Main-Group-Element Calix[4]arenes: Variable Coordination and Conformational Isomerism at Phosphorus and Silicon

Treatment of calix[4]arene (1b) with trichloromethylsilane yields two conformers of calix[4]SiMe(OH), the cone
2b-C and the partial cone 2b-PC. These are isolated and structurally characterized, and their thermodynamic
activation parameters are determined in solution [Ea = 117(3) kJ/mol, ΔH = 5(4) kJ/mol]. Similar parameters are
found for the p-tert-butylcalix[4]arene analogues 2a-C and 2a-PC. Deprotonation of 2b with butyllithium yields
calix[4]SiMe(OLi) (8b). The structure of 8b is solvent dependent:  8b contains a five-coordinate silicon in THF
and a four-coordinate silicon in benzene. Similar behavior is found for the p-tert-butylcalix[4]arene analogues.
The five-coordinate phosphorus analogue of the anion in 8, p-tert-butylcalix[4]PMe (11a), is synthesized from
the phosphonium triflate salt p-tert-butylcalix[4]PMe(OH)OTf (10a(OTf)) via treatment with butyllithium. The
structure of 11a shows the geometry around phosphorus to be very close to a pure trigonal bipyramid. The X-ray
structure of 10a(OTf) cannot be obtained, but its iodide analogue 10a(I) is synthesized and structurally characterized.
The cation in 10a(I) adopts the partial cone conformation in the solid state, similar to 2b-PC. Treatment of 8b
with methyl trifluoromethanesulfonate yields the methyl phenyl ether calix[4]SiMe(OMe) (3b). This species and
its p-tert-butyl analogue 3a are structurally characterized. Both adopt the partial cone conformation. The SiMe
group in 3b can be removed via treatment with fluoride to yield the monomethylated calix[4]arene 4b in 50−60% overall yield based on 1b.