The alkaline hydrolysis of several different phosphinate esters, R2P02R', was studied in 60 % dimethoxyethane-water or 33% dioxane-water. Rates are second order: v = k[ester][HO-]. Oxygen-18 experiments demonstrate that hydroxide attack occurs only at phosphorus and that the phosphinyl oxygen does not exchange I80 with solvent during hydrolysis. Activation enthalpies ranged from 12 to 16 kcal/mol, and activation entropies were near -30 gibbs. A comparison of rates for the strained, cyclic 2,2,3,4,4-pentamethyltrimethylenephosphinates with acyclic analogs shows acceleration of rate due to angle strain which indicates that the reaction involves a pentacoordinate intermediate. In methyl diisopropylphosphinate, an induction period preceded secondorder kinetics; the observed data for the rate ol hydrolysis fit a reaction mechanism with reversible formation of intermediate and rate-determining breakdown of intermediate to products. Rates were unusually sensitive to O-alkyl substituents: p* is 11 for (C6H5)2P02R and § is approximately 1. These unusual substituent effects appear to be due to the large change in the charge on the alkyl oxygen between ground state and transition state and to steric hindrance to solvation of the transition state which is an oxyanion. For acyl substituents in R2P02CH3, p* = 2.5 per substituent as is true in RC02C2H5, but = 2.5 which is considerably larger than for carboxylates. These results fit an addition-elimination mechanism for displacement at phosphorus in which the rate-determining step depends on the relative nucleophilicities of entering and leaving groups. In the alkaline hydrolysis of alkyl esters, the rate-determining step appears to be loss of alkoxide ion from the pentacoordinate intermediate.Nucleophilic displacement reactions at tetrahedral carbon and trigonal carbon have been studied extensively and reaction mechanisms are quite well understood.4 Displacement reactions at phosphorus have also received considerable attention, particularly the biochemically important phosphates and phosphonates." However, phosphorus being multivalent is subject to different energy constraints than carbon and many mechanistic details remain unclear. In order to have a better understanding of phosphorus chemistry, we have undertaken a broad study of the chemistry of functional groups, X, in phosphinates, R2-P(0)X.6 In this paper, we report our research on the alkaline hydrolysis of phosphinate esters,7 including hydrolyses in oxygen-18 enriched media which enable us to determine whether hydroxide attacks at alkyl carbon or acyl phosphorus and if the chemical dynamics along the reaction coordinate allow exchange of the (1) C.
The proton magnetic resonance spectra of several platinum-sulfide complexes have been studied at various temperatures. The rates of inversion at pyramidal sulfur are reported for cis-and mms-[(R2S)2PtCl2] complexes with three different sulfide ligands, trans complexes invert considerably more rapidly than cis complexes. Inversion at sulfur in these complexes is about 1018 times faster than inversion at sulfur in sulfoxides. The data are consistent with a mechanism for inversion in which the sulfur retains a distorted tetrahedral configuration and the platinum atom forms bonds alternately with the two sulfur lone pairs. The ratio of platinumproton coupling constants (Pt-S-C-H) is fairly constant: J,rans!Jcu = 0.82. The sensitivity of the coupling interaction to the nature of the trans ligand is discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.