Acid-Catalyzed Cleavage Of 2-Pyridyl and 4-Pyridyl Derivatives Of Aminomethylphosphonic Acid. Kinetic And Chemical Arguments For A Mechanism With A-S_E2 Character

Abstract

Studies of the acid-catalyzed cleavage of 2-pyridyl and 4-pyridyl(amino)methylphosphonic acids demonstrate that protonation has a profound effect on the cleavage of C-P bonds. In H₂SO₄, and other strong mineral acids the cleavage of aminophosphonic acids 1 and 4 exhibits a kinetic depedence on Zucker-Hammett acidity function (h _o), with rate law, v = k[phosphonate]·h _o, and also fulfil the Bunnett approach on acidity function. The measured solvent isotope effect K _H/K _D is small, and varied from 1.46 for 1b to 1.18 for 4b. The isotope effect shows that protons are involved on the rate-determining step of the cleavage. All of this evidence indicates that the cleavage of 1 or 4 shows A-S_E2 character substitution reaction. Calculated activation parameters and some chemical experiments indicate that the protonated phosphonate molecule is split by a dissociative mechanism, combined with formation of a positive-charged phosphorus moiety.

Key Words:

• Acidity
• Catalysis
• Cleavage reactions
• Protonations
• Phosphonates
• Pyridines