7-Substituted norbomadienes

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

The regio- and stereoselectivity of three different dienophiles in the HDA reaction with two different 7-substituted norbomadienes were investigated (Scheme 14). Both acrylonitrile (AN) and phenyl vinyl sulfone (PVS) give excellent yields of the HDA products. However, a modest drop in anti syn ratios was observed whereas the exo endo ratios decrease dramatically as the dienophile changes from MVK to AN or PVS (Table 10). 

Similar studies on the cobalt-catalyzed HDA reaction between various 7-substituted norbomadienes and an unactivated terminal acetylene, 1-hexyne, were also investigated (Scheme 16, Table 11). Although the yields remain high, the anti/syn selectivities are poor compared with the corresponding Ni-catalyzed cycloadditions. 

In Scheme 2.210, possible variants of intramolecular 1,3-dipole cycloaddition of norbomadiene derivatives with 2-substituted norbomadiene-tethered nitrones are presented. 

Y. Harel, A. W. Adamson, C. Kutal, P. A. Grutsch, K. Yasufuku. Photocalorimetry. 6. Enthalpies of Isomerization of Norbornadiene and of Substituted Norbomadienes to Corresponding Quadricyclenes. J. Phys. Chem. 1987, 91, 901-904. 

The regiochemistry of nickel mediated cycloadditions of substituted norbomadienes has been investigated in detail. The regioselectivity, exo/endo selectivity and site selectivity seem to depend strongly on the substituents on both diene and dienophile. Tetracya-noethene, for example, reacted with 2-acetyloxymethyl substituted norbomadiene on the distal side331. 

Adducts were obtained as exclusive adducts or as by-products in the nickel mediated reactions of some substituted norbomadienes with various dienophiles. The formation of these products was considered to result from an intermediate metallocy-clopentane species built up of the metal center, the dienophilic double bond and one of the double bonds of the norbomadiene moiety. 

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 norbomene 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 norbomadiene 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. 

As shown in Table 6 and Figure 1, the oxidation potentials of 2-substituted norbomadienes (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... 

Osmium tetraoxide has also been used in the oxidation of bicyclic and polycyclic dienes. Thus, oxidation of norbomadiene (26) in a stoichiometric reaction was found to yield the exo-cis diol exclusively. On the other hand, in the NMO catalytic system a mixture of the exo-cis and endo-cis products was reported. However, by use of the NMO catalytic procedure for the substituted norbomadiene 27, the exc-diol was formed exclusively at the sterically crowded unsubstituted double bond and this product was utilized in the synthesis of pentalenolactone. Somewhat surprisingly, oxidation of hexamethyl Dewar benzene (28) exclusively gave the endo-cis diol as sole product. The tricyclic compound 29 gave the usual c/s-diol oxidation product of one of the double bonds. ... 

The incorporation of sulphonyl groups does not inhibit the cyclization process as demonstrated by Gleiter and Ohlbach, who have reported the efficient synthesis of the quadricyclane 252 by irradiation of the norbomadiene derivatives 253. A study has also been made of the sulphonyl-substituted norbomadiene derivatives 254 ... 

Other catalytic systems were used recently, namely a Ddppi system (67, Scheme 8.14) for dicyclopentadiene and di(methylcyclopentadiene) [96] and imino-phosphine complexes (Scheme 8.19, 97 and 98) or hybrid terdentate carbene hgand complexes (99) [133] for norbornene. Furtlierrnore, the copolymerization of various substituted norbomadienes and norbomenes has been achieved with Rh6(CO)i6 under water gas shift reaction conditions (55 ""C, water and 100 atm CO) [134],... 

Cycloadditions. The Ni(0)-catalyzed intramolecular [4-1-2]-cycloaddition generates. eterocycles in a novel and efficient manner. Norbomadiene undergoes [2-i-2-t-2]-. .cloaddition with electron-deficient alkenes, while some substituted norbomadienes (e.g., 2-TMS) can form [2-F2]-cycloadducts. ... 

The goal to utilize photochemical valence isomerization between norbomadiene and quadricyclane for solar energy collection and storage was reported by others [185]. Nagai et al. synthesized five different polymers with trifluoromethyl-substituted norbomadiene moieties in the side chains and in the main chain [185] ... 

ROMP polymers from silyl-substituted norbomadienes and norbornenes Hg, H2, N2, O2, C02 CH4, C2H0 85-167 22 0.1-1 [168]... 

Similar studies of the regioselectivity in the HDA reaction between 2-substituted norbomadienes and unactivated terminal acetylenes catalyzed by a cobalt catalyst were also undertaken. Unfortunately, 2-sub-stituted norbomadienes are much less reactive with unactivated acetylenes in the cobalt-catalyzed HDA reaction than with electron-deficient olefins in the Ni-catalyzed HDA cycloadditions. When an electron-withdrawing group (Y = COOMe) or an electron-donating group (Y = OMe) is attached to the 2-position of the NBD, no desired [2 + 2 + 2] cycloadduct is observed with 1-hexyne, even in refluxing toluene for three days under the usual cobalt-catalyzed conditions (Scheme 12). Instead, some NBD dimers and acetylene trimers are detected. With a... 

During the studies of the nickel-catalyzed HDA reactions between substituted norbomadienes with various dienophiles, we noticed that some dienes undergo a [2 + 2] cycloaddition instead of a [2 + 2 + 2] cycloaddition. As shown earlier (Section IID, Eq. 14), 2-TMS-substi-tuted NBD reacts with acrylonitrile in the presence of Ni(COD)2/2PPh3, to afford a mixture of [2 + 2 + 2] and [2 -i- 2] adducts. In some cases, the... 

It is possible to rationalize the formation of the endo and exo [2 + 2] adducts of 2-substituted norbomadienes by the pathways shown in... 

Although our studies have not provided a full explanation for the regiose-lectivity of the substituted dienes, several key observations have been made. The selectivity depends on the substituent on the diene, the dienophile, and the ligand used in the cycloaddition. As described earlier, the reactivity of 2-substituted norbomadienes in the HDA reaction is much lower than NBD itself. Thus, when these 2-substituted norbomadienes react with reactive dienophiles, an alternative [2 + 2] cycloaddition pathway may predominate. 

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

Scheme 4-213. Synthesis of substituted (norbomadiene)rhodium complexes via iron-catalyzed cross-coupling of vinyl triflates with Grignard reagents.

Ikezawa, H., Kutal, C., Yasufuki, K., and Yamazaki, H., Direct and sensitized valence photoisomerization of a substituted norbomadiene. Examination of the triplet-state reactivities, /. Am. Chem. Soc., 108, 1589-1594, 1986. 

Nagai, X, Shimada, M., and Nishikubo, X, Synthesis of new photoresponsive polymers containing trifluoromethyl-substituted norbomadiene moieties, Chem. Lett., 1308-1309, 2001. 

Nagai, X, Xakahashi, I., and Shimada, M., Xrifluoromethyl-substituted norbomadiene, useful solar energy material, Chem. Lett., 897-898, 1999. 

A nickel-catalyzed similar reaction using substituted norbomadiene and maleic imide under more drastic conditions gives an exo/endo mixture of [2 + 2] adduct [115]. 

Abbreviations aapy, 2-acetamidopyridine Aik, alkyl AN, acetoniuile Ar, aryl Bu, butyl cod, 1,5-cyclooctadiene COE, cyclooctene COT, cyclooctatetraene Cp, cyclopentadienyl Cp , penta-methylcyclopentadienyl Cy, cyclohexyl DME, 1,2-dimethoxyethane DME, dimethylformamide DMSO, dimethyl sulfoxide dmpe, dimethylphosphinoethane dppe, diphenylphosphinoethane dppm, diphenylphosphinomethane dppp, diphenylphosphinopropane Et, ethyl Ec, feirocenyl ind, inda-zolyl Me, methyl Mes, mesitylene nb, norbomene orbicyclo[2.2.1]heptene nbd, 2,5-norbomadiene OTf, uiflate Ph, phenyl PPN, bis(triphenylphosphoranylidene)ammonium Pi , propyl py, pyridine pz, pyrazolate pz, substituted pyi azolate pz , 3,5-dimethylpyrazolate quin, quinolin-8-olate solv, solvent tfb, teti afluorobenzobaiTelene THE, tetrahydrofuran THT, tetrahydrothiophene tmeda, teti amethylethylenediamine Tol, tolyl Tp, HB(C3H3N2)3 Tp , HB(3,5-Me2C3HN2)3 Tp, substituted hydrotiis(pyrazol-l-yl)borate Ts, tosyl tz, 1,2,4-triazolate Vin, vinyl. 

Experimentally, this reaction sequence was observed with hydrogen 70) or a substituted octenyl group721 in position 7 of the norbomadiene. In the calculations a methyl group was used as marker, to show the course of the rearrangement. 

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