Aromatic chemistry 4 M. John Plater Department of Chemistry University of Aberdeen MestonWalk Aberdeen UK AB24 3UE 4 3 1 Theoretical and structural studies The existence of homoaromaticity in triquinacene 1 .rst suggested by Woodward in 1964 has been disproved. The enthalpy of formation was newly determined to be 57.51 kcalmol by measuring its energy of combustion in a microcalorimeter. This is identical to the value calculated at the RMP2/6-31G* level of theory and with AM1 semiempirical calculations. The previously reported value of 53.5 kcal mol was derived from the experimentalenthal py of hydrogenation of 1 to 4 of 78.0 kcalmol and the calculated enthalpy of formation of 4 of24.47 kcalmol . The lower than expected enthalpy of hydrogenation of 1 to 2 determined by Liebman is presumably an experimental error.2 1 The aromaticity and antiaromaticity of annelated .ve-membered ring systems pentalene 5 acepentalene 6 dicyclopenta[cd,gh]pentalene 7 and related compounds have been evaluated computationally using density functional theory (B3LYP/6- 31G*). The nucleus-independent chemical shifts (NICS) and magnetic susceptibility exaltations indicate 5 and 6 which have 8 and 10 electrons respectively to be antiaromatic as expected. In contrast 7 with 12 electrons is calculated to be aromatic. The dianions of all three have delocalised structures and are aromatic. 6 5 7 137 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 12 11 10 9 8 Homoaromaticity in some carbocations is well established such as for the 7- norbornenylcation 8 and the 8-endo-tricyclo[3.2.1.0]octylsystem 9.However until now no evidence for homoaromaticity in carbene intermediates has been found. An analysis of energies of stabilisation the molecular geometry and the singlet—triplet energy gap (E ) provides conclusive evidence that carbenes 10 11 and 12 are homoaromatic in character. The enhanced stabilities of singlet state carbenes 10 11 and 12 were evaluated at the B3LYP/6-31G*/B3LYP/6-31G* level correcting for zero-point energy di.erences. The stabilisation energies determined were 3.27 15.56 and 14.06 kcalmol respectively which compares with 20.91 kcal mol calculated for the 7-norbornenylcation 8. For carbene 11 the carbene carbon atom leans towards the double bond indicating the presence of conjugation.The singlet—triplet energy gaps for carbenes 10 11 and 12 were calculated as 8.33 27.82 and 25.74 kcal mol respectively. Hence for 10 only a modest stabilisation results but for 11 and 12 substantial stabilisation of the singlet carbene provides a large energy gap. Ha Cß Hb Hd Ca C1' C1 Hc 13a 13b The .rst synthesis of the molecular pinwheel [3 ](1,2,3,4,5,6)cyclophane 13 was reported by Sakamoto in 1996. This represented an important milestone in cyclophane chemistry. The trimethylene bridges invert rapidly at room temperature and a 10.9 kcalmol barrier for the degenerate interconversion 13a–13b was deduced from variable temperature NMR spectroscopy. It was proposed as a possible precursor to the unknown propella[3 ]prismane 14 which might be achieved by successive [22] 14 photochemical ring closures.Its molecular structure degenerate inversion and reaction to give 14 have now been studied by quantum chemicaltechniques. The geomet- 138 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 ries obtained for C symmetric 13 with various theoreticalmethods agree wellwith each other. In contrast to less symmetric cyclophanes which have bent or distorted benzene rings 13 has planar rings with equal C—C distances. Propella[3 ]prismane 14 which has six additional CC bonds is calculated to be 110.8 kcal mol higher in energy than 13. It is highly strained as its cyclohexane rings are severely distorted to achieve planarity.Calculations suggest that 13a interconverts with 13b by a sequential .ipping process with barriers at various levels which agree well with experiment. A synchronous mechanism involving a D symmetric structure is ruled out by its high energy of 43.5 kcalmol and seven imaginary vibrationalfrequencies. 15 17 16 18a 18b The X-ray crystallographic structures of .uorenylidenecycloproparenes 15 and 16 and of dibenzocycloheptatrienylidene 17 have been determined. Theoreticalstudies using ab initio methods have been used to study the structure charge distribution dipole moment and thermodynamic stability of these and related compounds. The X-ray crystalstructures of 15 and 16 reveala planar structure and a puckered structure is revealed for the seven-membered ring in dibenzocycloheptatrienylidene 17.The calculated dipole moments show that the cyclopropabenzene skeleton is positively charged as expected. However in benzotrifulvalene 18 the cyclopropabenzene ring is negatively charged owing to a stronger electron push from the non-benzo fused cyclopropene ring. 1H-Cyclopropa[b]naphthalene 19 was converted into novel polar ole.ns 21–25 by reaction of the disilyl derivative 20 with anthrone-like ketones (Scheme 1). The alkene products are crystalline polar compounds that range in colour from yellow to magenta. Calculated bond lengths and angles compared well with experimental values determined by X-ray crystallography. Calculated dipole moments for alkenes 21–23 and 25 have a positive pole on the cyclopropa[b]naphthalene ring.This is reversed for structure 24 which possesses the strongly electron-releasing amino group. A remarkably long bond length of 1.72Å was reported in 1994 for the C(sp )—C(sp ) bond in 1,1,2,2-tetraphenyl-3,8-dichloronaphthenocyclobutene 26. Variable steric and electronic factors have led to hexaarylethanes with bond lengths ranging from 1.54Å for 9,9-ditrypticyl 27 to 1.67Å for hexaphenylethane 28. A search of the Cambridge StructuralDatabase uncovered about 90 examples of C(sp )—C(sp ) benzocyclobutane bonds that are about 1.580.5Å in length. Owing to the exceptional world record for the above long bond length the crystallography was repeated collect- 139 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 SiMe3 (i) X SiMe3 19 20 21 X = CMe2 22 X = O 23 X = S 24 X = NMe 25 X = CO Scheme 1 Reagents (i) BuOK an anthrone.Cl Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph Cl Ph 26 29 28 27 ing data at 90 K. Re.nement of the data gave a CC bond length of 1.710(2)Å in excellent agreement with the previous value determined at room temperature. Computations on 29 in a C conformation similar to that found crystallographically for 26 predict a bond length of 1.708Å in excellent agreement with the experimental observation for 26. Further structural analyses helped the authors conclude that the long bond length can be explained by a classical steric argument without the need to consider orbitalinteractions such as through bond coupling.1,10-Diiodophenanthrene 32 was prepared from phenanthrene 30 by dilithiation with n-BuLi to give the lithiated intermediate 31 followed by quenching with iodine (Scheme 2). The X-ray crystalstructure showed the dihedralangl e I—C—C—I to be 63° and a distance of only 3.61Å between the two iodine centers. This amounts to only 84% of the sum of the van der Waals radii. Computational calculations gave estimates of about 30 kJ mol for the iodine—iodine repulsion and about 10 kJ mol for the reduced delocalisation in the extremely twisted skeleton of 1,10-diiodophenanthrene. 6b,10b-Dihydrobenzo[j]cyclobut[a]acenaphthylene33 is a precursor for the generation of pleiadene 34 a reactive hydrocarbon that spontaneously dimerises upon formation. Bis-pleiadene derivatives 36 have been prepared which polymerise upon heating. The presence of side chain substituents increases the solubility and .lm forming properties.The alkoxy groups dramatically enhance the .uorescence. 12,15-Dichloro[3.0]orthometacyclophane 43 a highly strained biphenylophane has been prepared by the route shown in Scheme 3. Benzosuberone 37 was converted to diene 38 in four steps by a Mannich reaction Wittig methylenation alkylation and Ho.mann degradation. Diene 38 was converted to cyclophane 43 by the four steps shown. The HNMRspectrum indicated the presence of a single endo conformer only. The molecular strain in 43 is evidenced by facile cycloaddition with tetracyanoethene (TCNE) to give adduct 44. The kinetically stabilised [1.1]paracyclophane 51 has been prepared by the route 140 Annu.Rep. Prog. Chem. Sect. B 1999 95 137—156 (ii) (i) Li Li I I 30 32 31 Scheme 2 Reagents (i) n-BuLi; (ii) I . 33 35 34 36 shown in Scheme 4. Irradiation of 50 with a low pressure mercury lamp at 20 ¡ÆC gave the desired paracyclophane 51 and some of isomer 52 resulting from a secondary transannular [44] addition within the molecule. The enhanced kinetic stability of compound 51 was demonstrated by heating a dilute solution in the dark in the absence of air. The intensity of the UV¡ªVIS absorptions remained unchanged for 2 h at 50 ¡ÆC and decreased by only 8%after the solution was heated at 100 ¡ÆC for 1.5 h. Irradiation of 51 in solution with light of wavelength greater than 420nm transforms 51 into 52.On standing in the dark at room temperature the characteristic UV¡ªVIS absorption of 51 slowly developed. This indicates the greater thermal stability of 51 compared with 52. 2 Me Me Si Si Me Me Li Si Me Si Me Me Me Si Me Me Si Me Me Si Si Si Si Me Me Me Me Me Me Me Me Me Me Si Me Si Me 54 53 Treatment of hexasilylbenzene 53 with excess lithium metal in dry deoxygenated THF gave dianion 54 which could be isolated as an amorphous black solid. 141 Annu. Rep. Prog. Chem. Sect. B 1999 95 137¡ª156 Cl O (ii) Cl (i) + Cl Cl 37 38 40 39 (iii) Cl Cl Cl Cl (v) Cl (iv) Cl Cl Cl 42 41 43 (vi) Cl NC NC Cl NC 44 CN Scheme 3 Reagents (i) 4 steps (see text); (ii) BuOK HCCl ; (iii) FVP 495 °C; (iv) HCCl NaOH PTC; (v) BuOK—DMSO; (vi) TCNE rt.Performing the reduction in the presence of quinuclidine followed by recrystallisation from toluene gave single crystals of 54 [Li(C H N)] 54a. The molecular structure was determined by X-ray crystallography. One lithium cation is located above and below the plane of the benzene ring. Each is coordinated by one quinuclidine. The benzene ring is virtually planar. The EPR spectrum of a powder sample of 54a at room temperature is characteristic of randomly orientated triplets having approximately three-fold symmetry. The temperature dependence of the signal intensity was measured. The signalintensity increases with increasing temperature between 100 and 300 K as expected for a thermally excited triplet state.The energy gap between the triplet and singlet ground state was estimated to be only 1.0 kcal mol determined by curve-.tting with the Bleany—Bowers equation. Ab initio calculations of C constrained to the planar ring geometry have shown that the triplet state is lower in energy than the singlet state. This discrepancy between theory and experiment is ascribed to silyl-substituent e.ects insu.cient levels of ab initio MO calculations or the ion-pair interactions between the lithium cations and the benzene dianion. H 4,7-Dihydroacepentalene 58 can be generated in situ by protonation of the readily accessible and stable dipotassium acepentalenediide 57. This was prepared from triquinacene 56 by treatment with n-BuLi and KOBu.Treatment of 57 with moist ether gave a single product which was shown to be the dimer 59 by X-ray crystallogra- 142 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 O O O R R (ii) (i) R R R R O 45 46 47 O O (iii) CONMe2 O CONMe2 R C PhSe R R R (iv) R (v) R R R R R R R CONMe2 SePh 49 CONMe2 48 50 C O (vi) CONMe2 CONMe2 R R R h¥í R R R R R CONMe2 51 52 CONMe2 Scheme 4 Reagents (i) h/RC¡ª¡ª ¡ª CR (R CH SiMe ); (ii) h/RC¡ª¡ª ¡ª CR; (iii) Diazo transfer then h; (iv) PhSeNMe ; (v) H O /; (vi) h. phy. The highly reactive monomer intermediate 58 could not be observed by NMR spectroscopy even at 80 ¡ÆC.The intermediacy of monomeric dihydroacepentalene 58 in the formation of the dimer 59 was proved by interception with two other dienes. Reaction with cyclopentadiene and anthracene gave the respective [42] cycloadducts 60 and 61 respectively. Reaction of diide 57 with chlorotrimethylsilane gave 4,7-bis(trimethylsilyl)-4,7-dihydroacepentalene 62 which was puried by distillation. The bulky trimethylsilyl groups sterically hinder the central double bond which prevents the molecule from dimerising. Organic acids and alcohols can be readily added to the centraldoubl e bond to give the addition products 63 and 64 respectively. Since 57 has a bowl shaped negatively charged carbon skeleton for which only one singlet can be observed in the LiNMRspectrum at 25 ¡ÆC it must undergo rapid bowl to bowlinversion.Ab initio calculations predict an inversion barrier of 5.4 kcal mol for the acepentalene dianion in solution which rises to 9.8 kcal mol with two lithium 143 Annu. Rep. Prog. Chem. Sect. B 1999 95 137¡ª156 H 2 H H H H H 57 56 58 55 [4 + 2] 2 anthracene 59 60 57 61 Me3Si SiMe3 RCO2H 2 Me3Si RCO2 Me3SiCl 63 SiMe3 62 57 Me3Si ROH SiMe3 RO 64 counterions present. The NMR chemicalshifts of the ring protons of 6.09 and 6.16ppm suggest that the molecule is aromatic. Dibenzo[a,g]corannulene (C H ) 65 and dibenzo[a,g]cyclopenta[kl]corannulene (C H ) 66 undergo two-electron reductions with lithium metal to give stable solutions of purple dianions that were studied by H C and Li NMR spectroscopy.The dianion of dibenzo[a,g]corannulene 65 was found to be paratropic whereas the dianion of dibenzo[a,g]cyclopenta[kl]corannulene 66 was found to be diatropic. Further reduction with lithium metal gave the trianion radicals but not the tetraanions. However reduction with the more electropositive potassium metal gave both the dianions or on further reduction the tetraanions. The tetraanion of 65 was found to be diatropic and the tetraanion of 66 was found to be weakly paratropic. The terms paratropic and diatropic refer to the summation of the overall changes in C chemicalshifts of the neutral starting material compared to the reduced anions.For example for 65 50 ppm and for 66 178 ppm. Taking into account 144 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 66 65 the two-electron reduction process this gives 25 ppm and 89 ppm per unit of charge for 65 and 66 respectively. These values di.er markedly from those found for the corannulene dianion which has a total carbon chemical shift of 336ppm and a value of 168ppm per unit of charge. This was rationalised as a consequence of the paramagnetic or shielding e.ect of the ring current which is inversely related to the size of the HOMO—LUMO gap. 3,6,9,12,15,18-Hexaphenyldodecahydro[18]annulene 74 and related derivatives have been prepared by the route shown in Scheme 5. The molecular structure was determined by X-ray crystallography which showed that 74 has a planar -system with D symmetry.The linear C—C—C—C linkage has a unique short bond average of 1.217Å and the other two longer bonds average 1.390Å. 2 Aromatic ring syntheses Fischer carbene complexes have proved to be versatile reagents for selective carbon —carbon bond formation. The formal[3 21] cycloaddition of alkoxycarbene complexes with alkynes known as the Do� tz benzannulation is a striking example. The new alkenyl(amino)carbene chromium complex 77 bearing an electronwithdrawing group at the C-position reacts smoothly with di.erent alkynes to give the benzannulation products 79 (Scheme 6). Electron-poor alkynes also reacted e.ciently. The intramolecular reaction of alkynes with a Fischer carbene complex has been exploited for the synthesis of cyclophanes (Scheme 7). Precursors 82 were prepared by aldol condensation of a Fischer carbene complex with appropriate alkynyl aldehydes catalysed SnCl .Thermolysis at 60 °C for 14—18 h or at 100 °C for 0.2—4h gave the corresponding cyclophanes 83 84 and 85 shown. Treatment of enediyne 86 with chromium methylcarbene complex 87 followed by toluene-p-sulfonic acid gave benzofuran 89 and an E—Z mixture of butenylbenzofurans 90 (Scheme 8). A mechanism was proposed involving coupling of the carbene complex to the less hindered alkyne to give an intermediate enyne-ketene 91. This then undergoes a Moore cyclisation to give an intermediate chromium complexed diradical 92. Hydrogen abstraction from the solvent would give a phenol-ether which could cyclise to benzofuran 89 upon acid treatment.Formation of alkene-benzofuran derivative 90 occurs by intramolecular hydrogen atom abstraction followed by acid catalysed cyclisation. —Zn dust gave the cycloaromatised products 96 98 and 100 (Scheme 9). The reaction of but-3-en-2-one 94 with terminally substituted diynes 95 97 and 99 and NiCl The addition of ZnCl and Et N increased the reaction yields. 145 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 OMe Ar SiMe3 H Ar (i),(ii) (iii) COAr Me3Si THPO MeO H 68 67 OMe CHO (iv) 69 Ar MeO 70 Ar CHO OMe Ar OMe Ar O OH HO O (v) Ar Ar Ar MeO Ar MeO OMe OMe O OTHP 72 71 (vi) Ar OMe Ar HO OH Ar Ar Ph Ph (vii) Ar Ar Ar MeO Ar OMe Ph OH Ar 74 Scheme 5 Reagents (i) ethynylMgBr; (ii) n-BuLi then MeI; (iii) EtMgBr; HCO Me; then 70; (v) PPTS then 73 dihydropyran—PPTS; TBAF; (iv) EtMgBr then CeCl Dess—Martin reagent; (vi) PhMgBr; (vii) SnCl .A .exible route to substituted triphenylenes has been developed (Scheme 10). o-Terphenylcarboxylic acid derivatives 104 were prepared via a 6 electrocyclisation of a diene-ketene generated from acid 103 by treatment with ethylchl oroformate and 146 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 N (CO)5Cr CH3 75 Scheme 7 Reagents (i) SnCl Scheme 6 Reagents (i) n-BuLi; EtO CCHO; MsCl—Et N; (ii) DBU; (iii) various acetylenes/; (iv) SiO H (CH2)n 80a-e n a 2 b 6 c 8 d 10 e 13 OMe OH (CH2)n 83 a-e + (CO)5Cr=C(OMe)CH2 H 81 OMe OH (CH2)n HO OMe 84 a-e ; (ii) in di.erent solvents.MeO Cr(CO)5 (i) (CH2)n 82 (ii) OMe (CH2)n MeO HO OH OH (CH2)n (CH2)n OMe 85 a-e N (i) (CO)5Cr OMs CO2Et 76 N R2 R1 OH 79a R1 = Ph R2 = H 79b R1 = Bu R2 = H 79c R1 = CO2Et R2 = H 79d R1 = Ph R2 = Ph . O (CH2)n Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 N (ii) (CO)5Cr CO2Et 77 (iii) N Cr(CO)3 R2 (iv) CO CO R1 2Et 2Et OH 78 147 Bu HO Bu Me Me Bu O O (i) (ii) + MeO Me 86 88 89 90 Bu Cr(CO)3 O Me (iii) Me O C 89 86 + 87 MeO MeO (CO)3Cr 92 91 (iv) Cr(CO)3 Me HO (v) MeO 90 H 93 Scheme 8 Reagents (i) Me(MeO)C——Cr(CO) 87; (ii) TsOH; (iii) solvent H atom abstraction/TsOH; (iv) H atom abstraction; (v) TsOH.Et N. The hydroxy group can be removed reductively to give o-terphenylderivatives 105 which are precursors to substituted triphenylenes. An e.cient regiocontrolled synthesis of substituted 1,4-dimethoxynaphthalenes as precursors for 1,4-naphthoquinone systems has been developed (Scheme 11). Polynuclear quinones attract long standing interest owing to their occurrence in many biologically active natural products. The route involves the addition of 2,5- dimethoxybenzylmagnesium chloride 107 to acyclic and cyclic -oxoketene dithioacetals 108 followed by cycloaromatisation to naphthalenes 109 with BF ·OEt .Dethiomethylated naphthalenes 110 were obtained by treatment with Raney Ni. 2-Unsubstituted hydroquinone monoacetates 113 were prepared by the addition of a lithiated alkene to a tert-butyl or trimethylsilyl substituted cyclobutenedione 111. The addition of unsaturated carbon nucleophiles proceeded regiospeci.cally at the carbonylgroup distant from the bulky tert-butyl or trimethylsilyl group. Thermolysis of the adducts followed by removal of the bulky group with ZnCl furnished the hydroquinone monoacetates 113. This methodology is useful for the synthesis of 148 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 O O (i) CO2Et Me CO2Et CO2Et CO2Et 96 94 95 O (i) 94 O + O 97 98 O (i) + 94 100 Scheme 9 Reagents (i) NiCl /Zn/ZnCl /Et N.R' R' R' (ii) (i) CO2H CHO CO2Me R R R 102 103 101 HO2C (iii) HO CO2Me CO2Me (iv) R R R' R' 104 105 Scheme 10 Reagents (i) ClCO Et/Et N/NaBH then MnO ; (ii) dimethylsuccinate/ NaOMe; (iii) ClCO Et/Et N then NaOH; (iv) 2-chloro-1-phenyltetrazole/NiCl . 99 highly functionalised aromatic molecules. Arene containing vinylic iodides and tri.ates 114 undergo regioselective carboannulation with alkynes 115 catalysed by Pd(0). This gives the corresponding polycyclic 149 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 OMe R1 O + R2 SMe OMe 106 ClMg t t O Bu SMe Runsatd R1 R1 O R1 = Me or iPrO 111 107 Scheme 11 Reagents (i) BF Bu O Runsatd OAc R1 R1 OH t R1 Bu R2 114a R1 = I 114b R1 = OTf fused naphthalenes 116. The generality of this methodology makes it an attractive synthetic route to a range of polycyclic aromatic hydrocarbons.Alkynes 117 are cyclotrimerised to substituted benzenes 119 and 120 in water¡ªmethanol mixtures by the cyclopentadienyl-COD cobalt catalyst 118. The electron withdrawing ketone group enhances the reactivity of the catalyst and the alcohol group enhances the water solubility. Protection of the alkyne functional groups is not required even for amines and carboxylic acids. Water-soluble transition metal catalysts are attractive because water is a cheap environmentally friendly solvent and hydrophobic eects can provide substantialrate enhancements chemoselectivity and stereoselectivity.116a R1 = Ph R2 = Ph 116b R1 = Ph R2 = tBu 116c R1 = Ph R2 = SiEt3 150 Annu. Rep. Prog. Chem. Sect. B 1999 95 137¡ª156 OMe OH (i) R1 R2 SMe OMe SMe 108 R1 R2 ; (ii) Raney Ni. R3 ZnCl2 R2 OAc 112 113a R1 = Me R2R3 = OCH2CH2 113b R1 = Me R2R3 = OCH2CH2CH2 113c R1 = iPrO R2R3 = OCH2CH2 113d R1 = iPrO R2R3 = OCH2CH2CH2 R1 R1 115a-e OMe R1 R2 SMe OMe 109 (ii) OMe OMe 110 OH H R3 R1 R2 OAc 113 R1 R1 R2 R2 R1 R2 2 + H R 118/MeOH/H2O R2 R1 R2 R1 117 R1 R2 R1 = COCH2CH2CH2OH 120a-e 119a R2 = COCH3 119b R2 = CO2CH3 119c R2 = (CH2)2OH 119d R2 = CH2NHCH3 Co 118 Cp Co CpCo(C COCH2CH2CH2OH 2H4)2 122 121 123 SiMe3 CpCo(CO)2/Ph3P/h¥í SiMe3 SiMe3 SiMe3 124 125 Treatment of triynes 121 with CpCo(C H ) gave high yields of the metallacyclopentadienes 122 bearing a -bound alkyne ligand. Thermaldecomposition gave the expected angular phenylenes 123.Intermediate complexes 122 are the missing link in transition metal-catalysed alkyne cyclotrimerisations and have not been isolated 151 Annu. Rep. Prog. Chem. Sect. B 1999 95 137¡ª156 C14H29O I C14H29O OC14H29 127 OC14H29 I Pd(0)(Ph3P)4/CuI 126 128 R R Mo(CO)6/ArOH CH3 CH3 CH3 n R R 129 130 R = alkyl branched alkyl alkoxy previously.The isolation of the novel metallated dibenzodehydro[10]annulene 122 is probably facilitated by the molecular strain of the .nal angular phenylene. Helicene 125 was prepared by an intramolecular [222] cycloisomerism of triyne 124 catalysed by CpCo(CO) and Ph P/h. Further examples illustrated the generality of the method. Helicenes are comparatively di.cult to prepare traditionally requiring high dilution photochemical methods. The route developed here o.ers numerous advantages to di.erent structural types with helical chirality. 3 Functional polycyclics The pentiptycene-derived polymer 128 was produced by the palladium-catalysed cross-coupling of disubstituted diiodobenzene 127 with pentiptycene diacetylene 126. The steric bulk of the polymer backbone prevents close packing of the chains increasing the porosity.Nitrated aromatics such as 2,4,6-trinitrotoluene (TNT) are absorbed into thin .lms within seconds and cause .uorescence quenching. Direct methods of TNT detection are needed owing to the existence of about 120 million unexploded land mines worldwide. Conjugated poly(p-phenyleneethynylene)s 130 were formed from 1,4-dipropynylbenzenes 129 by alkyne metathesis with but-2-yne and Mo(CO) —4-(tri- .uoromethyl)phenol. The degree of polymerisation was determined by NMR spectroscopy from the integration of the propyne end group singlet at 2.04. The catalyst system is less sensitive to air and moisture compared to Schrock’s (BuO) WCBu.This method known as acyclic diyne metathesis (ADIMET) has some advantages over more conventional palladium couplings such as the absence of butadiyne defect structures. Poly(p-phenyleneethynylene)s are of interest for photonic devices such as 152 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 OC4H9 OC4H9 CH3 CH3 CH3 CH3 CH3 CH3 H9C4O H9C4O 131 Hg(TFA)2/I2 OC4H9 OC4H9 I I CH3 CH3 CH3 CH3 CH3 CH3 I I H9C4O H9C4O 132 LEDs and polymer based lasers. The ethynyl-substituted quinquephenyl 131 was prepared by an iterative coupling strategy. Electrophile induced cyclisation with iodine gave the extended fused-ring polycyclic 132 containing nine annelated aromatic rings. Conjugated molecular and polymeric materials are currently being exploited in advanced technologies involving molecule based sensors non-linear optical electroluminescence and photovoltaic devices.The shape persistent macrocyclic amphiphiles 133 134 and 135 were prepared by a combination of palladium-catalysed Hagihara and copper-catalysed Eglinton—Glaser couplings. The isolation and puri.cation of these macrocycles was straightforward owing to their restricted solubility. The compounds are all unsaturated and are all yellow in colour. The molecular .exibility allows rotation of the aromatic rings so that the nanometre-sized cavity can be either more or less polar depending upon the solvent and binding to guest species. In this respect they resemble the polycyclic crown 153 Annu.Rep. Prog. Chem. Sect. B 1999 95 137—156 OPr OPr OH OH OH OH OPr OPr 133 OPr OPr OH OH OH OH OPr OPr ether antibiotics such as monensin which can change conformation depending upon the polarity of the medium to facilitate the uptake or release of metal cations. The polar phenolic groups are also available for further functionalisation. The triptycene-substituted [3]- and [4]helicenes 137 and 138 were prepared as possible molecular versions of mechanical ratchets 136 where the triptycene serves as the ratchet wheel a and the helicenes as pawl b and spring c. Proton NMR was used to study the rotation around the triptycene—helicene single bond. At 20 °C rotation is frozen but the NMR of 137 revealed a plane of symmetry indicating that it cannot function as a unidirectional ratchet.In contrast NMR revealed that triptycyl[4] helicene 138 lacks the symmetry of 137 and has an energy barrier to rotation of 24.5 kcalmol . However spin polarisation transfer NMR experiments indicated that 138 rotates equally in both directions. Thermodynamics o.ers an explanation on the 135 154 Annu. Rep. Prog. Chem. Sect. B 1999 95 137—156 PrO OPr HO OH OH HO PrO OPr 134 OPr OPr OH OH OH OH OPr OPr H C C (CH2)n H (i) EtO HOH2C CH2 OH 2C CO2Et EtO2C CO2Et H O (CH2)n H (CH2)n 141 139 140 (ii) (iii) ) ( CH2Cl ClCH2 H (CH2)n (CH2)n H 143 (n = 6,8,10) 142 ; (ii) SOCl ; (iii) BuOK. Scheme 12 Reagents (i) LiAlH grounds that it is only the height of the summit and not the steepness that matters.The principle of microscopic reversibility applies. A new family of soluble poly(p-phenylene vinylenes) 143 have been prepared by the route shown in Scheme 12. PPV polymers are of interest for light emitting diodes. The synthetic route involved cycloaddition of a long chain acetylene with cyclopentadienone 139 followed by reduction of the ester groups and conversion to chlorines with thionyl chloride. 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