Diradicals from ketones

On the basis of the general mechanistic concept used in Charts 11 and 12, the transformations of the steroid ketones 21 and 22 (Chart 3) can be rationalized as follows photoproducts 23, 24, and 26 are derived from ketone 21 via the common intermediate species 93, which may be produced upon 1,5-bridging in the excited state 91 and subsequent homolytic fission of the 1,10-cyclopropane bond in the resulting diradical 92 (Chart 13). Bonding between C-1 and C-6 in the key intermediate 93 (->- 94) and opening of the three-membered ring gives access to struc-... 

Diradicals, including 1,3- and 1,4-types incorporating the cyclopropylmethyl unit 39 and 40, have been generated from ketones, cyclic diazenes, and in various other ways (see Section 2.4.1.5.2.7.). The intra- and intermolecular reactions of the 2-(spiro[2.5]octadien-6-yl)ethyl diradical 41 lead to some novel products. ... 

Ketones such as 2,2,5,5-tetraphenylcyclopentanone and 2,2,6,6-tetraphenylcyclohex-anone decarbonylate rapidly because of the stabilization afforded by the phenyl groups. The products result from recombination, disproportionation, or fragmentation of the diradical intermediate. ... 

Intramolecular hydrogen-atom abstraction is also an important process for acyclic a,/ -unsaturated ketones. The intermediate diradical then cyclizes to give the enol of a cyclobutyl ketone. Among the by-products of such photolyses are cyclobutanols resulting from alternative modes of cyclization of the diradical intermediate ... 

Acetylenedimagnesium bromide, 66, 84, 137 Acyl-alkyl diradical disproportionations, 299 Acyl-alkyl diradical recombination, 296 Alkaline hydrogen peroxide, 10, 12, 20 Alkylation of formyl ketones, 93 Alkylation via enolate anions, 86 17a-Alkynyl steroids from 17-ketones, 67 2a-Al]yl-17jS-hydroxy-5a-androstan-3 -one, 9 5 Allylic acetoxylation, 242 Allylmagnesium bromide, 64 17 -Aminoandrost-5-en-3 -ol, 145 17 a-Aminomethy 1-5 a-androstane-3, 1718-diol, 387... 

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 ... 

In cyclohexane the same two ketones (12) and (13) are obtained from the photolysis of (11) but in aqueous dioxane two phenols are isolated as well as the bicyclic ketone (12). Swenton(10) suggested that the gas-phase reaction involves diradical species, whereas in polar solvents zwitterionic intermediates are favored ... 

The photochemistry of cyclobutanone presents a special case since the Norrish type-I cleavage to give an acylalkyl diradical intermediate releases ring-strain energy. Thus the energy available for subsequent reactions is reduced correspondingly, compared to the energy retained in an acyl radical from an acychc ketone, or less strained cyclic ketones. 

The formation of the tra/w-fused 4/ ,5a-ethylene adduct (262) has analogies in the addition of olefins to simple a,/ -unsaturated ketones. The possibility is that this compound derives in a concerted antara-antarcfi cial process from an excited state of the dienone (260) in which the a,j8-bond is nonplanar. Alternatively, it has been argued that upon a stepwise addition of the olefin to the excited (di)enone system a diradical is formed (e.g., through the formation of a single bond to the /2-carbon in the first step), and that part of the excess energy of this intermediate is absorbed in the formation of the strained trans-fused system. A two-step reaction is consistent with the result that the addition of either cis- or traws-dichloroethylene to enone (267) affords the same 1 1 mixture of 16a,17a-adducts [(269) and (270)]. 

The comments made about the diradical hypothesis with respect to the photochemistry of cyclopentanone are equally applicable to cyclohexanone. Since the formation of none of the products listed in reactions 15-18, and (15,29) is quenched by even 10-20 mm. of oxygen the existence of diradical intermediates in this system is subject to question. The alternative mechanism would be one that causes a concerted split of the ketone molecule in the excited state into two (in the cases of reactions 15 and 16) or three (reaction 17) molecular fragments. Both 16 and 17 are analogous to reactions 3 and 2 in the photochemistry of cyclopentanone and do not involve a shift of hydrogen atom from one... 

In addition some p.opane and n-hexane are formed. These are also believed to originate from the trimethylene diradical. A direct comparison of these results with the results reported by other workers is not possible since the only other study in the presence of a foreign gas involved water vapor at a pressure of 70 mm. (5). In the latter instance the ketone pressure was 136 mm., which is 5 to 10 times greater than the pressure used in ref. (17). The presence of water vapor does not seem to have affected the yields of propylene and of cyclopropane. Since the diradical mecha-... 

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

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