Shine light on your flask! New opportunities for organic synthesis

Key words:Photochemistry, sustainable chemistry, visible light, free radical chemistry, boron chemistry.

Sunlight driven organic reactions are among the most important processes in Nature, sustaining essential biochemical processes such as photosynthesis, crucial for our biosphere. In plants, a green coloured pigment -known as chlorophyll -acts as a light harvesting antenna, capturingthe radiative energy of the sunand convertingit into chemical energy. During the photosynthesis CO2, a fairly stable gas, is transformed intooxygen and sugars. This process is strongly endothermicas it requires external energy to occur, withthe energy provided by sunlight. The sugars synthesized in this process are the essentialenergy sourcefor all the animal species in Nature.

Photochemical reactions are not limited to the vegetal world, as animals also use these transformations for the synthesis of important molecules such as vitamins. Exposing our skin to solar irradiation, 7-dehydrocholesterol is converted to cholecalciferol(Vitamin D3)whose role iscrucial for the health of our bones and the absorption of important co-factors such as calcium and phosphate ions.

Figure 1. a. The photosynthesis is a chemical process which allows the conversion of carbon dioxide into sugars and oxygen.The energy for this process is obtained from sunlight thanks to chlorophyll, a green coloured dye which acts as a light harvesting antenna. b. The synthesis of vitamin D is a photochemical process which occurs in our skin when we expose our body to the sun. Vitamin D is crucial for the health of our bones, since it is involved in the absorption of calcium and phosphorus.

In recent research at the University of Bristol we focused on the development of visible-light driven reactions, exploiting the energy of the sun for the chemical synthesis of organic compounds. As organic compounds are seldom able to absorb visible light, and taking inspiration fromphotosynthesis, we used a compound which acts as a light harvesting antenna: an intensely red coloured dye known as Ru(bpy)3Cl2.[JH1]

The Ru(bpy)3cation absorbs visible light, and use the radiative energy for the generation of highly reactive species known as free radicals.Free radicals are highly reactive species that have unpaired electrons. The energy captured from light fuels the reaction depicted in Scheme 1, where a boron-derived compound 1 reacts with an alkyl iodide2 to furbish compound 3, where two new C-C bonds are formed.

Scheme1. The use of intensely red coloured Ru(bpy)3 as light harvesting antenna allows us to exploit the energy of sunlight for the development of new reactions. The generation of reactive free radicals from compound 2 is involved in the process.

We have taken our experiences learnt in this scheme and are currently working on the development of other light-catalysed reactions for the synthesis of complex organic molecules.

Dr. Mattia Silvi obtained his PhD in 2015working in the Institute of Chemical Research of Catalonia(Spain) and in the University of Michigan (United States of America).He worked in the fields of asymmetric catalysis, photochemistry, organometallic chemistry and alkaloid functionalization. He is currently working as a Marie Skłodowska-Curie Fellow in the University of Bristol where he is developing new photochemical reactions aimed at the development of new sustainable strategies in organic synthesis.

Shine light on your flask! New opportunities for organic synthesis

Questions

  1. What group 2 element is found in chlorophyll? (1 mark)
  2. In Scheme 2 what group of atoms does the ‘Et’ represent? (1 mark)
  3. An unbalanced photosynthesis equation can be found in Figure 1. Rewrite the equation in a balanced from. (2 marks)

  4. What is (a) the name of the metal ion found in the ‘intensely red coloured dye known as Ru(bpy)3[JH2]’ ?(1 mark)
  5. The Ru(bpy)3 is intensely red. What colour(s) of light does it absorb?(1 mark)
  6. What is the essential structural feature of a free radical? (1 mark)
  7. In Scheme2 suggest what product, not shown, the iodine atom forms? (1 mark)
  8. How many covalent bonds would you normally expect a boron atom to form? (1 mark)

Extension Question

Why would the topic of this research topic be considered an aspect of ‘green chemistry’’? (2 marks)

Shine light on your flask! New opportunities for organic synthesis

Questions

  1. What group 2 element is found in chlorophyll? (1 mark)
  1. In Scheme 2 what group of atoms does the ‘Et’ represent? (1 mark)
  1. An unbalanced photosynthesis equation can be found in Figure 1. Rewrite the equation in a balanced from. (2 marks)

  1. What is (a) the name of the metal ion found in the ‘intensely red coloured dye known as Ru(bpy)3[JH3]’ ?(1 mark)
  1. The Ru(bpy)3 is intensely red. What colour(s) of light does it absorb? (1 mark)
  1. What is the essential structural feature of a free radical? (1 mark)
  1. In Scheme2 suggest what product, not shown, the iodine atom forms? (1 mark)
  1. How many covalent bonds would you normally expect a boron atom to form? (1 mark)

Extension Question

Why would the topic of this research topic be considered an aspect of ‘green chemistry’’? (2 marks)

Answers

These will be added later

[JH1]Can you put in a structure of this-I don’t really want to nick one from Wikipedia  I assume it’s the chloride you use?

[JH2]Can you put in a structure of this-I don’t really want to nick one from Wikipedia  I assume it’s the chloride you use?

[JH3]Can you put in a structure of this-I don’t really want to nick one from Wikipedia  I assume it’s the chloride you use?