Aromatic and Heteroaromatic Chemistry Volume 7

A Review of the Literature Abstracted between July 1977 and June 1978

By H. Suschitzky, O. Meth-Cohn

The Royal Society of Chemistry

Copyright © 1979 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-600-0

Contents

Chapter 1 Three-and Four-membered Ring Systems By R. C. Storr, 1,
Chapter 2 Five-membered Ring Systems By G. V. Boyd, 13,
Chapter 3 Six-membered Homocyclic Compounds By A. W. Somerville, 91,
Chapter 4 Six-membered Heterocyclic Systems By R. K. Smalley, 145,
Chapter 5 Seven-membered Ring Systems By G. R. Proctor, 218,
Chapter 6 Medium-sized Rings and Macrocycles By O. Meth-Cohn, 227,
Chapter 7 Electrophilic Substitution By D. J. Chadwick, 241,
Chapter 8 Nucleophilic Substitution Reactions By G. M. Brooke, 287,
Chapter 9 Aromatic Substitution by Free Radicals, Carbenes, and Nitrenes By R. S. Atkinson, 307,
Chapter 10 Porphyrins and Related Compounds By A. H. Jackson, 319,
Author Index, 345,

CHAPTER 1

Three-and Four-membered Ring Systems

BY R. C. STORR

1 Three-membered Carbocyclic Systems

Calculations of the potential surfaces of the planar cyclopropenyl radica and anion indicate that both are subject to Jahn–Teller distortions such that the lowest energy configuration of the radical has one short and two long bonds whereas that of the anion has two short and one long bond. For the carbene (1), INDO calculations reveal a large singlet–triplet separation, the singlet state being stabilised by 2π aromatic character of the ring.

Symmetry-allowed extrusion of the cyclopropenyl cation is not observed on loss of nitrogen from (2). Instead the ether (3) is formed in methanol, possibly via the pyramidally bridged cation (4).

Yoshida and his co-workers have continued to contribute greatly to the field of cyclopropenium ion chemistry. In addition to describing a large number of cyclopropenium ions, cyclopropen-ones, and -thiones in the patent literature they have produced the first triafulvalene dication (5). This was obtained by treatment of (6; X = H) with butyl-lithium and then allowing the resulting carbenoid (6; X =Li) to react with (6; X =Cl). The transformation illustrates the use of intermediates such as (6; X =Li) to effect formal electrophilic substitution in the cyclopropenium ion system. The related (6; X = MgBr) produced from the iodocyclopropenium salt (6; X =I) and PhMgBr behaves as a typical Grignard reagent with electrophiles. In contrast to the iodo-cation (6;X =I), the chloro-cation (6; X =Cl) gives the aryldiamino-cyclopropenium cation (6; X =Ph) with PhMgBr.

Hydride abstraction from (7) with trityl perchlorate does not give the dication (8), but rather the monocation (9) resulting from deprotonation of (8). 13C n.m.r. spectroscopy reveals that the central carbon possesses a considerable amount of electron density, indicating that the dipolar form (9a) makes a significant contribution. There is evidence that formation of dicyclopropene from cyclopropenyl cations under conditions of two-electron reduction proceeds through coupling of cyclopropenyl radicals produced by electron transfer from anion to cation.

The cyclopropenyl ether (10; R=Et), with FeCl3, in refluxing ethanol, gives triphenylcyclopropene and indenone (11). The former results from reduction of the triphenylcyclopropenyl cation by ethanol; the latter is thought to involve oxidation of the hydroxy-cyclopropene (10; R = H) to the radical (12) (or further to the cation), which cyclises.

The cation (13), prepared from 11-methyl-11-bromotricyclo[4.4.1.0]-undecane in SbF5-SO2ClF at -120°C, is neither a symmetrical cyclopropyl cation nor an allylic cation, but is best considered as a bent cyclopropyl cation showing significant 2π-homoaromatic nature. An excellent review of the concept and experimental evidence for homoaromaticity includes sections on mono-, bis-, and tris-homocyclopropenium cations.

Full details of the preparation of cyclopropenone have appeared in Organic Syntheses. Under basic conditions, diphenylcyclopropenone reacts with malononitrile and ethyl cyanoformate at the carbonyl group to give the butadienes (14). The key step in the oxidation of cyclopropenones (15) by peracid appears to be formation of the acetylene from attack by peracid at the carbonyl group, as shown, rather than formation of an oxabicyclobutanone. The acetylene then undergoes further oxidation. CNDO/2 calculations have been reported for a number of cyclopropenones and cyclopropenethiones. The first e.s.r. spectrum of a cyclopropenone radical anion has been observed for diphenyl-cyclopropenone. The anion is relatively unstable and undergoes loss of CO to give the radical anion of diphenylacetylene. No spectra were obtainable for dialkyl-cyclopropenones.

A novel activation of diphenylcyclopropenone involving ring cleavage of the C–C double bond has been observed with Pt3(Me3CNC)6.

The reaction of cyclopropenones (16) with compounds containing active methylene groups leads to methylenecyclopropenes (17), which on oxidation give a new family of alkylidenequinocyclopropanes (18). These are highly coloured, strongly dichroic, and powerful oxidising agents. Syntheses and properties of the quinocyclopropanes (19) and quinoiminocyclopropanes (20) have also been reported.

The o-tropoquinonecyclopropenide (21) is somewhat less stable than the previously prepared para-isomer. Spectral data indicate that it is nearly planar; however, the mono-and di-cations appear to be non-planar, owing to steric hindrance and electronic repulsion. In agreement with previous X-ray studies, 35Cl quadrupole resonance data for the cyclopentadiene cyclopropenide (22) indicate that the cyclopentadiene ring is largely dienoid. The 13C n.m.r. spectra of the highly stabilised thiocarbonyl ylides (23) suggest that they possess appreciable C=S character, as shown.

2 Three-membered Heterocyclic Systems

Non-empirical SCF MO calculations with full geometry optimisation suggest that oxiren (24) is less stable than the carbene (25), but of similar energy to formyl-carbene, which is ~80 kcal mol-1 less stable than keten. The role of oxiren in oxocarbene rearrangements is well established. Use of the labelled diazo-compounds (26) and (27) indicates that the equilibrium between the two oxocar-benes (28) and (29) via the oxiren must be largely displaced to the right, since (26) gives products involving a high degree of O migration on photolysis, whereas those from (27) involve practically no migration. Labelling studies with formyl-carbene that had been produced by photolysis of formyldiazomethane revealed that formation of oxiren is a minor process compared with direct Wolff rearrangement. Attempts to generate oxirens by retro-Diets–Alder reactions in such ideal precursors as (30) were largely unsuccessful, although some evidence for the formation of small amounts of keten was found from flash pyrolysis of (30) at 600–800°C.

Ab initio calculations for the C2H2S system indicate that thiiren (35) is the least stable species in the series (31)–(35). However, clear evidence for the presence of thiiren in the photolysis of matrix-isolated 1,2,3-thiadiazole (and, more surprisingly, isothiazole) has appeared. The distribution of the label in products derived from labelled thiadiazole established that a species with thiiren symmetry was involved, and further support came from direct spectroscopic observation. Seleniren was generated similarly. The formation of both 2,7-and 2,8-dicar-bomethoxythianthrenes in the pyrolysis of 6-carbomethoxybenzo-1,2,3-thiadiazole is consistent with the intervention of a benzothiiren intermediate. A thiiren intermediate has also been suggested in the thermolysis of the thiadiazole (36). Examples of stable thiirenium salts have appeared.

3 Four-membered Carbocyclic Systems

New calculations for the potential surfaces of the singlet and triplet states of cyclobutadiene are essentially in agreement with the generally accepted picture, namely that the square triplet is of higher energy than the rectangular singlet molecule. The rectangular singlet is more stable than the square singlet, which corresponds to a transition state between the two possible rectangular configurations. More surprisingly, the square singlet is estimated to be lower in energy than the square triplet, this apparent violation of Hund’s rule being explained by dynamic spin polarisation.

One of the great problems of recent years has been to reconcile the theoretical picture of parent cyclobutadiene with the observed i.r. spectrum of the matrix-isolated species, which suggested that the molecule is square. One explanation advanced was· that the species actually observed was a metastable triplet. MINDO/3 has been used to calculate the vibrational frequencies for singlet and triplet cyclobutadiene and their mono-deuteriated derivatives (the method was shown to be reliable for several simple systems) and the spectrum calculated for the triplet is consistent with the experimental spectrum. However, it now appears that this whole problem has been due to the failure to observe certain bands in the original i.r. spectrum. Generation of cyclobutadiene in a matrix from several precursors, under conditions where the spectrum can be more clearly observed, reveals new absorptions which mean that the symmetry of the species is less than D4h and that it is therefore very probably rectangular.

Tetra-t-butyltetrahedrane has been prepared by irradiation of tetra-t-butyl-cyclopentadienone as the first example of this long-sought system. It is remarkably stable (m.pt. 135°C) because lengthening of any of the four C-C bonds causes increased compression of the t-butyl groups, and it is only transformed into tetra-t-butylcyclobutadiene on heating to 130°C. Some evidence for the formation of tetralithiotetrahedrane has also been claimed. Several other attempts to generate tetrahedranes led mostly to cyclobutadienes. Interestingly, irradiation of the ozonide (37) gave the symmetrical tetra(trifluoromethyl)cyclo-butadiene, which had a half-life of several hours at 145 K. Significantly, its i.r. spectrum is consistent with a rectangular rather than a square geometry.

Cyclobutenylidene (38) rearranges to give vinylacetylene with no evidence for the formation of cyclobutadiene or methylenecyclopropane. This preferred pathway also emerges in MINDO/3 calculations for the system.

All five oxidation states in the reversible redox system involving the [4]radialene (39) and the cyclobutadiene (40) have been observed. As expected, introduction of anti-aromatic character is reflected in the potential for the final electron transfer. The high reactivity of cyclobutadiene towards sterically hindered dienophiles has been utilised in a synthesis of cis,trans-octa-1,5-dienes. Fragmentation to benzene and cyclo-butadiene is a minor process in the photolysis of syn-tricyclo[4.4.0.0]deca-3,7,9-triene.

Further evidence that butalene (41) is produced from the chloro-Dewarbenzene (42) with base comes from the use of a methyl-labelled analogue. The final distribution of the label reveals that about half of the reaction proceeds through a butalene.

A comprehensive review of cyclobutadiene-metal complexes has appeared. The first bi(cyclobutadiene)nickel complex of the sandwich type has been reported. σ-Bonded intermediates (43) have been isolated at low temperature from [PdCl2(PhCN)2] and acetylene, thus giving insight into the mechanism of formation of palladium-cyclobutadiene complexes. Further examples of formation of (7T-cyclobutadiene )cobalt complexes from [(π-C5H5)CoL2] and acetylenes and further examples of ‘conventional aromatic chemistry’ for cyclobutadiene that is co-ordinated to iron tricarbonyl have appeared. 1H n.m.r. spectra of (cyclobutadiene)iron tricarbonyls indicate that substituents affect the ring in much the same way as they do benzene. The iron tricarbonyl complex (44) behaves as a typical cyclopentadiene and can be deprotonated (pKa ≈ 17) to give the complex of anion (45). Evidence has appeared for a (cyclobutadiene)nickel complex in the NiBr2-catalysed polymerisation of hex-3-yne, and the structure of cyclobutadienedicobalt hexacarbonyl has been established by X-ray diffraction. Complex (46), incorporating the common type of η4-co-ordinated cyclobutadiene, can be readily converted into the less common type of η2-complex (47). The newly reported di-t-butoxyethyne can be transformed into squaric and deltic acids. Full details of a new synthesis of semisquaric acid, involving hydrolysis of the photodimers of chlorovinylene carbonate, have appeared. A number of papers have been concerned with systems related to squaric acid and cyclobutenedione. The synthesis of 1,3,4-triamino-2-oxacyclobutenylium cations (48) has been described.

4 Fused Four-membered Carbocyclic Systems

An X-ray structure determination for the stable benzocyclobutadiene (49) indicates, as expected, that the canonical form shown best describes the structure. Sophisticated calculations have been carried out for a wide range of benzo-fused cyclobutadiene derivatives. The agreement between calculated and experimental transition energies and moments for the few known compounds is impressive enough to suggest that predictions for the (as yet) unknown compounds are reliable. Because of the discrepancy between the calculated spectra and the data reported for the tentatively claimed 1,2-diphenylphenanthro[1]cyclobutene (50), the alternative structure (51) has been proposed.

Some 13C n.m.r. spectra for benzo- and dibenzo-cyclobutadiene dications indicate that these species are fully delocalised six- and ten-electron aromatic systems respectively. Flash-pyrolytic extrusion of nitrogen from benzocinnoline analogues has been employed in the generation of the fused cyclobutadiene derivatives (52), (53), and (54). 1,2-Dirnethylbenzocyclobutadiene gives the expected angular dimer (55), which rearranges to the semibullvalene (56) on heating at 250°C. A bathochromic shift in the electronic spectrum of (57) reflects the increased strain in this molecule compared with 2,3-dimethyl-biphenylene. Mixed’ biphenylenes have been prepared successfully from simultaneous flash pyrolysis of different benzyne precursors.

Full details of the formation of 3,4-cyclobuta[ 1,2]cyclohepten-6-one (58) derivatives from the dichlorocarbene ring-expansion reaction of 1- and 2-methoxybiphenylenes have appeared. Spectral data suggest that there is considerable bond fixation in the seven-membered ring, to minimise cyclobutadienoid character in the four-membered ring. Similar considerations apply to the fused tropolones (59) and (60), which exist in the tautomeric forms shown and undergo methylation only at the points indicated.

A new synthesis of 9,10-diphenylbicyclo[6.2.0]decapentaenes (61) has been reported. Preliminary spectral data suggest a nearly planar structure. The benzocyclobutene oxide (63) can be isolated from the reaction of (62) with 3O2 in solution. With further O2 this gives ozonide (64), which on heating gives (65), (66), and (67), so providing some evidence that the oxidation of benzocyclobutene proceeds through analogous intermediates. In the solid state, (62) gives (68) with oxygen, cyclisation of this o-quinonemethide being inhibited by the development of cyclobutadienoid character.

Cycloaddition of TCNE occurs at the benzene nucleus of (62; p-halogenophenyl for Ph) to give the propellane (69).

5 Four-membered Heterocyclic Systems

Photolysis (>270 nm) of oxazinone (70) at 7 K gives the β-lactone (71), which undergoes further fragmentation to give MeCN, Bu’CN, propyne, and t-butylacetylene. That such products can be isolated strongly suggests the intermediate formation of the azete (72), which undergoes spontaneous cycloreversion via the two modes indicated overleaf.

CHAPTER 2

Five-membered Ring Systems

BY G. V. BOYD

1 Introduction

This chapter deals with aromatic and heteroaromatic compounds containing five-membered rings. Monocyclic systems, their benzo-analogues, and compounds containing two or more five-membered rings, whether fused or linked, are reviewed, but those with an annelated six-membered heterocycle, such as purines and indolizines, are omitted. These will be found in Chapter 4. Syntheses, physical properties, and reactions other than substitutions, such as thermolysis, photolysis, and additions and cycloadditions, are discussed. As in previous volumes, the term ‘aromatic’ has been interpreted rather liberally, the criterion for inclusion being full conjugation rather than compliance with Hiickel’s rule. Accordingly, compounds such as fulvenes, pyrazolinones, oxazolinones, and meso-ionic compounds, and even the outright anti-aromatic cyclo-pentadienones and pentalenes, are dealt with.

(Continues…)Excerpted from Aromatic and Heteroaromatic Chemistry Volume 7 by H. Suschitzky, O. Meth-Cohn. Copyright © 1979 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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