Mechanism of addition of aryltrimethyl-silanes and -stannanes to tricarbonyl(cyclohexadienyl)ruthenium(II) cation

Abstract

Kinetic studies of the reaction between the organometallic complex [Ru(CO)₃(1–5-η-C₆H₇)]BF₄(1) and aryltrimethyl-silanes and -stannanes of the type XC₆H₄M′Me₃(M′= Si or Sn; X = H, 4-NMe₂, 4-OMe, 1 -OMe, 4-Me, 4-SMe, 4-F, 4-Cl, 4-Br, 3-F, or 3-CF₃) reveal the rate law, rate =k[ Ru][XC₆H₄M′Me₃]. This rate law has been rationalised in terms of a rate-determining electrophilic attack by complex (1) on the XC₆H₄M′Me₃ substrates to form Wheland type Σ-complex intermediates of the type [Ru(CO)₃{1–4-η-C₆H₇(XC₆H₄M′Me₃)}](2) followed by cleavage of the Me₃M–C(dienyl) bond to give the new compounds [Ru(CO)₃{1–4-η-C₆H₇(C₆H₄X)}](3). These reactions are of considerable synthetic interest as they offer useful routes to otherwise inaccessible substituted benzenoid derivatives. For the Sn reactions, a Hammett plot of log (k_X/k_H)versusΣ(the standard Hammett substituent constant) is linear with a slope of –2.9 suggesting a common mechanism for each of the reactions. A plot of log (k_X/k_H) for the reactions of (1) with XC₆H₄SnMe₃ in MeNO₂(at 45 °C)versus log (k_X/k_H) for the protiodemetallation of XC₆H₄Sn(cyclo-C₆H₁₁)₃ by HClO₄ in aqueous ethanol (at 50 °C) is also linear indicating that organometallic complexes such as (1) behave in a similar manner to more classical electrophiles.