Kinetics and mechanism of tetrazole-catalyzed phosphoramidite alcoholysis



: Nurminen EJ, Mattinen JK, Lonnberg H

Publisher: ROYAL SOC CHEMISTRY

: 1998

Royal Society of Chemistry. Journal: Perkin Transactions 2

JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2

: J CHEM SOC PERK T 2

: 7

: 1621

: 1628

: 8

: 0300-9580

DOI: https://doi.org/10.1039/a801250d


Kinetics of the tetrazole-catalyzed reaction of diisopropyl N,N-diisopropylphosphoramidite (Ib) with tert-butyl alcohol has been studied by P-31 NMR spectroscopy in THF, and the results obtained have been compared to those observed for the possible partial reactions involved, viz, the formation of diisopropyl tetrazolylphosphite (2b) and its subsequent alcoholysis, The stoichiometry of the processes was first examined with dimethyl N,N-diisopropylphosphoramidite (la) in MeCN. The tetrazole-promoted disappearance of Ib is as fast in the absence and in the presence of the alcohol: the alcoholysis of Ib is zero-order in the concentration of alcohol and second-order in the concentration of tetrazole, The reaction of Ib with tetrazole is independent of the concentration of the tetrazolide anion and second-order in that of tetrazole, while the reverse reaction, aminolysis of 2b is first-order in the concentration of the amine, The alcoholysis of 2b is, in turn, brst-order in the concentration of alcohol and second-order in that of tetrazole, but it also proceeds, although slowly, in the absence of tetrazole, The time-dependent product distribution of the alcoholysis of Ib shows intermediary accumulation of 2b, but at a lower level than could be predicted by applying the rate constants determined independently for the assumed partial reactions. Accordingly, tetrazole-catalyzed alcoholysis of Ib is shown to proceed at least mainly via 2b, but an additional pathway not involving 2b as an intermediate is proposed, Mechanisms of the partial reactions are discussed on the basis of the formal kinetics observed.



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