Fragmentation process ketones

As shown in Figure 42 for the Norrish II reactions of a simple ketone, 2-nonanone, not only do the shapes of the products differ from those of the reactant, but so do their molecular volumes [265]. Interestingly, the volume of the fragmentation products, 1-hexene and 2-hydroxypropene (which ketonizes to acetone), are closer in volume to 2-nonanone than is either of the cyclization products. They are also capable of occupying more efficiently the shape allocated by a stiff solvent matrix to a molecule of 2-nonanone in its extended conformation the cross-sectional diameter of either of the cyclobutanols is much larger than that of extended 2-nonanone or the fragmentation products when spaced end-on. Both of these considerations should favor fragmentation processes if isomorphous substitution for the precursor ketone in the reaction cavity is an important requirement for efficient conversion to photoproducts. 

The mass spectra of the ketone (XXXI) and macroline show considerable similarity, indicating that the two bases suffer analogous fragmentation processes on electron impact. Thus, fragmentation of XXXI by a route analogous to the sequence XVIII XXc XXb leads to the formation of the 11-methoxy derivative of XXb at m/e 227, which is the base... 

The mass spectra of ketones show an intense molecular ion peak. Loss of the alkyl groups attached to the carbonyl group is one of the most important fragmentation processes. The pattern of fragmentation is similar to that of aldehydes. 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

Another example of a [4S+1C] cycloaddition process is found in the reaction of alkenylcarbene complexes and lithium enolates derived from alkynyl methyl ketones. In Sect. 2.6.4.9 it was described how, in general, lithium enolates react with alkenylcarbene complexes to produce [3C+2S] cycloadducts. However, when the reaction is performed using lithium enolates derived from alkynyl methyl ketones and the temperature is raised to 65 °C, a new formal [4s+lcj cy-clopentenone derivative is formed [79] (Scheme 38). The mechanism proposed for this transformation supposes the formation of the [3C+2S] cycloadducts as depicted in Scheme 32 (see Sect. 2.6.4.9). This intermediate evolves through a retro-aldol-type reaction followed by an intramolecular Michael addition of the allyllithium to the ynone moiety to give the final cyclopentenone derivatives after hydrolysis. The role of the pentacarbonyltungsten fragment seems to be crucial for the outcome of this reaction, as experiments carried out with isolated intermediates in the absence of tungsten complexes do not afford the [4S+1C] cycloadducts (Scheme 38). 

Due to the retractive forces in stretched mbber, the aldehyde and zwitterion fragments are separated at the molecular-relaxation rate. Therefore, the ozonides and peroxides form at sites remote from the initial cleavage, and underlying mbber chains are exposed to ozone. These unstable ozonides and polymeric peroxides cleave to a variety of oxygenated products, such as acids, esters, ketones, and aldehydes, and also expose new mbber chains to the effects of ozone. The net result is that when mbber chains are cleaved, they retract in the direction of the stress and expose underlying unsaturation. Continuation of this process results in the formation of the characteristic ozone cracks. It should be noted that in the case of butadiene mbbers a small amount of cross-linking occurs during ozonation. This is considered to be due to the reaction between the biradical of the carbonyl oxide and the double bonds of the butadiene mbber [47]. 

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. 

To select between these two alternative structures it was necessary to synthesize a labeled analog. Three hydrogen atoms of the methyl moiety of the ester group were substituted for deuterium. One of the principal pathways of fragmentation of [M N2]+ ions involves the loss of CH3 radical. Since all R substitutes in diazo ketones 4-1 were also methyls it was important to detect what group exactly is eliminated from the [M N2]+ ion. The spectrum of deuterated sample has confirmed that the methyl radical of the ester moiety leaves the parent ion. As a result the cyclic structure 4-2 was selected as the most probable. The ketene structure 4-3 is hardly able to trigger this process, while for heterocyclic ion 4-2 it is highly favorable (Scheme 5.22). 

Cyclobutanol formation is not usually an efficient process for simple aliphatic ketones. It has, however, been shown12 that irradiation of the urea inclusion complex of 5-nonanone is more effective, providing l-butyl-2-methylcyclobutanol in 40% yield, with the balance of the ketone undergoing photochemical fragmentation. The cyclobutanol product is a 97 3 cisjtrans mixture. In the absence of urea, photolysis proceeds to give the cyclobutanol in 24% yield, as a 60 40 cisjtrans mixture. Photocyclization has also been improved by inclusion in zeolites13. 

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