The synthesis and metal ion catalysed hydrolysis of methyl 4,5-diaminopentanoate

Abstract

The diamino acid ester, methyl 4, 5-diaminopentanoate has been prepared for the first time. The successful five step synthesis starting from 4-hydroxybutanoic acid lactone, and two other possible pathways via ethyl 3-formylpropanoate, are described. pKₐ values for 4,5-diaminopentanoic acid dihydrochloride and its a methyl ester dihydrochloride (determined by potentiometric titration at 25°C and I= 0.10molℓ⁻¹) are discussed in relation to those for other 1,2-diamines. Stability constants of Cu(II) complexes of 4,5-diaminopentanoic acid and its methyl ester were evaluated from potentiometric titration data by the program SCOGS on a PDP 11/70 computer. Both compounds form high stability constant 1:1 and 1:2 complexes with Cu(II). (For 4,5-diaminopentanoic acid log K₁ = 10.94, log K₂ = 9.40 and for methyl 4,5-diaminopentanoate log K₁ = 9.84, log K₂ = 8.51, at 25°C and I= 0.10molℓ⁻¹). A variety of protonated complexes were also present at lower pH. pH-stat and spectrophotometric investigations showed that in the alkaline hydrolysis of methyl 4,5-diaminopentanoate, the simple Bᴀс2 reaction is slow and masked by a competing, extremely rapid, pair of intramolecular aminolysis (lactamisation) reactions. These involve competition between five and six membered lactam ring closure. Overall constants of the order 10³ℓmol⁻¹s⁻¹ at 25°C and I = 0.10molℓ⁻¹ are observed. The Cu(II) catalysed hydrolysis of methyl 4,5-diaminopentanoate was studied at constant pH using a pH-stat. The kinetics of hydrolysis of solutions containing a 1:2 mole ratio of Cu(II): ester are consistent with the two consecutive second order (pseudo-first-order at constant pH) reactions (1) and (2) (where E is the ester and A⁻ the anion, of 4 ,5-diaminopentanoic acid). CuE₂²⁺ + OH⁻ → CuEA⁺ + CH₃OH 1 CuEA⁺ + OH⁻ → CUA₂ + CH₃OH 2 The relative values for the two rate constants obtained (k (CuE₂²⁺) = 2.41ℓmol⁻¹s⁻¹ and k (CuEA⁺) = 0.80ℓmol⁻¹s⁻¹ at 25°C and I = 0.10molℓ⁻¹) can be explained by charge and statistical effects. The value of k(CuEA⁺) was confirmed independently from studies on solutions containing a 2.5:1:4 mole ratio of Cu(II): ester: acid. The value of k(CuEen²⁺) = 0.96ℓmol⁻¹s⁻¹ obtained from solutions containing Cu(II): ester: 1,2-diaminoethane = 2.5:1:4 is approximately k(CuE₂²⁺)/2 as expected statistically. 1:1 Solutions of Cu(II) and methyl 4,5-diaminopentanoate disproportionate, so no values for k(CuE²⁺) could be measured. The absolute values for the Cu(II) catalysed rate constants are discussed in comparison with those for methyl 2,3-diaminopropanoate and other diamino acid esters. The results for methyl 4,5-diaminopentanoate can be explained by the attenuation of inductive and positive charge effects of the metal ion by the additional methylene groups of the methyl 4,5-diaminopentanoate alkyl chain. Direct methoxycarbonyl-metal ion interaction, presumably absent in methyl 4,5-diaminopentanoate must, therefore, be unimportant in the Cu(II) catalysed hydrolysis of methyl 2,3-diarninopropanoate.