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31.E: Transition Metal Organic Compounds (Exercises)

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    84770
  • Exercise 31-1 If the ferrocene rings in \(3\) were not free to rotate, how many different dichloroferrocene isomers would be expected (including chiral forms)? How could the substitution method (Section 1-1F) be used to determine which of the isomers was which?

    Exercise 31-2 The cyclobutadiene iron complex, \(10\), has been prepared optically active, and when oxidized with \(\ce{Ce}\)(IV) in the presence of tetracyanoethene gives a mixture of cyclobutadiene cycloadducts, all of which are optically inactive.

    a. Draw the other chiral form of \(10\).

    b. Write structures for the cycloadducts that would be expected to be formed if \(10\) were oxidized with \(\ce{Ce}\)(IV) in the presence of tetracyanoethene

    c. How does formation of optically inactive products indicate that the cycloadducts are formed from the cyclobutadiene corresponding to \(10\)?

    d. Is cycloaddition of an alkene to cyclobutadiene best regarded as a [2 + 2] or a [4 + 2] reaction?

    Exercise 31-3* Assuming the molecular formula of \(9\) is established as \(\ce{C_{40}H_{60}N_6Zr_2}\), explain how the proposed structure is consistent with \(\ce{^{15}N}\) NMR spectra as follows. Made with \(\ce{^{15}N \equiv ^{14}N}\), \(9\) shows three widely separated resonance lines of equal intensity. However, when \(9\) is made with \(\ce{^{15}N \equiv ^{15}N}\), two of the peaks become doublets with a spacing of \(6 \: \text{Hz}\).

    Exercise 31-4 The diphenylethyne complex with \(\ce{Pt}\)(0), analogous to \(12\), has been shown by x-ray diffraction analysis to have \(\ce{C-C \equiv C}\) bond angles of about \(140^\text{o}\) and a central \(\ce{C-C}\) bond distance of \(1.32 \: \text{Å}\). Explain which of the formulations, \(11a\), \(11b\), or \(11c\), seems most reasonable to account for the x-ray data for this complex.

    Exercise 31-5 Write the sequence of steps whereby \(\ce{(Cp)_2ZrClH}\) reacts with 2-methyl-2-pentene to form \(\ce{(Cp)_2Zr(Cl)CH_2CH_2CH_2CH(CH_3)_2}\). Why is there no appreciable amount of \(\ce{(Cp)_2Zr(Cl)CH_2CH(CH_3)CH_2CH_2CH_3}\) in the product?

    Exercise 31-6 Show how \(\ce{(Cp)_2ZrClH}\) could be used to achieve the following conversions:

    a.

    b. \(\ce{CH_3CH_2CH_2CH_2C \equiv CH} \rightarrow \ce{CH_3CH_2CH_2CH2CH=CHCHO}\)

    c.* \(\ce{(CH_3)_3CC \equiv CH} \rightarrow \ce{(CH_3)_3CCH_2CH_2COCH_3}\)

    Exercise 31-7* The stereochemistry of reactions in which \(\ce{Zr-C}\) bonds are formed and cleaved can be deduced from the results of the following reactions, where \(\ce{D}\) is hydrogen of mass 2.

    The \(\ce{CH-CH}\) coupling constants in the proton NMR spectra of \(14\) and \(15\) are about \(13 \: \text{Hz}\). Work out the favorable conformations and the likely configurations of \(14\) and \(15\) and the stereochemistry of the addition and cleavage reactions. (Review Section 9-10H.)

    Exercise 31-8 Explain how 2-methylpropanal could be formed in substantial amount in the cycle of Figure 31-3 with propene as the starting alkene.

    Exercise 31-9 Explain how an alkene-metathesis catalyst might convert a cycloalkene into (a) a long-chain unsaturated polymer, (b) a mixture of large-ring polymers, and (c) a catenane (interlocking carbon rings like two links in a chain).

    Exercise 31-10 The NMR spectrum of 2-propenylmagnesium bromide in ether is shown in Figure 31-4. With the aid of the discussion in Sections 9-10C and 9-10E and the knowledge that the \(\ce{CH_2}\) resonance of ethylmagnesium bromide comes at \(38 \: \text{Hz}\) upfield from tetramethylsilane, sketch the NMR spectrum you would expect for \(\ce{CH_2=CHCH_2MgBr}\). Consider possible ways of reconciling your expected spectrum with the actual spectrum shown in Figure 31-4. (Review Section 27-2.)

    Figure 31-4: NMR spectrum of 2-propenylmagnesium bromide in diethyl ether solution at \(60 \: \text{MHz}\) with reference to tetramethylsilane at \(0 \: \text{Hz}\). The off-scale bands are due to the diethyl ether, and the signals designated \(\ce{C_6H_{10}}\) are due to 1,5-hexadiene (coupling product resulting during formation of the Grignard reagent).

    Exercise 31-11 When one mole of azabenzene (pyridine), which is a good ligand, is added to a solution of one mole of \(20\) in diethyl ether, a complex of composition \(\ce{(C_3H_5)_2NiNC_5H_5}\) is formed in which the very complex proton spectrum of the \(\ce{C_3H_5}\) groups of \(20\) becomes greatly simplified and essentially like that of Figure 31-4. Explain how complexation of one mole of azabenzene with nickel in \(20\) could so greatly simplify the proton NMR spectrum.

    Exercise 31-12* \(\pi\)-Propenyl(ethyl)nickel decomposes at \(-70^\text{o}\) to give propene and ethene. If the ethyl group is labeled with deuterium as \(\ce{-CH_2CD_3}\), the products are \(\ce{C_3H_5D}\) and \(\ce{CD_2=CH_2}\). If it is labeled as \(\ce{-CD_2-CH_3}\), the products are \(\ce{C_3H_6} + \ce{CD_2=CH_2}\). Are these the products expected of a radical decomposition, or of a reversible hydride-shift followed by decomposition as in the mechanism of Section 31-2B? Suppose the hydride-shift step were not reversible, what products would you expect then?

    Exercise 31-13 Palladium has many interesting uses in organic syntheses. The following sequence of reactions also could be achieved by forming and carbonating a Grignard reagent, but would not be stereospecific as it is with palladium. Devise mechanistic steps for the reaction that account for the stereochemical result [\(\ce{L}\) is \(\ce{(C_6H_5)_3P}\)]. Review Sections 31-2, 31-3, and 31-4.

    Exercise 31-14*

    a. When a metal is complexed with an alkene, there are two possible ways for nucleophiles to become attached to carbon, as illustrated here with palladium:

    Show how these mechanisms in combination with others described in this chapter can explain how \(\ce{PdCl_2}\) can convert \(\ce{CH_2=CH_2}\) to \(\ce{CH_3CHO}\) (Wacker process). Your mechanism must be in accord with the fact that, when the reaction is carried out in \(\ce{D_2O}\), there is no deuterium in the ethanal formed.

    \[\ce{CH_2=CH_2} + \ce{PdCl_2} + \ce{H_2O} \rightarrow \ce{CH_3CHO} + 2 \ce{HCl} + \ce{Pd} \left( 0 \right)\]

    [This reaction is used for large-scale production by oxidizing the \(\ce{Pd}\)(0) back to \(\ce{Pd}\)(II) with \(\ce{Cu}\)(II). Thus \(\ce{Pd} \left( 0 \right) + 2 \ce{Cu} \left( II \right) \rightarrow \ce{Pd} \left( II \right) + 2 \ce{Cu} \left( I \right)\), and then the \(\ce{Cu}\)(I) is converted back to \(\ce{Cu}\)(II) with \(\ce{O_2}\). The overall result is \(\ce{CH_2=CH_2} + \frac{1}{2} \ce{O_2} \rightarrow \ce{CH_3CHO}\).]

    b. The balance between the competitive nucleophilic reactions described in Part a is a delicate one as judged from the following results:

    Write mechanistic steps that will account for the difference in stereochemical results of these reactions, noting that in one case there is a single carbonylation reaction and in the other a dicarbonylation reaction.

    Contributors

    • John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."