49. When heated, the structures shown to the right equilibrate by a Cope rearrangement mechanism; the numbered carbons should help you to see the necessary bond-making and bond-breaking changes which occur. The position of equilibrium in this reaction (as a function of substituents) has been discussed in a recent article. Use MM to calculate the relative energies of the equilibrating partners for the parent case (R1 = R2 = H); for the monomethyl derivative (R1 = CH3, R2 = H); and for two esters (R1 = H, R2 = COOCH3; R1 = CH3, R2 = COOCH3). Compare your results with the experimental results and with the MM calculations cited (see p. 1496, 2nd paragraph).
[Lange, J. H. M.; Klunder, A. J. H.; Zwanenburg, B. Tetrahedron 1991, 47, 1495.]
92. Compound 1,
(CH)14, has recently been synthesized. It undergoes Cope rearrangement
at 80oC and is completely transformed into isomer 2;
absolutely no 3 is detected. Upon irradiation, 1 is transformed
into 4. Do MM calculations on these compounds; try to explain why
the equilibrium between 1 and 2 is entirely on the side of
product and why 1 gives no 3. Then do MM calculations on
the related (CH)12 isomers 5-7.
[Fessner, W.-D.; Rodriguez, M. Angew. Chem., Int. Ed. Engl. 1991, 30, 1020.]
116. The Cope rearrangement of 1,5-dienes is a well-established reaction. Experiments have established that acyclic compounds react via a chair-like transition state in preference to the boat-like (as illustrated here for the parent case). The difference between the chair and boat transition states has been attributed to various factors. In a recent publication, it is argued that the difference is steric in origin, not electronic. Following the directions on p. 2639-40 of the cited reference, calculate the energies of the chair and boat transition states for 1,5-hexadiene itself (use FXDIS to set the proper C1 to C6 separation) and for compounds 20 and 21 in the article (the first of which can react only via the chair, the second by the boat). Compare your answers with those in Table II (calculated) and from Table I (experimental). Discuss the factors which make the chair transition state more stable.
[Shea, K. J.; Stoddard, G. J.; England, W. P.; Haffner, C. D. J. Am. Chem. Soc. 1992, 114, 2634.]
121. The Cope rearrangement between 5-hexen-1-yne and 1,2,5-hexatriene (and various derivatives) has been studied experimentally and by computation. Calculate the heats of formation of the parent compounds; of the derivatives with one methyl at C6; of the gem-dimethyl (at C6); and of the trimethyl (at C6 and C5). Compare your values with those based on Benson's tables of group equivalents, as reported in the article.
[Hopf, H.; Wachholz, G.; Walsh, R. Chem. Ber. 1992, 125, 711.]
196. The isomeric compounds shown can interconvert by a Cope rearrangement, a concerted process involving the 1,5-hexadiene unit within each caged structure. The left-hand isomer suffers from the strain of the four-membered ring, but benefits from having two conjugated unsaturated ketones. Compute for the pair shown here and for the isomeric compoounds in which the (CH2)2 and (CH2) bridges are interchanged (see structures 8, 10, and 12 in the article). Compare your computed enthalpy changes with those in the reference; comment on the factors that are responsible for the enthalpy difference.
[Mehta, G.; Reddy, S. H. K.; Pattabhi, V.; Bhanumathi, S.; Pramanik,
A.; Chandrasekhar, J. J. Chem. Soc., Perkin Trans. 1 1993, 1539.]
207. Pyrolysis of the dienone to the right gives two products; both are formed (supposedly) by a stepwise Cope rearrangement. Do MMX calculations on all three of these compounds as well as on selected other compounds (especially the trienes) in the cited article; determine if the products and rates (see Table I) can be correlated with the computed energies.
[Gleiter, R.; Sigwart, C. J. Org. Chem. 1994, 59, 1027.]
208. Triene 1 is, in principle, capable of undergoing two different Cope rearrangements. Only the rearrangement involving bonding of C1 to C6 is observed - the exclusive product is 3 which, presumably, arises by a second Cope rearrangement on undetectable intermediate 2. (Note that two views of 2 are shown: in 2a, the carbon numbers correspond to those in reactant 1; in 2b, the carbons are numbered in anticipation of the second Cope rearrangement.) It's interesting that 1 does not give the alternative Cope rearrangement (bonding of C1 to C6') leading to 4. Compute the energies of all of the structures shown. Try to formulate an explanation for the observed exclusivity of the first Cope reaction. Comment on the authors' claim that compound 2 has a very short lifetime because of "substantial steric compression ... an olefinic carbon in the six-membered ring is forcibly compressed against a carbon atom from the trans double bond ..."
[Guevel, R.; Paquette, L. A. J. Am. Chem. Soc. 1994, 116, 1776.]
237. The twistatrienes, to the right, undergo Cope rearrangement in stepwise fashion to the tricyclic trienes (cf. Problem No. 207). The highly resonance-stabilized diradical intermediate is shown from a distorted perspective in order to understand its generation; closure between atoms a and b generates the product. There is an enormous difference in rate, depending upon the number of CH2 groups in the bridge. For x = 1, t1/2 = 30 min at 30 oC; for x = 2, t1/2 = 560 hr at 80 oC. Do MMX computations on reactant, diradical, and product for both x = 1 and x = 2 in an effort to rationalize the huge rate difference.
[Gleiter, R.; Herb, T.; Borzyk, O.; Hyla-Krypsin, I. Liebigs Ann. Chem. 1995, 357.]
246. There are three
possible stereoisomers for the tetracyclic diene whose two-dimensional
representation is shown to the right. Do MMX calculations on all three
and assess their relative energies. One of these isomers, the endo,endo
(shown to the far right), is a derivative of 1,6-heptadiene (see the numbered
carbons); upon heating, it gives the first reported example of a "homo-Cope"
rearrangement. Use MMX to calculate the structure and energy of this rearrangement
[Hochstrate, D.; Klärner, F.-G. Liebigs Ann. Chem. 1995, 745.]
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