Cyclobutanol, photochemical formation

Yang photocyclization refers to the photochemical formation of cyclobutanols following Norrish type II y-hydrogen atom abstraction and was first reported by Yang NC, Yang DH. J. Am. Chem. Soc. 1958 80 2913. 

The photochemical formation of cyclobutanols is substantially favored if the ethylene that would be formed in a type II elimination must have a bridgehead double bond.120) Jn 1-adamcUitylacetone, 77, the strain is sufficient to substantially inhibit the elimination of acetone from the molecule ion. 12 ) Cyclobutanol formation is the virtually exclusive photochemical pathway for 71 and related bridgehead acetone derivative. 

Upon irradiation in solution aliphatic ketones containing 7-hydrogens from cyclobutanols as well as undergo photoelimination Cc g-) Eq. (46) Two mechanisms have been considered to account for the photochemical formation of cyclobutanols. The first is a stepwise mechanism, Eq. (59), and the second is a concerted process, Eq. (CO). 

Photochemical C —H insertion of ketone 1 proceeds by initial photoexcitation to give an excited state that can be usefully considered as a 1,2-diradical. Intramolecular hydrogen atom abstraction then proceeds to give a 1,4- or 1,5-diradical, which can collapse to form the new bond. This approach has been used to construct both four- and ftve-membered rings12 11. Photochemical-ly mediated cyclobutanol formation is known as the Norrish Type II reaction. 

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. 

Diketones cyclize much more efficiently than simple ketones. Photochemical cyclobutanol formation is the key step in a novel route14 to 1,3-cyclopentanediones, as exemplified by the conversion of l-bromo-5-phenyl-2,3-pentanedione to 4-phenyl-l,3-cyclopentanedione. 

The photochemical addition of cyclic 1,3-diones such as dimedone, 1,3-cylohexandione 62, or their respective silyl enol ethers leads to the formation of two fused furanylfullerenes, (1) achiral 63 and (2) chiral 64 [244], The latter having an unusual bis-[6,5] closed structure. In the initial step of this reaction, [2 + 2] photocycloaddition across a [6,6] bond to form cyclobutanols or the corresponding TMS ethers is involved (Scheme 26). Oxidation with 02 yields in the formation of the radical 65a. Cleavage to 66a followed by cyclization gives furanyl radical 67a. H abstraction by 102 or a peroxy radical finally leads to product 63. In competition, formation of fullerene triplets by absorption of a... 

Due to the very limited amount of experimental data available, a choice between the two mechanisms is even more difficult for reaction III than for reaction II. Orban et investigated the photochemical rearrangement, in pentane, of an aliphatic, optically active ketone with a single asymmetric carbon atom in the y position. Their results demonstrate a partial retention of configuration during photochemical cyclobutanol formation, which can be explained neither by a biradical mechanism alone, if this involves a long-lived radical, nor by a concerted mechanism alone. The results are reconcilable with the competitive participation of both mechanisms, but they are just as compatible with the assumption of the production of a short-lived biradical whose rates of racemization and of cyclization are comparable. 

A study of the photochemical reactivity of salts of the amino ketone (44) with enantiomerically pure carboxylates has been reported. The irradiations involved the crystalline materials using A, > 290 nm and the reactions are fairly selective which is proposed to be the result of hindered motion within the crystalline environment. Some of the many results, using (S)-(—)-malic acid, R-(+)-malic acid and (2R,3R)-(+)-tartaric acid, are shown in Scheme 1. The principal reaction in all of the examples is a Norrish Type II hydrogen abstraction and the formation of a 1,4-biradical. This leads mainly to the cis-cyclobutanol (45) by bond formation or the keto alkene (46) by fission within the biradical. A very minor path for the malate example is cyclization to the trn 5-cyclobutanol (47). A detailed examination of the photochemical behaviour of a series of large ring diketones (48) has been carried out. Irradiation in both the solid phase and solution were compared. Norrish Type II reactivity dominates and affords two cyclobutanols (49), (50) and a ring-opened product (51) via the conventional 1,4-biradical. Only the diketone (48a) is unreactive... 

Under normal conditions, excited carbonyl compounds isomerize to cyclobutanols (see Section 7. A.1.1.5.) via intramolecular y-hydrogen abstraction and closure of the intermediate 1,4-biradical. Nevertheless, a similar, formally /i-hydrogen abstraction, step leading to cyclopropanols occurs with several A-functionalized ketones (see Houben-Weyl, Vol. 4/5 b, p 797, literature up to 1971, and Vol. 6/la, part 2, pp 788-799, literature up to 1975 for a general mechanistic description of the formation of alcohols by photochemical reduction of carbonyl compounds, see Vol. 6/1 b, p432). 

V-Hydrc en abstraction dominates the photochemistry of the ketone (58) affording the cyclobutanol (59) as the principal product. A comparison of the photochemical behaviour of the ketone (60) in benzene or acetonitrile with its behaviour in the solid state has been carried out. In benzene solution the irradiation affords the two cyclobutanols (61a) and (61b) in a ratio of 2.6 1. The formation of the more hindered cyclobutanol (61b) predominates when the reaction is carried out in the solid state. The ratio of products in this instance is 0.5 1. The authors suggest that in solution conformational isomerism in the biradical is faster than bond closure while in the solid state the biradical is restricted to the conformation which gives rise to the more hindered product. ... 

Cyclobutanol formation in type II photochemistry was first reported by Yang, N. C. and Yang, D. H., Photochemical reactions of ketones in solution,/. Am. Chem. Soc., 80, 2913, 1958. 

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