Kinetic and Mechanistic Studies of Base-Catalyzed Phenylselenoetherification of (Z)- and (E)-Hex-4-en-1-ols

Abstract

The mechanism of phenylselenoetherification of (Z)- and (E)-hex-4-en-1-ols using some bases (triethylamine, pyridine, quinoline, 2,2'-bipyridine) as catalysts and some solvents [tetrahydrofuran (THF) and CCl4] as reaction media was examined through studies of kinetics of the cyclization by UV-vis spectrophotometry. It was demonstrated that the intramolecular cyclization is facilitated in the presence of bases as a result of the hydrogen bond between the base and the alkenol's OH group. The rate constants in the base-catalyzed reactions are remarkably influenced by the bulkiness and basicity of the base used and the nature of the considered nitrogen donors. The obtained values for rate constants show that the reaction with triethylamine is the fastest one. THF with higher polarity and higher basic character is better as a solvent than CCl4. Quantum-chemical calculations [MP2(fc)/6-311+G**//B3LYP/6-311+G** + ZPE(B3LYP/6-311+G**] show that the cyclization of (Z)-hex-4-en-1-ol to a tetrahydrofuranoid five-membered ring is kinetically controlled, while the cyclization of (E)-hex-4-en-1-ol to the tetrahydropyranoid six-membered ring is thermodynamically controlled. This is in accordance with previous experimental findings.

Kinetic and mechanistic insight into Lewis base and acid-mediated phenylselenoetherification of 2,6-dimethyl-hept-5-en-2-ol

“…In accordance with earlier findings [18,19], the distance between the entering H 3 NÁÁÁHO group and the formally sp 2 -C-atom, dedicated to form the new bond, in the furanoide transition states are consequently shorter (2,6-dimethyl-hept-5-en-2-ol: 2.08 Å , 6-methyl-hept-5-en-2-ol: 2.04 Å ) than in the pyranoide's form transition state (2,6-dimethyl-hept-5-en-2-ol: 2.16 Å , 6-methyl-hept-5-en-2-ol: 2.14 Å ).…”

“…As we investigated the possible reaction pathways already before [18,19], we focused here on a comparison of the transition states leading to the five-and sixmembered ether applying the smallest possible base NH 3 as in our earlier studies [18,19]. As alkenoles, we utilized of course 2,6-dimethyl-hept-5-en-2-ol and 6-methyl-hept-5-en-2-ol to get an impression of the influence of an extra methyl group next to OH.…”

“…As a part of our ongoing research, directed toward the intramolecular cyclization of unsaturated alcohols by means of phenylselenenyl halides as reagents, we have examined the chemo-, regio-, stereoselectivity and reaction kinetics as a function of substitution pattern at the double bond and at the carbinol carbon atom of the used alkenol substrate. The influence of some Lewis acids and bases as catalysts on mechanistic pathway and reaction rate has also been evaluated [12][13][14][15][16][17][18][19][20][21].…”

A mild and efficient procedure for alkenols oxyselenocyclization by using ionic liquids

“…Mechanistic studies of cycloetherifications of pent‐4‐en‐ols by treatment with PhSeCl in the presence of a base (pyridine, trimethylamine, quinolone, and 2,2'‐bipyridine) have shown that the catalytic effect of the base is caused by the hydrogen bond formed with the hydroxyl group, which increases the oxygen nucleophilic character . It has been also reported that solvents with higher polarity can better stabilize the charged transition state for the cyclization, increasing the reaction rate.…”