Chemical Change Chapter 2 Dr. Suzan A. Khayyat1

Chemical reactions Photochemical Reaction Photooxidation Reaction Photoaddition Reaction Photohydrogenation Pericyclic Reaction Photodissociation Thermal chemical Reaction types of chemical reaction Dr. Suzan A. Khayyat2

The Jablonski Diagram The energy gained by a molecule when it absorbs a photon causes an electron to be promoted to a higher electronic energy level. Figure 3 illustrates the principal photophysical radiative and non-radiative processes displayed by organic molecules in solution. The symbols So, S1, T2, etc., refer to the ground electronic state (So), first excited singlet state (S1), second excited triplet state (T2), and so on. The horizontal lines represent the vibrational levels of each electronic state. Straight arrows indicate radiative transitions, and curly arrows indicate non- radiative transitions. The boxes detail the electronic spins in each orbital, with electrons shown as up and down arrows, to distinguish their spin. Note that all transitions from one electronic state to another originate from the lowest vibrational level of the initial electronic state. For example, fluorescence occurs only from S1, because the higher singlet states (S2, etc.) decay so rapidly by internal conversion that fluorescence from these states cannot compete. Dr. Suzan A. Khayyat3

Jablonski energy diagram Dr. Suzan A. Khayyat4

Jablonski diagram Figure 3. The basic concepts of this Jablonski diagram are presented in the Basic Photophysics module. This version emphasizes the spins of electrons in each of the singlet states (paired, i.e., opposite orientation, spins) compared to the triplet states (unpaired, i.e., same orientation, spins). Dr. Suzan A. Khayyat5

Photochemical reactions with singlet Oxygen

Photooxygenation Reaction Dr. Suzan A. Khayyat9

( 1 O 2 ) 10

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Criteria of an ideal sensitizer It must be excited by the irradiation to be used, small singlet triplet splitting. High ISC yield. It must be present in sufficient concentration to absorb more strongly than the other reactants under the condition. It must be able to transfer energy to the desired reactant, low chemical reactivity in Triplet state.

Types of singlet oxygen reactions Dr. Suzan A. Khayyat15

Cis cyclic mechanism for the reaction of 1O2 with mono-olefins. 1- Ene Reaction Dr. Suzan A. Khayyat16

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2-Cycloaddition Reaction (Diels Alder) Dr. Suzan A. Khayyat22

Direct addition reaction to produce(1,2-dioxetane) Dr. Suzan A. Khayyat23

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Photosensitized oxidation C2H5O-CH=CH-OC2H5 Dr. Suzan A. Khayyat27

Photodissociation: processes and examples Hydrocarbons: Dr. Suzan A. Khayyat28

Carbonyl Compounds 1- Keetones: Norrish Type I: The Norrish type I reaction is the photochemical cleavage or homolysis of aldehydes and ketones into two free radical intermediates. The carbonyl group accepts a photon and is excited to a photochemical singlet state. Through intersystem crossing the triplet state can be obtained. On cleavage of the α-carbon carbon bond from either state, two radical fragments are obtained. Dr. Suzan A. Khayyat29

Norish Type I Processes of Ketones Basic Concepts

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Norrish type II A Norrish type II reaction is the photochemical intramolecular abstraction of a γ-hydrogen (which is a hydrogen atom three carbon positions removed from the carbonyl group) by the excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct Dr. Suzan A. Khayyat34

Norish type II photoelimination of ketones: Cleavage of 1,4-biradicals formed by γ - hydrogen abstraction

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Dr. Suzan A. Khayyat40 Complete the next equations

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2- Esters: Dr. Suzan A. Khayyat42

Photocycloaddition 2+2 Intermolecular cycloaddition Dr. Suzan A. Khayyat43

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Dr. Suzan A. Khayyat Intramolecular cycloaddition

Dr. Suzan A. Khayyat Cycloaddition

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Di-pi-methane rearrangement The di-pi-methane rearrangement is a photochemical reaction of a molecular entity that contains two π-systems separated by a saturated carbon atom (a 1,4-diene or an allyl- substituted aromatic ring), to form an ene- (or aryl-) substituted cyclopropane. The rearrangement reaction formally amounts to a 1,2 shift of one ene group (in the diene) or the aryl group (in the allyl-aromatic analog) and bond formation between the lateral carbons of the non- migrating moiety. 57

Oxa-Di-π-Methane rearrangement A photochemical reaction of a β, γ-unsaturated ketone to form a saturated α-cyclopropyl ketone. The rearrangement formally amounts to a 1,2-acyl shift and ‘bond formation’ between the former α and γ carbon atoms. 58

Mechanism I

Photoaddition and photocyclization reactions Dr. Suzan A. Khayyat61

Direct and photosensitized reactions Dr. Suzan A. Khayyat62

Isomerization and rearrangements Dr. Suzan A. Khayyat63

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Photochemical synthesis of oxetans Paternò-Büchi Reaction

Photo Fries rearrangement Dr. Suzan A. Khayyat84

a Fries Rearrangement is photochemical excitation Dr. Suzan A. Khayyat85

86 Synthetic applications of electrocyclisation reactions: The conversion of ergosterol to vitamin D2 proceeds through a ring-opening (reverse) electrocyclisation to give provitamin D2, which then undergoes a second rearrangement (a [1,7]- sigmatropic shift). Stereochemical control in the sigmatropic shift process will be described in a later section of this course. Dr. Suzan A. Khayyat

Photochemistry in solution Dr. Suzan A. Khayyat90

Photodimerization Dr. Suzan A. Khayyat91

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Factors determining reactivity 1- The excess energy possessed by the species (which may help overcome activation barriers). 2-The intrinsic reactivity of the specific electronic arrangement. 3-The relative efficiencies of the different competing pathways for loss of the particular electronic state. 4-The type of orbital (s, p, σ, or, π, etc.) and its symmetry. 5-Explicit in the correlation rules for orbital symmetry and spin that are introduced first at the end of this section.