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54 Science of Synthesis 1.1 Organometallic Complexes of Nickel Scheme 37 Ynal Cyclizations in Pumiliotoxin Synthesis [80] N H H O OBn Et 3SiH Ni(cod) 2 2 Bu3P N H OTES OBn N H OH OH 58 allopumiliotoxin 267A Hodgson has developed a variant on the Nozaki–Kishi coupling [81] in which a haloalkene, an alkyne, and an aldehyde undergo coupling to produce allylic alcohols with a tetrasubstituted alkene (Scheme 38). [82] Only the fully intramolecular procedure is reported. Scheme 38 Aryl Iodide–Aldehyde–Alkyne Coupling [82] I O H CrCl 2, NiCl 2 DMF, 25 o C 2-Methyl-4-propyloct-4-en-3-one (54): [77] In a 50-mL stainless steel autoclave were placed under N 2, THF (8.50 mL), a soln of [Ni(cod) 2] (2; 14 mg, 0.050 mmol) in THF (1.20 mL), and trioctylphosphine (0.046 mL, 0.10 mmol). The mixture was stirred for several min, then oct-4-yne (0.147 mL, 1.00 mmol) and isobutyraldehyde (0.136 mL, 1.50 mmol) were added. The mixture was magnetically stirred for 20 h at 808C. The soln was concentrated to give a residue which was purified by preparative layer chromatography (hexane/Et 2O 15:1) to give a 7:1 mixture of 54 and a 2:1 adduct; yield: 0.124 g (60%). Alkylative Cyclization of Ynals; General Procedure: [79] A 0.5–0.6 M soln of ZnCl 2 (2.5–3.0 equiv) in THF was stirred at 0 8C, and the organolithium or Grignard reagent (3.7–4.5 equiv) was added by syringe followed by stirring for 10– 15 min at 08C. A 0.02–0.04 M soln of [Ni(cod) 2](2; 0.05–0.20 equiv) in THF was added and the resulting mixture was immediately transferred by cannula to a 0.1–0.2 M soln of the ynal (1.0 equiv). After consumption of starting material as measured by TLC analysis (typically 0.25–0.5 h at 08C), the mixture was subjected to an extractive workup with NH 4Cl/ NH 4OH buffer (pH 8) and Et 2O followed by flash chromatography (silica gel). Reductive Cyclization of Ynals; General Procedure: [79] A 0.04–0.05 M soln of Bu 3P {4 equiv relative to [Ni(cod) 2]} in THF was added to [Ni(cod) 2](2; 0.05–0.20 equiv) at 25 8C followed by stirring for 3–5 min. The nickel soln was transferred to a 0.5–0.6 M soln of commercial Et 2Zn (2.5–3.5 equiv) in THF at 0 8C, and the resulting mixture was immediately transferred by cannula to a 0.10 M soln of the ynal (1.0 equiv) in THF at 08C. After consumption of starting material as measured by TLC analysis (typically 0.25–2.0 h at 0 8C), the mixture was subjected to an extractive workup with NH 4Cl/ NH 4OH buffer (pH 8) and Et 2O followed by flash chromatography (silica gel). Three-Component Couplings; General Procedure: [79] A 1.0 M soln of ZnCl 2 (2.5–3.0 equiv) in THF was stirred at 0 8C, and the organolithium or Grignard reagent (4.5–5.4 equiv) was added by syringe followed by stirring for 10–15 min HO
1.1.3 Nickel–Alkyne Complexes 55 at 0 8C. A 0.05 M soln of [Ni(cod) 2](2; 0.20 equiv) in THF and a soln containing the aldehyde (3.0 equiv) and alkyne (1.0 equiv, 0.3–0.4 M relative to the alkyne) in THF were added sequentially to the organozinc reagent. After consumption of starting material as measured by TLC analysis (typically 0.25–0.5 h at 0 8C), the mixture was subjected to an extractive workup with NH 4Cl/NH 4OH buffer (pH 8) and Et 2O followed by flash chromatography (silica gel). With the alkyne as the limiting reagent, the product derived from direct addition of the organozinc to the aldehyde was observed as a significant byproduct. In cases in which separation of this byproduct was problematic, slightly lower yields were obtained by employing the aldehyde as the limiting reagent; however, the purification was simpler. 1.1.3.5 Method 5: Coupling of Two Alkynes The couplings of diynes with a third unsaturated component are described in several subsections throughout Section 1.1.3. However, several other processes merit discussion here that do not fall into the other categories described. The hydrosilylation of diynes provides an excellent route to cyclic dienylsilanes. A mechanism was proposed that involves Si-H oxidative addition, alkyne silylmetalation, alkyne insertion, and C-H bond reductive elimination to give 59 (Scheme 39). [83] A related process ensues when diynes are treated with hydrodisilanes, and bicyclic silacyclopentadienes 60 are produced (Scheme 40). [83] The intermolecular version of this process was first reported by Lappert. [84] Scheme 39 Diyne Hydrosilylation [83] R 1 R1 SiX3 Ni H + HSiX 3 Ni(acac) 2 1, DIBAL-H L nNi R 1 H SiX3 H Scheme 40 Diyne Hydrosilylation with Hydrodisilanes [83] R 1 R 2 + HSiR 3 2SiX 3 Ni(acac) 2 1, DIBAL-H, R 1 3P H R 59 1 SiX3 H R 1 R 60 2 SiR 3 2 H SiX3 Ni R1 Ln H Cheng has demonstrated that spirocyclic cyclopentadienes such as 61 may be produced upon treatment of an iodoalkene with an alkyne (Scheme 41). [85] A mechanism involving oxidative addition of nickel(0) to the iodoalkene followed by two sequential alkyne insertions has been proposed. for references see p 79
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4.4.27 Product Subclass 27: Æ-Halo
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4.4.27 Æ-Haloalkylsilanes 147 Sche
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4.4.27 Æ-Haloalkylsilanes 149 dros
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4.4.27 Æ-Haloalkylsilanes 151 4.4.
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4.4.27 Æ-Haloalkylsilanes 153 In g
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4.4.27 Æ-Haloalkylsilanes 155 4.4.
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4.4.27 Æ-Haloalkylsilanes 157 Æ-H
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References 159 References [1] Haman
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References 161 [93] Bruno, J. W.; S
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166 Biographical Sketches Alan Spiv
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168 5.1.22 Product Subclass 22: Ary
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170 Science of Synthesis 5.1 German
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176 References [43] Eaborn, C.; Var
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178 Abbreviations Chemical (cont.)
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180 Abbreviations Ligands (cont.) 2
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182 Abbreviations General (cont.) q
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