1.4- Diaryl-2-ethylenes

Hexamethylphosphorous triamide general reaction with aromatic and heteroaromatic aldehydes to give diaryl ethylene oxides, 46,... 

During the past two decades the photocyclization of substituted diaryl ethylenes (analogous to A) has assumed considerable synthetic importance. In such cases photocyclization is carried out simultaneously with dehydrogenation (e.g., by iodine), the aim being the one step formation of the fully aromatic system ... 

The procedure developed in the initial NMR study of 12 has been used in subsequent studies of 61, 62 ° of 41 and 44 of and of 64 The DHP derivative can be obtained only in photostationary concentrations (at most), in reaction mixtures containing both cis- and trans-isomers of the 1,2-diaryl ethylene. Under such conditions the NMR signals due to the nuclei of the DHP derivative are identified as those which disappear following photochemical ring cleavage (process B). This process yields only the cis-isomer of the parent ethylenic compound. In addition to their structural value, NMR studies in this field allow to verify the conversion estimates obtained from optical studies In 61, 62 and 64 proton NMR proves that photocyclization takes place between 1 and 1 atoms and not for instance between atoms 1 and 8 or between atoms 8 and 8 (numbering as in 61 in Table 8). Table 10 provides a summary of the chemical shifts of 12, 25, 41, 44, 61, 62, and 64. The atoms of the DHP moiety are numbered as in 7 (see Tables 1—9 for details). 

As stated previously 4a,4b-dihydrophenanthrenes cannot be isolated pure but can be obtained only in reaction mixtures containing both cis- and trans-isomers of the parent diarylethylene. Thus studies of 4a,4b-dihydrophenanthrenes always depend on the prior development of conditions providing considerable conversion of diaryl-ethylenes into their DHP photoisomers. One requirement for obtaining maximum conversion consists in minimizing the rates of decomposition processes such as photochemical ring cleavage,... 

In addition to the definite conclusions of the theoretical studies (see Sect. VIC) which rule out a hot ground state process there are numerous experimental find-it which clearly suggest that 4a,4b-dihydrophenanthrene-like photocyclizations (with few exceptions, see below) take place in the first excited singlet state of diaryl-ethylenes Among those findings we should note the complementary... 

Depending on the symmetry of the diaryl ethylenes one to four cyclization products may be formed. The photoreaction is, however, very selective e.g. on irradiation of 2-styrylbenzo[c]phenanthrene (4) hexahelicene (6) is obtained in 85 % yield, whereas no trace of benzo[a]naphtho[l,2-h]-anthracene (9) is formed. 

The photocyclodehydrogenation of thienyl ethylenes is well-defined when both thiophene rings are bound via a C(2) atom to the ethylenic bond as in (70). In other cases, however, more cyclization products are possible. To predict the photocyclization mode for heterohelicenes the F s rule fails in many cases, because correction factors for the hetero atoms in the Huckel MO calculation have to be introduced and the systems are not well comparable with carbocyclic diaryl ethylenes. A better reaction parameter in these cases is the Mulliken overlap population (nrs)51), introduced by Muszkat52) for these cases. The overlap populations of the atoms r and s in ground and excited state (nIiS and n s), are calculated using the extended Huckel method. Cyclizations should not occur when nr>s and An s (= nr>s — n s) have negative values. (This method can also be used for diaryl olefins, but in these cases calculation of F s is more simple.). 

Curtin, D. Y., Flynn, E. W., Nystrom, R. F. (1958). Reaction of Stereoisomeric C14-Labeled l-Bromo-2,2-diaryl-ethylenes and P-Bromostyrenes with Butyllithium. Journal of the American Chemical Society, 80, 4599—4601. 

The above cycloaddition process consists of two separate [3-1-2] cycloaddition steps and represents a 1,3-2,4 addition of a multiple bond system to a hetero-1,3-diene [7S7]. The structure ot the azomethine imine intermediate has been proved unequivocally by X-ray analysis [195] Ethylene [194], acetylene [/iS2] . many alkyl- and aryl- as well sgemmal dialkyl- and diaryl-substituted alkenes [196,197, 198, 199], dienes [200], and alkynes [182, 201], certain cyclic alkenes [198, 199,... 

Nitrile oxides are generated by photolysis of 1,2-diaryl-substituted nitro-ethylenes through the formation of an oxazetine 2-oxide and its fragmentation (Scheme 1.7) (89). 

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