Rate constants for the decarboxylation of thet-butoxicarbonyl radical and concurring radical terminations directly obtained by kinetic ESR

Fragment Coupling and the Construction of Quaternary Carbons Using Tertiary Radicals Generated From tert-Alkyl N-Phthalimidoyl Oxalates By Visible-Light Photocatalysis

The coupling of tertiary carbon radicals with alkene acceptors is an underdeveloped strategy for uniting complex carbon fragments and forming new quaternary carbons. The scope and limitations of a new approach for generating nucleophilic tertiary radicals from tertiary alcohols and utilizing these intermediates in fragment coupling reactions is described. In this method, the tertiary alcohol is first acylated to give the tert-alkyl N-phthalimidoyl oxalate, which in the presence of visible-light, catalytic Ru(bpy)3(PF6)2, and a reductant fragments to form the corresponding tertiary carbon radical. In addition to reductive coupling with alkenes, substitution reactions of tertiary radicals with allylic and vinylic halides is described. A mechanism for the generation of tertiary carbon radicals from tert-alkyl N-phthalimidoyl oxalates is proposed that is based on earlier pioneering investigations of Okada and Barton. Deuterium labeling and competition experiments reveal that the reductive radical coupling of tert-alkyl N-phthalimidoyl oxalates with electron-deficient alkenes is terminated by hydrogen-atom transfer.

Factors Affecting the Relative and Absolute Rates of β-Scission of Alkoxythiocarbonyl Radicals and Alkoxycarbonyl Radicals

High-level ab initio calculations demonstrate that alkoxy-thiocarbonyl radicals (ROC*=S) undergo beta-scission significantly faster than alkoxycarbonyl radicals (ROC*=O) despite having similar exothermicities. The relatively low reactivity of the ROC*=O radicals is reduced further by electron-donating R groups and arises from the large polarization of the C*-O bonds of the reactant radicals. The results suggest that the generation of alkyl radicals from ROC*=S should be particularly efficient when the R group bears radical-stabilizing and/or electron-accepting groups, such as CN.