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Title:Synthesis and kinetic studies of tricyclic phosphates and phosphonates: Effect of phosphorus-oxide stereochemistry on allylic rearrangements
Author(s):Rotto, Nelson Torr
Doctoral Committee Chair(s):Coates, Robert M.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Organic
Abstract:The role of allylic pyrophosphates in terpene biosynthesis is thought to involve S$\sb{\rm N}$1-type ionization leading to an ion-pair. In this work, kinetic studies are reported on tricyclic phosphates and phosphonates which were prepared from 11 and 12 step syntheses. First order rate constants for solvolysis (k$\sb{\rm s}$) and loss of optical activity (k$\sb\alpha$) were measured for 12a and 12b under solvolytic conditions (30% D$\sb2$O/70% acetone-d$\sb6$, 25$\sp\circ$C). For phosphate 12a, k$\sb\alpha$/k$\sb{\rm s}$ = 1.7 while for 12b, k$\sb\alpha$/k$\sb{\rm s}$ = 1. Phosphates 12a and 12b cleanly interconvert when heated in nitrobenzene at 145$\sp\circ$C. Rate constants for isomerization (k$\sb{\rm i}$) and loss of optical activity (k$\sb\alpha$) were measured, and for 12a k$\sb\alpha$/k$\sb{\rm i}$ = 295 while for 12b k$\sb\alpha$/k$\sb{\rm i}$ = 2.4. The rate ratios of each phosphate isomer are clearly influenced by the stereochemistry of the P-oxide group. A mechanism for solvolysis and interconversion involving symmetrical and unsymmetrical "bidentate" ion-pairs is discussed.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)
Solvent effects on the rearrangement were investigated. The relative polarimetric rates (k$\sb\alpha$) of 12a in mesitylene, bromobenzene, and nitrobenzene are 1.0, 1.7 and 6.2 respectively, and these rate increases seem to be consistent with an ion-pair mechanism. Substituent effects are also consistent with an ionic mechanism since phosphonate 13a gave a polarimetric rate (k$\sb\alpha$) in nitrobenzene 22 times slower than phosphate 12a. Furthermore, phosphonate 15a undergoes allylic rearrangement to 17a 239 times faster than the analogous rearrangement of 13a (CH$\sb3$/H rate ratio).
Unlike 15a and 17a which undergo relatively fast allylic interconversion upon heating in nitrobenzene, phosphonates 15b and 17b give predominately elimination under the same conditions. Again, the stereochemistry of the P-oxide group has a great effect on the chemical reactivity of the tricyclic phosphonates.
Issue Date:1989
Rights Information:Copyright 1989 Rotto, Nelson Torr
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9011003
OCLC Identifier:(UMI)AAI9011003

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