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Organic Reaction Mechanisms Organic Reaction Mechanisms Fourth Edition V.K. Ahluwalia Rakesh Kumar Parashar ® Alpha Science International Ltd. Oxford, U.K. Contents Preface to the Fourth Edition vii Preface to the First Edition ix 1. Chemical Kinetics and Reaction Pathways 1 1.1.1 Homolytic bond fission 1 1.1.2 Heterolytic bond fission 2 1.1.3 Nucleophiles and electrophiles 2 1.1.4 Leaving group 4 1.1.5 Solvent 5 1.2 Chemical Kinetics, equilibria and energetics of reactions 7 1.2.1 Rates of reactions (Chemical kinetics) 7 1.2.2 Transition state and activation energy 7 1.2.3 Equilibrium constant and free energy difference between reactants and products 9 1.2.5 Temperature and equilibrium constant 10 1.3 Nucleophilic substitution at saturated carbon atom 10 1.3.1 SN1 (Substitution Nucleophilic Unimolecular) 12 1.3.2 SN2 (Substitution Nucleophilic Bimolecular) 18 1.3.3 Competetion between SN1 and SN2 22 1.3.4 Competition between substitution reaction and elimination reaction 23 1.3.5 Low reactivity of vinyl and aryl halides 23 1.3.6 High reactivity of allyl and benzyl halides 24 1.3.7 SNi (Substitution Nucleophilic internal) 25 1.3.8 Neighbouring group participation in displacement reactions 26 1.4 Addition reactions 34 1.4.1 Electrophilic additions 34 1.4.2 Nucleophilc additions 42 1.4.3 Free radical additions 46 1.4.4 Concerted additions 48 1.5 Elimination reactions 50 1.5.1 Bimolecular elimination reactions (E2) 51 1.5.2 Unimlecular elimination reactions (El) 57 1.6 Electrophilic substitution in aromatic systems 59 References 75 Problems 76 xii Contents 2. Reaction Intermediates, Ylides and Enamines 78 2[A] Reaction intermediates 78 2.1 Introduction 78 2.2 Carbocations 78 2.2.1 Stability of carbocations 79 2.2.2 Generation of carbocations 82 2.2.3 Reactions of carbocations 83 2.2.4 Applications 86 2.2.5 Non-classical carbocations 87 2.3 Carbanions 89 2.3.1 Stability of carbanions 90 2.3.2 Generation of carbanions 91 2.3.3 Reactions of carbanions 92 2.4 Free radicals 96 2.4.1 Stability of free radicals 96 2.4.2 Generation of free radicals 97 2.4.3 Reactions of free radicals 100 2.4.4 Mechanism of free radical reactions 102 2.4.5 Applications of free radicals 103 2.5 Carbenes 107 2.5.1 Stability of carbenes 108 2.5.2 Generation of carbenes 109 2.5.3 Reactions of carbepes 111 2.6 Nitrenes 116 2.6.1 Stability of nitrenes 116 2.6.2 Generation of nitrenes 116 2.6.3 Reactions of nitrenes 117 2.7 Benzynes 118 2.7J Generation of benznes 119 2.7.2 Reactions of benzynes 120 2.[B] Ylides and enamines 123 2.8 Ylides 123 2.8.1 Generation of ylides 124 2.8.2 Reactions of ylides 124 2.9 Enamines 128 2.9.1 Generation of enamines 129 2.9.2 Reactions of enamines 129 2.9.3 Metalloenamines 132 References 133 Problems 134 3. Oxidation 137 3.1 Introduction 137 Contents xiii 3.2 Manganese (VII) oxidants 138 3.2.1 Potassium permanganate 138 3.2.1.1 Oxidation of alcohols 139 3.2.1.2 Oxidation of alkenes 139 3.2.1.3 Oxidation of alkynes 142 3.2.1.4 Oxidation of aromatic side chains and aromatic ring systems 142 3.2.1.5 Oxidation of aldehydes and ketones 144 3.2.1.6 Oxidation of amines into nitro compounds 145 3.2.1.7 Oxidation of nitro compounds into carbonyl compounds 145 3.2.2 Manganese dioxide 146 3.3 Chromium (VI) oxidants 147 3.3.1 Oxidation of alcohols/Phenols 148 3.3.1.1 Chromic acid and sodium or potassium dichromate 148 3.3.1.2 Jones reagent 150 3.3.1.3 Chromium trioxide-pyridine complex 151 3.3.1.4 Pyridinium chlorochromate (PCC) 152 3.3.1.5 Pyridinium dichromate 153 3.3.2 Oxidation of alkanes 154 3.3.3 Oxidation of alkenes 154 3.3.4 Oxidation of aromatic side chains and aromatic nucleus 156 3.4 Oxidation with peracids 158 3.4.1 Oxidation of alkenes 159 3.4.2 Oxidation of ketones 163 3.4.3 Oxidation of N-heterocycles 164 3.5 Miscellaneous oxidants 165 3.5.1 Oxygen 165 3.5.2 Singlet oxygen (photochemical oxidation) 167 3.5.3 Ozone 170 3.5.4 Hydrogen peroxide 172 3.5.5 t-Butyl hydroperoxide 174 3.5.6 Aluminium tri-isopropoxide and aluminium tri-t-butoxide 176 3.5.7 Lead tetra-acetate 177 3.5.8 Selenium dioxide 177 3.5.9 Osmium tetroxide 178 3.5.10 Periodic acid 179 3.5.11 Potassium persulphate 180 3.5.12 Nitric acid 181 3.5.13 Dimethyl sulfoxide 181 3.5.14 Silver carbonate 184 3.5.15 Silver (I) oxide 186 v Contents 3.5.16 Silver (II) oxide 187 3.5.17 Bismuth oxide 187 3.5.18 Fusion with alkali 188 3.5.19 Halogens-sodium hydroxide 188 3.5.20 Iodine-pyridine 188 3.5.21 Iodine-silver carboxylates 189 3.5.22 N-Bromosuccinimide 191 3.5.23 Ruthenium tetroxide 192 3.5.24 Thallium nitrate 192 3.5.25 2, 3-Dichloro-5, 6-dicyano- 1,4-benzoquinone (DDQ) 3.5.26 Mercuric oxide 197 3.5.27 Potassium bromate 198 3.6 Enzymatic or microbial oxidations (Bio-oxidations) 198 3.7 Correlation tables 204 3.7.1 Oxidation of hydrocarbons 204 3.7.1.1 Oxidation of alkanes 204 3.7.1.2 Oxidation of alkenes 204 3.7.1.3 Oxidation of alkynes 208 3.7.1.4 Oxidation of aromatic hydrocarbons 208 3.7.2 Oxidation of alcohols 210 3.7.2.1 Oxidation of 1° alcohols 210 3.7.2.2 Oxidation of 2° alcohols 211 3.7.2.3 Oxidation of 3° alcohols 212 3.7.2.4 Oxidation of diols 213 3.7.2.5 Oxidation of phenols 213 3.7.3 Oxidation of carbonyl compounds 214 3.7.3.1 Oxidation of aldehydes 214 3.7.3.2 Oxidation of ketones 214 3.7.4 Oxidation of nitro compounds 216 3.7.5 Oxidation of amines 216 3.7.6 Oxidation of cyanides 216 3.7.7 Dehydrogenations 216 References 216 Problems 219 4. Reduction 4.1 Introduction 221 4.2[A] Heterogeneous hydrogenation 222 Platinum 222 Palladium 222 Nickel 223 Copper chromite 223 4.2.1 Reduction of alkenes 223 4.2.2 Reduction of alkynes 225 4.2.3 Reduction of aromatic compounds 225 4.2.4 Reduction of aldehydes and ketones 228 Contents xv 4.2.5 Reduction of nitriles, oximes and nitro compounds 228 4.2.6 Hydrogenolysis 229 4.2 [B] Homogeneous hydrogenation 231 4.3 Reduction with metal hydrides 233 4.3.1 Lithium aluminium hydride 233 4.3.2 Sodium borohydride 235 4.3.3 Sodium cyanoborohydride 236 4.3.4 Diborane 237 4.4 Reduction by dissolving metals 237 4.4.1 Sodium-alcohol 237 4.4.2 Sodium-liquid ammonia 240 4.4.3 Magnesium 240 4.4.4 Zinc-hvdrochloric acid 241 4.5 Reduction by miscellaneous reducing agents 241 4.5.1 Hydrazine 241 4.5.2 Di-imide 241 4.5.3 Formic acid 243 4.5.4 Silanes 244 4.5.5 Stannous chloride 244 4.5.6 Tin-hydrochloric acid 245 4.5.7 Zinc-acetic acid 246 4.5.8 Zinc-sodium hydroxide 246 4.5.9 Sodium metabisulphite 247 4.5.10 Sodium dithionite 247 4.5.11 Magnesium-alcohol 248 4.5.12 Sodium hydrogen sulphide 248 4.6 Photoreduction 248 4.7 Enzymatic or microbial reductions (Bio-reductions) 249 4.8 Correlation tables 251 4.8.1 Reduction of alkens 251 4.8.2 Reduction of alkynes 254 4.8.3 Reduction of aromatic hydrocarbons 255 4.8.4 Reduction of carbonyl compounds 257 (a) Reduction of aldehydes 257 (b) Reduction of ketones 258 4.8.5 Reduction of nitro and nitroso compunds 259 4.8.6 Reduction of nitriles, azides, oximes, halides and epoxides 260 4.8.7 Miscellaneous reduction 261 4 8.8 Hydrogenolysis 262 References 263 Problems 264 5. Some Reactions, Mechanisms and Appplications 266 Introduction 266 xvi Contents 5.1 Acetoacetic ester synthesis 267 5.2 Aldoi condensation 270 5.3 Algar-Flynn-Oyamada reaction 273 5.4 Arndt-Eistert synthesis 275 5.5 Auwers flavone synthesis 276 5.6 Baeyer-Villiger oxidation 277 5.7 Baker-Venktaraman rearrangement 277 5.8 Bamford-Stevens reaction 278 5.9 Barbier-Wieland degradation 280 5.10 Barton reaction 281 5.11 Beckmann rearrangement 283 5.12 Benzidine rearrangement 284 5.13 Benzilic acid rearrangement 286 5.14 Benzoin condensation 287 5.15 Birch reduction 288 5.16 Bouveault-Blanc reduction 290 5.17 Cannizzaro reaction 291 5.18 Chichibabin reaction 293 5.19 Claisen condensation 295 5.20 Claisen reaction 297 5.21 Claisen rearrangement 298 5.22 Claisen-Schmidt condensation 298 5.23 Clemmensen reduction 299 5.24 Cope elemination 301 5.25 Cope rearrangement 303 5.26 Dakin reaction 306 5.27 Darzens glycidic ester condensation 306 5.28 Dieckmann condensation 308 5.29 Diels-Alder reaction 310 5.30 Debner-Miller synthesis 314 5.31 Duff reaction 315 5.32 Enamine reaction 316 5.33 Ene reaction 318 5.34 Elbs persulfate oxidation 320 5.35 Etard reaction 322 5.36 Haloform reaction 323 5.37 Hantzsch pyridine synthesis 324 5.38 Hantzsch pyrrole synthesis 326 5.39 Hell-Volhard-Zeiinsky reaction 327 5.40 Henery reaction 329 5.41 Hinsberg oxyindoel synthesis 329 5.42 Hinsberg thiophene synthesis 330 5.43 Hofmann elimination 331 5.44 Hofmann isonitrile synthesis (Carbylamine reaction) 333 5.45 Hofmann-Martius rearrangement 333 Contents xvii 5.46 Hofmann rearrangement 334 5.47 Houben-Hoesh reaction 334 5.48 Hunsdiecker reaction 335 5.49 Jacobsen rearrangement 337 5.50 Jones oxidation 337 5.51 Kiliani-Fischer synthesis 339 5.52 Knoevenagel condensation 340 5.53 Knorr pyrazole synthesis 342 5.54 Knorr pyrrole synthesis 344 5.55 Knorr quinoline synthesis 345 5.56 Kolbe electrolytic reaction 346 5.57 Kolbe-Schmitt reaction 347 5.58 Kostanecki-Robinson acylation 349 5.59 Madelung indole synthesis 349 5.60 Malonic ester synthesis 350 5.61 Mannich reaction 352 5.62 Meerwein-Ponndorf-Verley reduction 354 5.63 Michael addition 354 5.64 Mukaiyama reaction 356 5.65 Nef reaction 357 5.66 Nencki reaction 358 5.67 Norrish type cleavage 359 5.68 Oppenauer oxidation 361 5.69 Paal-Knorr synthesis 361 5.70 Paterno-Buchi reaction 363 5.71 Pechmann condensation 364 5.72 Perkin reaction 365 5.73 Pinacol-pinacolone rearrangement 366 5.74 Reformatsky reaction 367 5.75 Reimer-Tiemann reaction 369 5.76 Ritter reaction 371 5.77 Rosenmund reduction 372 5.78 Rosenmund-Von Braun synthesis 372 5.79 Ruff-Fenton degradation 373 5.80 Sabatier and Senderens reduction 374 5.81 Sarett oxidation 374 5.82 Schiemann reaction (Balz-Schiemann reaction) 376 5.83 Schotten-Baumannn reaction 376 5.84 Simmons-Smith reaction 377 5.85 Skraup synthesis 379 5.86 Stephen reaction 380 5.87 Stobbe condensation 381 5.88 Strecker amino acid synthesis 382 5.89 Swern oxidation 383 5.90 Thiele acetylation 384 xviii Contents 5.91 Thorpe (Ziegle:) reaction 385 5.92 Tischenko reaction 386 5.93 Ullmann reaction 387 5.94 Vilsmeier reaction (Vilsmeier-Haack reaction) 388 5.95 Williamson's ether synthesis 390 5.96 Wohl-Ziegler reaction 391 5.97 Wolff-Kishner reduction 392 5.98 Wurtz reaction 393 Problems 396 6.
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