Organometallics CHEM 534 Final TAKE HOME exam Dec. 2004 Prof. Malachowski’s portion of the 2004 final exam 1. An important area of research interest is the identification of catalysts for the polymerization of ethylene. Chromium catalysts have recently been reported as efficient for the synthesis of 1-hexene. Shown below is a proposed catalytic cycle for this trimerization process as published in a recent literature article. Use our class discussions to answer the following questions about the proposed catalytic cycle. CrPPCrPPACr+PPCr(III)CrPPCrPP"activation"21) _____________________2) _____________________3) _________________4) _________________5 a) Based on our class discussions of organometallic reactions, label steps 1-4 with the reaction type. b) The last step, 5, lacks detail as drawn, but details have recently emerged from mechanistic studies. Step 5 actually involves two reactions. The first is a β-hydride elimination. Draw the product of a β-hydride elimination of intermediate A in the space below. CrPPAβ-hydride elimination c) What type of reaction would then take your intermediate to the final products, 1-hexene and the Cr-bisphosphine catalyst? d) What are the formal oxidation states of chromium in the catalytic cycle as drawn?2. Polymerization of ethylene and other simple alkenes is an enormously important area of organometallic chemical activity as it is the source of polymers used in a range of applications. One of the most famous catalytic process for polymerization is the Ziegler-Natta process. This reaction uses high valent forms of titanium or zirconium as the catalyst. a) Why might a high valent, early transition metal be more likely to make polymers instead of a trimer such as 1-hexene shown for chromium-catalyzed ethylene polymerization? b) One of the major controversies in organometallic polymerization is the mechanism of the reaction. There are primarily two proposed mechanisms. One mechanism suggests that the polymerization occurs via metallacycle intermediates as shown in the 1-hexene synthesis, while a second suggests polymerization occurs through a 1,2-ligand insertion. Using a hypothetical L3Ti-CH3 organometallic catalyst, sketch a series of reactions that show how the ethylene polymerization would proceed without metallacycle intermediates. c) The Ziegler-Natta polymerization is a heterogeneous catalytic process. What does this mean and what implications does this have for determining the mechanism of the reaction?3. Suggest a mechanism for the following reaction. Show all important intermediates and electron flow arrows where appropriate. Your mechanism should explain why the alkene is isomerized in the reaction process. Hint: a Pd-η3-allyl complex is an intermediate. CH3CH3OAcH3CH3COHNaCH(CN)2CH3CH3CH3H3CHONCCN+ Pd(PPh3)4THF, 65oC, 15 h 4. A couple weeks ago, I ran the following reaction in lab to make the biaryl product shown. a) Identify the name and type of reaction shown and show a "textbook" catalytic cycle for the synthesis of the product. ONR2OMeBrB(OH)2ONR2OMePd(PPh3)2+b) I actually added Pd(OAc)2 and PPh3 (3 eq.) to get to the catalytically active Pd(PPh3)2. This is a key, but overlooked and misunderstood step. Suggest a mechanism for this reaction. Hint: the actual catalytic Pd complex may be Pd(PPh3)2(OAc)-, the reaction is run in an aqueous solution and Ph3P=O is a side product. 5. a) Draw the products of the following reaction in the box provided. RuHLLLHMeOMeODCl+ b) Suggest a reasonable mechanism for the reaction leading to the products that you have drawn. c) Why doesn't a reaction occur without the addition of DCl?6. The titanacyclopentane article that we discussed in class on Tuesday, December 7 reported the titanacyclopropane below and offered characterization information in the experimental section. a) 1H NMR information provided for this compound showed four signals: δ 7.36-6.87, 1.72, 0.51, 0.22 ppm. Assign these four signals to protons in the structure. TiPMe3ArOArO b) The paper also reported the following splitting information:a) 1.72, triplet of doublets; b) 0.51 triplet; c) 0.22 doublet. Explain the splitting pattern seen for these three signals by identifying coupling constants. c) The related titanacyclopentane shown below did not show any clear splitting patters (all signals were reported to be broad). Suggest an explanation for this result.