The Norrish Type II Reaction

In alkyl aryl ketones spin inversion is so fast that no singlet state reaction is observed (Wagner and Hammond, 1965). 

Evidence that the triplet reaction is not concerted, but rather proceeds via the 1,4-biradical, has been obtained from the photoracemization of ke- 

The reaction is not concerted and does not yield a triplet olefin, even when this process would be exothermic, as in the case of 42. Triplet stilbene decays to a 60 40 mixture of cis- and trans-stilbene, but in the reaction of 42, 98.6% /ra/t. -stilbene was observed (Wagner and Kelso, 1969). 

Intersystem crossing to the So surface is believed to occur at the 1,4-biradical stage, and to yield one of the three possible types of singlet product (reactant, olefin, cycloalkanol) depending on the geometry at which it occurs, as discussed in Section 4.3.4. 

The singlet reaction also proceeds at least partially via a biradical, as was shown indirectly. If the reaction of 43 were concerted, the transfer of H should yield the deuterated cis isomer and the transfer of D the nondeuter-ated trans isomer  

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The photolysis of the following steroid system resulted in two products corresponding to the Norrish type II reaction and one product due to a-cleavage (Norrish type I cleavage)<101) ... 

The photolysis of an azetidine has been found to yield pyrole derivatives by the Norrish type II reaction/1075... 

Observation of the Norrish Type II reaction presents some difficulty in that generation of the biradical intermediate 12 requires a six-membered transition state and this is in conflict with the linear guest arrangement normally expected in the channel. However, as noted earlier, accommodation of planar six-membered rings in urea inclusion complexes has been observed 38. It appears that in this case the necessary six-membered transition state can be produced in the channel without destruction of the crystal structure. 

Abstract After a brief introduction and summary of various methods of asymmetric induction in organic photochemistry, the main part of the review covers the solid-state ionic chiral auxiliary approach to asymmetric photochemical synthesis. Application of this technique to the Norrish type II reaction, as well as to the di-n-methane and oxa-di-n-methane photorearrangements, and the cis,trans-photoisomerization of diarylcyclopropane derivatives is presented and discussed. 

Photochemical C —H insertion of ketone 1 proceeds by initial photoexcitation to give an excited state that can be usefully considered as a 1,2-diradical. Intramolecular hydrogen atom abstraction then proceeds to give a 1,4- or 1,5-diradical, which can collapse to form the new bond. This approach has been used to construct both four- and ftve-membered rings12 11. Photochemical-ly mediated cyclobutanol formation is known as the Norrish Type II reaction. 

To compare with. Figure 1.4 shows the first step of the Norrish type II reaction, namely, the y-H transfer to the C=0 group. 

A second example from Salem s work which illustrates the role of symmetry is the photochemical hydrogen abstraction by ketones, termed the Norrish type II reaction (Norrish, 1937). A typical case is illustrated in (69). 

Photochemistry. One might deduce that since the lowest electronic transition corresponds to transfer of an electron from an oxygen atom to a carbon atom, the nn state should have substantial diradical character and should react also by a McLafferty-type rearrangement or a cleavage, as in the mass spectrometer. This is indeed the case. The photochemical a cleavage is called the Norrish type I reaction, and the rearrangement is called the Norrish type II reaction. Both are discussed in Chapter 15. 

Figure 15.5. Dauben-Salem-Turro analysis of the photochemical step of the Norrish Type II reaction. The reaction is efficient on the 3(nn ) surface to yield triplet diradical products. It is less efficient on the 1(nn ) surface since IC is relatively efficient.

As with the Norrish Type II reaction, the lower electronic states of the reactants (left-hand side) are those of the carbonyl group shown in Figure 14.8c. The local group... 

Figure B15.1. (a) Intermediate diradical, (b) State correlation diagram for the Norrish Type II reaction.

Norrish Type II biradicals have also been trapped. For example, irradiation of a benzene solution of 58, to which a high concentration of butanethiol-S-Norrish Type II reaction and extensive incorporation of deuterium at the y carbon of recovered 58. The thiol, a good hydrogen donor, supplies a deuterium to the radical 59 before 59 can give products or return to starting material as shown in Scheme 6.93... 

In the case of the sample with PDK, acetophenone and dec-l-ene were isolated, showing that the Norrish type-II reaction is operating. A hypothesis that can be put forward is that, with different efficiency, the aniline and the ketone additives act essentially as a light screen without transfer of energy to the polymer. 

Scheffer et al. provided another unimolecular asymmetric transformation involving the Norrish type II reaction, a well-known excited state process of ketones that is initiated by an intramolecular hydrogen atom transfer from carbon to oxygen through a six-membered transition state (Scheme 5). [19a] An adamantyl ketone derivative 27 was found to crystallize from ethanol in very large prisms in the chiral space group P2 2 2. Upon irradiation of these crystals to approximately 10% conversion, the chiral cyclobutanol derivatives 28 were afforded as the major products in 80% ee. 

Ariel, S., Askari, S.H., Scheffer, J.R., and Trotter, J. (1986) Cyclopropanol formation via p-hydrogen atom abstraction the five membered transition state analog of the Norrish type II reaction. Tetrahedron Letters, 27 (7), 783—786. 

One of the most common photochemical reaction pathways of carbonyl compounds is the formation of a diradicaloid excited state which is able to abstract a hydrogen atom at the y (or, more rarely, e) position, followed by either fragmentation or recombination. This process, which is known as the Norrish type II reaction, has a parallel in the photochemistry of nitro groups the intramolecular hydrogen abstraction of excited ortho-nitrotoluene is actually one of the very early synthetic photochemical transformations [9]. It has been exploited in a family of photolabile protecting groups, most prominent among which are derivatives of ortho-nitrobcnzyl alcohol, as introduced in 1966 by Barltrop et al. (Scheme 13.1) [10, 11],... 

The Norrish type II reaction is occasionally used in synthetic processes, mostly when other procedures leading to a desired product are difficult or costly. Simple type II elimination has been put to use in several ways ... 

Recently, Hu and Neckers reported that triplet excited states of alkyl phenyl glyoxylates react rapidly and with high chemical yields with electron-rich alkenes forming oxetanes 24 with high regio and stereoselectivity (Sch. 6) [28]. The intramolecular y-hydrogen abstraction (Norrish type II) cannot compete with intermolecular reactions in most cases. When less electron-rich alkenes were used, the Norrish type II reaction became competitive. 

The Norrish Type II reaction is well known to produce a wide variety of diradicals, including 1,4-diradicals such as in the case of the following scheme ... 

Takagi, K., Harata, E., Shichi, T., Kanoh, T. and Sawaki, Y. (1997). Intercalation and control of the Norrish type II reactions of aromatic ketocarboxylates in hydrotalcite clay interlayers. J. Photochem. Photobiol. A-Chem. 105, 47. 

Carroll FA, Strouse GF, Hain JM. A visual manifestation of the Norrish type II reaction. The cyclohexanone sunburn dosimeter. J Chem Educ 1987 64 84—86. 

Wagnerhas reviewed the influence of conformational flexibility on the outcome of photochemical reactions, especially hydrogen abstraction reactions of the Norrish Type II kind and related processes. Kinoshita and Naito have published a short review dealing with 1,4-biradicaIs in the Norrish Type II reaction. A study of Norrish Type II degradation in polymers has been reported. ... 

Meador and Wagnerhave reported an efficient photochemical synthesis of indanols (75) by the cyclization of the ketones (76). The process ultilizes the variant of the Norrish Type II reaction involving 6-hydrogen abstraction by the excited carbonyl group. The kinetic information collected for the reactions... 

If a molecule has two y hydrogens available, in the Norrish type II reaction the transfer proceeds over the lower of the two barriers, and according to Scheme 10 a preference for cleavage of the weaker secondary CH bond results (Coxon and Halton, 1974). 

Figure 7.25. Stereoelectronic effects on the Norrish type II reaction. Presumed optimal orbital alignments a) for cyclization and b) for elimination.

An examination of the photoreaction of valerophenone in aqueous solution as a function of temperature, pH, and wavelength has appeared.Quantum yields have been measured for the Norrish Type II reaction, as well as for the cleavage to acetophenone and propene, and for cyclisation to two cyclobuta-... 

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