chemical change-2

8/10/2019 Chemical Change-2

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Chemical Change

Chapter 2

Dr. Suzan A. Khayyat 1


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Chemical

reactions

PhotochemicalReaction

Photooxidation Reaction

PhotoadditionReaction

Photohydrogenation

Pericyclic Reaction

Photodissociation

Thermal chemicalReaction

types of chemical reaction



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The Jablonski Diagram

The energy gained by a molecule when it absorbs a photon causes an electron tobe promoted to a higher electronic energy level. Figure 3 illustrates the principalphotophysical radiative and non-radiative processes displayed by organicmolecules 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 soon. 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, withelectrons 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, fluorescenceoccurs only from S1, because the higher singlet states (S2, etc.) decay so rapidly byinternal conversion that fluorescence from these states cannot compete.



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Absorption

Fluo

rescence

Phosphorescence

photochem. &singlet oxygen

n

1(n,

Singlet State

(S1,S2, ......) Triplet State(T1, T2, ...)

ISC

Biological Response

Photochem.

Ground StateSo

Jablonski energy diagram

Jablonski energy diagram



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Jablonski diagram

Figure 3. The basic concepts of this Jablonski diagram are presented in the BasicPhotophysics module. This version emphasizes the spins of electrons in each ofthe singlet states (paired, i.e., opposite orientation, spins) compared to thetriplet states (unpaired, i.e., same orientation, spins).



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Photochemical reactions with singlet Oxygen

1

O2


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1Sens (S0

)1Sens*(S1)

hv

1Sens*(S1) 3Sens*(T1)3Sens*(T1)

1Sens (S0) + 1O

2

+3O2

Photooxygenation Reaction



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(1O2

)

1+g

-g

3

1g

22.4

37.5 Kcal/mo

Kcal/mol

Hig hest occup ied mol ecu la r orb i ta l o f1

O2



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N

N

N

N

H

H

C6H5

C6H5

C6H5

C6H5

Tetraphenylporphyrine(TPP)

N

N

N

N

H3C CHCH3

OH

CH3

CHCH3OH

HOOCH2C-H2C

HOOC-H2C-H2C CH3

H

H

H3C

Hematoporphyrine( HP)

O

Cl

ClCl

Cl

I

OI

ONa

I

I

COONa

Ros Bengal(RB)



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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.


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Types of singlet oxygen reactions

3)

2)

1)

H

X

+

+

+

1

O2

O2

1O2

1

A

B

C

OOH

X

OO

O O



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O2*

C

C C

H

C

C C

O OH

Cis cyclic mechanism for the reaction of 1O2 withmono-olefins.

1- Ene Reaction



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C C

CH

+1O2 C C

OOH

C



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2-Cycloaddition Reaction (Diels Alder)



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Direct addition reaction to produce(1,2-dioxetane)



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Photosensitized oxidation

OCH3H3C

+ O2hv , sens

OCH3H3C

OO

C C

CH3

CH3

H3C

H3C

+ O2hv , sens

C C

CH2

CH3

H3C

H3C

+ O2hv , sens

C2H5O-CH-CH-OC2H5

O O

C2H5O-CH=CH-OC2H5



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Photodissociation: processes and examples

Hydrocarbons:

RCH2R/ + hv RCR

/+ H2

CH2=CH2+ hv H2 + H2C=C: ( HC CH)

2H + H2C=C:

H2 + HC CH

2H + HC CH



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Carbonyl Compounds

1- Keetones:

Norrish Type I:

The Norrish type I reaction is the photochemical cleavage or homolysisof aldehydes and ketones into two free radical intermediates. Thecarbonyl group accepts a photon and is excited to a photochemical

singlet state. Through intersystem crossing the triplet state can beobtained. On cleavage of the -carbon carbon bond from either state,two radical fragments are obtained.



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Norish Type I Processes of Ketones Basic

Concepts

R

O

C

O

C

h

+


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O O O

O OO

O

OMe

O

O

2 X 106 3 X 107 1 X 108

2 X 1082 X 107

1 X 107

7 X 105not measured

>109

# Norish type I reaction is much faster for n-

* compared to

* excited states

# n-

* reactivity is due to the weakening of the

-bond by overlap of this bond with the half

vaccant n-orbital of oxygen.

# This overlap is not possible for

* excited states

# Electron releasing group at para position lead to stabilization of

* excited states hence decrease in reactivity


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Norrish type II

A Norrish type II reaction is the photochemical intramolecular abstraction

of a -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



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Norish type II photoelimination of ketones:

Cleavage of 1,4-biradicals formed by -

hydrogen abstraction

O1O*

h


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RR'

RR'

1

RR'

1O*

R

R'OH

n

R

OH R'

R R'

O

R

R'

1O*

RR'

1O*

RR'

3O*

RR'

O

RR'

3O*

R

R'

OH

n

R

OH R'

R'

OH

R

RR'

O

h

1KHa

1Kd

Kisc3Kd

3

KH


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RCHO + hv RH + C

C=O + hv

2C2H4 + CO

+ CO

CH2=CHCH2CH2CHO



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H2C

O

H2C hv

Ohv

Complete the next equations


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H3C

CH2

CH3

O

hv

H3C

CH3

CH3

CH3

O

hv


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2- Esters:


RCH2CH2CH2COOR\

hv

RCH=CH2 + CH3COOR

\

hv

RCOOCH CH R\RCOOH + CH2=CHR

\


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Photocycloaddition

2+2 Intermolecular cycloaddition


R

R\

+

O

H3CO

OCH3

O

hv

H3CO

O

R

R\

OCH3

O

O


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O

hv

O O

+

O

O

2


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hv

2+2 Intramolecular cycloaddition


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+


2+4 Cycloaddition


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hv +


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O OEtCN O

OEt

OCN

O O

OEt OEtO

OEt

CN N O

CN

Regiochemistry of enone cycloaddition

-

h

reversal of polarity

head to tail

head to head

-

-

OAc


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O

OMe

OMe

O

O

nBu

OAc

nBu

O

OAc

nBu

nBu

O

OEtEtO

CO2Et

O

CO2Et

OEt

OEt

OOEt

OEt

CO2Et

O

SiMe3

OSiMe

3

OSiMe

3

O

OAc

OOOO

OAc

O

O OOAc

O

O

OAc

O

O OO

98%

+

+

only

+

82.5 17.5

+

1 1

+

95 5

96%

81 19

O


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O

OH

H

OH

H

OH

H

OH

H

always cis

always cis


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O O

OH OH

O

H

O

H

O O

O

CuOTf, h

exo pdt

The observed selectivity is assumed to arise from

a preferential formation of the less sterically crowded

copper (I)-diene complex, leading to exo pdt.

NaIO4/RuO4

CuOTf, h

CuOTf, h


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R

O

H

R

OH

CH3

R

O

R

OH

CO2Me

CO2Me

CO2Me

CO2

Me

R OHR

CO2Me

CO2

Me

H-Transfer

spin-inversion

+

Photoenolization


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O

Ph

.

C OH

Ph

Me

.

C

Me

OH

OH

PhOH

Ph

OH

PhO

Ph

h

O OH Oh (-)Ephidrine


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O

OO

OMe

OMe

MeO

O O

CO2Et O

OOH

OMe

OMe

MeO

O O

CO2Et

O

O

OMe

OMe

MeO

O O

OHCO

2Et

h

Norish II, Cleavage

(-)Ephidrine

Enantioselective

H-transfer

h

Photoenolization

4+2

Podophyllotoxin derivative


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Di-pi-methane rearrangement

The di-pi-methane rearrangement is a photochemical reaction

of a molecular entity that contains two -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 thediene) or the aryl group (in the allyl-aromatic analog) and

bond formation between the lateral carbons of the non-

migrating moiety.

57

hv

O Di M th t


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Oxa-Di--Methane rearrangement

A photochemical reaction of a , -unsaturated

ketone to form a saturated -cyclopropylketone. The rearrangement formally amounts to

a 1,2-acyl shift and bond formation between

the former and carbon atoms.

58

O

hv

O


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Mechanism I


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Photoaddition and photocyclization


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Photoaddition and photocyclization

reactions

+

NH2

hv

HN

+

HN

+



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Direct and photosensitized reactions

trans

cis

direct

sensitized



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Isomerization and rearrangements



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R h


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N N

R

RN N

R R

h

R = Me R = CHMe

R = R =

R = R =


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NN

N N

N NN N

C C

h

h(405nm)

h

(436nm)/heat

h

(313nm)

-N2 h (313nm)

-N2

Ci T i i i f lk


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A

B

D

E

A

B

E

D

Cis-Trans isomerization of alkenes

3S**3


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h

triplet

donor

h


69/95

h

sens

h

sens


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H

H

h

185 nm

sens

heath

h


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72/95

direct

Tripletsensitized



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hv

HH

hv+ +

Benzvalene bicyclo-

hexadiene

fulvene



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CN

C6H5C6H5

C6H5 CNC6H5

hv


75/95

Photochemical synthesis of

oxetansPatern-Bchi Reaction

O


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O

O

EtO

OEt

CO2H

O N

N

OOH

OH

N

N

NH2

O

O

NH2

NH

NH2

O

O

O

OO

O

OAc

OR

H

OBz

OOAc

OH

+

Paterno and Chieffi (1909), Buchi in 1954 mechanistic analysis

Insecticidal activity

Thromboxane A2 Oxetanocine

Bradyoxetin

Merrilactone A

Palitaxel

Reaction mechanism


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CHO

C

O

H

O

C

C

O

C

C

OO

h[PhCHO] S1

ISC[PhCHO] T1

(n-*)

Kisc aromatic>> Kisc aliphatic (>>1010/s)

responsible

+

electrophile nucleophile

+

Major Minor

Biradical intermediate

O O O

Enones and Ynones


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O

Me CCl3 Me CCl3

OO

O

Me Me

F

O

F

O

F

O

Me Me

Cl

O

Cl

O

Cl

+ +

42% 47%

+ +Low T

3% oxetane

+ +

10% 9 0%

+ +

90% 10%

O O

SiMe3

h


79/95

O

PhPh SiMe3

O

SiMe3

Ph

Ph

O

Ph

Ph

O

PhPh OTMS

O

OTMS

Ph

Ph

O

Ph

Ph

OTMS

O

PhPhH SMe

H

O

H

Ph

Ph

SMe

O

H

Ph

Ph

SMe

+

h

+

24 1

+

h+

94 6

+

h

+

100 0


80/95

R1R2

R3 R4O

R

R4

R3

R1

R2

OX

R4

R3

CHRYR1

R2

O

PhR

OTMSOH

R OTMS

Ph OH

R OH

Ph

O

OPh O

OH

Ph

+ R CHOh XY

Carboxydroxylation strategy by reductive cleavage of oxetanes

H2

H2

OH

Total synthesis of (+)-Preussin


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N

Ph

H

O

Ph N

PG

N

PG

O

Ph N

PG

OH

Ph

N

CO2Me

R

N

CO2Me

R

O

H

H

Ph NR

OH

Me

Ph

N

CO2Me

N

CO2Me

O

Ph N

OH

Ph

+

Carbohydroxylation strategy fo N-containing unsaturated heterocycles

PhCHO/h

MeCN

H2, Pd(OH)2/C

LAH/THF

endo

MeCN

17%

H2, Pd(OH)2/C

LAH/THF

Chem.Eur.J, 2000, 6, 3838-48

PhCHO/h


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+orthopara

meta

1

2

3

45

6

1

2

1

4

1

3

Possible modes of addition in the arene-alkene photocycloaddition reactions

RR


83/95

R

HH

R

+

endo exciplex

h

h i


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Photo Fries rearrangement

http://localhost/var/www/apps/conversion/tmp/scratch_5//upload.wikimedia.org/wikipedia/commons/5/5d/Fries_rearrangement_(photo).svg


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a Fries Rearrangement is photochemical

excitation


Synthetic applications of electrocyclisation reactions:


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86

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.

HHHO

ergosterol

sunlight

photochemically-promoted electrocyclisation(antarafacial, conrotation)

H

HOprovitamin D2

H

HO

H

[1,7]-sigma-tropic shift.

vitamin D2

Dr. Suzan A. Khayyat

DNA photochemistry


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NH

N

O

O

R

R'

N

N

NH2

O

R

N

N N

N

NH2

R

NH

N N

N

R

O

NH2

p y

Ura R ' = H R = H

Urd R ' = H R = ribose

UMP R ' = H R = ribose phosphate

Thy R ' = Me R = H

Thd R ' = Me R = deoxyribose

TMP R ' = Me R = deoxyribose phosphate

Cyt R = H

Cyd R = ribose

CMP R = ribose phosphate

PYRIMIDINES

Ade R = H

Ado R = ribose

AMP R = ribose phosphate

Gua R = H

Guo R = ribose

GMP R = ribose phosphate

PURINES

260 nm (*)270 nm (*)

NH2 OH

O


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N

NH

N

N

O

OH

NH2

O

O P

O

OO

O

OH

O

O

N

NH

N

N

O

OH

N2

O

O P

O

O

O

O

OH

O

O

H

HH

O

N

NH

N

N

O

OH

NH2

O

O P

O

O

OO

OH

O

N

NH

N

NH

O

OH

O

O P

O

O

O

O

OH

O

O

H

HH

O

N

N

N

N

O

OH

NH2

O

O P

O

O

O

OHO

OH

h

heat

Possible photoreaction at dipyrimidine sequences (CT); cyclobutane and oxetane formation

h

O O


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N

N

N

NH

O

OH

O

O P

O

O

O

O

OH

N

N

NH2

O

N

NH

O

OH

O

O P

O

O

O

N

NNH

2

N

NOH

N

N

N

N

O

OH O P

O

O

O

O

OH

N

N

NH2

N

N

NH2

N

N

O

O

O

PO O

O

OH

N

N

N

N

NN

NH2

NH2

h

Cycloadditions involving adenine; Cyclobutane and azetidine dimer formation

h

Ph t h i t i l ti


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Photochemistry in solution

(CH3)H2C C

O

H2C (CH3) CO + C3H8

liq+H3C CHCHO

gas

CHH2C

OC

OC

H2C (CH3)2


Ph t di i ti


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Photodimerization


hv

in open air

,CHCl3

Scheme 1

1

4

CHO CHO

OHC

O

O

OO

OHC

CHO1

2

3

45

1\\

2\\3\\

4

\\

5\\ 6\\

1\

2\ 3\

4\

5\6\


92/95


hv

inopenair ,CHCl3

Scheme 2

2

5

H3CO

HO

H3CO

HO

OCH3

OH

H3CO

HO OCH3

OH1\

2\

3\

4\

5\6

\

32

1

4


93/95


O

O

hv

in openair ,CHCl3

O

O

Scheme 3

3

O

O

O

6

O

OO

1 2

3

45

6 1\

2\

3\4\

5\6\

O

O

O O

Factors determining reacti it


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Factors determining reactivity

1- The excess energy possessed by the species (whichmay 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, , or, , 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.

ONOO


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H

H

O

H

H

H

O

H

O

C

H

HH

O

H

HH

NOH

O

h

chemical change-2

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