Carbonyl group, excited

The excited carbonyl groups have radical character at both carbon and oxygen, and the oxygen is rather similar in its reactivity to alkoxyl radicals. The MO diagrams for the n-n and 7t-7t states can be readily depicted ... 

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... 

The process of intramolecular abstraction of an hydrogen atom from the y carbon atom by the excited carbonyl group is commonly referred to as Norrish type II reaction. The resulting diradical can close to a cyclo-... 

A dienol is also formed via hydrogen abstraction by the excited carbonyl group from a second enone molecule in (4.14). This dienol tautomerizes in C6F6 to give the (3,y-unsaturated ketone selectively, the overall reaction thus being deconjugation of the a,(3-unsaturated ketone415K... 

Cycloaddition Reactions between Excited Carbonyl Groups in Carbohydrate Derivatives and Unsaturated, Non-Carbohydrates... 

Note 2 Some main-chain scissions are classified according to the mechanism of the scission process hydrolytic, mechanochemical, thermal, photochemical, or oxidative scission. Others are classified according to their location in the backbone relative to a specific structural feature, for example, a-scission (a scission of the C-C bond alpha to the carbon atom of a photo-excited carbonyl group) and P-scission (a scission of the C-C bond beta to the carbon atom bearing a radical), etc. 

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>. 

The luminescence arises from dye molecules which are sensitized by energy transfer from the excited carbonyl group. 

A reaction of particular interest is the photoinduced ring expansion in 95% yield of cis-l-<-butyl-2-phenyl-3-benzoylazetidine (32) to l- -butyl-2,4-diphenylpyrrole (33).36 This is regarded as the first example of the migration of an alkyl group from the a-position to the excited carbonyl group (n->n ). 

A final example of an n-n reaction will be cited to illustrate still another type of reactivity of the n—r excited state. This is the photolysis of frans-dypnorie oxide (XXXII) to afford as the major product the unsaturated alcohol XXXIV (22). In this case the mechanism, as outlined in Chart VI, involves hydrogen abstraction from the methyl group by the m-situated excited carbonyl group the odd electron containing... 

The basis of the 7-hydrogen transfer lies in the proximity of such hydrogens to the reactive carbonyl group. If by virtue of structure other hydrogens are closer to the excited carbonyl group, they will be transferred to the carbonyl group. An example of such an effect is found in the photochemical rearrangement of cyclodecanone (Formula 257) to the bicyclic alcohols [Formulas 258 (42%) and 259 (10%)] (108,109). Cyclo-... 

This applies to triplet diradicals only, not to singlet diradicals whose lifetimes are too short for conformational equilibration. This may be used to conserve chirality information of the reactant as depicted in Scheme 5b. Accordingly, the singlet excited carbonyl group of a ketone 20 abstracts a hydrogen atom from a chirality center whose chirality information is preserved in the planar chirality of the extremely short-lived singlet diradical 21. On cyclization to 22 this information is... 

The other way to prepare cyclopropanes is based on the spin-center-shift. Thus, aryl-alkyl ketones, 26, bearing a leaving group X adjacent to the excited carbonyl group undergo a smooth cyclization to benzoylcyclopropanes 27 in moderate to good yields (Scheme 6b) [4a, b]. The reaction tolerates a variety of functional groups, often proceeds stereoselectively, and can be extended to the preparation of bicyclo[n.l.O]alkane derivatives. 

For reasons discussed in detail in the preceding chapter, the y-position with respect to the excited carbonyl group is preferably attacked by the oxygen atom and 1,4-biradicals are formed. This chapter deals with reactions that are initiated by hydrogen abstraction from a non-y-position... 

The regioselective attack of nonactivated C-H-bonds by an excited carbonyl group is demonstrated with the transannular cyclization of bicyclic diketones 76, which can easily be prepared from cyclic ketones 75 according to Stetter et al. [61]. Upon irradiation, the selectively excited benzoyl... 

The Paterno-Biichi reaction between excited carbonyl groups and olefins leads to the formation of four-membered cyclic ethers (oxetanes). This reaction has been reviewed by Arnold [94], Jones [95], Wilson [96]. The scope of the Paterno-Buchi reaction is impressive. It might be initiated by either singlet or triplet, n, re or, less frequently re, n excited carbonyls, and both electron-rich as well as extremely electron-poor olefins participate. It is now widely accepted that the initial intermediate in the oxetane formation is an exciplex between the excited carbonyl triplet and the olefin. The exciplex is then thought to decay to the triplet 1,4-diradical which, upon ISC to the singlet diradical, may either fragment back to the starting components or cyclize to oxetanes. 

A. oc-Cleavage is the common unimolecular, intrachromophoric radical-producing reaction of the excited carbonyl group and is closely analogous to the /S-cIeavage of alkoxy radicals 16>. Acyclic compounds yield a pair of radicals... 

When one takes up a polyfunctional molecule which contains at least one carbonyl group, one knows which reactions of that carbonyl group are possible. One does not know how well these reactions will compete with various physical and chemical interactions of that excited carbonyl group with other functional groups in the molecule. It is this aspect of carbonyl photochemistry which requires and deserves extensive future research. The uniquely well understood chemistry of the carbonyl group can serve as a monitor for studying interactions in electronically excited polyfunctional molecules. 

In the absence of y- and 8-hydrogen atoms, the abstraction of an activated hydrogen by the n,7r -excited carbonyl group becomes possible, and cyclopro]... 

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