Guia Alulas Pericyclic Reactions

8/11/2019 Guia Alulas Pericyclic Reactions

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Pericyclic reactions are defined as the reactions that occur by a concerted cyclic shift

of electrons. This definition states two key points that characterise a pericyclic

reaction. First point is that reaction is concerted. In concerted reaction, reactantbonds are broken and product bonds are formed at the same time, without

intermediates. Second key point in pericyclic reactions involves a cyclic shift of

electrons. The word pericyclic means around the circle. Pericyclic word comes from

cyclic shift of electrons. Pericyclic reactions thus are characterised by a cyclic

transition state involving the bonds.

The energy of activation of pericyclic reactions is supplied by heat Thermal

Induction!, or by "# light Photo Induction!. Pericyclic reactions are stereospecific

and it is not uncommon that the two modes of induction yield products of opposite

stereochemistry.

$lthough most organic reactions take place by way of ionic or radical intermediates, a

number of useful reactions occur in one%step processes that do not form reactive

intermediates.

Pericyclic reactions require light or heat and are completely stereospecifc; that is, a

single stereoisomer of the reactant forms a single stereoisomer of the product. &e

will consider two categories of pericyclic reactions' electrocyclic reactions and

cycloadditions. $n electrocyclic reaction is a reversible reaction that can involve ring

closure or ring opening. Two features determine the course of the reactions' the

number of bonds involved and whether the reaction occurs in the presence of heat

thermal conditions! or light photochemical conditions!. These reactions follow a set

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of rules based on orbitals and symmetry ) rst proposed by *. +. &oodward and *oald

offmann in (-/, and derived from theory described by 0enichi Fukui in (-/1. To

understand pericyclic reactions we must review and e2p

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Pericyclic reactions are defined as the reactions that occur by a concerted cyclic shift

of electrons. This definition states two key points that characterise a pericyclic

reaction. First point is that reaction is concerted. In concerted reaction, reactantbonds are broken and product bonds are formed at the same time, without

intermediates. Second key point in pericyclic reactions involves a cyclic shift of

electrons. The word pericyclic means around the circle. Pericyclic word comes from

cyclic shift of electrons. Pericyclic reactions thus are characterised by a cyclic

transition state involving the bonds.

The energy of activation of pericyclic reactions is supplied by heat Thermal

Induction!, or by "# light Photo Induction!. Pericyclic reactions are stereospecific

and it is not uncommon that the two modes of induction yield products of opposite

stereochemistry.

$lthough most organic reactions take place by way of ionic or radical intermediates, a

number of useful reactions occur in one%step processes that do not form reactive

intermediates.

Pericyclic reactions require light or heat and are completely stereospecifc; that is, a

single stereoisomer of the reactant forms a single stereoisomer of the product. &e

will consider two categories of pericyclic reactions' electrocyclic reactions and

cycloadditions. $n electrocyclic reaction is a reversible reaction that can involve ring

closure or ring opening. Two features determine the course of the reactions' the

number of bonds involved and whether the reaction occurs in the presence of heat

thermal conditions! or light photochemical conditions!. These reactions follow a set

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of rules based on orbitals and symmetry ) rst proposed by *. +. &oodward and *oald

offmann in (-/, and derived from theory described by 0enichi Fukui in (-/1. To

understand pericyclic reactions we must review and e2p

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&e shall concern with four ma4or types of pericyclic reactions. The first type is the

electrocyclic reaction' a reaction in which a ring is closed or opened! at the e2pense

of a con4ugated double or triple bond! bond. The second type of reaction iscycloaddition reaction' a reaction in which two or more electron systems react to

form a ring at the e2pense of one bond in each of the reacting partners. In this

reaction formation of two new sigma bonds takes place which close a ring. 5verall

there is loss of twopi bonds in reactants and gain of two sigma bonds in a product.

The third type of reaction is the sigmatropic rearrangement or reaction!' a reaction

in which a sigma bond formally migrates from one end to the other end ofpi electron

system and the net number of bonds remains the same. The fourth type of reaction

is the group transfer reaction' a reaction in which one or more groups or atoms

transfer from one molecule to another molecule. In this reaction both molecules are

4oined together by sigma bond.

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&e shall concern with four ma4or types of pericyclic reactions. The first type is the

electrocyclic reaction' a reaction in which a ring is closed or opened! at the e2pense

of a con4ugated double or triple bond! bond. The second type of reaction iscycloaddition reaction' a reaction in which two or more electron systems react to

form a ring at the e2pense of one bond in each of the reacting partners. In this

reaction formation of two new sigma bonds takes place which close a ring. 5verall

there is loss of twopi bonds in reactants and gain of two sigma bonds in a product.

The third type of reaction is the sigmatropic rearrangement or reaction!' a reaction

in which a sigma bond formally migrates from one end to the other end ofpi electron

system and the net number of bonds remains the same. The fourth type of reaction

is the group transfer reaction' a reaction in which one or more groups or atoms

transfer from one molecule to another molecule. In this reaction both molecules are

4oined together by sigma bond.

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Three features of any pericyclic reaction are intimately interrelated. These are'

(. Activation: Pericyclic reactions are activated either by thermal energy or by "#

light.owever, many reactions that re7uire heat are not initiated by light and vice%versa.

3. The number ofpi bonds involved in the reaction.

6. The stereochemistry of the reaction.

8onsider the following three reactions'

First two reactions are thermal reactions activated by heat and third reaction is

photochemical reaction activated by light. The relationship between the mode of

activation and the stereochemistry is e2emplified by a comparison of reactions ii!

and iii!. &hen starting material is heated it gives cis product and when starting

material is irradiated the product is trans.

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The 9 bond in ethylene 83::83! is formed by side%by%side overlap of twop

orbitals on ad4acent carbons. Twop orbitals can combine in two different ways. $s

shown in Figure 8.(, when twop orbitals of similar phase overlap, a 9 bondingmolecular orbital designated as ;(! results.

Two electrons occupy this lower%energy bonding molecular orbital. &hen twop

orbitals of opposite phase combine, a 9< antibonding molecular orbital designated as

;3

combine.

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The 9 bond in ethylene 83::83! is formed by side%by%side overlap of twop

orbitals on ad4acent carbons. Twop orbitals can combine in two different ways. $s

shown in Figure 8.(, when twop orbitals of similar phase overlap, a 9 bondingmolecular orbital designated as ;(! results.

Two electrons occupy this lower%energy bonding molecular orbital. &hen twop

orbitals of opposite phase combine, a 9< antibonding molecular orbital designated as

;3

combine.

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m-Symmetry. Some molecular orbitals have the symmetry about the mirror plane m!

which bisects the molecular orbitals and is perpendicular to the plane of the

molecule Fig. >!. +oth orbitals in Fig. > are mirror images to each other hence in this@5 there is mirror plane symmetry, abbreviated as mS!. In Fig. ? a! both orbitals are

not mirror images to each other. Thus in this @5 there is mirror plane asymmetry,

abbreviated as m$!.

+oth orbitals in Fig. ? b! are symmetrical with respect to centre of the molecular a2is.

Thus in this @5 there is centre of symmetry, abbreviated as C3S!.

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5n the basis of the above two e2amples we can conclude the following very

important points for linear con4ugated systems '

(. The wave function An will have n: (! nodes.3. &hen n is odd, An will be symmetric with m and asymmetric with C3.

6. &hen n is

8on4ugated polyenes always contain even number of carbon atoms. These polyenes

contain either 1n! or 1n B 3! con4ugated electrons. The filling of electrons in the

molecular orbitals of a con4ugated polyene is summarised below '

(. Cumber of bonding @5s and antibonding @5s are same.

3. Cumber of electrons in any molecular orbital is ma2imum two.

6. If a molecular orbital contains two electrons then both electrons are always paired.

1. @olecular orbitals follow $ufbau principle and undDs rule.

/. Energy of the molecular orbital is directly proportional to the number of the

nodal planes.

. There will be no degenerate molecular orbitals in any energy level, i.e., each and

every energy level contains one and only one molecular orbital.

even, An will be symmetric with C3 and asymmetric with m. Table%(.

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*egra de &oodward e offman

Principle of conservation of orbital symmetry' the transformations in which the

symmetry of the 5@ is conserved orbitals remain in phase and thus maintain adegree of bonding during the process! involve a relatively low energy TS and are

called Symmetry $llowed.

The transformation in which the symmetry of the orbitals is destroyed by bringing

one or more orbitals out of phase, the energy the TS become very high due to nan

antibonding interaction and the reaction is Symmetry Forbidden.

Thermally allowed transformation is forbidden photochemically and a

photochemically allowed process is forbidden thermally : moreover thermal and

photochemical reactions give opposite stereochemistry.

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Notice that a molecular orbital is bonding if the number of bonding interactions is

greater than the number of nodes between the nuclei, and a molecular orbital is

antibonding if the number of bonding interactions is fewer than the number of nodesbetween the nuclei.

In a thermal reaction the reactant is in its ground state; in a photochemical reaction

the reactant is in an excited state.

Some molecular orbitals are symmetric and some are asymmetric also called

dissymetric!, and they are easy to distinguish. If thep orbitals at the ends of the

molecular orbital are in%phase both have blue lobes on the top and green lobes on

the bottom!, the molecular orbital is symmetric. If the two endp orbitals are out%

ofphase, the molecular orbital is asymmetric. In Figure 3-.3, and are symmetric

molecular orbitals and and are asymmetric molecular orbitals. Cotice that as the

molecular orbitals increase in energy, they alternate in being symmetric and

asymmetric. Therefore, the ground-state !"! and the excited-state !"! always

have opposite symmetriesGone is symmetric and the other is asymmetric. $

molecular orbital description of (,6,/%he2atriene, a compound with three con4ugated

double bonds, is shown in Figure 3-.6. $s a review, e2amine the figure and note

H the distribution of electrons in the ground and e2cited states

H that the number of bonding interactions decreases and the number of nodes

increases as the molecular orbitals increase in energy

H that the molecular orbitals alternate from symmetric to asymmetric as the

molecular orbitals increase in energy

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$lthough the chemistry of a compound is determined by all its molecular orbitals, we

can learn a great deal about the chemistry of a compound by looking at only the

highest occupied molecular orbital (HOMO and the lo!est unoccupied molecular

orbital ("#MO. These two molecular orbitals are known as the frontier orbitals. &e

will now see that simply by evaluating one of the frontier molecular orbitals of the

reactants! in a pericyclic reaction, we can predict the conditions under which the

reaction will occur thermal or photochemical, or both! and the products that will be

formed.

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Pericyclic reactions are defined as reactions that occur by concerted cyclic shift of

electrons. $ccording to the &oodward and offmann symmetry of the molecular

orbitals that participate in the chemical reaction determines the course of thereaction. They proposed what they called the principle of the conservation of orbital

symmetry in the concerted reactions. In the most general terms, the principle

means that in concerted pericyclic reactions, the molecular orbitals of the starting

materials must be transformed into the molecular orbitals of the product in smooth

continuous way. This is possible only if the orbitals have similar symmetry, i.e.,

orbitals of the reactant and product have similar symmetries. In concerted reaction

product formation takes place by formation of cyclic transition state. The transition

state of pericyclic reactions should be intermediate between the electronic ground

states of the starting material and product. 5bviously, the most stable transition state

will be one which conserves the symmetry of the reactant orbitals in passing to

product orbitals. In other words, a symmetric S! orbital in the reactant must

transform into a symmetric orbital in the product and that an asymmetric $! orbital

must transform into an asymmetric orbital. If the symmetries of the reactants and

product orbitals are not the same, the reaction will not take place in a concerted

manner. If symmetry is conserved during the course of the reaction then reaction will

take place and process is known as symmetry allowed process. If symmetry is not

conserved during the course of the reaction, the reaction is known as symmetry-

forbidden process. The energy of the transition state i.e., energy of activation of the

transition state! of symmetry allowed process is always lower than the symmetry%

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forbidden process.

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*egra de &oodward e offman

Principle of conservation of orbital symmetry' the transformations in which the

symmetry of the 5@ is conserved orbitals remain in phase and thus maintain adegree of bonding during the process! involve a relatively low energy TS and are

called Symmetry $llowed.

The transformation in which the symmetry of the orbitals is destroyed by bringing

one or more orbitals out of phase, the energy the TS become very high due to nan

antibonding interaction and the reaction is Symmetry Forbidden.

Thermally allowed transformation is forbidden photochemically and a

photochemically allowed process is forbidden thermally : moreover thermal and

photochemical reactions give opposite stereochemistry.

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An electrocyclic reaction is a reversible reaction that involves ring closure of a

con$ugatedpolye ne to a cycloal%ene& or ring opening of a cycloal%ene to a

con$ugated polyene. For e2ample, ring closure of (,6,/%he2atriene forms (,6%cyclohe2adiene, a product with one more )bond and one fewer 9 bond than the

reactant. *ing opening of cyclobutene forms (,6%butadiene, a product with one fewer

) bond and one more 9 bond than the reactant. To draw the product in each reaction,

use curved arrows and begin at a 9 bond. @ove the 9 electrons to an ad4acent

carbon:carbon bond and continue in a cyclic fashion. In a ring%forming reaction, this

process forms a new ) bond that now 4oins the ends of the con4ugated polyene. In a

ring%opening reaction, this process breaks a ) bond to form a con4ugated polyene with

one more 9 bond. &hether the reactant or product predominates at e7uilibrium

depends on the ring si=e of the cyclic compound. Jenerally, a si2%membered ring is

favored over an acyclic triene at e7uilibrium.

In contrast, an acyclic diene is favored over a strained four%membered ring.

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'lectrocyclic reactions are completely stereospeci c. For e2ample, ring closure of

3#,1$,#!%3,1,%octatriene yields a single product with cis methyl groups on the ring.

*ing opening of cis%6,1%dimethylcyclobutene forms a single con4ugated diene withone$ alkene and one # alkene.

@oreover, the stereochemistry of the product of an electrocyclic reaction depends on

whether the reaction is carried out under thermal or photochemical reaction

conditionsGthat is, with heat or light, respectively. 8ycli=ation of 3#,1#!%3,1%

he2adiene with heat forms a cyclobutene with trans methyl groups, whereas

cycli=ation with light forms a cyclobutene with cis methyl groups.

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To understand these results, we must focus on the 5@5 of the acyclic con4ugated

polyene that is either the reactant or product in an electrocyclic reaction. In

particular, we must e2amine thep orbitals on the terminal carbons of the 5@5, anddetermine whether like phases of the orbitals are on the same side or on opposite

sides of the molecule.

The product of an electrocyclic reaction results from the formation of a new sigma

bond. For this bond to form, thep orbitals at the ends of the con4ugated system must

rotate so they overlap head%to%head and rehybridi=e to sp6!. *otation can occur in

two ways. If both orbitals rotate in the same direction both clockwise or both

counterclockwise!, ring closure is conrotatory. If the orbitals rotate in opposite

directions one clockwise, the other counterclockwise!, ring closure is disrotatory.

The mode of ring closure depends on the symmetry of the 5@5 of the compound

undergoing ring closure. 5nly the symmetry of the 5@5 is important in determining

the course of the reaction because this is where the highest energy electrons are.

These are the most loosely held electrons and therefore the ones most easily moved

during a reaction.

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To form a bond, thep orbitals on the terminal carbons must rotate so that like phases

can interact to form the new ) bond. Two modes of rotation are possible.

The mode of ring closure depends on the symmetry of the 5@5 of the compoundundergoing ring closure. 5nly the symmetry of the 5@5 is important in determining

the course of the reaction because this is where the highest energy electrons are.

These are the most loosely held electrons and therefore the ones most easily moved

during a reaction.

To form the new bond, the orbitals must rotate so that in%phasep orbitals overlap,

because in%phase overlap is a bonding interaction. 5ut%of%phase overlap

would be an antibonding interaction. If the 5@5 is symmetric the end orbitals are

identical!, rotation will have to be disrotatory to achieve in%phase overlap. In other

words, disrotatory ring closure is symmetry%allowed, whereas conrotatory ring

closure is symmetry%forbidden. If the 5@5 is asymmetric, rotation has to be

conrotatory in order to achieve inphase overlap. In other words, conrotatory ring

closure is symmetry%allowed, whereas disrotatory ring closure is symmetry%forbidden.

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Cotice that a symmetry)allo!ed path!ay is one in which in%phase orbitals overlap a

symmetry)forbidden path!ay is one in which out%of%phase orbitals would overlap. $

symmetry%allowed reaction can take place under relatively mild conditions. If areaction is symmetry%forbidden, it cannot take place by a concerted pathway. If a

symmetryforbidden reaction takes place at all, it must do so by a nonconcerted

mechanism. Cow we are ready to learn why the electrocyclic reactions discussed at

the beginning of this section form the indicated products, and why the configuration

of the product changes if the reaction is carried out under photochemical conditions.

The ground%state 5@5 of a compound with three con4ugated bonds, such as

3#,1$,#!%octatriene, is symmetric Figure 3-.6!. This means that ring closure under

thermal conditions is disrotatory. In disrotatory ring closure of 3#,1$,#!%octatriene,

the methyl groups are both pushed up or down!, which results in formation of the cis

product. In disrotatory ring closure of 3#,1$,$!%octatriene, one methyl group is

pushed up and the other is pushed down, which results in formation of the trans

product.

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If the reaction takes place underphotochemical conditions, we must consider the

e2cited%state 5@5 rather than the ground%state 5@5. The e2cited%state 5@5 of

a compound with three bonds is asymmetric Figure 3-.6!. Therefore, underphotochemical conditions, 3#,1$,$!%octatriene undergoes conrotatory ring closure,

so both methyl groups are pushed down or up! and the cis product is formed.

To e2plain the stereochemistry observed in electrocyclic reactions, we must e2amine

the symmetry of the molecular orbital that contains the most loosely held 9

electrons. In a thermal reaction, we consider the HOMO of the ground state

electronic con guration. *otation occurs in a disrotatory or conrotatory fashion so

that like phases of thep orbitals on the terminal carbons of this molecular orbital

combine. Two e2amples illustrate different outcomes. Thermal electrocyclic ring

closure of 3#,1$,#!%3,1,%octatriene yields a single product with cis methyl groups

on the ring.

*ycli+ation occurs in a disrotatory fashion because the 5@5 of a con4ugated triene

has like phases of the outermostp orbitals on the same side of the molecule Figure

8.6!. $ disrotatory ring closure is symmetry allowed because like phases of thep

orbitals overlap to form the new ) bond of the ring. In the disrotatory ring closure,

both methyl groups are pushed down or up!, making them cis in the product. This is

a speci) c e2ample of the general process observed for con4ugated polyenes with an

odd number of 9 bonds. The 5@5 of a con4ugated polyene with an odd number of 9

bonds has like phases of the outermostp orbitals on the same side of the molecule.

$s a result'

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If the reaction takes place underphotochemical conditions, we must consider the

e2cited%state 5@5 rather than the ground%state 5@5. The e2cited%state 5@5 of

a compound with three bonds is asymmetric Figure 3-.6!. Therefore, underphotochemical conditions, 3#,1$,$!%octatriene undergoes conrotatory ring closure,

so both methyl groups are pushed down or up! and the cis product is formed.

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The series of reactions in Figure 3-.1 illustrates 4ust how easy it is to determine the

mode of ring closure and therefore the product of an electrocyclic reaction. The

reactant of the first reaction has three con4ugated double bonds and is undergoingring closure under thermal conditions. *ing closure, therefore, is disrotatory Table

3-.(!. Kisrotatory ring closure of this reactant causes the hydrogens to be cis in the

ringclosed product. To determine the relative positions of the hydrogens, draw them

in the reactant and then draw arrows showing disrotatory ring closure Figure 3-.1a!.

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Table 8.( summari=es the rules, often called the ,ood!ard-Hoffmann rules& for

electrocyclic

reactions under thermal or photochemical reaction conditions. The number of 9bonds

refers to the acyclic con4ugated polyene that is either the reactant or product of an

electrocyclic

reaction.

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A cycloaddition is a reaction bet!een t!o compounds !ith 9 bonds to form a cyclic

product !ith t!o ne! ) bonds. Like electrocyclic reactions, cycloadditions are

concerted, stereospeci) 8 reactions, and the course of the reaction is determined bythe symmetry of the molecular orbitals of the reactants. 8ycloadditions can be

initiated by heat thermal conditions! or light photochemical conditions!.

8ycloadditions are identi) ed by the number of 9 electrons in the two reactants.

he /iels-Alder reaction is a thermal 01 2 34 cycloaddition that occurs between a

diene with four 9 electrons and an alkene dienophile! with two 9 electrons Sections

(.(3:(.(1!. A photochemical 03 2 34 cycloaddition occurs bet!een t!o al%enes&

each with two 9 electrons, to form a cyclobutane. Thermal M3 B 3N cycloadditions do

not take place.

In a cycloaddition reaction, two different bond:containing molecules react to form

a cyclic molecule by rearranging the electrons and forming two new O bonds. The

Kiels:$lder reaction is one of the best known e2amples of a cycloaddition reaction

Section ?.?!. 8ycloaddition reactions are classified according to the number of

electrons that interact in the reaction. The Kiels:$lder reaction is a M1 B 3N

cycloaddition reaction because one reactant has four interacting electrons and the

other reactant has two interacting electrons. 5nly the electrons participating in

electron rearrangement are counted.

In a cycloaddition reaction, the orbitals of one molecule must overlap with the

orbitals of the second molecule. Therefore, the frontier molecular orbitals of both

reactants must be evaluated to determine the outcome of the reaction. +ecause the

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new O bonds in the product are formed by donation of electron density from one

reactant to the other reactant, we must consider the 5@5 of one of the molecules

and the L"@5 of the other because only an empty orbital can accept electrons. It

does not matter which reacting moleculeDs 5@5 is used. It is re7uired only that we

use the 5@5 of one and the L"@5 of the other.

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To understand cycloaddition reactions, we e2amine thep orbitals of the terminal

carbons of both reactants. +onding can take place only when like phases of both sets

ofp orbitals can combine. Two modes of reaction are possible. +ecause of thegeometrical constraints of small rings, cycloadditions that form four) or

si5membered rings must ta%e place by suprafacial path!ays. Since cycloaddition

involves the donation of electron density from one reactant to another, one reactant

donates its most loosely held electronsGthose occupying its 5@5Gto a vacant

orbital that can accept electronsGthe L"@5Gof the second reactant. The 5@5 of

either reactant can be used for analysis.

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To e2amine the course of a M1 B 3N cycloaddition, letDs arbitrarily choose the 5@5 of

the diene and the L"@5 of the alkene, and look at the symmetry of thep orbitals on

the terminal carbons of both components. Since two bonding interactions result fromoverlap of the like phases of both sets ofp orbitals, a 01 2 34 cycloaddition occurs

readily by suprafacial reaction under thermal conditions. 01 2 34 cycloaddition

occurs readily by suprafacial reaction under thermal conditions. This is a speci) c

e2ample of a general cycloaddition involving an odd number of 9 bonds three

9 bonds total, two from the diene and one from the alkene!.

+ecause a Kiels:$lder reaction follows a concerted, suprafacial pathway, the

stereochemistry of the diene is retained in the /iels-Alder product. $s a result,

reaction of 3#,1#!%3,1%he2adiene with ethylene forms a cyclohe2ene with cis

substituents *eaction M(N!, whereas reaction of

3#,1$!%3,1%he2adiene with ethylene forms a cyclohe2ene with trans substituents

*eaction M3N!.

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To e2amine the course of a M1 B 3N cycloaddition, letDs arbitrarily choose the 5@5 of

the diene and the L"@5 of the alkene, and look at the symmetry of thep orbitals on

the terminal carbons of both components. Since two bonding interactions result fromoverlap of the like phases of both sets ofp orbitals, a 01 2 34 cycloaddition occurs

readily by suprafacial reaction under thermal conditions. 01 2 34 cycloaddition

occurs readily by suprafacial reaction under thermal conditions. This is a speci) c

e2ample of a general cycloaddition involving an odd number of 9 bonds three

9 bonds total, two from the diene and one from the alkene!.

+ecause a Kiels:$lder reaction follows a concerted, suprafacial pathway, the

stereochemistry of the diene is retained in the /iels-Alder product. $s a result,

reaction of 3#,1#!%3,1%he2adiene with ethylene forms a cyclohe2ene with cis

substituents *eaction M(N!, whereas reaction of

3#,1$!%3,1%he2adiene with ethylene forms a cyclohe2ene with trans substituents

*eaction M3N!.

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In contrast to a M1 B 3N cycloaddition, a M3 B 3N cycloaddition does not occur under

thermal conditions, but does take place photochemically. This result is e2plained by

e2amining the symmetry of the 5@5 and L"@5 of the alkene reactants. In athermal M3 B 3N cycloaddition, like phases of thep orbitals on only one set of terminal

carbons can overlap. For like phases to overlap on the other terminal carbon, the

molecule must twist to allow for an antarafacial pathway. This process cannot occur

to form small rings. In a photochemical M3 B 3N cycloaddition, light energy promotes

an electron from the ground

state 5@5 to form the e2cited state 5@5 designated as ;3< in Figure 8.(!.

Interaction of this e2cited state 5@5 with the L"@5 of the second alkene then

allows for overlap of the like phases of both sets ofp orbitals. Two bonding

interactions result and the reaction occurs by a suprafacial pathway.

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Table 8.3 summari=es the &oodward:offmann rules that govern cycloaddition

reactions. The

number of 9 bonds refers to the total number of 9 bonds from both components ofthe cycloaddition.

For a given number of 9 bonds, the mode of cycloaddition is always opposite in

thermal

and photochemical reactions.

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In a sigmatropic rearrangement, a O bond in the reactant is broken, a new O bond is

formed, and the electrons rearrange. The O bond that breaks is a bond to an allylic

carbon. It can be a O bond between a carbon and a hydrogen, between a carbon andanother carbon, or between a carbon and an o2ygen, nitrogen, or sulfur.

SigmatropicQ comes from the Jreek word tropos, which means change,Q so

sigmatropic means sigma%change.Q

The numbering system used to describe a sigmatropic rearrangement differs from any

numbering system you have seen previously. First, mentally break the O bond in

the reactant and give a number ( label to the atoms that were attached by the bond.

Then look at the new O bond in the product. 8ount the number of atoms in each of

the fragments that connect the broken bond and the new O bond. The two numbers

are put in brackets with the smaller number stated first. For e2ample, in the following

M3,6N sigmatropic rearrangement, two atoms C, C! connect the old and new O bonds

in one fragment and three atoms 8, 8, 8! connect the old and new O bonds in the

other fragment.

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The numbering system used to describe a sigmatropic rearrangement differs from any

numbering system you have seen previously. First, mentally break the O bond in

the reactant and give a number ( label to the atoms that were attached by the bond.Then look at the new O bond in the product. 8ount the number of atoms in each of

the fragments that connect the broken bond and the new O bond. The two numbers

are put in brackets with the smaller number stated first. For e2ample, in the following

M3,6N sigmatropic rearrangement, two atoms C, C! connect the old and new O bonds

in one fragment and three atoms 8, 8, 8! connect the old and new O bonds in the

other fragment.

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In the transition state of a sigmatropic rearrangement, the group that migrates is

partially bonded to the migration origin and partially bonded to the migration

terminus. There are two possible modes for rearrangement. If the migrating groupremains on the same face of the system, the rearrangement is suprafacial. If the

migrating group moves to the opposite face of the system, the rearrangement is

antarafacial.

Sigmatropic rearrangements have cyclic transition states. If the transition state has si2

or fewer atoms in the ring, rearrangement must be suprafacial because of the

geometric constraints of small rings

$ M(,6N sigmatropic rearrangement involves a bond and a pair of O electrons, or we

can say that it involves two pairs of electrons. $ M(,/N sigmatropic rearrangement

involves two bonds and a pair of O electrons three pairs of electrons!, and a M(,>N

sigmatropic rearrangement involves four pairs of electrons. The symmetry rules for

sigmatropic rearrangements are nearly the same as those for cycloaddition

reactionsGthe only difference is that we count the number of pairs of electrons

rather than the number of bondsR 8ompare Tables 3-.6 and 3-.1.! %ecall that the

ground-state !"! of a compound with an even number of con&ugated double bonds

is asymmetric, whereas the ground-state !"! of a compound with an odd number

of con&ugated double bonds is symmetric.

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$ M(,6N sigmatropic rearrangement involves a bond and a pair of O electrons, or we

can say that it involves two pairs of electrons. $ M(,/N sigmatropic rearrangement

involves two bonds and a pair of O electrons three pairs of electrons!, and a M(,>Nsigmatropic rearrangement involves four pairs of electrons. The symmetry rules for

sigmatropic rearrangements are nearly the same as those for cycloaddition

reactionsGthe only difference is that we count the number of pairs of electrons

rather than the number of bondsR 8ompare Tables 3-.6 and 3-.1.! %ecall that the

ground-state !"! of a compound with an even number of con&ugated double bonds

is asymmetric, whereas the ground-state !"! of a compound with an odd number

of con&ugated double bonds is symmetric.

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$ *ope3 rearrangement is a M6,6N sigmatropic rearrangement of a (,/%diene. $

*laisen6 rearrangement is a M6,6N sigmatropic rearrangement of an allyl vinyl ether.

+oth rearrangements form si2%membered%ring transition states. The reactions,therefore, must be able to take place by a suprafacial pathway. &hether or not a

suprafacial pathway is symmetry%allowed depends on the number of pairs of

electrons involved in the rearrangement Table 3-.1!. +ecause M6,6N sigmatropic

rearrangements involve three pairs of electrons, they occur by a suprafacial pathway

under thermal conditions. Therefore, both 8ope and 8laisen rearrangements readily

take place under thermal conditions.

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&hen a hydrogen migrates in a sigmatropic rearrangement, the s orbital of hydrogen

is partially bonded to both the migration origin and the migration terminus in the

transition state. Therefore, a M(,6N sigmatropic migration of hydrogen involves a four%memberedring transition state. +ecause two pairs of electrons are involved, the

5@5 is asymmetric. The selection rules, therefore, re7uire an antarafacial

rearrangement for a (,6%hydrogen shift under thermal conditions Table 3-.1!.

8onse7uently, (,6%hydrogen shifts do not occur under thermal conditions because the

four%membered%ring transition state does not allow the re7uired antarafacial

rearrangement. (,6%ydrogen shifts can take place if the reaction is carried out under

photochemical

conditions because the 5@5 is symmetric under photochemical conditions, which

means that hydrogen can migrate by a suprafacial pathway.

M(,/N Sigmatropic migrations of hydrogen are well known. They involve three pairs of

electrons, so they take place by a suprafacial pathway under thermal conditions.

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photochemical





/(


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M(,>N Sigmatropic hydrogen migrations involve four pairs of electrons. They can

take place under thermal conditions because the eight%membered%ring transition

stateallows the re7uired antarafacial rearrangement.

/3


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photochemical





/6


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"nlike hydrogen, which can migrate in only one way because of its spherical s orbital,

carbon has two ways to migrate because it has a two%lobedp orbital. 8arbon can

simultaneously interact with the migration origin and the migration terminus usingone of its lobes. 8arbon can also simultaneously interact with the migration source

and the migration terminus using both lobes of itsp orbital.

If the reaction re7uires a suprafacial rearrangement, carbon will migrate using one of

its lobes if the 5@5 is symmetric and will migrate using both of its lobes if the

5@5 is asymmetric. &hen carbon migrates with only one of itsp lobes interacting

with the migration source and migration terminus, the migrating group retains its

configuration because bonding is always to the same lobe. &hen carbon migrates

with both of itsp lobes interacting, bonding in the reactant and bonding in the

product involve different lobes. Therefore, migration occurs with inversion of

configuration.

/1


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The following M(,6N sigmatropic rearrangement has a four%membered%ring transition

state that re7uires a suprafacial pathway. The reacting system has two pairs of

electrons, so its 5@5 is asymmetric. Therefore, the migrating carbon interacts withthe migration source and the migration terminus using both of its lobes, so it

undergoes inversion of configuration.

//


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