Norrish hydrogen abstraction

Typical chemical reactions of photoexcited aldehydes and ketones are cleavage reactions, usually designated as Norrish Type I [equation (54)], II [equation (55)] and III [equation (56)], hydrogen abstraction [equation (57)] and cycloadditions, such as the Paterno-Buchi reaction [equation (58)]. Of these, Norrish Type II cleavage and the related... 

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

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... 

Explain how intramolecular hydrogen abstraction in carbonyl compounds can lead either to cleavage (Norrish type 2 reaction) or to the formation of cyclic compounds (Yang cyclisation). 

Intramolecular hydrogen abstraction from a g carbon occurs when certain ketones are irradiated. For e.g., vapours of Hexan-2-one on photolysis gives an alkene and a ketone (via the enol form). It is referred to as Norrish type II cleavage. 

A second example from Salem s work which illustrates the role of symmetry is the photochemical hydrogen abstraction by ketones, termed the Norrish type II reaction (Norrish, 1937). A typical case is illustrated in (69). 

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

Rather phase-insensitive Norrish II photoproduct ratios are reported from irradiation of p-chloroacetophenones with a-cyclobutyl, a-cyclopentyl, a-cycloheptyl, a-cyclooctyl, and a-norbonyl groups [282], In each case, the E/C and cyclobutanol photoproduct ratios are nearly the same in neat crystals as measured in benzene or acetonitrile solutions. On this basis, we conclude that the reaction cavity plays a passive role in directing the shape changes of these hydroxy-1,4-biradicals. As long as the initial ketone conformation within the cavity permits -/-hydrogen abstraction (and these ketones may be able to explore many conformations even within their triplet excited state lifetime), the cavity free volume and flexibility allow intramolecular constraints to mandate product yields. 

In a 5-nonanone/urea complex whose molar ratio suggests that the channels of the host are nearly filled with guest molecules [294], it was found that the time required to effect photoconversion is similar to that required in solution consistent with 5-A diameter channels (see Figure 1) attenuating only slightly the motions necessary for the initial y-hydrogen abstraction, the total quantum efficiency in the complex does not appear to be reduced appreciably. However, the photoproduct ratios clearly indicate that the fate of the hydroxy-1,4-biradicals is controlled by the urea channels. Only Norrish II photoproducts could be isolated, but they did not include the cis cyclobutanol (Eq. 12). 

Although all the products can be rationalized on the basis of 7-hydrogen abstraction followed by cyclization or rearrangement-cyclization of 1,4-biradical intermediates, the mechanism has been shown to involve analogous zwitterionic intermediates [299b, 301], Although they are not strictly Norrish II reactions, transformations of 98 will be considered so for the purposes of discussion since the photoproducts and the mechanisms of their formation are very similar to those expected of Norrish II processes. 

Intramolecular hydrogen abstraction of this type is a well-documented process (the Norrish type-2 process) in the photochemistry... 

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 photocyclization reaction is more efficient with p-anomers, and the new C—C bond created at the anomeric center is obtained by preferential axial anomeric hydrogen abstraction followed by carbocyclization. Norrish II reactions generally result from hydrogen abstraction at the y-position according to Scheme 31 to give cyclobutanols or degradation products. For 68, (n = 1) photoabstraction at the 5-position of the anomeric hydrogen leads to cyclopentanols 69 (Scheme 37) [67]. 

The E-anomeric hydrogen of compound 74 is too far away to permit abstraction and the Norrish II process results from the y-hydrogen abstraction of the aglycone chain. In this way, O-vinylglycosides 75 can be formed [74] (Scheme 39). 

M425>. Here the selectivity was only low with respect to the site of addition, which could be either the benzoyl or 4-substituted benzoyl group. Phenyl glyoxylates can also be successfully utilized as reactive carbonyls in the Paterno-Btichi reaction as demonstrated by Hu and Neckers <1997JOC564>. Oxetanes were formed in very high yields with electron-rich (e.g., polyalkylated) alkenes, but with monosubstituted alkenes there was no oxetane formation due to the prevalence of Norrish II type hydrogen abstraction (Scheme 22). 

The Norrish-Yang reaction [63] is a widely used photochemical reaction, which consists of an intramolecular hydrogen abstraction by a photoexcited ketone, followed by carbon-carbon bond formation in the (l,n)-biradical intermediates (Scheme 9.38). 

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 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. 

---