New insight into the chemistry of allymetals

The present thesis work provides new insight into the generation of allylmetal species and into their reactivity toward electrophiles. endo-Allylpotassiums are known to be more stable than their corresponding exo-isomers. The ring strain of methylenecycloalkanes was used to modulate the relative stabilities of endo- and exo-(cycloalk-1-enyl)methylpotassiums. A series of methylenecycloalkanes was metalated with the superbasic "LIC-KOR" mixture of butyllithium and potassium tert-butoxide in tetrahydrofuran at -75 °C or in hexanes at 25 °C, in other words under conditions precluding any torsional isomerization. The eight-membered ring afforded exclusively the unstrained exo-isomer and the open-chain reference the priviledged endo-isomer. However, mediocycles revealed a strikingly strong solvent effect, favoring the exo-isomers in hexanes and the endo-isomers in tetrahydrofuran. Pinenylmetals were then allowed to react with various electrophiles. Pinenylpotassium reacted mostly, if not exclusively (as in the case of iodomethane and carbon dioxide), at the exocyclic position. The opposite selectivity was observed with pinenylmagnesium bromide which reacted mainly at the endocyclic position (with one exception, halotrialkylsilanes). Pinenyllithium gave mixtures of the two possible regioisomers, except with trimethylvinylsilane which was added exclusively at the endocyclic position. As shown by competition experiments, oxirane, and presumably other electrophiles, dock at the metal-bearing face of typical allylpotassium species. Whereas the nucleophile thus favors a retention mode, epoxides react with inversion of the configuration. A "conveyer belt" (or "closed loop") mechanism appears to be the most plausible way to combine the geometrical requirements of the retention and inversion processes. In hexanes, allylpotassium species were found to react with iodotrimethylsilane faster than with bromo-, chloro- and fluorotrimethylsilanes. An SN2-like reaction pathway was postulated. In tetrahydrofuran, the halide had no effect on the reaction rates. A diffusion-controlled formation of a pre-complex as the rate-limiting step of the reaction was considered as a possible explanation. Finally, substituent effects on the reaction of benzaldehydes with 2-methylallylpotassium were determined. Amazingly, electron-rich substituents increased the reaction rate and electron-withdrawing groups decreased it. Interactions between the metal and the electrophile at the transition state were postulated to rationalize this finding.


Advisor(s):
Schlosser, Manfred
Year:
2005
Publisher:
Lausanne, EPFL
Other identifiers:
urn: urn:nbn:ch:bel-epfl-thesis3250-1


Note: The status of this file is: EPFL only


 Record created 2005-04-06, last modified 2018-05-01

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