Norrish type II photochemistry

Irradiation of 103 inside carcerand 104 gave a 5.6 1 mixture of Norrish Type II cleavage products 105 and 106 and Yang product 107, which is very similar to the ratio found in solution. On the other hand, incarcerated hexyrophenone was completely stable to irradiation even though it shows similar photoieactivity as 103 in solution. The inertness of 105 is a result of its inner phase conformation, in which the distance between /-protons and carbonyl oxygen precludes abstraction. 

Both Norrish Type II photoproducts remained incarcerated. Furthermore, the surrounding host prevented ketonization of 105, which requires acid or base catalysis. In fact, the resistance of incarcerated photoenol to ketonize is remarkable. Incarcerated 105 required 

---

Keywords czv-9-dccalyl aryl ketone, Norrish type II photochemistry, cyclopropanol, cyclobutanol, cyclopentanol... 

The topochemical principles have also been applied to the interpretation of mframolecular photochemical reactions of polymorphic materials depending on polymorphic form. 1,14-cyclohexacosanedione 6-XLII can be prepared as conformational dimorphs that undergo Norrish type II photochemistry which can be correlated with the molecular conformations in the two modifications, one leading to a cw-butanol, while the other leads to a trans product (Gudmunsdottir et al. 1996). Similarly, these principles can be invoked to understand the relative photochemical stability of two polymorphs of the active ingredient in a compound developed for the treatment of psoriasis (Lewis 2000). 

The opposite behavior is evident in the Norrish type II photochemistry of arylglyoxylates that Neckers and co-workers have studied (Scheme The 1,4-biradicaI intermediate resulting from triplet-state... 

Experimental evidence on the relationship between structure and reactivity in Norrish type II photochemistry of spirobenzoyladamantanes in the solid state has been produced. Geometric factors clearly determine partitioning between cleavage, cychzation, and hydrogen transfer reactions of the 1,4-biradical intermediate Cleavage becomes predominant when the singly occupied orbitals overlap poorly with one another but well with the central CC bond, which, as earlier sections indicate, has been presumed for years based on ketone structure. 

Majeti11 has studied the photochemistry of simple /I-ketosulfoxides, PhCOCH2SOCH3, and found cleavage of the sulfur-carbon bond, especially in polar solvents, and the Norrish Type II process to be the predominant pathways, leading to both 1,2-dibenzoylethane and methyl methanethiolsulfonate by radical dimerization, as well as acetophenone (equation 3). Nozaki and coworkers12 independently revealed similar results and reported in addition a pH-dependent distribution of products. Miyamoto and Nozaki13 have shown the incorporation of protic solvents into methyl styryl sulfoxide, by a polar addition mechanism. 

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. 

Since the photochemistry of many compounds that have been used as triplet sensitizers has been well studied, we will not attempt to cover these reactions in detail. Unless the investigator is unaware of them, common photochemical processes such as the Norrish Type II cleavage are not ordinarily a complication and as will be mentioned later, they can actually serve as mechanistic probes. A discussion of the mechanisms of triplet energy transfer1,3,9 is beyond the scope of this review as are other specific reactions which have been recently covered elsewhere. 

Table 1.1 clearly shows that the major pathway in the photochemistry of pentanal is the y-H transfer, followed by the C—C cleavage. The H detachment is only a minor pathway. A high percentage of trajectories are unreactive in this timescale. The relative yield of Norrish type I versus Norrish type II reaction from this table is 66% Norrish type II reaction and 34% Norrish type I reaction. This compares well to the observed experimental yield of 80% for Norrish type II reaction [16, 70]. 

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. 

Carbonyl compounds, 121-128 acids and electrophiles, 97 electrophilic attack, 121-123 photochemistry Norrish type I, 215-217 Norrish type II, 213-215 stability of, 127-128 UV spectra, 214 Carbonyl group... 

Use of chiral single crystals to convert achiral reactants to chiral products in high optical yield application to die di-Jt-methane and Norrish type II photorearrangements, J. Am. Chem. Soc., 108, 5648-5649. (b) Chen, J., Pokkuluri, P. R., Scheffer, J. R., and Trotter J. (1990) Absolute asymmetric induction differences in dual pathway photoreactions, Tetrahedron Lett., 31, 6803-6806. (c) Fu, T. Y., Liu, Z., Scheffer, J. R., and Trotter, J. (1993) Supramolecular photochemistry of crystalline host-guest assemblies absolute asymmetric photorearrangement of the host component, J. Am. Chem. Soc., 115, 12202-12203. (d) Leibovitch, M.,... 

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 photochemistry of these aliphatic ketones is distinguished from that of other aliphatic ketones by the occurrence of an intramolecular primary process which gives an olefin and a methyl ketone. This process is classified as a Norrish Type II process to distinguish it from the primary process which leads to free radicals. It occurs in the photolysis of many high aliphatic ketones and a similar process also occurs in the photolysis of many aldehydes 101 and esters.4 Primary processes which give rise to free radicals also occur. 

Photodecomposition. The photochemistry of 2-pentanone is distinguished by the possibility of "Y-hydrogen abstraction by the carbonyl oxygen (Norrish type II process). The biradical intermediate thus formed allows the possibility of cyclobutanol formation. 

The solvent dependency of the photochemistry of the dione (268) has been studied.The diketones (269a-f) undergo Norrish Type II processes to yield... 

---