Rate constants for cyclizations of α-hydroxy radical clocks

Area of Science:

• Organic Chemistry
• Physical Chemistry
• Radical Chemistry

Background:

• Understanding the kinetics of radical cyclization and hydrogen atom transfer is crucial for organic synthesis and reaction mechanisms.
• Diphenyl-substituted alkenyl radicals offer a unique structural motif for studying radical behavior.

Purpose of the Study:

• To synthesize novel 1-hydroxy-1-methyl-6,6-diphenyl-5-hexenyl radical (4a) and 1-hydroxy-1-methyl-7,7-diphenyl-6-heptenyl radical (4b).
• To directly measure the rate constants for the cyclization reactions of these radicals.
• To determine the rate constants for hydrogen atom trapping of these radicals by thiols.

Main Methods:

• Precursor PTOC esters were synthesized via coupling of carboxylic acids with thiohydroxamic acid.
• Radical generation and kinetic measurements were performed using laser flash photolysis.
• Rate constants were determined in acetonitrile and THF across a temperature range of -30 to 60 °C.

Main Results:

• Arrhenius functions for cyclization in acetonitrile were determined for both radicals (4a and 4b).
• Room temperature cyclization rate constants were found to be higher than those of analogous alkyl radicals.
• Hydrogen atom trapping rate constants with thiols were also measured and found to be modestly larger than for alkyl radicals.

Conclusions:

• The synthesized diphenyl-substituted hexenyl and heptenyl radicals exhibit enhanced reactivity in cyclization and H-atom trapping.
• The findings provide valuable kinetic data for radical reactions, contributing to the understanding of radical mechanisms.
• These results suggest potential applications in synthetic strategies where controlled radical reactions are desired.