Synthetic and mechanistic studies of electrophilic attack by the cations [Fe(CO)3(1–5-η-dienyl)]+(dienyl = C6H7 or 2-MeOC6H6) on aryltrimethyl-silanes and -stannanes

Abstract

Synthetic and kinetic studies are reported for the reactions of the organometallic cations [Fe(CO)₃(1–5-η-dienyl)]⁺(I)(dienyl = C₆H₇) and (III)(dienyl = 2-MeOC₆H₆) with a wide range of aryltrimethyl-silanes and -stannanes of the type XC₆H₄MMe₃(M = Si or Sn). Less extensive studies with related substrates (M = Pb or Ge), and with 2-trimethylsilyl-furan or -thiophen and allyltrimethylstannane, are also described. These provide novel routes to a variety of diene-substituted arenes and heterocycles, including the only current path to such derivatives of the less activated arenes XC₆H₅(X = MeO, MeS, Me, F, Cl, Br, or H). The general rate law, rate =k[Fe][XC₆H₄MMe₃], is rationalised in terms of electrophilic attack by cations (I) and (III) on the substrates XC₆H₄MMe₃. Similarly, with 2-trimethylsilylfuran the rate =k[Fe][2-Me₃SiC₄H₃O]. Linear free energy relationships reveal close similarities with related protiodemetallations. Rate data for the reactions of [Fe(CO)₃(1–5-η-C₆H₇)]⁺ with XC₆H₄SnMe₃ in CH₃CN are best fitted (r= 0.98) by the Yukawa–Tsuno equation, log (k_x/k_H)=ρ[σ+ 0.4(sigma;⁺–σ)], with a ρ value of –2.7. As expected, rates vary with the nature of the metal M in the order Pb Sn Ge Si. In general, the substrates XC₆H₄MMe₃ are considerably more reactive towards cations (I) than are the corresponding compounds XC₆H₅. The only exceptions are the species 4-Me₂NC₆H₄MMe₃(M = Si or Sn), which exhibit reactivities similar to Me₂NC₆H₅.