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Norrish Type Reaction

Norrish type reactions are a member of photolysis reactions. In general photolysis implies the homolytic σ-bond cleavage by the absorption of electromagnetic irradiation usually in UV-range. When there is no additional support structure, this process results in molecular fragmentation. In Figure 1 the general scheme of a photolysis reaction is depicted.[1]

A common photolysis reaction is the photoinduced cleavage of isoelectronic CO and N2 from carbonyl and azo compounds.[2] In chemical substances containing carbonyl and Azo functionalities, absorption of light with a wavelength of λ = 230 nm to 330 nm usually initiates a n→πexcitation. By entering the triplet state via intersystem crossing (ISC), those functional groups possess biradical characteristics (see Figure 2).[2;3]

From this state, several competitive, consecutive reactions can occur such as radical recombination, α-bond cleavages, intra- and intermolecular H-abstractions as well as the formation of a cycloadduct via addition to a C––C-double. The Norrish type I cleavage is a α-cleavage of an adjacent C–C σ-bond, yielding a set of fragmentation products. Norrish type II cleavage involves the abstraction of a γ-H atom, finally resulting in an enol and an alkene or cyclization.[4]

photolysis_schemeFigure 1: Scheme of a photolysis reaction.

 carbonyl

Figure 2: Excitation of an ketone and formation of the biradical.

 Norrish type I reactions

An example of the Norrish type I reaction is depicted in Figure 3:

  norrish1

Figure 3: Norrish type I reaction.

Norrish type I reactions usually occur in carbonyl compounds (ketones, aldehydes) under irradiation with light of a wavelength of λ = 230 nm to 330 nm in a non-reactive solvent. During

 norrish1_consecutive

Figure 4: Possible consecutive reaction pathways for a Norrish type I reaction.

the first reaction step, a α-bond cleavage, one acyl and one alkyl radical are generated. Due to movements of both radicals in the solvent cage by diffusion and rotation, three consecutive reactions are possible.[5] Firstly, CO can be released from the acyl radical (decarbonylation, see also section ??). During this process alkyl radicals are formed, that can be recombined or stabilized via disproportion. This typically results in the formation alkanes and alkenes. Secondly, by leaving the solvent cage a H-atom can be abstracted by the acyl radical yielding an alkene and an aldehyde. Thirdly, by presence of an α-H in the acyl radical, ketene formation by H-abstraction can occur(see Figure 4). Additionally, a back reaction to the reactant through recombination is also possible inside the solvent cage. On this account the quantum yield of the products for Norrish type I reactions is usually significantly below 1.[4;5]

The Norrish type I reaction can be influenced via three different parameters. One crucial parameter is the excitation level. It is assumed that the α-bond cleavage proceeds 100 to 1000 times faster from the nπ-excitation state than from a state. The multiplicity of the excited state constitutes an additional important parameter. The reaction can occur 100 times faster from a nπtriplet state than from a nπsinglet state. For aromatic ketones or aldehydes fast intersystem crossing rates are observed (> 108 s1). These reactions occur exclusively from triplet state. Aliphatic ketones and aldehydes show much lower ISC rates. For these substrates, the Norrish type I reaction could occur from singlet and triplet state. Finally, the possible substituents and molecular structures strongly affect the secondary reaction pathways. The first reaction step (αbond cleavage) as well as the decarbonylation, that corresponds to the first consecutive reaction pathway, can be favored by formation of stabilized radicals such as benzyl- or t-alkylradicals. Furthermore, sterical issues have to be considered. The reaction rate for the α-bond cleavage step increases when it is possible to reduce sterical tension through radical formation. On this account, the cleavage rate increases via cyclohexanones to cyclopentanones and cyclobutanones. cyclobutadiene

Figure 5: Synthesis of anti aromatic cyclobutadiene.

In a non-symmetrically substituted ketone usually the weakest C–C-σ-bond is broken, yielding the radical with the highest possible number of substituents.[2;4]

The Norrish type I reaction is a fundamental member of photoreactions in synthetic organic chemistry. One outstanding example for this reaction type is the photochemical formation of the unstable antiaromatic cyclobutadiene, which could be synthesized by additional use of low temperature techniques (see Figure 5).[4]

Another great example is the synthesis of tetrahedrane system demonstrating high intramolecular tension. Through irradiation of t-butyl-cyclopentadienone the desired tetra-t-butyl-tetrahedran was obtained through decarbonylation. t-butyl groups were used for stabilization.[4]

Norrish type II reactions

Norrish type II reactions are characterized by formation of a biradical through intramolecular γH-abstraction (see Figure 6). In the course of this a 6-ring-transition state is formed. Through dissociation of the αβ-positioned bond (referenced to the carbonyl group) the biradical turns into an olefine and an enole. Both products can be considered as coupled main products of the ketone fragmentation. In lower amounts (10% to 25%) cyclobutanole is formed as a recombination product. This process is known as the Yang-cyclization. Depending on the α substituent, the cyclization product fraction can raise up to 90%. This can occur for example for αdimethyl-butyrophenone. The back reaction through H transfer is also possible. By this, the educt ketone is released, which corresponds formally to a deactivation of the excited state. Because biradical formation is exothermic, the amount of energy released is high enough to initiate the back reaction. That is why the back reaction has always to be taken into account.

Laserphotolysis experiments showed that the lifetime of biradicals is in the range of 30 ns to 100 ns. The lifetime can be influenced by the polarity of the solvent cage. Higher lifetime are observed in alcohols or aqueous acetonitrile. This is due to hydrogen bond formation with solvent cage molecules. Lower values were observed in non polar solvents.[2;4]

 norrish2_consecutive

Figure 6: Norrish type II reaction mechanism.

Norrish type I reactions vs. Norrish type II reactions

The occurring cleavage type during irradiation of a ketone strongly depends on the substrates structure and the biradical stability. Lower aliphatic ketones without any γ−H-atoms as well as cyclic ketones undergo Norrish type I reactions. As an exceptional case, tert-butylketones undergo Norrish type I reactions despite of the existing γ−H-atoms. This is due to the high stability of intermediate tert-butylradicals.

Norrish type II reactions is observed for aliphatic ketones and higher alkyl phenyl ketones obtaining γ−H-atoms. Lower alkyl phenyl ketones react to pinacoles via external H-abstraction.

This is also applies for diaryl ketones, which can not be dissociated photochemically.[4]

 

 

 

  1. SMITH, M.: March’s advanced organic chemistry: reactions, mechanisms, and structure, 7th edition edn., 2013, Wiley, Hoboken, New Jersey, ISBN 978-0-470-46259-1.
  2. WÖHRLE, D.; TAUSCH, M. W.; STOHRER, W.-D.: Photochemie: Konzepte, Methoden, Experimente, im kolophon: milton keynes: lightning source, 2010 edn., 2010, Wiley-VCH, Weinheim, ISBN 978-3-527-29545-6.
  3. LOWRY, T. H.; SCHUELLER RICHARDSON, K.: Mechanismen und Theorie in der organischen Chemie, 1980, Verl. Chemie, Weinheim, ISBN 978-3-527-25795-9.
  4. 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.
  5. 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.


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