Toluene photocycloaddition reactions

There are few reports of photocycloaddition reactions of di- and poly-ynes which involve more than one of the alkyne groups, but the production of toluene or other alkylbenzenes in the irradiation of butadiyne with propene or other terminal alkenes (equation 60) is one such process. ... 

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

The formation of trans-products is observed to a lesser extent in the reaction of 3-alkoxycarbonyl-substituted cyclohexenones, in the reaction with electron-deficient alkenes and in the reaction with olefinic reaction partners, such as alkynes and allenes, in which the four-membered ring is highly strained (Scheme 6.11). The ester 26 reacted with cyclopentene upon irradiation in toluene to only two diastereomeric products 27 [36]. The exo-product 27a (cis-anti-cis) prevailed over the endo-product 27b (cis-syn-cis) the formation of trans-products was not observed. The well-known [2 + 2]-photocycloaddition of cyclohexenone (24) to acrylonitrile was recently reinvestigated in connection with a comprehensive study [37]. The product distribution, with the two major products 28a and 28b being isolated in 90% purity, nicely illustrates the preferential formation of HH (head-to-head) cyclobutanes with electron-acceptor substituted olefins. The low simple diastereoselectivity can be interpreted by the fact that the cyano group is relatively small and does not exhibit a significant preference for being positioned in an exo-fashion. 

Further examples of mixed [2 + 2] photocycloadditions, in which various chiral auxiliaries were attached to the alkene. are shown for the reactions of 3-methyl-2-cyclohexenone with the chiral enoates 6a-c75. Irradiation of a toluene solution gave the cis-anti-cis, head-to-head adducts as inseparable mixtures of diastereomers 7a-c and 8a-c. Reduction and spontaneous lactonization of the adduct mixture with sodium borohydride in refluxing methanol led to the formation of the lactone 9 as an unequal mixture of enantiomers. 

The 5,6-double bond in uracils is involved in several [2 + 2] photochemical reactions. The cyclo-octapyrimidine-2,4-dione (338) is formed by photocycloaddition of the A-substituted 6-chlorouracil (336) to benzenes in the presence of an acid, preferably TEA (Scheme 57). The reaction is assumed to proceed by orr//o-addition, with a cyclobutane (337) as intermediate. Without acid, 6-phenyluracil is formed in low yield. In toluene as solvent and participating reagent, a regioisomeric mixture with the methyl group in the 6-, 7-, 8-, or 10-position was formed. In fluorobenzene, all of the regioisomers... 

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