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CHM 404 : Stereoelectronics, Structure and Reactivity

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CHM 404 : Stereoelectronics, Structure and Reactivity CHEM40003: Aromatic Chemistry WORKSHOP PROBLEMS - ANSWERS Prof Alan Spivey; Office: 835 C1; e-mail: [email protected]; Tel.: 45841 2 Question 1 Compound 1 has 18 electrons of which 14 make up a contiguous cyclic set of p-orbitals perpendicular to the plane of the molecule and therefore constitute the aromatic system. The other 4 electrons are in p-orbitals in the plane of the ring as the result of the sp hybridisation of the carbons shown ‘dotted’ in the picture. NB. although this molecule must be represented by resonance forms in which there is a cumulene and an alkyne, it is in fact highly symmetrical and is correctly represented by the two resonance forms shown below. Hence it is referred to as a 1,8-bisdehydro[14]annulene. the symmetry is perhaps most clearly apparent from orbital representation. The 1H NMR spectrum shows just thee signals as indicated below. see: Sondheimer et al. J. Am. Chem. Soc. 1962, 84, 4595 (DOI) H H H H Ha Ha H H H H Hb Hb H H H H = Hc Hc H H H H Hb Hb H H H H Ha Ha (1) (1) 1H NMR: -5.54 (Hc, t), 8.43 (Ha, d), 9.65 (Hb, dd) 3 Question 2 Compound 2 is predicted to have a large dipole moment due to the significant contribution made to the structure by the dipolar resonance form shown below comprising an aromatic cyclopentadienyl anion portion and an aromatic cyclopropenyl cation portion. In fact, this compound, named calicene, has never been prepared although various derivatives have and these do indeed display large dipole moments (e.g. hexaphenylcalicene has a dipole moment of 6.3 D) see: Weber et al. Helv. Chim. Acta. 1989, 72, 41 (DOI) 6p electrons, aromatic 2p electrons, aromatic DIPOLE 1 4 Question 3(a-d) The following routes are suggested, alternatives are possible: + - NO2 NH2 N2 Cl CN CN c.H2SO4 Sn-HCl or NaNO2 c.H2SO4 (a) KCN (S 1) c.HNO3 S(NH4)2 HCl N c.HNO3 NO2 O Zn-HCl or Et Et Et O MeCOCl H2NNH2-KOH or MeCOCl HSCH CH SH-RaNi (b) AlCl3 2 2 AlCl3 + O Me O Zn-HCl or Et MeCOCl (c) H2NNH2-KOH or H SO -SO AlCl 2 4 3 3 HSCH2CH2SH-RaNi SO3H SO3H SO3H + - CuCl NO2 NO2 N2 Cl Cl Cl c.H2SO4 Br2 i) Sn-HCl or or c.H2SO4 (d) NO2 c.HNO3 FeBr3 S(NH4)2 KCl c.HNO3 Br ii) NaNO2- Br Br Br HCl 5 Question 4a see: Evans ‘Total syntheses of vancomycin and eremomycin aglycons’, Angew. Chem. Int Ed., 1998, 37, 2700 (DOI) 6 Question 4b see: Evans ‘Total syntheses of vancomycin and eremomycin aglycons’, Angew. Chem. Int Ed., 1998, 37, 2700 (DOI) Standard SNAr mechanism. Fluorine is highly electronegative (4.1, Pauling Scale) and facilitates initial nucleophilic attack on ipso-C (so-called ‘element effect’) which is the rate determining step (RDS). The nitro group stabilises the Meisenheimer intermediate by allowing negative charge to be delocalised onto its oxygens by resonance (NB. only possible at ortho- and para- positions relative to fluorine) 2 8 Question 4d see: Evans ‘Total syntheses of vancomycin and eremomycin aglycons’, Angew. Chem. Int Ed., 1998, 37, 2700 (DOI) 6 (X = F): use HBF4 to form diazonium salt and then heat to effect Bälz Scheimann 6 (X = CN): form diazonium salt then treat with CuCN (Sandmeyer reaction) ***** 3 .
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