Organic Synthesis Enabled by Light-Irradiation of EDA Complexes

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Organic Synthesis Enabled by Light-Irradiation of EDA Complexes: Theoretical Background and Synthetic Applications Carolina G. S. Lima, Thiago de M. Lima, Marcelo Duarte, Igor D. Jurberg, and Marcio W. Paixao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02386 • Publication Date (Web): 11 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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Organic Synthesis Enabled by Light-Irradiation of EDA Complexes: Theoretical Background and Synthetic Applications Carolina G. S. Lima,a Thiago de M. Lima,b Marcelo Duarte,c Igor D. Jurberg*,c and Márcio W. Paixão*,a a

Department of Chemistry, Federal University of São Carlos,

b

Department of Chemical

Engineering, Federal University of São Carlos, 13565-905, São Carlos, SP, Brazil. c

Institute of Chemistry, State University of Campinas, 13083-970, C.P. 6154, Campinas, SP,

Brazil. ABSTRACT. In the past decades, the physicochemical properties of electron donor-acceptor, EDA, complexes (also called charge-transfer, CT, complexes) have been extensively studied, while their synthetic applications have been somewhat limited. However, in recent years, this scenario has started to change as an increasing number of examples has been reported. In this regard, this review aims to present and discuss the main aspects associated to the physicochemical properties of these complexes, and a selection of synthetic photochemical applications in organic chemistry.

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KEYWORDS: EDA complexes, CT complexes, charge-transfer band, photochemistry, visible light, organic synthesis.

1. INTRODUCTION “Light for Change” is the motto of 2015, proclaimed as the “International Year of Light” by the United Nations. For chemists, “change” implies transformation, and perhaps, there is no greater transformation than photosynthesis, in which nature is able to convert simple molecules into more complex structures employing light as a renewable energy source. Indeed, for centuries photosynthesis has been driving scientists toward efficient light-harvesting technologies to promote chemical processes. 1,2,3,4,5,6 In the field of organic photochemistry, the use of visible light is increasingly growing in importance as it represents a safer and more practical source of photons.7,8 However, because most organic molecules do not absorb in the visible region, the use of an external photocatalyst (e.g. metal complexes with polyheteroaryl ligands 9 or organic dyes10) is often required in order to achieve a desired transformation (Scheme 1a). In contrast to this mainstream scenario, the diffusion controlled, ground state association between an electron rich (a donor, D) and an electron poor molecule (an acceptor, A), produces an electron donor-acceptor (EDA) complex, that can sometimes absorb light in the visible region. In this case, an electron transfer event can occur without the need of any photocatalyst (Scheme 1b).


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Scheme 1: General representations for a photoinduced electron transfer mediated by (a) a photocatalyst (PC) and (b) an electron donor-acceptor, EDA, complex.

EDA complexes have been studied since the 1950´s, when their physicochemical properties have attracted attention.11 Yet, their involvement in organic synthesis started to be investigated only in a more intensive manner twenty years later, for instance, in studies involving radical-nucleophilic aromatic substitutions12 and the nitration of aromatic compounds.13 In more recent decades, the search for more environmentally friendly processes have lead chemists toward the (re)investigation of photochemical reactions involving EDA complexes. Modern achievements include for instance biaryl coupling,14 intramolecular cyclization,15 and catalytic asymmetric alkylation protocols.16 In recognition to the growing importance of photochemical organic transformations promoted by EDA complexes, this review aims to describe their main physicochemical properties, and a selection of the most representative synthetic applications reported until the present date. As judged appropriate, a few comments are made to situate the chemistry performed with photoexcited EDA complexes in relation to the use of photocatalysts, but a more in-depth comparison is outside the scope of this review. In addition, thermal reactions of EDA complexes


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are also not covered. Although organometallic complexes are sometimes mentioned, the focus of this review is essentially posed on organic molecules.

2.

THEORETICAL

BACKGROUND:

PHYSICOCHEMICAL

ASPECTS

OF

EDA

COMPLEXES AND PHOTOCHEMICAL ELECTRON TRANFERS 2.1 The General Scenario Electron transfer events are ubiquitous in chemistry. Not only transformations in the fields of photochemistry, enzymatic processes, electrochemistry, but also many classic organic reactions exhibit a single-electron transfer (SET) component.17 Since the original work of Mulliken,18 then followed by contributions of Marcus,19 Taube,20 Hush,21 and Kochi,22 among others,23 electron transfer theory has been extensively studied. Today, different synthetic strategies have been established based on mechanistically distinct, yet efficient electron transfers. A light promoted electron transfer event between two molecules does not necessarily need a photocatalyst.9,10,24 The process can occur under rather general reaction conditions. The occurrence of this electron transfer event can be generally described as follows: a donor D, an electron-rich compound of low ionization potential, (i.e. a reducing agent, nucleophile), and an acceptor A, an electron-poor molecule of high electron-affinity (i.e. an oxidizing agent, electrophile), rapidly combine in solution (or in the gas phase) to form an encounter complex: [, ] . This weak, reversible, ground state association is generally called an electron donoracceptor, EDA, complex (also called a charge transfer, CT, complex)22b,25. Because these associations are less robust and less directional than other weak interactions, such as hydrogen bonding,26,27 they are generally very sensitive to variations of temperature, solvent and


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concentration.25,28 As a consequence of such complex formation, often the ensemble exhibits a new absorption band, the charge-transfer band, which is not observed at the UV/vis absorption spectra of its individual components. This band is generally shifted to longer wavelengths, sometimes in the visible light region, thus producing a characteristic strong coloration. An example of such a variation in the UV/vis absorption spectrum can be observed even from a simple mixture of I2 (acceptor) and EtOH (donor) in n-heptane (Figure 1). The appearance of the CT band due to the formation of the EDA complex [ , ] and the position of the local absorption band of I2 are indicated. In this case, one can also observe a "contact" CT band, appearing as a long wavelength shoulder on the UV iodine band. This is a consequence of the very weak interaction of I2 and the solvent employed.29 For the EDA complex considered, the CT band is shifted to longer wavelengths (ca. 225-300 nm), but does not reach the visible region (390-700 nm).

Figure 1. The molar absorptivity ( ) spectrum of I2 vapor (in black), of I2 in n-heptane (in red) and of I2 and EtOH (3.4M) in the same solvent (in blue). The CT band due to the complex formation [EtOH, I2]EDA appears in longer wavelengths29 Adapted with permission from ref. 29. Copyright 1969 Wiley-VCH.


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Upon light irradiation at the wavelength of its CT band, the [, ] can evolve, via a

reactive excited complex ([, ]∗ ), and produce an electron transfer event, thus generating a

radical ion pair trapped in the solvent cage [ , ] . Finally, this radical ion pair can

undergo different reaction pathways (e.g. rearrangements, additions, eliminations, etc.), escape the solvent cage to produce free radical ions or undergo back-electron transfer. Concurrently to this process, local irradiation of individual bands of either components, D (or A), leads to excited forms D* (or A*), which can combine with its non-excited counter-part A (or D), thus leading to exciplex D*(A) (or A*(D) ), respectively. This exciplex can produce an electron transfer, thus being converted to the same ion-radical pair previously described [ , ] (Scheme 2).23 30

Scheme 2. Kinetic profile derived from light irradiation of a reaction mixture containing an electron donor and an acceptor.

The excited complex (also called an exciplex) formed from local band irradiation is dynamically assembled in the excited state, and therefore, requires an important level of solvent reorganization. On the other hand, the exciplex derived from CT band irradiation requires only minor changes in the organization of solvent, as the dyad has been previously assembled in the


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ground-state. These two exciplexes are typically more polarized than their corresponding ground-state species and as a consequence, are better solvated in polar solvents (however, not in the same manner) (Scheme 3).30 diffusion

D*(A) A

h

D

D*

D

D

+

A

-

*

high reorganization

1 [D , A ]SOLV

= solvent D

h

A EDA

CT

D

low reorganization

'+

A

'-

*

2 [D,A]*

Scheme 3: A general representation of the requirement for solvent reorganization involved in the formation excited complexes due to local band and CT band irradiation, respectively (Adapted with permission from reference 30. Copyright 2013, Royal Society of Chemistry).

Because vibrational relaxation is typically very fast in condensed phases (10-12-10-13 s) electronically excited states 1 and 2 are generally under thermal equilibrium, and a common excited-state complex can be rapidly attained starting from both exciplexes, either the one derived from CT band irradiation, or the one derived from local-band irradiation (Figure 2).30,31


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Figure 2. General scenario representing arbitrary potential energy surfaces of exciplexes derived from both CT band irradiation (leading to [, ]∗ ) and local band irradiation of a single component (leading to D*(A) or A*(D) ). Equilibrium between both species is possible (Adapted with permission from reference 30. Copyright 2013, Royal Society of Chemistry).

Importantly, if within the lifetime of a certain excited state, a sufficiently high-energy barrier or potential surface crossing does not allow them to freely interconvert, it is possible that such exciplexes can actually exist as separate entities. In this case, these two exciplexes often display different properties in terms of structure and reactivity.30 In addition, because local bands of D and/or A may eventually overlap in some cases with the CT band, it is likely that, in such situations, the EDA complex will be also excited to a certain extent upon irradiation of a local band (and vice-versa).30 Finally, other main distinctive features of EDA complexes in relation to its individual components also include changes in dipole moment, solubility, conductivity and crystal structure.28 When feasible, the study of X-ray crystal structure of an EDA complex can be much informative. It allows to i) access the general orientation of the complex, in order to evaluate which orbitals of D and A are being used for CT stabilization, ii) it provides D-A distances, which can be compared to van der Waals radii and covalent bonds distances, and iii) it allows the evaluation of changes in the internal structures of D and A. Overall, the study of these properties provide a direct evidence for EDA complex formation.32,33


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2.2 Theory of Electron Transfer: Kinetics Considerations Two main pathways are generally employed to describe an electron transfer occurring within an excited encounter complex of D and A: outer- and inner- sphere electron transfers.20,28 These terms have been originally employed to describe redox reactions between metal centers. Indeed, the terms bonded and non-bonded electron transfer, corresponding to inner- and outersphere electron transfer, respectively, would be more appropriate in the case of organic molecules.22a The non-bonded (outer-sphere) electron transfer mechanism is typically a non-adiabatic multi-step process.19,34 In this case, the electron transfer event is most probably the ratedetermining step.17b On the other hand, the bonded (inner-sphere) electron transfer mechanism35 is generally an adiabatic multi-step process. Here, the electron transfer step might not be rate-determining .17b,34b (Scheme 4 ).

Scheme 4. Electronic transfers can proceed either via an inner- or outer-sphere mechanism.

According to Marcus theory,36 the structural change involved in the transformation of reactant into product can be represented by identical parabolas vertically shifted in relation to each other, depending on the strength of the driving force associated to the process, the standard Gibbs free energy ∆G0. Among the two possible regimes, non-adiabatic and adiabatic electron transfers, it is the value of the electronic coupling term between reactant (R) and product (P) states, given by the resonance integral V,30,37 that differentiates them (Figure 3).


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Figure 3. Potential-energy surfaces for electron transfers follow two possible regimes: nonadiabatic and adiabatic.

As an electron is transferred from one molecule to another, the reaction changes from one surface (R) to another (P), thus intersecting a point, which represents two configurations of identical electronic energies, but with a deformed nuclear geometry. The probability of electron transfer at this point is governed by the interaction between initial (R) and final (P) states. The extent of this interaction is established by the resonance integral V, which determines the splitting of energy surfaces.37 In one extreme situation, an important energy barrier can separate the passage of the electron from a donor to an acceptor orbital. As a consequence, this reflects in a very small electronic interaction (V is closer to 0), and the probability for the electron transfer is equally small. In this case, if and when an electron transfer occurs, the change of reaction coordinates from R to P is an abrupt event. This process is called non-adiabatic. On the other hand, if strong electron interaction energies are present, the electron transfer occurs smoothly. This process is named adiabatic. In a non-adiabatic regime (non-bonded system), the resonance integral V within an EDA complex is typically small, V ≈ 100-300 cm-1. This mechanistic frame is commonly observed for


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hindered [, ] , where the interplanar distance between the components is given by ≈

5 − 6Å. In addition, such EDA complexes are not significantly stabilized by resonance, thus making its formation constant (K ) small, and often hampering experimental measurements (Scheme 5).28

Scheme 5. Equilibrium formation of EDA complex.

Considering a non-adiabatic process, the formula for the rate of electron transfer is generally accepted as:38 #$% =

(, + 345 )( '( + $/0 1− 7 * ℏ ,#- . 6,#- .

Equation 1. Rate for electron transfer in a non-adiabatic process. In equation 1, 8 is the reorganization energy. It is defined as the energy needed for a vertical electron transition without changes in the nuclear configuration (cf. Franck-Condon principle, which is a consequence of the Born-Oppenheimer approximation: electronic transfer occurs much faster, ~10-16 s, than nuclear vibration, ~10-13 s). 9: is the Boltzmann constant, ℏ is the reduced Planck constant, T is the absolute temperature. As a corollary of equation 1, one can plot a function of ; with ΔGA , thus producing

a concave curve. This curve defines three domains: i) ΔGA > −8 , where ; increases if ΔGA

decreases (Normal Marcus region), ii) ΔGA = −8, where the reaction becomes barrierless,


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diffusion controlled, and iii) ΔGA < −8 , where ; decreases with the decrease of ΔGA (inverse Marcus region) (Figure 4).

Figure 4. Dependence of the logarithm of the electron transfer rate constant with ΔGA , according to Marcus theory.

Overall, Marcus theory can be generally applied for non-bonded (outer-sphere, nonadiabatic) transition states, thus successfully providing a theoretical guide for such electrontransfer rates.19,36,38 On the other hand, bonded (inner-sphere) electron transfers have no general theoretical model, due to the great difficulty in identifying structural parameters associated to the isolation of the putative encounter complex.28,39,40 Within an adiabatic regime (bonded system), the distance between D and A in the encounter complex [, ] is typically smaller, ≈ 3.0 − 3.3Å (appreciably shorter than the sum of the van der Waals radii of D and A), and the ressonance integral assumes values of V ≈ 1000-3000 cm-1 for medially coupled potential-energy surfaces, and higher values for strongly coupled potential-energy surfaces.28 In medially coupled potential energy surfaces, the reaction barrier is approximately half of the value in a non-adiabatic regime. Due to a more important stabilization of EDA complexes in this regime (with G ≈ 0.1 − 1.0IJ ), now they are


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observable in solution. In strongly-coupled potential-energy surfaces, the electron-transfer occurs in the absence of an activation barrier. The process is controlled by diffusive dynamics and leads the encounter complex [, ] directly to [ , ] .28

Remarkably, in photoredox catalysis promoted by metal complexes, such as KL(MNO) P ,

the electron transfer generally occurs via an outer-sphere mechanism.9b,24 In contrast, electrontransfers proceeding via excited EDA complexes often occur via inner-sphere pathways. Intriguingly, in such cases, the electron-transfer rates do not seem to follow the "bell-shaped" correlation of Figure 4.41 An additional, equally remarkable observation reported by numerous groups indicate that the electron-transfer event does not only depend on the standard Gibbs freeenergy ΔQ A , but also on the nature of the components D, A participating in the EDA association. For instance, the choice of either n- or π-donors, as well as the size of the π-system involved in such encounter complexes, are parameters that can play a pivotal role in the electron transfer event.42

2.3 Redox Potentials: Thermodynamic Considerations In solution, the strength of donors and acceptors can be estimated based on their standard A A () = A (/ ) and WXU () = A (/ ), oxidation and reduction potentials, i.e. RSTU

respectively.43 The overall standard potential associated to an electron transfer from a donor to A A () + RSTU an acceptor is then given by the expression ∆ A = WXU () (also formulated as

A A () − WXU ∆ A = WXU () ). If standard conditions are not employed (i.e. reaction temperature

of 25 oC, concentration of solutes in 1 mol.L-1 and gases at 1 atm), the standard reaction potential can be calculated from the non-standard value measured using the Nernst equation: ∆ = Z>

∆ A − [\ ;]^, where R = 8.31 J.mol-1.K-1 is the gas constant, T is the absolute temperature, n is


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the number of electrons involved in the redox reaction, F ≈ 96485 C.mol-1 is the Faraday constant and Q is the reaction quotient.44 A mathematical expression for the variation of the Gibbs free energy associated to an electron transfer within an excited EDA complex [, ]∗ can be deduced from the corresponding expression used for exciplex (D*)A (or ( A*)D ) (Equation 2):45 Δ_` Q A = a [b( A ( /) − A (/ )) + c( , ) − c(, ) ] − ΔA,A Equation 2. The expression of the change in the Gibbs free energy Δ_` Q A associated to the

electron transfer within an excited EDA complex,. a = 6.02 e 10 P mol-1 is the Avogadro h h X

number, b = 1.60e 10Jf g is the elementary charge, c(, ) = klmi mj W

n o ij

h h X

and c( , ) =

klmp mq W are the electrostatic work terms. They describe the Coulombic attraction of reactants n o ij

and products, respectively, where r , r are the charge numbers of donor and acceptor, respectively, before the electron transfer (for neutral molecules their values are zero); and r , r

are the corresponding charge numbers of those components after the electron transfer; W is the relative static permittivity of the solvent, A is the permittivity of vacuum, and is the average

distance between D and A. ΔA,A is the energy associated to the electronic transition between the two lowest vibrational levels of ground state and excited complex.

As evidenced by equation 2, both redox potentials and the energy of 0-0 excitation associated to the EDA complex (ΔA,A) are necessary to infer if a given electron transfer is a thermodynamically favored process. In terms of reaction design, because the values of ΔA,A


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associated to most EDA complexes are generally not available, the knowledge of standard redox potentials of D and A alone does not produce a direct indication. Nevertheless, being able to compare such potentials can a priori serve as an estimate in this direction (Figure 5).37,46,47,48,49,50,51,52

Figure 5. Presentation of some reduction potentials of electron-deficient and electron-rich substrates.

2.4 Quenching of Fluorescence When a molecule F (i.e. A or D) is irradiated with a wavelength falling inside its UV/vis absorption spectrum, it is excited, thus forming F*. This new species is a better reducing and


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oxidizing agent, as it is now easier for F* (in relation to F) to give an electron away or to accept an electron from other molecules (Figure 6).

better reductant

LUMO h

better oxidant

HOMO

F

1

F*

Figure 6. An excited molecule F* is simultaneously a better reductant and oxidant.

An excited molecule 1F*(in the singlet state) can naturally return to the ground state F via a number of relaxation mechanisms. For instance, a non-radiative process involves the dissipation of the excitation energy in the form of heat to molecules of surrounding solvent. 1F* can also relax via intersystem crossing (ISC) to a triplet state, 3F*, which may subsequently undergo further relaxation via phosphorescence. Alternatively, 1F* can also decay emitting a photon, (i.e. via fluorescence, F is called a fluorophore)53 (Figure 7).


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Figure 7. The Jablonsky diagram reveals the main possible changes in electronic states of a chemical species F involved in a photochemical process (IC: internal conversion, ISC: intersystem crossing).

In the presence of a different reactive molecule Q, the fluorescence intensity can be decreased according a variety of mechanisms. Two of them are especially important: energy and electron transfer.48 In these circumstances, one says that 1F* = F* is being quenched (the singlet state notation will be removed at this point, for simplification). As a consequence of this process, the lifetime of F* and the quantum yield of the photochemical process involved are both reduced. The mechanism and efficiency of fluorescence quenching can be experimentally evaluated by employing a Stern-Volmer plot.54,55 Three mechanistic situations are common for the quenching of a fluorophore F*: i) via a static quenching, where the quenching occurs due to a ground state EDA association between the fluorophore and the quencher, ii) via a dynamic quenching, where quenching occurs due to the collision of excited fluorophore with a groundstate molecule of the quencher, or iii) via a combination of both mechanisms (Figure 8).

Figure 8. Three common scenarios for the quenching of fluorophores: i) static quenching, ii) dynamic quenching or iii) a combination of both processes (F: fluorophore, Q: quencher).


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In the most general scenario, where both quenching mechanisms are occurring, a modified Stern-Volmer expression can be written as follows (Equation 3):55 s 5 t5 = = (u + vw' [x])(u + vyz{ [x]) s t

Equation 3: Modified Stern-Volmer equation. ΦA and Φ are the quantum yields in the absence and in the presence of the quencher, as well as A and are the intensities of fluorescence (at a

certain wavelength), in the absence and in the presence of a quencher Q, respectively. G =

9} ~A is the Stern-Volmer constant, that is, the product of 9} , the rate constant for fluorescence quenching and ~A is the lifetime of the excited fluorophore F* in the absence of the quencher Q.

KEDA is the equilibrium constant for the formation of the ground-state complex between Q and F. [Q] is the concentration of the quencher Q.

Associated to the plot of

n

x [Q], either a pure static or dynamic fluorescence quenching

mechanisms will produce straight lines corresponding to the formulas n

n

= (1 + G [^]) or

= (1 + G [^]), respectively.55 In both cases, the lines intercept 1, but their slopes, G or

G , have different meanings. A possible way to distinguish between each process (and therefore

to know which coefficient is actually being considered) is to verify the behaviors of these curves with the temperature. In the case of a dynamic quenching, an increase in temperature often causes G to also increase: under more intense thermal agitation, fluorophore and quencher

meet more rapidly, which reflects in the increase of 9} . In the case of a pure static quenching, the slope KEDA should decrease with the increase of temperature, as the formation of a ground-state

complex becomes less favorable.55 Alternatively, one can also distinguish between both


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mechanisms by verifying if the relative change in lifetime of fluorophore emission

n

follow the

change in relative fluorescence emission n , when the concentration of the quencher ([Q]) is increased. In the static case, only the plot of relative emission [Q]. The relative lifetimes of fluorophore emission of dynamic quenching, plots of relative emissions

n

n

n

will change with the increase of

will remain virtually constant. In the case

and relative lifetimes

n

are the same, thus

following the increase of the quencher concentration. (Figure 9).55

Figure 9. Plots of relative fluorescence intensities and fluorophore lifetimes with concentration of the quencher [Q] for two common regimes, static and dynamic.

In the scenario of dynamic quenching, occurring via a non-bonded (outer sphere) electron transfer (in the frame of Marcus theory), Rehm and Weller established an empirical correlation for the constant of fluorescence quenching 9} with ΔX G based in a study using a series of aromatics (acceptors) and amino- and methoxybenzene derivatives (donors) (Equation 4).45,56,57

# =

# # ‡ ⁄ 5 3 4 u $ $3 4 ⁄ v

, where 3 4 ‡ =

3 45 (

+

3 4 5 (

(

+ (3 4 ‡ (5))(

Equation 4. The Rehm-Weller equation describes the fluorescence quenching constant as a function of Δ_` G for a non-bonded (outer-sphere) electron transfer. 9U and 9U are the rates ACS Paragon Plus Environment

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associated to the formation and dissociation of exciplex D*(A) (or A*(D)), respectively.

GU = is the equilibrium constant for the formation of this complex and Z is the bimolecular q

collision frequency in the exciplex (Z ≈ 1011 s-1), Δ_` Q ‡ is the free energy of activation for

electron transfer and Δ_` Q ‡ (0) is the free energy of activation when Δ_` Q A = 0.

Although the Rehm-Weller equation was successful in describing the experimental results obtained, it was clearly in disagreement with the predicted inverted region of Marcus theory: the electron transfer rates 9X measured (and consequently log ket) did not decrease for

more negative values of Δ_` Q A (Figure 10, see also figure 4).45,56

Figure 10. Plot of the experimental data obtained by Rehm and Weller.45,56 The red curve represents what could be expected according to Marcus theory. (Adapted with permission from reference 56. Copyright 1970, Isr. J. Chem.).

2.5 Molecular Orbital Description of EDA Complexes A quantum-mechanical resonance structure description of the ground state ( ) and the

excited state ( ) of an EDA complex was first provided by Mulliken,18b-c,,58 who described that an EDA complex of composition 1:1 D:A can be written as a linear combination of a “nonbonded” component , and an ionic-bonded component p q (Equation 5): ACS Paragon Plus Environment

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w = z,{ + zp {q ,

yw = zp {q − z,{

Equation 5 . Molecular orbital description of an EDA complex. The ratio = M roughly defines the degree of charge transfer.

Interestingly, the energy gap ℎ> associated to the electronic transition from to of an EDA complex can be written as a linear combination of the ionization potential (IP) of the donor component D, and the electron affinity (EA) of the acceptor component A: ℎ> = − X

+ ω59 (in a first order approximation, ω is the coulombic work term − W , with b ≈ ij

1.6e 10Jf g being the elementary charge and being the mean separation between D and A

in the EDA complex).

18b-c,60

2.6 Importance of the Solvent The nature of the solvent is very important not only for the reactivity of the generated radicals,61 but also for the electron transfer step involved in their formation.62 When an electron-transfer occurs in a polar solvent, a change in the charge distribution follows, which forces a reorientation of the solvent dipoles around the reacting species, donor and acceptor. In this regard, the electronic transfer can be decomposed as a sequence of three events: i) solvent molecules are partly reoriented to form the transition state, ii) a fast electron transfer proceeds between donor and acceptor, and iii) the solvent is again reoriented to form the products. As a consequence of the first and final steps, the electron transfer strongly depends on the interactions derived from the different basicities/ acidities of the solvent, reactants and


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products involved in this process. (In this context, the reorientation of the solvent molecules during the electron transfer step can be neglected because of the Franck-Condon principle).63 Furthermore, the stabilization promoted by the solvent on the excited state can be measured by the shifts observed in the absorption spectra of the solutes. This phenomenon is called solvatochromic shift.64 This effect serves as an indication of the polarizability of the solvent and allows important correlations to a number of thermodynamic and kinetic properties of the system.65

3. SYNTHETIC APPLICATIONS The importance of photochemistry and electron transfer processes has been recognized since the 1950s, but it was only in the 1970s that the first synthetic applications started to appear. This delay is mostly due to the need of a more comprehensive understanding on the nature of reaction intermediates and their kinetics, which was only made possible with the development of highly time-resolved methods of transient analysis, such as flash spectroscopy, chemically induced dynamic nuclear polarization (CIDNP), Raman, and electron spin resonance (ESR) spectroscopy.66 It was only after the development of such techniques that one had the tools to study the kinetic dilemma of whether EDA complexes participated in the reaction as a key intermediate (Scheme 6a) or as a simple spectator (Scheme 6b)67


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Scheme 6. Kinetic dilemma previously faced by EDA complexes. In (a), the EDA complex is an active intermediate associated to product formation. In (b), the EDA complex is not involved in product formation.

Today, compelling evidences have been gathered supporting the direct involvement of EDA complexes in several reactions.67 An important number of these are promoted thermally, and therefore are outside the scope of this review.68 In this section, a selection of photochemical processes involving EDA complexes will be discussed as case-studies, according to their reactivity and chronology, in order to showcase the physicochemical properties previously presented, as well as the synthetic power and sophistication attained by these systems. Stoichiometric processes will be presented first, followed by more recent catalytic enantioselective transformations. All reactions discussed here are reported to be unproductive in the dark.

3.1 Stoichiometric Processes 3.1.1

Metal

Additions

to

Tetracyanoethylene

(TCNE):

Unifying

Thermal

and

Photochemical Mechanisms A TCNE is a very strong electron-acceptor in solution (WXU = 0.24 V vs SCE, in MeCN).46

This makes it an excellent partner to EDA complex formation, and numerous electron-donors have been used for studies in this context.69 In 1979, Kochi and co-workers developed an important study (the first one) comparing the mechanisms of thermal and photochemical electron transfers mediated by EDA complex of tetraalkyltin derivatives 3 (donor) and TCNE 4 (acceptor).70 As the authors highlight, this


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insertion reaction is of special importance as it is a rare example where both thermal and photochemical mechanisms can be separately identified and studied. Physicochemical parameters, including the wavelength of absorption maxima (λmax) from the corresponding CT bands, the equilibrium constants of EDA complex formation (KEDA) and molar absorptivities ( ) were measured. All CT bands were observed to fall at least in some extent in the visible region (with 345nm ≤ λmax ≤ 437nm). In addition, low formation constants (KEDA ≤ 7.7) and molar absorptivity ( ≤ 500 M-1cm-1) were observed, which was attributed to the weak interaction between the σ HOMO of R4Sn 3 and the π∗ LUMO of TCNE 4 (Scheme 7).

Scheme 7. Study on the thermal and photochemical electron transfer between tetraalkyltin derivatives 3 and TCNE 4.

The selectivity for the thermal insertion of TCNE into mixed tetraalkyltin derivatives, such as Et2Me2Sn, is remarked to follow the cleavage of the weakest alkyl-tin bond and could be measured by 1H NMR. Taking into account the statistical component, Et2SnMe2 and EtSnMe3 have demonstrated essentially the same selectivity for Me/ Et addition. Qualitatively, the authors observed the same trend for other tetraalkyltin derivatives.


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When performing the same addition photochemically, Kochi and co-workers employed reaction conditions in which the thermal process did not compete, namely using a 1000W high pressure Hg lamp irradiating at 436nm, at -35oC, in acetonitrile (reaction times typically in the range of 5-10h were observed) The photochemical reaction of TCNE 4 with numerous tetraalkyltin derivatives 3 was performed. Under 436 nm irradiation, Me4Sn (and Et4Sn) adds to TCNE with a quantum yield Φ = 1, where neither TCNE or Me4Sn exhibits any significant absorption (Scheme 8).

Scheme 8. Example of photochemical addition of Me4Sn to TCNE carried out during the work of Kochi and co-workers.

Irradiation of the EDA complex formed between tBu2SnMe2 and TCNE also using this same wavelength (436 nm) leads to the concomitant formation of tert-butyl and TCNE radicals, without any observation of methyl radicals (as noted via electron spin resonance, ESR), while irradiation under a shorter wavelength, 262 nm, produces the same radical species, but with a slightly higher quantum yield. Under visible-light irradiation (again at 436 nm), attempts to observe alkyl radicals using Et4Sn, iBu4Sn and secBu4Sn were unfruitful. Under similar reaction conditions, inside the experimental error, the photochemical reaction of Et2SnMe2 with TCNE exhibits the same selectivity for Et/ Me addition in respect to the corresponding thermal process.


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Overall, these previous observations (added to others),70a allowed the establishment of the EDA complex 5, derived from the combination of R4Sn 3 and TCNE 4, as a common intermediate in both processes, thermal and photochemical. Both reaction pathways have been remarked to proceed via the same general mechanism, that is, an electron transfer event leading to a radical ion pair 7, followed by the fragmentation of unstable tin radical cation (this one being characterized as a “dark” process) to afford triad 8. Finally, triad 8 combines to afford the addition product 6 (Scheme 9).

Scheme 9. Addition of tetraalkyltin compounds 3 to TCNE 4 through the photochemical or thermal process derived from EDA complex 5.

The authors corroborated the involvement of the EDA complex 5 in the electron transfer event (as opposed to a participation as a spectator species, cf. the kinetic dilemma of Scheme 5), because the rate for the overall transformation 9> correlates with the constant of formation G , and the fact that an intimate relationship is noted between both the photochemical and thermal activations in the electron transfer event (Scheme 10).

Scheme 10. The overall process for the electron transfer event involving tetraalkyltin 3 and TCNE 4.


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Based on the same selectivities observed for the insertion of mixed alkyl tin derivatives R12SnR22 into TCNE 4 (e.g. in the case of Me2SnEt2 previously discussed), for the thermal and photochemical processes, as well as the correlation between G and 9> , it was concluded that the same radical ion pair of type 7 must be involved in both processes. Therefore, this study unified the mechanism of thermal and photochemical electron transfer, as it strongly suggests the involvement of common intermediates: 5, 7, and 8.

3.1.2 Radical-Nucleophilic Substitution (SRN1) Reactions Photoinduced SRN1 reactions between nucleophiles (Nu-) and organic halides (RX, often R = Ar) have been well studied, especially in the 1970s,71 when the mechanism of this reaction received considerable attention (Scheme 11).

Scheme 11. Photoinduced SRN1 reaction between Nu- and ArX.

The mechanism of SRN1 reactions is generally accepted as involving a radical chain propagation step, that can be initiated by different events: i) the homolysis of the C-X bond of ArX; ii) SET from the Nu- to the excited ArX, thus generating a radical anion that enters the propagation chain; iii) photoejection of an electron from an excited Nu-; or iv) SET within an excited EDA complex between Nu- and ArX. Depending on the nature of the components, Nuand ArX, as well as on the reaction conditions, any of these mechanisms can be operative (Scheme 12).


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Scheme 12: Presentation of different mechanisms potentially involved in the initiation step of SRN1 reactions.

The initiation mechanism consisting on the photoexcitation of the EDA complex formed between Nu- and ArX can be supported by studies on the dependence of the wavelength of the incident radiation with the quantum yield (Φ). Examples of reactions proceeding via this initiation mechanism have been identified, for instance between acetone enolate 10 and PhI or PhBr in DMSO (Scheme 13a).12 In contrast, the reaction of potassium diethyl phosphite with PhI had two competing pathways identified: the homolysis of PhI and the photoinduced electron transfer (PET) within the EDA complex formed between these two molecules.71,72 Other radical substitutions implicating EDA complexes also include the reactions of para-nitrocumyl chloride 12 with sodium azide 13 and quinuclidine 15, which had quantum yields determined as 6000 and 3.5, respectively (Scheme 13b)73.


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Scheme 13: Examples of reactions where excited EDA complexes have been recognized as active intermediates involved in the initiation step of a radical chain. (a) Reaction between acetone enolate and PhI or PhBr in DMSO and (b) reactions between para-nitrocumyl chloride with sodium azide and quinuclidine.

Furthermore, a initiation step for a radical chain process based on excited EDA complexes has also been postulated in the reaction of nitrile carbanions with aryl halides, such as 2-bromonaphthalene or para-bromobiphenyl in liquid ammonia. The quantum yields observed for these reactions were in the range 7-31 (for λ > 313 nm).74 The intermediacy of an EDA complex has also been proposed in the photochemical cleavage of the C-S bond of ethyl phenyl sulfide in the presence of a diethyl phosphite ion.75

3.1.3 Alkylation and Nitration of Aromatic Compounds with Tetranitromethane In 1987, Kochi demonstrated that the irradiation of the CT band of EDA complex 19, A formed from the transient union of arene 17 (estimated RSTU ≈ -1.5V vs SCE, in MeCN, based

A on Figure 5) with tetranitromethane 18 (WXU ~ 0 vs SCE, in MeCN, see Figure 5), leads to the

solvent-caged triad 20 (Scheme 14a).13 Different molecules, in different classes of aromatic


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compounds exhibited different behaviors toward aromatic substitution, and their charge-transfer properties could be described individually. One of the most remarkable behaviors was observed for para-methylanisole 21, which displayed a striking dichotomous aromatic substitution pattern according to the solvent employed: in acetonitrile, C-2 nitration is the preferred pathway, thus producing nitrocompound 22 as the major outcome, while in dichloromethane, a C-2 alkylation process affords compound 23 selectively (Scheme 14b).13

(a) ArH

+ C(NO2)4

17

[ArH, C(NO2)4] EDA

18

h

CT

[Ar

, HC(NO2)3 , NO2 ]SOLV

19

20

(b) 500W Hg lamp ( > 425nm) 30oC, 6.5h MeCN HC(NO2)3

OMe +

O2N

OMe NO2 22: 66%y

NO2

O2N NO2 21

18 500W Hg lamp ( > 425nm) 30oC, 5h DCM

HNO2

OMe NO2 NO2 NO2 23: 95%y

Scheme 14: (a) Solvent-caged triad generated upon CT band irradiation of the EDA complex formed between an arene 17 and tetranitromethane 18. (b) Example of reaction dichotomy, depending on the solvent employed. In acetonitrile, nitration is the major process, which affords compound 22. In dichloromethane, an alkylation process affords the major compound 23.

The reason for such a dichotomy in outcome, can be rationalized based on the solvent polarity. In a polar solvent such as acetonitrile, the collapse of the radical-pair from triad 20 is the efficient pathway that leads to nitro arene 22. However, in a low polarity solvent, such as dichloromethane (hexane and benzene also show the same trend), the efficient pathway is the ion-pair collapse (from triad 20), that leads to alkylated compound 23.13


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3.1.4 Trifluoromethylation of Enolsilanes In 2008, the MacMillan group has established a novel strategy for the α-alkylation of aldehydes, named “photoredox organocatalysis”. This methodology consisted on the combination of a chiral imidazolidinone catalyst and a ruthenium photoredox co-catalyst to generate electrophilic alkyl radicals that could be trapped by a catalytically generated chiral enamine, under ambient conditions.76 Later, in 2011, the group envisioned that a similar A approach could be extended to the trifluoromethylation (WXU CF3I = -1.22V vs SCE, in DMF)9b,77

of enolsilanes and silylketene acetals. Although the photoredox methodology could be successfully

applied

to

a

variety

of

ketone-derived

enolsilanes

25

(e.g.

A RSTU

1-(Trimethylsiloxy)cyclohexene ≈ -1.2V vs SCE, in MeCN)78 to produce trifluoromethylated ketones 26 (Scheme 15a), the silylketene acetal of δ-valerolactone and other silylenolethers 27, derived from esters and amides, could also undergo trifluoromethylation to afford the corresponding trifluoromethylated products 28, without any catalyst (Scheme 15b).79 In this context, the authors suggested that this reaction could be occurring via photoexcited EDA complexes, but did not substantiate such hypothesis with any experimental evidences.


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Scheme 15. Trifluoromethylation of enolsilanes under visible light irradiation via (a) photoredox catalysis and (b) photoexcitation of EDA complexes (bpy: 2,2’-bipyridine, TBS: tertbutyldimethylsilyl).

3.1.5 Biaryl Coupling In 2013, the Chatani group reported the development of an Ir-photocatalyzed arylation A Ph2I+ = -0.2V vs SCE, method of arenes and heteroarenes 29 with diaryl iodonium salts 30 (WXU

in H2O)80 using white LEDs, λ = 400-750 nm (Scheme 16). While investigating the scope of A heteroarene acceptors, the authors noticed that the arylation of some pyrroles (RSTU ≈ -0.46V vs

SCE, in MeCN, see figure 5) could be achieved even in the absence of any photocatalyst. UV/vis spectroscopic analysis of the reaction mixture revealed a new absorption band, the CT band, attributed to the formation of an EDA complex between the arene and the diaryl iodonium reagent.14 The results obtained demonstrated that the nature of the aryl groups on the iodonium salt 30 had no dramatic effect in the reactions performed in the presence of the Ir-photocatalyst (yields in parentheses in Scheme 16) but the same could not be said for the reactions proceeding


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via EDA complexes. It is possible to note that the diaryl iodonium salts bearing electronwithdrawing groups (e.g. Cl 31a, CF3 31b) afforded the corresponding products 31 in higher yields in relation to the electron-donating substituents (e.g. Me 31c). This observation can be presumably related to the formation of stronger encounter complexes, when electronwithdrawing groups are present in the diaryl iodonium acceptor.

Scheme 16. EDA complex-photoinduced arylation of arenes using iodonium salts, under irradiation with white LEDs (λ = 400-750 nm). In parentheses are the yields of the corresponding photocatalyzed process promoted by [Ir(ppy)2(bpy)]PF6 (1 mol%) (ppy: 2-phenylpyridine, bpy: 2,2’-bipyridine).

3.1.6 Aromatic Perfluoroalkylation of α-Cyano Arylacetates In 2014, the Melchiorre group developed a methodology for the aromatic perfluoroalkylation of α-cyano arylacetates 35 employing perfluoroalkyl iodides 36 via the visible-light irradiation of the EDA complex formed between these two components (Scheme 17).81 The development of a strong yellow-orange solution, upon the mixture of reagents, indicated the formation of an EDA complex, which could be confirmed by the bathochromic


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shift in the visible region of the UV/vis absorption spectrum, from 360 nm for the isolated enolate of 35, to around 400 nm for the mixture. However, differently from an anticipated solvent-cage combination of two radicals, in this case, an approximate 2:1 ratio for the para-: ortho- substituted aromatic compound 37 and an above-unit quantum yield (Φ = 3.8) for this process suggested that an homolytic aromatic substititution (HAS) pathway, proceeding via a radical chain mechanism, was occurring. Interestingly, during the optimization studies of this transformation, the authors remarked only partial conversion of the starting material 35 into product 37, even after prolonged reaction times. In a series of control experiments, they realized that the anion of the perfluoroalkylated compound 37 was also being formed and it strongly absorbed in the visible region, which inhibited the reaction completion. In order to address this issue, an additional organic phase of tetradecafluorohexane (be\ ) was added (in a 1:5 ratio to MeCN) in order to sequestrate the generated perfluoroalkylated compound to a different phase. The choice of a biphasic system and 1,1,3,3-tetramethyl guanidine (TMG) improved the yields and shortened the reaction times, thus establishing the optimized conditions for this transformation (Scheme 17).


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Scheme 17. Aromatic perfluoroalkylation and trifluoromethylation of α-cyano arylacetates via visible-light irradiation of EDA complexes. (TMG: 1,1,3,3-tetramethylguanidine, be\ : tetradecafluorohexane, ATRA: atom transfer radical addition, SET: single electron transfer).

The protocol could be applied for a variety of α-cyano arylacetates bearing electrondonating substituents. The substitution pattern on the aryl ring of α-cyano arylacetates 35 turned out to be an important factor influencing selectivity: para-substituents leads to selective perfluoroalkylation, while substituents at ortho or meta positions generate lower selectivities. Corroborating a HAS pathway, the presence of electron-withdrawing groups on the aromatic moiety of 35 has a pronounced negative effect on the efficiency of the reaction (yields lower than 40%).


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3.1.7 Alkylation of Indoles In 2015, the Melchiorre group reported the alkylation of indoles via the visible light irradiation of EDA complexes formed between indoles 42 and an alkylating agent (benzyl or phenacyl bromides) 43. The UV/vis absorption spectrum of the solution containing the two components exhibits a bathochromic shift above 430 nm, where neither of individual components absorbs. An intense orange/red color is also observed for the reaction mixture. These facts suggest the involvement of an EDA complex between the indole 42 and the alkylating agent 43. Regarding the scope of the reaction, electron-deficient benzyl and phenacyl bromides reacted satisfactorily with 3-methyl indole. In the same manner, a wide range of 2- and 3substituted 1H-indoles and tetrahydrocarbazoles successfully reacted as donors. It is interesting A = -0.69V vs SCE, in MeCN, to note that, based on the redox potentials of 3-methyl indole (RSTU

A see Figure 5) and phenacyl bromide (WXU = -0.73V vs SCE, in MeCN, see Figure 5) it comes as

no surprise that no electron transfer occurs in the dark. One can see that in order for this photochemical process to become thermodynamic viable, the electrostatic work term (c), but most importantly, the 0-0 energy difference between vibrational ground and first excited state (ΔA,A ) of Equation 2 must compensate an important potential difference associated to Δ A = −1.42V.

The alkylation can be performed in either C-2 or C-3 positions of the indole ring, depending on which position is available. Interestingly, when both positions are monosubstituted, a second bonding event is preferred in C-3. In addition, polyfunctionalized indoles can be also engaged in radical cascades, thus successfully participating in alkylating cyclization sequences.


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In the proposed mechanism, the reaction starts with the formation of an EDA complex between the donor, indole 42, and the acceptor, bromide 43. Visible light irradiation of the transiently formed complex 45 induces an electron transfer, which is accompanied by the fragmentation of the alkyl bromide, thus generating a radical pair 46. This radical pair gets combined to produce a new C-C bond. After a re-aromatization step assisted by the base, the corresponding alkylated indole 44 is produced (Scheme 18).82

Scheme 18. Alkylation of indoles promoted by visible light irradiation of EDA complexes formed between indoles 42 and bromides 43.

In addition, the authors were able to isolate and characterize the EDA complex involved in the synthesis of 44a by X-ray analysis. A 1:1 composition of D:A was established, with the


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distance measured between planes of the dyad of ca. 3.33Å, significantly shorter than the usual van der Waals radii between two aromatic molecules, of 3.40 Å83 (Figure 11).

(a)

(b)

Figure 11. (a) ORTEP plot (50% probability) of EDA complex formed during the synthesis of 44a. (b) Crystal structure projection along the b axis and average interplanar spacing calculated based on the distances shown. C: gray, H: white, Br: orange, N: blue, O: red. (Reprinted with permission from reference 82. Copyright 2015, Wiley-VCH)

Remarkably, in the previously mentioned processes (cf. paragraphs 3.1.5, 3.1.6 and 3.1.7), it is possible to note that the fragmentation event of the electron acceptor (mesolysis) is a key event to circumvent the competing fast back electron transfer, which would regenerate the corresponding starting materials. It is the departure of iodobenzene (3.1.5), the iodide (3.1.6) and the bromide (3.1.7) ions, respectively, that allows in each case the formation of the productive radical species that combine with other radicals in order to create new C-C bonds.

3.1.8 Synthesis of 3-Methyl-Indoles and 3-Methyl Oxindoles In 2015, a report from the Paixão group described the synthesis of indole and oxindole scaffolds via the visible-light irradiation of an EDA complex formed between a functionalized


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aniline precursor 47, or 48, and tris(trimethylsilyl)silane 49 to form the corresponding cyclization products 50 or 51, respectively. A variety of 2-halobenzenesulfonamides containing terminal alkynes were employed (45-85% yield). Substrates bearing electron-donating groups at different positions of the aromatic ring generally afforded the corresponding indoles in higher yields than those bearing electron-withdrawing groups. This same protocol can be also extended for the synthesis of oxindoles, which can be accessed from the use of N-protected 2halophenylacrylamides, in yields ranging from 45 to 80%. The reaction mechanism is proposed to occur via an EDA complex 52 (which is corroborated by the observation of a change in the color of reaction mixture upon light irradiation from transparent to yellow/ orange), which under visible light irradiation, is excited and transfers energy to another molecule of tris(trimethylsilyl)silane 49. This silane undergoes homolysis to produce a silyl radical that abstracts a halide from substrate 47 (or 48), thus furnishing the corresponding aryl radical 53. This radical undergoes a 5-exo-trig cyclization, thus leading to intermediate 54. Hydrogen abstraction from ethanol and isomerization leads to the corresponding heteroaromatic 50 (or 51) (Scheme 19).84


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Scheme 19. Indoles 50 or oxindoles 51 can be accessed via the visible-light irradiation of EDA complexes 52 formed between functionalized anilines 47 or 48, respectively, and tris(trimethylsilyl)silane 49.

3.1.9 α-CH Functionalization of Tetrahydroisoquinolines α In 2015, Zeitler and co-workers have reported a photocatalyst-free protocol for the photochemical oxidation of tetrahydroisoquinoline (THIQ) 55 with BrCCl3, thus gaining access to the corresponding iminium ion intermediate 56, that can be trapped with different classes of nucleophiles 57, in order to produce the corresponding products 58.85 Preliminary mechanistic experiments suggest two plausible pathways for this oxidation process: i) homolytic cleavage of BrCCl3 under blue light irradiation (due to a weak C-Br bond, which has a bond dissociation energy, BDE = 55.3 kcal.mol-1), followed by a number of possible atom transfers/ oxidation steps or ii) an electron transfer event within an EDA complex 59,


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formed between BrCCl3 and tetrahydroisoquinoline 56 (characterized by a blue coloration and bathochromic shift in the UV/ vis spectrum, in the region 450-650 nm), to afford radical cation 60. This intermediate can either lose a proton to afford radical intermediate 61, which in turn can undergo an atom transfer or oxidation event to produce compound 56, or it can undergo hydrogen atom transfer (HAT) to directly produce the same intermediate 56. Within these possible scenarios, chloroform was observed as a byproduct in the reaction mixture, pointing out that HAT must be operative (Scheme 20).

Scheme 20: Photocatalyst-free protocol for the photochemical α-functionalization of tetrahydroisoquinolines 55.


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3.1.10 A Cyclization Procedure Involving N Centered Radicals In 2015, the Leonori group reported visible-light promoted hydroimination and iminohydroxylation reactions via the intramolecular cyclization of N-centered radicals, starting A from aryl hydroxymes 62 (WXU ≈ -0.55V vs SCE, in MeCN). The process can be triggered

either by an electron transfer promoted by eosin Y ( A (¡ ∗ / ¡ ) = 1.11V vs SCE, in MeCN), or by an electron transfer within the EDA complex 68 formed between the labile electron-poor A aryloxy group and triethylamine (RSTU = -0.76V vs SCE, in MeCN, see Figure 5), which serves

as an external reducing agent. Interestingly, a dichotomy in the reaction outcome can be attained, depending if eosin Y or triethylamine is employed, thus leading to compound 63 or 64, respectively, under the appropriate reaction conditions (Scheme 21).15

Scheme 21. Cyclization of N-centered radicals under the action of eosyn Y or under the frame of EDA complexes (HAT: hydrogen atom transfer).


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The group was also able to determine the stoichiometry of EDA complex 68 using Job’s method,86 thus revealing a 1:1 ratio for D:A. Titration experiments afforded a value for KEDA ~ 22 M-1 and DFT calculations confirmed that absorption in the region of 440 nm is caused by a transition from the nitrogen lone pair (n) to the π* orbital of the 2,4-dinitroaromatic moiety on the oxime. Concerning the scope of the reaction, a wide range of aryl oximes, bearing both electron-donating and withdrawing groups, could be successfully applied to this protocol, to afford the corresponding products in moderate to high yields (29-85%).

3.2 Catalytic Processes 3.2.1 Organocatalyzed Enantioselective α-Alkylation of Aldehydes In 2013, the Melchiorre group reported an important breakthrough in the interface of two fields, aminocatalysis and photochemistry. The group disclosed that catalytically generated chiral enamines 75, accessed from the condensation of the corresponding aldehydes 71 and the aminocatalyst 74, can participate in EDA complex formation with electron-poor alkylating agents 72. Under visible-light irradiation, this EDA complex 76 undergoes electron transfer to afford two ion-radicals confined in a solvent cage 77. After fragmentation of the alkyl halide, two radicals also transiently locked in a solvent cage 78 meet in proximity, thus rapidly combining to afford iminium ion 79. Finally, hydrolysis affords highly enantioenriched αalkylated aldehydes 73 (Scheme 22).16 In terms of substrate scope, not only a number of simple aldehydes could be successfully α-alkylated, but also the formation of α-all-carbon quaternary stereocenters and the remote functionalization at the γ-position of unsaturated aldehydes was also possible (Scheme 22).


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Scheme 22. A photochemical protocol using an EDA complex for the α-alkylation of aldehydes with electron deficient bromides.

Evidences for the formation of EDA complexes were obtained by the observation of color change in the reactions, which became yellow–orange immediately after mixing the reagents and the aminocatalyst in MTBE. A bathochromic shift to the visible region of the


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UV/vis absorption spectrum was also measured for numerous dyads D, A. For instance, the shift for the pair composed of an enamine derived from butanal and phenacyl bromide was observed up to 465 nm. Interestingly, the UV/vis absorption spectra recorded for the EDA complexes of some of the electron poor bromides with extended enamines (i.e. di- and trienamines) were all red shifted (~ 500-600 nm) when compared to simple enamines, bearing only alkyl chains. In contrast to the previous reported protocol, the α-alkylation of enamines with bromomalonates proceeds on a very different mechanistic basis, even if under similar reaction conditions. Indeed, under visible light irradiation, the catalytically generated enamine 75 can reach an electronically excited state (via local band irradiation, cf. 82) and act as a photosensitizer. As a consequence, quenching by bromomalonate 80 starts a radical chain reaction that is responsible for the formation of the corresponding α-alkylated aldehydes 81. Therefore, this process does not occur via the photoexcitation of an EDA complex (Scheme 23).87,88

Scheme 23: An example of photocatalyst-free visible light induced α-alkylation of aldehydes, that does not proceed via an EDA complex (TIPS: triisopropylsilyl).


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3.2.2 Organocatalyzed Enantioselective α-Alkylation of Ketones Extending their own previous work, the Melchiorre group developed a photoinduced protocol for the α-alkylation of ketones. The increased steric demand of ketones when compared to aldehydes limits the use of bulky secondary amine catalysts, thus making necessary the use of chiral primary amines, such as 86.89 During the development of this alkylation protocol with 2,4dinitrobenzyl bromide (a particular case of 84) and different cyclic ketones 83 (aliphatic ketones were less reactive in this context), the authors observed the deactivation of the catalyst under visible light irradiation when employing a 23W household light source at room temperature. However, when the temperature was lowered to 0 oC, the catalyst activity could be preserved (Scheme 24).90

Scheme 24. Organocatalyzed α-alkylation of ketones proceeding via light irradiation of EDA complexes formed between catalytic ketone-derived enamines and electron-poor alkylating agents (TFA: trifluoroactic acid).


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When the light source was changed to a Xenon lamp (300W, λ = 300-600 nm), other alkylating agents could be employed and the reactions could be conducted at room temperature. In a control experiment, the use of a cut-off filter at 385nm on a Xe lamp (i.e. only 600 nm > λ > 385 nm are allowed), prevented the alkylation with phenacyl bromides, thus pointing out that the UV range of the lamp is essential for this transformation. In contrast, the alkylation protocol with 2,4-dinitrobenzyl bromide only uses the visible region of the spectrum, because in this case, the employment of the same filter does not inhibit the reaction. The EDA complex formation was observed by means of the immediate appearance of a marked yellow color after mixing a toluene solution of the cinchona catalyst 86 with TFA, cyclohexanone 83a and 2,4-dinitrobenzylbromide 84a. In addition, the UV/vis absorption spectrum of this mixture showed a bathochromic displacement to the visible region, moved up to 400 nm. The study of the reaction scope conducted at 0 °C showed that the method could tolerate six-membered ring ketones with different substitution patterns, as well as five- and sevenmembered rings with moderate to good yields and stereoselectivities (38-94% yield, 62-95% ee). Both electron-deficient benzyl and phenacyl bromides carrying different functional groups can be applied in this reaction, in order to produce the corresponding compounds 85 in moderate yields (40-73%) and good to excellent stereoselectivities (76-92% ee). In a more complex setting, 5a-cholestan-3-one 83b can also be alkylated with 2-bromo acetophenone 84b. The corresponding product 85g is obtained in moderate yield (47%) but excellent regio- and stereoselective control (dr > 20:1) (Scheme 25).


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Scheme 25. Photochemical alkylation protocol applied to steroid 83b using phenacyl bromide 84b.

3.2.3 Organocatalyzed Perfluoroalkylation of β-Ketoesters. In 2015, the Melchiorre group also reported an efficient protocol for the asymmetric photo-organocatalytic perfluoroalkylation of β-ketoesters. In this protocol, catalytically generated chiral enolates 91, formed under phase transfer catalysis (PTC) , serves as the electrondonors and perfluoroalkyl iodides 36, as electron-acceptors. They combine in solution to form an EDA complex 90, as evidenced by the yellow solution and the corresponding UV/vis absorption spectrum, which exhibits a new CT band in 400-480 nm (obtained for the mixture of 87a, IC6F13 and catalyst 89). Visible-light irradiation of the ensemble allows the electron transfer between the components, consequently promoting the fragmentation of the alkyl iodide, which initiates a radical chain propagation step. In the sequence, the perfluoroalkyl radical 39 undergoes addition inside a chiral environment of electron rich olefine 91. The new radical formed 92 attacks a molecule of perfluoroalkyl iodide 36, which then fragments to regenerate the perfluoroalkyl radical 39 and propagates the radical chain. The intermediate 93 decomposes to provide the corresponding perfluoroalkylated β-ketoester 88 and regenerates the catalyst for a new cycle (Scheme 26).91


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Scheme 26. Photo-organocatalyzed enantioselective perfluoroalkylation of β-ketoesters 87 via catalytically generated chiral EDA complexes.

The use of a 300W Xe lamp with a band-pass filter for λ > 400 nm did not significantly change the reaction efficiency. This result refutes the hypotheses of a possible homolytic cleavage of the C-I bond promoted by high-energy photons, or the direct excitation of the chiral enolate 91, which does not absorb above 400 nm. Remarkably, the authors observed that the perfluoroalkyl radical 39, generated under the reaction conditions, could also add to the double bond of the original catalyst. This new catalyst obtained in situ was also identified as a competent promoter of the reaction, albeit producing slightly lower yields and enantiomeric excesses.


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In the study of the scope of β-ketoesters 87, the substrates bearing electron-withdrawing groups (e.g. F, Cl, Br, CF3) in different positions of the indanone aromatic ring reacted satisfactorily and the corresponding products were isolated in moderate to good yields (60-71%) and high stereoselectivities (82-90% ee). On the other hand, substrates possessing electrondonating groups (e.g. Me, OMe) preserved the levels of stereoselectivity (78-86% ee), but were produced in lower yields (38-55%).

4. CONCLUSIONS Mulliken provided the first pieces of evidences associated to the characterization and understanding of EDA complexes. Standing on his shoulders, Kochi and others performed comprehensive studies, thus establishing a sound theoretical basis, in which today, many synthetic transformations can rely on. The recent contributions of the Melchiorre group in this area are remarkable as they establish new possibilities for reaction design in asymmetric photo(organo)catalysis. If in some occasions, reactions proceeding through the photoexcitation of EDA complexes are not as efficient as photoredox catalyzed processes (e.g. biaryl coupling with diaryliodonium salts),14 they can also display an orthogonal nature (e.g. the N-centered radical cyclization).15 This potential complementarity can possibly elevate this novel reactivity to an unique position in organic synthesis.


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AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected]

ACKNOWLEDGMENTS C.G.S.L. and T.M.L. are grateful to CAPES and CNPq (158438/2012-4) for their PhD fellowships. M.W.P. acknowledges financial support from FAPESP (2014/50249-8 and 2015/17141-1), CNPq (INCT-Catálise and 475448/2013-8) and CAPES. I.D.J acknowledges FAPESP (2013/09680-4) and CNPq (458416/2014-2) for research grants. The authors would like to thank Prof. Jackson D. Megiatto Jr. (Unicamp, Brazil) for helpful comments on the manuscript.

Notes The authors declare no competing financial interest.

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(3) For examples in the purification of water, see: (a) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Nature 2008, 452, 301-310. (b) Chong, M. N.; Jin, B.; Chow, C. W. K.; Saint, C. Water Res. 2010, 44, 2997-3027. (c) Pichat, P. Photocatalysis and Water Purification: from Fundamentals to Recent Applications. Wiley-VCH: Weinheim, 2013. (4) For examples in the generation of energy via CO2 reduction, see: (a) Sahara, G.; Ishitani, O. Inorg. Chem. 2015, 54, 5096-5104. (b) Sastre, F.; Puga, A. V.; Liu, L.; Corma, A.; García, H. J. Am. Chem. Soc. 2014, 136, 6798-6801. (c) Mao, J.; Li, K.; Peng, T. Catal. Sci. Technol. 2013, 3, 2481-2498. (5) For examples in H2 production from water, see: (a) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253-278. (b) Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. Renew. Sust. Energy Rev. 2007, 11, 401-425. (c) Zou, Z.;Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625-627. (d) Maeda, K.; Domen, K. J. Phys. Chem. Lett. 2010, 1, 2655-2661. (6) For examples in H2 production from methanol, see: (a) Xu, C.; Yang, W.; Guo, Q.; Dai, D.; Chen, M.; Yang, X. J. Am. Chem. Soc. 2013, 135, 10206-10209. (b) Miwa, T.; Kaneco, S.; Katsumata, H.; Suzuki, T.; Ohta, K.; Chand Verma, S.; Sugihara, K. Int. J. Hydrogen Energy 2010, 35, 6554-6560. (c) Chiarello, G. L.; Aguirre, M. H.; Selli, E. J. Catal. 2010, 273, 182-190. (d) Wu, N.-L.; Lee, M.-S. Int. J. Hydrogen Energ. 2004, 29, 1601-1605. (7) (a) Albini, A.; Fagnoni, M. Green Chem. 2009, 11, 239-249. (b) Albini, A.; Fagnoni, M. Handbook of Synthetic Photochemistry. Wiley-VCH: Weinheim, 2010. (8) Yoon, T.; Ischay, M. A.; Du, J. Nature Chem. 2010, 2, 527-532. (9) (a) Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102-113. (b) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322-5363.


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(10) Nicewicz, D. A.; Nguyen, T. M. ACS Catal. 2014, 4, 355-360. (11) (a) Kosower, E. M. J. Am. Chem. Soc. 1956, 78, 5700–5701. (b) Nagakura, S. J. Am. Chem. Soc. 1958, 80, 520–524. (12) Fox, M. A.; Younathan, J.; Fryxell, G. E. J. Org. Chem. 1983, 48, 3109-3112. (13) Sankararaman, S.; Haney, W. A.; Kochi, J. K. J. Am. Chem. Soc. 1987, 109, 7824-7838. (14) Tobisu, M.; Furukaxa, T.; Chatani, N. Chem. Lett. 2013, 42, 1203-1205. (15) Davies, J.; Booth, S. G.; Essafi, S.; Dryfe, R. A. W.; Leonori, D. Angew. Chem. Int. Ed. 2015, 54, 14017-14021. (16) Arceo, E.; Jurberg, I. D.; Álvarez-Fernández, A.; Melchiorre, P. Nature Chem. 2013, 5, 750756. (17) (a) Kochi, J. K. Angew. Chem. Int. Ed. 1988, 27, 1227-1338. (b) Patz, M.; Fukuzumi, S. J. Phys. Org. Chem. 1997, 10, 129-137. (18) (a) Mulliken, R. S. J. Am. Chem. Soc. 1950, 72, 600-608. (b) Mulliken, R. S. J. Phys. Chem. 1952, 56, 801-822. (c) Mulliken, R. S. J. Am. Chem. Soc. 1952, 74, 811-824. (19) (a) Marcus, R. A. Angew. Chem. Int. Ed. 1993, 32, 1111-1222. (b) Marcus, R. A.; Rev. Modern Phys. 1993, 65, 599-610. (20) Taube, H. Angew. Chem. Int. Ed. 1984, 23, 329-394. (21) (a) Hush, N. S. Trans. Faraday Soc. 1961, 57, 557-580. (b) Hush, N. S. IntervalenceTransfer Absorption. Part 2. Theoretical Considerations and Spectroscopic Data. In Progress in Inorganic Chemistry; Cotton, F. A. (Eds.) Wiley-VCH: New York, 1967. Vol. 8, pp. 391-443. (22) (a) Hubig, S. M.; Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc. 1996, 118, 3842-3851. (b) Kochi, J. K. Pure Appl. Chem. 1991, 63, 255-264.


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(23) For instance: (a) Gould, I. R.; Moody, R.; Farid, S. J. Am. Chem. Soc. 1988, 110, 72427244. (b) Cave, R. J.; Edwards, S. T.; Kouzelos, J. A. J. Phys. Chem. B 2010, 114, 14631-14641. (c) Flurry Jr., R. L. J. Phys. Chem. 1969, 73, 2111-2117. (d) Levy, D.; Arnold, B. R. J. Phys. Chem. A 2005, 109, 8572-8578. (e) Szwarc, M. Acc. Chem. Res. 1972, 5, 169-176. (f) Andrews, L. J. Chem. Rev. 1954, 54, 713-776. (g) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100, 13148-13168. (24) Tucker, J. W.; Stephenson, C. J. Org. Chem. 2012, 77, 1617-1622. (25) Foster, R. J. Phys. Chem. 1980, 64, 2135-2141. (26) Zhao, G. J.; Han, K. L. Acc. Chem. Res. 2012, 45, 404-413 and references therein. (27) Among others, such interactions can be seen as being of intermediate strength, between one extreme of a scale, attributable to forces of van der Waals, which values typically less than 1 kcal.mol-1, and the other extreme, where chemical bonds are, which values typically

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kcal.mol-1 or more. (28) Rosokha, S. V.; Kochi, J. K. Acc. Chem. Res. 2008, 41, 641-653. (29) Mulliken, R.S.; Person, W. B. Molecular Complexes. Wiley-VCH: New York, 1969, pp. 78. (30) Mori, T.; Inoue, Y. Chem. Soc. Rev. 2013, 42, 8122-8133 and references therein. (31) Such exciplexes can be studied by time-resolved picosecond spectroscopy. See for instance: (a) Hilinski, E. F.; Masnovi, J. M.; Amatore, C.; Kochi, J. K.; Rentzepis, P. M. J. Am. Chem. Soc. 1983, 105, 6167-6168. (b) Masnovi, P. J. M.; Kochi, J. K.; Hilinski, E. F.; Rentzepis, P. M. J. Am. Chem. Soc. 1986, 108, 1126-1135. (32) Mulliken, R.S.; Person, W. B. Molecular Complexes. Wiley-VCH: New York, 1969, pp. 4358.


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(33) For examples of X-ray structures of EDA complexes, see: (a) Vasilyev, A. V.; Lindeman, S. V.; Kochi, J. K. New J. Chem. 2002, 26, 582-592. (b) Williams, R. M.; Wallwork, S. C. Acta Cryst. 1967, 22, 899-906. (c) Kim, J. H.; Hubig, S. M.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2001, 123, 87-95. (d) Blackstock, S. C.; Poehling, K.; Greer, M. L. J. Am. Chem. Soc. 1995, 117, 6617-6618. (e) Masnovi, J.; Baker, R. J.; Towns, R. L. R.; Chen, Z. J. Org. Chem. 1991, 56, 176-179. (f) Berionni, G.; Bertelle, B.-A.; Marrot, J.; Goumont, R. J. Am. Chem. Soc. 2009, 131, 18224-18225. (34) (a) Zusman, L. D. Chem. Phys. 1980, 49, 295-304. (b) Zwickel, A.; Taube, H. J. Am. Chem. Soc. 1961, 83, 793-796. (35) (a) Taube, H. Adv. Inorg. Chem. Radiochem, 1959, 1, 1-53. (b) Haim, A. Acc. Chem. Res. 1975, 8, 264-272. (c) Schwarz, C. L.; Endicott, J. F. Inorg. Chem. 1995, 34, 4572-4580. d) Haim, A.; Mechanisms of Electron Transfer Reactions: The Bridged Activated Complex. In Progress in Inorganic Chemistry: An Appreciation of Henry Taube. Lippard, S. J. (Ed.) Wiley-VCH, 1983; Vol. 30, pp 273-357. (36) (a) Marcus, R. A. J. Phys. Chem. 1968, 72, 891-899. (b) Marcus, R. A. J. Chem. Phys. 1956, 24, 966-978. (c) Marcus, R. A.; Zwolinski, B. J.; Eyring, H. J. Phys. Chem. 1954, 58, 432-437. (d) Marcus, R. A. J. Chem. Phys. 1957, 26, 867-871. (e) Marcus, R. A. Discuss. Faraday Soc. 1960, 29, 21-31. (37) The resonance integral ¢ represents the electronic coupling term. This parameter depends on the overlap of wave functions in the initial and final states of the electron transfer process. For more details, see: Kavarnos, G. J.; Turro, N. J. Chem. Rev.1986, 86, 401-449. (38) See for instance: Marcus, R. A. J. Electroanal. Chem. 2000, 483, 2-6 and references therein.


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(39) In the context of metal complexes: (a) Taube, H.; Myers, H. J.; Rich, R. L. J. Am. Chem. Soc. 1953, 75, 4118-4119. (b) Candlin, J. P.; Halpern, J.; Trimm, D. L. J. Am. Chem. Soc. 1964, 86, 1019-1022. (c) Candlin, J. P.; Halpern, J. Inorg. Chem. 1965, 4, 766-767. (d) Taube, H. Pure Appl. Chem. 1970, 24, 289-305. e) Taube, H. Science 1984, 226, 1028-1036. (40) In the context of organic molecules, see for instance: Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2007, 129, 3683-1697. (41) See for instance: (a) Asahi, T.; Mataga, N.; Takahashi, Y.; Miyashi, T. Chem. Phys. Lett. 1990, 171, 309-313. (b) Asahi, T.; Mataga, N. J. Phys. Chem. 1989, 93, 6575-6578. (c) Asahi, T.; Mataga, N. J. Phys. Chem. 1991, 95, 1956-1963. (d) Asahi, T.; Ohkohchi, M.; Mataga, N. J. Phys. Chem. 1993, 97, 13132-13137. (42) (a) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am. Chem. Soc. 1990, 112, 4290-4301. (b) Burget, D.; Jacques, P.; Vauthey, E.; Suppan, P.; Haselbach, E. J. Chem. Soc., Faraday Trans. 1994, 90, 2481-2487. (43) Redox potentials can be readily measured by cyclic voltammetry. See: Heinze, J. Angew. Chem. Int. Ed. 1984, 831-847. (44) See for instance: (a) Hamann, C. H.; Hamnett, A.; Vielstich, W. (Eds.) Electrochemistry Wiley-VCH: Weinheim, 2007, 2nd ed., pp.77-86. (b) Thompson, M. L.; Kateley, L. J. J. Chem. Educ. 1999, 76, 95-96. (45) Klán, P.; Wirz, J. Photochemistry of Organic Compounds: From Concepts to Practise Wiley: West Sussex, 2009, 1st ed., pp. 184-191. (46) Rathore, R.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 1997, 119, 9393-9404. (47) Fuchigami, T.; Inagi, S.; Atobe, M. (Eds.) Fundamentals and Applications of Organic Electrochemistry: Synthesis, Materials, Devices. Wiley-VCH, 2015, pp. 218-219.


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(48) Tanner, D. D.; Singh, H. K. J. Org. Chem. 1986, 51, 5182-5186. (49).Koch, D. A.; Henne, B. J.; Bartak, D. E. J. Electrochem. Soc. 1987, 134, 3062-3067. (50) Méndez-Hernández, D. D.; Gillmore, J. G.; Montano, L. A.; Gusta, D.; Moore, T. A.; Moore, A. L.; Mujica, V. J. Phys. Org. Chem. 2015, 28, 320-328. (51) Keech, P. G.; Chartrand, M. M. G.; Bunce, N. J. J. Eletroanal. Chem. 2002, 534, 75-78. (52) Fry, A. J. Synthetic Organic Electrochemistry, 1989, 2nd ed., Wiley-VCH pp. 95-98. (53) Anslyn, E. V.; Dougherty, D. A.; Modern Physical Organic Chemistry University Science Books, 2006, pp. 936-953. (54) Klán, P.; Wirz, J. Photochemistry of Organic Compounds: From Concepts to Practise Wiley: West Sussex, 2009, 1st ed., pp. 121-127. (55) Lakowicz, J. R. Principles of Fluorescence Quenching Springer: New York, 2006, 3rd ed., pp. 277-284. (56) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259-271. (57) (a) Rehm, D.; Weller, A.; Ber. Bunsen-Ges. Phys. Chem. 1969, 73,834-839. (b) Knibbe, H.; Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 839-845. (c) Knibbe, H.; Rehm, D.; Weller, A.; Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 257-263. (58) Bender, C. J. Chem. Soc. Rev. 1986, 15, 475-502. (59) This expression is especially applicable in the gas phase for weak EDA complexes. In A A solution, the corresponding expression is approximated to ℎ£> = RSTU () − WXU () + ¤ b ,

when the energies of solvation of related species are approximately the same. See: (a) Rosokha, S. V.; Kochi, J. K. J. Org. Chem. 2002, 67, 1727-1737. (b) Rabinowitch, E. Rev. Mod. Phys. 1942, 14, 112-131. (60) McGlynn, S. P. Chem. Rev. 1958, 58, 1113-1156.


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Chem. 1990, 55, 606-624. (d) Frey, J. E.; Andrews, A. M.; Combs, S. D.; Edens, S. P.; Puckett, J. J.; Seagle, R. E.; Torreano, L. A. J. Org. Chem. 1992, 57, 6460-6466. (e) Frey, J. E.; Aiello, T.; Beaman, D.N.; Combs, S. D.; Fu, S.-L.; Puckett, J. J. J. Org. Chem. 1994, 59, 1817-1830. (f) Frey, J. E.; Aiello, T.; Beaman, D.N.; Hutson, H.; Lang, S. R.; Buckett, J. J. J. Org. Chem. 1995, 60, 2891-2901. (g) Spange, S.; Maenz, K.; Stadermann, D. Liebigs Ann. Chem. 1992, 10, 10331037. (h) Britt, B. M.; McHale, J. L.; Friedrich, D. M.J. Phys. Chem. 1995, 99, 6347-6355. (i) Kulinowski, K.; Gould, I. R.; Myers, A. B. J. Phys. Chem. 1995, 99, 9017-9026. (70) (a) Fukuzumi, S.; Mochida, K.; Kochi, J. K. J. Am. Chem. Soc. 1979, 101, 5961-5972. See also: (b) Gardner, H. C.; Kochi, J. K. J. Am. Chem. Soc. 1976, 98, 2460-2469. (71) Rossi, R. A.; Pierini, A. B.; Peñéñory, A. B. Chem. Rev. 2003, 103, 71-168. (72) Hoz, S.; Bunnett, J. J. Am. Chem. Soc. 1977, 99, 4690-4699. (73) Wade, P. A.; Morrison, H. A.; Kornblum, N.; J. Org. Chem. 1987, 52, 3102-3107. (74) Wu, B.; Zeng, F.; Ge, M.; Cheng, X.; Wu, G. Sci. China, Ser. B 1991, 34, 777-786. (75) Cheng, C.; Stock, L. M. J. Org. Chem. 1991, 56, 2436-2443. (76) Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77-80. (77) Calculated value: (a) Bonesi, S. M.; Erra-Balsells, R. J. Chem. Soc., Perkin Trans. 2, 2000, 7, 1583–1595. This is in agreement with measurements: (b) Andrieux, C. P.; Gelis, L.; Medebielle, M.; Pinson, J.; Savéant, J. M. J. Am. Chem. Soc. 1990, 112, 3509–3520. (78) Value converted from electrode Ag/ AgCl: (a) Hammerich, O.; Speiser, B. (Eds.) Organic Electrochemistry CRC Press: Boca Raton, 2016, 5th ed. pp. 722. See also: (b) Rathore, R.; Kochi, J. K. Tetrahedron Lett. 1994, 35, 8577-8580. (79) Pham, P. V.; Nagib, D. A.; MacMillan, D. W. C. Angew. Chem. Int. Ed. 2011, 50, 61196122.


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(80) Bachofner, H. E.; Beringer, F. M.; Meites L.; J. Am. Chem. Soc. 1958, 80, 4269-4274. See also: (b) Dursun, C.; Degirmenci, M.; Yagci, Y.; Jockusch, S.; Turro, N. J. Polymer 2003, 44, 7389-7396. (81) Nappi, M.; Bergonzini, G.; Melchiorre, P. Angew. Chem. Int. Ed. 2014, 53, 4921-4925. (82) Kandukury,S. R.; Bahamonde, A.; Chatterjee, I.; Jurberg, I. D.; Escudero-Adan, E. C.; Melchiorre, P. Angew. Chem. Int. Ed. 2015, 54, 1485-1489. (83) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441-451. (b) Alvarez, A.; Dalton Trans. 2013, 42, 8617-8636. (84) da Silva, G. P.; Ali, A.; da Silva, R. C.; Paixão, M. W. Chem. Commun. 2015, 51, 1511015113. (85) Franz, J. F.; Kraus, W. B.; Zeitler, K. Chem. Commun. 2015, 51, 8280-8283. (86) Job, P. Annali di Chimica Applicata 1928, 9, 113-203. (87) Silvi, M.; Arceo, E.; Jurberg, I. D.; Cassani, C.; Melchiorre, P. J. Am. Chem. Soc. 2015, 137, 6120-6123. (88) For another example of a radical chain mechanism promoted by local band excitation of one of the components, see: Using visible-light: (a) Fillippini, G.; Nappi, M.; Melchiorre, P. Tetrahedron 2015, 71, 4535-4542. Other examples using visible-light that might also proceed via a somehow similar mechanism, although they have not been fully elucidated, are: (b) Creutz, S. E.; Lotito, K. J.; Fu, G. C.; Peters, J. C. Science 2012, 338, 647-651. Using blue light: (c) Sagadevan, A.; Ragupathi, A.; Hwang, K. C. Angew. Chem. Int. Ed. 2015, 54, 13896-13901. Using UV light: (d) Ziegler, Z. T.; Choi, J.; Muñoz-Molina, J. M.; Bissember, A. C.; Peters, J. C.; Fu, G, C. J. Am. Chem. Soc. 2013, 135, 13107-13112.


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(89) In this context, see: (a) Melchiorre, P. Angew. Chem. Int. Ed. 2012, 51, 9748-9770. (b) Jiang, L.; Chen, Y.-C. Catal. Sci. Technol. 2011, 1, 354-365. (90) Arceo, E.; Bahamonde, A.; Bergonzini, G.; Melchiorre, P. Chem. Sci. 2014, 5, 2438-2442. (91) Woźniak, Ł.; Murphy, J. J.; Melchiorre, P. J. Am. Chem. Soc. 2015, 137, 5678-5681.


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Organic Synthesis Enabled by Light-Irradiation of EDA Complexes

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