Type I Cleavage

Actually, type I cleavage occurs from both singlet and triplet excited states, as evidenced by the incomplete quenching of the photolysis of acetone by high pressures of biacetyl308 or 2-butene.243 The extent to which dissociation can be quenched is subject to large temperature and wavelength effects and depends markedly on the stability 

With ketones which can eject radicals more stable than methyl, fragmentation competes more successfully with all physical processes than in acetone, and unsymmetrical ketones preferentially eject the more stable alkyl radical.309 Thus both methyl ethyl ketone310 and methyl isopropyl ketone311 yield chiefly acetyl and ethyl or isopropyl radicals. Half of the diethyl ketone molecules excited by 3130-A irradiation at 25° decompose from the excited singlet state before they can undergo intersystem crossing, and another 40% fragment from the triplet state.312 Both fluorescence and phosphorescence are extremely weak. The more rapid decomposition in both excited states relative to that observed in acetone almost eliminates competition from physical-decay processes. 

The photochemistry of acetone is perhaps more complicated than that of any other ketone, because the rates of all the chemical and physical reactions of its excited states are closely competitive. Since 

The quantum yield of fluorescence from 3130-A excitation of gaseous acetone has been measured as 0.002,308 whereas a value of 0.01 has been measured in solution.316 If the rate of fluorescence, as calculated from the integrated absorbance intensity, is 4 x 10 sec-1,316 the rate of intersystem crossing of singlet acetone lies between 4 x 107 and 2 x 108 sec-1. 

The quantum yield for decomposition of acetone from the triplet state is about 0.40 at 40°, and the quantum yield of phosphorescence is about 0.02.308 At least 407o of the initially excited molecules are not accounted for by either emission or chemical reaction, and thus must undergo some kind of radiationless decay to the ground state, presumably mostly from the triplet state. Recent studies of hexafluoro-acetone317 indicate that approximately half the triplet molecules formed by intersystem crossing undergo radiationless decay. 

In Chapter 3 we discussed two photochemical reactions characteristic of simple carbonyl compounds, namely type II cleavage and photoreduction. We saw that photoreduction appears to arise only from carbonyl triplet states, whereas type II cleavage often arises from both the excited singlet and triplet states. Each process was found to occur from discrete biradical intermediates. In this chapter we will discuss two other reactions observed in the photochemistry of carbonyls, type I cleavage and oxetane formation. 

The various routes available to these radicals are shown in the following scheme  

Each of these reactions will be discussed in greater detail after we investigate the nature of the excited state involved in type I cleavage. 

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With strained cycloketones the type I-cleavage gives better yields, and can be used as a preparative method. For example photolysis of the bicyclic ketone 16 gives diene 17 in good yield ... 

In general however the various possible reaction pathways give rise to formation of a mixture of products. The type I-cleavage reaction is only of limited synthetic importance, but rather an interfering side-reaction—e.g. with an attempted Paterno-Buchi reaction, or when an aldehyde or ketone is used as sensitizer in a [2 -I- l -cy do addition reaction. 

Norrish Type I cleavage of cyclic ketones necessarily yields biradicals, and in certain cases (e.g., cycloheptanone, camphor) strong emissions due to T i S mixing have been reported (Gloss and Doubleday, 1972). 

When applied to ketones, this is called Norrish Type / cleavage or often just Type I cleavage. In a secondary process, the acyl radical R —CO can then lose CO to give R radicals. Another example of a category 1 process is cleavage of CI2 to give two Cl atoms. Other bonds that are easily cleaved by photolysis are the 0—0 bonds of peroxy compounds and the C—N bonds of aliphatic azo compounds R—N=N—R. The latter is an important source of radicals R , since the other product is the very stable N2. 

The mechanism probably involves a Norrish type I cleavage (p. 318), loss of CO from the resulting radical, and recombination of the radical fragments. 

Cyclohexanones undergo type I cleavage to produce a mixture of ketenes and aldehydes by hydrogen transfer,<1-3)... 

Barltrop and Coyle<12) have presented strong evidence for the participation of a discrete diradical intermediate in type I cleavage. In this study a... 

Ddla, quantum yield for disappearance of ketone j, quantum yields for type I cleavage n, u, quantum yields for type II cleavage cb. cb> quantum yields for cydobutane formation. 

In this section a brief review of the various reactions arising from type I cleavage and some synthetic applications of these reactions will be presented. 

Type I Cleavage Reactions Proceeding through Carbene Intermediates... 

Perhaps the first examples of this type of reaction resulting from type I cleavage were reported by Yates in a study of the photochemistry of cyclocamphanone and nortricyclanone,... 

From the reactions presented in this section one can conclude that cyclic acetal formation via addition to a carbene intermediate is a general reaction for type I cleavage of cyclobutanones, tricyclic compounds, and certain bridged bicyclics as minor products. No acetal has been isolated from photolyses of cyclopentanones or cyclohexanones except for the special case of an a-sila ketone previously discussed. 

The following reactions are believed to result from addition to ketene intermediates produced from type I cleavage reactions ... 

Type I Cleavage Reactions Leading to Molecular Rearrangement... 

Low-Temperature Observation of Intermediates in Type I Cleavage Reactions... 

By carrying out photolyses in liquid nitrogen- or liquid helium-cooled infrared cells using a special low-temperature apparatus (see Figure 4.2), one is often able to obtain direct spectroscopic evidence for intermediates of photochemical reactions. In this section we will briefly review how low-temperature techniques have been used to observe intermediates in type I cleavage reactions. 

In previous sections of this chapter we have seen many examples of type I cleavage reactions in which loss of carbon monoxide was not an important process. In the examples given above, however, decarbonylation is important, as evidenced by the high yields of decarbonylated products. Factors which facilitate decarbonylation include the presence of a suitably located cyclo-... 

Type A photorearrangement, 323, 329, 331 Type I cleavage of carbonyls, 135-171 application of extended Hiickel theory to, 179-181... 

Type I cleavage of ketones, 75 Type II cleavage of ketones effect of 7-substituents, 122-124 effect of ring substituents, 124 of 5 -(+)-4-methyl-l-phenyl-1- hexanone, 118... 

The photolysis of the following steroid system resulted in two products corresponding to the Norrish type II reaction and one product due to a-cleavage (Norrish type I cleavage)<101) ... 

The other photochemical reactions of simple carbonyls mentioned earlier in this chapter—type I cleavage (a-cleavage) and oxetane formation—will be discussed in Chapter 4. 

The Photochemistry of Simple Carbonyl Compounds Type I Cleavage and Oxetane Formation... 

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