Cyclobutane photolysis

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

In the analogous studies of the photolysis of sulfolane (31), the work of Honda and coworkers66 was carried out in the gas phase at 70-130 °C and established the formation of S02, ethylene, cyclobutane and acetylene as the major products, on mercury-sensitized photolysis. In considerable contrast, photolysis of sulfolane at 185 nm in the liquid phase67 produced ethylene( = 0.22), the sultine (32) (parallel experiments on aqueous solutions of sulfolane, a sulfinic acid is also believed to be formed. The authors believe that both four-membered (33) and six-membered (32) sultines may be formed during these photolyses. Further work in this area would appear to be necessary to unravel the full mechanistic details. 

Enhancement of the total butene yield is observed when various additives whose ionization potential falls below about 9.4 e.v. are present during ethylene radiolysis (35). This is consistent with the above interpretation (Figure 2). In the vacuum ultraviolet photolysis of cyclobutane the yield of butenes varies with the ionization potential of the additives in the same way as observed here (12). The maximum enhancement corresponds closely to the yield of C4H8+, as expected from our mechanism. 

Photoaddition of fra/w-stilbene to tetramethylethylene yields cyclobutane (42) in high yield. Cw-stilbene also results from the photolysis ... 

The photolysis of cyclic diazo ketones in hydroxylic solvents leads to ring contracted carboxylic acid derivatives via this ketocarbene -> ketene rearrangement. Examples of such reactions are given in (2.24)239) and (2.25) 240). In this last example a photoequilibrium between the diazo ketone and its valence isomer, a diazirine, has been observed, both products then eliminating nitrogen to afford the cyclobutane carboxylic acid. 

Though the triplet sensitized photolysis of isoprene (159) does, as noted above, produce a complex mixture of products, one of these adducts has been used in the context of complex molecule synthesis (equation 5)71. Cyclobutane 160, which was formed in ca 20% yield by the benzophenone sensitized photolysis of 159, could be easily transformed into fragrantolol, 161, an isomer of grandisol isolated from the roots of the Artemisia fragrans, by simple hydroboration/oxidation of the less hindered double bond. 

The multiplicity of products produced in the photolysis of aqueous solutions of bromouracil and l,3-dimethyl-5-bromouracil has been thoroughly investigated by Ishihara and Wang.67 The products, which in neither case included the cyclobutane-type dimer or the hydrate, are shown in Tables IV and V. 

Photolysis of the amide 31 in methylene chloride at room temperature results in the formation of the tricyclic lactam 32 in both syn and anti forms <99TL6001>. Treatment of 32 with BF3 etherate brings about cleavage of the cyclobutane ring in the syn isomer only, with the formation of 33. (Scheme 5). The corresponding esters undergo similar transformations, if somewhat less efficiently. 

However, attempts by Kiefer and Carlson59 to prohibit undesired bi-molecular reactions by irradiating 2,3,3-trimethyl-l-penten-4-one adsorbed onto silica gel were unsuccessful (due probably to steric inhibition of adsorption) the product composition was the same as that previously obtained in solution. Werbin and Strom80 attempted to restrain the freedom of movement of the radicals formed from the photolysis of vitamin K3 (2-methyl-1,4-naphthoquinone) by adsorption onto silica gel, but obtained the same mixture of dimers as that obtained from the irradiation in acetone solution, viz., syn and anticyclobutanes, an oxetane dimer, and a binaphthoquinone dimer. Photolysis of the solid substrate, however, produced only the syn isomer of cyclobutane, in this case no migration of radicals is possible, hence only one product. 

In contrast to If/-azepines the photolysis of N-substituted 5f/-dibenz[Z>,/]azepine under sensitized (PI12CO) conditions yields [2 + 2] cyclobutane dimers (77 R = Ac, CC Et, CONH2, COPh). The N-p- tosyl derivative, however, undergoes photo-Fries reaction to the 2-p-tosylamino derivative (74CRV101). 

The thermal decomposition of 8 in tetrachloroethene at 134 C gave a chromatographically separable mixture of cyclobutane 9 and 1,8-divinylnaphthalene 10 in 7 1 ratio. Although several experiments have been carried out to identify the spin multiplicity of the intermediate diradical, the results were inconclusive.17 A recent report stated that while triplet-sensitized photolysis resulted in predominant denitrogenation, laser/liquid jet photochemical reaction also gave cyclopentenes by 1,2-hydrogen shift.18 Indications are that the amounts of cyclopentenes increase with increasing lifetime of the intermediary 1,3-cyclopentadiyl triplet diradical.18... 

Cyclopropane derivatives, including spiropentanc, have proven to be virtually inert towards carbenes,1 For this reason, no literature report that describes cyclobutane synthesis from a C3 and a Cj building block by ring enlargement of cyclopropanes exists. However, due to the partial p character, as well as the increasing reactivity caused by its strain, the central bond of bicyclo[1.1.0]butane (l)2 has been found to react with carbenes.1 Photolysis of diazomethane in the presence of bicyclo[1.1.0]butane (1) at — 50 C provides a mixture of several compounds. The major fraction of the material (80%) was analyzed by means of NMR spectrometry and found to consist of penta-1,4-diene (2, 21%) and bicyclo[l.l.l]pentane (3, 1%), plus several other known compounds as well as some unidentified products.3 The mechanistic pathway for the formation of bicyclo[l.l.l]pentane (3) has not been addressed in detail, but it is believed that a diradical intermediate is involved, as shown below.3... 

Siloxanes 1 (R = Me, Ph synthesized from 3-phenylallyl alcohol and dimethyl- and diphenyldichlorosilane, respectively) on photolysis afforded one cyclobutane product, the all-p-am-isomer 2, in almost quantitative yield.2 Photolysis of 3-phenylallyl alcohol under the same conditions resulted in very little cyclobutane formation. 

Similarly to the nickelacyclopentanes, related complexes of platinum and iridium also yielded cyclobutanes on thermolysis,130 132 while palladacyclopentanes on thermolysis or photolysis generally gave cyclobutane in > 20% yield.135-136 Examples are given in Table 3. 

The failure of 3,6-diphenyl-3,4,5,6-tetrahydropyridazine to undergo elimination of nitrogen on irradiation11 has been attributed to facile isomerization to the corresponding hydrazone under slightly acidic conditions. However, photolysis with a Pyrex-filtered high-pressure mercury lamp in the presence of a trace of V, V-diisopropylethylamine gave the stereoisomcric diphenyl-cyclobutanes (cisjirans 1.7 1, 41%) and styrene (54%).12... 

Similarly, 2,3-diazabicyclo[2.2.2]oct-2-enes underwent photoelimination of nitrogen via singlet 1,4-diradicals15 to give bicyclic cyclobutanes (Houben-Weyl, Vol. 4/4, pp 40-44). However, the parent compound on photolysis at 185 nm gave bicyclo[2,2.0]hexane (11 a) in only 9% yield,10 while benzophenone sensitized or direct photolysis resulted in mixtures of bicy-clo[2.2.0]hexane and hexa-1,5-diene (27 73 or 42 58, respectively).16 In a separate study, a triplet 1,4-cyclohexadiyl was trapped with molecular oxygen.17... 

Several photochemically induced vinylcyclopropane to cyclopentene rearrangements of nor-carene derivatives to form bicyclo[3.2.0]heptenes can be understood as ring contractions of cyclohexenes to cyclobutanes. Upon direct irradiation of norcar-2-ene (bicyclo[4.1.0]hept-2-ene) at 214 nm (pentane solution), however, complex product mixtures were obtained containing only small amounts of bicyclo[3.2.0]hept-2-ene, while toluene sensitized photolysis in 50 millimolar solution in degassed pentane at 254 nm gave mainly the cyclobutane derivative 13 in addition to EjZ-isomeric hepla-l,3,6-trienes.72... 

Arc-generated carbon atoms abstract sulfur from tetrahydrothiophene, forming ethylene (91%) and cyclobutane (9%) (71JA3807). The photolysis of tetrahydrothiophene vapour with 2139 A radiation gave ethylene as the major product (71CJC1316). 

Exercise 28-10 Write a mechanism for formation of cyclobutane from the photolysis of cyclopentanone, and ketene from the photolysis of cyclobutanone. 

Photolysis of cyclopentanone leads to the formation of carbon monoxide, ethylene, cyclobutane (3), and 4-pentenal (28). An early report of the formation of butenes (27) has not been substantiated by later work. The yield of the gaseous products agrees with eq. 1 at 3130 A. and temperatures up to 125° (33) and at shorter wavelengths up to 300° (3). 

Fig. 1. Effect of methane on the photolysis of cyclopentanone at 3130 A. (33). Average ketone pressure 11.6 mm. room temperature , cyclobutane A, pentenal , ethylene O, cyclopentanone accounted for in products.

Gas Photolysis of cyclopentanone Thermal decomposition Cyclopropane13 Cyclobutane ... 

In a qualitative study of the photolysis of 2-ethyl cyclopentanone (XIX) (20) it lias been observed that ethyl cyclobutane is a product. 

It has been observed that the formation of the olefin and carbon monoxide, 45, is ten times more important than the formation of the bicyclic hydrocarbon and carbon monoxide, 46, at 80° and 80 mm. pressure even at 3130 A. The formation of the strained bicyclic hydrocarbon is evidently not a favorable reaction although this may not be the only consideration. In the case of camphor it should be interesting to find out if an optically active isomer of the ketone on photolysis will give rise to an optically active trimethyl bicyclo [2.1.1] hexane (XXVI). A concerted reaction, analogous to the formation of cyclobutane from cyclopentanone, may lead to only an optically active product. 

Keywords enone, photolysis, [2+2]-cycloaddition, head-to-tail-anti, cyclobutane, spiro compound... 

The structures and isomerization of C4Hg+ ions in connection with the problem of ethylene clusters has been the subject of many studies. Doepker and Ausloos192 studied the photolysis of cyclobutane, its deuterated isotopomer and mixtures thereof, and in their detailed product analysis they found cA-2-butene, trans-2-butene and 1-butene as major ionic products in the approximate ratio of 1 1 2. Lias and Ausloos193 determined... 

For the preparation of larger amounts of the phenanthrene the reaction should be repeated photolysis of more concentrated solutions is to be avoided in order to minimise the possibility of photodimerisation to give a cyclobutane derivative. 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

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