Environment-friendly organic synthesis. The photochemical approach

Abstract

Photochemical reactions are expected to have an increasing role in organic synthesis with the drive towards environment-friendly reactions. Some examples illustrating the scope and versatility of radical alkylation of electrophilic alkenes via photoinduced electron transfer are presented. A few applications in special fields are mentioned. PHOTOCHEMISTRY FOR ORGANIC SYNTHESISPhotoinitiated reactions are promising candidates for environment-friendly organic synthesis. The central issue of organic chemistry is to make quite stable organic molecules susceptible to reactions. This is obtained either by weakening a covalent bond (e.g., by complexation or adsorption on a catalyst surface) or by polarizing it (e.g., again by complexation or by forming a hydrogen bond) or by cleaving a covalent bond. The last choice is the one requiring more energy and more control of the medium, as typified by the deprotonation of a carbonyl or carboxyl derivative to form an enolate, probably the most largely used strategy for the formation of the carbon-carbon bond.Photochemistry is different, since electronically excited states (see eq. 1) differ from ground states not only for the high energy level, but also, and perhaps more importantly, for the dramatic change in the electronic distribution by which they are characterized as well as by the fact that such a change does not require the addition of a chemical reagent or-at least in principle-a special care of the medium. To take a simple example, a ketone is a weak electrophile, due to the polarization of the p C-O bond. As such, it reacts only with quite active (usually charged) nucleophiles. The electrophilicity can be increased by increasing the polarization (e.g., by complexation by means of a Lewis acid), and under these conditions weaker electrophiles add. However, electronic excitation brings about a more deepseated change. The np* excited state is no more a C-electrophile (3 electrons now crowd in the p space) but the single electron remaining in the n O orbital gives to this species a strong radical reactivity centerd at the oxygen atom (see Scheme 1). Indeed, the typical reactions are fast (typical rates ≥ 1 ¥ 10 6 M -1 s -1 ) H-abstractions from a C-H bond or addition to an alkene, i.e., the same reactions that an alkoxy radical would give.

Photochemistry in synthesis: Where, when, and why

“…Thus, the photon is the green reagent par excellence, or at least this is what photochemistry practitioners think. In order that this concept is generally accepted, suitable tests of its viability must be made [2,6].…”

Intermolecular Addition Reactions onto CC Multiple Bonds

“…In many cases, the procedures described compared favorably with thermal analogues, allowing a controlled synthetic approach to a wide variety of organic products. In addition, the above-discussed photochemical methods can be thought of as offering a clean (or green) alternative method [84,85], which represents an active area of current research.…”

Green chemistry: the emergence of a transformative framework

“…Photon absorption can occur independently of temperature and solvent, providing flexibility in environmentally friendly synthetic design (53). Solvent-free ''solid-to-solid'' photochemical reactions have been developed as a technique for synthesizing molecules with adjacent quaternary stereogenic centers (54) (Figure 8).…”