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62 Science of Synthesis 1.1 Organometallic Complexes of Nickel allowed to warm to 258C and worked up. The crude product was purified by flash chromatography (silica gel, hexane/Et 2O 20:1), affording the ester 68 as a white powder; yield: 1.13 g (71%); (Z/E) > 99:1. 1.1.4 Product Subclass 4: Nickel–Alkene Complexes Nickel complexes of alkenes are involved in many catalytic transformations. Many of the reaction classes of alkenes involve migratory insertion of an alkylnickel or a hydridonickel species. Alternatively, some transformations are initiated by the oxidative cyclization of nickel–alkene complexes with a second unsaturated component to produce five-membered metallacycles. Several examples of nickel–bis(alkene) complexes and nickel metallacyclopentanes are known, and the interconversion of these two structural classes has been studied (Scheme 55). [109,110] Scheme 55 Ligand Dependence of Nickel Metallacycle Decomposition [109,110] LNi L 2Ni L 3Ni L = trialkyl- or triarylphosphine Synthesis of Product Subclass 4 1.1.4.1 Method 1: Ligand Exchange with Nickel(0) Complexes 2 H 2C CH 2 Many crystal structures of nickel–alkene complexes have been reported. As demonstrated in Scheme 55, bis(alkene) complexes may exist in equilibrium with the corresponding metallacyclopentane complex. However, several alkene complexes which have the potential to undergo oxidative cyclization to a metallacycle have been fully characterized. The substitution chemistry of bis(ç 4 -cycloocta-1,5-diene)nickel(0) (2) is representative of most nickel(0)–alkene complexes, which are readily substituted by a variety of ligands. Bis(ç 2 -ethene)(tricyclohexylphosphine)nickel(0) has been prepared and fully characterized, [111,189] and a variety of complexes of electron-deficient alkenes such as 69 have been prepared which tend to be more stable than the complexes of ethene (Scheme 56). [112–114] The alkene complexes may be prepared directly from bis(ç 4 -cycloocta-1,5-diene)nickel(0) (2) [113] or from nickel(II) chloride [112] that is reduced by zinc metal. Scheme 56 Preparation of Bis(methyl acrylate)(pyridine)nickel(0) [112] NiCl 2 6H 2O + Zn + N + CO2Me THF, 60 oC 70% N Ni CO2Me CO2Me 69
1.1.4 Nickel–Alkene Complexes 63 Bis(methyl acrylate)(pyridine)nickel(0) (69): [112] To NiCl 2 •6H 2O (4.7 g, 20 mmol), methyl acrylate (5.0 mL, 55 mmol), and pyridine (5.0 mL, 61 mmol) in THF (50 mL) was added Zn powder (5.0 g, 76 mmol). The suspension was heated to 608C, and the oil bath was removed. After 2 h the mixture was filtered, and the solid residue was washed with THF (3 ” 15 mL). After removal of the solvent in vacuo, extraction with Et 2O (50 mL) left behind the insoluble zinc salts. Filtration and evaporation of the extract in vacuo gave a red oil which was treated with hexane to dissolve the excess pyridine and methyl acrylate. The hexane layer was discarded, and the residual oil was dissolved in Et 2O (50 mL). Cooling to –188C afforded 69 as a pale orange powder; yield: 4.3 g (70%); mp 808C (dec). Applications of Product Subclass 4 in Organic Synthesis 1.1.4.2 Method 2: Conjugate Addition to Electrophilic Double Bonds Nickel-catalyzed conjugate additions are among the most widely used synthetic applications of nickel chemistry. Conjugate additions of a broad range of main-group and transition-metal organometallics have been reported to be accelerated by nickel catalysis. Bis(acetylacetonato)nickel(II) (1) is the most widely used catalyst due to its low cost and air stability, although bis(ç 4 -cycloocta-1,5-diene)nickel(0) (2) is an excellent catalyst for most conjugate additions. Bis(acetylacetonato)nickel(II) (1) may be reduced by the nucleophilic organometallic reagent (organozinc, organoaluminum, etc.), or more typically it may be reduced with diisobutylaluminum hydride prior to treatment with the nucleophilic organometallic reagent. The primary advantages of nickel-catalyzed conjugate additions relative to organocuprates are increased thermal stability of the reagents and increased efficiency with sterically hindered substrates. Several asymmetric variants have been reported, but no general solution to the problem of asymmetric catalysis has been reported. The four variations listed below have received the most attention, although additions of triorganoindiums, [115] alkenylboranes, [116] and organotitaniums [117] have been reported. Mackenzie has reported a related procedure with organotins that likely bears mechanistic similarity to the procedures described here. [39] However, since this reaction class was demonstrated to involve nickel–ð-allyl complexes, its description is included in Section 1.1.2.7.2. It is worth noting that the variations described below may possibly involve ð-allyl complexes as reactive intermediates in direct analogy to the model proposed by Mackenzie. A comprehensive review of nickel-catalyzed nucleophilic additions to activated alkenes has appeared. [186] 1.1.4.2.1 Variation1: Organoaluminums The bis(acetylacetonato)nickel(II)-catalyzed addition of trimethylaluminum was the firstreported variant of nickel-catalyzed conjugate additions, [118,119] and a later report demonstrated that the process is efficient with sterically demanding substrates such as 70 that possess â,â-disubstituted enones (Scheme 57). [120] Significantly, alkynylaluminum reagents efficiently transfer the alkynyl unit in a conjugate fashion to enones such as 71 (Scheme 57). [121,122] This transformation is notoriously difficult with organocopper chemistry. Therefore, the nickel-catalyzed procedure is exceptionally useful. for references see p 79
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4.4.27 Æ-Haloalkylsilanes 153 In g
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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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