2.1.8. Michael Reaction 2245 2.1.9. Others 2247 2.2. Cyclization Reactions 2248 2.2.1. Carbon Diels−Alder Reactions 2248 2.2.2. Aza-Diels−Alder Reactions 2252 2.2.3. Other Hetero-Diels−Alder Reactions 2255 2.2.4. Ionic Diels−Alder Reaction 2256 2.2.5. 1,3-Dipolar Cycloadditions 2256 2.2.6. Other Cycloaddition Reactions 2258 2.2.7. Prins-type Cyclization 2259 2.3. Friedel−Crafts Acylation and Alkylation 2259 2.4. Baylis−Hillman Reaction 2263 2.5. Radical Addition 2264 2.6. Heterocycle Synthesis 2267 2.7. Diazocarbonyl Insertion 2270 3. C−X (X ) N, O, P, Etc.) Bond Formation 2271 3.1. Aromatic Nitration and Sulfonylation 2271 3.2. Michael Reaction 2272 3.3. Glycosylation 2273 3.4. Aziridination 2275 3.5. Diazocarbonyl Insertion 2276 3.6. Ring-Opening Reactions 2277 3.7. Other C−X Bond Formations 2280 4. Oxidation and Reduction 2280 4.1. Oxidation 2280 4.2. Reduction 2281 5. Rearrangement 2283 6. Protection and Deprotection 2285 6.1. Protection 2285 6.2. Deprotection 2288 7. Polymerization 2291 8. Miscellaneous Reactions 2291 9. Lanthanide(II) Triflates in Organic Synthesis 2295 10. Conclusion 2295 11. Acknowledgment 2295 12. References 2295

Rare-earth metal triflates in organic synthesis

The area of catalysis of radical reactions has recently flourished. Various reaction conditions have been discovered and explained in terms of catalytic cycles. These cycles rarely stand alone as unique paths from substrates to products. Instead, most radical reactions have innate chains which form products without any catalyst. How do we know if a species added in "catalytic amounts" is a catalyst, an initiator, or something else? Herein we critically address both catalyst-free and catalytic radical reactions through the lens of radical chemistry. Basic principles of kinetics and thermodynamics are used to address problems of initiation, propagation, and inhibition of radical chains. The catalysis of radical reactions differs from other areas of catalysis. Whereas efficient innate chain reactions are difficult to catalyze because individual steps are fast, both inefficient chain processes and non-chain processes afford diverse opportunities for catalysis, as illustrated with selected examples.

Catalysis of Radical Reactions: A Radical Chemistry Perspective.

II.2.1. Homogeneous Systems 2166 II.2.2. Heterogeneous Systems 2168 II.3. Hydride Transfer Reduction 2169 II.3.1. Homogeneous Systems 2169 II.3.2. Heterogeneous Systems 2171 II.4. Hydrosilylation 2172 III. C−O Bond Formation 2173 III.1. Epoxidation of Unfunctionalized Olefins 2173 III.1.1. Homogeneous Catalysis 2173 III.1.2. Heterogeneous System 2176 III.2. Dihydroxylation of Olefins 2178 III.2.1. Homogeneous Systems 2178 III.2.2. Heterogeneous System 2178 III.3. Ring Opening of Meso Epoxides 2180 III.4. Kinetic Resolution 2180 III.4.1. Terminal Epoxides 2180 III.4.2. Secondary Alcohols 2181 IV. C−H Bond Oxidation 2181 IV.1. Allylic and Benzylic Oxidation 2181 IV.2. Baeyer−Villiger-type Reaction 2181 IV.3. Wacker-type Cyclization 2182 V. S−O Bond Formation 2182 VI. C−N Bond Formation 2183 VI.1. Hydroboration/Amination 2183 VI.2. Enolate Amination 2183 VI.3. Aza-Claisen Rearrangement 2183 VI.4. Azide Synthesis 2184 VI.5. Aminohydroxylation 2184 VI.6. Aziridine Synthesis 2184 VI.7. C−N Bond Formation via S−N Bond Formation 2185

Nitrogen-containing ligands for asymmetric homogeneous and heterogeneous catalysis.

Claisen rearrangement over the past nine decades.

Chemistry of Acyl Radicals

Practical and Efficient Enantioselective Conjugate Radical Additions

Lewis acid-mediated intermolecular .beta.-selective radical additions to N-enoyloxazolidinones.

Lewis acid-mediated conjugate addition of alkyl radicals to a differentially protected fumarate 10 produced the monoalkylated succinates with high chemical efficiency and excellent stereoselectivity. A subsequent alkylation or an aldol reaction furnished the disubstituted succinates with syn configuration. The chiral auxiliary, 4-diphenylmethyl-2-oxazolidinone, controlled the stereoselectivity in both steps. Manipulation of the disubstituted succinates obtained by alkylation furnished the natural products (-)-enterolactone, (-)-arctigenin, and (-)-isoarctigenin. The overall yields for the target natural products were 20-26% over six steps. Selective functionalization of the disubstituted succinates obtained by aldol condensation gave the paraconic acid natural products (-)-nephrosteranic acid (8) and (-)-roccellaric acid (9). The overall yield of the natural products 8 and 9 over four steps was 53% and 42%, respectively.

Free-radical-mediated conjugate additions. Enantioselective synthesis of butyrolactone natural products: (-)-enterolactone, (-)-arctigenin, (-)-isoarctigenin, (-)-nephrosteranic acid, and (-)-roccellaric acid.

We have developed a highly diastereoselective method for the conjugate addition of carbon radicals to chiral α,β-unsaturated N-enoyloxazolidinones using Bu3SnH as chain carrier and Et3B/O2 as radical initiator. Lewis acids have been screened, and Yb(OTf)3 proved to give optimized results for both chemical yield (88% for 1a and 94% for 1b) and diastereoselectivity (25:1 for 1a and 46:1 for 1b). The selectivity is solvent-dependent, CH2Cl2-THF being an ideal combination. Scrupulously dry solvents or reaction conditions were not required. Substoichiometric amounts of Yb(OTf)3 provided efficient reaction with minimal sacrifice in diastereoselectivity. Carbon radicals with reasonable nucleophilicity were generally successful, including functionalized radicals such as acetyl or methoxymethyl. Electrophilic radicals were not successful. A model which accounts for most of our observations is presented.

Jianguo Ji

Papers

Practical and Efficient Enantioselective Conjugate Radical Additions

Lewis acid-mediated intermolecular .beta.-selective radical additions to N-enoyloxazolidinones.

Free-radical-mediated conjugate additions. Enantioselective synthesis of butyrolactone natural products: (-)-enterolactone, (-)-arctigenin, (-)-isoarctigenin, (-)-nephrosteranic acid, and (-)-roccellaric acid.

Free Radical-Mediated Intermolecular Conjugate Additions. Effect of the Lewis Acid, Chiral Auxiliary, and Additives on Diastereoselectivity

Acyclic Stereocontrol in Radical Reactions: ρ‐Selectivity with Oxazolidinone Auxiliories