Rings phosphetane

The phosphetane ring is a useful synthon in the preparation of optically active ligands. Chiral l,2-bis(phosphetano)benzenes 38 are easily prepared from dilithiophenylphosphine 36 by reaction with a cyclic sulfate 37 <00T95>. 

The C2-symmetric, bidentate phosphetanes 71, 80, 81, and 82 are especially suited for use in ruthenium- and rhodium-catalyzed hydrogenations of prochiral substrates (Figure 16). These ligands are easily available from optically pure anti-1,3-diols, and their catalytic properties can be modulated and optimized by variation of the alkyl substituents on the phosphetane rings. 

In addition to the conventional McBride synthesis using simple alkenes, a few other unsaturated substrates serve as starting materials for the construction of phosphetane rings. For example, the reaction of norbornadiene with chlorophosphines (ClP(OEt)2), instead of dichlorophosphines, afforded tetracyclic phosphetane oxides (Equation 23) <1996JHC979>. 

The direct synthesis of a phosphetane ring by the alkylation-cyclization method using dilithium phosphide and 1,3-dihalopropane resulted in a low yield (13%), because of thermally favored chain-forming side reactions and the instability of the final product in the reaction mixture (Equation 25) <1996OM1301>. 

A further approach to phosphetane rings is available in a single step by double Arbuzov reaction of bis(trimethyl-siloxy)phosphine with dibromopropane <1995JOC6076>. Bis(trimethylsiloxy)phosphine is generated in situ from hexamethyldisilazane and ammonium phosphite. The proposed mechanism for this reaction is shown in Scheme 8. 

Phosphorus ylides, P=C compounds with a pentavalent phosphorus atom, can give phosphetanes as well as phosphaalkenes. By the reaction of a stabilized phosphoms ylide with activated alkenes, the phosphetane ring is formed via a zwitterionic betaine <1997HAC157>. 

Similar phosphetane ring formation by thermal rearrangement of spirodiphosphirane has been reported <1994IC596>. In this reaction, the phosphetane 100 with an exocyclic phosphaalkene moiety was obtained as a major product (70%) with 1,4-diphosphanorbornadiene 101 as a minor product (30%) (Equation 36). 

For an example of nucleophilic displacement in the phosphetane ring system which has been unambiguously determined to proceed with retention of configuration at phosphorus, see Ref. 

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