CONTENTS Tetra-amines

The contents of the test-tube are mixed and warmed at 55 °C for 1-5 h. The mixture is cooled and an aliquot portion is spotted on to a TLC plate for separation. Two-dimensional chromatography is carried out on silica gel layers with cyclohexane-ethyl acetate (1 1) and light petroleum-chloroform-diethyl ether-acetic acid (33 33 33 1). Chromatography on polyamide layers is accomplished with heptane-ethyl acetate-butanol (8 1 1). The Rp values of six NBD-amines in these systems [99] are given in Table 4.15. Amounts of less than 15 ng of NBD-amine per spot can be detected. HPLC of some NBD-amines has been carried out using Zipax coated with 0.5% 0,/3 -oxydipropionitrile and 1% tetra-hydrofuran in hexane as the mobile phase (see Section 4.2.4.2.2). 

Mechanistic Studies. - The mechanism of the reaction of tetra-zole-activated phosphoramidites with alcohols has been studied. A series of diethyl azolyl phosphoramidites (85) was prepared from diethyl phosphorochloridite and fully characterized, and the same compounds shown to be formed from the phosphoramidite (86) and azole. The degree of formation of (85) from (86) increases with the acidity of the azole, and the proposed mechanism is a fast protonation of (86), followed by a slow, reversible formation of (85) and a fast reaction of (85) with alcohols. Another study was concerned with the influence of amine hydrochlorides on the rate of methanolysis of the phosphoramidites (87) or (88), or tris(diethylamino)phosphine.The chloride content was measured to be 10-20 mM in doubly distilled samples which explains that "uncatalysed alcoholysis is possible. Intensive purification, including treatment with butyllithium and distillation from sodium, brought the chloride content down to 0.1-1 mM. The methanolysis reaction, in methanol as the solvent, was found to be first-order in catalyst concentration. An aJb initio calculation on N- and P-protonated aminophosphine (89) gave similar proton affinities for N and P this contrasts with earlier MNDO calculations which had ff-protonated species as the most stable. The M-protonated compound had an electronic structure reminiscent of a phosphenium ion-ammonia complex. 

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