Chemical Change

Dr. Suzan A. Khayyat 1

Chemical Change

Chapter 2


Chemical reactions

Photochemical Reaction

Photooxidation Reaction

Photoaddition Reaction

Photohydrogenation

Pericyclic Reaction

Photodissociation

Thermal chemical Reaction

types of chemical reaction


• 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.


Absorption

Fluorescence

Phosphorescence

photochem. & singlet oxygen

n

1(n,

Singlet State(S1,S2, ......)

Triplet State(T1, T2, ...)

ISC

Biological ResponsePhotochem.

Ground StateSoJablonski energy

diagram

Jablonski energy diagram


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).

Photochemical reactions with singlet Oxygen

1O2

1Sens (S0) 1Sens* (S1)hv

1Sens* (S1) 3Sens* (T1)3Sens* (T1) 1Sens (S0) + 1O2

+3O2

Photooxygenation Reaction


( 1O2)

1+g

-g

3

1g

22.4

37.5 Kcal/mol

Kcal/mol

Highest occupied molecular orbital of 1 O2


N

N

N

N

H

H

C6H5

C6H5

C6H5

C6H5

Tetraphenylporphyrine (TPP)

N

N

N

N

H3C CH CH3

OH

CH3

CH CH3OH

HOOCH2C-H2C

HOOC-H2C-H2C CH3

H

H

H3C

Hematoporphyrine( HP)

O

ClClCl

ClI

OI

ONa

I

I

COONa

Ros Bengal(RB)


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

3)

2)

1)

H

X

+

+

+

1O2

O2

1O2

1

A

B

C

OOH

XOO

O O


O2*

C

C C

H

C

C C

O OH

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

1- Ene Reaction


C C

CH

+ 1O2 C C

OOH

C


2-Cycloaddition Reaction (Diels Alder)


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


Photosensitized oxidation

OCH3H3C

+ O2hv , sens

OCH3H3C

OO

C C

CH3

CH3

H3C

H3C

+ O2hv , sens

C C

CH2

CH3

H3C

H3COOH

+ O2hv , sens C2H5O-CH-CH-OC2H5

O O

C2H5O-CH=CH-OC2H5


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


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.

Norish Type I Processes of Ketones Basic Concepts

R

OC O

C h

+

O O O

O OO

O

OMe

O

O

2 X 106 3 X 107 1 X 108

2 X 108 2 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 halfvaccant 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


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

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

RR'

O

RR'

1O*

RR'

1O*

R

R'OH

n

R

OH R'

RR'

OR

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

3KH


RCHO + hv RH + CO

C=O + hv

2C2H4 + CO

+ CO

CH2=CHCH2CH2CHO


H2C

O

H2C hv

Ohv

Complete the next equations


H3CCH2

CH3

O

hv

H3C

CH3

CH3

CH3

O

hv


2- Esters:

RCH2CH2CH2COOR\hv

RCH=CH2 + CH3COOR\

hv

RCOOCH2CH2R\ RCOOH + CH2=CHR\


Photocycloaddition

2+2 Intermolecular cycloaddition

R

R\

+

O

H3CO

OCH3

O

hvH3CO

O

R

R\

OCH3

O

O


O

hv

O O

+

O

O

2


hv

2+2 Intramolecular cycloaddition


+

2+4 Cycloaddition


hv +

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

-

-

O

OMe

OMe

O

O

nBu

OAc

nBu

O

OAc

nBu

nBu

O

OEtEtO

CO2EtO

CO2Et

OEt

OEt

OOEt

OEt

CO2Et

O

SiMe3

OSiMe3

OSiMe3

O

OAc

OO OO

OAc

O

O OOAc

O

O

OAc

O

OAc

O O O98%

+

+

only

+

82.5 17.5

+1 1

+

95 5

96%

81 19

O

OH

H

OH

H

OH

H

OH

H

always cis

always cis

O O

OH OH

O

H

O

H

O O

O

CuOTf, h

exo pdt

The observed selectivity is assumed to arise froma preferential formation of the less sterically crowdedcopper (I)-diene complex, leading to exo pdt.

NaIO4/RuO4

CuOTf, h

CuOTf, h

R

O

H

R

OH

CH3

R

O

R

OH

CO2Me

CO2Me

CO2Me

CO2Me

R OH RCO2Me

CO2Me

H-Transfer

spin-inversion

+

Photoenolization

O

Ph

.

C OH

Ph

Me

.

C

Me

OH

OH

PhOH

Ph

OHPhO

Ph

h

O OH O

O

O O

OMe

OMe

MeO

O OCO2Et O

O OH

OMe

OMe

MeO

O OCO2Et

O

O

OMe

OMe

MeO

O O

OHCO2Et

h

Norish II, Cleavage

(-)Ephidrine

EnantioselectiveH-transfer

h

Photoenolization

4+2

Podophyllotoxin derivative

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

hv

Oxa-Di-π-Methane rearrangementA 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

O

hv

O

Mechanism I


Photoaddition and photocyclization reactions

+

NH2

hv

HN

+

HN

+


Direct and photosensitized reactions

trans

cis

direct

sensitized


Isomerization and rearrangements

N NR

RN N

R Rh

R = Me R = CHMe

R = R =

R = R =

NNN N

N N N N

C C

h

h (405nm)

h(436nm)/heat

h (313nm)-N2 h (313nm)

-N2

A

B

D

E

A

B

E

D

Cis-Trans isomerization of alkenes

3S**3

h

tripletdonor

h

h

sens

h

sens

H

H

h

185 nm

sens

heath

h


direct

Tripletsensitized


hv

HH

hv+ +

Benzvalene bicyclo-hexadiene

fulvene


CN

C6H5C6H5

C6H5 CNC6H5

hv

Photochemical synthesis of oxetans

Paternò-Büchi Reaction

O

O

O

EtO

OEt

CO2HO N

N

OOH

OH

N

N

NH2

O

O

NH2

NH

NH2

O

OO

OO

O

OAc

OR

HOBz

OOAc OH

+

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

Insecticidal activity

Thromboxane A2 Oxetanocine

Bradyoxetin

Merrilactone APalitaxel

CHO

C O

H

O

C C

O

C C

O O

Reaction mechanism

h[PhCHO] S1

ISC[PhCHO] T1

(n-*)

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

+

electrophile nucleophile

+

Major Minor

Biradical intermediate

O O O

O

Me CCl3 Me CCl3

O O

O

Me Me

F

O

F

O

F

O

Me Me

Cl

O

Cl

O

Cl

Enones and Ynones

+ +

42% 47%

+ +Low T

3% oxetane

+ +

10% 9 0%

+ +

90% 10%

O

PhPh SiMe3

O

SiMe3

Ph

Ph

O

Ph

Ph

SiMe3

O

PhPh OTMS

O

OTMS

Ph

Ph

O

Ph

Ph

OTMS

O

PhPhH SMe

HO

H

Ph

Ph

SMeO

H

Ph

Ph

SMe

+h

+

24 1

+h +

94 6

+h

+

100 0

R1R2

R3 R4O

R

R4

R3

R1

R2

OXR4

R3

CHRYR1

R2

O

PhR

OTMS OHR OTMS

Ph OHR OH

Ph

O

OPh O

OH

Ph

+ R CHOh XY

Carboxydroxylation strategy by reductive cleavage of oxetanes

H2

H2

N

OH

Ph

H

O

Ph N

PG

N

PG

O

Ph N

PG

OH

Ph

N

CO2Me

RN

CO2Me

R

O

H

H

Ph N R

OH

Me

Ph

N

CO2Me

N

CO2Me

O

Ph N

OH

Ph

Total synthesis of (+)-Preussin

+

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

+orthopara

meta

1

2

3

45

61

2

1

4

1

3

Possible modes of addition in the arene-alkene photocycloaddition reactions

R

R

HH

R

+

endo exciplex

h


Photo Fries rearrangement


• a Fries Rearrangement is photochemical excitation

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.

HH

HO

ergosterol

sunlight

photochemically-promoted electrocyclisation(antarafacial, conrotation)

H

HOprovitamin D2

H

HO

H

[1,7]-sigma-tropic shift.

vitamin D2

Dr. Suzan A. Khayyat

NH

N

O

O

R

R'

N

N

NH2

O

R

N

N N

N

NH2

R

NH

N N

N

R

O

NH2

DNA photochemistry

Ura R ' = H R = HUrd R ' = H R = riboseUMP R ' = H R = ribose phosphate

Thy R ' = Me R = HThd R ' = Me R = deoxyriboseTMP R ' = Me R = deoxyribose phosphate

Cyt R = HCyd R = riboseCMP R = ribose phosphate

PYRIMIDINES

Ade R = HAdo R = riboseAMP R = ribose phosphate

Gua R = HGuo R = riboseGMP R = ribose phosphate

PURINES

260 nm ( *)270 nm ( *)

N

NH

N

N

O

OH

NH2

O

O P

O

O

O

O

OH

O

O

N

NH

N

N

O

OH

NH2

O

O P

O

O

O

O

OH

O

O

H

H H

O

N

NH

N

N

O

OH

NH2

O

O P

O

O

OO

OHO

N

NH

N

NH

O

OH

O

O P

O

O

O

O

OH

O

O

H

H H

O

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

N

N

N

NH

O

OH

O

O P

O

O

O

O

OH

O

N

N

NH2

O

N

NH

O

OH

O

O P

O

O

O

O

N

NNH2

NN

OH

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 OO

OH

N

N

N

N

NN

NH2

NH2

h

Cycloadditions involving adenine; Cyclobutane and azetidine dimer formation

h


Photochemistry in solution

(CH3)H2C C

O

H2C (CH3) CO + C3H8

liq+ H3C CHCHO

gas

(CH3)2H2C

OC

OC

H2C (CH3)2


Photodimerization

hv

in open air ,CHCl3

Scheme 1

1

4

CHO CHO

OHC

O

O

OO

OHC

CHO12

3

451\\

2\\3\\

4\\

5\\ 6\\

1\

2\ 3\

4\

5\6\


hv

in open air ,CHCl3

Scheme 2

2

5

H3CO

HO

H3CO

HOOCH3

OH

H3CO

HO OCH3

OH1\

2\

3\

4\

5\6\

32

14


O

O

hv

in open air ,CHCl3

O

O

Scheme 3

3

O

O

O

6

O

OO

1 2

3

45

6 1\

2\

3\ 4\

5\6\

O

OO O

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.

H

H

O

H

ONO

H

H

O

H

O

C H

H

O

H

O

H

H

O

H

NOHO

h

Chemical Change

Documents

Transcript of Chemical Change