2- butene photochemical cycloaddition

It is possible that some of these photochemical cycloadditions take place by a lA + A] mechanism, which is of course allowed by orbital symmetry when and if they do, one of the molecules must be in the excited singlet state (5i) and the other in the ground state.The nonphotosensitized dimerizations of cis- and trans-2-butene are stereospecific,making it likely that the [n2s + n2s] mechanism is operating in these reactions. However, in most cases it is a triplet excited state that reacts with the ground-state molecule in these cases the diradical (or in certain... 

Photochemical cycloaddition of 2-arylisoindoline-l-thiones 8 to electron-poor alkenes 9, such as methacrylonitrile, 2-butene-2-nitrile and methyl methacrylate, affords the corresponding tricyclic isoindolines 10 as two diastereomers143. 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

Photochemical [2h-2] cycloadditions of olefins occur with retention of configuration according to the Woodward-Hoffmarm rule [6,7], These are excited-state reactions in the delocalization band of the mechanistic spectrum. A striking example of the symmetry-allowed reaction was observed when the neat cis- and tran -butenes were irradiated (delocahzation band in Scheme 3) [8],... 

As mentioned in chapter 4.2.3, aliphatic imines are photochemically rather unreactive. When the C—N double bond is conjugated to an electron withdrawing group (e.g. carbonyl group), as in O-alkyl derivatives of succinimide and phthalimide, the reactivity increases and azetidines are obtained in cycloadditions to olefins486). A somehow similar example is the photoaddition of a 6-azauracil derivative to 2,3-dimethyl-2-butene... 

The initial example of a photochemical cross [2+2] cycloaddition reaction of a stilbene was the reaction of t-1 and 2,3-dimethyl-2-butene to yield the cyclobutane AO reported in... 

Mechanistic investigations by Chapman and co-workers (99) indicated that these reactions occurred via a nonfluorescent singlet exciplex intermediate. While the rate constant for quenching of - -t5 by 2,3-dimethy 1-2-butene is slower than the rate of diffusion (Table 8), the limiting quantum yield for cycloaddition is 1.0. Thus, highly efficient exciplex cycloaddition may account for the absence of exciplex fluorescence, as in the case of t-1 photodimerization. Photochemical [2+2] cycloaddition reactions have also been observed to occur upon irradiation of the cyclic c-1 analogues diphenylcyclobutane (7) and diphenyl-vinylene carbonate (10) with 2,3-dimethyl-2-butene (96) however, the mechanistic aspects of these reactions have not been investigated. 

The final series of five procedures presents optimized preparations of a variety of useful organic compounds. The first procedure in this group describes the preparation of 3-BROMO-2(H)-PYRAN-2-ONE, a heterodiene useful for (4+2] cycloaddition reactions. An optimized large scale preparation of 1,3,5-CYCLOOCTATRIENE, another diene useful for [4+2] cycloaddition, is detailed from the readily available 1,5-cyclooctadiene. Previously, the availability of this material has depended on the commercial availability of cyclooctatetraene at reasonable cost. A simple large scale procedure for the preparation of 3-PYRROLINE is then presented via initial alkylation of hexamethylenetetramine with (Z)-1,4-dichloro-2-butene. This material serves as an intermediate for the preparation of 2,5-disubstituted pyrroles and pyrrolidines via heteroatom-directed metalation and alkylation of suitable derivatives. The preparation of extremely acid- and base-sensitive materials by use of the retro Diels-Alder reaction is illustrated in the preparation of 2-CYCLOHEXENE-1.4-DIONE, a useful reactive dienophile and substrate for photochemical [2+2] cycloadditions. Functionalized ferrocene derivatives... 

The photochemical 2 + 2-cycloaddition of 2(5//)-furanones to symmetrical 1,4-disubstituted 2-butenes yielded 1,2,3,4-tetrasubstituted cyclobutanes with a high diastereo-selectivity. The ruthenium-catalysed 2 + 2-cycloadditions of a 2-oxa- (g) 3-azabicyclo[2.2.1]hept-5-ene with unsymmetrical alkynes formed cycloadducts. 

Photolysis of 3-buten-l-ol nitrite affords no cyclized products (Cy5/Cy4) neither does 5-hexen-l-ol nitrite (Cy6/Cy7). The same result is obtained on peroxydisulphate oxidation of 5-hexen-l-ol. In the Cy6/Cy7 case an important competitive pathway is probably 1,5-intramolecular ally lie hydrogen abstraction and, indeed, esr spin trapping by nitrosodurene " provides evidence of this. Cyclization in the Cy6/Cy7 case was considered to explain the reaction products of tetrahalogeno-o-benzoquinones with 2,3-dimethylbut-2-ene but was discarded in favor of a direct cycloaddition process on the basis of spin trapping and deuteration experiments. As discussed before, cyclization in the Cy3/Cy4 case must be difficult to observe because of the high j5-scission rate of oxyranylalkyl radicals. Nevertheless, this pathway has been used recently to explain the formation of diepoxides in the thermal-, photochemical-, or ferrous-salt-induced decomposition of unsaturated cyclic peroxides. In view of the multistep scheme involved this conclusion must await further confirmation. 

Scheme 9.26. An example of a photochemically induced 2 + 2 cycloaddition reaction between benzophenone (diphenylketone [(C6H6)2C=0]) and 2-butene [an (E)- and (Zj-mixture of isomers]).

Non-stereospecific photochemical [2+2]-cycloadditions occur in the dimerization of phenyl cyclohexene 23 in the presence of a sensitizer to produce 24 and 25 [17], and in reactions of Z/E-2 butene with cyclohexenone 26 to give 27 and 28 [18] through the formation of intermediate diradicals. The photoaddition of cyclohexene to an enolised form of 1,3 diketone 29 gives 30 in a concerted process via the formation of an unstable cycloadduct [18]. 

Z- and -2-Butenes in liquid neopentane undergo ci5—/ra/is-isomerization and it is competitive with their photochemical [2+2]-cycloadditions [3]. When the alkene solution is sufficiently diluted with an inert solvent, the isomerization becomes the dominant reaction. The triplet energy of sensitizer PhH is higher ( 84 kcal/mol) than that for 2-butene ( 80 kcal/mol) and hence triplet energy transfer is... 

Coyle has summarized the photochemistry of carboxylic acid derivatives. For arene carboxylic acid esters it has been shown that [2-1-2]-cycloaddition competes with hydrogen abstraction by the excited ester from an allylic position of the alkene. The addition of methyl benzoate 17 to 2-methyl-2-butene gave a 1 1 mixture of the Paterno-Bilchi adduct 18 and the coupling product 19. Less electron-rich alkenes (e.g., cyclopentene) did add preferentially toward the benzene ring of 17 in an ortho- and metacycloaddition manner. Furans could also be added photochemically to methyl benzoate and other aren-ecarboxylic acid esters. The resulting bicyclic oxetanes could be transformed into a series of synthetically valuable products. [2-1-2]-Cycloadducts and/or their cleavage or rearrangement products have also been described for photoreactions of alkenes with diethyl oxalate,benzoic acid, " and carbamates. ... 

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