4 Aromatic chemistry By M. JOHN PLATER Department of Chemistry University of Aberdeen Meston Walk Aberdeen UK AB24 3UE 1 Theoretical and structural studies Computational calculations have demonstrated that the D 6) symmetry of benzene is determined by the r-framework and that the p-electrons would cause distortion to a localised D 3) structure.1 An experimental probe supporting this theory is provided by the cyclohexatriene motif 1 which shows alternating double and single bonds in the X-ray single crystal structure. However the excited state 1B 26 owing to a p–p* electron transition is calculated to be almost symmetrical. This suggests that p- bonding in the ground state of 1 which is disrupted in the excited state does indeed cause bond fixation and gives the D 3) structure. 1 2 3 The isomeric syn-2 and anti-bismethano[14]annulenes 3 are known to have di§erent structures and magnetic properties.2 In syn-annulene 2 the p-electrons are delocalised and the C–C double bonds are of nearly equal length.The diamagnetic ring current causes deshielding of the perimeter protons (d\7.4–7.9 ppm) while the signals of the methano bridge protons are shifted upfield (d\0.9 and [1.2 ppm). No ring current is apparent from the chemical shifts of protons in the anti-isomer 3. The perimeter protons occur at d\6.2 ppm and the methano bridge protons at 2.5 and 1.9 ppm. The delocalised and localised structures of syn-2 and anti-3 have now been investigated with common computational methods. Semiempirical PM3 and ab initio RHF/6-31G* methods favour bond localised structures for both isomers whereas MP2/6-31G*//RHF/6-31G* single point calculations favour the delocalised structure for both isomers.These conflicting results illustrate how predictions of delocalised or localised structures should be made with caution. The magnitude and direction of errors for each type of calculation should be considered. The authors conclude that aromatic stabilisation energies of planar p-systems are not altered drastically either by bond localisation or by modest out of plane deformations as long as the p-overlap is maintained. Similarly diamagnetic ring currents decrease only modestly upon bond 129 localisation and disappear only in twisted structures such as 3 in which p-overlap is eliminated. Bond localisation in 2 can occur with a comparatively small loss in energy (6.0 kcal mol~1) and also in benzene (4.3 kcal mol~1) by a simple ‘breathing’ distortion.4 5 6 Molecular mechanics calculations semiempirical calculations and full geometry optimisations have been performed on [14]annulene 4 and on [18]annulene 5.3 The X-ray structure of [14]annulene 4 showed it to be aromatic with C–C bond lengths in the range 1.350 to 1.407Å. However calculations have not reproduced the small bond lengths which are thought to have arisen as a consequence of the small number of reflection data points in the structure refinement. The X-ray structure of [18]annulene 5 shows it to have approximately D 6) symmetry with 1.385Å inner and 1.405Å outer C–C bond lengths confirming the aromatic structure. The bond length di§erence of 0.020Å is much smaller than the value of 0.037Å previously extracted from X-ray data and smaller than the value of 0.052Å from a recent theoretical calculation.It is however well reproduced by BLYP B3LYP and MP2 methods. One important observation of electron correlation calculations on [14]annulene 4 and [18]annulene 5 is that irrespective of the chosen starting symmetry the geometry optimisations invariably gave delocalised more symmetrical structures. This observation casts doubt on the validity of using HF and semiempirical methods for determining relative stabilities of localised and delocalised structures of conjugated molecules. The suitability of RHF/DF methods to predict the structure of annulenes has been demonstrated by calculations on [18]annulene 5.4 The molecular geometry predicted by correlated computations agrees well with the recently determined disordermodelled crystal structure of 5 at 110 K.The use of correlated methods and extensive polarisation functions is necessary to predict the molecular geometry with experimental accuracy. The authors predicted that annulene 6 which is on the border of the breakdown of the Hu� ckel rule would possess significant bond localisation and loss of ring current. Computational calculations have been used to predict the structure of a series of fulvenes 7–9 and fulvalenes 10–15.5 Fulvenes 7–9 were all found to be planar. Pentaheptafulvalene 14 was slightly nonplanar while heptafulvalene 15 was predicted to have an anti-folded C 2) structure in accord with X-ray crystal structure data. Calculations predict that the unknown smallest fulvalene 10 is destabilised with localised bonding and that triapentafulvalene 11 is stabilised with delocalised electrons in line with Hu� ckel theory.1,4-Biphenylenequinone 17 was generated by teatment of 16 with Et 3 N.6 Its antiaromatic character precluded isolation; only the dimer 18 was obtained. It could be intercepted by cyclopentadiene to give the two isomers 19 and 20 (Scheme 1). 130 M. John Plater H Br O O 16 O O 17 O O O 18 O O O O + i ii 19 20 O Scheme 1 Reagents i Et 3 N rt; ii cyclopentadiene 7 8 9 10 11 12 13 14 15 Alkali metal reduction of 1,3,5,7-tetra-tert-butyl-s-indacene 21 with lithium or potassium metal gives the corresponding dianions [(Li`) 2 (212~)(thf) 4 ] 22 and [(K`) 2 (212~)(18-crown-6) 2 ] 23.7 The 1HNMRchemical shifts for the methine protons of [(Li`) 2 (X2~)(thf) 4 ] 22 are d\6.77 and 8.28ppm for the C-2,6 and C-4,8 positions respectively.These protons are significantly deshielded in comparison to the corresponding shifts in the starting material 21 (d\5.29 and 6.90 ppm respectively) probably owing to an increase in the ring current. The 13C NMRsignals for dianion 22 are significantly shielded (d\103.7 108.4 111.7 122.8) compared to those of the starting material (d\124.9 129.1 132.0 164.3 ppm) also owing to the increased electron 131 Aromatic chemistry O2N N NO2 NO2 NHTs O2N NO2 NO2 – O2N NO2 NO2 H 27 28 i 26 Scheme 2 Reagents i NaH THF heat But But But But 21 M But But But But 22 M = Li 23 M = K 2– 2M+ H H 24 25 – density. The crystal structure of the indacene dianion [(Li`) 2 (212~)(OEt 2 ) 2 (thf) 2 ] (prepared by recrystallisation of dianion 22 from diethyl ether) reveals a planar structure in which each lithium cation is bonded symmetrically to all five carbon atoms of the terminal indacene rings.The average Li–C bond length is 2.34Å. This contrasts with most previous metal s-indacene complexes in which the metal is bonded o§-centre from the centroid five-membered ring. Computational calculations on dianion 22 show good agreement for bond lengths and angles compared with the experimental data. The gas phase acidity of benzocyclopropene 24 has been measured experimentally and calculated as *H0 !#*$ \386 kcal mol~1.8 This is only 4 kcal mol~1 more acidic than the value for toluene and 34.5 kcal mol~1 more acidic than that for cyclopropene. However the benzocyclopropene anion 25 is significantly more stable than the benzyl anion in solution.This is explained by the greater electronegativity of the methylene carbon owing to the greater percentage of s character and owing to the diminished importance of delocalisation in solution owing to solvation. The anion 25 is calculated to show more bond fixation than benzocyclopropene 24 as shown in the canonical form drawn for compound 25. This may be to avoid an unfavourable 4p interaction in the three-membered ring. The cyclopropenyl anion 28 was calculated to have a triplet ground state 74 kJ lower in energy than the lowest single state.9 The precursor trinitrotriphenylcyclopropene 27 132 M. John Plater 29 30 31 • • 32 i i Scheme 3 Reagent i hl was prepared by decomposition of the tosylhydrazone 26 (Scheme 2). However all attempts to form the anion 28 by treatment of precursor 27 with either potassium hydride and 18-crowlithium tetramethylpiperidide or BunLi were unsuccessful.Irradiation of hydrocarbon 29 in methyltetrahydrofuran at 89K generates the coloured species 30 but the EPR spectrum gave no signals assignable to a triplet.10 However irradiation of hydrocarbon 31 under the same conditions gave a persistent species 32 whose EPR spectrum was diagnostic for a randomly orientated triplet (Scheme 3). This is the first Kekule� hydrocarbon with a triplet ground state observed at 89 K and is representative of an exciting new class of high spin molecules. R1 R1 R1 R1 33a R1 = CN 33b R1 = H hn heat R1 R1 R1 R1 34a 34b The thermal transformation of a strained paracyclophane into the corresponding Dewar isomer has been observed for the first time.11,12 Irradiation of Dewar isomer 33a at 365nm in isopentane–diethyl ether at 77K gave the [4]paracyclophane 34a which was su¶ciently stable at[50 °C to allow its 1HNMRspectrum to be recorded.When the mixture was thawed briefly warmed to room temperature and recooled to 77K a nearly quantitative thermal conversion back to Dewar isomer 33a had occur- 133 Aromatic chemistry Cl Cl Cl Cl Cl Cl 35 Cl Cl Cl Cl 36 Cl OH Cl Cl CH3 38 37 i Scheme 4 Reagents i Bu5OK DMSO rt red. The half-life of paracyclophane 34a was determined as 15^5 min at[20 °C. Treatment of metacyclophane precursor 35 with Bu5OK in DMSO gave two products 37 and 38 along with some polymeric material presumably via the strained intermediate 7,14-dichloro[1.1]metacyclophane 36 (Scheme 4).13 The parent compound [1.1]metacyclophane still remains elusive.Compound 36 was studied in the hope that it might have greater thermal stability. The strained compounds 39 and 40 have unusually long C–C bond lengths. Compound 39 has a C–C bond length of 1.720(4)Å and compound 40 has long C–C bond lengths of 1.710(5)Å and 1.724(5)Å. These are the longest C–C bond lengths reliably determined to date. However semiempirical calculations underestimate the long C–C bond length of 39 by 0.05Å showing the inadequacy of these methods. Discrepancies between experiment and theory on the long bond lengths in compounds 39 and 40 Ph Ph Ph Ph Cl Cl 39 Ph Ph Ph Ph Ph Ph Ph Ph Cl Cl Cl Cl 40 O O Me Me Me 41 have now been resolved by the use of full geometry optimisations based on Hartree –Fock and density functional theory.14 Becke’s 1988(B) and his three parameter hybrid (B3) functionals incorporating exact exchange were used as gradient-corrected density functionals in combination with the Lee Yang and Parr (LYP) correlation functional in the calculation.Good agreement between experimental and theory is obtained by these methods which include electron correlation e§ects. The X-ray crystal structure of dimer 41 has long C–C bond lengths of 1.602(7)Å and 1.649(6)Å holding it together.15 The dimer undergoes a rapid degenerate [3,3] Cope rearrangement in solution. These may be the longest C–C bond lengths determined for a molecule that can dissociate in solution. 134 M. John Plater NNH2 H2NN 42 i I I 43 44 ii I NC NC I 46 NC NC iv NC NC 45 NC NC Li But NC NC But Li + 47 49 CN NC X But CN NC But X 48a X = Li 48b X = H 50a X = Li 50b X = H v iii Scheme 5 Reagents i I2 Et3 N; ii dicyanoacetylene 100 °C 96 h; iii ButLi; iv 1,2-dimethylenecyclopentane; v ButLi 3,4-Dicyanotricyclo[4.2.2.2]dodeca-1,3,5,7,9,11-hexaene 45 was generated by treatment of the diiodo precursor 44 with tert-butyllithium in THF at [78 °C.16 The cleaving bonds in precursor 44 are ideally disposed antiperiplanar to each other.The formation of products 48b and 50b was explained by the addition of ButLi to intermediate hexaene 45 followed by electrocyclic ring opening to the lithio species 48a and 50a and subsequent protonation (Scheme 5). Cycloadduct 46 was formed by interception of hexaene 45 with dimethylenecyclopentene. Tetradehydrodianthracene (TDDA) 51 reacts with tetrazines 52 at 20 °C in CH 2 Cl 2 in a 1 1 molar ratio to give the monoadducts 53a–d (Scheme 6).17 The unsubstituted tetrazine also forms the 2 1 products 54a–d with each of the adducts 53a–d under the same conditions.Cycloaddition of TDDA 51 with a-pyrone and 1,2-diazine gives Kammermeierphane 1 58 via the loss of either CO 2 or N 2 to give intermediate 57 followed by electrocyclic ring opening (Scheme 7).18 The X-ray crystal structure of compound 58 shows that the bridging ethene unit is syn with respect to the two bridging quinoid double bonds. This is in agreement with a thermochemically allowed 135 Aromatic chemistry 51 N N N N R R R R + N2 N N N N R R H H N N N N H H 53a R = H 53b R = CH3 53c R = CO2CH3 53d R = CF3 54a R = H 54b R = CH3 54c R = CO2CH3 54d R = CF3 52 N N Scheme 6 ring opening of diene 57.Irradiation of Kammermeierphane 1 58 and TDDA51 with a 150W high-pressure mercury lamp leads directly to cyclophane 60 of molecular formula C 60 H 36 via ring opening of the initially formed 2]2 adduct 59. The two butadiene units have the s-trans configuration. According to AM1 calculations the s-cis/s-trans and s-cis/s-cis configurations are not possible for steric reasons. The dimensions of the cavity are calculated to be 7.9]4.8Å. Treatment of dibromo bicyclooctene-annelated benzene 61 with BunLi at[78 °C in THF generated the aryne 62 which gave the cycloadduct 63 in the presence of furan and the dimer 64 in the absence of an intercepting reagent (Scheme 8).19 Decomposition of the diazonium carboxylate 65 in refluxing dichloroethane gave none of the dimer 64 but a low yield of the acridone 66.Dimer 64 showed a reversible oxidation wave at E 1@2 \]0.33V versus a ferrocene–ferricenium couple indicating the formation of a stable radical cation. The anthracene derivative 67 also showed a low oxidation potential at E 1@2 \]0.21 V.20 Pronounced stabilisation results from inductive hyperconjugative and steric e§ects of the bicyclo[2.2.2]octene framework. Coloured solutions of the radical cations were generated by treatment with NO`SbCl 6 ~ in CH 2 Cl 2 . The radical cations could be isolated and gave the same EPR spectrum upon redissolution in CH 2 Cl 2 . 68a 68b S S Me S S Me 9-Methyl-2,11-dithia[3.3](1,4)triphenylenometacyclophane was synthesised as a mixture of syn-68a and anti-68b isomers.21 The major isomer was confirmed as the syn-isomer 68a owing to the upfield shift of the methyl group of the anti-isomer 68b to 136 M.John Plater O O 55 O O 51 N N N N 56 57 –CO2 –N2 58 H H 59 51 / hn 60 Scheme 7 d 0.85 ppm. The cyclisation reaction was kinetically controlled as calculations suggested that the anti-isomer 68b was the more stable isomer. Treatment of tetrachlorocyclopropene 70 with 6 equivalents of the lithio carbanion of the bisarylmethanes 69a–e in THF–DMSO at 0 °C followed by aerial oxidation gave the corresponding radialenes 71a–e (Scheme 9).22 These were isolated as stable crystalline solids. The X-ray crystal structure of radialene 71e showed that the three radialene alkenes were all in the same plane. 137 Aromatic chemistry N CO2 N + – Br Br i ii O 63 62 61 64 65 D N O H 67 66 Scheme 8 Reagents i BunLi; ii furan R R 69a R = H 69b R = Br 69c R = I 69d R = CO2Me 69e R = CN i ii iii R R R R R R Cl Cl Cl Cl 70 71a–e Scheme 9 Reagents i CH 3 SOCH 2 Li THF–DMSO; ii 70; iii O 2 0 °C 2 Fullerene fragments Fullerene fragments are generally di¶cult to form by pyrolysis of unfunctionalised polycyclic aromatic hydrocarbons.A study of the pyrolysis of halogenated benzo[c] phenanthrenes 72a–e was therefore carried out by the author.23 They were expected to 138 M. John Plater R4 R3 R2 R1 72a R1 = H R2 = H R3 = H R4 = Cl 72b R1 = Cl R2 = H R3 = H R4 = Cl 72c R1 = F R2 = H R3 = H R4 = Cl 72d R1 = Cl R2 = H R3 = H R4 = I 72e R1 = F R2 = H R3 = H R4 = F R3 R2 R1 73a (53%) 73b 73c 73b 73c (38%) (46%) (75%) (23%) 72e (32%) X –X 72 X = halogen –HX –H 74 75 73 FVP 950–1150 °C • Scheme 10 undergo easier ring coupling reactions by the generation of a reactive intermediate such as an aryradical 74 or benzyne 75 at the ring coupling site (Scheme 10).Each benzo[c]phenanthrene contains a halogen in the hindered fiord region. Flash vacuum pyrolysis gives the corresponding benzo[ghi]fluoroanthenes 73a–c far more easily than by pyrolysis of the parent compound benzo[c]phenanthrene. This model study has now been exploited for the synthesis of larger fullerene fragments. Pyrolysis of 6,12,18-tribromobenzo[c]naphtho[2,1-p]chrysene 76 gave the ring coupled half-bowl 77 by three consecutive ring coupling reactions.24 The ring coupling reactions could either proceed by the elimination of HBr to generate a benzyne as in the above model study or by fragmentation of a carbon–bromine bond followed by a 1,2-hydrogen shift to give an aryl radical at the ring coupling position.Pyrolysis of the dibromo precursor 78 gave C 27-semibuckminsterfullerene 79 by two consecutive ring coupling reactions (Scheme 11).25 Pyrolysis of 7,10-bis(2-bromophenyl)benzo[k]fluoranthene 80 gave bucky bowl 81 by four ring coupling reactions.26 The crystal structure of the fullerene shaped hydrocarbon 82 has been obtained.27 The bowls have a depth of 3.107Å and radius of 4.068Å. The bowls are stacked inside each other and are all aligned in one direction with the non-symmetric space group R 3#. 139 Aromatic chemistry Br Br Br 76 77 1050 °C N2 mbar Br Br 1150 °C N2 0.5 mm 78 79 80 81 Br Br 1100 °C 82 Scheme 11 140 M. John Plater 3 Molecular boards The soluble polycyclic molecular boards 86 and 87 were prepared by the cycloaddition of bis-dienophile 83 with cyclopentadienones 84 and 85 respectively.28 Compound 87 is probably the largest fully characterised polycyclic aromatic hydrocarbon known to date.Compound 87 is green with j.!9 \611nm and compound 86 is red with j.!9 \582 nm. R R R R R R O R R O CO2R CO2R R R R R R R R R R R R R R R R R CO2R CO2R CO2R CO2R 83 84 85 86 87 Cycloaddition of the arylacetylenes 88 91 and 94 with tetraarylcyclopentadienones 97a or 97b gave the corresponding polyaromatic compounds 89 92 and 95 respectively (Scheme 12).29–31 Treatment of the unsubstituted precursors 89a 92a and 95a with Cu(CF 3 SO 3 ) 2 and AlCl 3 gave some of the cyclodehydrogenated polycyclics 90a 141 Aromatic chemistry 88 R R R R R R R R R R R R R R R R 91 R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R O R2 R1 R1 R2 R 89a R = H 89b R = Bu t 90a R = H 90b R = Bu t 92a R = H 92b R = Bu t 93a R = H 93b R = Bu t 95a R = H 95b R = Bu t 94 96a R = H 96b R = Bu t 97a R1 = R2 = H 97b R1 = H R2 = Bu t i ii ii i i ii Scheme 12 Reagents i 97a or b Ph 2 O 250 °C; ii CuCl 2 AlCl 3 CS 2 rt or FeCl 3 CH 2 Cl 2 rt 142 M.John Plater C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 C6H13 99 C6H13 (RO)2B C6H13 Br 98a R = H 98b R = –C(CH3)2C(CH3)2– i Scheme 13 Reagents i [PdMP(o-tol) 3N3 ] Na 2 CO 3 toluene * RO RO O O RO O O O O RO + OR OR O O + RO O OR O O i i 100a R = C12H25 100b R = (Pri)3Si 102a R = CH3 102b R = n-C12H25 103a,b 101a,b O Scheme 14 Reagents i toluene basic alumina * 143 Aromatic chemistry OH CHO CHO HO OR RO + H2N NH2 105 104 O O RO RO N M N O OR RO RO RO RO OR O N M N O O O OR OR N M N O RO OR OR OR OR RO M N N M N etc.etc. 106 Scheme 15 93a and 96a respectively whose presence was shown by laser-desorption time-of-flight mass spectrometry (LD-TOF-MS). Cyclodehydrogenation of the tert-butyl substituted precursors 89b 92b and 95b with FeCl 3 in CH 2 Cl 2 gave some of the desired polycyclic boards 90b 93b and 96b respectively along with some chlorinated products. Presumably loss of the solubilising tert-butyl groups might also occur under these conditions. The novel cyclodehydrogenated products were too insoluble to obtain 1H NMR spectra. The driving force for smooth cyclodehydrogenations is probably the production of a stable conjugated polycyclic aromatic hydrocarbon from a more energetic strained oligophenylene precursor.144 M. John Plater The soluble macrocyclic oligophenylene 99 was prepared by Suzuki cross-coupling of the bromo boronic acids 98a–b (Scheme 13).32 This and related compounds are of interest for their aromaticity host–guest chemistry aggregation behaviour and for further molecular construction. [5] and [6]Helicenebisquinones 101a–b and 103a–b were prepared by the reaction of enol ethers of 1,4-diacetylbenzene 100a–b or 2,7-diacetylnaphthalene 102a–b with p-benzoquinone (Scheme 14).33 The yields are greatly improved over those obtained from the use of diethenyl aromatics which have no alkoxy groups either on the double bond or have the alkoxy groups on the aromatic ring.The helicenes can be made in quantity (30 g scale) which is a great improvement over previous small scale photochemical syntheses. A conjugated helical ladder polymer 106 was prepared by condensation of helicene 104 with ortho-phenylenediamine 105 (Scheme 15).34 Conjugation is maintained by the novel metal salophen units that bind adjacent helicenes. Evidence for the polymeric structure was provided by the TOF mass spectrum of the nickel containing polymer which consisted of clusters of peaks whose first members (at m/z 1890 2900 3910 and 4917) are separated by 1010 Da which is the mass of the repeat unit C 58 H 66 NiO 10 of the polymer. 4 Polyyyne chemistry The chemistry of cyclic polyynes and of substructures of graphyne 107 and graphdiyne 108 are attracting considerable interest.35,36 Hexaethynyltribenzocyclynes 109a,b were prepared and converted to the circularly conjugated oligophenylenes 110a,b and 111a,b by treatment with [CpCo(CO) 2 ] in refluxing xylene.35 Complete ring closure to the as yet unknown circular phenylene 112 did not occur.Circulene 112 is of interest owing to its 4n electron count or antikekulene structure. The substructures 116a,b of graphdiyne 108 were prepared by the selective deprotection of acetylenes 113a,b palladium catalysed coupling with 1,2,4,5-tetraiodobenzene followed by ring closure under Eglington coupling conditions (Scheme 16).36 Treatment of precursors 113a,b under standard Eglington coupling conditions with potassium carbonate gave the precursors 117a,b by a one-pot selective desilyation–dimerisation reaction which were then cyclised to the [32]annulenes 118a,b (Scheme 17).37 A series of bicyclo[2.2.2] octene fused dehydroannulenes 121–126 were prepared from acetylene precursors 119 and 120 by copper or palladium catalysed coupling reactions.38 The X-ray crystal structure of annulene 122 showed that the system was planar while that of annulene 126 was tub-shaped like that of cyclooctatetraene.The 1H NMR signal for the bridgehead proton of the BCO unit in annulene 122 indicated diatropic ring current. [2.2.2]Metacyclophane-1,9,17-triyne128 was prepared by bromination of triene 127 followed by dehydrobromination with ButOK in diethyl ether (Scheme 18).39 Triyne 145 Aromatic chemistry R R R R R R R R R R R R R R R R 109a R = Pr 109b R = CH2C6H11 110a R = Pr 110b R = CH2C6H11 111a 111b 112 R R 146 M.John Plater R SiMe3 SiPri 3 113a R = Bu t 113b R = Dec n I I I I i 114 R R R R R R R R R R R R 115a R = Bu t 115b R = Dec n 116a R = Bu t 116 b R = Dec n ii iii Scheme 16 Reagents i KOH [Pd(PPh 3 ) 3 ] [Pd(PPh 3 ) 3 Cl 2 ] CuI Et 3 N; ii Bu 4 NF; iii Cu(OAc) 2 pyridine 147 Aromatic chemistry SiMe3 SiPri 3 113a R = Bu t 113b R = Dec n R SiPri 3 117a R = Bu t 117b R = Dec n R SiPri 3 R i R R R R ii iii 118a R = Bu t 118b R = Dec n Scheme 17 Reagents i Cu(OAc) 2 K 2 CO 3 pyridine; ii Bu 4 NF; iii Cu(OAc) 2 pyridine i ii iii 127 128 129 Scheme 18 Reagents i Br 2 HCCl 3 rt; ii ButOK Et 2 O *; iii cyclopentadiene 148 M. John Plater H Br 119 H 120 H 122 123 124 125 126 121 149 Aromatic chemistry But But H H But But 130 But But But But But But But But But But But But 131a n = 0 131b n = 1 131c n = 2 n i Scheme 19 Reagents i CuCl TMEDA acetone O 2 128 is a fairly stable colourless crystalline substance whose structure was solved by X-ray crystallography.The average sp bond angle was 158.6° similar to that of cyclooctyne (158.5°). Triyne 128 reacts at room temperature with cyclopentadiene to give the bis-adduct 129. Octaalkynyldibenzooctadehydro[12]annulenes131a–c were prepared by Eglington coupling of hexaethynylbenzene 130 (Scheme 19).40 Compound 131a is a stable yellow crystalline solid. The higher homologues 131b,c were obtained as an inseparable mixture. 1,3,5/2,4,6 Di§erentiality functionalised hexaethynylbenzene derivatives 133 and 134 were prepared by palladium catalysed coupling of 4-bromonitrobenzene and 4- dimethylaminophenylacetylene with derivative 132 respectively (Scheme 20).41 These compounds could lead to new discotic liquid crystals and may exhibit second order non-inear optical properties.PhS SPh PhS PhS SPh PhS SPh SPh SPh PhS 135 PhS SPh PhS PhS SPh PhS SPh PhS SPh PhS SPh SPh SPh PhS 136 150 M. John Plater Et3Si SiEt3 SiEt3 Br NO2 NO2 NO2 NO2 i Et3Si SiEt3 SiEt3 Br Br Br NMe2 i H Et3Si SiEt3 SiEt3 NMe2 Me2N NMe2 134 133 132 Scheme 20 Reagents i (PPh 3 ) 2 PdCl 2 CuI Pr* 2 NH Diacetylene linked nanoscale poly(phenylsulfonyl)-substituted benzenes 135 and 136 are of interest as reducible molecular wires for the development of molecular and supramolecular electronic and photonic devices.42 They may serve as connectors between components and may possess novel non-linear optical properties owing to extended p-conjugation.The nanoscale molecular wires 140 were prepared by a novel iterative coupling strategy (Scheme 21).43 This involved dividing the precursor 137 into two portions. In one portion the triazene was converted to an aryl iodide 138 and in the other portion the silylacetylene was deprotected to give 139. The two portions were then mixed and coupled. The selective deprotection and coupling was repeated to give longer chains. 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