Rauk Orbital Interaction Theory of Organic Chemistry

EXERCISES 309

Figure B.5. (a) Interaction of two molecules in their ground state. (b) three-electron and oneelectron bonding in an exciplex. (c) Bonding in an excimer.

sensitizer, a twisted binaphthyl (Kim, J.-I.; Schuster, G. B., J. Am. Chem. Soc., 1992, 114, 9309±9317).]

Answer. The interaction between two ground-state molecules in close approach is shown schematically in Figure B.5a. The energy interaction is dominated by the four-electron, two-orbital case, which is repulsive. The interaction of the LUMOs, which may be large, has no consequence to the energy, unless one of the molecules is photoexcited. In this case, shown in Figure B.5b, the LUMO±LUMO and the HOMO±HOMO interactions become attractive. The overall energy gain depends on the energy separations and the extent of orbital overlap in the two interactions. A perfect match in nodal characteristics and energies is achieved when the two molecules are identical (Figure B.5c.). Therefore an excimer would be expected to be more stable in general than an exciplex.

19.Use orbital interaction theory to develop the p orbitals of the 2-oxaallyl system, R2CÐOÐCR2, also known as a carbonyl ylide. Show why 2-oxaallyl readily reacts with alkenes and alkyne in a 4 + 2 cycloaddition reaction (an example may be found in El-Saidi, M.; Kassam, K.; Pole, D. L.; Tadey, T.; Warkentin, J., J. Am. Chem. Soc., 1992, 114, 8751±8752).

20.The photochemistry of previtamin D3 has been intensively studied (for leading references, see Dauben, W. G.; Disanayaka, B.; Funho¨, D. J. H.; Kohler, B. E.; Schilke, D. E.; Zhou, B., J. Am. Chem. Soc., 1991, 113, 8367, and Enas, J. D.; Shen, G.-Y.; Okamura, W. H., J. Am. Chem. Soc., 1991, 113, 3873). The thermal and photoreactions are summarized in Figure B.6. Discuss the various conversions using the descriptive terminology of pericyclic reactions.

21.Cyclobutadiene has been shown to have a rectangular geometry by competitive trapping of the two valence tautomeric 1,2-dideuteriocyclobutadienes using methyl 3-cyanoacrylate in a Diels±Alder reaction. [(a) Whitman, D. W.; Carpenter, B. K., J. Am. Chem. Soc., 1982, 104, 6473±6474. (b) Whitman, D. W.; Carpenter, B. K.,

J. Am. Chem. Soc., 1980, 102, 4272±4274.]

310 EXERCISES

Figure B.6. Photoreactions of previtamin D3.

Show the products expected from the Diels±Alder reactions. Use orbital interaction theory to develop a bonding scheme for rectangular cyclobutadiene and explain why rectangular cyclobutadiene may be exceptionally reactive as a diene in Diels±Alder reactions.

22.The operation of the anomeric e¨ect and the stabilization of carbocations are beautifully illustrated in a conformational study of 2-oxanol (2-oxacyclohexanol) (Smith, B. J., J. Am. Chem. Soc., 1997, 119, 2699±2706). 2-Oxanol prefers the OH axial form by 12 kJ/mol and, upon protonation of the OH group, spontaneously loses water to form the oxonium ion. Use principles of orbital interaction theory to explain:

EXERCISES 311

(a)The stabilization of the carbocation center by the oxygen

(b)The preference for the OH axial form (the anomeric e¨ect)

(c)Why it is plausible that loss of water would be easy from the protonated form

23.Vinylboronic acids have found use in a multicomponent one-step synthesis of a-

amino acids (see question 24). The HOMO and LUMO of the parent compound, CH2 ÐCHB(OH)2, from an SHMO calculation are

eHOMO ˆ a ÿ 1:153jbj: fHOMO ˆ 0:62w1 ‡ 0:72w2 ‡ 0:28w3 ÿ 0:10w4 ÿ 0:10w5 eLUMO ˆ a ‡ 0:343jbj: fLUMO ˆ ÿ0:62w1 ‡ 0:21w2 ‡ 0:74w3 ÿ 0:11w4 ÿ 0:11w5

(a)Draw the HOMO and LUMO.

(b)Would you expect vinylboronic acid to be more of less reactive toward electrophilic attack than ethylene? Where would be the site of attack? Explain.

(c)Would you expect vinylboronic acid to be more of less reactive toward nucleophilic attack than ethylene? Where would be the site of attack? Explain.

(d)De®ne a, b, and w1.

24.Petasis and Zavialov have developed the following new synthesis of a-amino acids 4 (Petasis, N. A.; Zavialov, I. A., J. Am. Chem. Soc., 1997, 119, 445±446):

312 EXERCISES

The reaction is initiated by addition of the amine 2 to the a keto acid 3, catalyzed by the vinyl boronic acid derivative 1. Use orbital interaction theoretical arguments to explain the following features of this synthesis:

(a)The amine attacks the carbonyl of the keto group of 3 rather than the carbonyl of the acid group.

(b)The nucleophilic attack on carbonyl may be catalyzed by the vinylboronic acid 1. How?

(c)The amine attacks the a keto acid 3 rather than the vinylboronic acid 1. Why? (By SHMO, the LUMO of an a keto acid is at a ‡ 0:122jbj; see also question 23).

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