Alkynophilicity of Group 13 MX₃ Salts: A Theoretical Study

Abstract

The concept of alkynophilicity is revisited with group 13 MX 3 metal salts (M = In, Ga, Al, B; X = Cl, OTf) using M06-2X/6-31+G(d,p) calculations. This study aims at answering why some of these salts show reactivity toward enynes that is similar to that observed with late-transition-metal complexes, notably Au(I) species, and why some of them are inactive. For this purpose, the mechanism of the skeletal reorganization of 1,6-enynes into 1-vinylcyclopentenes has been computed, including monomeric ("standard") and dimeric (superelectrophilic) activation. Those results are confronted with deactivation pathways based on the dissociation of the M−X bond. The role of the X ligand in the stabilization of the intermediate nonclassical carbocation is revealed, and the whole features required to make a good π-Lewis acid are discussed.

Theoretical Investigation of the Mechanism of the Hock rearrangement with InCl3 as Catalyst

“…8.8 kcal/mol vs. 11.0 kcal/mol for substrate 5a). Therefore, unlike previous studies highlighting superelectrophilic dimeric species such as A2X6 or AX2 + AX4among the group 13 elements, 33,35,44 our study shows the absence of superelectrophilicity for the dimeric In2Cl6 species during this Hock rearrangement. These species are however central in our catalytic cycle, serving as a reservoir of active monomeric catalytic species.…”

“…Interestingly, IXa can reorganize to structure Xa bridged by a single chlorin atom between the two indium atoms, thus adopting the Cl2In-Cl-InCl3 scaffold. Unexpectedly, unlike Ga2Cl6 catalysis displaying gallium superelectrophilic species during the reorganization of 1,6 enynes 33 or the methylation of benzene, 44 the dimeric species Cl2In-Cl-InCl3 is not a superelectrophile able to promote the Hock rearrangement. Indeed, not only Xa is not significantly more stable than IXa (-0,4 kcal/mol), but more importantly the activating barrier for the Hock rearrangement is in this case higher (+11.0 kcal/mol from Xa to TSXa-XIa) than with monomeric InCl3 (+8.8 kcal/mol from Ia to TSIa-VIa).…”

“…Recent studies showed that the catalytically active species of Lewis acids of type AX3 (A = Al, Ga, Fe, In and X = H, Cl) are in the dimeric form A2X6, or in the form of a superelectrophile like X2A-X-AX3. [33][34][35]43,44 The Hock rearrangement of 5a was thus investigated in the presence of dimeric InCl3 (Figure 4, not all intermediates are presented, see Supporting information for details). The dimerization of InCl3 gives rise to a stable homodimer (In2Cl6) favored by 34.5 kcal/mol.…”

“…The relationship between the structure of the hydroperoxide substrate and the efficiency of the reaction remains underinvestigated. The active form of the Lewis acid catalyst InCl3 is another point of interest, as it can potentially act in a monomeric or dimeric form, [30][31][32][33] or as a cationic InCl2 + species, 34 which raises the question of its superelectrophilicity. 35 Herein, we wish to fill these gaps by reporting the first computational study of the Hock rearrangement at the DFT level.…”

Halogen Bond Catalysis on Carbonyl–OlefinRing‐ClosingMetathesis Reaction: Comparison with Lewis Acid Catalysis

“…Although BCl 3 shows good catalytic performance, it is reluctant to pyramidalization, [ 17 ] and the cyclization products will not converge in the form of BCl 3 adducts. [ 18 ] The spontaneous transfer of chlorine to the carbon cation center leads to the formation of alkydichloroboron. This group transfer has been reported in the case of cyclopropanation/carboboration of enynes promoted by B(C 6 F 5 ) 3 , [ 19 ] so the yield of olefin metathesis catalyzed by BCl 3 is not high in the experiment.…”