chemical change chapter 2 dr. suzan a. khayyat1. chemical reactions photochemical reaction...
TRANSCRIPT
Dr. Suzan A. Khayyat 1
Chemical Change
Chapter 2
Dr. Suzan A. Khayyat 2
Chemical reactions
Photochemical Reaction
Photooxidation Reaction
Photoaddition Reaction
Photohydrogenation
Pericyclic Reaction
Photodissociation
Thermal chemical Reaction
types of chemical reaction
Dr. Suzan A. Khayyat 3
• 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. Khayyat 4
Absorption
Fluorescence
Phosphorescence
photochem. & singlet oxygen
n
1(n,
Singlet State(S1,S2, ......)
Triplet State(T1, T2, ...)
ISC
Biological ResponsePhotochem.
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 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
Dr. Suzan A. Khayyat 9
( 1O2)
1+g
-g
3
1g
22.4
37.5 Kcal/mol
Kcal/mol
Highest occupied molecular orbital of 1
O2
Dr. Suzan A. Khayyat 10
N
N
N
N
H
H
C6H5
C6H5
C6H5
C6H5
Tetraphenylporphyrine (TPP)
N
N
N
N
H3C CH CH3
OH
CH3
CH CH3
OHHOOCH2C-H2C
HOOC-H2C-H2C CH3
H
H
H3C
Hematoporphyrine( HP)
O
ClClCl
ClI
OI
ONa
I
I
COONa
Ros Bengal(RB)
Dr. Suzan A. Khayyat 12
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.
Dr. Suzan A. Khayyat 15
Types of singlet oxygen reactions
3)
2)
1)
H
X
+
+
+
1O2
O2
1O2
1
A
B
C
OOH
X
OO
O O
Dr. Suzan A. Khayyat 16
O2*
C
C C
H
C
C C
O OH
Cis cyclic mechanism for the reaction of 1O2 with mono-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
+ O2
hv , 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
Dr. Suzan A. Khayyat 28
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
Dr. Suzan A. Khayyat 29
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 1071 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
Dr. Suzan A. Khayyat 32
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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
• 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'
O
RR'
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
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RCHO + hv RH + CO
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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H3CCH2
CH3
O
hv
H3C
CH3
CH3
CH3
O
hv
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2- Esters:
RCH2CH2CH2COOR\hv
RCH=CH2 + CH3COOR\
hv
RCOOCH2CH2R\ RCOOH + CH2=CHR\
Dr. Suzan A. Khayyat 43
Photocycloaddition
2+2 Intermolecular cycloaddition
R
R\
+
O
H3CO
OCH3
O
hvH3CO
O
R
R\
OCH3
O
O
Dr. Suzan A. Khayyat 44
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O
hv
O O
+
O
O
2
Dr. Suzan A. Khayyat 46
hv
2+2 Intramolecular cycloaddition
Dr. Suzan A. Khayyat 47
+
2+4 Cycloaddition
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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
CO2Et
O
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
Dr. Suzan A. Khayyat 53
R
O
H
R
OH
CH3
R
O
R
OH
CO2Me
CO2Me
CO2Me
CO2Me
R OHR
CO2Me
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 O
CO2Et O
O OH
OMe
OMe
MeO
O O
CO2Et
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
Dr. Suzan A. Khayyat 61
Photoaddition and photocyclization reactions
+
NH2
hv
HN
+
HN
+
Dr. Suzan A. Khayyat 62
Direct and photosensitized reactions
trans
cis
direct
sensitized
Dr. Suzan A. Khayyat 63
Isomerization and rearrangements
N NR
RN N
R R
h
R = Me R = CHMe
R = R =
R = R =
NN
N N
N NN 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
Dr. Suzan A. Khayyat 72
direct
Tripletsensitized
Dr. Suzan A. Khayyat 73
hv
HH
hv+ +
Benzvalene bicyclo-hexadiene
fulvene
Dr. Suzan A. Khayyat 74
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
O
O
OO
O
OAc
OR
HOBz
OOAc OH
+
Paterno and Chieffi (1909), Buchi in 1954 mechanistic analysis
Insecticidal activity
Thromboxane A2 Oxetanocine
Bradyoxetin
Merrilactone A
Palitaxel
CHO
C O
H
O
C C
O
C C
OO
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
OO
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
R
N
CO2Me
R
O
H
H
Ph NR
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
Dr. Suzan A. Khayyat 84
Photo Fries rearrangement
Dr. Suzan A. Khayyat 85
• 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
HH
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
HH
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 O
O
OH
N
N
N
N
NN
NH2
NH2
h
Cycloadditions involving adenine; Cyclobutane and azetidine dimer formation
h
Dr. Suzan A. Khayyat 90
Photochemistry in solution
(CH3)H2C C
O
H2C (CH3) CO + C3H8
liq+ H3C CHCHO
gas
(CH3)2
H2C
OC
OC
H2C (CH3)2
Dr. Suzan A. Khayyat 91
Photodimerization
hv
in open air ,CHCl3
Scheme 1
1
4
CHO CHO
OHC
O
O
OO
OHC
CHO1
2
3
451\\
2\\3\\
4\\
5\\ 6\\
1\
2\ 3\
4\
5\6\
Dr. Suzan A. Khayyat 92
hv
in open air ,CHCl3
Scheme 2
2
5
H3CO
HO
H3CO
HO
OCH3
OH
H3CO
HO OCH3
OH1\
2\
3\
4\
5\
6\
32
14
Dr. Suzan A. Khayyat 93
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