To ensure a realistic chemical model, we first considered the aggregation state of the organolithium reagents in solution. a-Silylated organolithium reagents are known to crystallise as polymeric, hexameric, 27 tetrameric, 28 dimeric, or monomeric structures -depending on the nature of the supporting ligand on lithium. 2•TMEDA has previously been reported to be monomeric in the solidstate, but it is not immediately clear if it remains monomeric in solution. Single crystals of 1 and 2•THF1.5 were obtained and shown to be tetrameric and dimeric in the solid-state respectively (Figure ). DOSY studies on C6D6 NMR samples of 1 and 2•THF1.5 support deaggregation in the presence of chelating ligands in solution. Hence while 1 and 2•THF1.5 possess diffusion coefficients consistence with retention of tetrameric and dimeric structures respectively, 1•PMDETA and 2•TMEDA diffuse at much slower rates and can be modelled as monomeric species (Figure ). Confident of the aggregation state of the organolithium reagent in solution, DFT calculations were carried out using B3PW91-D3 functional with energies corrected with the def2-TZVPP basis-set. Experimental conditions were modelled using Grimme's Goodvibes correction. Previous computational studies have concluded that HCF3 reacts with organolithium, 36 lithium silanides, and lithium boryl reagents through an initial deprotonation to generate a trifluoromethanide anion, which is subject to further SN2-type nucleophilic attack by a second equivalent of the lithium reagent. The reaction mechanism invokes two roles for the organometallic. A catalytic role as a general base to deprotonate HCF3 and protonate the intermediate, and a stoichiometric role in the SN2 step. Based on this precedent, it was assumed that 1•PMDETA reacts with HCF3 through an initial deprotonation and SN2 sequence to generate an organolithium intermediate Int-1 (Figure ). Two staggered conformers Int-1anti and Int-1gauche were identified which were almost identical in energy (DG°298K = 0.7 kcal mol -1 ). Int-1anti and Int-1gauche differ by the orientation of the sterically demanding Li•PMDETA and SiMe2Ph substituents about the carbon-carbon bond, adopting anti and gauche conformations respectively. From Int-1gauche a transition state, TS-1, for a concerted 1,2-silyl group migration and a-elimination of LiF was located (Figure ). TS-1 has a low energy barrier of DG ‡ 298K = 16.4 kcal mol -1 and is expected to be accessible at 25 °C. TS-1 connected directly to the experimentally observed product 3 with extrusion of LiF•PMDETA (DG°298K = -39.0 kcal mol -1 ). Further analysis of TS-1 provides additional insight. The reaction occurs with evolution of carbene character at the site of a-elimination. This carbene character is quenched by simultaneous 1,2-migration and formation of a C=C p-bond. In the transition state, both the aand b-carbon deform toward sp 2 -hybridisation. The migrating group adopts cationic character during the migration and is thus expected to follow known migratory aptitudes of SiR3 > H established for cationic 1,2-rearrangements. NBO calculations evidence charge depletion at the migrating group through comparison of the NPA charges, supporting the hypothesis that this group adopts cationic character as the reaction proceeds. The calculated mechanism is reminiscent of that proposed for the Fritsch-Buttenberg-Wiechell rearrangement -with the key difference being that TS-1 leads to construction of alkene rather than an alkyne functional group. A series of alternative pathways diverging from Int-1 were also considered but were calculated to be non-competitive (Figure ). For example, pathways involving a concerted 1,2-proton migration were located and give rise to two stereoisomeric fluoroalkene products through TS-2 (DG ‡ 298K = 26.1 kcal mol -1 ) and TS-3 (DG ‡ 298K = 33.8 kcal mol -1 ) respectively. Both were found to be higher in energy than the 1,2-silyl migration process. Similarly, the reprotonation of Int-1 by HCF3 was calculated to occur through TS-4 with a larger activation barrier (DG ‡ 298K = 27.5 kcal mol -1 ).