Substituted norbornadienes

As shown in Table 6 and Figure 1, the oxidation potentials of 2-substituted norbornadienes (1), 2-substituted bicyclo[2.2.2]octa-2,5-dienes (2) and 4-substituted [2.2]paracyclophanes (3) clearly indicate that the transannular interaction between two double bonds contributes already at the stage of the first electron transfer. Namely, in compounds 1-3, the electron is transferred from the unsaturated bond which is not substituted by the electron-withdrawing group, Figure 1 shows the... 

The incorporation of sulphonyl groups does not inhibit the cyclization process as demonstrated by Gleiter and Ohlbach133, who have reported the efficient synthesis of the quadricyclane 252 by irradiation of the norbornadiene derivatives 253. A study has also been made of the sulphonyl-substituted norbornadiene derivatives 254134 135. 

Recently an indication of path A has been shown by investigating solvolysis of the 7-substituted norbornadiene complexes of Cr, Mo, and W 90). It was found that in an 80% acetone-water solution at 25°C,(t7-C7H7C1)Mo (CO)4 gives free 7-norbornadienol instead of the expected (t7-C7H70H)Mo (CO)4- Moreover, the solvolysis rate constant was of the same order of magnitude as that for free 7-chloronorbornadiene. All these data support fast dissociation of the type... 

The regioselectivity in the reactions of 7-substituted norbornadienes with substituted acetylenes generally proved low. The reactions of 563 and 2-methoxy norbornadiene with 1-hexyne (198) did not proceed. With 2-trunethylsilylnorbornadiene (574), adducts 575 and 576 were obtained, albeit in low to moderate yields (equation 165). The best regioselectivity S15IS16 = 92/8) was obtained when the reaction was performed at room... 

An unique dimerization of norbornadienes involving C—C bond activation by Ru(cod)(cot) has been reported recently (eq (49)) [200]. This reaction occurs under mild conditions (85 °C) to give a single species with high yield. This reaction is also applicable to substituted norbornadienes to give corresponding dimers. 

Dimerization of norbornadiene, a formal [2 + 2 + 2 + 2] cycloaddition with stereospecific cis-endo addition, is possible with molybdenum carbonyl complexes20 and other transition metal compounds21 23. With unsymmetrically substituted norbornadienes (e.g., 1-tert-butoxynorbornadiene), the corresponding substituted cage molecules of heptacy-clo[6.6.0.02,6.03 13.04 11.05,9.0lo,14]tetradccane (cydooetaquinaue) are formed20,24,25. 

The rate and the yield of the reaction shown in equation (128) depend on the nature of R and of the alkene to which the S3 unit is to be transferred. If this norbornene is also substituted in the 2-position the yields differ greatly depending on the substituent phenyl results in high, methyl in low, and methoxycarbonyl in intermediate yields. When an unsymmetrically-substituted norbornadiene is used as acceptor molecule, the two double bonds react at the same rate to give equal amounts of the two trithiolanes. Most probably, the transfer reaction proceeds by a concerted bimolecular mechanism. 

Substituted norbornadienes isomerize at 170°C to substituted cyclo-heptatrienes , viz. 

The goal to utilize photoehemieal valenee isomerization between norbornadiene and quadrieyelane for solar energy eolleetion and storage was reported by others. Nagai and eoworkers synthesized five different polymers with trifluoromethyl-substituted norbornadiene moieties in the side ehains and in the main ehain... 

Norcaradienes are intermediates in several types of reaction, including intramolecular and intermolecular additions of carbenes to aromatic rings, the chromic acid oxidation of cycloheptatriene to benzaldehyde, the thermal isomerization of 7-substituted norbornadienes to cycloheptatrienes, °° and the reaction of pyridine iV-oxides with benzynes. An interesting feature of the latter reaction is the transfer of oxygen from the pyridine to the benzene nucleus (Scheme 61). 

The heptafulvalene (266) gives with TCNE a [12 + 2] adduct (267) only, whereas with dimethyl butynedioate a [4 -i- 2] adduct (268) is formed. A -Ray studies on both adducts show considerable distortions. Peculiar stereoselectivities in cycloadditions to 7-substituted norbornadienes (269) are reported. On reaction with diazoethane (269) (R = Cl, Br, I, or OAc) gives a single 1 1 adduct with endo,anti geometry (270) which reacts further to give a 2 1 adduct (271). However, when R = OBu an exo-adduct is formed and when R = OH a mixture of three products, (270), (271), and the endo,syn-adduct, is produced. The orientation is explained by a n a interaction in (269 R = halogen) between the anti double bond and the... 

An asymmetric synthesis of 2-substituted norbornadienes relies on the diastereoselective cycloaddition of cyclopentadiene to the activated alkenyl sulphoxldes (220 X C02Me or S02Ph)These dienophiles were obtained by a self-induced chiral oxidation of the corresponding sulphide. 

The ionization potentials of some substituted norbornadienes have been measured by photoelectron spectroscopy. The values which pertain to the tt orbitals are shown ... 

D. Homo Diels-Alder Reaction with 2-Substituted Norbornadienes... 

Unlike the Diels-Alder reaction (where predictable and high regiose-lectivity is expected in a cycloaddition between an electron-rich diene and an electron-poor dienophile), little is known about the regiochemi-cal outcome of an analogous unsymmetrical HDA reaction. In fact, before our studies, reports of successful cycloadditions using substituted norbornadienes are rare in the literature.In one instance, TCNE, a symmetrical and highly reactive dienophile, reacts on the unsubstituted side of the 2-substituted NBD 17 to give the substituted cyclopropane derivative 18 (Eq. 12). Other attempts to promote the... 

Nagai, T., Shimada, M., Ono, Y., Nishikubo, T. Synthesis of new photorespon-sive polymers containing trifluoromethyl-substituted norbornadiene moieties. Macromolecules, 36,1786—1792 (2003). 

A series of 7-substituted norbornadienes have been synthesized by quenching the 7-norbornadienyl cation with various nucleophiles of sulphur (SMOj), arsenic (AsPhj), and antimony (SbPhg). 7-t-Butylnorbornadiene, prepared by reaction of t-butyl-lithium with 7-t-butoxynorbornadiene, has been transformed to, inter alia, (128) and (129). By a similar route, (20) and related compounds, required for study of their photoelectron spectra, were prepared (Scheme 5). Propenylidene-norbornadienes were accessible by the routes shown in Scheme 6. They provide a novel photochemical synthesis of dihydro-azulenes, azulenes, and of vinylogous heptafulvenes. 

Nagai, T, Takahashi, I., and Shimada, M., Trifluoromethyl-substituted norbornadiene, useful solar energy material, Chem. Lett., 897-898,1999. 

As mentioned in the previous section, substituted norbornadienes can proceed [2+2] cycloaddition with highly reactive dienophiles (maleic imide or strained alkene) in the presence of a nickel (0) catalyst leading to the formation of cyclobutane derivatives. With less reactive dienophiles, the reaction seems to be prone to [2+ 2+ 2] cycloaddition giving homo-Diels-Alder reaction products. These two types of reactions appear to be competitive in certain cases to give a mixture of products [130-133]. 

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