Buttressing effects responsible for the meta-deprotonation of trialkylsilyl-substituted haloarenes by organometallic reagents an unprecedented phenomenon

(2,6-Dichlorophenyl)trimethyl- and triethylsilane exhibit untypical reactivity patterns toward strong bases: when treated at -100 °C with lithium 2,2,6,6-tetramethylpiperidide two, with sec- or tert-butyllithium even three organometallic intermediates are generated and were trapped with dry ice. One of these intermediates carries the metal at the halogen-remote 4-position, the other at the 2-position and the third one carries the metal at the halogen-adjacent 3-position. At -75 °C, it spontaneously eliminates lithium chloride thus setting free a didehydrobenzene ("aryne") which is trapped by still intact aryllithiums. The behavior of the aryne toward nucleophiles was studied. The dibromo analog gave rise to two intermediates when treated with lithium amide-type bases. One carries the metal at the 4-position and the second one at the 2-position. Three (2,6-difluorophenyl)trialkylsilanes were prepared, metalated, carboxylated and analyzed to find out what is the role of the smallest halogen in the transmission of the buttressing effects. In all these cases, the buttressing effects were extremely weak compared with (2,6-dichlorophenyl)triethylsilane and (2,6-dibromophenyl)triethylsilane. The metalation at the halogen-remote 4-position was only perceptible with (2,6-difluorophenyl)-triisopropylsilane. Eight 1,3-dichlorobenzenes congeners were selected as model compounds to assess the relative rates of proton abstraction from the 4- and 5-position ("ortho" vs. "meta" metalation). Using sec-butyllithium in tetrahydrofuran at -75 °C, attack at the chlorine adjacent 4-position proved to be always predominant. However, attack at the halogen-remote 5-position became significant when bulky or even just moderately sized substituents such as methoxy, dimethylamino or ethyl occupied the 2-position, 4-/5- ratios ranging then from 97 : 3 to 75 : 25. Competitive reactions between (2-fluorophenyl)trimethylsilanes and fluorobenzenes were performed in order to prove that a trimethylsilyl group reduces the reactivity compared to the non silylated compound. Another series of competitive experiments have shown the same for the chlorinated analogs. Finally, equilibration reactions have demonstrated that the phenomenon of buttressing effects has no thermodynamic origin. Is is a kinetic phenomenon, in other words its appearance is restricted to transition states.

Schlosser, Manfred
Lausanne, EPFL

Note: The status of this file is: EPFL only

 Record created 2005-03-16, last modified 2020-10-28

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