Abstract
From calculations of a model reaction scheme for base-catalyzed RNA
hydrolysis (which also represents the base-catalyzed methanolysis of
ethylene phosphate monoanion in reverse), a pentacoordinate dianionic
intermediate 2a (Storer et al. J. Am. Chem. Soc. 1991, 113, 5216-5219)
as well as two transition states, TS1 and TS2, to the intermediate have
been located by ab initio calculations at the 3-21G* level. However, the
intermediate, which has a well depth on the order of k(B)T, is unlikely
to be kinetically significant. The endocyclic P-O(2') bond is found to
be much weaker than the exocyclic P-O(5') bond. In agreement with this
finding, calculations on 2a at the 6-31+G* level abolishes TS1 and the
pentacoordinate intermediate, leaving only TS2 as the sole transition
state. Thus, for all the cases examined, the rate-limiting
transition-state structure is TS2 which has an extended P-O(5') breaking
bond. These results and the mode of cleavage of a simpler compound 3b
are in accord with stereoelectronic predictions (see text for the
definition). Moreover, solvation appears to stabilize the
pentacoordinate intermediate. In the gas phase, the simplest
oxyphosphorane 3b has the least tendency to form a pentacoordinate
intermediate. However, 3b does form a pentacoordinate intermediate when
it is solvated with six water molecules. These results support the
hypothesis that phosphoryl-transfer reactions take place via
pentacoordinate intermediates not only in acidic but also in basic
media.
Original language | English |
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Pages (from-to) | 3009-3017 |
Journal | Journal of Organic Chemistry |
Volume | 58 |
Issue number | 11 |
DOIs | |
Publication status | Published - 1993 |
MoE publication type | A1 Journal article-refereed |