Synthesis and reactivity of some β-substituted alkylphosphonates
The ability of the β-trimethylammonioethylphosphonic acid dianion to undergo fragmentation in alkaline conditions to produce trimethylamine, ethylene and metaphosphate was investigated. The results obtained were to give an indication of the susceptibility of (C₆H₃Cl₂)OCH₂CH₂N⁺R₂CH₂CH₂P(OR')O₂⁻ to analogous fragmentation to release 2-(3,4- dichlorophenoxy)ethyldialkylamine and ethylene, both well documented plant growth regulating hormones. Since the trimethylammonioethyl-substituted species was found to be stable to fragmentation, studies were embarked upon to convert this molecule into one which would react as required. The substitution of an alkyl group at the β-position of such a β-substituted alkyl phosphonate is reported to accelerate the rate of fragmentation. β-trimethylammoniopentylphosphonate was therefore expected to fragment where its ethyl analogue did not. Attempts at its synthesis were made by various approaches. 2- chloropentylphosphonic acid, a precursor, was successfully prepared and esterified (in order to protect the acid from premature fragmentation) so that it might be reacted with trimethylamine (a nucleophile), to produce diethyl β-trimethylammoniopentylphosphonate, which was then to be hydrolysed to the phosphonic acid form. Instead of the required substitution of Cl by Me₃N, the action of base on the ester produced diethyl 1-pentenylphosphonate. Only traces of the substitution product were observed. It was thought that the replacement of chlorine by a better leaving group such as bromine or iodine would favour the substitution product over the elimination product. Attempts at the synthesis of diethyl 2-iodopentylphosphonate and diethyl 2-bromopentylphosphonate by various means are described, but no satisfactory syntheses have been achieved, since once again the dehydrohalogenation product predominates. During an attempted synthesis of diethyl 2-iodopentylphosphonate by reaction of diethyl 2-chloropentylphosphonate with NaI, an unusual interaction between the sodium cation and the substrate in acetone solution was discovered. Evidence gleaned from ¹H, ¹³C and ³¹P nmr experiments points to the formation of a loose complex between the phosphonate part of the substrate and Na⁺. This phenomenon is also observed when other salts such as CaI₂, KSCN and NaClO₄ are present in solution.
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