Martin Newcomb

The generation of nitrogen radicals and their cyclizations for the construction of the pyrrolidine nucleus

Publisher Summary Many of the conditions necessary for the incorporation of nitrogen centered radicals in synthesis have been met. Several precursors exists that can be used to generate monoalkyl and dialkyl aminium cation radicals as well as amidyl radicals under relatively mild conditions. An important advancement was the extension of Bartons pyridine-2-thioneoxycarbonyl group (PTOC) protocol to the production of various nitrogen centered radicals. The PTOC precursors to nitrogen radicals appear to be as useful as the PTOC ester precursors for carbon radicals, especially in light of the variety of trapping agents available to functionalize the product carbon radicals formed by cyclizations and addition reactions of nitrogen radicals. The relative rate constants for competitive reactions involving nitrogen radicals and, in some cases, absolute rate constants are known; these allow design of efficient nitrogen radical chain reactions. Based on the many ring constructions demonstrated in this chapter, it is expected that the applications of nitrogen radical reactions in synthesis will become more common.

HYDROGEN DONOR ABILITIES OF THE GROUP 14 HYDRIDES

Publisher Summary Collection of kinetic data for reactions of the group 14 hydrides with various radicals provides essential information to chemists in both the synthetic and physical communities. This chapter focuses on the kinetics of reactions of the silicon, germanium, and tin hydrides with radicals. The absolute kinetics of radical reactions is determined. The quantitation of the kinetics of radical reactions involving the Group 14 metal hydrides in condensed phase and the type of metal hydride are discussed. Kinetic studies have been performed with many types of silicon hydrides, and much of the data can be interpreted in terms of the electronic properties of the silanes imparted by substituents. A number of approaches based on calculations are applied to obtain information on the factors that influence the reactivity of radical reactions. The hydrogen-atom transfer, the applied methodologies have spanned from high-level ab initio to empirical calculations. The heats of reactions and the methods used for the kinetic determinations are reviewed. Some important bond dissociation energies followed by kinetic methods used for determining radical reaction rate constants are discussed—namely, bond dissociation energies and radical kinetic methods.

Amidyl radicals from N-(phenylthio)amides☆

Preparations on N-(phenylthio)amides from secondary amides are described, and these species are shown to be efficient precursors for amidyl radicals in Bu3SnH mediated reactions; cyclizations of δ,e-unsaturated amidyl radicals from these precursors and their use in measurement of relative rate constants are reported.

Polar substituent and solvent effects on the kinetics of radical reactions with thiols

Abstract The rates of thiol trapping of radicals depend upon polar substituents (Table 1) and solvent effects (Table 2). The rate accelerating effect of water on the reactions of biologically relevant radicals is of special importance.

N-Acyl-N-alkylcarbamoyloxy radicals: Entries to amidyl radicals by decar☐ylation and to α-amide radicals by radical translocation

N′-Acyl-N-hydroxypyridine-2-thione carbamates react in radical chain reactions to give the title radicals which can decar☐ylate or react by intramolecular hydrogen atom transfer; the competing reaction pathways are controlled by the structure of the alkyl group and the conformation of the precursor which can be influenced by addition of a Lewis acid.

Hypersensitive radical probe studies of Gif oxidations

Abstract Hypersensitive probes were employed in mechanistic studies of Gif oxidations. The results indicate that, unlike the case in enzyme catalyzed hydroxylation reactions, diffusively free radicals are formed in Gif oxidations of these substrates.

Rate constants for aminyl radical reactions

Abstract Rate constants for cyclization of the N -methyl-5,5-diphenyl-4-pentenaminyl radical ( 4 ) and for reaction of this radical with t -BuSH were determined by direct methods.

Dimethyldioxirane hydroxylation of a hypersensitive radical probe: Supporting evidence for an oxene insertion pathway

Abstract Dimethyldioxirane oxidation of the hypersensitive radical probe (rans-2-phenylcyclopropyl)ethane gave non-rearranged hydroxylation products consistent with an oxene insertion mechanism for the reaction.

α-Ethoxycarbonyl and α-methoxy substituted radical clocks

Abstract Rate constants and Arrhenius functions for 5-exo cyclizations of the 1-(ethoxycarbonyl)-5-hexenyl radical, the 1-(ethoxycarbonyl)-1-methyl-5-hexenyl radical and the 1-methoxy-5-hexenyl radical were determined by indirect kinetic methods.

β-(Phosphatoxy)alkyl and β-(acyloxy)alkyl radical migrations studied by laser flash photolysis

Abstract A number of β-(phosphatoxy)alkyl and β-(acyloxy)alkyl radicals have been generated by laser flash photolysis of the corresponding Barton PTOC esters. Time resolved studies permitted the determination of rate constants and kinetic parameters for their rearrangement in benzene, THF, and acetonitrile. Migrations are accelerated by electron-donating groups on the alkyl moiety, electron-withdrawing groups on the migrating group, and more polar solvents. All reactions showed strictly first order kinetics. The results are interpreted in terms of concerted mechanisms occurring through three electron, three center and five electron, five center mechanisms.