6. A stepwise mechanism
has been suggested for the thermal conversion of cis-bicyclo[4.2.0]oct-7-
ene into Z,Z-1,3-cyclooctadiene. What does molecular mechanics
indicate about the feasibility of this? What about the analogous transformation
of the [3.2.0] system? How well does molecular mechanics (a non-orbital
calculation) mimic the orbital symmetry calculation?
11. Exo-fused
norbornyl derivatives are know to undergo transannular hydride transfers
in the carbocations. How close are the hydrogens in the hydrocarbons shown?
What are the relative strain energies vs. the non-fused analogs?
35. Use molecular
mechanics to calculate the energy, geometry, strain, etc. of the bicyclic
alkene (shown to the right) and of its conjugate acid after protonation;
do the calculations for n = 4, 5, and 6. Determine the distance from the
"inside" H to the double bond and to the cationic center to see
if there is a possibility of having a hydrido-bridged structure; see the
cited article for the meaning of the preceding phrase.
[Sorensen, T. S.; Whitworth, S. M. J. Am. Chem. Soc. 1990, 112, 8135.]
43. The electrocyclic
opening (with heat or with light) of cis- or trans- bicyclo[5.2.0]non-8-ene
can produce cis,cis and/or cis,trans-1,3- cyclononadiene.
Calculate the ground- state geometries and energies of both stereoisomers
of the bicyclic reactant. Then, calculate the energies and geometries of
both the cis,cis and cis,trans monocyclic dienes, not only
in their lowest energy form but also in the conformation in which the four
carbons of the diene are coplanar. See the cited article (Scheme II and
p. 1578, left column) for details on this.
[Leigh, W. J.; Zheng, K.; Clark, K. B. J. Org. Chem. 1991, 56, 1574.]
86A. Upon heating
(E)-1,3,5-hexatriene cyclizes to 1,3- cyclohexadiene, presumably
by first undergoing electrocyclic ring closure to 3-vinylcyclobutene which
then opens to (Z)-1,3,5-hexatriene, the immediate precursor to the
six-membered ring. (Note: neither acyclic triene is shown in the conformation
needed for ring closure to occur.) Calculate the heats of formation of
all four compounds and compare your results with those shown in Figure
1 and Table 3 of the reference.
86B. (E,E)-1,3,5-heptatriene
cyclizes to a six- membered cyclic diene. Unlike the case above, this initial
diene can undergo a set of [1,5] sigmatropic shifts of H to give two isomeric
cyclic dienes. Calculate the heats of formation of the four structures,
and compare your results with those in Tables 2 and 3 of the reference.
[Doering, W. von E.; Roth, W. R.; Bauer, F.; Boenke, M.; Breuckmann, R.; Ruhkamp, J.; Wortmann, O. Chem. Ber. 1991, 124, 1461.]
88. The ene reaction
can occur inter- (Eqn (l)) or intramolecularly (Eqn (2)). Calculate deltaH
for Eqn (1) and for the intramolecular cases (Eqn (2)) in which n = 1,
2, or 3. Discuss the factors which lead to the variation observed for deltaH.
Compare your answers with the calculate and experimental results in the
cited article.
[Berson, J. A. Acc. Chem. Res. 1991, 24, 215.]
94. The ability of
a series of ene-diynes to undergo thermal closure to a fused aromatic product
(in a stepwise process via a diradical which abstracts hydrogens from solvent)
depends critically on the distance between the termini of the two triple
bonds. Use MM to calculate the bond distances, bond angles, dihedral angles,
etc. of the hydrocarbons with X = CH2 and n = 0 or 1 or 2; of
the hydrocarbon with X absent and n = 0; and of the heterocycle with X
= S and n = 1. Compare your results with the calculated and experimental
(X-Ray crystal structure) results in the cited article.
[Sakai, Y.; Nishiwaki, E.; Shishido, K.; Shibuya, M.; Kido, M. Tetrahedron Lett. 1991, 32, 4363; see also Sakai, Y.; Bando, Y.; Shishido, K.; Shibuya, M. Ibid. 1992, 33, 957; Nicolaou, K. C.; Zuccarello, G.; Riemer, C.; Estevez, V. A.; Dai, W.-M. J. Am. Chem. Soc. 1992, 114, 7360.]
100. Bromine adds
to alkenes in stereospecific anti fashion. Consider the bromination
of steroids 2- cholestene (1) and cholesterol (2, X = OH),
shown as partial structures to the right. Anti addition of Br2
to each can (in principle) produce either of two diastereomers; in fact,
only one diastereomer is initially formed (for reasons which will be discussed
later in lecture), but it can rearrange to the other. Your job is to determine
which diastereomer predominates for the case of trans-2,3-dibromocholestane
and for three cases of addition to 2: X = H, OH, or Br.
105C. Consider the
stepwise hydrogenation of the triene shown to the right. Calculate the
energy of the triene; of the three possible dienes; of the monoene; and
of the saturated final product. Discuss differences and similarities in
the energy changes as each double bond is hydrogenated; compare your answers
with those in the article.
105D. Consider the
stepwise hydrogenation of "birdcage" dienes A, B,
and C. Calculate the energies of these molecules as well as of the
resulting monoenes and saturated hydrocarbons. Discuss differences and
similarities in the energy changes as each double bond is hydrogenated;
compare your answers with those in the article.
[Roth, W. R.; Adamczak, O.; Brueckmann, R.; Lennartz, H.-W.; Boese, R.; Chem. Ber. 1991, 124, 2499; see, also, Turner, R. B.; Jarrett, A. D.; Goebel, P.; Mallon, B. J. J. Am. Chem. Soc. 1973, 95, 790.]
108. Cyclization
by intramolecular nucleophilic substitution of the anion to the right can
produce either fused (1,2) or bridged (1,4) products, each of which can
be either cis or trans in stereochemistry. One MM project
would be to calculate the energies and structures of all of the 1,2-products
for n =2 through n = 5. Another project would be to do the same for the
1,4-products (n = 2 through 5). For either project, compare your results
with those found in the cited article.
[Auchter-Krummel, P.; Krummel, G.; Lex, J.; Müllen, K. Chem. Ber. 1991, 124, 2819.]
109. By calculating
the energies of the alkenes and alkanes shown to the right, determine the
heat of hydrogenation for each case; compare your results with those in
the cited reference. Do calculations on the systems where n = 0, 1, and
2 and, for reference purposes, on the "unstrained" parent system
where there is no bridge (i.e., bicyclo[3.3.0]- 1(5)-octene). Discuss
the strain energy as a function of pyramidalization of the alkene unit.
[Branan, B. M.; Paquette, L. A.; Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 1992, 114, 774.]
144.
Cleavage of a C-C bond in the cubylcarbinyl radical begins a "cascade"
sequence in which a second and then a third bond C-C are broken. Note that
the third and fourth structures are allylic radicals (even though only
one contributor is indicated). Calculate the energies and geometries of
these four radicals and discuss the factors that are involved in each of
the three exothermic steps.
[Choi, S.-Y.; Eaton, P. E.; Newcomb, M.; Yip, Y. C. J. Am. Chem. Soc. 1992, 114, 6326.]
146.
One can imagine a set of steps leading from "pagodane" (1)
(see, also, problem 83) to "dodecahedrane"
(2) via either (a) [2+2] cleavage of the central ring followed by
hydrogenation steps and then by dehydrogenation or (b) alternating hydrogenation
and dehydrogenation steps. Some of these "steps" are chemically
unreasonable, but one can imagine real examples corresponding to
them. The authors contend (see Scheme I and Fig. 1 in the article) that
one sequence is inherently preferred to the other; confirm or disprove
this by calculating the energies of all of the species along the two pathway.
[Fessner, W.-D.; Murty, A. R. C.; Spurr, P. R.; Pinkos, R.; Melder, J.-P.; Fritz, H.; Prinzbach, H. Chem. Ber. 1992, 125, 1697.]
147. Consider the
possibility of introducing two double bonds into the "pagodane"
skeleton after cleavage of the central four-membered ring. Do MMX calculations
on six dienes: double bonds at b and c; at a and
c; at a and d; at a and e; at a
and f; and at a and g. Compare your answers with those
in Table 1. Discuss the factors which are responsible for the very different
energies of these dienes.
[Murty, A. R. C.; Pinkos, R.; Spurr, P. R.; Fessner, W.-D.; Lutz, H.; Fritz, H.; Hunkler, D.; Prinzbach, H. Chem. Ber. 1992, 125, 1719.]
153. Friedel-Crafts
acylation of cyclohexene by 1-cyclopentenylacetyl chloride produces a mixture
of racemic diastereomers that can interconvert via keto-enol type equilibria.
Calculate the structure and energies of the four possible diastereomers
based on the structure shown; also, for the all cis-fused isomer,
do calculations on both of the double- chair conformations. Compare your
results with those cited in the article; comment on differences, if any.
[Faure, R.; Pommier, A.; Pons, J.-M.; Rajzmann, M.; Santelli, M. Tetrahedron 1992 48, 8419.]
156. This problem
has to do with the application of the Winstein-Holness-Eliel equation to
a particular elimination reaction. Calculate the energies of all three
staggered conformations (about the central C-C bond) of both the erythro
and threo stereoisomers of 1,2-diphenyl-1-bromopropane [Ph-CH(CH3)-CH(Br)-Ph],
and use this to estimate which stereoisomer eliminates more rapidly.
165. Dimerization
of 1,3-diphenylallene occurs exclusively in [2+2] head-to-head fashion
to give three (of six possible) stereoisomers. To simplify the problem,
consider the dimerization of 1,3-dimethylallene - the six stereoisomers
(not counting enantiomers) correspond to the various arrangements at the
two double bonds and at the two stereogenic centers in the ring. Compare
the relative energies with those reported for the diphenyl series.
[Brattesani, A. J.; Maverick, E.; Muscio, O. L., Jr.; Jacobs, T. L. J. Org. Chem. 1992, 57, 7346.]
172. This problem
is reminscent of Nos. 83 and 146.
The series of dienes (to the right) can, in principle, undergo intramolecular
[2+2] cycloaddtion to the caged adduct. Compute the heats of formation
and strain energies for both structures for the series n = 0, 1, 2, and
3. Compare your calculated energies and geometrical parameters with those
in the article. Comment on the likely success or failure of this reaction.
[Wollenweber, M.; Hunkler, D.; Keller, M.; Knothe, L.; Prinzbach, H. Bull. Soc. Chim. Fr. 1993, 130, 32.]
185. Birch reduction
of trans-fused aromatic compound 1 followed by methylation
gives a mixture of diastereomers. Similar reaction of cis-fused
2 gives another pair of diastereomers. Do MMX calculations on the
four diastereomers; note that those derived from 2 can, because
of the cis fusion, exist in either of two conformations. Discuss
the factors that lead to differences in energy of these various stereoisomers
and conformations.
[Vila. A. J.; Cravero, R. M.; González-Sierra, M. Tetrahedron 1993, 49, 4511.]
186. The diene (to
the right) can conceivably undergo [2+2] cycloaddition as shown, even though
the ring strain should increase. Compute the change in enthalpy and in
strain energy for this reaction and for the series of reactions
in which the C=C's are replaced, singly or in pairs, by benzo and naphtho
groups. For these aromatic compounds, the increase in strain will be accompanied
by a loss in resonance energy. Also compute the distance between the reacting
pi systems in the reactants. Compare your results with those in
the Table of the cited reference.
[Thiergardt, R.; Keller, M.; Wollenweber, M.; Prinzbach, H. Tetrahedron Lett. 1993, 34, 3397.]
191. Thermal [2+2]
dimerization of the all s- trans linear triene can lead to cyclobutane-
containing dimers in which the decalin-like units have either a syn
or anti relationship. When optically pure reactant (say R
configuration) reacts, three dimers are possible (see Scheme IV in the
reference). When racemic material reacts, the R + S combination
leads to three other possible dimers (Scheme V). Calculate the MMX energies
of these six structures and compare your results with the MM2 values in
the reference (Table I).
[Doering, W. von E.; Belfield, K. D.; He, J. J. Am. Chem. Soc. 1993, 115, 5414.]
205. The [2+2] syn
dimer of acenaphthalene is hydrogenated under vigorous conditions to its
H16 derivative; further hydrogenation to the H18
and H20 derivatives proved impossible. Do calculations on all
four compounds and discuss why complete hydrogenation does not occur. Compare
your structural parameters with those in the cited article.
[Boldt, P.; Döring, D.; Jones, P. G.; Köpper, H.; Trog, R.-S. Chem. Ber. 1994, 127, 219.]
209. In a problem
reminiscent of No. 144, generation of the 9- basketyl
radical, C10H11·,
leads via a cascade to new radicals by the cleavage of one, two, and then
three of the C- C bonds to the "upper" cyclobutane. Calculate
the energies of all of these species and comment on the feasibility of
this proposed sequence of steps. Speculate on why the last radical does
not undergo cleavage of the one remaining C-C bond.
[Binmore, G. T.; Della, E. W.; Elsey, G. M.; Head, N. J.; Walton, J. C. J. Am. Chem. Soc. 1994, 116, 2759.]
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