Norrish type II cyclization

Several absolute asymmetric Norrish type II cyclizations have been reported in the solid state. Achiral a-(3-methyladamantyl)-/ -chloroacetophenone 114 formed a chiral crystal in space group P2i2i2i. Irradiation of these crystals caused Norrish/Yang cyclization to afford the cyclobutenol-type photoproduct 115 (six chiral centers) with respectable enantiomeric excess at low conversion (Scheme 27) [101]. Photolysis in solution phase led to mixtures of four of the six possible cyclobutanols, and no trace of optical activity could be detected in the mixtures. 

Scheme 28 Absolute asymmetric Norrish type II cyclization to 0-lactams.

Fleming, M., Basta, R., Fisher, P.V., Mitchell, S., and West, F.G., Synthesis of bridged bicyclic ethers and fused oxetanes from pyran-4-ones via tandem solvent trapping and Norrish type II cyclization, /. Org. Chem., 64, 1626, 1999. 

Frequently B will also undergo a back hydrogen transfer which regenerates the parent ketone, as well as cyclization (in most cases a minor reaction) as a result of this competition the quantum yields of fragmentation are typically in the 0.1-0.5 range in non-polar media. When the Norrish Type II process takes place in a polymer it can result in the cleavage of the polymer backbone. Poly(phenyl vinyl ketone) has frequently been used as a model polymer in which this reaction is resonsible for its photodegradation, reaction 2. 

Norrish Type II photocyclizations have been employed in many instances in the synthesis of nitrogen heterocycles. Type II cyclizations are the result of an intramolecular hydrogen abstraction by an excited carbonyl group followed by cyclization of the resulting biradical. Hydrogen abstraction from the y-carbon atom is normally preferred. The introduction of a nitrogen atom... 

When an excited aldehyde or ketone has a y hydrogen, intramolecular hydrogen abstraction via a six-membered ring transition state usually occurs. The resulting 1,4-biradical may either cleave or cyclize to give the Norrish Type II products of Scheme 4. 

The first three chapters by Wagner, Wessig, and Griesbeck deal with typical carbonyl chemistry Norrish type II reactions followed by Yang-cyclization, homologous Norrish type II reactions (i.e. hydrogen abstractions from non y-positions), and Paterno-Buchi [2+2]-photocycloadditions. The enantiomerically pure (S-am ido-cycl o butanol 1 is formed from a chiral... 

There are four distinct processes initiated by y-hydrogen abstraction in excited carbonyl compounds Norrish type II photoelimination, Yang photo-cyclization (cyclobutanol formation), Yang photoenolization (o-xylylenol formation), and (3-cleavage of radicals from carbons adjacent to the radical sites of the 1,4-biradicals. Some of these require unique structures and generate distinct products. 

Substituents in a-Position. To rationalize the ratio between Norrish type II cleavage and cyclization (see preceding chapter) the influence of a-substituents on cyclization were studied exclusively on straight-chain aryl ketones with preferential y-hydrogen abstraction. 

However, the Norrish Type II cleavage doesn t matter in carbonyl compounds with (y )-hydrogen abstraction. In this case, one has to bear in mind that electron-donating a-substituents can promote the Norrish Type I cleavage by stabilizing the resulting radical. For that reason, a-cleavage can compete with the desired cyclization. 

PET cyclizations of donors-substituted phenylgloxylates have been intensively studied by the groups of Neckers and Hasegawa. For the thioether derivatives 1 the efficiency of the cyclization depended on the linker chain length (Sch. 5) [18]. With increasing carbon tether (>7 = 2-10) the yield dropped steadily and secondary reductive dimerizations or Norrish Type II processes became competitive. Consequently, the Cn-linked substrate solely underwent dimerization and cleavage [18b]. 

The host-guest and ionic chiral auxiliary approaches have been most intensively applied for solid-state asymmetric induction. A number of achiral organic compounds could be converted into chiral compounds in high enantioselectivities. However, all the photoreactions in themselves are well-known intramolecular photoreactions photocyclization, [2 + 2] photocyclization, Norrish type II photo-cyclization, di-ir-methane photorearrangement and photoisomerization. New types of asymmetric photoreactions have never been reported. 

A laser flash study of the photoreactions of hexan-2-one and 5-methylhexan-2-one has provided evidence for the existence of the triplet 1,4-biradicals produced by the y-hydrogen abstraction typical of Norrish Type II reactivity. The photochemical behaviour of the alkanone, nonan-5-one, in urea inclusion compounds has been studied. In solution, irradiation of nonan-5-one yields hexan-2-one, propylene, and two cyclobutanols. In the clathrate, the fragmentation products were essentially the same but only one cyclobutanol was observed. The cyclization fragmentation ratio was established as 0.67, compared with 0.32 in methanol. The authors suggest that the CIS-cyclobutanol has less stringent rotational requirements and that it is this isomer (43) which is formed in the clathrate. 

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). 

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... 

The N-aryl-N, N -dibenzylureas (252) undergo photochemical hydrogen abstraction of a Norrish Type II kind to afford biradicals (253) which subsequently cyclize and undergo dehydration to the imidazolidinones (254). "... 

Coyle and Bryant have reported the efficient synthesis of a triazabicyclo[3.3.0]octane system by the photocyclization of hydantoin derivatives. Thus irradiation of (295), synthesized by standard Mannich procedures, afforded the bicyclooctanes (296) in moderate to good yields. The process, akin to Norrish Type II behaviour, is a 1,6-hydrogen transfer followed by bonding in the resultant biradical (297). The bis-Mannich base compound (298) also photocyclizes, yielding (299). Coyle and his co-workers have also successfully applied the known reactivity of phthalimides to yield polycyclic compounds. Thus irradiation of (300) affords the cyclized compound (301). This can be converted into the protoberberine alkaloid skeleton (302) by treatment with aqueous HCl. The crown ether analogues (303) and (304) have... 

Norrish Type I fission of the side chain carbonyl group again at C-4. - Laser flash irradiation has been used as a aethod for the production of n-butylkotene from cyclohexanone. The chemistry of this ketene was studied in detail. The cyclohexanones (9a) undergo both Norrish Type I and II processes on irradiation. The fluorinated compounds (9b) showed a preference for Norrish Type II behaviour. Within the Norrish Type II biradical fluorine substitution leads to a preference for cyclization rather than cleavage. The Norrish Type I biradical afforded a ketene rather than an alkenal. A study of the photochemical reactivity of the diones (10) has shown that both Norrish Typo I and Type II reactivity can take place. The Typo I Type II product ratio is dependent upon ring size. Thus dione (10a) affords the Type II products (11) and (12) while dione (10c) yields the Norrish type I products (I3c-15c) and low yields of the Norrish Type II products (11) and (12). Compound (10b) is intermediate between these results affording a Type I Type II ratio of 0.3. A mechanistic study of the reactions was carried out. - ... 

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