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Transcript
14.11 Alkane Synthesis Using Organocopper Reagents Lithium Dialkylcuprates Lithium dialkylcuprates are useful synthetic reagents. They are prepared from alkyllithiums and a copper(I) halide. 2RLi + CuX R2CuLi + LiX [customary solvents are diethyl ether and tetrahydrofuran (THF)] How? the alkyllithium first reacts with the copper(I) halide R Cu Li R Li+ I Cu I– then a second molecule of the alkyllithium reacts with the alkylcopper species formed in the first step R Li+ R Li R Cu R Cu – Lithium diorganocuprates are used to form C—C bonds R2CuLi + R'X R R' + RCu + LiX Ar2CuLi + R'X Ar R' + ArCu + LiX Example: Lithium dimethylcuprate (CH3)2CuLi + CH3(CH2)8CH2I diethyl ether CH3(CH2)8CH2CH3 (90%) primary alkyl halides work best (secondary and tertiary alkyl halides undergo elimination) Example: Lithium diphenylcuprate (C6H5)2CuLi + CH3(CH2)6CH2I diethyl ether CH3(CH2)6CH2C6H5 (99%) Vinylic halides can be used (CH3CH2CH2CH2)2CuLi + diethyl ether CH2CH2CH2CH3 (80%) Br Aryl halides can be used (CH3CH2CH2CH2)2CuLi + diethyl ether CH2CH2CH2CH3 (75%) I 14.12 An Organozinc Reagent for Cyclopropane Synthesis Iodomethylzinc iodide formed by reaction of diiodomethane with zinc that has been coated with copper (called zinc-copper couple) Cu CH2I2 + Zn ICH2ZnI reacts with alkenes to form cyclopropanes reaction with alkenes is called the Simmons-Smith reaction Example CH2CH3 H2C CH2I2, Zn/Cu CH2CH3 C CH3 diethyl ether CH3 (79%) Stereospecific syn-addition CH3CH2 CH2CH3 C H CH2I2, Zn/Cu CH3CH2 H C H diethyl ether CH2CH3 H Stereospecific syn-addition CH3CH2 H C H CH2I2, Zn/Cu CH3CH2 H C CH2CH3 diethyl ether H CH2CH3 14.13 Carbenes and Carbenoids Carbene name to give to species that contains a divalent carbon (carbon with two bonds and six electrons) •• C Br Br dibromocarbene Carbenes are very reactive; normally cannot be isolated and stored. Are intermediates in certain reactions. Generation of Dibromocarbene Br Br C – •• H + •• OC(CH3)3 •• Br Br Br – • C• Br + H •• OC(CH3)3 •• Generation of Dibromocarbene •• C Br Br Br Br C •• Br – + Br – Carbenes react with alkenes to give cyclopropanes KOC(CH3)3 Br (CH3)3COH Br + CHBr3 (75%) CBr2 is an intermediate stereospecific syn addition 14.14 Transition-Metal Organic Compounds Introduction Many organometallic compounds derived from transition metals have useful properties. Typical transition metals are iron, nickel, chromium, platinum, and rhodium. 18-Electron Rule The number of ligands attached to a metal will be such that the sum of the electrons brought by the ligands plus the valence electrons of the metal equals 18. When the electron-count is less than 18, metal is said to be coordinatively unsaturated and can take on additional ligands. 18-Electron rule is to transition metals as the octet rule is to second-row elements. Example CO OC Ni CO CO Nickel carbonyl Ni has the electron configuration [Ar]4s23d8 Ni has 10 valence electrons Each CO uses 2 electrons to bond to Ni 4 CO contribute 8 valence electrons 10 + 8 = 18 (Benzene)tricarbonylchromium OC Cr CO CO Cr has the electron configuration [Ar]4s23d4 Cr has 6 valence electrons Each CO uses 2 electrons to bond to Cr 3 CO contribute 6 valence electrons benzene uses its 6 electrons to bind to Cr. Ferrocene Fe Fe2+ has the electron configuration [Ar]3d6 Each cyclopentadienide anion contributes 6 electrons Total 6 + 6 + 6 = 18 Organometallic compounds with cyclopentadienide ligands are called metallocenes. 14.15 Homogeneous Catalytic Hydrogenation Wilkinson’s Catalyst Wilkinson’s Catalyst Ni, Pt, Pd, and Rh can act as a heterogeneous catalyst in the hydrogenation of alkenes. However, tris(triphenylphosphine)rhodium chloride was found to be soluble in organic solvents. This catalyst was developed by Sir Geoffrey Wilkinson, who received a Nobel Prize in 1973. P(C6H5)3 (C6H5)3P Rh Cl P(C6H5)3 Wilkinson's Catalyst Mechanism of Homogeneous Hydrogenation Steps 1 and 2: Catalyst is converted to the active form. + H2 Cl then - (C6H5)3P P(C6H5)3 Cl Rh (C6H5)3P H P(C6H5)3 Rh (C6H5)3P P(C6H5)3 then - (C6H5)3P H + H2 This is the active form of the catalyst. Mechanism of Homogeneous Hydrogenation Step 3: Alkene bonds to rhodium through electrons. Cl Cl H P(C6H5)3 Rh P(C6H5)3 Rh (C6H5)3P H H (C6H5)3P H + CH2=CHCH3 CH2=CHCH3 Rhodium-alkene complex Mechanism of Homogeneous Hydrogenation Step 4: Rhodium-alkene complex rearranges. Cl H P(C6H5)3 Cl Rh (C6H5)3P H Rhodium-alkene complex P(C6H5)3 Rh (C6H5)3P CH2=CHCH3 H CH2CH2CH3 Mechanism of Homogeneous Hydrogenation Step 5: Hydride migrates from Rh to carbon. Cl H P(C6H5)3 Rh (C6H5)3P CH2CH2CH3 [(C6H5)3P]2RhCl + CH3CH2CH3 Mechanism of Homogeneous Hydrogenation Step 6: Active form of the catalyst is regenerated. Cl [(C6H5)3P]2RhCl + H2 H P(C6H5)3 Rh (C6H5)3P H 14.16 Olefin Metathesis Olefin Metathesis In crossed-olefin metathesis, one alkene is converted to a mixture of two new alkenes. catalyst 2 CH3CH=CH2 CH2=CH2 + CH3CH=CHCH3 The reaction is reversible, and regardless of whether we start with propene or a 1:1 mixture of ethylene and 2-butene, the same mixture is obtained. Olefin Metathesis The reaction is generally catalyzed a transition metal complex. Typically Ru, W, or Mo are used.Shown below is Grubb’s catalyst. Cl PCy Ru Cl PCy CH Cy = Ring-Opening Metathesis Ring-opening metathesis is used as a method of polymerization. Usually, it is applied most often when ring opening creates a relief of strain, as in some bicyclic alkenes. =HC CH= catalyst -80oC n Norbornene Polynorbornene 14.17 Ziegler-Natta Catalysis of Alkene Polymerization The catalysts used in coordination polymerization are transition-metal organic compounds. Ethylene oligomerization n H2C CH2 Al(CH2CH3)3 CH3CH2(CH2CH2)n-2CH CH2 Triethylaluminum catalyzes the formation of alkenes from ethylene. These compounds are called ethylene oligomers and the process is called oligomerization. Karl Ziegler (1950) n H2C CH2 Al(CH2CH3)3 CH3CH2(CH2CH2)n-2CH CH2 Ziegler found that oligomerization was affected differently by different transition metals. Some gave oligomers with 6-18 carbons, others gave polyethylene. Giulio Natta n H2C CHCH3 Al(CH2CH3)3 polypropylene Natta found that polymerization of propene under Ziegler's conditions gave mainly isotactic polypropylene. This discovery made it possible to produce polypropylene having useful properties. Karl Ziegler (1950) n H2C CH2 Al(CH2CH3)3 CH3CH2(CH2CH2)n-2CH CH2 The ethylene oligomers formed under Ziegler's conditions are called linear -olefins and have become important industrial chemicals. Karl Ziegler (1950) n H2C CH2 Al(CH2CH3)3 CH3CH2(CH2CH2)n-2CH CH2 The polyethylene formed under Ziegler's conditions is called high-density polyethylene and has, in many ways, more desirable properties than the polyethylene formed by free-radical polymerization. Ziegler-Natta Catalysts Early Ziegler-Natta catalyst were a combination of TiCl4 and (CH3CH2)2AlCl, or TiCl3 and (CH3CH2)3Al. Currently used Ziegler-Natta catalyst combinations include a metallocene such as bis(cyclopentadienyl)zirconium dichloride. Cl Zr Cl Ziegler-Natta Catalysts Early Ziegler-Natta catalyst were a combination of TiCl4 and (CH3CH2)2AlCl, or TiCl3 and (CH3CH2)3Al. Currently used Ziegler-Natta catalyst combinations include a metallocene such as bis(cyclopentadienyl)zirconium dichloride. Ziegler-Natta Catalysts The metallocene is used in combination with a promoter such as methyl alumoxane (MAO) O—Al—O—Al CH3 CH3 n Mechanism of Coordination Polymerization Cl Zr MAO Cl Zr Cl CH3 – Cl– This is the active form of the catalyst. + Zr CH3 Mechanism of Coordination Polymerization CH3 + Zr CH2 H2C + Zr CH2 CH2 H2C CH3 + Zr CH2 CH3 Mechanism of Coordination Polymerization + Zr H2C CH2CH2CH3 CH2 CH2 H2C + Zr CH2 CH2 + Zr CH3 CH2CH2CH2CH2CH3 Mechanism of Coordination Polymerization etc. H2C CH2 + Zr CH2CH2CH2CH2CH3