"Front Matter". In: Organosilanes in Radical Chemistry - Index of

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Application to Tandem and Cascade Radical Reactions 177 The plan for a tandem cyclization as the key step in the synthesis of morphine alkaloids has been successfully performed [95]. Three examples are given in Scheme 7.10. Specifically, the initially formed aryl radical, generated by bromine abstraction from compound 74, underwent a tandem cyclization to construct the desired carbocyclic skeleton. Depending on the nature of the Z substituent to the double bond, the product radicals either abstracted hydrogen from silane (Z ¼ CO2Me or CN) or eliminated thiyl radical (Z ¼ SPh). MeO O H H H Z (TMS) 3SiH MeO AIBN, 125 �C Z = CO2Me or CN O Br 74 Z (TMS) 3SiH AIBN, 125 �C Z = SPh MeO 45% 85% Scheme 7.10 Examples of tandem radical cyclizations Novel bridged spirolactones have been synthesized via tandem radical cyclizations of enol ether radical. In Reaction (7.85) the first 5-exo spirocyclization is followed by a 6-endo cyclization to give the bridged derivative as a single diastereoisomer [96]. O O Br (TMS) 3SiH AIBN, 80 �C O O H Me O 76% O O H H H (7.85) Tandem strategies have been successfully applied to synthesis of indole alkaloids, starting from aryl radical generation [97–99]. Iodoarylazides have been thoroughly used in protocols where the aryl radical formation is coupled with radical addition to alkenes and eventually a further cyclization onto an azide, with the elimination of nitrogen gas, as a driving force. The skeletons of ( )-horsfiline, ( )-aspidospermidine and ( )-vindoline have been achieved by this route. For example, cyclization of the iodoarylazide, such as 75, mediated by (TMS) 3SiH under standard experimental conditions, produced the N w Si(TMS) 3 protected alkaloid 76, which after washing with dilute acid afforded the amine 77 in an overall 83 % yield from 75 (Reaction 7.86). The
178 Consecutive Radical Reactions formation of the labile N w Si(TMS) 3 bond was thought to arise from the reaction of the product amine 77 with the by-product (TMS) 3SiI. Similar results were obtained for some substituted analogues of 75 [98]. It is worth underlining that the field of alkaloid synthesis via tandem cyclizations has favoured the application of (TMS) 3SiH over other radical-based reagents, due to its very low toxicity and chemoselectivity. I N 3 N SO2Me 75 (TMS) 3SiH AIBN, 80 �C N H MeO2S R N 76 R = Si(TMS) 3 77 R = H, 83% (7.86) A radical annulation strategy for the synthesis of heterocyclic fused quinolines has been proposed, starting from the readily available N-aryl thiocarbamates, thioamides and thioureas of general structure 78 (Reaction 7.87) [100]. (TMS) 3SiH (4 equiv) together with AIBN (1 equiv) were used under UV irradiation in benzene solutions, thus giving the cyclized products in good to excellent yields. The reaction mechansim consists of the addition of silyl radical to the thiocarbonyl function to give an athioalkylamino, which undergoes a tandem radical cyclization prior to rearomatization. X N H Z S 78 Y (TMS) 3SiH AIBN, hν, 20h Y = CH 2 , NH, O X Z N 44-88% Y (7.87) As model studies and radical reactivity control have improved, the so-called cascade (or domino) reactions have emerged as a very powerful method for natural product synthesis, since they offer a unique route to prepare complex backbones from appropriately designed but quite simple precursors. A few selected reactions will be presented here. Treatment of iodo derivative 79 with (TMS) 3SiH starts a cascade of 6-endo, 6-endo, and 6-exo cyclizations to afford exclusively 80 in a 77 % yield (Reaction
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Organosilanes in Radical Chemistry
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DEDICATION To my parents XrZstoB an
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viii Contents 3.6 Hydrogen Atom: An
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PREFACE A large number of papers de
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ACKNOWLEDGEMENTS I thank Keith U. I
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2 Formation and Structures of Silyl
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10 Formation and Structures of Sily
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2 Thermochemistry 2.1 GENERAL CONSI
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Bond Dissociation Enthalpies 21 Tab
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Bond Dissociation Enthalpies 23 2.2
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Ion Thermochemistry 25 Table 2.4 Re
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Ion Thermochemistry 27 Table 2.5 El
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References 29 3. Goumri, A., Yuan,
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32 Hydrogen Donor Abilities of Sili
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4 Reducing Agents 4.1 GENERAL ASPEC
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General Aspects of Radical Chain Re
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Tris(trimethylsilyl)silane 53 Therm
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Tris(trimethylsilyl)silane 55 4.3.1
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Tris(trimethylsilyl)silane 57 A goo
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Tris(trimethylsilyl)silane 59 C(O)C
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Tris(trimethylsilyl)silane 61 radic
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Tris(trimethylsilyl)silane 63 86% y
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Tris(trimethylsilyl)silane 65 RO RO
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Tris(trimethylsilyl)silane 67 O AcO
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Tris(trimethylsilyl)silane 69 Tris(
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Other Silicon Hydrides 71 The reduc
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Other Silicon Hydrides 73 The decre
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Other Silicon Hydrides 75 Ph MeS O
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Other Silicon Hydrides 77 Table 4.6
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Silicon Hydride / Thiol Mixture 79
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Silylated Cyclohexadienes 81 and (4
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References 83 34. Kawashima, E., Uc
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References 85 104. Gimisis, T., Bal
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88 Addition to Unsaturated Bonds te
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90 Addition to Unsaturated Bonds ap
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92 Addition to Unsaturated Bonds 5.
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94 Addition to Unsaturated Bonds R
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96 Addition to Unsaturated Bonds Ph
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98 Addition to Unsaturated Bonds EP
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100 Addition to Unsaturated Bonds 5
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102 Addition to Unsaturated Bonds T
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104 Addition to Unsaturated Bonds R
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106 Addition to Unsaturated Bonds a
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110 Addition to Unsaturated Bonds S
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112 Addition to Unsaturated Bonds T
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114 Addition to Unsaturated Bonds I
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120 Unimolecular Reactions 1 Si(H)M
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122 Unimolecular Reactions t-Bu t-B
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124 Unimolecular Reactions MeO Si O
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Page 141 and 142: 136 Unimolecular Reactions Br O Si(
Page 143 and 144: 138 Unimolecular Reactions R3Si C C
Page 145 and 146: 140 Unimolecular Reactions Me3Si O
Page 147 and 148: 142 Unimolecular Reactions 43. Albe
Page 149 and 150: 144 Consecutive Radical Reactions c
Page 151 and 152: 146 Consecutive Radical Reactions O
Page 153 and 154: 148 Consecutive Radical Reactions a
Page 155 and 156: 150 Consecutive Radical Reactions d
Page 157 and 158: 152 Consecutive Radical Reactions T
Page 159 and 160: 154 Consecutive Radical Reactions M
Page 161 and 162: 156 Consecutive Radical Reactions F
Page 167 and 168: 162 Consecutive Radical Reactions i
Page 169 and 170: 164 Consecutive Radical Reactions A
Page 173 and 174: 168 Consecutive Radical Reactions r
Page 175 and 176: 170 Consecutive Radical Reactions E
Page 179 and 180: 174 Consecutive Radical Reactions A
Page 181: 176 Consecutive Radical Reactions P
Page 187 and 188: 182 Consecutive Radical Reactions 1
Page 189 and 190: 184 Consecutive Radical Reactions 8
Page 191 and 192: 186 Silyl Radicals in Polymers and
Page 225 and 226: 220 List of Abbreviations PED photo
Page 227 and 228: 222 Index Carbonyl sulfide 111 Carb
Page 229 and 230: 224 Index Organosilicon boranes 2,
Page 231 and 232: 226 Index Triethylsilane as mediato
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