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Photo-Nitrosylation

Introduction of heteroatoms is of great synthetic interest. A photochemical reaction type providing this possibility is the photonitrosylation to build up nitroso functionalities in organic compounds. Under suited reaction conditions, the nitroso moiety can isomerize to yield an oxime. A typical reaction procedure is the irradiation of a mixture of the organic reactant with NOCl, yielding the nitrosylated product and HCl. The presence of HCl leads to isomerization and formation of the oxime[1]: nitrolysationeq1

This reaction path is of great interest for chemical industries, because oximes can be further converted to lactams, which are important intermediates. Consequently, photooximations were realized on an industrial scale. An outstanding example of photooximations, which found extended industrial use, is the photooximation of alkyl compounds. The photochemical ε-caprolactam synthesis, a precursor to Nylon 6, using cyclohexane and nitrosyl chloride is one of the most popular examples of this reaction type. This process is conducted in Japan since the 1950s.[2] With this reaction type the photochemical synthesis of lauryllactam is possible as well, which is a precursor to Nylon 12. Other examples for photonitrosylations include the use of diverse aromatic hydrocarbons (e.g. side chain nitrosylation), paraffins and polymers as well as the option for other nitrosylations agents such as tert-butyl nitrite or sodium nitrite.[2]

Two mechanisms are discussed for the photonitrosylation. First, a radical mechanism, initiated by homolytic dissociation of NOCl, and secondly a mechanism involving a pre-dissociated excited state of nitrosyl chloride. Both mechanisms were found to occur in parallel. Depending on the reaction conditions and most importantly the wavelength of irradiation either the one or the other mechanism is favored.[2;3]

photo_caprolactam

Figure 1: Photochemical production of ε-caprolactam.

For the radical reaction pathway, the first step is the homolytic cleavage of nitrosyl chloride. After that a H-abstraction by the chlorine atom is provoked. The formed alkyl radical subsequently combines with the nitrosyl radical. Due to HCl formation during H-abstraction the nitrosylated compound can isomerize to an alkanoneoxime.[2;4]

nitrolysationeq2

The mechanism involving a pre-dissociated excited state of nitrosyl chloride occurs mainly when NOCl is irradiated by light of a wavelength larger than 500nm. Interaction of the excited NOCl leads to H-abstraction instead of an interaction with a chlorine radical:[2;3] nitrolysationeq3

The following reaction steps are similar to that discussed for the radical mechanism. As a consequence of these two mechanisms, reaction control is important to gain high selectivities and yields.

For industry the costs of the process are the most relevant parameter. On this account, the costs associated with the generation of photons as well as with the chemical engineering aspects can disqualify the photochemical route when compared to a thermal alternative. For photooximations the possibility to gain for example cyclohexanoneoxime through a direct route together with the absence of any coupled byproduct renders this process attractive to produce the required intermediates for Nylon 6. The main advantage compared to the conventional process is the absence of any stochiometric byproduct during the photoreaction (see Figure 1). Converting the produced oxime to the required ε-caprolactam by the Beckmann rearrangement directly yields the intermediate of interest.[1;2] This reaction is conducted analogous to the thermal process. Although the conventional pathway possesses a simple reaction path as well, the production of large amounts of ammonium sulfate is a problem. Compared to the photochemical process, (NH4)2SO4 is not only produced during the Beckmann rearrangement, but also during the production of the cyclohexanoneoxime by use of hydroxylamine sulfate (see Figure 2). With this, the total production of (NH4)2SO4 per amount of ε-caprolactam is lower for the photochemical route. An additional benefit is the high purity of the (NH4)2SO4 produced by the photochemical process, which makes it possible to directly sell (NH4)2SO4 as fertilizer without any further processing.[2].

 thermal_caprolactam

Figure 2: Thermal production of ε-caprolactam.

 

  1. NAYLOR, M. A.; ANDERSON, A. W.: Synthesis of Cylcohexanone Oxime by Photoreaction of Nitrosyl Chloride with Cyclohexane, The Journal of Organic Chemistry, 1953January, 18 (1), 115–120, DOI: 10.1021/jo01129a018.
  2. BRAUN, A. M.; MAURETTE, M.-T.; OLIVEROS, E.: Photochemical technology, 1991, Wiley, ISBN 0471926523, DOI: 10.1002/ange.19921041147.
  3. MUELLER, E.: Mechanism of the Tübingen photooximation reaction, Pure and Applied Chemistry, 1968-January, 16 (1), DOI: 10.1351/pac196816010153.
  4. MUELLER, EUGEN; BOETTCHER, A. E.: Photooximation of the methyl groups in saturated aliphatic hydrocarbons, Tetrahedron Letters Issue35 Pages3083-6, 1970.

 

 
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