Pi-Pi star transition.pptx

Photochemistry of alkenes

1. Photochemistry of alkenes 12. 1 The main  ,  * absoption of simple alkenes is located at  < 200 nm, thus limiting the usefulness of most conventional light sources. h   ,  * lowest state • h   ,  Rydberg state * carbene leading rearrangements cis-trans isomerizations

2. Electronic absorption spectra of alkene consist of intense broad band from 140 nm to 190 nm for ethylene, with absorption threshold of 200 nm for ethylene to 240 nm for 2,3 dimethyl -2- butene. The diffuse bands in the spectrum of ethylene are attributed to pi-pi* transitions. The absorption at 174 nm is first singlet Rydberg transition. This Rydberg transition is due to the 2p 3s excitation The excited state of alkene are complex because different electronic states have different electronic configuration. Although the nature of most electronic states of alkenes is not known in detail, some are well understood. Alkenes have two low lying excited singlet states: 1 [2p , 3s] Rydberg state and the 1 [ , *] valence state. Calculation indicates that there is apparently an additional state i.e. the [2p , 3p] Rydberg state. The lowest triplet state is practically pure 3 [ , *] and T2 is essentially a pure Rydberg 3 [2p , 3s] state  

3. A Rydberg state is a state of an atom or molecule in which one of the electrons has been excited to a high principal quantum number orbital. Classically, such a state corresponds to putting one electron into an orbit whose dimensions are very large compared to the size of the leftover ion core. Among the novel properties of these states are extreme sensitivity to external influences such as fields and collisions, extreme reactivity, and huge probabilities for interacting with microwave radiation

4. Spectroscopy The two lowest absorptions of alkenes involve  ,  * and  ,3s (Rydberg) states For ethylene the two transtions are very close and around 170 kcal/mol For substituted alkenes the Rydberg transition tends to be at lower energies. For example for cis -2-butene the energies based on the absorption spectra are: 12. 4 singlet  ,  * singlet (Rydberg) triplet  ,  * 164 kcal mol 141 kcal/mol 97 kcal mol The lowest triplet state has  ,  * character and energies around 94-99 kcal/mol

5. S3 ( π , 3P) S2( π , π *) 180 Kcal/mol T2 ( π , 3S) S1( π , 3S) 175 Kcal/mol 150 Kcal/mol T1 ( π , π *) 164 Kcal/mol 101 Kcal/mol S0 Different Electronic States of Ethylene

6. For Ethylene, Singlet-Triplet splitting is large (70 Kcal/mol). As a result, ISC is slow and inefficient Therefore, sensitization is required for triplet state reaction Excitation of Planar alkene like ethylene results initially in the formation of planar excited state molecule according to Frank Condon Principle. The initially formed planar excited state π, π * species, wherever as a singlet or a triplet relaxes by rotation of the terminal group-methylene in case of ethylene- through 90° around the central bond to give the lowest energy conformation possible.

7. Geometry changes Biradical –almost tetrahedral Excimer like interaction between two rings.

8. Thus the molecule can rotate from the planar configuration., produced by the frank condon excitation to reach an energy minimum. This energy minimum is arrived by rotation about the central bond so that the methylene groups are at right angle to each other so relieving the unfavourable interactions between the singly filled orbitals on the carbon atoms.

9. . C C h ν C C (Frank Condon planar state) rotation C C ( Relaxed Twisted State) . . . . Formation of Relaxed Twisted State ( Orthogonal State)

10. Spectroscopic evidence shows that there is additional distortion of the methylene groups from Planarity as the carbon atoms tend towards tetrahedral (SP 3 ) hybridization/ In this energy minimum there is essentially no Pi bond, the electronic interaction is at minimum and the terminal carbons are orthogonal From the excited state of olefin, several mode of reactions can be predicted. These are cis-trans isomerization, hydrogen abstraction by the half-filled orbital on carbon, addition to other alkenes and rearrangement.

11. The cis-trans (Z-E) isomerization 12. 11 S1-trans So-trans S1-cis So-cis S1-p sta te T1-cis T1-trans T1-p state R H H R h  H R H R Stilbenes

12. This phenomenon can occur by both direct and sensitized irradiation. This isomerization is usually associated with π , π * excited states. By Direct Irradiation : Below 200nm is difficult to achieve as a result of high energy adsorptions. However, with more substituted alkenes the UV absorptions are pushed above 200nm and so direct excitation can be more readily achieved. The cis-trans (Z-E) isomerization

13. Example of effect of geometry change in excited state :Isomerisation of stilbenes Ph-CH=CH-Ph trans cis

14. Isomerisation of Stilbenes The irradiation of cis/trans stilbene at 333nm results in formation of 93& cis and 7% trans stilbenes. Ratio of the product doesn’t vary no matter how long the irradiation is continued, and such a state is photo stationary state. The composition of the photo stationary state is determined by the extinction coefficient of the two isomers at the exciting wavelength. For most of the alkenes ( Є max ) trans is greater than ( Є max ) Cis . Therefore, in an equilibrium mixture more molecules of trans than cis isomer will reach the excited singlet state, followed by rapid relaxation to S At photo stationary state, the rate of cis trans and trans cis isomerization become equal; no change in the composition of the reaction mixture occurs upon further irradiation

15. Control of the photostationary state 12. 15 The photostationary state is a wavelength dependent property = [t] [c] s  c  ct  t  tc Control of the photostationary ratio excitation ratio decay ratio

16. The energy of π π * excited species is a function of the angle twist about the carbon-carbon ( σ ) sigma bond and trans to cis isomerization is believed to be affected by the distortion of the trans excited state initially produced to an excited state common to both cis and trans isomers. The excited state is termed as a phantom state . The mechanism by which the cis- trans isomerization occurs, involves the excitation of an electron to a planar excited state which subsequently relaxes to the twisted state. A small activation energy of 2Kcal/mol is required to twist the planar trans singlet to the twisted singlet state. Decay from this twisted state can give either cis or trans stilbene.

17. . . H Ph Ph H . . Ph H Ph H . . H Ph Ph H Common Intermediate Ph-CH=CH-Ph Ph-CH=CH-Ph Cis Stilbene trans Stilbene

18. However, as mentioned earlier the trans form absorbs more light at the exciting wavelength ( Є max trans= 16300) than does the cis form ( Є max cis= 2800) and consequently the trans isomer is converted almost completely into cis. The synthetic utility value of trans-cis isomerization lies in the fact that the more stable alkene can be readily converted into less stable alkene.

19. Examples of Direct Irradiation C 6 H 5 H C 6 H 5 CH 3 C C h ν C C H CH 3 H H CH 3 C 6 H 5 H CH 3 C 6 H 5 COOH C C h ν C C H COOH H H

20. Examples of Direct Irradiation C 6 H 5 CH 2 CH 2 H C 6 H 5 CH 2 CH 2 CH 3 C C h ν C C H CH 3 H H Cl CH 2 Cl Cl Cl C C h ν C C H Cl H CH 2 Cl

21. Ketones can promote cis-trans isomerization by two different mechanisms Energy transfer from a triplet ketone (why not a singlet?) can lead to triplet alkene that then follows the normal isomerization pathway, independent of the ketone sensitizer The sensitised isomerization of stilbene is a typical example. The composition of the photo stationary state of the sensitized isomerization varies with the energy of the sensitizer employed. In this process if the donor species fulfils the condition for effective sensitization and its triplet energy is greater than that of either of the geometrical isomers, then triplet energy transfer to both cis and trans isomers and isomerization takes place. The initially formed cis & trans triplet excited state undergo distortion to a common phantom triplet, which on collapsing to the ground state affords a mixture of isomers. S ENSITIZATION

22. As the sensitizer can excite both isomers, the proportion of cis-trans isomer in the photostationary state from a sensitized reaction is lower than that obtained from direct irradiation. Sensitisers of high energy give photostationary state with approximately the same composition of isomers (55% cis) while direct photolysis results in higher proportion of cis isomer (93% cis) in case of stilbenes. As the energy of sensitizer is reduced, anomalous results are observed Sens (S0) Sens (S1) ISC Sens (T1)

23. Sens (T1) + R R R R + Sens (S0) R R T1 . . H R R H

24. The amount of photostationary state depends very much on the triplet energy sensitizer. With sensitizer having triplet energy above 60Kcal/mol, cis-trans ratio is slightly more than one but a range of sensitizer having triplet energy of 52-58 Kcal/mol offered much higher cis-trans ratio in the photostationary state. The higher cis trans ratio in this region results from the fact that energy required for excitation of trans isomer is less than that for the excitation of cis isomer The sensitiser having triplet energies in the range of 52-58 Kcal/mol, selectively excited the trans isomer. Since the rate of trans-cis conversion is increased, the composition of photostationary state is enriched in cis isomer

25. Photochemical cis-trans isomerization may also take place in the presence of halogens: X 2 h ν X° + X° R R R R R H C C + X° X C C ° + X C C ° H R H H H R Cis form R R R H C C + C C H H H R

26. Dimerisation of Alkenes Reaction that compete with cis-trans isomerization at increased concentration of the alkenes are photocycloaddtion reactions. An alkene in its S1 or T1 state reacts with either the same (photodimerization) or a different (photocycloaddition) alkene in the ground state to produce a cyclobutene derivative. It is difficult for simple alkene to dimerise ( λ max is equal to or less than 200nm) is technically difficult. Increased alkyl substitution of ethene causes a shift to higher λ and thus dimerisation by direct irradiation

27. The dimerization reaction is stereospecific, cis-2-butene when irradiated yields all cis dimer and cis-anti-cis dimer while trans-2-butene gives cis-anti-cis and trans-anti-trans dimer λ 214nm + Cis-2-butene All Cis dimer 57% Cis-anti-Cis 43% 2 hν 2

28. 2 λ 214 nm + trans-2-butene Cis-anti-Cis 39% Trans-anti-Trans 60% Cyclohexene also dimerise giving mixture of dimeric product, norborene dimerise to give two dimers (A) & (B) in ratio 1 : >10 2 (A) + (B)

29. With cycloalkenes photosensitized dimerization occurs easily, particularly with small ring alkenes because they cannot undergo cis-trans isomerization. Aliphatic ketones are commonly used as sensitisers for the dimerization of alkenes and cycloalkenes. h ν H H 2 acetone H H cis-trans-cis h ν 2 acetone + +

30. Norborene can also be made to dimerise on sensitization and that is preferred for the formation of exo -trans-endo isomer h ν H H 2 acetone In case of Indene, four dimers that differ in regio and stereochemistry are formed by photodimerization. The ratio of dimers depends on the irradiation conditions. Triplet sensitized dimerization produces more stable exo product from biradical intermediate (Head to head)

31. h ν H H acetone H H +

32. Intramolecular Dimerisation Non conjugated dienes with isolated double bond give intramolecular [2+2] cycloaddition reactions to give bicyclic products. The product depends upon the number of CH 2 groups between the two double bonds. If the number of intervening groups is odd, the major product is from normal [2+2] cycloaddition, otherwise two crossed sigma bonds are formed CH=CH 2 h ν CH-CH 2 (CH 2 )n sense (CH 2 )n CH=CH 2 CH-CH 2 n= 1,3,5…. Normal [2+2] Adduct

33. Intramolecular Dimerisation In case n is even CH=CH 2 h ν CH-CH 2 ° (CH 2 )n sense (CH 2 )n CH=CH 2 CH°-CH 2 n= 2,4,6…. CH-CH 2 (CH 2 )n CH-CH 2 x[2+2] Adduct

34. This feature is more prominent in cyclic dienes n=m=2 (1, 5 cyclooctadiene) in which x[2+2]adduct is the only product 1 2 ° 1 2 8 3 3 h ν acetone 7 4 4 6 5 ° 6 5 8 1 2 7 3 6 5 4

35. ° ` hν acetone ° Here n=1 and m=3 or vice versa, thus both odd and therefore normal [2+2[ adduct is formed The photoreaction of 1,5-hexadiene from T1 state follows the rule of Five which states that if triplet cyclisation can lead to rings of different sizes, the one formed by 1, 5 addition is preferred kinetically. If there are several possibilities for 1,5 addition leading to different biradicals, the most stable one is formed

36. 5 ° 3 hν acetone 2 ° 1 minor behaves as 1,6 dienes ° h ν acetone ° behaves as 1,5 dienes Product as per rule of Five (major)

37. Cage structures from intramolecular [2+2] cycloaddition are the molecules of theoretical interest with respect to strain and bond angles h ν 254nm Norborandiene is an example of such reaction. UV spectrum of the compound exhibits absorption at 205 nm, 214nm, and a shoulder at 230 in ethanol. Such a spectrum is not expected for non-conjugated diene. Molecular model also indicates that bonding is feasible between C2 & C6 and between C3 & C5 without too much of strain. Compound can undergo [2+2] cycloaddition both by direct irradiation as well as sensitizer.

38. hν C 6 H 5 COCH 3 ° ° Spin Inversion The [2+2] cycloaddition reaction of norbornadiene in the presence of sensitizer is a reversible reaction.The introduction of gp other than CH 2 at C7 does not change the relationship between isolated double bonds. Whereas, in such compounds the reaction is not reversible by irradiation or sensitisation

39. O O COOH COOH h ν COOH COOH COOMe COOMe h ν COOMe COOMe

40. Photochemistry of Conjugated dienes Can give either type of photochemical reaction: Cis-trans isomerization Sigmatropic Shift Disrotatory Electrocyclic ring closure Intramolecular x[2+2] cycloaddition Depending upon the excited state population, and on the phase in which reaction is performed. Singlet excitation generally leads to intramolecular process ( Disrotatory Electrocyclic ring closure, Sigmatropic rearrangement), whereas dimerization and addition reaction are more common from triplet excited state. Reactions in the gas phase at low pressure often leads to greater fragmentation than in solution.

41. Solution phase photolysis of butadiene leads to cyclobutene and bicyclobutane, whereas vapour phase gives, 1- butyne, methylallene , acetylene, ethylene, methane, hydrogen and polymeric materials Cyclohexane + hν 10 : 1 Vapour Phase CH 3 -CH 2 -C = CH + CH 3 -CH=C=CH 2 + C 2 H 2 + C 2 H 4 + CH 4 +H 2 + +

42. Photochemistry of Conjugated dienes in Solution Intramolecular x[2+2] cycloaddition 1,3-butadiene in solution exist as rapidly equilibrating mixture of S- transoid (95%) and S- cisoid (5%) conformers. ° h ν ° S-trans Trans ° ° h ν S-cis Cis

43. S1 107-124 Kcal/mol S1 T1 T1 59.6 Kcal/mol 53.3Kcal/mol S0 S0

44. 1 ° ° Electrocyclic Ring Closure

45. Bicyclobutane formation is particularly important in systems that contain a rigid S-trans diene geometry. ° ° However x[2+2] cycloaddition also occurs with cis fused dienes particularly in case of cyclopentadiene hν

46. ° h ν Cyclohexane °

47. Some dimer formation (10%) can occur if conc. Solution of 1,3-butadiene is irradiated Dimer formation is Conc. Dependent as reaction is bimolecular of E.S butadiene with G.S butadiene. Formation of Cyclobutene and bicyclobutane (ring closure) are on the other hand are unimolecular. Thus dimerization is possible with high conc. Of butadiene 1 [CH 2 =CH-CH=CH 2 ] + CH 2 =CH-CH=CH 2 + S1 Excited State S0 Ground State ( i ) (ii) (30%) (50%) + (8%)

48. In the presence of sensitizer, reaction occurs from triplet state, and is quite different from that of the S1 state A mixture of dimer is formed in good yield (approx. 75%) and neither cyclobutene, bicyclobutane can be detected in sensitized reaction. The dimer composition in triplet state depends upon the energy of sensitizer used to populate T1. Sensitiser E T ( Kcal/mol) C 6 H 5 COCH 3 74 82% 14% 4% C 6 H 5 COC 6 H 5 57 49% 8% 43%

49. The remarkable change in product composition is due to relative energies of sensitisers . Acetophenone with E T (74 Kcal/mol) is sufficiently energetic to transfer energy either to s-cis or s-trans butadiene. Due to predominant amount of s-trans being there, energy gets transfer at almost every collision, the dimer composition from the acetophenone sensitized experiment must represent primarily the reaction of s-trans-T1 Benzil with E T (54 Kcal/mol) is energetic enough to transfer energy to s-cis butadiene but not s-trans butadiene. Thus the dimer composition from benzil sensitized experiment represent primarily the reaction of s-cis-T1 If a sensitizer with a triplet energy of less than 54 Kcal/mol were used no dimer formation is observed.

50. ° ° Cis & trans 3 ° ° Thus when acetophenone is the sensitizer, s-trans-T1 is the reacting species, which reacts with ground state s-trans to give dimer.

51. In benzil sensitized reaction s-cis-T1 adds to the s-trans ground state butadiene ° 3 ° ° ° ° ° ° °

52. Benzene and substituted benzene undergo valence isomerization by irradiation. Selective excitation into S1 gives preferentially meta and ortho product while excitation into S2 gives para bonded products by valence isomerization. These processes are described by biradical intermediate for case of visualisation Photo- isomerisation of Benzene and Substituted Benzene

53. S2 S1 1 * 1 * ° ° ° ° ° ° ° ° ° °

54. Irradiation of liquid benzene under nitrogen at 254 nm causes excitation to S1 state and the products benzvalene, fulvene are formed via 1,3-biradical H ° H ° ° ° S1 Biradical (a) 1,2-H shift H H H H Bond Breaking Fulvene °

55. Formation of benzvalene ° 1,3 bonding ° (A) Benzvalene Dewar Benzene is formed via S2 state upon short wavelength irradiation ° S2 ° Dewar Benzene

56. Dewar benzene gets converted into prismane either by concerted path or by formation of biradical [2+2] Cycloaddition h ν Prismane ° ° All these strained intermediates are thermally labile and ultimately isomerises into benzenoid compounds

57. Monocyclic aromatic compounds undergo remarkable photochemical rearrangements. E.g. o-xylene on irradiation gives mixture of o, m, p-xylene CH 3 CH 3 CH 3 h ν 1,2 alkyl shift CH 3 h ν CH 3 h ν 1,2 Alkyl Shift 1,3 Alkyl Shift CH 3 Conversion of o-xylene to m-xylene and m-xylene to p-xylene is due to 1,2-alkyl shift. Similarly conversion of o-xylene to p-xylene and vice-versa is due to the 1,3-alkyl shift. 1,2 alkyl shift takes place by benzvalene as well as prismane intermediate whereas 1,3-alkyl shift takes place only by prismane intermediate

58. 1,2-Alkyl Shift Via benzvalene and prismane as reaction intermediate Mechanism of 1,2-Alkyl shift via benzvalene intermediate R R R Bond Formation Between R C1-C3 & C2-C4 Benzvalene R R 1 3 1 2 R 2 R 3 Reorganization of carbons of the ring by ( i ) bond breaking between c1-c2 and c3-c4 and (ii) bond formation between c1-c3 and c2-c4

59. 1,2-Alkyl Shift Via benzvalene and prismane as reaction intermediate (b) Mechanism of 1,2-Alkyl shift via Prismane intermediate R R R R R R [2+2] Cycloaddition R 1 R 2 4 3 Reorganization of carbons of the ring by ( i ) bond breaking between c1-c2 and c3-c4 and (ii) bond formation between c2-c3 and c1-c4

60. 1,3-Alkyl Shift Via prismane as reaction intermediate [2+2] Cycloaddition Reorganization of carbons of the ring by ( i ) bond breaking between c5-c6 and c3-c4 and (ii) bond formation between c4-c5 and c3-c6

61. The photo isomerization of 1,3,5 trimethyl benzene to 1,2,4-trimethyl benzene is due to 1,2 alkyl shift and has been confirmed by isotopic labelling experiment ° hν ° ° ° ° °

62. Similarly 1,3,5 tri-t-butyl benzene and 1,2,4-trit-butyl benzene are interconvertible to each other by 1,2 alkyl shift and this interconversion takes place via benzvalene intermediate hν °