Comparison of Cu(II)-Promoted Leaving Group Stabilization of the Cleavage of a Homologous Set of Phosphate Mono-, Di-, and Triesters in Water, Methanol, and Ethanol
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- 作者：Mark A. R. Raycroft ; C. Tony Liu ; R. Stan Brown
- 刊名：Inorganic Chemistry
- 出版年：2012
- 出版时间：March 19, 2012
- 年：2012
- 卷：51
- 期：6
- 页码：3846-3854
- 全文大小：405K
- 年卷期：v.51,no.6(March 19, 2012)
- ISSN：1520-510X
文摘
The cleavage of a set of phosphate mono-, di-, and triesters having a Cu(II)-complexed 2-phenanthrolyl group at the ortho-position of a departing phenoxide was studied in water and ethanol. Experimentally observed pH/rate profiles, solvent deuterium kinetic isotope effects, and activation parameters are compared with those obtained in methanol. The pH/rate profile in each solvent exhibits an extended plateau due to solvent attack on forms designated as [Cu(II):1b/c]0 for the monoester, [Cu(II):2b]+, for the diester, and [Cu(II):3a]2+ for the triester. The solvent dkie values (kH/kD) for the three complexes are 0.91, 0.95, and 0.83 for decomposition of [Cu(II):1b/c]0 in water (W), methanol (M), and ethanol (E), 1.22, 1.09, and 1.29 for [Cu(II):2b]+ in W, M, and E, and 1.94, 2.2, and 1.96 for [Cu(II):3a]2+ in W, M, and E. Near unit, or slightly inverse values for the monoester are taken as evidence for little involvement of solvent in a highly dissociative TS for P鈥揙Ar cleavage, with slightly higher solvent dkie values for the diester signifying the onset of some solvent participation in assisting the nucleophilic displacement. The larger primary dkie for the triester gives evidence for a solvent-assisted delivery of ROH in the cleavage through a more associative mechanism. Activation parameters for each substrate in the solvents are compared, indicating that the transition from methanol to ethanol for each substrate involves a near cancellation of the 螖螖H鈥?/sup> and 鈥?i>T螖螖S鈥?/sup> values (25 掳C) so that the respective rates in both solvents are very similar. The transition from alcohol to water produces variable effects, with 螖螖H鈥?/sup> and 鈥?i>T螖螖S鈥?/sup> values canceling for cleavage of the triester and being additive for the mono and diester, explaining their 100鈥?00 rate reduction in passing from methanol to water. The rate enhancing effects of the Cu(II)-promoted leaving group assistance in all three solvents are substantial and estimated at 1012鈥?015 for the monoester, 1012鈥?014 for the diester, and 105 for the triester relative to their background reactions.