The Study of the Kinetics of Intramolecular Radical Cyclizations of Acylsilanes via the Intramolecular Competition Method

Abstract

The kinetics of the radical cyclizations of acylsilanes is probed by the construction of an intramolecular competition system. In this system, known rate constants of olefin cyclizations are used as the internal clock. At benzene reflux temperature (80 °C), the cyclization rate constants for 5-exo cyclizations of acylsilanes with primary radicals are on the order of 10 6 s -1 . The corresponding rate constants for 5-exo cyclizations with secondary radicals and 6-exo cyclizations with primary radicals lie on the order of 10 5 s -1 . The 6-exo cyclizations of acylsilanes with secondary radicals are slower and fall in the range of 10 4 s -1 . Substituents on the silicon of the acylsilanes affect the rates of cyclization. Large silyl groups reduce the rate, and this is more serious for the 6-exo type of cyclizations. Electron-withdrawing groups increase the rates of cyclization.

The Application of Intramolecular Radical Cyclizations of Acylsilanes in the Regiospecific Formation of Cyclic Silyl Enol Ethers

“…When 1.2 equiv of tributyltin hydride was used, we only isolated 83% of straight reduction product 23 (Scheme ). Previously, we found that 1,6-radical cyclizations of acylsilanes are sensitive toward steric effect . Presumably, with a bulky α-tributylstannyl radical the cyclization of acylsilane 17 becomes very slow.…”

“…Thio- and selenoesters also undergo similar reactions . The complementary acylsilane cyclizations give cyclic alcohols in the form of silyl ethers through irreversible radical-Brook rearrangements − of β-silyl alkoxy radical intermediates. In the case of tributyltin hydride mediated pinacol coupling developed by Hays and Fu, the cyclized γ-tributylstannyloxy alkoxy radical was trapped via an intramolecular homolytic substitution at tin.…”

Regioselectivity Patterns in Radical Cyclization of Diosphenol Derivatives with Different Ring Size: A Combined Experimental and DFT Study

“…Cyclization reactions of radicals are one of the substantial topics in organic synthesis [1–5] and have been well documented [6,7] . These reactions include the selective formation of cyclic structures.…”

Tandem Cyclization of α-Cyano α-Alkynyl Aryl Ketones Induced by tert-Butyl Hydroperoxide and Tetrabutylammonium Iodide

“…During the past decade, a variety of annulation processes based on α-activated cross-conjugated cycloalkenone systems have been developed in our laboratories . As indicated below, α-cyano ketone 2 thus obtained from 1 via 1,4-addition was found to undergo autoxidative cyclization under air to afford product 3 in a significant amount. , …”