Activation of Pentafluorophenylsilanes by Weak Lewis Bases in Reaction with Iminium Cations

Abstract

The Lewis base mediated carbon-carbon bond forming reactions between pentafluorophenylsilanes and iminium cations were studied theoretically and experimentally. The complexation of silanes with anionic Lewis bases was analyzed computationally using DFT methods at the B3LYP/6-31+G(d) level. The pentafluorophenyl group was found to exhibit a significant stabilizing effect on the formation of pentacoordinate silicon species, with (C6F5)3SiF and C6F5SiF3 being the strongest Lewis acids. Comparison of geometrical isomers of trigonal bipyramidal siliconates R2(C6F5)SiXY- (R = Me, F, Cl; X, Y = F, Cl, ClO4) revealed that the heteroatoms and the C6F5 group prefer to occupy apical and equatorial positions, respectively. Calculations of C6F5 group transfer processes from silanes to N,N-dimethyliminium cation lead to the following conclusions: (1) when employing weak Lewis bases X- = Cl-, ClO4- as activators, R2(C6F5)SiF are more reactive than R2(C6F5)SiX (X = Cl, ClO4); (2) the C6F5 group is much more reactive in an apical position than in an equatorial position of the siliconate; (3) a solvent greatly increases the reaction barrier by stabilizing the reactants; and (4) the efficiency of C-C bond formation can be improved by rendering the process intramolecular. It was shown experimentally that reactions of (C6F5)3SiF with PhCH=NMe2+TfO- can be promoted by a series of weak Lewis bases, from which the chloride anion was the most effective. Even more facile was the reaction of N-(silyloxyethyl)iminium ion (C6F5)3SiOCH2CH2N(Me)=CHPh+TfO-, likely proceeding via intramolecular transfer of a C6F5 group. The optimal conditions for pentafluorophenylation of iminium ions involved the use of benzyltriethylammonium chloride in refluxing dichloroethane.

Comparison of σ‐/π‐Hole Tetrel Bonds between TH3F/F2TO and H2CX (X=O, S, Se)

“…It has been accepted that a Group IV atom particularly a silicon atom is taken as a Lewis acid to bind with a base . In silicon‐containing molecules with Si−O−N units, there often exists an intramolecular Si⋅⋅⋅N interaction, which increases the basicity at the oxygen atom through a narrow Si−O−N angle and thus two such molecules are bonded by an intermolecular Si⋅⋅⋅O interaction .…”

Reaction of Baylis–Hillman Adducts with Fluorinated Silanes

“…For example, silanes Me 2 Si(C 6 F 5 ) 2 and Me 3 SiC 6 F 5 reacted readily but provided complex mixtures, whereas the use of (EtO) 3 SiC 6 F 5 afforded lower yields. Notably, the use of fluorosilane FSi(C 6 F 5 ) 3 , which has been shown to be the most effective silane in reactions mediated by weak Lewis acids,11 was completely unproductive.…”

“…Another important variable is the nature of the silicon reagent. Indeed, we have previously shown that the efficiency of reactions involving silanes is strongly influenced by the number of perfluorinated and heteroatom groups borne by the silicon 11. In the case of the trifluoromethyl group, the only available silane is Me 3 SiCF 3 (Ruppert–Prakash reagent).…”

Reactions of Aldehydes and Ketones and their Derivatives

“…57 Silane reagents, (F 5 C 6 ) 4−n SiF n (n = 1, 3) have been investigated, particularly with reference to their activation by weak Lewis bases. Pentafluorophenyl significantly stabilizes pentacoordinate silicon species, but as a group it is much more reactive in an apical than an equatorial position.…”

“…166 Using Lewis acid catalysts in DCM at −78 • C, highly selective versions of this reaction have been developed: with scandium(III) triflate, near-quantitative yields with an E:Z ratio of 1:20 result, and a chiral (salen)chromium(III) auxiliary gave 92% ee. (57) (60) is a bifunctional organocatalyst: in addition to the phenolic Brønsted acid groups, it has a Lewis base unit attached via a spacer moiety. 167 This ee particular combination holds the groups in a conformational lock, where they can doubly activate a substrate while giving a high level of stereocontrol.…”