Benzonorbomadiene

The JV-benzyl aziridines 69, 92-95 which differ only in the nature of the norbomane bridge (CH2, spirocyclopropyl, isopropylidene, oxygen, substituted nitrogen) (Scheme 14), have been prepared and reacted with each of the corresponding benzonorbomadienes 36-40 from which they were derived. All 25 reactions were conducted to produce 14 of the possible 15 different... 

Synthesis. The unsymmetrical norbomadiene substituted pz (34) (Scheme 7) (34%) (36), was prepared by a mixed cyclization of benzonorbomadiene phthalonitrile (32) (93) with a sevenfold excess of dipropylmaleonitrile (33) (28) (Scheme 7). 

The magnesium complex 34 was demetalated with trifluoroacetic acid to produce the H2[pz(A3B1)] (35) (98%) followed by conversion into the corresponding copper complex Cu[pz (A3B x)] (36, 95%) with Cu(II) acetate. Alternatively, Compound 36 can be prepared directly from the magnesium complex 34 by treatment with trifluoroacetic acid in the presence of Cu(II) acetate (72%). All forms of the norbomadiene substituted pz precursors, M[pz (A3Bj)] A = dipropyl, B = benzonorbomadiene M = Mg, H2, Cu, 34-36, have... 

Although it is generally accepted that the exo approach to norbomene is favored over the endo, the magnitude of the AAE is rarely measurable experimentally unless some fraction of the endo product can be detected. The exo approach to norbomene is favored over the endo orientation by nearly 3 kcalmol for both PFA and DMDO, while the exo TS for benzonorbomadiene is favored by nearly 5 kcalmol (Figure 27). The preferred exo approach does not appear to result from steric interactions. It is of particular interest that the AE for norbomene is only 0.9 kcalmol greater than that for cyclohexene despite the SE = 19.2 kcalmol" for the strained bicyclic alkene. Thus, in the absence of twist-strain we observe little rate enhancement due to strain energy. 

Figure 44 The enantioselective photoreaction of benzonorbomadiene. The reaction originates from the triplet state. The cation Tl+ helps to generate the triplet state of the substrate.

The polymerization of norbornene, Eq. (19), is stopped by cooling the reaction mixture to room temperature. The active polymer 11 can be stored for long periods of time. Heating 11 to temperatures above 65 °C in the presence of monomer causes renewed chain propagation. The subsequent addition of different cyclic olefins, such as endo- and exo-dicyclopentadiene, benzonorbomadiene and 6-methylbenzonorbornadiene resulted in the formation of well-defined AB- and ABA-type block copolymers, Eq. (21) [38]. Triblock copolymers 13 with narrow molecular weight distributions (polydispersity = 1.14) were prepared. Thus, the living character enables the preparation of new uniform block copolymers of predictable composition, microstructure and molecular weight. 

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

Salt crystals of the benzonorbomadiene carboxylic acid 68 with optically pure triplet sensitizer amines 70, 71 also underwent enantioselective di-TT-meth-ane photorearrangement to afford the optically active tetracyclo[5.4.0.02,4.03 6]undecane derivative 69 in high enantiomeric excess [81]. The 4-acetylbenzyl ester of L-phenylalanine 70 and the 4-benzoylphenyl ester of l-valine 71 perform the dual roles of asymmetric induction and triplet energy sensitization. 

Recent efforts have focused also on the possible realization of enantioselective reactions. The use of organized chiral media has met with some success. Benzonorbomadiene forms stoichiometric complexes... 

Even mote complex is the mixture of products which arises from the direct irradiation of benzotricy-clo[3.1.0.0 ]hex-3-ene (15), the di-ir-methane product of benzonorbomadiene (equation 12). In cyclohexane as solvent, 2-vinylindene and benzonorcaradiene are the major primary products (equation SS). ... 

Another modified metal hydride, lithium triethylborohydride, the so-called superhydride , has been introduced as a powerful reducing agent especially suitable for trisubstituted, tetrasubstituted and bicy-clic epoxides (Table 3). With trisubstituted epoxides the regiochemistry is completely controlled to give only tertiary alcohols. No skeletal rearrangement is observed for benzonorbomadiene oxide. 

Donor-substituted benzonorbomadienes 128 generally yield the product of a di-a -methane rearrangement corresponding to m-bridging, while acceptor-substituted compounds 129 give the p-bridged product ... 

In comparison with acyclic and cyclic substrates, polycyclic systems are far more reactive. The di-ir-methane unit par excellence is benzonorbomadiene, which affords on triplet sensitization the tricyclic product (15) (equation 13). ° The favorable geometry of this substrate, which by virtue of the methylenic bridge possesses the two rr-bonds fixed at an optimal interacting distance, is largely responsible for the... 

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