Norrish Type II hydrogen abstraction

The two ketones (28) and (29) are known to undergo the Norrish Type II hydrogen abstraction process, and their photochemical reactivities have now been studied in chirally modified zeolites. The zeolites were modified by stirring them with known amounts of ( —)-ephedrine. Irradiation of the ketones in the zeolites brought about some enantiomeric enhancement. However, the various zeolites studied behaved differently and the NaX zeolite favoured the (+)-isomer of the product (30) while the NaY favoured the ( —)-isomer. The other ketone (29) showed only low enantiomeric enhancement and gave both the cis and the trans cyclobutanols (31) and (32) in a ratio of 4 1. ... 

Photoreactions of Thymines, etc. - Irradiation at 254 nm of the pyrimidine derivative (153) induces a Norrish Type II hydrogen abstraction from a methyl group of the t-butyl substituent. The resultant 1,4-biradical (153a) undergoes cyclization to afford an unstable cyclobutanol. Elimination of water from this species affords the final product identified as the cyclobutane derivative (154). The structure of this product was verified by X-ray diffraction techniques. The Norrish type II reactivity of the pyrimidine derivative (155) at 254 nm in water follows the analogous path to that observed for (153) and yields the cyclized product (156) in 52 % yield. - °... 

Pincock and his co-workers have studied the photochemical fragmentation reactions of the esters (31). This system has an in-built electron accepting sensitiser. When (31a-c) are irradiated in methanol the principal reaction is fission to yield the styrene (32) and p-cyanobenzoic acid. The other products formed from the reactions are the styrene addition products (33)-(35). The authors propose that the Norrish T) e II process in this instance involves a proton transfer and this occurs within the zwitterionic biradical formed as the primary intermediate on electron transfer. Further proof of the authenticity of this mechanism was obtained by irradiation of the deuteriated derivatives (31 d, e). The results of a study of the photochemical decomposition of benzyl phenylacetate, as a suspension in water over Ti02, have been reported. Bond fission is the result of irradiation of (36) in cyclohexane/ethyl acetate. A Norrish Type II hydrogen abstraction occurs with the elimination of the enone moiety. This affords a path to the CD ring system (37) of vitamin D. 

A detailed study of the photochemical reactions of the ketones (38) and (39) in the solid phase has been reported. " Both of these systems readily undergo Norrish Type II hydrogen abstraction in solution and it was this fact that attracted the authors to the systems. One of the facets of the work focussed... 

Upon was the use of chiral auxiliaries as counter ions of the carboxylate examples in (38a) and (39a). The authors conclude that the ionic chiral auxiliary approach is a viable general method for asymmetric synthesis. The irradiation of the biphenyl ketoamide (40) at 340 nm affords the two products (41) and (42) via the conventional Norrish Type II hydrogen abstraction process. When the reaction is carried out in the presence of an antibody microenvironment the reaction follows a different route and yields the tetrahy-dropyrazine derivative (43). The authors reason that there is interplay between conformational control and chemical catalysis that results in this high specificity. 

A study of the photochemical reactivity of salts of the amino ketone (44) with enantiomerically pure carboxylates has been reported. The irradiations involved the crystalline materials using A, > 290 nm and the reactions are fairly selective which is proposed to be the result of hindered motion within the crystalline environment. Some of the many results, using (S)-(—)-malic acid, R-(+)-malic acid and (2R,3R)-(+)-tartaric acid, are shown in Scheme 1. The principal reaction in all of the examples is a Norrish Type II hydrogen abstraction and the formation of a 1,4-biradical. This leads mainly to the cis-cyclobutanol (45) by bond formation or the keto alkene (46) by fission within the biradical. A very minor path for the malate example is cyclization to the trn 5-cyclobutanol (47). A detailed examination of the photochemical behaviour of a series of large ring diketones (48) has been carried out. Irradiation in both the solid phase and solution were compared. Norrish Type II reactivity dominates and affords two cyclobutanols (49), (50) and a ring-opened product (51) via the conventional 1,4-biradical. Only the diketone (48a) is unreactive... 

The dienones (106) are well known to undergo photochemical conversion into the cyclopentenones (107). Fleming et al. have shown that irradiation of the enones (106) in methanol and ethanol yields the two products (108) and (109). This transformation involves a Norrish Type II hydrogen abstraction within the cyclopentenone derivatives (107). The 1,4-hydrogen transfer results in the formation of the biradical (110) which cyclises by two paths to give (108) and (109). 

Irradiation (quartz filtered) of the oxazolone derivative (199) in acetonitrile results in decarbonylation and the formation of the imine (200). When this imine is formed in the presence of allyl alcohols, trapping (a thermal reaction) results in the formation of the ethers which are also photochemically reactive and are transformed by a Norrish type II hydrogen abstraction process into the isomeric compounds (201). Oxy-Cope rearrangement of (201) yields the second product (202) isolated from the initial irradiation. 

Hydrogen abstraction by an excited carbonyl group is the most typical photoreaction in both solution and solid states. A number of intramolecular Norrish type II hydrogen abstraction reactions in the crystalline state are already known, and the geometric requirements have been precisely discussed by Scheffer [58]. Solid-state asymmetric induction in the Norrish type II photocyclization of carbonyl compounds using supramolecular approaches has been also intensively studied. 

Moorthy and MaP have reported that irradiation of the benzoyl ketones (38) results in photochemical conversion to the mixture of cyclobutanes (39) and (40). The yields are in the 31-43% range and, as can be seen from the ratios of products, there is a good degree of selectivity when the reactions are carried out in non-polar solvents. The ratios change when polar solvents are used. This change is more dramatic with the ketones (38, R = Ph), where the observed selectivity is reversed from non-polar to polar solvents. The keto derivatives (41, R = H or Ac, n = 1) undergo Norrish Type II hydrogen abstraction on irradiation at 300 nm in t-butanol as solvent. Cyclization results in the formation of the imidazolidinones (42 and 43) by cyclization within the resultant biradicals. 

A study of bischromones such as (131) has been carried out. Irradiation (254 nm) in CH3CN of the derivative (131, n = 2) results in the formation of the two products (132) and (133) in low yields of 10% and 12%, respectively. The cyclization reaction is the result of Norrish Type II hydrogen abstraction and cyclization. The analogous reaction occurs with the corresponding phenyl derivatives. The cyclization is chain-length dependent and the reaction fails with (131, n = 0). ... 

The gibberellin derivative (55) is photochemically reactive and on irradiation affords the lactone (56). The reaction presumably involves a photoreaction akin to the Norrish Type II hydrogen abstraction process, in thii case a 1,8-hydrogen transfer, from the allylic site to yield the biradical (57). Bond... 

Irradiation of 2-methoxy- and 2-ethoxy-1,4-naphthoquinone in acetic anhydride solution gave the dimers (297).181 The formation of dimer (297b) was best brought about at 436 nm when a 90% yield was obtained. Irradiation of this dimer (297b) at 365 nm converted it into the bisoxetanol (298b) by a Norrish Type II hydrogen abstraction path. A similar reaction has been described for the dimer of... 

Norrish Type II hydrogen abstraction to afford the usual biradicals, which can cyclize into cyclobutanols. Both the c/ -(13) and the ra -isomeric forms are possible. This particular investigation has studied the influence of antibodies (12B4, 20F10 ad 21H9) on the cyclization reaction. The authors observed that the most reactive antibody, 20F10, catalyses the formation of the cw-product... 

Norrish Type II hydrogen abstraction is the predominant reaction on irradiation of the silylated ketones (27). This affords the dealkylated product (28). There is some Norrish Type I reactivity that results in the formation of the isomerized product (29) and the two ring-opened products (30) and (31). The ratio of the two reactions varies with the silyl group, with a 32 1 ratio of (28) (29) obtained from (27, R = Me) and a 13 1 ratio from (27, R = Ph or R3 = MePh2). ... 

The Oxa-Di-n-methane reaction and Related Processes. Interestingly, some P,y-unsaturated ketones do not undergo 1,3-acyl shifts or the oxa-di-Ti-methane rearrangement. An example of this is compound (61), which on irradiation undergoes only a Norrish Type II hydrogen abstraction. ... 

An example of a bimolecular photoinitiator that generates radicals by a Norrish type II hydrogen abstraction process is the combination of camphorquinone with a benzyl alcohol (eqn [47]). The carbon-centered radical, 74, is capable of reducing a diaryliodonium salt in a similar manner as shown previously in Scheme 24. 

Studies of the mechanism of photolysis of S,S-dialkyl-S-(phenacyl)sulfonium salts showed that irradiation at 248 nm produces Bronsted adds by a reversible intramolecular Norrish type II hydrogen abstraction process as shown in Scheme 27. Ylides are also formed as by-products. However, once the light is extinguished, the two photolysis products recombine to regenerate the starting sulfonium salt. Estimates of the quantum yields for these photoinitiators are 0.4-0.5. ... 

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