**6. Conclusions**

104 Artificial Photosynthesis

formation of pyrrolo[2,1-]isoquinolines from ethyl 2-(3,4-dihydroisoquinolin-2(1*H*-yl)

THF - H2O - visible light <sup>+</sup> R''' CF3I R'

We have already reported the photocatalytic activity of tetrabutylammonium decatungstate salt ((*n*-Bu4N)4W10O32) in selective photoxidations. TBADT has been also used by Albini and coworkers to promote the photocatalytic radical conjugate addition of electron-poor olefins by cycloakanes (Eq. 22) (Dondi et al., 2006) and the acylation of ,-unsaturated nitriles, ketones

The same group has recently shown that irradiated TBADT can also effectively catalyze the alkylation at position 2 of 1,3-benzodioxoles, making this moiety more biological active and

CH3CN / h

Another significative example of potentials of semiconductor photocatalysis is represented by the artificial photosynthesis design, that is the fixation of CO2 molecules to afford higher

For example, many studies have concentrated on the fixation of CO2 in carboxylic acids to produce intermediates in key cellular processes. Recently Guzman and Martin have reported that a glyoxylate can be methylated to produce the corresponding lactate, directly involved in the reductive tricarboxylic acid cycle, by photocatalytic fixation of CO2 mediated

Nevertheless, it has been recently outlined (Yang et al., 2010) that many results reported in the literature and related to these studies could be influenced by the presence of carbon residues left over from the synthesis of metal oxide semiconductors. In other words, there could be experimental artefacts affecting reports and final conclusions, so that more

CH3CN - visible light <sup>+</sup> <sup>N</sup> OEt

and esters (Eq. 23) (Esposti et al., 2007), affording the desired products in good yields.

N Ph O O

CH3CN / h <sup>+</sup> <sup>O</sup>

C CH3CN / h 6H13 H

+ EWG EWG

R4

investigations in the field of artificial photosynthesis is still mandatory.

R O

Ru(bpy)3Cl2 / *i*-Pr2NEt

Ru(bpy)3Cl2 / O2

(*n-*Bu4N)4W10O32

CF3

N O

Ph

CO2Et

(20)

(21)

(23)

R'' R'''

N

O

O

C6H13

O

R2 R3 EWG

R4

<sup>R</sup> R1

(*n-*Bu4N)4W10O32 (24)

EWG

EWG

(*n-*Bu4N)4W10O32 (22)

O

O

acetate (Eq. 21)

R'

R''

O

enzyme-specific (Eq. 24) (Ravelli et al., 2011).

O

organic compounds (Hoffmann et al., 2011).

by ZnS (Guzman & Martin, 2010).

+ R3

R1

R2

O EWG

O

OSiR3

In the last decades a growing interest has been devoted to the development of photocatalytic processes both in the homogeneous and in the heterogeneous phase. Particularly, concerning the heterogeneous systems, great interest has aroused the use of photosensitive semiconductors as catalysts for organic processes, due to their ease to use, recycle and low environmental impact. Although most of the actual applications are restricted to the decomposition of organic pollutants, semiconductors are becoming more and more important for the development of new photocatalyzed organic protocols, as an alternative to the conventional metal-catalyzed thermal processes. Generally, TiO2 has a dominant role in all the semiconductor-phtocatalyzed applications, including the organic synthesis, however, in the last decades many others transition metals photocatalysts, have been developed. Actually, in the scientific landscape, big challenges are represented by the reduction of energy consumption and environmental impact, and photocatalysis could be one of the winning answers in the chemistry field.
