Phosphoramidite alcoholysis

B. Studies of Equilibria and Reactions.—N.m.r. spectroscopy is being increasingly employed to study the mode and course of reactions. Thus n.m.r. has been used to unravel the mechanism of the reaction of phosphorus trichloride and ammonium chloride to give phosphazenes, and to follow the kinetics of alcoholysis of phosphoramidites. Its use in the study of the interaction of nucleotides and enzymes has obtained valuable information on binding sites and conformations and work on the line-widths of the P resonance has enabled the calculation of dissociation rate-constants and activation energies to be performed. 

Mechanistic Studies.- The kinetics and mechanism of alcoholysis of some cyclic phosphoramidites (126)-(129) has been studied in... 

The function of the acidic catalyst in alcoholysis reactions of phosphoramidites has not been elucidated yet, but low reactivity of... 

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. 

Triphosphates (13) may be formed using O-ethyl or O-isopropyl phosphoramidites. These esters do not undergo hydrolysis by concentrated aqueous ammonia solution applied for deprotection after synthesis. Another option for their synthesis involves use of 3 -0-phos-phorodimorpholidites, which are subjected to alcoholysis prior to the final oxidation step [90]. By choice of specific phosphoramidites this technology may be used to insert conjugated functional groups into an oligonucleotide. 

The P-N bond in phosphoramidites is more unstable than P-O bonds toward cleavage reactions. Alcoholysis of phosphoramidites is therefore one of the most useful synthetic methods for the construction of a P-O bond. The reaction is supported by acids. On the other hand, this feature is responsible for the high hydrolysis sensitivity of several phosphoramidites, which may cause serious problems when applied in hydroformylation. 

Phosphoramidites undergo alcoholysis in the absence of acids only in protic solvents [42]. The rate depends on the pK, value of the alcohol. l//,l//-Heptafluorobutanol [pK j o) = 12]. with similar acidity as phenol [P arHjO) 11 ] replaces the amino group [43] in contrast to tert-butyl alcohol [P arHjO) = 19.2], which does not react [44]. An aniline substituent was displaced faster than a more basic alkyl amine. This result could be a counter-argument against the first protonation of the amine, but it should be kept in mind that the aniUnium salt is more acidic than the ammonium salt containing the amidite [45]. 

In oligonucleotide chemistry, alcoholysis of phosphoramidites is widely used for the construction of the polymers from monomeric sugars (Scheme 2.136) [46]. The reaction is usually catalyzed by Lff-tetrazol (TetH), which in the first step protonates the amino group. Subsequently, the tetrazole anion replaces the amine. The dinucleoside phosphite is formed by the final reaction with the alcoholic group of a second nucleoside. Since the formation of the tetrazole phosphoramidite from the intermediate salt is rate-determining, the catalyst... 

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