Aromatic and Heteroaromatic Chemistry Volume 5

A Review of the Literature Abstracted between July 1975 and June 1976

By C. W. Bird, G. W. H. Cheeseman

The Royal Society of Chemistry

Copyright © 1977 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-793-9

Contents

Chapter 1 Ring Systems of Topical Interest By P. J. Garratt, 1,
Chapter 2 Intermolecular and lntramolecular Cyclization Reactions in Ring Synthesis By C. W. Bird and G. W. H. Cheeseman, 65,
Chapter 3 Cycloaddition Reactions ByG. V. Boyd, 123,
Chapter 4 Ring Transformations By H. C. van der Plas and J. W. Street, 163,
Chapter 5 Electrophilic Substitution on Carbon By J. H. Ridd, 260,
Chapter 6 Electrophilic Substitution on Heteroatoms By J. H. Lister, 299,
Chapter 7 Nucleophilic Substitution By G. M. Brooke, 314,
Chapter 8 Aromatic Substitution by Free Radicals, Carbenes, and Nitrenes By S. R. Challand, 339,
Chapter 9 Addition Reactions By G. V. Boyd, 359,
Chapter 10 Ring-cleavage Reactions By T. L. Gilchrist, 411,
Chapter 11 Reactions of Substituents By B. C. Uff, 424,
Chapter 12 Porphyrins and Related Compounds By K. M. Smith, 456,
Chapter 13 Naturally Occurring Oxygen-ring Compounds By R. D. H. Murray, 472,
Chapter 14 Other Naturally Occurring Compounds By J. R. Lewis, 502,
Author Index, 535,

CHAPTER 1

Ring Systems of Topical Interest

BY P. J. GARRATT

1 Introduction

There has been continued interest in methods by which the number of Kekulé structures are counted as a means of determining the aromaticity of a polycyclic system. A general discussion of the application of graph theory to conjugated molecules has been made, and a number of examples have been discussed. Wilcox has introduced a topological definition of resonance energy, which can be related to that of Hess and Schaad. The p.e. and electronic spectra of polycyclic systems have been correlated with the number of aromatic sextets. An extension of the Hückel rule to polycyclic systems has been proposed. The connection between aromaticity and the generalized Woodward–Hoffman rules for pericyclic reactions has been explored. Reviews have appeared on the use of the bis-Wittig reaction in the synthesis of non-benzenoid systems, on the role of cyclobutadieneiron tricarbonyl in the cyclobutadiene problem, on cyclobutadienoids, and on non-classical condensed thiophens. Pyramidal mono-and di-cations have been suggested as bridges between organic and organometallic chemistry’ and the argument against non-classical ions has been vigorously propounded.

2 Valence Isomers

The vibrational i.r. and Raman spectra of Dewar-benzene (1a) and benzvalene (2) have been reported. Dewar-benzene has a wing-flapping mode at 381 cm-1. The electronic spectra of (1a) and (2) have also been recorded, and assignments made to the bands. A gas-phase electron-diffraction study of (la) gave the structural parameters shown in Figure 1. A MINDO/3 calculation suggests that the rearrangement of benzvalene to benzene is allowed. The interaction between bicyclo-butane and an ethene fragment is calculated to be greater than that between cyclobutene and ethene, which may explain why benzvalene is converted into benzene more slowly than is Dewar-benzene. MINDO/3 calculations indicate that halogen bridge-substituted Dewar-benzenes should convert more readily into halogenobenzenes than Dewar-benzene into benzene, in accord with experimental findings (see Vol. 3). Photoirradiation of 5,6-dideuteriobenzvalene (3) in the presence of acetone gave o-dideuteriobenzene (4). The low quantum yield is due to a degenerate benzvalene photoisomerization (3)[??](5). The substituted pentafluoro-Dewar-benzenes (1b) rearranged to the corresponding substituted benzenes in a first-order unimolecular reaction, and the resulting benzenes are reconverted into (lb) on photoirradiation. The equilibrium between (6),and (7) has been studied over a temperature range, and (7) is favoured above 551 K. The optically active Dewar-benzene (10) was prepared by the reaction of the cyclobutenyl cation (8) with (9), followed by hydrolysis to remove the menthyl group. The c.d. spectrum of (10) suggests that there is no π,π overlap between the two cyclobutene rings. The stereo-and regio-specificity of the partial reduction of (11) has been investigated, using a range of Group VIII metals. The photochemical interconversion of o-, m-, and p-perfluoroxylenes has been investigated in detail, and a number of Dewar-benzenes, but no prismanes or benzvalenes, have been isolated. The addition of furan and 2,4,6-trimethylbenzonitrile oxide to polyfluoro-Dewar-benzenes has been reported.

The thermal conversion of 1,1′-dimethylbicyclopropenylidene (12) into xylene gives some of the latter in the triplet state. Intermediate ground-state Dewar-benzenes probably intervene. Perfluorohexamethylbicyclopropenyl (13), the last missing isomer of the C6(CF3)6 group, has been prepared as shown in Scheme 1. Although (13) is the kinetically least stable of the isomers, it is thermochemically the most stable, being converted into perfluorohexamethylbenzene with a half life of 2 h at 360°C. With Et3N in MeOH, (13) yields (14). Photoirradiation of the bicyclo-propenyl (15) gave the substituted benzenes (16) and (17). This rearrangement did not proceed via the prismane derivative but by scrambling of the bicyclopropenyl, as was shown by the conversion of (18a) into (19a) and (18b) into (19b). The 1,4-bridged Dewar-benzene (20) has been prepared via both the silver perchlorate and the Ramberg-Backlund routes [see Vol. 4, p. 4, Scheme 2. Structure (26) should be the sulphone]. Flow thermolysis of (20) at 282°C gave (21) and (22), the latter possibly via [5]paracyclophene. Treatment of (23) with AgClO4 gave (24) together with other products, but rearrangement of (25) did not give the two-carbon-bridged Dewar-benzene. A full paper on the formation and properties of (26) has appeared. Photoirradiation of the substituted pyridine (27) gave (28) and two azaprismanes (29) and (30). These azaprismanes are thermally quite stable, and are only slowly transformed into pyridines. A formalism for tracing the fate of ring atoms in phototranspositions, such as those described above, has been outlined.

The reaction of the dibromocarbene adduct of benzvalene (31) with methyl-lithium in the presence of styrene gave (33), possibly via the allene (32).

3 Polybenzenoid Systems

The out-of-plane deformations of some cyclic polybenzenoid systems have been calculated. The p.e. spectra of a number of polycyclic aromatic hydrocarbons have been reported. An investigation of the reaction of dichlorocarbene, produced by Makosza’s two-phase method, with naphthalene, anthracene, and phenanthrene has been made. The reaction of octamethylnaphthalene with dichlorocarbene has also been investigated. Photoirradiation of the triptycene (34) gave the 2,3-benzofluoranthrene (35).

The interest in arene oxides has continued. General synthetic routes for the synthesis of K-region oxides (Scheme 2) and for non-K-region oxides (Scheme 3) have been described. Treatment of phenanthrene oxide (36) with phenolate anions gave (37) and (38), and similar compounds could be obtained under acidic conditions. The reaction of the tetra-aldehyde (39) with P(NMe2)3 gave the di-oxide (40), and pyrene di-oxide could be made similarly. The aziridines (41) and (42) have been prepared.

4 Helicenes

The theoretical optical rotation of oriented hexahelicene has been calculated, and it has been found to have the same sign of rotation in the three axes. The cyclization of the menthyl esters (43a, b) gave, after reductive removal of the menthyl group, the optically active hexahelicenes (44a, b). The synthesis of a hexahelicene, using a solid support, has been reported. A stirred suspension of the resin was photoir-radiated in the presence of iodine, and (45) was recovered after hydrolysis from the resin. Irradiation of (46) with circularly polarized light gave the optically active hexahelicene. No asymmetric synthesis occurred for homologues above decahelicene. Coupling of (47) with (48) gave (49), which on thermolysis gave the phenanthracene (50), which can be converted into the dicyanohexahelicene (51) by photoirradiation in the presence of I2. The preparation of [11]-, [12]-, and [14]-helicenes has been reported. The synthesis of [11]helicene involved a Wittig reaction between the phenanthracene dicarboxaldehyde (52) and the phosphonium salt (53) to give (54), which was cyclized by photoirradiation. [12]-and [14]-helicenes were prepared in an analogous manner from (55) and (56) respectively.

Examples of intramolecular reactions in helicene involving the carbon skeleton have been described. Treatment of (57) with toluene-p-sulphonic acid in refluxing benzene gave (58), and the reaction of (59) with (60) gave (61), presumably via a Diels–Alder reaction of the initially formed allyl ester. Compound (61) was formed predominantly as the stereoisomer (61a).

Irradiation of the pentahelicene (62) in ethyl acetate solution gave (63), while irradiation in benzene in the presence of iodine gave (64), in which a phenyl substituent has been lost.

Hexa[7]circulene (65) has been prepared. A full paper on the preparation of heterocirculenes has appeared. Treatment of the heterohelicene (66) with aluminium trichloride at 140°C gave the dehydrohelicene (67). The reaction is restricted to hetero-[5]-and -[6]-helicenes, i.e. to the formation of six-or seven-membered rings. The reaction of (68) with N-bromosuccinimide at 160°C gave the methano-bridged non-planar heterohelicene (69).

5 Sterically Overcrowded Molecules

The p.e. spectra of biphenyls indicate a steric inhibition of resonance. A study of the effect of high pressure on the electronic spectra of bisanthrones has been reported. The bisanthrones (70a, b) have low barriers to interconversion with the stereoisomers (71a, b). The aryl rings of cis-diarylacenaphthalenes are locked, owing to steric interaction. The barrier to rotation in (72a) is between 23 and 26 kcal mol-1 and for (72b) it is greater than 26 kcal mol-1. Stable cis-and trans-rotamers of 1,8-di-o-tolynaphthalene (73a, b) have been isolated. They interconvert with a half life of one day in solution at room temperature, and at 40°C the equilibrium constant for (73b): (73a) is 3.21.

Photoirradiation of (74) gave (75), and a mechanism involving two consecutive concerted reactions has been proposed. Photoirradiation of (76) gave (77), which on thermolysis gave (78) by a 1,5-hydrogen shift. The cyclopentadienone dimer (79) shows only two methyl signals in the 1H n.m.r. spectrum, the methyl groups equilibrating by a [3,3] Cope rearrangement involving rupture of the 1,2-bond and formation of a 4,8-bond. In the unsubstituted system the Cope rearrangement proceeds only slowly at 80°C. The diol (80) rearranges under acidic conditions to (81).

6 Bridged Aromatic Compounds

A review on layered compounds has appeared. The electron and p.e. spectra of [2,2]metacyclo-2,6-pyridinophane indicate an interaction between the lone pair of nitrogen and the other ring. The chemiluminescence of 1,4-diketophthalazine cyclophane has been reported fully.

[2,2]Paracyclophanes. — Simple MO calculations on the quinone-hydroquinone (82) have been reported. The e.s.r. and 1H n.m.r. spectra of the nitroxide (83) indicate an interaction between the unpaired electron and the proton that is pseudo-geminal to the nitroxide group. Birch reduction of (84) gave (85). The preparation of thirty-nine disubstituted [2,2]paracyclophanes has been reported, all of which have one substituent in each ring. The π-electron spin distribution in the radical anion of [2,2]paracyclophane and related systems has been discussed. The reaction of (86) with diazomethane gave the cyclopropanated compounds (87) and (88). Treatment of the sulphide (89) with benzyne gave (90), which on oxidation with m-chloroperoxybenzoic acid and subsequent thermolysis of the sulphoxide gave the diene (92).

[2,2]Metacyclophanes. — Photoirradiation of the disulphide (93) in the presence of trimethyl phosphite gave [2,2]metacyclophane (94) in 34% yield. The preparation of sulphides from disulphides was also described. 4,12 and 4,14-disubstituted [2,2]metacyclophanes have been prepared. The conformational changes in [2,2]metacyclophanes of type (95) have been examined by 1H n.m.r. spectroscopy. The benzo[2,2]metacyclophanediene (96) has been prepared via the sulphide route, and it can be converted into the 14,15-dihydropyrene derivative on irradiation (see p. 35).

Other Bridged Ring Systems. — The 2-bromo-substituted metacyclophanes (99) have been prepared by ring expansion and aromatization of the corresponding bicyclo[n,3,0]-derivatives (97). Photoirradiation of (99; n=6) gave (100), and (99; n = 7) underwent a similar reaction. A number of heterocyclic analogues of (99) have been synthesized, e.g. (101) and (102). The related chiral systems (103) have also been prepared. The through-space effect of a nitrogen or sulphur pole on the rate of nitration of a benzene ring has been investigated by examining compounds of the type (104). A nickel-catalysed coupling of 2,6-dichloropyridine (105) with a bis-Grignard reagent (106) gave the metapyridinophanes (107). Using m-dichlorobenzene, cyclophanes could be prepared, but in lower yield.

Treatment of the metathiophane (108) with t-butyl chloride and tin(IV) chloride gave the rearranged di-t-butyl derivative (109). The differences in the 1H n.m.r. spectra of a series of paracyclophanes with acetylenes in the bridge have been accounted for in terms of the relative geometries of the rings and acetylene groups.

The reaction of the dithiol (110) with N-bromosuccinimide gave the disulphide (111). The octafluoro-derivative (112) has its electronic absorption maximum shifted 20 mm hypsochromically from the parent system. The reaction of (113) with sodium selenide in DMSO gave (114) in 5 — 10% yield. The [2,2]-(2,5)pyrrolophane (117) was prepared from the tetraketone (115) by reaction with benzylamine to give (116), which was reductively cleaved by sodium in liquid ammonia to (117). The compound (117), unlike the corresponding furanophane, did not undergo conformational interchange, as observed in the 1H n.m.r. spectrum. The dimethyl ether of alnusone (119), a bridged biphenyl obtained from the wood of Alnus japonica Steud., has been prepared by a nickel-promoted closure of (118). Compound (121), prepared by desulphurization of (120) with triethyl phosphite, gave coronene on treatment with AlCl3. The 1H n.m.r. spectrum of (122) suggests that it is either flipping fast, or else the ring systems are perpendicular. A full paper on [2,2](2,7)naphthalenophane-1, 11-diene has appeared. The wavelength dependence of the photochemical intramolecular dimerization of [2,2] (9,10)anthracenophane has been examined, and the direct excitation of the trans-annular excited state appears the most efficient. The [2,2](2,7)pyrenophane (123) and the corresponding diene (124) have been prepared by the sulphur route. Both are yellow compounds, and show red shifts, compared to pyrene, in the long-wavelength regions of the electronic spectrum. The [2,2](1,3)pyrenophane (125) has also been prepared.

The reaction of p-xylylene chloride with sodium and tetraphenylethylene in THF gave [2,2;2]paracyclophane (126) plus higher oligomers. The related systems (130) were prepared by treatment of the benzylic halide (127) (or thiol) with the thioalkoxide (128) (or halide) to give (129), which, on oxidation and pyrolysis, gave (130). This is an extension in scope of the sulphide route in that only one of the adjacent methylene groups was benzylic. The 1H n.m.r. spectra of (131) are temperature-dependent. [2,2,2,2](1,2,4,5)Cyclophane (132) has been prepared by a combination of the sulphide and p-quinodimethane routes. The 1H n.m.r. spectrum shows a four-proton singlet at τ 4.06, and a sixteen-proton A2B2 system at τ 6.5-7.40. The upfield shift of the aromatic protons is due to the distortion of the benzene rings, but the shift is less than that in [2,2,2]-(1,3,5)cyclophane.

The reaction of (133) with pyridinium bromide perbromide gave (134) and (135), which could be converted into the pyrene (136) and into (137), respectively. A cross-coupling between (138) and (139) gave (140), which presumably arises from an intramolecular Diels-Alder reaction of the initially formed ‘triple-decker’. The ‘triple-decker’ quinones (141) and (142) have been prepared, and show broad charge-transfer bands which are bathochromically shifted.’ The reaction of the benzylic bromide (143a) with the corresponding thiol(143b) gave (144)’ which under Steven’s conditions gave (145).

7 Heterocyclic Compounds

The controversy regarding the method of calculation of ring currents in five-membered heterocycles (Vol. 4, p. 13) appears to have been resolved. A modified ring-current model for pyrrole has been proposed. Calculations on the bonding in thiophen, its S-oxide, and its dioxide have been reported.

Boron and Silicon. — It has been suggested, from the X-ray crystallographic data, that the benzene ring in (146) has fixed bonds. The silacyclobutene (147) reacts with acetone upon photoirradiation to give (148), presumably via the butadiene.

(Continues…)Excerpted from Aromatic and Heteroaromatic Chemistry Volume 5 by C. W. Bird, G. W. H. Cheeseman. Copyright © 1977 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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