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38 Science of Synthesis 1.1 Organometallic Complexes of Nickel Scheme 12 Preparation of a Nickel–ç 3 -Allyl Complex from Nickel(0) and Allyl Electrophiles [35] Ni(cod) 2 + 2 OMe 20 Br Br MeO Ni Ni OMe Br 21 benzene 4−25 oC 55% Br MeO Ni Ni OMe Br 21 + Ph3P Et2O 97% MeO Br Ni PPh3 22 Di-ì-bromobis[(ç 3 -2-methoxyallyl)nickel] (21): [35] [Ni(cod) 2](2; 2.63 g, 9.60 mmol) was suspended in argon-sat. benzene (40 mL) in a 100-mL two-necked flask fitted with a stopcock and a dropping funnel and was cooled to 4 8C. 3- Bromo-2-methoxypropene (20; 1.45 g, 9.6 mmol) in benzene (5 mL) was added dropwise to the stirred slurry. A deep red color developed immediately, and some metallic nickel precipitated. The mixture was allowed to warm to 25 8C and was stirred at this temperature for 0.5 h. It was then filtered under argon and concentrated to ~ 20 mL under aspirator vacuum, and petroleum ether (75 mL) was added. The resulting red slurry was cooled to –20 8C to complete crystallization, and the supernatant was removed with a syringe. Compound 21 was obtained as a brick-red solid; yield: 1.00 g (55%). Bromo(ç 3 -2-methoxyallyl)(triphenylphosphine)nickel (22): [35] Ph 3P (0.29 g, 1.10 mmol) in Et 2O (5 mL) was added to complex 21 (0.23 g, 0.55 mmol) in Et 2O (10 mL) under argon at 25 8C. An orange precipitate formed immediately. After stirring at 258C for 0.5 h the slurry was allowed to settle, and the supernatant was removed with a syringe. Washing with Et 2O (10 mL) and drying under vacuum afforded compound 22; yield: 0.50 g (97%). 1.1.2.2 Method 2: Additionof Allylmagnesium Halides to Nickel(II) Salts Electrophilic nickel(II) salts, when treated with allyl organometallics, afford nickel(II)–ðallyl complexes. [8,36] Rather than producing dimeric halo-bridged complexes as observed by the oxidative addition route (Section 1.1.2.1), monomeric bis(allyl) complexes are instead obtained (Scheme 13). Although both the method described in Section 1.1.2.1 and this method afford structurally different ð-allyl complexes, disproportionation may, in some instances, allow the interconversion of a bis(allyl) complex 23 and the corresponding halo-bridged dimer 24 if a dihalonickel species is present. Scheme 13 Preparation of a ç 3 -Allylnickel Complex from Nickel(II) and Allyl Nucleophiles [8,36] MgBr Ni 23 + NiBr2 + NiBr 2 Et2O, −10 o C Ni 23 Br Ni Ni Br 24 + MgBr2
1.1.2 Nickel–Allyl Complexes 39 Bis(ç 3 -2-methylallyl)nickel (23): [8] Following the general procedure of Wilke, [37,190] anhyd NiBr 2 (18.7 g, 0.086 mmol) was suspended in anhyd Et 2O (100 mL) in a 1-L three-necked flask fitted with a 500-mL dropping funnel, a mechanical stirrer, and a Claisen head containing a low-temperature thermometer and a three-way stopcock. The system was alternately evacuated to aspirator vacuum and filled with inert gas (N 2 or argon) several times, and cooled to –108C. 2-Methylallylmagnesium bromide (27.1 g, 0.17 mol) in Et 2O (300 mL) was added dropwise to the vigorously stirred soln over 2 h. The mixture was stirred for an additional 0.4 h at –108C, warmed to 258C, and filtered under inert gas. (The simplest technique is to use a reaction flask with a fritted disk and two-way stopcock attached to the bottom; the soln can be forced through the frit with positive pressure into a second flask which is also under inert gas.) The residue was washed with Et 2O (50 mL), filtered, and the combined Et 2O filtrates were concentrated under aspirator vacuum at –308C. Pentane (125 mL) was added, the resulting slurry was filtered under inert gas (most of the pentane was removed from the filtrate at aspirator vacuum), and the remaining brown soln was cooled to –788C. Impure brown crystals (mp 358C) of 23 were isolated by decanting the supernatant liquid under inert gas. (It is convenient to store the complex at this stage at –208C and then sublime the requisite quantity into the vessel for further reaction.) Sublimation at 258C/0.1 Torr with a –788C receiver afforded yellow crystals. No precise yield has been reported for this method. 1.1.2.3 Method 3: Oxidative Addition of Nickel(0) with Enones in the Presence of Lewis Acids An advance by Mackenzie has made nickel–ð-allyl complexes accessible from enals and enones. [38,39] In a reaction that is mechanistically analogous to Method 1 in Section 1.1.2.1, enals, when treated with bis(ç 4 -cycloocta-1,5-diene)nickel(0) (2) in the presence of chlorosilanes, afford chloro-bridged dimeric ç 3 -allylnickel complexes such as 25 (Scheme 14). Enones are less reactive in the process and require pyridine to facilitate the oxidative addition. Rather than using bis(ç 4 -cycloocta-1,5-diene)nickel(0) (2), a more convenient and less expensive alternative involves the in situ reduction of dichlorotetrakis(pyridine)nickel(II) (26) with sodium metal in the presence of cyclooctadiene to give enone-derived ç 3 -allylnickel complexes (e.g., 27). Scheme 14 Preparation of Enone-Derived ç 3 -Allylnickel Complexes [38,39] H O NiCl2 6H2O + Ni(cod) 2 2 py + TMSCl NiCl2(py)4 26 86% TMSO 1. 2Na, excess cod 2. cyclopent-2-enone TBDMSCl Cl Ni Ni Cl 25 OTMS NiCl(py) 2 27 OTBDMS Di-ì-chlorobis{[ç 3 -1-(trimethylsiloxy)allyl]nickel(II)} (25): [38] In a drybox, a 50-mL Schlenk tube was charged with [Ni(cod) 2](2; 500 mg, 1.82 mmol, 1.00 equiv). On a Schlenk line, a soln of propenal (244 ìL, 3.64 mmol, 2.00 equiv) in benzene (8mL) in a 25-mL Schlenk vessel was treated with (MeO)(TMSO)C=CMe 2 (185 ìL, 0.909 mmol, 0.500 equiv; added as a proton-scavenging reagent), stirred for 5 min, and for references see p 79
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Page 86 and 87: 86 Biographical Sketches Rinaldo Po
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88 2.6.4.2.2 Variation 2: Two-Elect
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90 2.6 Product Class 6: Organometal
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92 Science of Synthesis 2.6 Complex
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100 Science of Synthesis 2.6 Comple
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141 Science of Synthesis Houben-Wey
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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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