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2 [2 + 2]-Photocycloaddition of Nonconjugated Alkenes

2 [2 + 2]-Photocycloaddition of Nonconjugated Alkenes

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j 5 Formation of a Four Membered Ring



138











(1)



Conversion

80%

1

MeO



MeO



OMe



70%



5%



2



3



OMe







(2)



Acetone

95%



Pentaprismane



5



4

EtO2C

MeO



N



O



6



7



O



O



hν (λ>290nm)

+



N



hν or

EtO2C



OMe

8a



+



N



EtO2C



OMe

8b



O

N

EtO2C



(3)



OMe



84%

9

Scheme 5.1 Intramolecular [2 ỵ 2] photocycloaddition.



alkene functions are arranged close to each other, produced in good yields the

cyclobutane derivative 2 (Scheme 5.1, reaction 1) [11]. The reaction was carried out

with an irradiation at l ¼ 254 nm in cyclohexane as solvent. These conditions indicate

that the reaction took place at the first excited singlet state (S1), and the cage

compound 3 was obtained as a byproduct. The same type of [2 þ 2] cycloaddition

can also be carried out under photosensitization; thus, compound 5 was obtained in

high yields from transformation of the diene 4 (Scheme 5.1, reaction 2) [12], the

reaction being carried out in acetone as solvent. Under these conditions, acetone acts

as triplet sensitizer. The transformation was also performed as part of the tentative

synthesis of hexaprismane and similar compounds [13]. The [2 þ 2] photocycloaddi

tion is frequently observed as a consecutive reaction; for instance, in the transfor

mation of the pyridine derivative 6 with furan 7, a [4 ỵ 4] cycloaddition rst takes

place leading to the adducts 8a,b (Scheme 5.1, reaction 3) [14]. This step is reversible

and the primary products are not stable enough to be isolated in pure form. However,

two alkene functions are well orientated to react readily in a [2 ỵ 2] photocycloaddi

tion, leading to a high overall yield of the final cage hydrocarbon 9. These reactions are

also frequently observed with condensed aromatic compounds, such as naphthalene

derivatives [15].



5.2 [2 ỵ 2] Photocycloaddition of Nonconjugated Alkenes



A [2 ỵ 2] photocycloaddition with two alkenes can also be induced by photochem

ical electron transfer [16, 17]. In such cases, sensitizers are frequently used and the

reactions therefore occur under photocatalysis [18]. Under photochemical electron

transfer (PET) conditions, the diene 10 yielded in an intramolecular reaction the

cyclobutane 11 (Scheme 5.2) [19], such that in this reaction a 12 membered cyclic

polyether is built up. The reaction starts with excitation of the sensitizer 1,4

dicyanonaphthalene (DCN); only 0.1 equivalents of the sensitizer are added to the

reaction mixture. Electron transfer occurs from the substrate 10 to the excited

sensitizer, leading to the radical cation I. This intermediate then undergoes cycli

zation to the radical cation of the cyclobutane (II). Electron transfer from the radical

anion of the sensitizer to the intermediate II leads to the final product 11, and

regenerates the sensitizer. In some cases, for example the cyclodimerization of

N vinylcarbazole, the efficiency is particularly high because a chain mechanism

is involved [20].

Copper catalyzed [2 ỵ 2] photocycloadditions are related to the latter reactions.

These transformations have been extensively studied, frequently in the context of

application to organic synthesis [21]. When irradiated in the presence of copper(I)

triflate, norbornene 12 was efficiently transformed into its dimer 13 (Scheme 5.3,

reaction 4) [22]. Although complexes such as III are involved in the reaction

mechanism [22, 23], it is unclear whether MLCT (metal to ligand charge transfer)

or LMCT (ligand to metal charge transfer) excitation induces the transformation.



O



O



O



O



h, DCN

80%



10



O



O



O



O



11

CN



DCN:



O



O



O



10



DCN *



CN



O



h



DCN



O



DCN



O



O



O



O



O



O



O



O



I



O

II



O



O



11



Scheme 5.2 [2 ỵ 2] Photocycloaddition induced by photochemical electron transfer.



j139



j 5 Formation of a Four Membered Ring



140



h, CuOTf



(4)



88%

12



13



Cu+

III

h, CuOTf



(5)



48%

14



15



h, acetone



(6)



48%

14



16



Scheme 5.3 Intermolecular [2 ỵ 2] photocycloadditions catalyzed by CuOTf.



Interestingly, when using copper(I)triflate, the cyclopentadiene dimer 14 reacts in an

intermolecular way, leading to the cyclobutane 15 (reaction 5) [22]. When the same

substrate is transformed in the presence of the triplet sensitizer acetone, an

intramolecular [2 ỵ 2] cycloaddition takes place and the cage hydrocarbon compound

16 is formed. Obviously, the formation of a copper complex intermediate involving

both alkene double bonds of the substrate is unfavorable in this case.

Many intramolecular reactions of this type have been described, mainly in the

context of applications to organic synthesis or to the synthesis of natural products.

The irradiation of compound 17 in the presence of CuOTf leads to the stereoisomers

18a,b (Scheme 5.4, reaction 7) [24]. In this case, the exo isomer 18a is formed in slight

excess. This stereochemistry was explained by an equilibrium between the two

copper complexes IV and V. In IV, the copper atom is orientated in an exo position;

due to steric hindrance, this structure is generally discussed in such reactions.

Nevertheless, in the present case, structure V with the copper atom in an endo

position is also formed in considerable amounts, which can be explained by a

complexation of the hydroxyl function. In the corresponding transformation of

the enantiomerically pure allylalcohol 19, the endo isomer 20b is formed in excess;

this is explained by the increased stability of intermediate VI (reaction 8) [25].

Compound 20b was transformed into ( ) grandisol, which is part of the insect

pheromone of the boll weevil (Anthonomus grandis). The same strategy was used

previously for a further asymmetric synthesis of grandisol [26].

More complex ring systems have been built up using the CuOTf catalyzed [2 þ 2]

photocycloaddition. For instance, transformation of the cyclopentene derivative 21

leads in high yields to the tricyclic compounds 22a,b (Scheme 5.5, reaction 9) [27],

with the endo isomer 22a being obtained in excess. The reaction was applied to



5.2 [2 ỵ 2] Photocycloaddition of Nonconjugated Alkenes



HO



HO



HO



H



hν, CuOTf



H



+



(7)



84%

H

18a 4 : 3



17



HO



Cu+



HO



H

18b



Cu+

V



IV

HO



HO



HO



H



hν, CuOTf



H



+



(8)



70%

20a



19



Cu+



1 : 7



OH



20b



H



VI

HO

(–)-Grandisol

Scheme 5.4 Intramolecular [2 ỵ 2] photocycloaddition of

3 hydroxy 1,6 heptadiene derivatives catalyzed by CuOTf.



the synthesis of analogues of angular triquinane sesquiterpenes, where various

functional groups are tolerated. In the case of compound 23, a vinyl substituent is

added to one of the olefinic double bonds (reaction 10), and this compound was then

selectively transformed into the cyclobutane derivative 24 [28]. No Diels Alder

reaction was observed which may occur under PETor Lewis acid catalysis. Enolethers

such as 25 react in the same way (reaction 11), the resultant product 26 being

transformed into b necrodol [29], an insect repellent of the defense spray of the

red carrion beetle (Necrodes surinaminsis), and into the sesquiterpene herbertene [30].

A similar transformation of an enolether was used as key step in the synthesis of the

sequiterpene a cedrene [31]. Esters [32], carbamates [33], or carbohydrate deriva

tives [34] possessing two alkene double bonds have all been successfully transformed,

and the resulting products applied to organic synthesis.

Due to its versatile applicability, the CuOTf catalyzed [2 ỵ 2] photocycloaddition

was used successfully to study the topology of the intermolecular and intramolecular

dimerization of norbornene derivates. When a racemic mixture of compound 27 is

transformed in the presence of CuOTf, a 1 : 1 mixture of two stereoisomers (28a,b) is



j141



j 5 Formation of a Four Membered Ring



142



hν, CuOTf



OH



+



93%

HO

21



22a



OH

72 : 28



(9)



22b



H

hν, CuOTf

O



(10)



O



87%



n -Bu H



n-Bu



24



23



OEt



OEt

h, CuOTf

O



88%



O



(11)

H



25



26



HO

-Necrodol



Herbertene

Scheme 5.5 Intramolecular CuOTf catalyzed [2 ỵ 2]

photocycloaddition, and its application to organic synthesis.



obtained (Scheme 5.6, reaction 12) [35]. In both isomers, the polycyclic system

possess an exo trans exo configuration. Compound 28a results from a dimerization of

molecules of the same absolute configuration (one enantiomer), while 28b is formed

from two molecules possessing the opposite absolute configuration (two enantio

mers). When the molecules are linked together by an adamantane tether, two

substrate molecules are obtained (reaction 13). In 29a, norbornene moieties have

the same absolute configuration, while in 29b they possess opposite absolute

configurations. The transformation of a 1 : 1 mixture of 29a,b yielded a 1 : 1 mixture

of compounds 30a,b. The polycyclic molecule 30a possesses an exo trans exo con

figuration (compare 28a,b), and the norbornene moieties have the same absolute

configuration; therefore, 30a results from the transformation of 29a. The meso

isomer 29b is transformed into an achiral exo cis exo polycyclic product 30b. In the

case of compound 31, which possesses a larger tether derived from adamantane, one

diastereoisomer (the racemic product) was also obtained. By using the CuOTf

catalyzed [2 ỵ 2] photocycloaddition, this compound can be selectively transformed

into the polycyclic exo trans exo product 32 (reaction 14). It should also be mentioned

here that the irradiation time is significantly shorter in the case of the intramolecular

reactions. In these cases, under identical reaction conditions, an irradiation of only



5.2 [2 ỵ 2] Photocycloaddition of Nonconjugated Alkenes



OMe

h, CuOTf

62%



+



OMe



(12)



MeO

MeO

27

(racemic)



28a

(racemic)



MeO

1



:



1



28b

(racemic)



O



O

O



O



O



O



O



O

1

29a

(racemic)

:



+



1



h, CuOTf



30a

(racemic)



80%



+



:



1



1



O



O



O



O

O



O



O



O



30b



29b



h, CuOTf



O

O



O



O



80%



O

O



O

O



31

(racemic)



(13)



32

(racemic)



Scheme 5.6 CuOTf catalyzed [2 ỵ 2] photocycloaddition used for

a topology study of inter and intramolecular dimerization of a

norbornene derivates.



(14)



j143



j 5 Formation of a Four Membered Ring



144



3 h was required (see reactions 13 and 14), while the intermolecular reaction was

complete after 56 h (reaction 12).

The inter and intramolecular CuOTf catalyzed [2 ỵ 2] photocycloaddition of two

alkenes was also performed in ionic liquids [36]. The best results were obtained when

trimethyl(butyl)ammonium bis(trifluoromethylsulfonyl)imide was used as the re

action medium. This ionic liquid does not absorb light to any significant degree

around 254 nm. Imidazolium salts, which are frequently used for ground state

reactions, are not appropriate for this transformation because they absorb light in

the range between 200 and 350 nm, which in turn induces a significant decompo

sition of the ionic liquid.

A large number of [2 ỵ 2] photocycloadditions of vinylarene compounds have been

reported in the literature [37], and these are particular suitable for the synthesis of

cyclophanes. Based on the fact that conjugation between the olefinic and aromatic p

systems is rather low, these reactions can be compared to corresponding reactions

with nonconjugated alkenes.



5.3

[2 ỵ 2]-Photocycloaddition of Aromatic Compounds



A variety of four membered ring compounds can be obtained with photochemical

reactions of aromatic compounds, mainly with the [2 þ 2] (ortho) photocycloaddition

of alkenes. In the case of aromatic compounds of the benzene type, this reaction is

often in competition with the [3 ỵ 2] (meta) cycloaddition, and less frequently with the

[4 ỵ 2] (para) cycloaddition (Scheme 5.7) [38 40]. When the aromatic reaction partner

is electronically excited, both reactions can occur at the pp singlet state, but only the

[2 ỵ 2] addition can also proceed at the ppà triplet state. Such competition was also

discussed in the context of redox potentials of the reaction partners [17]. Most

frequently, it is the electron active substituents on the aromatic partner and the

alkene which direct the reactivity. The [2 ỵ 2] photocycloaddition is strongly favored

when electron withdrawing substituents are present in the substrates. In such a

reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41], and

only one regioisomer (35) was obtained in good yield. In this transformation, the



R

[2+2]

ortho

R

+

R







R

R



[2+3]

R



meta

[2+4]



R



para

R

Scheme 5.7 Photocycloadditions of alkenes with benzene.



5.3 [2 ỵ 2] Photocycloaddition of Aromatic Compounds



aromatic reaction partner was excited and the addition occurred at the ppà singlet

state. When an additional methoxy group was present on the benzene ring, PET took

place and products possessing no cyclobutane ring were formed.

The [2 ỵ 2] cycloaddition of benzene derivatives with alkenes was also carried out

using photosensitization. Maleimide 36 was added to benzene in high yields

(Scheme 5.8, reaction 16) [42]. In this case, the sensitizer acetophenone 37 transfers

its triplet energy to 36, after which the cycloadduct VII reacts immediately with an



OMe



OMe







CN

+



CN

(15)



74%

34



33



35



O

+



O





NH



NH



O



36 O



VII



Ph



37



(16)



O



O

O



H

N



HN

O



O

O

38



NH

O



82%



h

solid state



O

H

N



HO



O



HO



(17)



conversion: 66%

yield: 100%



O

39

O



O



OH

OH



H

N

O



O

O

HO



N

H



O

OH



40



Scheme 5.8 Intermolecular [2 ỵ 2] photocycloaddition of benzene derivatives.



j145



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