instructor supplement 27 - purdue...

Instructor Supplemental Solutions to Problems © 2010 Roberts and Company Publishers

Chapter 27 Pericyclic Reactions

Solutions to In-Text Problems

27.1 (b) This is a sigmatropic reaction; two electrons are involved.

(d) This is a sigmatropic reaction; six electrons are involved.

27.3 Because there are ten p electrons, there are five occupied MOs in this alkene, each containing two electrons. The

MOs alternate in symmetry; the odd-numbered ones are symmetric, and the even-numbered ones are antisymmetric.

Hence, p*6 is antisymmetric (A); it is antibonding; and it is the LUMO and is thus one of the frontier orbitals.

27.5 (a) The resonance structures of the allyl radical show that the unpaired electron is shared between the terminal

carbons:

(b) Figure 27.4, text p. 1341, shows that the unpaired electron of the allyl radical resides in the nonbonding MO,

which has a node at the central carbon. The unpaired electron density therefore exists only at the carbons on

either side of a node, that is, at the terminal carbons, as the resonance structures show.

27.7 The two different conrotatory processes are as follows: (The curved arrows indicate atomic motion, not electron

flow.)

INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 27 2

They are equally likely because they are enantiomeric; enantiomers have equal energies, and enantiomeric pathways

have identical energies at all points and therefore identical rates (Sec. 7.8A, text p. 301).

27.9 The opening of the cyclobutene is conrotatory and therefore gives (1E,3Z)-cyclodecadiene. The trans double bond

does not introduce as much strain into the large ring as was present in the starting cyclobutene.

Here is one way to use models to visualize this transformation. To avoid having to build two models, construct a cyclodecane (the cyclic alkane with a ten-membered ring); don’t worry about the missing double bond. Now connect two carbons to make a cis-fused four-membered ring. Identify the “up” hydrogens on these two carbons. Using a pencil, draw a single line across the bond that is a double bond in the starting material (even though it is a single bond in your model). Now draw two lines across each of the bonds that will become double bonds as a result of the reaction. Now break the cyclobutane ring and turn the carbons bearing the “up” hydrogens in a conrotatory manner. Examine the stereochemistry at the bonds marked with two lines; one should have an E configuration and the other a Z configuration. Voila!

27.11 Use the HOMO of the 4p-electron component and the LUMO of the 2p-electron component. The [4a + 2s] process

would involve the overlap shown in (a) below. The [4s + 2a] process would involve the overlap shown in (b). In

both cases an out-of-phase overlap between the p-electron systems is required at one end. Consequently, these

processes are not allowed.

27.13 The four products are the diastereomeric 1,2,3,4-tetramethylcyclobutanes. Each of the pure alkene stereoisomers

can undergo an allowed photochemical [2s + 2s] cycloaddition in two distinguishable ways. Cis-2-butene reacts to

give compounds A and B; trans-2-butene reacts to give compounds B and C; and the mixture of cis- and trans-2-

butene can give these three compounds plus a fourth, compound D, that results from the [2s + 2s] cycloaddition of

cis-2-butene to trans-2-butene.


27.14 (b) Two other starting materials that would give the same product as in part (a):

27.15 (c) This is a [5,5] sigmatropic rearrangement.

27.17 If the migration were antarafacial, the opposite stereochemical result would have been observed; that is, the 3E

stereoisomer of the product would have the S configuration at the asymmetric carbon stereocenter, and the 3Z

stereoisomer of the product would have the R configuration at the asymmetric carbon stereocenter. This is the result

that is not observed.

27.18 (a) Think of the five-carbon p-electron system as a 2,4-pentadienyl cation, whose MOs are shown in Fig. 27.5,

text p. 1342, and think of the migrating group as a carbon anion in which the unshared electron pair resides in

a 2p orbital. The orbitals involved in the rearrangement are the LUMO of the pentadienyl system and the

HOMO of the carbon anion. The LUMO of the pentadienyl system, as shown by Fig. 27.5, is symmetric; that

is, at each end of the system, the LUMO has the same phase on a given face. The HOMO of the carbon anion

is simply the filled 2p orbital. The migration is allowed only if the carbon anion migrates suprafacially such

that each end of the pentadienyl LUMO interacts with the same lobe of the carbon anion 2p orbital.


Alternatively, you can think of this problem in terms of the 2,4-pentadienyl anion and a methyl cation. The orbitals involved are now the HOMO of the anion and the LUMO of the cation. But these are the same orbitals used in the foregoing solution (only their occupancies are different); hence, the stereochemical result is the same.

27.19 (b) The “hydrogen walk” on 2,3-dimethyl-1,3-cyclopentadiene should proceed as shown in the following

equation. Although a hydrogen that starts out on carbon-1 could migrate to four other carbons, only two of

the possible products are unique. It can be argued that compounds A and B should be the predominant

alkenes at equilibrium, and that compound C should be present in least amount. (Why? See Sec. 4.5B, text

p. 144.) Remember that the selection rules have nothing to say about which compound is favored at

equilibrium, only whether the equilibrium can be attained at a reasonable rate.)

27.20 (b) Deduce the starting material by drawing the curved arrows for the reverse of a Claisen rearrangement:

27.21 See Fig. 27.4, text p. 1341, for the relevant orbital diagram. The SOMOs of the two allylic radicals (p2) interact

because it is these molecular orbitals that contain the unpaired electrons. Since the two orbitals are the same, they

have the same symmetry, and therefore interact suprafacially on both components with positive overlap.

27.22 (b)


27.24 (a) The s bond that “moves” is indicated with an asterisk (*). It moves suprafacially on both p systems; [3,3]

sigmatropic rearrangements are thermally allowed processes.

27.26 Heating should promote a disrotatory reaction; that is, the reaction should have a stereochemical course which is

opposite to that of the photochemical reaction.

27.28 A [1,7] thermal sigmatropic hydrogen shift should be antarafacial. That is, a hydrogen from the methyl group

migrates from a conformation in which it is “up” to the lower face of the ring, or a hydrogen from the methyl group

migrates from a conformation in which it is “down” to the upper face of the ring, or both.

Problem 27.51 (text p. 1373) describes an effort to elucidate the stereochemistry of the previtamin D rearrangement.


Solutions to Additional Problems

27.29 (b) Think of the Diels–Alder reaction as a prototype for an allowed thermal cycloaddition. If 4n + 2 electrons are

involved, a cycloaddition is allowed thermally. If 4n electrons are involved, then the thermal cycloaddition is

forbidden and the photochemical cycloaddition is allowed. Therefore, a photochemical [8s + 4s] process (12

electrons) is allowed.

27.30 The pericyclic selection rules say absolutely nothing about the position of equilibrium in each case. These rules

refer to rates of reactions, not to equilibrium constants. Other considerations must be used to decide on the position

of equilibrium. The point of the problem is for you to use what you have learned about the relative energies of

molecules; the molecule of lower energy is favored in each equilibrium.

(b) The left side of the equation is favored at equilibrium because the double bonds have more alkyl branches.

(Alkyl substitution stabilizes a double bond.)

(d) The right side of the equation is favored at equilibrium because one product is aromatic and therefore

particularly stable, and the other product is volatile (ethylene is a gas).

27.31 (b) The stereoisomer of compound A that also gives compound C on heating is the one in which both of the

terminal double bonds have the Z configuration.

27.32 (b) Because a thermal suprafacial [1,9] or [1,13] methyl migration must occur with retention of configuration at

the migrating carbon, the methyl group that migrates, when isotopically substituted, must have the S

configuration in both starting material and product.

27.34 (a) The structure of the ozonolysis product C shows that compound B is a cyclobutene, which must be formed in

a disrotatory photochemical electrocyclic reaction. This defines the stereochemistry of B, which, in turn,

defines the stereochemistry of C:

(b) Compound D is the trans-fused stereoisomer of the cyclobutene derivative, which undergoes conrotatory

opening upon heating to give compound A.

The reason that compound B is inert under the same conditions is that conrotatory ring opening of B would

give a stereoisomer of A in which one of the cyclohexene rings would contain a trans double bond within a

six-membered ring. This introduces so much strain that the reaction does not occur, even though it is allowed

by the selection rules.


27.36 This cycloaddition reaction involves sixteen electrons; hence, it must be suprafacial on one component and

antarafacial on the other. Thus, it must be a [14s + 2a] or a [14a + 2s] cycloaddition. The heptafulvene molecule is

large enough that its p-electron system can twist without introducing too much strain or without losing too much p-

electron overlap; hence, the cycloaddition is a [14a + 2s] process. In either case, the product has the following

stereochemistry:

27.37 (a) Toluene has added stability associated with aromaticity; compound A, although conjugated, is not aromatic.

Because any equilibrium favors the more stable compound, the added stability of toluene causes it to be

strongly favored in the equilibrium.

27.38 (b) This reaction superficially appears to involve internal rotation about both double bonds, a process that is

virtually impossible. However, a more reasonable pericyclic mechanism is available: a sequence of two

conrotatory electrocyclic reactions, the first one a ring closing, and the second one a ring opening. (The

curved arrows refer to the clockwise rotations of groups and not to electron flow.)

27.41 These transformations are very much like the last two in the solution to Problem 27.40, text p. 1370. A Diels–Alder

reaction is followed by a reverse Diels–Alder reaction. The driving force for the reverse Diels–Alder is the

formation of a product that is stabilized by aromaticity.


27.43 (c) The intermediate trapped by maleic anhydride is the tetraene Z formed by a [1,9] (presumably suprafacial)

sigmatropic hydrogen migration.

27.44 Compound A is a secondary alcohol, and the aldehyde B is formed from an enol intermediate (shown in parentheses)

that results from a [3,3] sigmatropic (oxyCope) rearrangement.

Evidently in this case the Grignard reagent undergoes mostly 1,2-addition to the a,b-unsat-urated aldehyde; none of the 1,4-addition product is formed. (See Sec. 22.10A, text p. 1101.)

27.46 (b) A key step in this synthesis is a Claisen rearrangement of the starting material, which is prepared in the

solution to part (a)

27.47 (b) Two successive photochemical disrotatory electrocyclic reactions account for the products.


The product, the very unstable antiaromatic cyclic diene 1,3-cyclobutadiene (see text p. 729), was generated in 1972 by this sequence of reactions carried out at a temperature of 8 K in the laboratory of Professor Orville L. Chapman (1932–2004), then at Iowa State University, and later at the University of California, Los Angeles. This technique allowed the observation and characterization of 1,3-cyclobutadiene by infrared spectroscopy.

27.48 (b) Given that germacrone assumes a chairlike conformation in its transition state when it reacts to give

b-elemenone, deduce the structure of germacrone by mentally reversing the Cope rearrangement.

Now re-examine (or work) Problem 27.42, text p. 1370, to see why the product in that problem has E stereochemistry.

27.50 (b) Oxidation of the secondary alcohol and cleavage of the ether gives estrone:

27.51 This is a [1,7] sigmatropic rearrangement, which, according to the selection rules, should be antarafacial. (See

Problem 27.28, text p. 1367, and its solution on p. 5 of this chapter.) Migration of hydrogen gives B and migration

of deuterium gives C. (Presumably, the predominance of B is due in part to a primary isotope effect, which should

favor more rapid migration of hydrogen, all things being equal.) If the rearrangements are indeed antarafacial, the

stereochemistry of the products should be as follows:


This was indeed the stereochemistry observed.

This work was carried out to adduce evidence for the stereochemistry of the previtamin D rearrangement to vitamin D (Eq. 27.39, text p. 1366). Compound A was constructed to be a “model” for previtamin D. As noted in Problem 27.28, the structure of the previtamin itself does not reveal the stereochemistry of the rearrangement.

27.54 (a) If the formation of benzene were concerted, it would have to be a thermal disrotatory electrocyclic ring-

opening reaction involving four electrons.

(b) The reaction that gives benzene is not allowed by the selection rules; therefore, it does not occur. In spite of

the high degree of strain of Dewar benzene, and in spite of the great stability of benzene, Dewar benzene is

effectively constrained to exist by the selection rules!

Prismane, or Ladenburg benzene, which is discussed on text p. 718 and 1348, is another very unstable constitutional isomer of benzene that is effectively trapped into existence because its concerted conversion into benzene would violate the selection rules for pericyclic reactions.

instructor supplement 27 - purdue...

Documents