Solid-supported radical reaction

Solid-supported radical reactions started to be exploited in carbon-carbon bond formations due to the increased ability of the chemist to control radical processes. A deeper knowledge of the thermodynamic and kinetic aspects of the radical chains allows the minimization of unwanted side reactions. Radical reactions lead mainly to kinetically determined products. Favored reactions are those involving proximal functionalities, along with reactions leading to cyclized products. In these cases, a radical generated by a selective reaction is allowed to react with a non-radical, usually a double bond. The radical character in such a reaction is not destroyed during the process therefore, only catalytic amounts of radical initiator are required. The products generated in radical reactions are mostly not diffusion-... 

Clerici and Porta reported that phenyl, acetyl and methyl radicals add to the Ca atom of the iminium ion, PhN+Me=CHMe, formed in situ by the titanium-catalyzed condensation of /V-methylanilinc with acetaldehyde to give PhNMeCHMePh, PhNMeCHMeAc, and PhNMeCHMe2 in 80% overall yield.83 Recently, Miyabe and co-workers studied the addition of various alkyl radicals to imine derivatives. Alkyl radicals generated from alkyl iodide and triethylborane were added to imine derivatives such as oxime ethers, hydrazones, and nitrones in an aqueous medium.84 The reaction also proceeds on solid support.85 A-sulfonylimines are also effective under such reaction conditions.86 Indium is also effective as the mediator (Eq. 11.49).87 A tandem radical addition-cyclization reaction of oxime ether and hydrazone was also developed (Eq. 11.50).88 Li and co-workers reported the synthesis of a-amino acid derivatives and amines via the addition of simple alkyl halides to imines and enamides mediated by zinc in water (Eq. 11.51).89 The zinc-mediated radical reaction of the hydrazone bearing a chiral camphorsultam provided the corresponding alkylated products with good diastereoselectivities that can be converted into enantiomerically pure a-amino acids (Eq. 11.52).90... 

Gas-Solid Heterogeneous Reaction Mixtures. Gas-solid heterogeneous reaction mixtures may be advantageously irradiated in annular (immersion-type) photochemical reactors. Again, the content of solid particles is limiting the size and the productivity of the reactor system. This is of particular importance when the solid support is used to specifically adsorb substrates or products of the photochemical reaction the first to enhance specificity of radical substitution reactions [20], the latter to reach better photostability and to ensure optimal purity. 

Until now, the chemistry of radicals on solid supports has been investigated mostly in respect to intramolecular radical cyclizations and radical chain reactions. One reason for refraining from free radical transformations is the chemical nature of the polystyrene with its abundance of benzylic positions that are prone to H-radical abstraction and oxidation. 

Comparing the results from radical release in solution and on solid support, it was determined that the amounts of recombined product were reduced significantly in the matrix-supported reaction even at higher concentrations (0.1 M solution 2.6%, 0.16 M polymer support 0.4%). On the other hand, in both solution and polymer gel experiments, the... 

Once a chiral silane is synthesized, intramolecular reaction such as the Heck reaction and the radical cyclization reaction with transfer of chirality from the silicon center can be pursued. Other reactions of interest include the stereoselective hydrosilylation of aldehydes and ketones. Our method could be used to tether chiral silanes to solid support for uses in solid support... 

Harrowven [33] has reported two methods for carrying out the radical cyclization reactions of dienes attached to a Wang polystyrene support. Treatment of solid-supported diene 195 with either thiophenol or p-tolylbenzeneselenosulfonate, in the presence of AIBN, triggers 5-exo-trig... 

Although a large variety of reactions were successfully transferred to the solid phase there are only a few examples of free radical reactions on solid phase [173,174]. The free allyl radical transfer was achieved when the carbohydrate auxiliary was linked to a noncross-linked polystyrene polymer (NCPS) [175]. The use of NCPS instead of cross-linked supports has several advantages, including complete solubility in many organic solvents [176]. The polymer 269 was obtained in a radical polymerization of two equivalents of styrene and one equivalent /7-chloromethylstyrene. The protected D-xylose 270 was covalently linked to the support by a Williamson synthesis generating compound 271 (Scheme 10.88). 

Sibi, M P, Chandramouli, S V, Intermolecular free radical reactions on solid support, allylation of... 

Water has been shown to be an effective solvent in some chemical reactions such as free radical bromination. Supercritical fluids such as liquified carbon dioxide are already commonly used in coffee decaffeination and hops extraction. However, supercritical carbon dioxide can also be used as a replacement for organic solvents in polymerization reactions and surfactant production. Future work may involve solventless or neat reactions such as molten-state reactions, dry grind reactions, plasma-supported reactions, or solid materials-based reactions that use clay or zeolites as carriers. 

Solid-phase intermolecular radical reactions, as previously seen for supported cross-metathesis reactions, should offer the advantage of limiting side reactions, i.e. potential homo-coupling of the immobilized counterpart, due to the unfavorable distance between the reaction centers. 

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