Mapping transition state structures for thiophosphinoyl group transfer between oxyanionic nucleophiles in water and aqueous ethanol solvents

Abstract

Second-order rate constants were measured for thiophosphinoyl group transfer from the substrates 3a-g to oxygen nucleophiles in 50% water-50% ethanol and 30% water-70% ethanol mixtures, to obtain solvent-independent Brønsted coefficients...

Solvent and solvation effects on reactivities and mechanisms of phospho group transfers from phosphate and phosphinate esters to nucleophiles

“…Some recent work, especially from our laboratories ( Buncel et al, 2004 ; Onyido, et al, 2005a ; Onyido, et al, 2005b ; Kalu, et al, 2021 ; Kalu, et al, 2022 ), discussed in greater detail in Section 3 below, complementing earlier results in the literature ( Haake et al, 1968 ; Douglas and Williams, 1976 ; Williams and Naylor, 1976 ; Eliseeva, et al, 1978 ; Istomin et al, 1982 ; Cook and Rahhal-Arabi, 1985 ; Bourne, et al, 1988 ; Dunn and Buncel, 1989 ; Dunn et al, 1990 ; Pregel, et al, 1995 ; etc. ), demonstrates a change in the mechanism of phospho transfer from aryl phosphinates and phosphinothioates of the types shown as 6 to oxyanionic nucleophiles (HO − and PhO − ), from a concerted mechanism to an associative one, when solvent is changed from water to the less polar ethanol.…”

“…The reaction was first investigated by the same group, Istomin and Eliseeva (1981) to obtain a Hammett ( ) value of 1.72 as evidence for an associative mechanism. A Brönsted linear free energy relationship (LFER) reinvestigation of the same reaction system ( Kalu et al, 2022 ) yielded the Brönsted exponents = 0.35 and = −0.64 and the Hammett parameter = 1.54 (see entry 15b), which are consistent with a concerted reaction with substantial dissociative character. As in water, the reactions of this substrate with moderately to strongly basic alkoxides as well as with phenoxide ion and its weaker analogues were all shown to proceed by the concerted mechanism in 50% water-50% ethanol ( Kalu et al, 2022 ).…”

“…The transfer of the phosphoryl group (PO 3 ) from phosphate esters to nucleophilic entities and fragments at the cellular level is a fundamental process in biology, which plays indispensable roles in the biochemistry of life, such as bioenergy transductions, signal transductions, and in genome processing ( Knowles, 1980 ; Westheimer, 1987 ; Cleland and Hengge, 2006 ; Lassila, et al, 2011 ). It is the mechanistic aspects of the solution chemistry of these compounds, which are uncatalyzed reactions, that attention has been focussed on in our laboratories ( Omakor, et al, 2001 ; Buncel et al, 2004 ; Onyido, et al, 2005a ; Onyido, et al, 2005b ; Kalu, et al, 2021 ; Kalu, et al, 2022 ), which forms the base for this review.…”

“…General solvent effects have been empirically parameterized ( Catalán, 2009 ) by Kosower’s Z scale ( Kosower, 1958a ; 1958b ; 1958c ), Kamlet, Abboud and Taft’s dipolarity and polarizability scale ( Taft et al, 1985 ), Brooker’s scale ( Brooker et al, 1965 ), Dimroth-Reichardt’s (30) scale ( Dimroth et al, 1963 ; Reichardt, 1994 ), Buncel’s scale ( Buncel and Rajagopal, 1989 ; 1990 ), etc. The dielectric constant of the solvent has also been deployed in correlating rates of reactions with solvent polarity ( Moelwyn-Hughes, 1936a ; 1936b ; Laidler and Eyring, 1939 ; Laidler and Landskroener, 1956 ; Kalu, et al, 2022 ). On the other hand, Kamlet and Taft’s and scales ( Kamlet and Taft, 1976a ; 1976b ), Arnett’s scale ( Arnett, et al, 1974 ; 1976 ), Catalán’s SA and SB scales ( Catalán, 2009 ), etc.…”

“…), demonstrates a change in the mechanism of phospho transfer from aryl phosphinates and phosphinothioates of the types shown as 6 to oxyanionic nucleophiles (HO − and PhO − ), from a concerted mechanism to an associative one, when solvent is changed from water to the less polar ethanol. Lowering the polarity of the medium by the incremental addition of ethanol to the aqueous solution ( Kalu et al, 2021 ; Kalu et al, 2022 ) enabled the tracking of the loosening of the concerted TS to looser TS structures, until a solvent-independent concerted TS was attained in ethanol-rich aqueous ethanol solvents. The fact that the reactions proceed via two different mechanisms in water or aqueous ethanol solvents on the one hand and in pure ethanol solvent on the other hand, shows that the potential energy surfaces are different in the two solvent systems.…”