Rauk Orbital Interaction Theory of Organic Chemistry

290 EXERCISES

(g)The following rearrangement of cycloheptatrienes is not concerted but proceeds by thermally allowed steps. Suggest a mechanism.

(h) Propose a mechanism for the following reaction.

3. Classify the following reactions by the component analysis method (e.g., p4s ‡ s2a ‡ ). Decide whether the reaction as shown is thermally allowed. Show clearly the orbitals on which you base your analysis.

(a)The cycloaddition of acetylene to cyclobutadiene to give Dewar benzene:

(b)The rearrangement of Dewar benzene to benzene:

(c)The electrocyclic opening of the steroid-derived cyclohexadiene:

(d)The opening of cyclopropyl cations to allyl cations:

(e)The sigmatropic rearrangement of bicyclo[2.1.1]hex-2-ene to bicyclo[3.1.0]hex-2- ene:

EXERCISES 291

(f ) The following stereochemistry was recently established for the thermal vinylcyclopropane to cyclopentene rearrangement (Gajewski, J. J.; Olson, L. P.; Willcott III, M. R., J. Am. Chem. Soc., 1996, 118, 299±306). Using whatever method you wish, decide whether the experimental result is consistent with a concerted allowed [1,3] sigmatropic rearrangement.

4.Use orbital interaction diagrams to explain the following observations:

(a)Benzyne is an excellent dienophile in Diels±Alder reactions.

(b)A mixture of cyclopentadiene and ethene yields only dicyclopentadiene and not norbornene (bicyclo[2.2.1]hept-2-ene).

(c)2-Methylpropenyl cation adds to cyclopentadiene to form 1.

5.Show the expected products of the following Diels±Alder reactions. Pay careful attention to stereochemistry and regioselectivity if these considerations are appropriate.

292 EXERCISES

6. Predict the product of each of the following reactions:

D

(a) H2CÐCHÐCO2H ‡ (E )-H2CÐCHÐCHÐCHCH3 ÿ!

(b)

7.The origin of the diastereoselectivity found for the Diels±Alder reaction, has been attributed to a dominance of interactions between the occupied MOs of the reactants, that is, to four-electron, two-orbital interactions rather than to the usual secondary aspect of the HOMO±LUMO interaction. Show by suitable orbital interaction diagrams why this may be the case. The relevant references are found in Ashton, P. R.; Brown, G. R.; Isaacs, N. S.; Giu¨rida, D.; Kohnke, F. H.; Mathias, J. P.; Slawin, A. M. Z.; Smith, D. R.; Stoddart, J. F.; Williams, D. J., J. Am. Chem. Soc., 1992, 114, 6330±6353. Note: The process shown in the following reaction forms the basis for the construction of larger stereoregular molecules and has been termed molecular LEGO.

Chapter 13

1.(a) An optically active complex, PtCl2(Am*)(C2H4) 1 (Am* ˆ a-methylbenzylamine), derived from Zeise's anion 2, was used in the ®rst optical resolution of transcyclooctene. The ethylene could be exchanged for trans-cyclooctene to give both diastereomers of PtCl2(Am*)(trans-C8H14), which could be separated (Cope, A. C.; Ganellin, C. R.; Johnson, Jr., H. W.; Van Auken, T. V.; Winkler, H. J. S., J. Am. Chem. Soc., 1963, 85, 3276±3279). Derive a bonding picture for the metal-to-distorted alkene part of the complex 1.

(b)Exchange of the alkene in complexes such as 1 or 2 have been shown (Plutino, M. R.; Otto, S.; Roodt, A.; Elding, L. I., Inorg. Chem., 1999, 38, 1233±1238) to take place by an associative mechanism involving displacement of the labile trans ligand. On the basis of the frontier orbitals of 2, suggest a structure for the initial associative complex between 2 and ethylene.

2.The commercially important Wacker process for the oxidation of ethylene is shown in Figure B13.1 (see Shriver, D. F.; Atkins, P. W.; Langford, C. H., Inorganic Chemistry, Oxford University Press, Oxford, 1994, p. 728).

EXERCISES 293

Figure B13.1. Schematic of the Wacker process for the oxidation of ethylene.

(a)In step 2, show the orbitals involved in the nucleophilic addition of water to the complex.

(b)Discuss step 3 in terms of anchimeric assistance by the hydroxyl group.

(c)Compare steps 3r and 4. Which should be the more probable based on orbital interaction theory?

(d)Discuss step 5 in terms of anchimeric assistance by the hydroxyl group.

3.A catalytic system based on bisphosphine-substituted Pd(II) in the presence of both CO and an alkene leads to the formation of alternating ole®n±CO copolymers, (CH2CH2C(O)Ð)n, rather than homopolymers. The system has been studied theoretically (Margl. P.; Ziegler, T., J. Am. Chem. Soc., 1996, 118, 7337±7344). A schematic is shown in Figure B13.2. According to the study, the alternation arises from the lower activation energies for steps 5 (48 kJ/mol) and 6 (58 kJ/mol) compared to step 7 (65 kJ/mol) and the greater exothermicity of CO attachment (step 1, ÿ219 kJ/ mol) over ethylene attachment (step 2, ÿ200 kJ/mol). Apply principles of orbital interaction theory to rationalize the trends in activation energies and binding energies.

Answer. Three aspects of the scheme shown in Figure B13.2 may be examined by orbital interaction theory: (a) the binding of CO compared to ethylene; (b) the reactivity of coordinated carbonyl compared to ethylene with respect to reactivity toward intramolecular nucleophilic attack; and (c) the migratory aptitude of alkyl versus carbonyl groups.

294 EXERCISES

Figure B13.2. Schematic of the copolymerization of CO and ethylene. The dashed arrows (steps 5±8) represent intramolecular rearrangement processes.

(a)Figure B13.3a shows the possible interactions between a tricoordinated Pd(II) complex and ethylene or CO. The binding of ethylene is entirely analogous to

Zeise's salt, which was discussed in Chapter 13 (Figure 13.7). Only the positions

of the ethylene donor …pCC† and acceptor …pCC† are shown for reference. Carbon monoxide, having a higher donor orbital …nCO†, and a pair of lower energy ac-

ceptor orbitals …pCO† will bind more strongly to the metal, in spite of the less favorable four-electron, two-orbital interaction with the n dz2 orbital. In other words, the carbonyl donor and acceptor interactions, i and ii, respectively, will both be more favorable than the corresponding interactions for ethylene, iii and iv, respectively, but the repulsive interaction with n dz2 will also be stronger for CO. Thus, with reference to Figure B13.2, step 1 is more exothermic than step 2, and step 3 is more exothermic than step 4.

(b)For an assessment of the acceptor aptitude of ethylene compared to carbonyl

(acyl or CO) ligands on the metal, one needs to consider the energy of the pCC and pCO orbitals and the possible s-type overlap with a group in the cis position

EXERCISES 295

…b†

…c†

…a†

Figure B13.3. (a) Comparison of bonding of CO and ethylene to Pd(II). (b) Distortion of bound ethylene to favor interaction between pCC and a cis ligand. (c) Compared to interaction with pCO of ligated CO or acyl group.

of the metal. In consideration of the relative energies, you may ignore the presence of the metal since p backbonding in these cationic complexes is weak. Energy considerations dictate that the acceptor ability of the acyl or carbonyl group should be better than that of ethylene simply because the pCO orbital is at lower energy than pCC. However, and unfortunately, geometric considerations have the opposite e¨ect, as shown graphically in Figures B13.3b,c. The ethylene can readily shift to improve overlap of the pCC orbital with a cis ligand while maintaining bonding to the metal through the opposite C atom.

(c)The migratory aptitude of the alkyl group should be greater than of acyl because the s bond to alkyl (which resembles a nonbonded orbital on sp3 hybridized carbon) is higher in energy than the s bond to acyl (which resembles a nonbonded orbital of sp2 hybridized carbon). Thus, with reference to Figure B13.2, step 5, in which an alkyl group migrates to carbonyl, is more facile than step 7, in which an alkyl group migrates to ethylene. To obtain the observed alternation in copolymerization, step 6, in which an acyl group migrates to ethylene, yielding a longer alkyllike segment, must be easier than step 7, and also step 8, in

296 EXERCISES

which an acyl group migrates to carbonyl, yielding adjacent carbonyl groups in the polymer chain. The theoretical calculations yield this result, but it cannot be deduced from simple orbital interaction considerations.

Chapter 14

1.A key step in one route to the synthesis of hexamethyl Dewar benzene is the cycloaddition of 2-butyne to tetramethylcyclobutadiene (stabilized by Al cation). Using the parent compounds (no methyls), develop a Woodward±Ho¨mann orbital correlation diagram for the reaction and determine whether the reaction is thermally allowed.

2.This question requires you to construct an orbital correlation diagram of the Woodward±Ho¨mann type.

(a)Show that the rearrangement of Dewar benzene to benzene is a thermally forbidden process.

[Breslow et al. (Breslow, R.; Napierski, J.; Schmidt, A. H., J. Am. Chem. Soc., 1972, 94, 5906±5907) have determined that the activation energy for the rearrangement is 96.2 kJ/mol.]

(b)Show that the photochemical electrocyclic ring opening of 1,3-cyclohexadiene to cis-1,3,5-hexatriene should occur by conrotatory motion.

[Although long predicted by the Woodward±Ho¨mann rules, this was ®rst demonstrated experimentally in 1987 (Trulson, M. O.; Dollinger, G. D.; Mathies, R. A., J. Am. Chem. Soc., 1987, 109, 586±587).]

(c)Cyclopropyl cations open to allyl cations spontaneously and stereospeci®cally. Predict the stereochemical course of ring opening, disrotatory or conrotatory?

(d)Further to (c) above, it has been shown both experimentally and theoretically that 2,3-dialkyl-1,1-di¯uorocyclopropanes isomerize thermally in a disrotatory manner (Tian, F.; Lewis, S. B.; Bartberger, M. D.; Dolbier, Jr., W. R.; Borden, W. T., J. Am. Chem. Soc., 1998, 120, 6187±6188). Explain why these compounds would be expected to behave analogously to cyclopropyl cations.

Answer to 2(b). The orbital correlation diagram for the rearrangement of Dewar benzene to benzene is shown in Figure B14.1. This is a special case of electrocyclic ring opening. The bridgehead carbon atoms must rotate in a disrotatory fashion, preserving a single mirror plane of symmetry, s1. A second mirror plane, s2, is also preserved, but this does not determine the allowedness of the reaction.

298 EXERCISES

Answer to 2(d). This question illustrates that it is the number of electrons, not the number of nuclei, that is important. The orbital correlation diagram is shown in Figure B14.2. In disrotatory opening, a mirror plane of symmetry is preserved. This correlation is with bold symmetry labels and solid correlation lines. Italic symmetry labels and dotted correlation lines denote the preserved rotational axis of symmetry for conrotatory ring opening. For the cation, the disrotatory mode is the thermally allowed mode. It corresponds to a s2s ‡ o0s pericyclic reaction.

Chapter 15

1.In a recent article on photoexcited ketones, Wagner reported the following overall reaction (Wagner, P. J., Acc. Chem. Res., 1989, 22, 83), a mechanism thought to involve a diradical intermediate:

(a)Draw the structure of the intermediate diradical and explain how each of the products could be derived from it.

(b)Produce a state correlation diagram of the Dauben±Salem±Turro type and analyze the reaction in terms of the carbonyl electronic states which are likely to be involved (you may ignore the e¨ect of the phenyl group on the carbonyl and radical orbitals; it will not change the relative energies of the states).

Answer. All of the products can be directly derived from the diradical shown in Figure B15.1a. The state correlation diagram for the Norrish Type II reaction is shown in Figure B15.1b. The insert shows the two product orbitals and the con®g- uration of the 3D state. Both the singlet and triplet np states of the carbonyl group descend to product diradical ground states. E½cient intersystem crossing (IC) on the singlet manifold may account for the 16% yield of recovered starting material.

2.In the article mentioned in question 1, Wagner reported the following overall reaction, a mechanism thought to involve two di¨erent diradical intermediates:

(a)Draw the structures of the intermediate diradicals and explain how each of the products could be derived from it.

(b)Produce a state correlation diagram of the Dauben±Salem±Turro type and analyze the reaction in terms of the carbonyl electronic states which are likely to be involved (you may ignore the e¨ect of the phenyl group on the carbonyl and radical orbitals; it will not change the relative energies of the states).

3.This question involves the analysis of carbonyl photochemistry using Dauben± Salem±Turro state correlation diagrams.