Tải bản đầy đủ - 0 (trang)
Paterno` -Bu¨ chi photocycloadditions

Paterno` -Bu¨ chi photocycloadditions

Tải bản đầy đủ - 0trang

transfer makes the carbonyl oxygen more nucleophilic.77 The photoreaction of heterocyclic aldehydes with furan gives the corresponding exo

oxetanes through the excited triplet state. In the case of the 2-formyl

compounds, in situ metathesis leads to butadienyl formates. This is

explained through participation of the p aromatic orbitals in the C–O

bond cleavage.78

R3



O

R



( )n

O

O

( )n



O



N



O O



R2



O



O



O



O



R1



O



O



O



R

O



(23)



(24)



(25)



R1

R2

O



O



O



N

OR



O



HO



R



R = H, CH3, COCH3

(26)



(27)



(28)



Photocycloaddition of aldehydes to methylated isoxazoles leads to

the exo-adducts (29) with high regio- and diastereoselectivities. Ring

methylation of the isoxazole substrates results in higher conversions and

product stability. The obtained oxetanes show type T photochromism.79

The diastereoselective photoreaction of allenoates and trifluoromethylketones to give the corresponding 2-alkylideneoxetanes (30) is eciently

catalyzed by DABCO.80 In the Paterno`-Buăchi reaction of chiral pcyanobenzoates with 1,1-diphenylethene, the reactivity observed upon

selective excitation of the charge transfer complex is different from that of

the conventional exciplex generated through direct excitation. The activation parameters obtained by Eyring analysis of the diastereoselectivity

indicate that the conventional exciplex is relatively flexible and susceptible

to environmental factors, whereas the charge transfer complex is better

p-p stacked and more rigid both in the ground and in the excited state.

The stereochemical outcome of this photochirogenic reaction can be

controlled by appropriate selection of irradiation wavelength, temperature, and solvent.81

The Paterno`-Buăchi reaction of 1,3-dimethylthymine and 1,3-dimethyluracil with benzophenones gives rise to two regioisomeric oxetanes (31).

Substituent, temperature and heavy atom effects on the reaction are discussed in terms of entropy vs. enthalpy control.82 Irradiation of uracil

in frozen aqueous solution produces two diastereomeric (6-4) products.

Photochemistry, 2012, 40, 146–173 | 155



In fluid solution, in the presence of acetone as triplet photosensitizer, only

cyclobutane dimers are formed.83

Polymer end group modications via Paterno`-Buăchi reactions have been

achieved with polyisobornyl acrylate and polystyrene, employing an aldehyde function as initiator.84



R2

R1

N O



O



ROCO



Ph

O

R3



O

Ar



O



R

X



N

N



Y



CF3

X = C(Ar)2, Y = O or X = O, Y = C(Ar)2



(29)



4



(30)



(31)



Photoreactions of enones and quinones



4.1 Enones

The dynamics of acrolein photochemistry at 193 nm has been investigated

by means of time-resolved Fourier transform IR absorption spectroscopy.

In solution, the major channel is 1,3-hydrogen migration from the pp*

excited triplet state, which results in isomerization to methyl ketene,

whereas in the gas phase CO–H bond dissociation predominates.85

Theoretical CASSCF/MRMP2 calculations on the photorearrangement of

4-phenyl-4H-pyran to endo 6-phenyl-2-oxabicyclo[3.1.0]hexene are in

agreement with the previously proposed three-step mechanism. Initial

s-bond formation from the p,p* triplet excited state gives a primary phenylbridged biradical, which is converted into a 1,3-biradical. Ring closure of

the singlet biradical formed after intersystem crossing is almost barrierless

and yields the final product.86

Irradiation of 3-(alk-1-ynyl)cyclohept-2-en-1-ones (32) leads to selective

formation of tricyclic head-to-head dimers. However, in the presence of

2,3-dimethylbuta-1,3-diene, [2 ỵ 2], [4 ỵ 2] or [4 ỵ 4] cycloadducts can be

obtained, depending on the enone structure.87 In the case of cyclohex-2enones, photocycloaddition to the diene leads to cyclobutane adducts. A

special case are 3-(alk-1-ynyl)-cyclohex-2-enones, which undergo [2 ỵ 2]

cycloaddition exclusively at the CC bond to aord 3-cyclobutenylcyclohex-2-enones (33).88

Photochemical oxa-di-p-methane rearrangement of bicyclo[3.2.1]octanoid scaffolds affords multifunctional, donor-acceptor cyclopropanes

(34).89 Application of this reaction to bicyclo[2.2.2]octenones has been used

as a key step in the total synthesis the hirsutane-type sesquiterpenes ()connatusin A (35), (ỵ)-connatusin B (36), and the marine sesquiterpenoid

2-isocyanoallopupukeanane (37).90–92

Intramolecular photocycloaddition of 6-alkenyl-3-phenylcyclohex-2en-1-ones affords tricyclic ‘‘parallel’’ or ‘‘crossed’’ [2 þ 2] products (38)

(39); in addition, the 6-propenyl compound gives a bicyclic enone

156 | Photochemistry, 2012, 40, 146–173



resulting from hydrogen abstraction in the biradical intermediate.93

A similar process has been conveniently performed with 4-alkenyloxycoumarins in a flow-through photochemical reactor.94 Likewise, the

diastereoselective [2 ỵ 2] photocycloaddition of cyclopentene with a

cyclohexenone containing a chiral substituent at position 3 has been

conducted using a continuous microflow reactor in a shorter reaction time

than in a batch reactor.95 Broănsted acids promote the photocatalytic

reductive cyclization of 1,9-disubstituted nonan-2,7-dien-1,9-diones to

1,2-diacylmethylcyclopentanes with high diastereoselectivity, via neutral

b-ketoradicals.96

In solution, 3-(9-anthracenyl)-1-(pyridin-4-yl)propen-1-one undergoes

(E) to (Z)-photoisomerization, whereas 3-(9-anthracenyl)-1-(pyridin-4yl)propen-3-one does not. The driving force for the reaction is intramolecular p-stacking of the pyridine and anthracene rings.97 Photoisomerization

of the (E)-enone moiety in 14-membered macrolactones leads to the (Z)enone (40). This process has been applied to the synthesis of resorcylic acid

lactones.98

O

O



O

H



H

O



H

R1



Ph



R2



OCH3



(32)

H



HO



H



(33)



(34)

H



OH



CN



O

HO



OH



H



O

HO

OH



(35)



Ph



(36)



Ph



O

COOCH3

(38)



(37)



( )n



OH



O

O



O

COOCH3

(39)



O



CH3O

(40)



Irradiation of 5-benzoylbicyclo[2.2.2]octenone oximes (41) in the

presence of triethylamine affords the corresponding bicyclo[3.2.1]octane

derivatives via an electron transfer pathway.99 The synthesis of 2-vinylcyclopropanecarbaldehydes, precursors of cyclopropane components present in

pyrethroids, has been achieved by using the oxa-di-p-methane

Photochemistry, 2012, 40, 146–173 | 157



rearrangement with 4-phenylbenzophenone as photosensitizer.100 Photochemical oxa-di-p-methane rearrangement of tricyclo[5.2.2.02,6]undeca4,10-dien-8-ones has been exploited for the synthesis of linear triquinanes,

including (Ỉ)-D9(12)-capnellene (42).101

The photocatalytic intramolecular hetero-Diels-Alder reaction of

tethered bis(enones) (43) via radical anions constitutes the formal coupling

of an electron-deficient heterodiene with an electronically mismatched

enone dienophile and affords dihydropyrans with high diastereoselectivity

and regioselectivity.102 In aqueous solution, irradiation of a trans-2hydroxychalcone bearing a carboxylate group at 2 0 -position leads to

isomerization to the cis chalcone, followed by hemiacetal formation and

lactonization, to give (44). Photoinduced ring-opening of the lactone in rigid

matrixes gives a red o-quinone allide, providing a new photochromic

system.103



NOH

O



R2

O



R2



O



COPh

R1



COPh



COPh



(41)



(42)



(43)



(44)



The brakelike performance of a molecule containing a pentiptycene rotor

and a 2-methyleneindanone brake unit (45) has been reported. The repeated

switching between the brake-on and brake-off states is conducted by a

combination of photochemical and electrochemical E/Z isomerization.104

Dienone-ether macrocycles (46) may be reversibly switched from the thermodynamically stable E,E,E,E-isomers, with an open central cavity, to

globular, cavity-closed E,E,Z,Z- isomers by the action of sunlight. They are

restored to their open forms by gentle heating.105



R1



O



R2



R2



R



O



O



O



O



O



O

R2



2



OC8H17

R1

(45)



158 | Photochemistry, 2012, 40, 146–173



( )n



(46)



4.2 Quinones

The photoacylation of benzoquinone with arylaldehydes has been applied

to the synthesis of phenylaminophenanthridinequinones (47).106 Under

solar irradiation, the reaction between benzoquinone or naphthoquinone

and heteroaromatic carbaldehydes gives the corresponding heteroacylated

hydroquinones.107

A model reaction of hydrogen transfer has been proposed that describes

well the results of kinetic studies of the photoreduction of o- and p-quinones

and fluorenone in the presence of p-substituted N,N-dimethylanilines and

polyalkylbenzenes.108 Photosubstitution of the sulfo group for hydrogen is

observed upon irradiation of sulfonated derivatives of hydroquinone. The

reaction can be promoted by visible light, with eosin as sensitizer.109

Intramolecular [2 ỵ 2] photocycloaddition of the quinone moiety of

quinopimaric acid (48) to the isopropyl-substituted double bond leads to a

polycyclic cage compound.110 2-Hydroxynaphthoquinones undergo a formal [3 ỵ 2] photocycloaddition with dierent cyclic alkenes, delivering

chiral products (49). In the presence of a chiral template, a limited enantioselectivity is observed.111

Irradiation of the herbicide metamifop (50) leads to N-(2-fluorophenyl)N-methyl-2-hydroxypropionamide as major product. Minor amounts of

N-methyl-2-fluoroaniline and N-methyl-N-phenyl-2-oxopropionamide are

also obtained.112 Laser flash photolysis of b-lapachone-3-sulfonic acid (51)

gives the triplet excited state, which is efficiently quenched by electron

rich amino acids via electron transfer followed by proton transfer. No

measurable quenching is observed in the presence of thymine or thymidine;

by contrast, the reaction with 2 0 -deoxyguanosine is fast, although not

diffusion-controlled. The singlet oxygen quantum yield, determined by

time-resolved near-IR emission, is ca. 0.7.113

O



O



O



O



R2



O

N



R3

O



O



R1



COOR



(47)



R

( )n



O



(48)



(49)

O

O



O

Cl



O



N

O



N



O

F



O

(50)



SO3H



(51)



Bodipy dyads incorporating covalently attached hydroquinone/quinone

groups (52) show various levels of fluorescence depending on the oxidation

state of the appended group. Femtosecond transient absorption

Photochemistry, 2012, 40, 146–173 | 159



spectroscopy indicates that electron transfer from the hydroquinone unit to

the first-excited singlet state of the Bodipy center is followed by ultrafast

charge recombination.114 This technique has been employed for the study of

different benzoquinones, showing that intersystem crossing from the lowest

singlet excited state is operative for duroquinone (53), where the T2 (p,p*)

state is likely below S1 (n,p*). By contrast, for ubiquinone 0 (2,3-dimethoxy5-methyl-1,4-benzoquinone) and thermoquinone (54), this process is not

efficient because of the higher energy of T2. Instead, the triplet state is

generated upon excitation at 266 nm, showing the involvement of an upper

excited singlet state in the intersystem crossing process.115

In order to study the viscosity effect on the quenching of triplet excited

state of (53) by TEMPO, chemically induced dynamic electron polarization

and transient absorption spectra have been measured in ethylene glycol,

1,2-propanol and their mixtures. The results indicate that the quenching

rate constant is viscosity-dependent and decreases linearly with the increase

in solvent viscosity.116 The spectroscopy and dynamics of near-threshold

excited states of the isolated chloranil radical anion have been studied

using photoelectron imaging taken at 480 nm, which clearly indicates

resonance-enhanced photodetachment via a bound electronic excited state.

Time-resolved photoelectron imaging reveals that the excited state rapidly

decays on a timescale of 130 fs via internal conversion.117



O

N

F



B



N



N



R1



CH2CH2OH

CH2CH2OH



n



F

O

(52)



5



O



(53)



O

(54)



Photoelimination



5.1 Photodehydration

4-Hydroxy- and 4-methoxy-6-methylene-2,4-cyclohexadien-1-one, generated by photodehydration of the corresponding 2-hydroxymethyl

precursors, can be quantitatively trapped by azide ion or ethyl vinyl ether.

Alternatively, the 4-hydroxy derivative may undergo tautomerization to

produce 2-methyl-1,4-benzoquinone. Quinone methides of this type can

also be generated by photolysis of the 2-ethoxymethyl or N,N,N-trialkylammoniummethyl analogs; their short-lived benzoxete precursors (55) have

been detected by laser flash photolysis.118 2,3-Naphthoquinone methides,

generated by photodehydration of 3-(hydroxymethyl)-2-naphthol,

undergo hetero-Diels-Alder cycloaddition to electron-rich polarized olefins,

such as vinyl ethers and enamines, in aqueous solution. This has been

exploited to achieve photolabeling or photoligation of two substrates by

means of a light-induced Diels-Alder ‘‘click’’ reaction. The kinetics of

this process is appropriate for time-resolved measurements.119 Irradiation of

1,8-naphthalimide derivatives (56) at 355 nm leads to quinone methides,

160 | Photochemistry, 2012, 40, 146–173



which can be trapped by nucleophiles or by ethyl vinyl ether. A photoinduced

electron transfer mechanism is proposed based on detection of the triplet

excited state, the radical ion pair, and the phenoxy radical by laser flash

photolysis.120 2,6-Naphthoquinone methide and binol quinone methides (57)

have been photogenerated by water-mediated excited state proton transfer

from appropriate precursors. The latter are formed more efficiently and

react faster with N and S prototype nucleophiles. Their reactivity with

nucleobases makes them potential purine-selective DNA alkylating

agents.121 Intramolecular proton transfer of hydroxymethylphenols with a

2-hydroxy-2-adamantyl substituent, followed by dehydration, leads to longlived adamantylidene quinone methides (58). They can be detected by laser

flash photolysis and trapped by nucleophiles, to give addition products.122

Water-assisted excited state proton transfer in 4-phenylphenol derivatives,

coupled with dehydration, delivers quinone methides (59). These species

react with methanol, yielding photosolvolysis products. The steric hindrance

introduced by an adamantyl moiety, or the additional stabilization provided

by two phenyl rings, result in increased lifetimes of the quinone methides

and higher selectivity of their reactions with nucleophiles.123 In hydroxyadamantyl derivatives of 2-phenylphenol, excited state intramolecular

proton transfer from the phenol to the carbon atom of the adjacent phenyl

ring and from the phenol to the hydroxyl group, to give (60), are competitive

processes.124 In dihydroxyphenyl anthracenes, an analogous mechanism of

quinone methide formation has been confirmed by deuterium exchange.

In addition, 9-(2,5-dihydroxyphenyl)anthracene undergoes photocyclization

in organic solvents, to give a bridged product (61).125



OH

N(CH3)3+I–



O



H2C



O

N



O



X



O



OR



(55)



O



OH



( )n



(56)



(57)



(58)

HO



O



O



O



(59)



(60)



(61)



5.2 Photodecarbonylation

The synthesis of 1,4,8,11-tetraphenylpentacene and 1,4,8,11-tetra(2 0 thienyl)pentacene has been achieved via photodecarbonylation of the corresponding a-diketone precursors (62) (Strating-Zwanenburg reaction).126

Photochemistry, 2012, 40, 146–173 | 161



Octacene and nonacene have been obtained by photochemical stepwise

bisdecarbonylation of bridged a-diketones (63).127

O

O



R



R



R

O



O



O



O



R

(62)



n



(63)



The photodecarbonylation of diphenylcyclopropenone from the S2

singlet excited state has been shown to proceed non-adiabatically, to give

the electronic ground state of diphenylacetylene. The transient absorption

of electronically excited diphenylacetylene is caused exclusively by photoexcitation of its ground state.128 Photodecarbonylation of hexasubstituted

meso- and d,l-ketones (64) gives a mixture of products in solution, but takes

place chemo- and diastereospecifically in the solid-state.129 The solid state

photochemistry of aliphatic, dispiro-substituted 1,4-cyclobutanediones (65)

has been studied in solution, bulk (powder) crystals, and nanocrystals. All

of them react efficiently, to give 1,3-biradicals, whose lifetime may become

remarkably long in crystals.130 The solid state photodecarbonylation of

triphenylmethyl alkyl ketones proceeds via a stepwise mechanism and

results in the formation of radical-radical combination products.131

Photodecarbonylation of crystalline 1,3,3-triphenyl-1-hydroxy-2-indanone

results in the exclusive formation of benzocyclobutane (66), while in solution leads to the isomeric photoenols, which subsequently tautomerize. The

1,4-biradical is detected as transient species, both in solution and in the solid

state. While enols revert to ketones in time scales that range from a few

hundred nanoseconds to tens of microseconds, benzocyclobutanol remains

kinetically trapped in the crystal lattice, but undergoes a thermal ring

opening when dissolved.132

R



R2



R2



R1



R1



Ph



Ph



O

R1 = CH3, CH2CH3

R2 = CN, COOCH3, COON(CH3)2



O

( )n



( )n



R



Ph

Ph

Ph

OH



O

n = 0, 1, 2; R= H

n = 2, R = C(CH3)3



(64)



(65)



(66)



The polymerizable a-keto ester methacryloylethyl phenylglyoxylate and its

homopolymer, have been tested for their photoinitiation capabilities in a

crosslinking monomer resin system containing bis-phenol A-glycidyl

methacrylate and triethylene glycol dimethacrylate. The evolution of CO

resulting from the photoinduced decarbonylation leads to a significant

reduction in the volume shrinkage of the resin. Dispersion of CO is uncontrolled, resulting in large voids which are likely to be detrimental to material

162 | Photochemistry, 2012, 40, 146–173



properties.133 The photoreactions of polymers bearing N-acetylcarbazole

and N-formylcarbazole groups have been investigated. While the former

undergo a partial photo-Fries rearrangement, the latter photodecarbonylate

smoothly. These phototransformations produce changes in the refractive

index, which are of potential interest for practical applications.134

5.3 Photodecarboxylation

Decarboxylation of N-Boc-L-valine has been performed in polar solvents,

using electron acceptors such as dicyanobenzenes, methyl 4-cyanobenzoate,

and 1,4-dicyanonaphthalene as photosensitizers, in combination with several

arenes (phenanthrene, naphthalene, 1-methylnaphthalene, biphenyl, triphenylene, and chrysene) as co-sensitizers. The best result is achieved using

biphenyl and 1,4-dicyanonaphthalene in aqueous acetonitrile.135 This type of

reaction has been applied to the photodecarboxylation of N-Boc protected

amino acids and other free carboxylic acids, in the presence of thiol and a

small amount of D2O, to obtain products with high deuterium content.136

A related application is the sysnthesis of a-substituted a-amino esters by

addition of the intermediate alkyl radicals to glyoxylic oxime ethers.137

Trifluoromethyl-substituted phenylacetic and mandelic acids undergo

efficient photodecarboxylation in basic aqueous solution, to give the

corresponding trifluoromethyltoluenes or trifluoromethylbenzyl alcohols.

This is consistent with formation of benzylic carbanions that subsequently

react with water. Quenching studies support a reaction mechanism involving the singlet excited state.138

In weakly acidic aqueous solution, uranyl is able to slowly photolyze

gluconic acid, to form D-arabinose, under ambient laboratory light.139

Visible light-absorbing tris(bipyridyl)ruthenium(II) has been used to mediate

electron transfer photodecarboxylation of N-methylpicolinium carbamates

(67), resulting in the release of free primary amines.140 Hydroxycarboxylic

acids are converted into the corresponding carbonyl compounds under

aerobic conditions, in the presence of a mesoporous silica material (FSM-16)

as photocatalyst, under visible light irradiation.141 During the photosensitized cleavage of coumarin dimers (68), photodecarboxylation is

occasionally observed, along with formation of the expected monomers.142

The photodecarboxylation of o-acetylphenylacetic acid has been theoretically studied with CASSCF and DFT. Excitation to the S1 (np*) state is

followed by rapid relaxation and efficient intersystem crossing to the T1

(np*) state via the S1/T2/T1 three-surface intersection. On the T1 pathway,

1,5-H shift leads easily to the triplet 1,4-biradical, which undergoes intersystem crossing to the singlet biradical. Parent acid-catalyzed bimolecular

decarboxylation is responsible for the experimentally obtained products,

namely CO2 and o-acyltoluene.143

Either direct or sensitized photolysis of 3-(N-phthalimido)adamantane-1carboxylic acid leads to population of the triplet excited state, which

decarboxylates in the presence of a base, giving N-(1-adamantyl)phthalimide. The intermediate radical adds regiospecifically to electron deficient

alkenes. This type of reaction can be extended to related compounds, where

the electron donor (carboxylate) and the acceptor (phthalimide) are separated by a rigid spacer.144 The photodecarboxylation of phthalimides

Photochemistry, 2012, 40, 146–173 | 163



sensitized by 4,4 0 -dimethoxybenzophenone or acetone has been performed

under microflow conditions. In general, the conversions, yields, and

chemoselectivities achieved with microreactors are better than those

obtained using batch photoreactors.145,146 Photodecarboxylative addition

of phenylacetates and N-acylated a-amino acid salts to phthalimides

gives the corresponding alkylated hydroxyphthalimidines. Likewise, photodecarboxylative addition of a-thioalkyl-substituted carboxylates to alkyl

phenylglyoxylates, to give (69), has been reported. In some cases, the use of a

micro-structured reactor is advantageous.147–149 The fluorescence of caged

phthalimide-serine systems is up/down modulated by decarboxylative

photorelease with fluorescence decrease versus moderate fluorescence

increase, serving as reporter function.150 The photochemistry of arene-linked

phthalimides incorporating a carboxylate or thioether donor group has been

investigated. Photodecarboxylative cyclization is observed in catechol-linked

derivatives and o-phthalimido-m-phenoxy carboxylates (70). The reaction

has been applied to the synthesis of macrocyclic target compounds.

Photocyclization products are also obtained with phthalimides containing a

thioether branch at the ortho-position of the arene linker.151

Theoretical calculations on the photochemistry of ketoprofen, ibuprofen,

and naproxen derivatives, using hybrid and time-dependent DFT, show

that these compounds should have lower propensity to decarboxylate from

the first triplet excited state than the parent drugs.152 The pH- and wavelength-dependent pathways for the photodecarboxylation of ketoprofen

have been mapped by CASSCF/CASPT2 computations. Decarboxylation

of the basic form is attributed to a long-distance charge transfer excited

state, populated by excitation at 330 nm. Conversely, a short-distance

charge transfer excited state, populated by excitation at 260 nm, appears to

be responsible for decarboxylation of the acidic form.153 Photoproduct

studies, as well as nanosecond laser flash photolysis, do not show chiral

discrimination in the photodecarboxylation of (S)- and (R)-ketoprofen

within the cavity of b-cyclodextrin.154 Upon sunlight exposure, in aqueous

media, indomethacin undergoes oxidative decarboxylation, to give an

alcohol and an aldehyde.155 The photocatalytic degradation of diclofenac

has been achieved using commercial and synthesized TiO2, as well as

functionalized multi-walled carbon nanotubes. The major photoproducts

correspond to photocyclization, decarboxylation, and dehalogenation.156

O



NHR

O



O



O

O



O



R3



R2

S

OR1



HO

TfO



N

CH3

R



(67)



R



(68)



O



(69)



OCH2COOK

O

O

N ( )m

O



(70)



A phase-change thermochromic system has been designed through

the reversible transformation of the 4-substituted flavylium dye

164 | Photochemistry, 2012, 40, 146–173



4-(2-carboxyphenyl)-7-diethylamino-4 0 -dimethylamino-1-benzopyrylium

into its leuco form (71). Photodecarboxylation of the latter, to give

4-phenyl-7-diethylamino-4 0 -dimethylamino-1-benzopyrylium, erases the

thermochromic effect.157

Irradiation of 4,6-dimethyl-a-pyrone immobilized in a guanidiniumsulfonate-calixarene gives rise to a 4,6-dimethyl-Dewar-b-lactone, a

carboxyl zwitterion and 1,3-dimethylcyclobutadiene, both in the solid

crystalline state and in aqueous solution.158 Diarylmethyl carbocations have

been generated within the cavities of non-protic zeolites (LiY, NaY, KY,

RbY, CsY, and NaX) via laser-mediated decarboxylation of diarylacetic

acids.159

The photochemistry of nitrophenylacetates has been investigated by

UV-pump/IR-probe spectroscopy, combined with quantum chemical calculations. For the meta and para isomers, decarboxylation proceeds via a

triplet state with subnanosecond lifetime. In the case of the ortho derivative,

photodecarboxylation is nearly suppressed by excited state proton or

hydrogen atom transfer, but it can still be investigated due to the isolated

spectral position of the CO2 band. Data analysis reveals that a weak

ultrafast release channel represents the main photodecarboxylation pathway for this compound. Understanding CO2 uncaging mechanisms can be

useful for the design of improved nitrobenzyl cages.160,161



6



Photo-Fries and photo-Claisen rearrangements



Irradiation of 1,5-bis(4-chlorobutanoyl)naphthalene leads exclusively to the

mono-rearranged photo-Fries product at position 2 of the naphthalene

ring.162 The photo-Fries rearrangement of 8-alkylnaphth-1-yl acetates has

been used as a tool to generate aryloxy radicals, for a combined experimental and theoretical study on the mechanisms and energies associated

with intramolecular hydrogen atom transfer to oxygen atoms.163 An

efficient one-pot method for the synthesis of functionalized quinolines has

been developed based on the photo-Fries rearrangement of p-substituted

anilides, followed by in situ reaction of the resulting o-amino ketones

with acetylenic Michael acceptors.164 The photo-Fries rearrangement of

(hetero)aryl 3-methyl-2-butenoates has been achieved under biphasic

(cyclohexane/aqueous KOH) catalysis. This reaction leads to the formation

of chroman-4-one derivatives in one-pot.165 A convergent total synthesis of

the ansamycin antibiotic (À)-kendomycin has been achieved, using the

photo-Fries rearrangement of macrolactone (72) as a key step.166

Irradiation of 1,n-dibenzyloxy-9,10-anthraquinones gives rise to a complex mixture of photoproducts, including those derived from photo-Claisen

rearrangement.167 Direct photolysis of the herbicide cyhalofop (73) in water

affords the photo-Claisen product.168

Serum albumins have been employed as hosts for the photo-Fries

rearrangement of 4-methoxy-1-naphthyl esters, at site I (acetate) and site II

(monoglutarate) of the proteins. A species- and site-dependent quantum

yield of product formation is observed; the best results are obtained within

site I of bovine serum albumin.169

Photochemistry, 2012, 40, 146–173 | 165



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Paterno` -Bu¨ chi photocycloadditions

Tải bản đầy đủ ngay(0 tr)

×