Norrish type n reaction

The following types of events were observed in the simulation of PA. (i) About one-third of the simulations show no reaction even after 100 ps simulation time, (ii) An H atom transfer from methyl group to the carbonyl group (Figure 1.6a), the initial step in Norrish type n reaction, occurred in some trajectories. There was no clear preference for early versus late timing of this event in the simulation time of up to 100 ps. (hi) Loss of CH3C(0) radical corresponding to Norrish... 

Sucholeiki [152] has recently suggested the use of benzyl sulfide photocleavage for detachment from a solid support. His results are consistent with Fleming s work. The pam-phenyl-substituted benzyl sulfide, 108 R = Ph, has enhanced benzylic cleavage compared to benzyl sulfide, 108 R = H. The increase in benzyl cleavage precludes the competitive Norrish type n reaction pathway which is shown in Eq. (40). However, the wavelength of excitation was... 

The photodecomposition of alkanones included within urea complexes has been reported by Casal et al. Urea, when crystallized in the hexagonal lattice, possesses long channels of approximately 5 A diameter into which guest molecules are incorporated. For 5-nonanone the possible reactions which may occur are shown in Scheme 6.15. In solution photodecomposition occurs principally via the Norrish Type n reaction, leading to 2-hexanone, propylene, and two isomeric cyclobutanols. The crystalline hexagonal urea complex gave, on the other hand, essentially only one cyclobutanol isomer — the cis. The formation of the cis isomer requires less stringent rotational requirements and is, therefore, the preferred product in the constrained channel environment. 

The behaviour of triplet acyl-diphenylmethyl biradicals 0=C -(CH2) 2-C Ph2, generated from the Norrish type-I reaction of 2,2-diphenylcycloalkanones (CK) with various ring sizes, n = 6, 7, 9, 11, 12, 13, was the subject of a study. Eor 2,2-diphenylcycloalkanones where n = 6 and 7 an intramolecular disproportionation takes place giving rise to a diphenylalkenal (94). The primary products in the photolysis... 

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. 

Norrish Type I reactions occur from 7 only if the state is 3(rc,7r ). Compound 53 has a phosphorescence lifetime of 10 3 sec, and there is a mirror-image relationship between its closely spaced phosphorescence and absorption spectra. Compound 54, however, has a phosphorescence lifetime of 5.5 sec, and there is a large Stokes shift between its dissimilar absorption and phosphorescence spectra. Apparently both compounds have a lowest (n, r ) singlet,... 

TABLE 7 Norrish type I reactions of alkanones in the (n, n ) state ... 

A state-symmetry diagram for the Norrish type II reaction (Section 6.3.4) shows that it is allowed only from the n,7t singlet and triplet states to give the covalent biradicals (Section 5.4.4) 1DC7l and 3DC7l, respectively, but not from 7t,7i states (Figure 4.33).331... 

One method of generating enols involves photohydration of phenylacetylene, while another method utilizes the Norrish Type n cleavage of y-hydroxybutyrophenone. Photochemical reactions will be discussed in Chapter 12. 

Equations 12.46 and 12.47 do not show the spin states of the unpaired electrons. In general, a cleavage can occur from both the singlet and triplet n,n states, but quantum yields are much greater for the triplet state reactions. Zewail and co-workers reported femtosecond studies and theoretical calculations of the Norrish type I reaction of acetone. They found a barrier of ca. 18 kcal/mol for a cleavage from the Si state of acetone but a barrier of only 5 kcal/mol for dissociation from the Ti state. Thus, they concluded that the photodissociation occurs from the triplet state and that intersystem crossing from Si to Ti is the rate-limiting step in the Norrish type I reaction of acetone. ... 

Another interesting example is provided by the photochemical conversion of o-methylbenzophenone (62) to the enol (63). The evidence suggests very strongly that (63) is formed directly and not by a secondary photochemical cis-trans isomerization of the enol (64) that would be formed by a simple Norrish type II reaction, by collapse of the intermediate (65 ). The reason is that the reaction involves the triplet n- n excited state of... 

Later,triplet alkylititrenes 12a-c were generated by intramolecular sensitization in solution. The photochemistry of three a-benzoyl-ft)-azidoalkanes (PhCO(CH2)oN3 (13a-c, n = 3-5) was studied and two competitive processes were found to proceed from the triplet ketone (Scheme 11.8) energy transfer to azide to yield triplet alkyl nitrene and 7-hydrogen abstraction to yield Norrish type II products. Photolysis of azides with a longer methylene chain (13b,c) yields mainly acetophenone, a product of Norrish type II reaction. The major products of the azide 13a photolysis were substituted pyrrole and pyr-roline derived from triplet nitrene 12a. ... 

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