Reductive cleavage of the carbon‐phosphorus bond with alkali metals. I. Cleavage of functionalised triphenylphosphines; formation of secondary and primary phosphines

Abstract

AbstractThe reductive cleavage reaction of functionalised triphenylphosphines 1–34 with Na/NH3 and Li/THF depends strongly on the nature of the functionality and on the reducing agent. No reduction occurs with 11, 24, 30, 31 and 32 in Na/NH3. Compounds 3, 4, 5, 10, 12, 13, 15, 19, 23, 25, 26 and 27 cleave to give the secondary phosphide in high yield with Na/NH3, whereas 2, 7 and 9 give a high yield with Li/THF. Reduction occurs but cleavage is poor with 6, 7, 14, 29 and 34 and Na/NH3, or with 11 and Li/THF. Primary ortho‐functionalised phenyl phosphines are obtained by a double cleavage reduction from 2, 5, 12, 25, 26 and 27 with Na/NH3. This unprecedented reaction proceeds via the secondary phosphine, which is formed by protonation of the corresponding phosphide with NH3. It occurs when the aryl group contains a strongly electron‐donating substituent. Multiple cleavage of aryl groups with extended π systems occurs with 7 and 34 when they are made to react with Li/THF. Halogens are cleaved from the phenyl group (16, 17, 18, 28 and 33, with Na/NH,), whereas SCH, groups are converted to the corresponding mercapto group (20, 21 and 22). Birch reduction (2 and 10) can take place in NH, but not in the aprotic solvent THF; it occurs only when other reactions are slow. Sodium amide is obtained via reaction of 8 in Na/NH,. Restricted Hartree‐Fock calculations were carried out for a number of substituted phenylphosphines. From the correlation between the energies and coefficients of the LUMO (always an aryl π* orbital) and the experimental cleavage data, it was concluded that there are three requirements for successful cleavage. The LUMO energy should be neither too high (no reduction) nor too low (radical anion too stable) and, further, the coefficient of the LUMO on the carbon attached to phosphorus must be large.