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52 Science of Synthesis 1.1 Organometallic Complexes of Nickel 1.1.3.3 Method 3: Coupling of Alkynes with Isocyanides Little is known about the mechanistic details of isocyanide–alkyne couplings. However, a useful stoichiometric cyclization of enynes and diynes with isocyanides was developed by Tamao and Ito, [75] and Buchwald later developed a catalytic variant (Scheme 34). [76] The resulting cyclopentenimines may be hydrolyzed to cyclopentenones. A variety of carbocyclic and heterocyclic templates are tolerated in the sequence. Scheme 34 Cyclizations of Enynes and Isocyanides [75,76] O Ph + NC Ni(cod) 2 2, bipy Cyclization of Enynes To Form Bicyclic Cyclopentenones; General Procedure: [76] In an inert-atmosphere glovebox, [Ni(cod) 2](2; 14 mg, 0.05 mmol), bis(diphenylmethylene)ethylenediamine (24 mg, 0.06 mmol), the enyne (1.0 mmol), and TIPSCN (201 mg, 1.1 mmol) were dissolved in DMF (46 mL) in a 100-mL sealable Schlenk flask. The Schlenk flask was sealed, removed from the glovebox, and heated at 110–135 8C until the enyne was consumed (as determined by GC, 8–36 h). The flask was cooled to 0 8C, treated with sat. aq oxalic acid (10 mL), and stirred at rt for 12–24 h. Et 2O (80 mL) and sat. aq NaHCO 3 (40 mL) were added to the soln and the layers were separated. The aqueous layer was extracted with Et 2O (3 ” 50 mL) and the combined Et 2O extracts were washed with H 2O (3 ” 40 mL) and brine (40 mL). The Et 2O soln was dried (MgSO 4) and concentrated in vacuo to give an oily residue which was purified by flash chromatography (silica gel, Et 2O/hexane 1:1–2) to give the desired cyclopentenone, typically as a colorless to pale yellow oil. 1.1.3.4 Method 4: Coupling of Alkynes with Aldehydes Couplings of alkynes and aldehydes have been investigated in a number of contexts. Although no metallacycles derived from oxidative couplings of one alkyne and one aldehyde have been isolated, the reaction probably proceeds in a fashion similar to the coupling of alkynes and carbon dioxide (Section 1.1.3.2). Tsuda and Saegusa have reported the formal hydroacylation of alkynes by a process that involves the coupling of an alkyne and an aldehyde (Scheme 35). [77] Two possible mechanisms are proposed for the formation of the enone 54, and one involving the formation of an oxametallacycle is depicted. The corresponding coupling of diynes and aldehydes is also reported by the same investigators to afford the dihydropyran 55. [78] 80% O Ph N
1.1.3 Nickel–Alkyne Complexes 53 Scheme 35 Nickel-Catalyzed Aldehyde–Alkyne Couplings [77,78] X O H + Pr Pr Ni(cod) 2 2 L L Ni Pr O L L H Ni Pr R Ni(cod) 2 2, Cy3P + 1 O R2 X R1 R H O 2 O Pr Pr R 1 R 1 55 60% O H Pr Montgomery has developed a procedure for the three-component coupling of aldehydes, alkynes, and organozincs to produce allylic alcohols (Scheme 36). [79] Two-component couplings involving ynals are also reported. If no phosphine is employed, the organozinc ligand is incorporated into the allylic alcohol product 56. However, if bis(ç 4 -cycloocta-1,5diene)nickel(0) (2) and tributylphosphine are employed and if the organozinc possesses a â-hydrogen, a hydrogen atom is instead introduced from the organozinc to generate product 57. Triethylsilane may also be employed as the reducing agent, and this variant has been utilized in the total synthesis of allopumiliotoxin 267A (58; Scheme 37). [80] The ynal cyclization method is quite general with functionalized substrates. Scheme 36 Three-Component Aldehyde, Alkyne, Organozinc Couplings [79] O R 1 H + R 2 L L Ni R O 3 R2 R1 H R 3 ZnR 4 2 L nNi(0) R 4 ZnO R 1 R LnNi 4 R 1 H L L O Ni H OH R 4 R2 56 R2 H R1 L = THF R 3 R 2 R 3 R 3 large substituent small substituent (or tether chain) L = Bu3P R 4 ZnO H LnNi R 1 H 54 R 3 R 2 Pr OH H R3 R2 R1 H 57 (intramolecular variant <strong>only</strong>) for references see p 79
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Page 86 and 87: 86 Biographical Sketches Rinaldo Po
Page 88 and 89: 88 2.6.4.2.2 Variation 2: Two-Elect
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102 Science of Synthesis 2.6 Comple
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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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