Propellane photolysis

The reaction of EtsSiH with [l.l.l]propellane under photolytical decomposition of di-tert-butyl peroxide afforded products 17 and 18 in 1 3 ratio (Reaction 5.15) [36]. A rate constant of 6.0 x 10 M s at 19 °C for the addition EtsSi radical to [l.l.l]propellane was determined by laser flash photolysis [37]. Thus, it would appear that [l.l.l]propellane is slightly more reactive toward attack by EtsSi radicals than is styrene, and significantly more reactive than 1-hexene (cf. Table 5.1). 

If R1 and R2 are a tri- or tetra-methylene bridge then the products (127) are heterocyclic propellanes (78CI(L)95>. Photolysis of the sulfone (128) eliminates sulfur dioxide to give a 47r-electron system which can be trapped by N- phenylmaleimide (Scheme 41) (78JOC3374). 

Radical attack on the central bond in [1.1.1 [propellane 1 occurs 2-3 times faster than attack on styrene96 and yields bridgehead bicyclo[l.l.l]pent-l-yl radicals6. Laser flash photolysis techniques were used to measure the rate constants for the reactions of la with five different radicals (Table 12)96"99. The addition of the phenylthiyl radical to la is... 

The parent Dewar furan (49) was generated in a similar fashion from the corresponding propellane precursor (48) and trapped with isobenzofuran to give the cycloadducts (51) and (52) (Scheme 7) <85JA7176, 86PACI61). The proportion of isobenzofuran-Dewar furan adducts (51) and (52) was increased at the cost of the cyclopropene aldehyde adducts (53) and (54) as the photolysis temperature was lowered, indicating that (49) is indeed the primary photolysis product. 

A less reactive bicyclo[3.2.0]hept-l(7)-ene substructure has been found in 164 (149). This compound was prepared from propellane 163 by photolysis and subsequent reduction. In contrast to 163, irradiation of the parent tricyclic ketone 165 in ethanol gives the saturated ketone 167. The formation of 167 is taken as evidence for the intermediate formation of the ketone 166. When 168 was treated with CsF in the presence of 1,3-diphenylisobenzofuran, an adduct was isolated, which was assigned the structure of the expected [2 + 4]cycloaddition product of 169 (150). 

Recently, the formation of bicyclo[4.2.2]decapentaene (213) as a highly unstable intermediate in the photolysis of the propellane 212 has been reported (166). Classical formulae are used to interpret the spectroscopic and trapping results in terms of the valence isomer 213 rather than 214. It remains to be seen whether 213 and 214, the % system of which is apparent in the tetrabenzannulated derivative 211, are true valence isomers. 

Mercury-sensitized irradiation of (193c) produces a triplet biradical (194), and predictions that this should collapse to [2,2,2]propellane have now been verified by reaction of the products of irradiation with bromine, which afforded 4—5% of the dibromide (195 X = Br) after 600 h photolysis. 

Z,Z,Z)-Cyclo-octa-l,3,5-triene reacts with SO2 to give 9-thiabicyclo[4,2,l]nona-2,7-diene dioxide which was converted into 9-thiabicyclo[4,2,l]nona-2,4-diene dioxide. Thermal expulsion of SO2 had a free energy of activation lOkcalmol" greater for the latter dioxide than for the former dioxide so showing that linear extrusion of SO2 is favoured over non-linear extrusion or sequential bond cleavage. Photolysis of 1,2,5,6-tetramethylenecyclo-octane gave a mixture of the [3,3,2] propellane (230) and the tricyclodiene (74). ... 

The NOs- radicals can also be generated by flash photolysis of CAN in acetonitrile (Del Giacco et al., 1993). These photochemically produced nitrate free radicals have been used to study the one-electron oxidation of methylbenzenes. Depending on the oxidation potential of the substrate, radical cations or benzyl-type neutral radicals were formed. Formation of radical cations was observed for most of the methylbenzene derivatives that were studied, whereas formation of the benzyl radical was observed with toluene and with ortho- and meta-xylene. This reaction is not useful for synthetic applications. The reactivity of propellane C-C bonds towards free nitrate radicals photochemically generated from ammonium hexanitrato-eerate(IV) in acetonitrile was studied from both theoretical and experimental points of view (Fokin et al., 2000). Baciocchi and coworkers reported that the photochemical reaction of CAN in acetonitrile with cyclohexene, 1-octene and styrene derivatives leads to the formation of 1,2-dinifrate adducts in high yields (scheme 65) (Baciocchi et al., 1988b). 

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