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Photolysis of Azido Compounds

Organic compounds containing a functional N3-group are called azides. In inorganic chemistry azides are the salts of hydrazoic acid. Depending on the counter ion, azides can have a propensity to explode. Driving force for this degradation is the release of nitrogen. On this account, heavy metal azides such as Pb(N3)2 are used as primary explosives. This is also applies for most organic azides. Nevertheless, organic azides are important intermediates in synthetic organic chemistry for heterocycles such as triazoles or tetrazoles. This is also the reason for the interest of chemical industry in this kind of substances.[1;2]

The N3 group is linear, forming polar mesomeric structures. Additionally, it can be stabilized by conjugation with adjacent aromatic systems. The polar mesomeric structures implies the reaction behavior of azides as 1,3 bipole as well as the regioselectivity in reactions with nucleophiles or electrophiles.[1]

azidoeq1Besides conventional organic reactions such as Schmidt and Curtius reactions, azides can undergo photochemical degradation. This process is called photolysis. By absorption of UVlight azides decompose by releasing N2, finally forming nitrenes. Alkyl azides absorb in the range of 260nm to 290nm and aryl azides in the range of 300nm to 400nm. On account of this, azides are usually used as precursors for nitrenes in synthethic organic chemistry.[1]

azidoeq2                                                              

Nitrenes are analogues of carbenes with only 6 electrons in the outer orbitals. Despite this, their properties differ from their carbon analogues. Nitrenes are uncharged, high reactive intermediates. Nitrenes stabilize themselves through inter- and intramolecular H-abstraction, [1,2]shift, addition or coupling reactions. Nitrene chemistry is therefore an own field of research that contains reactions from cycloadditions to rearrangements. A distinction is made between singlet and triplet nitrenes (Equation 3).[3] Usually singlet nitrenes are not observed. Triplet nitrenes are more stable and considered to be the ground state. azidoeq3

A direct photolysis of most azides leads to imine formation. Characteristic nitrene reaction products are usually not observed. On this account it is assumed that a [1,2]-rearrangement occurs simultaneously to N2 abstraction. The quantum yield of alky azide photo reactions is usually above 1. This results from the radical chain reactions.[3]

azidoeq4

This way, even high reactive compounds, which could not be synthesized conventionally, are accessible (for an example see Figure 1).[4]

azides_compounds

Figure 1: Reaction products of 1-azidonorbornane upon irradiation.

Despite the mentioned differences, many carbene reactions can be transferred to nitrenes. Similar to carbenes, most nitrene reactions strongly depend on the spin state. Singlet nitrenes undergo stereospecific additions with olefins, [1,2]-rearrangements as well as insertions into σ-bondings. Triplet nitrenes add in a non stereospecific way to double bonds. They undergo H-abstractions as well as additions to radicals.[4]

Direct photolysis leads to singlet nitrenes first, which then change into to the more stable triplet Nitrene through inter system crossing. By triplet sensitization triplet nitrenes are accessible directly.[4]

 

  1. MORTIMER, E.;      MÜLLER, U.:  Chemie, 2015, Thieme Georg Verlag, ISBN 3134843129.
  2. BRÄSE, S.; GIL, C.; KNEPPER, K.; ZIMMERMANN, V.: Organic Azides: An Exploding Diversity of a Unique Class of Compounds, Angewandte Chemie International Edition, 2005August, 44 (33), 5188–5240, DOI: 1002/anie.200400657.
  3. BECKER, H.; BÖTTCHER, H.; DIETZ, F.; REHOREK, D.; ROEWER, G.; SCHILLER, K.; TIMPE, H.-J.: Einführung in die Photochemie, 3. bearb. aufl. edn., 1991, Deutscher Verlag der Wissenschaften GmbH, ISBN 978–3–3260–0604–8.
  4. KLESSINGER, M.; MICHL, J.; KLESSINGER, M.: Lichtabsorption und Photochemie organischer Moleküle, no. hrsg. von Martin Klessinger ; 3 in Physikalische organische Chemie, 1990, VCH, Weinheim, ISBN 978-3-527-26085-0.