Trifluoromethyl‐Substituted Phenylsilanes: The Regiochemical Course of Their Metalation Dictated by Buttressing Effects

Abstract

Triethyl [(2-trifluoromethyl)phenyl]silane reacts with the superbasic LIC-KOR mixture of butyllithium and potassium tert-butoxide exclusively at the 4-position ("meta-metalation") and not at all at the 3-position ("ortho-metalation"). Two further substrates which simultaneously contain two trifluoromethyl groups, triethyl[2,4-and 2,5-bis(trifluoromethyl)phenyl]silane, undergo deprotonation at the 5-and

Substituent effects on the relative rates and free energies of ortho-lithiation reactions: families of fluorobenzenes as the substrates

“…In some cases (e.g. with X = SiMe 3 ), buttressing effects [34][35][36][37][38][39][40][41] (steric effects exerted by the X group through the fluoro group) 42 may compromise the reactivity of the 1,2-isomers 2. More importantly, the acidifying effect of substituents at meta positions is no longer the same as at ortho positions.…”

Proton Mobility in 2‐Substituted 1,3‐Dichlorobenzenes: “ortho” or “meta” Metalation?

“…Buttressing effects compromising the ortho ‐metalation of (2,6‐dibromophenyl)trialkylsilanes,1,2 (2,6‐dichlorophenyl)trialkylsilanes1,2 and even (2,6‐difluorophenyl)trialkylsilanes6 have been discovered a short while ago. So far, this mysterious phenomenon seemed to be restricted to substrates carrying very bulky substituents, in particular trialkylsilyl groups, as emitters and neighboring halogen atoms or trifluoromethyl groups as transmitters of steric pressure 7,8. It is surprising if not alarming to recognize now that clean ortho ‐metalatation is also menaced with substrates as simple as 1,3‐dichloro‐2‐ethylbenzene or 2,6‐dichloro‐ N , N ‐dimethylaniline if sec ‐butyllithium or other alkyllithium compounds serve as the reagent.…”

Selective Metalation of 4,6-Dibromoresorcinol Dimethyl Ether with LiTMP

“…9 A more subtle effect termed 'steric buttressing' has been invoked by Schlosser and co-workers to rationalize the novel metalation reaction shown in Scheme 3. 10 In the absence of the triethylsilyl group, metalation occurs ortho to the trifluoromethyl group.…”