Lewis acid-mediated radical

Curran, D. R Porter, N. A. Giese, B. Stereochemistry of Radical Reactions, VCH, Weinheim, 1995. For an excellent recent review on Lewis acid mediated radical rcaclions see Rcnaud, R Gerster, M. Angew. Chem., Int. Ed. Engl. 1998, 37, 2562-2579. 

Mechanistic Perspectives on Stereocontrol in Lewis Acid-Mediated Radical Polymerization Lessons from Small-Molecule Synthesis... 

Scheme 5 The necessary requirements for effective isotactic control in Lewis acid-mediated radical polymerization.

Benjamin Noble and Michelle Coote s chapter on stereocontrol in Lewis acid-mediated radical polymerization brings much-needed order into the important area of polymer synthesis the lack of tacticity control is now the greatest obstacle to the synthesis of precisely controlled macromolecules by versatile and efficient radical-based processes. They bring mechanistic insights from small-molecule synthesis to bear on the problem, as they outline and assess some of the key factors likely to undeilie the success or failure of Lewis acid-mediated isotactic control in radical polymerization. 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

Chiral Lewis acid mediated reductions at carbon atom a to a carbonyl group can be carried out either by generation of the radical from o -halo car-... 

A potential method for the preparation of novel amino acids via the highly selective addition of radicals to the glyoxylic oxime derivative of Oppolzer s camphor sultam (88) has been reported.181 Both Lewis acid and non-Lewis acid-mediated reaction conditions for the addition of alkyl radicals generated from alkyl iodides and Et3B/Bu3SnH were examined. A new chiral auxiliary based upon (R,R)-2,5-diphenylpyiTolidine has been used in the addition of phenylthiyl radicals to unsaturated methacrylamides. The selectivity was found to be better than that reported for the structurally related 2,5-dimethylpyrrolidine derivative.182... 

In one of the earliest reports on enantioselective radical reactions, chiral Lewis acid mediated conjugate addition followed by enantioselective H-atom transfer a to a carbonyl was reported by Sato and co-workers (Scheme 3) [22], The single point binding chiral aluminum complex presumably coordinates to the carbonyl oxygen of the lactone as shown in 10. The strong Lewis acidity of the aluminum complex activates the substrate 7 to nucleophilic conjugate addition, which is followed by an enantioselective H-atom transfer from BuaSnH in a chiral environment provided by BINOL ligand in 8. Only 28% ee was observed for product 9. 

An enantioselective reduction of an a-iodolactone under radical conditions has been reported [95CC481]. Treatment of 207 with tin hydride, magnesium(II) iodide and in the presence of a chiral amine gave the 8-lactone 208 in good yield and moderate enantioselectivity. This is one of the first examples of chiral Lewis acid mediated enantioselective radical reactions. 

Although copper reagents, hahdes and triflates, are widely used in atom-transfer polymerization reactions (ATRP) [63], these processes do not fall under the category of Lewis acid-mediated reactions. Sherrington and co-workers have shown that a vinyl monomer coordinated to a chiral copper Lewis acid (122) undergoes stereoselective polymerization (Sch. 29) [64]. A chiral block-copolymer 124 was prepared under radical conditions. 

Sibi and Ji reported that acyl radicals, generated from acyl bromides, can participate in Lewis acid-mediated diastereoselective radical addition reactions (Scheme 4-26) [49]. Using triethylborane/02 as a radical initiator, the reaction was conducted at -78 °C. 

The success in diastereoselective Lewis acid-mediated conjugate radical additions using chiral oxazolidinones led to the development of enantioselective variants. The achiral template selection was based on literature precedents for rotaraer control s-cis vs s-trans) and the requirement for a bidentate Lewis acid chiral ligand combination for obtaining selectivity (Eq. (13.51)). 

Lewis Acidic Trialkylborane Radical Mediator and Terminator as Well as Initiator... 

Lewis Acid-Mediated Diastereoselective Radical Reactions... 

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