Pericyclic Reactions and Organic Photochemistry

347 Pages·2008·12.62 MB·English

by  Sharmar

PERI CYCLIC REACTIONS
and
ORGANIC PHOTOCHEMISTRY
VINAY P. SHARMA RAKESH KUMAR
M.Sc., Ph. D. MSc., Ph. D.-Scholar
Department of Chemistry, Department of Chemistry,
J.Y. (P.G.) College, Baraut (U.P.) J.Y. (P.G.) College, Baraut (U.P.)
~
PRAGATI PRAKASHAN
PRAGATI PRAKASHAN
First Edition : 2008
Educational Publishers
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Contents
PERICYCLIC REACTIONS 1-184
•
1111
INTRODUCTION ................................................ 1-13
1.1 General Introduction 3
1.2 Singlet and Triplet States 6
1.3 Classification 6
1.3.1 Electrocyclic Reactions 6
1.3.2 Cycloadditions 9
1.3.3 Sigmatropic Rearrangements 11
IFII
MOLECULAR ORBITAl, ORBITAL SYMMETRY AND FRONTIER
MOLECULAR ORBITALS •••••••••••••••••••••••••••••••••••••••• 14-23
2.1 Molecular orbital 14
2.2 Molecular Orbital Symmetry 15
2.3 Frontier Molecular Orbitals (FMO) 21
[III
PREDICTIONS ABOUT FEASIBILITIES OF PERICYCLIC REACTIONS ••••• 24-35
3.1 Woodward-Hoffmann Correlation-diagram Approach 24
3.1.1 Precautions in the construction of Correlations Diagrams 28
3.2 Frontier Molecular Orbital (FMO) Approach 28
3.3 Perturbational Molecular Orbital (PMO) Approach 30
1111
ElECTROCYClIC REACTIONS •••••••••••••••••••••••••••••••••••. 36-46
4.1 Predictions of Feasibility 39
4.1.1 Correlation Diagram Method 39
4.1.2 Frontier Molecular Orbital (FMO) Method 42
4.1.3 Stereochemical Outcome of Electrocyclic Reactions 43
4.1.4 Perturbational Molecular Orbital (PMO) Method 46
1111
CYCLOADDITION REACTIONS •••••••••••••••••••••••••••••••••••• 47-81
5.1 Suprafacial and Antarafacial Process 49
5.2 Correlation Diagram Method 49
5.2.1 Diels-Alder Reaction 51
5.2.2 Cycloadditions of Benzene and its Derivatives 51
5.2.2.1 Reactions between Benzene and Alkenes 51
5.2.2.2 Reactions between Benzene and 1,3-butadiene 57
5.3 Frontier Molecular Orbital (FMO) Method 59
5.4 Perturbational Molecular Orbital (PMO) Method 62
5.5 Mechanism of Cycloaddition Reactions 64
5.5.1 2 + 2 Cycloadditions 64
5.5.2 4 + 2 Cycloadditions 66
5.6 2 + 2 Thermal Cycloaddition of Ketenes 70
5.7 1,3-Dipolar Cycloadditions 72
5.8 Cheletropic Reactions 77
1111
SIGMATROPIC REARRANGEMENT ••••••••••••••••••••••••••••••• 82-98
6.1 Correlation Diagram Method 82
6.2 Frontier Molecular Orbital Method 82
6.3 Peripatetic Cyclopropane Bridge 83
6.4 1,3-Sigmatropic Shift 85
6.5 Perturbational Molecular Orbital Method 86
6.6 Important Facts about Sigmatropic Rearrangements 88
6.7 Some Important Examples of Sigmatropic Rearrangement 89
6.7.1 The Cope Rearrangement 89
6.7.2 The Claisen Rearrangement 91
6.7.3 The Ene Reaction 94
6.7.4 Fluxional Tautomerism 94
6.7.5 Conversion of Triene to Toluene 95
6.7.6 Vinylcyclopropane ~ Cyclopentene Conversion 96
6.7.7 1,5-Shift in 1,3-pentadiene 96
6.7,8 Precalciferol ~ Calciferol Conversion 96
6.7.9 [1,5]-Thermal and [1,7]-photochemical sigmatropic hydrogen
shifts in the isomers of 1,4~di (cycloheptatrienyl) benzene 97
6.8 Sequential Sigmatropic Shifts 98
6.9 Sigmatropic Migration of Carbon 98
lal
MISCElLANEOUS •••••••••••••••••••••••••••••••••••••••••••• 99-118
7.1 Group Transfers and Eliminations 99
7.1.1 Group Transfer 99
7.1.2 Group-elimination 100
7.2 Pericyclic Reactions Involving Ionic Transition States 102
7.2.1 Electrocyclic Reactions 102
7.2.1.1 FMO Method 102
7.2.1.2 PMO Method 105
7.2.2 Cycloadditions 105
7.2.2.1 PMO Method 106
7.2.3 Sigmatropic Reactions 107
7.2.3.1 FMO Method 107
7.2.3.2 PMO Method 107
7.3 Electrocyclic and Sigmatropic Reactions as Cycloadditions 109
7.3.1 Electrocyclic Reactions 109
7.3.2 Sigmatropic Reactions 110
(vii)
7.4 Woodward-Hoffmann's Generalized Rules for
Peri cyclic Reactions 111
7.5 Secondary Effects 114
7.6 2 + 2-Cycloadditions in the Photochemistry of the
cyclohexadienones and Cyclohexenones 115
7.7 Aza-Cope Rearrangement 117
1111
PROBLEMS AND SOLUTIONS ••••••••••••••••••••••••••••••••• 119-184
8.1 Electrocyclic Reactions 119
8.2 Cycloaddition Reactions 137
8.3 Sigma tropic Rearrangements 152
8.4 Miscellaneous Problems 157
8.5 Synthetic Applications of Pericyclic Reactions 173
ORGANIC PHOTOCHEMISTRY 185-324
•
IllllNTRODUOION •••••••••••••••••••••••••••••••••••••••••••• 181-192
1.1 Activation Energy 189
1.2 Energy Intake (Thermal or Photochemical) 190
1.3 Brief History of Photochemistry 191
IfI I
BASIC PRINCIPLES •••••••••••••••••••••••••••••••••••••••••• 193-215
2.1 Interaction of Electromagnetic Radiation with Matter 193
2.2 Laws of Photochemistry 195
2.3 Fate of Excited Molecule 198
2.4 1Ypes of Excitations 203
2.5 Transfer of Excitation Energy: [Sensitisation and Quenching] 205
2.6 Quantum YieldiQuantum Efficiency 210
2.7 Actinometry 212
1111
DETERMINATION OF REACTION MECHANISM ••••••••••••••••••• 216-228
3.1 Classification: [Thermodynamic Pathways] 217
3.2 Rate Constants and Their Relation with the Life-Time of
Reactive Excited States 218
3.3 Determination of Rate Constants of Photochemical Reaction 221
3.4 Effect of Light Intensity on the Rate of Photochemical Reactions 223
3.5 1Ypes of Photochemcial Reactions 223
3.6 Photodissociation 224
3.7 Gas Phase Photolysis 425
IIII
ELECTROCYCUC REACTIONS •.••••••..•••••••..•••••••.•.•.••• 229-241
4.1 Geometrical Isomerisation 230
4.2 Cyclisation Reactions: Valence Isomerisation 233
4.3 Photochemical Rearrangements of Alkenes 234
4.4 Rearrangement of 1,4-Diene (Di-1t-Methane) Rearrangement 235
4.5 Rearrangement of 1,5-Diene 238
IIII
PHOTOCHEMISTRY OF AROMATIC COMPOUNDS •••••••••••••••• 242-253
5.1 Photochemical Isomerisation of Aromatic Compounds 242
5.2 Photochemical addition Reactions Shown by
Aromatic Compounds 245
5.3 Photochemical Substitution on Aromatic Compounds 249
IIII
PHOTOCHEMISTRY OF CARBONYL (OMPOUNDS .••• 254-280
0 •••••••••••
6.1 Saturated Acyclic Carbonyl Compounds 255
6.2 Norrish Type I (a-Cleavage) Reaction 255
6.3 Norrish TYpe II Reactions 257
6.4 Photoreduction 260
6.5 Saturated Cyclic Carbonyl Compounds 262
6.6 ~, y-Unsaturated Carbonyl Compounds 265
6.7 a, ~-Unsaturated Carbonyl Compounds 268
6.8 Cyclohexadienone 271
6.9 Intermolecular Cycloaddition Reactions 273
6.10 Photodimerisation of Carbonyl Compounds 274
6.11 Oxextane Formation 277
1111
MISCELLANEOUS PHOTOCHEMICAL REACTIONS ••••••••••••••••• 281-299
7.1 Photo-Fries Rearrangement 281
7.2 Photo-Fries Reaction of Anilides 283
7.3 Barton Reaction 283
7.4 Singlet Molecular Oxygen Reaction 286
7.5 Photochemical Formation of Smog 292
7.6 Photochemical Degradation of Polymers 293
7.7 Photochemistry of Vision 295
IIII
PROBLEMS AND SOLUTIONS ••••••••••••••••••••••••••••••••• 300-324
8.1 Introduction 300
8.2 Basic Principles 301
8.3 Determination of Reaction Mechanism 305
8.4 Photochemistry of Alkenes 306
8.5 Photochemistry of Aromatic Compounds 310
8.6 Photochemistry of Carbonyl Compounds 315
8.7 Miscellaneous Photochemical Reactions 322
>- APPENDIX ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 325-333
>- INDEX •••••••••••••..•••••••••••••••••••••••••••••••••••••.••••••••••••••••
>- LOG TABLES ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
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GENERAL INTRODUCTION
A large number of chemical reactions of unsaturated organic compounds
(both olefinic as well as acetylenic) are such that they proceed without any
involvement of intermediates like carbonium ion, carbanion or free radical,
rather they involve cyclic transition state (T.S.). As they involve cyclic T.S.,
hence they are known as peri cyclic reactions. These reactions were discovered by
Woodward and Hoffmann in 1965. Their characteristics are:
(i) They are insensitive to the presence or absence of catalysts and
solvents.
(ii) They are feasible both thermally as well as photochemically.
(iii) Products through thermal and photochemical transformations are
generally different in their stereochemistry.
(iv) Most important observation about them is that during their occurance
symmetry properties of orbitals are conserved, i.e., a symmetric orbital
transforms into a symmetric orbital and an asymmetric orbital is
transformed into an asymmetric orbital.
(v) They occur in a concerted manner, i.e., bond making and bond
breaking both are synchroneous.
(vi) They always follow lowest energy path even if difference between
symmetry allowed or symmetry forbidden transiiion is very small;
otherwise they will not occur in concerted manner.
An interesting example of pericyclic reactinn is cyclization of precalciferol
to steroisomeric I and II under thermal condition; both of which are cis-products.
Similar reaction under photochemical conditions, i.e., upon irradiation gives
ergsterol(III), which is trans-product. Thus pericyclic reaction may result is
different products under thermal and photochemical conditions.
These reactions are generally equilibrium reactions. The extent of
equilibrium is governed by entropy and enthalpy of reacting systems. Though
these reactions are not affected by catalyst, but some transition metals such as
Pt(II), Pd(II), Ag(I), Ir(I), Rh(I) and Ni(I) etc., sometime catalyse these reactions in
unusual way. However, they are unaffected by acids and bases.
Under thermal condition activation energy required for peri cyclic reactions
is supplied by increasing the reaction temperature as a result of which most ofthe
molecules have nearly same amount of energy and the molecules which have
(3)