Bromo- paracyclophane

Gade and Bellemin-Laponnaz have reported the synthesis, in good yields, of chiral oxazoline-imidazoliums salts 10a (Scheme 8) obtained by reaction of 2-bromo-4(S)-t-butyl oxazoline with several mono-N-substituted imidazoles [16]. Similaly an imidazolium salt 10b bearing a paracyclophane substituent was prepared by Bolm [17]. 

The resonance structures 65 suggest that the pseudo-meta and pseudo-geminal positions are the preferred sites for substitution however, it has been observed (e.g. in the bromination and acetylation of 4-bromo[2.2]paracyclophane (67)) that there is predominant formation of pseudo-ortho and pseudo-para products 8 3b>8 5> (see Table 3) ... 

From the experimental results it can be assumed that in the [2.2]-paracyclophane system substitution predominantly occurs pseudo-geminally to the positions of the most basic prior substituents in the ring. In the bromo compound, for example, the most basic positions of the substituted ring are the ortho and para positions (see Fig. 10). Bromination and acetylation actually do occur predominantly in the pseudo-ortho and pseudo-para positions. 

Fig. 10 Canonical structures of 4-bromo[2.2]paracyclophane (67) 83b>. Cram called the polar formulae intraannular resonance structures ...

Scheme 10. Palladium-catalyzed assembly of [2.2]paracyclophane-l-ene and its 1-bromo derivative attempted synthesis of dihydrotrifoliaphane (a) Pd(OAc)2, K2C03, Bu4NBr, DMF, 85°C,4h [30]...

In view of the successful palladium-catalyzed [2 + 2 + 2] assembly of a cyclo-hexa-1,3-diene derivative from one molecule of norbornene and two molecules of /i-bromostyrene [30 a], a rationally designed synthesis of dihydrotrifoliaphane 33 as a precursor to 29 appeared to be plausible. However, when a 1 2 mixture of [2.2]paracyclophane-l-ene (32) and its 1-bromo derivative 4 was subjected to the reported conditions, the novel hydrocarbon 34 with a different array of three [2.2]paracyclophane units was obtained in 62% yield [30b]. Trifoliaphane 29, arising from oxidation of the primarily formed dihydro derivative 33, was obtained as a minor by-product (2 % yield) in this coupling. The constitution of 34 was established by an X-ray crystal structure analysis. 

Such a rigorous proof of structure is still lacking for an equally interesting hydrocarbon with the molecular formula C60H38, which was isolated in 52% yield after subjecting a 1 2 mixture of 1,2-benzo [2.2]paracyclophane-1,9-diene (35) and its bromo derivative 36 to the same conditions as applied to 32 + 4. The NMR ( H and 13C) spectroscopic data, however, indicate a close structural analogy to 34 and suggest the constitution 37 for this product [30b]. Whether... 

In essence, these domino-coupling reactions form cyclohexadiene fragments from three two-carbon fragments. The 1 2 coupling of norbomene and iodobenzene discovered by Catellani and Chiusoli can also be adopted to couple norbomene with /3-bro-mostyrene.f hi an attempt to apply this Pd-catalyzed [2+2 + 2] assembly for an alternative and more productive access to Hopf s trifoliaphane, a 1 2 mixture of [2.2]paracyclophan-l-ene and l-bromo[2.2]paracyclophan-l-ene was tfeated with palladium acetate under Jeffery conditions. The main product was the hydrocarbon consisting of three [2.2]paracyclophane units linked by a common bicyclo[3.3.0]octene unit (Scheme Apparently, the key intermediate formed via a palladacycle preferen-... 

CiBHaoCrIa, rj -[ 3.3 ]Paracyclophane )chromium( I) tri iodide, 46B, 914 C1 bH2 ftBBrFijMOaOa, M Bromo-di-/Li hydroxo-bi s ((T -cycloheptatrienyl)mo-lybdenumdl)) tetraf luoroborate tetrahydrofuran, 43B, 1099 C1bH2 nCrOa, (1 -t-Butyl-2, 2 -dimethylpropyl)-Tr-tricarbonylchromium-benzene, 42B, 693... 

Figure 8.2 Stereochemical assignation and numbering of bromo-[2.2]paracyclophane.

In a sealable glass vial, bromo[2.2]paracyclophane (1.02 g, 3.56 mmol), (rac)-BlNAP (55 mg, 0.08 mmol), tris(dibenzylideneacetone)dipalladium(0) (22 mg, 0.03 mmol), and sodium tert-butoxide (840 mg, 8.75 mmol) was dissolved in dry toluene (11 mL), and benzophenone imine (0.9 mL, 5.2 mmol) was added. The reaction was stirred at reflux for 3 d. The same amounts of BINAP, tris(dibenzylideneacetone)dipalladium(0), and sodium /ert-butoxide were added, and the reaction was stirred at reflux temperature for another 4 d. The mixture was allowed to cool to ambient temperature and was filtered over silica with ethyl acetate. The filtrate was concentrated under reduced pressure and the residue was dissolved in methanol (5 mL). Hydrogen chloride (2 M, 2 mL) was added and the suspension was stirred at 100 °C for 3 h. The reaetion was allowed to cool to ambient temperature and sat. sodium carbonate (5 mL) was added. The reaction was extracted with diethyl ether (3 x 10 mL), and the combined organic phases were dried over sodium sulfate, filtered, and concentrated under reduced pressure. The residue was subjected to column chromatography (silica, cyclohexane/dichloromethane 1 1) to yield the title compound as a brown solid (0.31 g, 1.30 mmol, 37%). The compound can be recrystallized from dichloromethane/hexane mp 243 °C (capillary) Rf 0.28 (cyclohexane/ethyl acetate, 1 1). ... 

The bromo-, formyl-, vinyl-, and ethynyl-substituted [2.2]paracyclophanes serve as versatile precursors for the preparation of a variety of jt-stacked oligomers. Arylene vinylene arms may be installed onto the [2.2]paracyclophane core by a Wittig... 

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