1,2-Diarylethylenes radical cation

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

As noted in Section 3, the lifetimes of arylalkene radical cations in the absence of added nucleophiles or alkenes are frequently determined by their reaction with excess neutral alkene in solution to generate an adduct radical cation that ultimately gives the 1,2-diarylcyclobutane or substituted naphthalene products described above. Rate constants for the addition of the radical cation to its neutral precursor have been measured recently for both styrene and diarylethylene radical cations using either flash photolysis or pulse radiolysis methods. - ... 

Other olefinic substrates known to dimerize through photo-induced electron transfer sensitization include enamines (72), diarylethylenes (73-75), vinyl ethers (76), styrenes (77,78), and phenyl acetylenes (79). Alternate ring closures (besides cyclobutanes) are sometimes observed, probably via 1,4-radical cationic intermediates. For example, a tetrahydronaphthalene is formed from the radical cation of 1,1-diphenylethylene, eq. 

Alternatively, Lewis acids such as SbCl5 may initiate oligomerization directly by electron transfer from extremely reactive alkenes such as 1,1-diphenylethylene and 1,1 -di(p-methoxyphenyl)ethylene [28,143,144]. The dimeric tail-to-tail carbenium ion of 1,1-diphenylethylene shown in Eq. (32) was observed, and its formation explained by a radical cation intermediate. Because 1,1-diarylethylenes can not polymerize, only oligomerization was observed. 

The DCNB-sensitized addition reactions of 1,1-diarylethylenes with ammonia or primary amines yield the a ri-Markovnikov adducts. The mechanism is analogous to that shown in Scheme 7 for addition to sthbene. The regioselectivity is determined by nucleophilic attack of the amine on the alkene cation radical to yield the more stable benzyl radical intermediate. The mechanism and dynamics of the reactions of p-methoxystyryl radical cations with amines have been investigated. Anihne and EtjN are found to react as electron donors with rate constants near the diffusional Hmit. Primary amines react as nucleophiles, with somewhat slower rate constants. 

The appeaience of a long wavelength band (600—700 nm) in these systems is neither due to the formation of acid-olefin ir-complexes nor to cation radicals derived from the diarylethylene but rather to minor side reactions producing fluorenyl-type carbenium ions, as diown by Sauvet ... 

The ET-sensitized photoamination of 1,1-diarylethylenes with ammonia and most primary amines yields the anti-Markovnikov adducts. Photoamination of unsymmetrically substituted stil-benes yields mixtures of regioisomers 15 and 16. Modest re-gioselectivity is observed for p-methyl or p-chloro substituents however, highly selective formation of adduct 15 is observed for the p-methoxy substituent (Table 5). Selective formation of 15 was attributed to the effect of the methoxy substituent on the charge distribution in the stilbene cation radical. This re-gioselectivity has been exploited in the synthesis of intermediates in the preparation of isoquinolines and other alkaloids." Photoamination of 1-phenyl-3,4-dihydronaphthalene yields a mixture of syn and anti adducts 17 and 18 (Scheme 5)." Use of bulky primary amines favors formation of the syn adduct (Table 5), presumably as a consequence of selective anti protonation of the intermediate carbanion. 

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