2-Butene photodimerization

The photodimerization of simple isolated olefinic bonds is rarely observed because of the absorption of these compounds in the high-energy or vacuum-ultraviolet region. One case reported is that of the photo-dimerization of 2-butene.<2) Irradiation of liquid cw-2-butene with light from a cadmium (A = 229, 227, 214 nm) or zinc (A = 214 nm) lamp was reported to lead to dimers (1) and (2) ... 

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of cw-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

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 direct irradiation of the parent coumarin in the presence of alkenes results only in an inefficient photodimerization and [2 + 2]-photocycloaddition. Lewis acid coordination appears to increase the singlet state lifetime, and leads to improved yields in the stereospecific [2 + 2]-photocycloaddition [95]. Alternatively, triplet sensitization can be employed to facilitate a [2 + 2]-photocycloaddition. Yields of intramolecular [2 + 2]-photocycloadditions remain, however, even with electron-rich alkenes in the medium range at best. The preference for HT addition and for formation of the exo-product is in line with mechanistic considerations discussed earlier for other triplet [2 + 2]-photocycloadditions [96, 97]. Substituted coumarins were found to react more efficiently than the parent compound, even under conditions of direct irradiation. 3-Substituted coumarins, for example, 3-methoxy-carbonylcoumarin [98], are most useful and have been exploited extensively. The reaction of 3-ethoxycarbonylcoumarin (100) with 3-methyl-l-butene yielded cleanly the cyclobutane 101 (Scheme 6.36) with a pronounced preference for the exo-product (d.r. = 91/9). Product 101 underwent a ring-opening/ring-closure sequence upon treatment with dimethylsulfoxonium methylide to generate a tetrahydrodibenzofur-an, which was further converted into the natural product ( )-linderol A (102) [99]. 

Turro [108] proposed a similar mechanism to explain the photodimerization of 2-butene either concerted addition of Spirit ) to 2-butene, or formation of a singlet exciplex by interaction of SJjiji ) and 2-butene, followed by direct collapse... 

Open-chain alkenes undergo dimerizations in stereospecific [nl + tt ] paths in low yields on irradiation. For example, photodimerization of Z-2-butene 26 gives two products in which cw-geometry of methyl groups is retained. Similarly, the cycloadducts from irradiation of -2-butene 27 retains the trans-geovasXry of methyl groups [29]. Irradiation of Z-2-butene also gives cycloadduct with -stil-bene in a similar manner [29]. 

Incorporation of a triplet sensitizer benzophenone into the macrocycle 33c has little influence on the self-assembly nor the dimension of the resulting nanotube, but the reaction environment of the cavity is totally changed. Photodimerization of the above enones could not happen within 33c-tube. Instead, the nanotube was able to act as a triplet sensitizer for the cis-trans photoisomerization of trans- >-mQ hy -styrene and selective oxidation of 2-methyl-2-butene to a primary allylic alcohol (Scheme 3.14) [70, 109]. As well, an enlarged nanotube 33d-tube was shown to facilitate the selective photodimerization of coumarin to the awr/-head-to-head product [71]. 

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