Pericyclic reactions : a mechanistic and problem solving approach

371 Pages·2016·64.64 MB·English

by  Kumar

Pericyclic Reactions
A Mechanistic and Problem-Solving Approach
Sunil Kumar
Department of Chemistry
F.G.M. Govt. College
Haryana, India
Vinod Kumar
Department of Chemistry
Maharishi Markandeshwar University
Haryana, India
S.P. Singh
Department of Chemistry
Kurukshetra University, Kurukshetra
Haryana, India
AMSTERDAMlBOSTONlHEIDELBERGlLONDON
NEWYORKlOXFORDlPARISlSANDIEGO
SANFRANCISCOlSINGAPORElSYDNEYlTOKYO
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To Our Families
Sunil Kumar
Parents
Dr. Meenakshi, Ayush, Neerav
Vinod Kumar
Parents
Sushma, Mohit, Vignesh
S.P. Singh
Pushpa, Sunny, Romy
Preeti, Preety
Poorva, Uday, Adi, Veer
Preface
EversincetheappearanceoftheclassicTheConservationofOrbitalSymmetry
byWoodwardandHoffmannin1970,therehasbeenasurgeinthepublicationof
many books and excellent review articles dealing with this topic. This was
natural as after having established mechanisms of ionic and radical reactions,
focus had shifted to uncover the mechanisms of the so-called “no-mechanism
reactions.” The uncovering of the fact that orbital symmetry is conserved in
concerted reactions was a turning point in our understanding of organic re-
actions. It is now possible to predict the stereochemistry of such reactions by
followingthesimplerulethatstereochemicalconsequencesofreactionsinitiated
thermallywillbeoppositetothoseperformedunderphotochemicalconditions.
Studyofpericyclicreactions,astheseareknowntoday,isanintegralpartofour
understandingoforganicreactionmechanisms.
Despitethepresenceofmanyexcellentbooksonthisvibranttopic,therewas
anabsenceofabookthatconcentratesprimarilyonaproblem-solvingapproach
forunderstandingthistopic.Wehadrealizedduringourteachingcareerthatthe
mosteffectivewaytolearnaconceptualtopicisthroughsuchanapproach.This
bookiswrittentofillthisimportantgapinthebeliefthatitwouldbehelpfulto
students to have problems pertaining to different types of pericyclic reactions
compiledtogetherinasinglebook.
Thebookopenswithanintroduction(Chapter1),which,besidesproviding
background information needed for appreciating different types of pericyclic
reactions, outlines simple ways to analyze these reactions using orbital sym-
metry correlation diagram, frontier molecular orbital (FMO), and perturbation
molecularorbital(PMO)methods.Thischapteralsohasreferencestoimportant
publishedreviewsandarticles.
Electrocyclic, sigmatropic, and cycloaddition reactions are subsequently
describedinChapters2,3,and4,respectively.Chapter5isdevotedtoastudyof
cheletropic and 1,3-dipolar cycloaddition reactions as examples of concerted
reactions. Many group transfer reactions and elimination reactions, including
pyrolyticreactions,areincludedinChapter6.Therearesolvedproblemsineach
chapterthataredesignedforstudentstodevelopproficiencythatcanbeacquired
only by practice. These problems, about 450, provide sufficient breadth to be
adequatelycomprehensive.Solutionstoalltheseproblemsareprovidedineach
chapter. Finally, in Chapter 7, we have compiled unworked problems whose
xi
xii Preface
solutionsareprovidedseparatelyintheAppendix.Theaimbehindintroducing
theseunsolvedproblemsistoletthestudentsdeveloptheirownskills.
Assumingthatastudenthastakencoursesinorganicchemistrythatinclude
reaction mechanisms and stereochemistry, the bookis meant to be taught as a
one-semestercoursetograduateandseniorundergraduatestudentsmajoringin
chemistry.Onehastorememberthatabookdesignedforaone-semestercourse
cannot include all the reactions reported in the literature; rather, only repre-
sentativeexamplesofeachofvariousreactiontypesaregiven.Ageneralindexis
included,whichitishopedwillbeofhelptoreadersinsearchingforthetypesof
reactionsrelatedtoaparticularproblem.
We hope that our book will be well received by students and teachers.
We encourage all those who read and use this book to contact us with any
comments,suggestions,orcorrectionsforfutureeditions.Ouremailaddresses
are: [email protected], [email protected], and shivpsingh@
rediffmail.com.
We thank our reviewers for carefully reading the manuscript and offering
valuablesuggestions.Finally,wethanktheeditorialstaffofElsevierforbringing
thebooktofruition.
July2015 SunilKumar
VinodKumar
S.P.Singh
Chapter 1
Pericyclic Reactions and
Molecular Orbital Symmetry
Chapter Outline
1.1 ClassificationofPericyclic 1.5 AnalysisofPericyclic
Reactions 2 Reactions 13
1.2 MolecularOrbitalsof 1.5.1 OrbitalSymmetry
AlkenesandConjugated Correlation Diagram
PolyeneSystems 3 Method 13
1.3 MolecularOrbitals 1.5.2 FrontierMolecular
ofConjugatedIons OrbitalMethod 15
orRadicals 7 1.5.3 PerturbationMolecular
1.4 SymmetryProperties ofpor OrbitalMethod 17
s-MolecularOrbitals 11 FurtherReading 19
Inorganicchemistry,alargenumberofchemicalreactionscontainingmultiple
bond(s) do not involve ionic or free radical intermediates and are remarkably
insensitivetothepresenceorabsenceofsolventsandcatalysts.Manyofthese
reactionsarecharacterizedbythemakingandbreakingoftwoormorebonds
inasingleconcertedstepthroughthecyclictransitionstate,whereinallfirst-
order bondingsare changed. Such reactions are namedas pericyclicreactions
by Woodward and Hoffmann.
The word concerted means reactant bonds are broken and product bonds
are formed synchronously, though not necessarily symmetrically without the
involvement of an intermediate. Theword pericyclic means the movement of
electrons (p-electrons in most cases) in a cyclic manner or around the circle
(i.e., peri¼around, cyclic¼circle or ring).
They are initiated by either heat (thermal initiation) or light (photo
initiation) and are highly stereospecific in nature. The most remarkable
observation about these reactions is that, very often, thermal and photo-
chemical processes yield products with different stereochemistry. Most of
these reactions are equilibrium processes in which direction of equilibrium
depends on the enthalpy and entropy of the reacting species. Therefore, in
general, three important points that should be considered while studying the
PericyclicReactions.http://dx.doi.org/10.1016/B978-0-12-803640-2.00001-4
Copyright©2016ElsevierInc.Allrightsreserved. 1
2 PericyclicReactions
pericyclicreactionsare:involvementofp-electrons,typeofactivationenergy
required (thermal or light), and stereochemistry of the reaction.
There is a close relationship between the mode of energy supplied and
stereochemistry for a pericyclic reaction, which can be exemplified by
considering the simpler reactions shown in Scheme 1.1.
SCHEME1.1 Stereochemicalchangesinpericyclicreactionsunderthermalandphotochemical
conditions.
When heat energy is supplied to the starting material, then it gives one
isomer,whilelightenergyisresponsibleforgeneratingtheotherisomerfrom
the same starting material.
1.1 CLASSIFICATION OF PERICYCLIC REACTIONS
Pericyclicreactionsare mainlyclassified intothe four mostcommon types of
reactions as depicted in Scheme 1.2.
SCHEME1.2 Commontypesofpericyclicreactions.
In an electrocyclic reaction, a cyclic system (ring closure) is formed
through the formation of a s-bond from an open-chain conjugated polyene
system at the cost of a multiple bond and vice versa (ring opening). These
reactions areunimolecular innature astherate ofreactions dependsuponthe
PericyclicReactionsandMolecularOrbitalSymmetry Chapter j1 3
presence of one type of reactant species. Such reactions are reversible in
nature, but the direction of the reaction is mainly controlled by thermody-
namics. Most of the electrocyclic reactions are related toring closingprocess
instead of ring opening due to an interaction between the terminal carbon
atoms forming a s-bond (more stable) at the cost of a p-bond.
Sigmatropicrearrangementsaretheunimolecularisomerizationreactions
inwhichas-bondmovesfromonepositiontoanotheroveranunsaturatedsystem.
Insuchreactions,rearrangementofthep-bondstakesplacetoaccommodatethe
news-bond,butthetotalnumberofp-bondsremainsthesame.
Incycloadditionreactions,twoormorecomponentscontainingp-electrons
come together to form the cyclic system(s) through the formation of two or
morenew s-bondsat the costofoverall two ormore p-bonds, respectively, at
least one from each component. Amongst the pericyclic reactions, cycloaddi-
tions are known as the most abundant, featureful, and valuable class of the
chemical reactions. The reactions are known as intramolecular when cycload-
dition occurs within the same molecule. The reversal of cycloaddition in the
same manner is known as cycloreversion. There are some cycloaddition
reactionsthatproceedthroughthestepwiseionicorfreeradicalmechanismand
thus are not considered as pericyclic reactions.
These reactions are further extended to cheletropic and 1,3-dipolar
reactions, which shall be discussed in detail in Chapter 5.
Group transfer reactions involve the transfer of one or more atoms or
groups from one component to another in a concerted manner. In these
reactions,twocomponentsjointogethertoformasinglemoleculethroughthe
formation of a s-bond.
Itisveryimportanttonotethatinstudyingthepericyclicreactions,thecurved
arrowscanbedrawninclockwiseoranticlockwisedirection(Scheme1.3).The
directionofarrowsdoesnotindicatetheflowofelectronsfromonecomponentor
sitetoanotherasinthecaseofionicreactions;rather,itindicateswheretodraw
thenewbonds.
SCHEME 1.3 Clockwise and anticlockwise direction of the curved arrows in pericyclic
reactions.
1.2 MOLECULAR ORBITALS OF ALKENES AND
CONJUGATED POLYENE SYSTEMS
In order to understand and explain the results of the various pericyclic re-
actions on the basis of different theoretical models, a basic understanding of
the molecular orbitals of the molecules, particularly those of alkenes and
conjugated polyene systems and their symmetry properties, is required.
4 PericyclicReactions
According to the molecular orbital theory, molecular orbitals (MOs) are
formed by the linear combination of atomic orbitals (LCAO) and then filled
by the electron pairs. In LCAO when two atomic orbitals of equivalent
energy interact, they always yield two molecular orbitals, a bonding and a
corresponding antibonding orbital. The bonding orbital possesses lower
energyandmorestabilitywhileantibondingpossesseshigherenergyandless
stability as compared to an isolated atomic orbital. Let us consider the
simplest example of H molecule formed by the combination of 1s atomic
2
orbitals (Figure 1.1).
nodal plane
Eσ* (σ*) antibonding
E 1s 1s
Eσ
H H
(σ) bonding
molecular orbitals
FIGURE1.1 FormationofmolecularorbitalsinthecaseofanH2molecule.
Thebondingmolecularorbitalisaresultofpositive(constructive)overlap,
andhenceelectrondensityliesintheregionbetweentwonuclei.However,an
antibonding molecular orbital is formed as a result of negative (destructive)
overlap and, therefore, exhibits a nodal plane in the region between the two
nuclei. The bonding and antibonding molecular orbitals exhibit unequal
splitting pattern with respect to the atomic orbitals because a fully filled
molecular orbital has higher energy due to interelectronic repulsion.
We now consider molecular orbital theory with reference to the simplest
p-molecular system, ethene. As already discussed, the number of molecular
orbitals formed is always equal to the number of atomic orbitals combining
together. Similarly, in the case of an ethene molecule, sideways interaction
betweenp-orbitalsofthetwoindividualcarbonatomsresultsintheformation
of the new p bonding and p* antibonding molecular orbitals that differ in
energy (Figure 1.2). In the bonding orbital of ethene, there is a constructive
overlap of two similar lobes of p-orbitals in the bonding region between the
nuclei. However, in the case of an antibonding orbital, there is destructive
overlapoftwooppositelobesinthebondingregion.Eachp-orbitalconsistsof
two lobes with opposite phases of the wave function.
We ignore s-bond skeleton in this treatment as sigma molecular orbitals
remain unaffected during the course of a pericyclic reaction.
The conjugated polyenes constitute an important class of organic
compounds exhibiting a variety of pericyclic reactions. On the basis of the
PericyclicReactionsandMolecularOrbitalSymmetry Chapter j1 5
node nodal plane
• π*
antibonding
node
• nodal plane E
p-orbital π p-orbital
p-orbital
bonding
Ethene
FIGURE1.2 Formationoftwomolecularorbitals(pandp*)ofethene.
number of p-electrons, such compounds are classified into two categories
bearing4nor(4nþ2)p-electronsystems.Inordertoconstructthemolecular
orbitals for such polyene systems, let us consider buta-1,3-diene as the
simplest example.
Inthemoleculeofbuta-1,3-diene,therearefourp-orbitalslocatedonfour
adjacentcarbonatomsandhencethisgeneratesfournewp-molecularorbitals
on overlapping. The way to get these new p-molecular orbitals is the linear
combination of two p-molecular orbitals of ethene according to the
perturbationmolecularorbital(PMO)theory.Likethecombinationofatomic
orbitals,overlappingofthebonding(sorp)orantibondingmolecularorbitals
(s* or p*) of the reactants (here, ethene) results in the formation of the new
molecular orbitals that are designated as J , J , etc. in the product (here,
1 2
buta-1,3-diene).
AccordingtoPMOtheory,linearcombinationalwaystakesplacebetween
the two orbitals(two molecularorbitals ortwo atomic orbitals, or oneatomic
andonemolecularorbital)havingminimumenergydifference.Thus,herewe
need to consider pep and p*ep* interactions (constructive or destructive)
instead of interactions between p and p* orbitals (Figure 1.3). In buta-
1,3-diene, 4p-electrons are accommodated in the first two p-molecular or-
bitals, and the remaining two higher energy p-molecular orbitals will remain
unoccupied in the ground state of the molecule.
The lowest energy orbital (represented as wave function J ) of buta-
1
1,3-dienedoesnothaveanynodeandisthemoststableduetothepresenceof
three bonding interactions. However, the second molecular orbital J
2
possessesonenode,twobondingandoneantibondinginteractions,andwould
be less stable than J . The J has two nodes and one bonding interaction.
1 3
Due to the two antibonding interactions, J possesses overall antibonding
3
characterandthusenergyofthisorbitalismorethantheenergyofJ .TheJ
2 4
orbital is formed by the interaction between p* and p* of two ethene mole-
cules. It bears three nodes and the highest energy.
Similarly,inthecaseoflongerconjugatedsystemslikeahexa-1,3,5-triene
system, there are six p-orbitals on six adjacent carbon atoms, which can