Quadricyclanes formation

Triplet-sensitised photocycloaddition of norbornadiene (N) leads to formation of the energy-rich quadricyclane (Q) by an intramolecular reaction ... 

Like the cyclization of the parent system, the photocyclization of hetero analogues of norbornadiene was reported many years ago. Recent interest in this area will serve to illustrate the potential of the conversions. Thus the irradiation of the oxanorbomadiene system 259 brings about conversion to the oxepine 260138. The formation of 260 presumably arises by quadricyclane 261 formation, followed by secondary photolytic ring-opening. Other research has shown that irradiation of the oxanorbomadiene 262 follows the same reaction mode and it undergoes (2 + 2)-cycloaddition to the quadricyclane derivative 263. Apparently, in this instance, cycloreversion affords the ylide 264 that can be trapped by suitable addends, giving the adducts illustrated in Scheme 3139. 

It appears likely that transient metallacyclobutanes are involved in a variety of organic reactions which are catalyzed by transition metal complexes. Thus, cycloadditions of activated alkenes to strained hydrocarbons such as quadricyclane and bicyclo[2.1.0]pentane are catalyzed by complexes such as Ni(CH2=CHCN)2 and probably involve initial formation of a nickelacyclobutane (Scheme 2) (79MI12200). The nature of the organometallic intermediates in related metal-catalyzed rearrangements (72JA7757) and retro-cyclo-addition reactions (76JA6057) of cyclopropanoid hydrocarbons, e.g. bicyclo[n.l.O]alkanes, has been discussed. 

The unsaturated lactones (341) behave in a similar fashion and yield the isomers (342).283 The formation of the oxygen-bridged quadricyclane (343) is a consequence of intramolecular addition in 7-oxa-norbornadiene (344)284 many analogous transformations have been reported in substituted 7-oxanorbornadienes285 and in l,4-epoxy-l,4-dihydronaphthalenes,286 which are further converted to benzoxepins. 

This suggests that the formation of quadricyclane during the thermal decomposition of 70 exo results from the competing [2s+2s+2s] retrocycloaddition process rather than via intramolecular trapping of the diradical intermediate by a C-C bond. Thus, the thermal decomposition of 70 exo resulted from [2s+2s+2s] cycloreversion reaction rather than from a biradical pathway. If the latter pathway was active, both exo and endo isomers should have furnished quadricyclane 72. In fact, 70 endo is geometrically incapable to participate in the cycloreversion process. 

A similar reaction of 2,2,4,4-tetrafluoro-l,3-dithietane 69 at elevated temperature resulted in formation of the mixture of products 70-72 along with some polymeric materials (Equation 10) <2005JFC(126)1332>. The proposed mechanism assumes a single electron transfer from electron-rich quadricyclane to the electron-deficient cyclic sulfides <2005JFC(126)1332>. 

Quadricyclane effectively contains two cyclopropyl rings and it can be converted to bicyclo [2.2. l]hepta-2,5-diene when treated with numerous transition metal complexes (equation 9). The reaction is of current interest as an energy storage system since bicyclo[2.2.1]hepta-2,5-diene can be photolyzed to quadricyclane. The most hkely mechanism for the decychzation reaction is initial interaction between the strained cyclopropyl rings and the transition metals d 2 orbitals followed by formation of the metallocycle (1). The isomerizations of heavily substituted quadricyclanes (equation 10) are slow. ... 

Treatment of P3C3Bu 3 with [(CO)sW <—PMe], generated / si/u by thermal decomposition of 56b (R = Ph) at 110°C, furnished a 2 1 mixture of quadricyclane 58b and the tricyclic compound 59, which were separated by fractional crystallization as colorless or yellow crystals in 39% and 25% yield, respectively. The presence of a three-membered ring in 59 agrees with an initial attack of the phosphinidene complex at a P=C bond of the triphosphinine to give transient 60. An intramolecular electrophilic aromatic substitution furnished product 59, whereas rearrangement of 60 to tetraphosphanorbornadiene 57a and its intramolecular [2+2] cycloaddition would rationalize the formation of 58b (Scheme 22) <2001CEJ3545>. 

Another example of triplet recombination involves the formation of quadricyclane upon electron transfer from norbornadiene to 1-cyanonaphthalene (1-CNN) [226]. Once again, the radical ion pair energy lies above 3 eV, so an intermediate on the... 

Quadricyclane in Csl or KBr matrices, prepared by deposition in the salt under conditions that yield single-molecule isolation, is rapidly converted into norbornadiene under conditions that induce color center formation in the alkali halide rapid-growth vapor deposition, or UV or X-ray irradiation. The reaction proceeds only at temperatures at which color centers of the missing electron type (H center) are mobile. At lower temperatures (T < 90 K), UV irradiation of norbornadiene converts it into quadricyclane in the usual fashion (Kirkor et al., 1990). 

Nortricyclanylidene (tricyclo[2.2.1.02,6]heptan-3-ylidene, 46) was originally generated via the Bamford-Stevens reaction of the corresponding 3-nortricyclanone p-tosylhydrazone sodium salt (49) (Scheme 9).126 The formation of quadricyclane (tetracyclo[2.2.1.02 6.03 5]heptane, 48) was anticipated, from an intramolecular... 

Photolyses of 3-azinortricyclane (45) were conducted within CyDs and FAU zeolites to alter the selectivity of the carbene intermediate, 3-nortricyclanylidene (46). It was hypothesized that constriction of carbene 46 within each host would suppress the formation of 4-ethynylcyclopentene (47) and concomitantly enhance that of quadricyclane (48). But only 47 was observed. Since CyD innermolecular products were also formed, control experiments in relevant solvents were performed. Both triplet-state participation and the Bronsted basicity of carbene 46 were hinted at. 

The norbomadiene derivatives (157) undergo efficient ring closure to give the corresponding quadricyclanes (158). The ester functions of these products were elaborated by reduction and ether formation to provide a path to so-called tentacle molecules. 

Electron-withdrawing substituents on the norbornadiene C-C double bond facilitate the valence isomerization and also stabilize the quadricyclane formed by photoisomerization, as demonstrated by the selective formation of the methoxycarbonyl-substituted quadricyclane 4 (mp 50-52 C) from both 2 and 3 on direct irradiation. ... 

Difiuorocarbene underwent addition to quadricyclane to give enr/u-6-(2,2-difluorovinyl)bi-cyclo[3.1.0]hex-2-ene together with three other products in 3% yield.The proposed formation of an insertion product into one of the cyclopropane C — C bonds was not verified in this reaction, but was observed in the reaction with other carbenes such as dichlorocarbene and bis(methoxycarbonyl)carbene. The major product with these carbenes possessed a bi-cyclo[3.2.1]octa-2,6-diene structure as the result of the insertion of the carbene into the cyclopropane bond next to the methylene bridge. ... 

A similar system with additional strain on the cyclopropane ring is quadricyclane. Its reaction with benzeneselanyl chloride in apolar solvents gave quantitatively addition products with preference for the formation of stereoisomeric 3-chloro-2-phenylselanylbicyclo[2.2.1]hept-5-enes 4 and 5. In polar or protic solvents such as acetonitrile, methanol or acetic acid, the competing opening of only one of the cyclopropane rings became dominant. The benzeneselanyl cation cleaved the most strained C — C bond in cyclopropane with concomitant or subsequent reaction with a nucleophilic solvent molecule. ... 

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