Norbornadiene isomerization

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

Asymmetric hydrogenolysis of epoxides has received relatively little attention despite the utility such processes might hold for the preparation of chiral secondary alcohol products. Chan et al. showed that epoxysuccinate disodium salt was reduced by use of a rhodium norbornadiene catalyst in methanol/water at room temperature to give the corresponding secondary alcohol in 62% ee (Scheme 7.31) [58]. Reduction with D2 afforded a labeled product consistent with direct epoxide C-O bond cleavage and no isomerization to the ketone or enol before reduction. 

Cassar and Halpern (85) provided evidence that the Rh -catalyzed valence isomerization of quadricyclene to norbornadiene also proceeds through a nonconcerted mechanism ... 

The facile thermal isomerization (17) of norbornadiene derivatives [71]-[77] to cycloheptatrienes in polar solvents has been explained in terms of the initial heterolytic cleavage of the strained C(l)-C(7) bond (Hoffmann and Hauser, 1965 Lemal et al., 1966 Hoffmann, 1971, 1985 Lustgarten and Richey, 1974 Hoffmann et al., 1986 Bleasdale and Jones, 1993). The resulting zwitterion intermediates are stabilized by the cation-stabilizing groups attached to C(7) and the cyclohexadienyl-type delocalization of the negative charge. 

Isomerizations are another category of reaction where Co complexes act as catalysts. Mixed S,P donor complexes Co(SCN)2(PR3)2 catalyze the isomerization of 1-butene to 2-butene in the presence of NaBH4, with CoH(SCN)(PR3)2 proposed as the active species.1366 A cyclopentadienyl complex (272) is active in the isomerization of quadricyclene to norbornadiene. 

The characteristics of the hydrogenation of norbornadiene, substituted butadienes and conjugated and cyclic dienes are all very similar. In the case of conjugated dienes, there appears to be hardly any isomerization activity, while in the case of 1,4-dienes an isomerization step to form the corresponding 1,3-diene is assumed prior to hydrogenation. The catalyst behavior changes after the diene has been completely converted to the monoene, whereupon the rhodium catalyst resumes its normaF monoene hydrogenation behavior. 

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 variety of derivatives that undergoes cyclization is almost limitless, as shown by the conversion of 239 into 240. Here, one of the functional groups on the norbornadiene is ketonic122. The isomerization of other keto or aldehydo derivatives of norbornadiene into... 

Fig. 12.3 Sequential (right) vs a non-reactive four-center ring closure in the isomerization of norbornadiene to quadricydane inset). The two new C-C bond distances and the two bonds that start as the C =C bonds of norbornadiene are shown vs time in fsec. The two trajectories...

The results for quadricyclane (QC) shown in Fig. 4.6 are qualitatively similar to those for norbornadiene, except that the onset of decomposition for quadricyclane is about 100 K lower. QC is stable on the available time scale at temperatures up to 523 K, above which decomposition sets in rapidly. The QC is completely decomposed within 2 ms (Table 4.1) by about 750 K. As with norbornadiene, the dominant low-temperature decomposition channel is fragmentation to C5H6 + C2H2, while isomerization to more stable isomers (TOL) becomes increasingly important at higher temperatures. There is some evidence for production of norbornadiene in the 700-800 K temperature range, but at higher... 

In 5-47 we used the principle of conservation of orbital symmetry to explain why certain reactions take place readily and others do not. The orbital-symmetry principle can also explain why certain molecules are stable though highly strained. For example, quadricyclane and hexamethylprismane1002 are thermodynamically much less stable (because much more strained) than their corresponding isomeric dienes, norbornadiene and hexamethylbicy-clo[2.2,0]hexadiene (105).1,103 Yet the former two compounds can be kept indefinitely at... 

The first report of triplet photochemistry of three-membered rings concerned the reversibility of the photosensitized isomerization of norbornadiene to quadricyclane.270 Since no spectroscopic triplet levels of cyclopropane are known, it was felt that this reaction represents an extreme sort of nonvertical energy transfer with actual bond breaking in the acceptor. 

Exo addition is not always the rule with cyclic dienes, and endo adducts have been identified in several instances. Although in earlier work phenyl azide was thought to yield with excess norbornadiene only a single exo adduct,25 later work indicates that the endo isomer is also formed in an exo-endo ratio of 11/1 (Scheme 12).97 In excess azide, four isomeric bistriazolines have been... 

The monoadduct from norbornadiene and the bulky trimethylsilyl azide is less stable than the bisadduct and spontaneously yields a 1-silylated triazole that isomerizes to a 2-silyltriazole (Scheme 139).104... 

A new and readily available catalytic system has been developed to open the cyclopropane ring in 4 + 2-1- 2-homo-Diels-Alder cycloadducts formed by reaction of norbornadienes and buta-1,3-diene.47 The cobalt-mediated homo-Diels-Alder reaction followed by this PtCB-promolcd isomerization is a key step in the efficient route to bicyclo[5.3.0]decanes. 

Figure 9.8 Endergonic and reversible electrocyclic reactions obeying Woodward-Hoffman rule, (a) Valence isomerization, (b) cycloaddition, (c) sigmatropic effect, and (d) norbornadiene to quadricyclene conversion.

In this context it is useful to remember that the concept of the possible recombination of triplet radical ion pairs is not an ad hoc assumption to rationalize certain Z - E isomerizations, although the CIDNP effects observed during an isomerization reaction played a key role in understanding this mechanism. Triplet recombination has been accepted in several donor-acceptor systems as the mechanism for the generation of fast (optically detected) triplets [169-171], and invoked for several other reaction types [172]. The CIDNP technique is a sensitive tool for the identification of this mechanism, for example, in the geometric isomerization of Z- and E-1,2-diphenylcyclopropane and in the valence isomerization of norbornadiene (vide infra). Most of these systems have in common that the triplet state can decay to more than one minimum on the potential surface of the parent molecule. 

Titanacycle 10 ring-opens photochemically at temperatures well below 0°C, Eq. (27) [41]. The unstable titanium carbene 20 can be trapped by exo-trans-exodiene 21, derived from (2 + 2) cycloaddition of norbornadiene, to produce a mixture of two isomeric bis(titanacyclobutanes), 22a, b [42]. Bis(titanacycle) 22a, b could not be synthesized cleanly by thermolysis of 10 because formation of 22a, b competes with ring-opening and the addition of a second equivalent of diolefin. 

A variety of bis(dithiolene) complexes has been reported as catalysts for the isomerization between quadricyclane and norbornadiene (Eq. 9) (86). 

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