Photo-Bergman cyclization

This analysis confirms that the effect of cyclic constraints is not purely steric but also has an electronic component. Another aspect of this dichotomy is shown in Fig. 11 which illustrates the decrease in the energy gap between the frontier in-plane rc-MOs. The decrease in the C1-C6 distance destabilizes the occupied MO where the interaction between the end orbitals is antibonding and, at the same time, stabilizes the empty MO where the 7i -orbitals overlap constructively. As a result, the efficiency of the photochemical Bergman cyclization should increase and, indeed, the most efficient photo-Bergman cyclizations reported in the literature involve cyclic enediynes.43 Again, the analogy with interrupted [2 + 2] photocycloaddition is instructive. 

Irradiation of diethynyl sulfide 408 at 300 nm in hexane in the presence of 1,4-cyclohexadiene gives 3,4-diphenylthiophene 409 (photo-Bergman cyclization). The authors describe this as being the first five-membered ring cycloaromatization reaction. The low yield of the thiophene 409 is due to the production of many side products (Scheme 68) <20030L2195>. Naphthothiophene 411 and dinaphthothiophene 413 can be prepared from 410 and 412, respectively, by use of this method <2004T7191>. 

Photo-Bergman cyclization have been carried out on diethynyl sulfides 4 to produce 3, 4-disubstituted thiophenes 5. Matzger reported this as being the first five-membered ring cycloaromatization reaction <03OL2195>. The reaction is found to produce many side products thereby lowering the yield of the desired heterocycle. 

The diyne ene (84) undergoes photochemical cyclization on irradiation at 350 nm. The product obtained was identified as the phenanthrene derivative (85). " A variant of the photo-Bergman cyclization has been reported following the irradiation of the enediynes (86). The reactions are brought about using electron transfer from cyclohexa-1,4-diene in acetonitrile. This affords the products (87) in the yields reported under the structure. The products are formed by cyclization via the fulvene radical anion intermediate (88). " ... 

The development of photochemical variants of the Bergman cyclization would contribute to the broadening of its scope and utility. However there are relatively few examples of photoinduced cyclizations, despite the promise that such an approach would have in the photodynamic therapy of cancer. Funk and co-workers reported that certain cyclic ortho-dialkynylarenes, such as 9, underwent the photo-Bergman cyclization on irradiation in Pyrex. ° In 2000 Russell and co-workers showed that 10-membered cyclic pyrimidine enediynes could undergo cyclization efficiently (82-83%) when exposed to light (310 nm) in /-PrOH for 2-24 hours. 

Turro and co-workers showed that simple acyclic aromatic enediynes, such as 11, could also undergo Bergman cyclization under photochemical conditions. They proposed that the photo-Bergman cyclization of these substrates involved a diradical intermediate 2 identical to that of the thermal reaction."... 

Hirama and co-workers demonstrated that the photo-Bergman cyclization could be extended to both cyclic and acyclic aliphatic enediynes. 1,2-Dipropynylcyclopentene 13 in hexane was converted to indane derivative 14 in 71% yield when irradiated with a low-pressure mercury lamp at room temperature. 

SCHEME 30.13 Top photo-Bergman cyclization of copper metalloenediynes via metal-ligand charge transfer complex. Bottom relative energies of copper and ligand MOs proposed to be involved in the photoelectronic Bergman cyclization of Cu(I) and Cu(II)-metalloenediynes. Adapted with permission from Benites et al. [27b]. American Chemical Society. 

SCHEME 30.17 Inhibition of photo-Bergman cyclization upon Ru complexation. 

It was found that cyclization of benzannulated enediynes can be initiated by irradiation to give the same products as were obtained by thermal Bergman cyclization [238]. Photo-Bergman cyclization, which is also related to the chemistry of benzyne, has not been so extensively investigated. Terminal acyclic enediynes capable of cycloaromatization by thermal C -C route do not undergo photochemical initiation. However, introduction of one or two phenyl substituents in the acetylenic branches facilitates the C -C photocyclization of acyclic enediynes, but any increase in steric hindrance raises the activation barrier and the photochemical yield is significantly lower than in the case of their cyclic... 

SCHEME 8 Pyrene enediyne Photo-Bergman cyclization. 

SCHEME 14 Preparation and Photo-Bergman cyclization of phe-enediyne hybrid. 

SCHEME 19 Theoretical examination of Photo-Bergman cyclization. 

Although the seminal study of Bergman reported that no photochemical cyclization occurs during irradiation of the parait aiediyne [8], a photochanical transformation that can be classified as a photo-Bergman reaction had been reported as early as 1968 by Campbell and EgUnton (Scheme 30.1) [9]. 

In addition to the aforementioned transformations, the photo-induced Bergman cyclization was utilized by Zhu and coworkers to form intramolecular polymeric nanoparticles.The appropriate reactive diyne unit was incorporated into the polymer using both direct controlled radical polymerization and aHqme protection strategies. The resulting linear polymers were subjected to photochemical Bergman cyclization conditions using a continuous addition strategy, similar to the case previously discussed. ... 

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