Elimination, Norrish

Other syntheses of the spiro[3.3]hepta-l,5-diene system by Cope elimination. Norrish type II reaction and dehydrobromination, respectively, are shown below.ig... 

The synthesis of l,3-oxazin-4-ones of type 464 is the first example of the formation of a C-O bond in the course of the Norrish-Yang reaction. Upon treatment with 1-hydroxy-l-phenyl-A -iodanyl mesylate, /3-keto amide 460 was converted to the corresponding a-mesyloxy-/3-keto amide 461 in excellent yield. On ultraviolet (UV) irradiation (A>300nm) of 461, 5-hydrogen transfer to the excited carbonyl group occurred and the diradical 462 thus formed underwent MsOH elimination to enolate diradical 463, cyclization of which resulted in formation of 3-methyl-6-phenyl-3,4-dihydro-277-l,3-oxazin-4-one 464 (Scheme 89) <2001S1258>. 

Based on the known photoreduction chemistry of Rose Bengal [275], one would anticipate that electron transfer would reduce the xanthene skeleton of RBAX and that the radical anion thence formed might decay by the elimination of an acetyl radical. Acetyl is totally analogous to benzoyl, the radical that initiates chains in the case of most Norrish type I UV photoinitiators, that is, benzoin ethers or acetophenone acetals. The putative scheme is shown in Scheme 7. 

In a lecture presented to the Faraday Society,332 Norrish commented on why the higher vibrational levels of NO were observed in the nitrosyl halide experiments but not by absorption in NO irradiation experiments. He reasoned that, as the emission of NO from the AZH + state populates the first five vibrational levels of NO almost equally,341 the fast-exchange reaction (4) can quickly eliminate all the vibrational levels above the first. However, in the nitrosyl halide experiments, the NO may be formed preferentially in very high levels, such as v — 10 or 11, almost exclusively. Thus, reaction (4) cannot occur initially, and depopulation must be by reaction (2). Reaction (2) is considerably slower than reaction (4), because of the increased difference in vibrational energy between the reactants and products resulting from an-harmonic effects. 

It was first noted by Spence (56), that mercury had an accelerating effect on the formaldehyde oxidation. Lewis and von Elbe (32) suggest that the absence of an induction period in the experiments of Axford and Norrish (/), compared to those of Snowdon and Style (55), was to be attributed to the destruction of peroxides by mercury vapor from heated mercury cutoffs in the Axford-Norrish experiments. This was confirmed in Schecr s work, where mercury vapor eliminated the induction period. A freshly cleaned surface had an effect similar to mercury vapor. It was found that although the induction period represents only a slow pressure rise, extensive reaction occurs, the A ft during this time being unrelated to the formaldehyde consumption. 

In view of the evidence cited, it is clear that reactions of formaldehyde make a substantial contribution to the kinetics of the high-temperature oxidation of ethylene. However, attention should be drawn to one further observation of Harding and Norrish 24). The maximum pressure of formaldehyde developed at 400° C. approximates 6 mm., whereas the amount just necessary to eliminate the induction period at this temperature is close to 18 mm. This result suggests that another intermediate may play a part without affecting the kinetics of the reaction. In this connection it was observed that ethylene oxide was formed and built up to a small steady-state concentration during the induction period. The relative ineffectiveness of prior addition of ethylene oxide in reducing the induction period is consistent with the conclusion that formaldehyde plays the dominant role. 

One of the most interesting molecular elimination reactions was first discovered by Norrish and Appleyard65 in 1934 and studied further by Bamford and Nor-rish66-69 in papers appearing in 1935 and 1938. These authors found that, on photolysis, aliphatic ketones with hydrogen atoms on carbons in the gamma position to the carbonyl yielded olefins and a methyl ketone. An early example was found in 2-hexanone, viz. 

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 ... 

The idea of a spin center shift as an extension of the synthetic scope of the Norrish-Yang reaction was already discussed twice in this chapter. A third example applies to the preparation of six-membered rings. Thus, by treating (3-ketoamides 42 (Sec. 3.4.2.1, Ih) with hypervalent iodine(III) reagents the oc-methanesulfonyloxy-(3-ketoamides 59 are obtained in excellent yields. On irradiation, the 1,5-biradicals 60 are formed, which are converted into the biradicals 61 by elimination of methanesulfonic acid. 

As mentioned above, the photooxidation was discovered by exposure of compound 22 to sunlight. The reaction proved to be of great value for angucycline synthesis because the -hydroxy group present in most natural products which is easily eliminated under basic or acidic conditions (see Scheme 2) and the carbonyl group at C-1 can thus be introduced under mild neutral conditions. We assume that the reaction is initiated by Norrish type II y-hydrogen abstraction of the excited carbonyl in 25 to yield a diradical 26 as shown in Scheme 8 with 1-deoxyrabelomycin (25) as the example [39]. The H-abstraction requires a very definite steric environment in which the benzylic protons have to be in proximity of the excited carbonyl group. Subsequent addition of the diradical 26 with... 

Norrish reaction Pericyclic elimination Tandem cationic cyclization Bergmann cycloaromatization... 

The importance of alkylperoxy radicals as intermediates had long been realized (see Sect. 2) and their subsequent reaction to yield the alkyl-hydroperoxide or decomposition products such as aldehydes and alcohols had been reasonably successful in describing the mechanism of the autocatalytic oxidation of alkanes. However, even though 0-heterocycles (which cannot be derived from intermediate aldehydes) had been found in the products of the oxidation of n-pentane as early as 1935 [66], the true extent of alkylperoxy radical isomerization reactions has been recognized only recently. Bailey and Norrish [67] first formulated the production of O-heterocycles in terms of alkylperoxy radical isomerization and subsequent cyclization in order to explain the formation of 2,5-dimethyl-tetrahydrofuran during the cool-flame oxidation of n-hexane. Their mechanism was a one-step process which involved direct elimination of OH. However, it is now generally formulated as shown in reactions (147) and(I67)... 

Direct irradiation of the ester (46) affords isobutyrophcnonc and the cis-cyclobutanol (47) in 35% and 26% yields, respectively. The elimination and cyclization products are the result of Norrish Type II behaviour. However, the selectivity in the cyclization is controlled by the ester function, presumably via hydrogen bonding in the intermediate biradical (48). The ester (49) also cyclizes selectively on irradiation to yield the trans-cyclobutanol (50). 

Bamford and Norrish observed that the free radical formation is the sole primary process in the photolysis of cyclohexanone, while step II is the major reaction occurring in the photolysis of 1-menthone. These results are rather difficult to interpret if reaction II occurs through a four-centred ring complex however, if a six-centred complex is involved, the consideration of the steric factors leads to a conclusion which is reconcilable with the results of Bamford andNorrish. The significance of steric factors (stereoelectronic requirements) appears from the fact that type II elimination is the major intramolecular path in the photolysis of ciy-2- -propyl-4-t-butyl cyclohexanone, while the photolysis of the tram compound yields the cis isomer as the major product The difference has been explained... 

The amide (38a) is photochemically inert on irradiation in ether. The related compound (38b) is, however, photochemically reactive and undergoes fission by a Norrish Type II process to yield a mixture of products.The results of a study of the enantioselective photodeconjugation reactions of the lactones (39) have been published. The behaviour of the ketones (40) and (41) in the isotropic and the two solid phases of heneicosane (CcxH ) has been evaluated. The influence of the various phases on the ratio of elimination to cyclization products of the ketones was discussed. The modification of the photoreactivity of ketones (42) in cyclodextrin has been... 

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

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