The chemistry of heteroarylphosphorus compounds. Part VI. Alkaline hydrolysis of 1-methylpyrrol-2-yl- and 1-methylpyrrol-2-yl-methyl-phosphonium salts. A comparison with 2-furyl, 2-thienyl, phenyl, and related heteroarylmethyl and benzyl derivatives. Relative stabilities of forming carbanions

Abstract

Studies of the products of alkaline hydrolysis of phenyl- and benzyl-phosphonium salts bearing 1-methylpyrrol-2-yl substituents show that while the forming 1-methylpyrrol-2-yl carbanion is more stable than the forming phenyl carbanion, in contrast to the 2-furyl and 2-thienyl analogues it is less stable than the forming benzyl carbanion. The order of cleavage of heteroaryl substituents from phosphorus is established as 2-thienyl > 2-furyl > benzyl > 1-methylpyrrol-2-yl > phenyl. Kinetic studies show that methyltris-1-methylpyrrol-2-ylphosphonium iodide (VII) undergoes hydrolysis considerably more slowly than the 2-furyl- and 2-thienyl analogues, but slightly faster than methyltriphenylphosphonium iodide. The ability of the 1-methylpyrrol-2-yl substituent to stabilise a forming negative charge on an adjacent carbon has been investigated by studies of the kinetics of hydrolysis of (1-methylpyrrol-2-ylmethyl)triphenylphosphonium iodide (XI). Comparison of the rate data with those for the 2-furylmethyl-, 2-thenyl-, and benzyl analogues indicates that the relative order of stability of the forming 2-heteroarylmethyl carbanions is 2-furylmethyl > 2-thenyl > benzyl > 1-methylpyrrol-2-ylmethyl.