Rearrangements norbornadiene-quadricyclane

The catalytic rearrangement of strained alkanes is common. Cassar and Halpern showed that in the [RhCl(CO)2]2-catalyzed rearrangement of quadricyclane to norbornadiene, the admission of CO led to the formation of an acyl complex that seemed to be formed by trapping the initial oxidative addition product by a migratory insertion (equation 28). On the other hand, the Ag -catalyzed rearrangement of strained alkanes goes via carbonium ion intermediates which can be trapped by such nucleophiles as MeOH. ... 

Norbornadiene 24 rearranged to quadricyclane 25 upon UV irradiation in the presence of a sensitizer. Thermal isomerization of quadricyclane to norbornadiene proceeded at 140 °C with = 14 h. The reaction rate was dramatically increased by a transition metal-olefin complex (Scheme 2.25). For instance, t 2 at -26 Cis... 

The skeletal rearrangement of various strained cyclic compounds is carried out with a catalytic amount of soluble complexes of PdCl2. Namely, the rearrangements of bulvalene (67) to bicyclo[4.2.2]deca-2,4,7,9-tetraene (68)[54], cubane (69) to cuneane (70)[55], hexamethyl Dewar benzene (71) to hexa-methylbenzene (72)[56], and 3-oxaquadricyclanes[57] and quadricyclane (73) to norbornadiene[58-60] take place mostly at room temperature. Reaction of iodocubane (74) with a terminal alkyne catalyzed by Pd(0) and CuBr unexpectedly affords an alkynylcyclooctatetraene 75, without giving the desired cubylalkyne 76. Probably the rearrangement is a Pd-catalyzed reaction[61]. 

Optical spectroscopy has merits in identifying radical cations, particularly when their spectra are known independently. For example, the radiolysis of quadricyclane led to the observation of the known spectrum of norbornadiene radical cation. In another study, irradiation of cyclooctatetraene radical cation caused the color of the sample to change from bright red to royal blue, suggesting the conversion to a different species, the previously identified semibullvalene radical cation. Further irradiation of the latter led to a characteristic banded (vibrationally resolved) spectrum the nature of this spectrum suggested that the rearranged species may be a linear conjugated radical cation and helped in its identification as 1,4-dihydropen-talene radical cation. ... 

The opposite substrate-induced diastereoselectivity is observed in the cycloadditions to norbor-nenc and norbornadiene, since the adducts exo-9 and endo-10 are obtained, respectively6. The e.w-adduct 10, prepared from norbornene, can also be obtained, together with a rearrangement product, by cycloaddition to quadricyclane followed by double-bond hydrogenation8. 

Both the acid induced and the photochemical rearrangement of the norbornaidiene derivative (146) affords the hydroxynaphthalene (147). The photo-process is not affected by triethylamine and it is thought to involve benzy lie-ally lie C-O bond cleavage. No evidence was put forward for the intermediacy of a quadricyclane. Prinzbach smd his coworkers have reported the synthesis of the oxanorbornadienes (148) and their photoconversion into the corresponding oxaquadricyclane derivatives (149) on irradiation. The azaquadricyclanes (150) have been prepared by the irradiation of the corresponding norbornadiene derivatives (151). ... 

Metal-catalysed Isomerizations Because of the wide range of values previously available, Wiberg and Cannon have accurately determined the enthalpy of the quadricyclane- norbornadiene conversion as —26.16 0.24kcal.mol in solution and —27.13 0.24 kcal mol" in the gas phase by studying the metal-catalysed rearrangement. The same authors have studied the metal-catalysed tricyclo[4,1,0,0 ]-heptane- 3-methylenecyclohexene reaction [A/T/soIution) = — 29.99 0.08 kcal mol" cf. 25.8 kcal mol" for bicyclobutane butadiene)]. 

Several theoretical studies have been performed on thermal reactions. Ab initio electronic-structure calculations at the 6-3IG level have been performed to determine the mechanism of the thermal isomerization of buta-1,2-diene to buta-1,3-diene. The thermal interconversion of the norbornadiene and quadricyclane radical cations has been studied, with geometries optimized at the UMP2/6-31G and UHF/6-31G levels. Two transition structures are proposed with C and C2 symmetry, respectively. They conclude by suggesting the true transition state is the Ci symmetric one whereas the second is a second-order saddle point. The thermal rearrangement of diformyl peroxide has been studied using the UHF/AMl MO method to determine the mechanism of the isomerization and the MP2 method to improve the barrier energies of the reaction. A stepwise radical reaction is proposed. ... 

Skeletal rearrangements of some strained compounds, such as bulvalene to bicyclo[4.2.2]deca-2,4,7,9-tetraene, cubane to cuneane, hexamethyl Dewar benzene to hexamethylbenzene (eq 52), and quadricyclane to norbornadiene (eq 53), are catalyzed by derivatives of PdCl2. 

Photoinitiated Quadricyclane and Norbornadiene Cycloaddition Reactions and Rearrangements... 

Goez, M. and Frisch, I., Rearrangements of photogenerated 1,5-biradicals in the Patemo-Buchi reaction of quinones with norbornadiene or quadricyclane, /. Inf. Rec., 25, 287-293, 2000. 

Weng, H. and Roth, H.D., Electron transfer photochemistry of norbornadiene and quadricyclane. Nucleophilic capture of radical cations, free-radical rearrangements, and hydrogen abstraction, . Org. Chem., 60, 4136-4145, 1995. 

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