocatalyzed, and biocatalytic reactions methods for the formation of carbon-heteroatom and heteroatom-heteroatom bonds like asymmetric hydroamina-tion and reductive amination, carboamination and alkylative cyclization, cycloadditions with carbon-heteroatom bond formation, and stereoselective halogenations and methods for the formation of carbon-sulfur and carbon-phosphorus bonds, asymmetric sulfoxidation, and so on. 

Direct metallation of o-halogenophenoxyelement derivatives of silicon, tin, and phosphorus leads to an unstable metallated intermediate which undergoes a rapid 1,3-rearrangement under element-carbon bond formation. This type of reaction seems to be a general method for the synthesis of hydroxyphenyl element derivatives [1-4], We have studied the influence of different organoelement groups on the reaction pathway. The yield increases in the sequence R3Sn < R2P < RjSi P(0)(0R)2. 

By analogy, the formation of a double bond between carbon and phosphorus, which in our opinion, however is improbable, is assumed to be the third stage. Finally, this is followed by the addition of water and tautomeric rearrangement to the primary phosphine. In this way, phosphine, which is present in technical acetylene, and because of its good solubility in actone concentrates in the commercial steel cylinders, forms with acetone, isopropylphosphine oxide and possibly, secondary products... 

Rearrangement processes of alkyltitanocene dichlorides that occur under electron impact have been investigated using deuterium labelling. A novel type of zirconium-mediated coupling reaction of alkynes with vinyl bromide to afford 2,3-disubstituted dienes has been reported (see Scheme 105), and an inter-intramolecular reaction sequence has been proposed for the observed formation of vinylcyclohexadienes and/or methylenecycloheptadienes from the copper-catalysed reaction of zirconacyclo-pentadienes with allylic dichlorides. The essential step in these processes appears to be transmetallation of the zirconium-carbon bond of the zirconacyclopentadiene to produce a more reactive copper-carbon bond. New phosphorus heterocycles, e.g. (417), have been constructed by the thermal rearrangement of a [l,4-bis(trimethylsilyl)->/ -cyclooctatetraene]- ,3,5-triphospha-7-hafhanorbomadiene complex (416). 

The synthesis of various new chiral (o-hydroxyaryl)oxazaphospholidine oxides (139), derived from (S)-proline derivatives, from precursors (140) have been elaborated. This two-step reaction involves an unstable metallated intermediate that undergoes a fast 1,3-rearrangement with the formation of phosphorus-carbon bond. These catalysts have been successfully applied to the catalytic asymmetric borane reduction of numerous prochiral ketones with enantiomeric excess up to 84% ee (Scheme 35). ... 

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