Low-coordinate potassium alkoxide – an efficient trap for arenes: the role of ηn non-covalent bonding in substrate activation for C–H bond metalation

Abstract

Metalation of bulky tris(2-(piperidin-1-yl-methyl)phenyl)methanol [(C₅H₁₀N)CH₂C₆H₄-o]₃COH with (Me₃Si)₂NK in Et₂O results in a dimeric potassium alkoxide {[(C₅H₁₀N)CH₂C₆H₄-o]₃C(μ²-O)K(Et₂O)}₂ (1). The Et₂O molecule can be removed from the K⁺ coordination sphere affording coordinatively unsaturated alkoxide species which readily traps π-donor molecules. In the presence of excess arene, the reactions result in ηⁿ–π-complexes, retaining in the crystal state a dimeric core {[(C₅H₁₀N)CH₂C₆H₄-o]₃C(μ²-O)K(ηⁿ-arene)}₂ (arene = C₆H₆ (2), CH₃C₆H₅ (3), C₁₀H₈ (4)). With C₆H₅OMe and C₆H₅NMe₂ molecules containing competing n- and π-donating sites, the reactions proceed differently: the former coordinates to K⁺ through an oxygen lone pair resulting in {[(C₅H₁₀N)CH₂C₆H₄-o]₃C(μ²-O)K(κ¹-O(Me)C₆H₅)}₂ (5) while for the latter, π-arene interaction turns out to be preferable, yielding {[(C₅H₁₀N)CH₂C₆H₄-o]₃C(μ²-O)K(η²-C₆H₅NMe₂)}₂ (6). The reactions with equimolar amounts of benzene or thiophene afford coordination polymers [{[(C₅H₁₀N)CH₂C₆H₄-o]₃C(μ²-O)K}₂(μ-C₆H₆)]_n (7) and [{[(C₅H₁₀N)CH₂C₆H₄-o]₃C(μ²-O)K}₂(μ-C₄H₄S)]_n (8), in which benzene and thiophene molecules are μ-bridging two K⁺ ions. The treatment of {[(C₅H₁₀N)CH₂C₆H₄-o]₃C(μ²-O)K(η²-CH₃C₆H₅))}₂ with Me₃SiCH₂Li or n-BuLi (1.2 eq.) in hexane at 20 °C results in the facile metalation of the Me group of toluene, forming [PhCH₂K]_n and lithium alkoxide. This model reaction provides a deeper insight into the probable mechanism of metalation of CH bonds under Lochmann–Schlosser superbasic conditions, and the role and the nature of the synergistic effect of two metals. The calculations and QTAIM analysis were performed for 1–8 and model molecules as well.