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Mesoionic Compounds an Unconventional Class of Aromatic Heterocycles

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Mesoionic Compounds an Unconventional Class of Aromatic Heterocycles GENERAL I ARTICLE Mesoionic Compounds An Unconventional Class of Aromatic Heterocycles Bharati V Badami Mesoionic compounds are distinct types of heterocycles which belong to the class of non-benzenoid aromatics. Sydnone, the representative mesoionic compound has been extensively studied because of its unusual structure, chemi­ cal properties and synthetic utility. Sydnone is used as a versatile synthon in heterocyclic synthesis. This article gives a brief account of the comparative studies of the Bharati V Badami was a Professor of structural features of mesoionic compounds and other Organic Chemistry heteroaromatic systems. Some synthetically useful tandem Kamatak University, reactions of sydnones are also discussed. Dharwad. Her research interests are synthesis, Aromatic Compounds reactions and synthetic utility of sydnones. She is Benzene has played an important role in the development of the currently working on ideas concerning 'aromaticity'. Though benzene and its deriva­ electrochemical and insecticidal/antifungal tives were considered to be the best examples of aromatic com­ activities for some of pounds, a number of heterocyclic systems are known to exhibit these compounds. properties sufficiently similar to the benzenoid compounds, so that they are also classified as aromatic. There are, in addition, a large number of aromatic compounds structurally unrelated to benzene, and are classified as 'non-benzenoid aromatic com­ pounds'. The criteria to identify the aromatic character of a planar, fully conjugated cyclic system are as follows. • All the atoms comprising the ring must be Sp2 hybridised with a p-orbital for sideways overlapping . • The ring must have (4n+2)n electrons (Huckel's rule) de­ localised over all the atoms of the ring. Keywords Aromaticity, mesoionic hetero­ The stability caused by this cyclic delocalisation of (4n + 2)n cycles, sydnones, tandem re­ electrons, makes the ring susceptible to substitution rather than actions. addition by electrophiles. However, this chemical reactivity is -40----------------------------~-~~----------R-E-S-O-N-A-N-C-E-I--O-ct-o-be-r--2-0--o6 GENERAL I ARTICLE not a deciding property in several cases. Instead, proton N MR Box 1. property has been used to determine whether a compound is aromatic or not. When an external mag­ netic field is applied to an Mesoionic compounds are structurally very different from ben­ aromatic compound, the zenoid compounds, but they fulfill most of the criteria of aroma­ circulation of (4n+2)1t ticity and form a part of a variety of aromatic compounds, which electrons in a particular can be classified as follows. direction produces a tiny magnetic field opposing A) Benzenoid Compounds the applied field in the middle ofthe ring but aug­ i) Heterocyclic analogs of benzene in which one or more ring menting the applied field carbons are replaced by heteroatoms (Figure la). at the periphery where the ring hydrogens are lo­ ii) When the heteroatom is oxygen or sulfur, the ring shows cated. The latter is known aromatic properties only in its ionic form (Figure 1b). as 'deshielding effect' and the peripheral hydrogens iii) For ring with an exocyclic atom, aromaticity is explained by require lower applied field the dipolar forms (Figure Ic). to resonate compared to the vinylic hydrogens. B) Nonbenzenoid Compounds: Rings other than six-membered. This has allowed chem­ ists to recognise many i. Ionic hydrocarbons (Figure 2) structurally variant mol­ ecules as aromatic. (a) N N I ~ I I) () etc. o ON OCN N N (b) ~x- ~x etc. +.,9 +.,9 oo Oo oS OS pyran pyrilium cation thiopyran thiopyrilium cation (c) ~) f ~ Q (~-() Figures 1a-c. Benzene O 0 I c?~ ~"o- o and benzene-like aro­ pyran-2-one pyran-4-one matic compounds. -R-ES-O-N--A-N-C-E-I-O--ct-o-be-r--2-0-0-6------------~~------------------------------4-1 GENERAL I ARTICLE Ring size (4n + 2)7l' electrons Compounds Name [>+~~ Three 2 Cyclopropenium cation Q-® Five 6 Cyclopentadienyl anion. Seven 6 Cycloheptarienyl cation. o-@ (Tropylium cation) Eight 10 O~® Cyclooctatetraenyl dianion. Figure 2. Ionic compounds ii. Even a large ring compound like [18]annulene, (Figure 3) with (4n+2)1C electrons. which follows Ruckel's rule, is regarded as aromatic due to the presence of ring current inferred from its lR-NMR spectrum. C) Heterocycles: Aromatic heterocycles comprise 2/3 of organic compounds and the majority of them are 5-membered. D) Mesoionic Compounds: A number of 5-membered ionic heterocycles, with unusual structural features, have been recognised as members of a vast family of nonbenzenoid hetero­ cycles, known as the 'Mesoionic Compounds'. Mesoionic com­ Figure 3. [18Jannulene. pounds are defined as 'five membered heterocycles which cannot be satisfactorily represented by anyone covalent or dipolar structure, but only as hybrids ofpolar structures and they possess a sextet ofelectrons'. 000 ON o ttI S tyj ' h pyrazole oxazole pyrrole thlOp ene etc, Figure 4. 5-membered het­ oxazole erocycles. imidazole thiazole oxadiazole -42--------------------------------~~-----------R-ES-O-N-A--N-C-E-I-O-c-t-ob-e-r--2-0-0-6 GENERAL I ARTICLE H R-~ -H ~~L-H R-;;J r H R-N-----i:- II~' ~ ~II \ ~ -I ~ -I \ ~ 1/ "-\\ r- "N 0=0 ··N 0=0 ··N ",/0=0 "N~ .•/C-O ~O/ ~O/ "0/ o .. .. .. I'~ Ib Ie Id This definition, which makes them a distinct class of hetero­ Figure 5. Resonance struc­ cycles, is illustrated with the sydnone ring - a representative tures of sydnone. meso ionic compound. As can be seen in the above resonance structures (Figure 5), it is not possible to write a covalent struc­ ture for sydnones without separating the positive and negative A large ring charges. The term 'Mesoionic' (mesomeric+ ionic) was coined compound like by Baker et al in 1949 to indicate this peculiar ionic character of [18]-annulene, such heterocycles. which follows The aromaticity of the ring is explained by the classical sextet Huckel's rule, is theory. A total of seven 2pz electrons are contributed by the five regarded as atoms of the ring with one 2pz electron on the exocyclic atom. A aromatic due to sextet of electrons will be obtained when one of the seven 2pz the presence of electrons is paired with the single electron on the exocyclic atom. ring current in its The ring will be positively charged, balanced by the negative 1 H-NMR spectrum. charge on the exocyclic atom. The circle indicates the delocalisation of 6 electrons which is detected as ring current by 1 H -N MR spectroscopy. This polarization of charges is evidenced by large dipole moments (4- 6 D) for the mesoionic rings. Figure 6. Overlap of p­ orbitals in sydnone ring. -R-ES-O-N-A-N-C-E--I-O-c-to-b-e-r--20-0-6------------~-------------------------------43- GENERAL I ARTICLE -w_c- I ®'c-b I® y_ -r®c--,+\ - (X&r= c ...... / ~ N-G-{) -c, /c--o ~ ---0 7 at ° mesoionic - pyrazole imidazole thiazole oxazole -N-C-- -N-C--O -N-C-- -,NcB\ _ '(±) , '0\ N, /(r N,'0' /0 N, /N N" /G--{) S F F ~ 0_ Q tetrazole thiadiazole oxadiazole triazole (isosydnone) Figure 7. Some mesoionic Mesoionic heterocycles contain two or more heteroatoms with compounds. an exocyclic heteroatom (oxygen, nitrogen, sulfur). All the 52 mesoionic heterocycles known so far are ofsynthetic origin, though they contain naturally occurring heterocyclic rings viz: pyrazole, imidazole, thiazole, oxazole, thiadiazole, oxadiazole, triazole, tetrazole etc. Sydnones Of all the mesoionic compounds known, Sydnone ring is the most extensively studied because of ease of its synthesis from primary amines and also it is the only meso ionic ring which undergoes a wide variety of chemical reactions of synthetic utility. This lactone, which is a 1,2,3-oxadiazole-5-one heterocycle was named as Sydnone in honour of University of Sydney (Sydney + lactone) where it was prepared for the first time by Earl and Mackney in 1935. Substitution Reaction of Sydnone The aromatic character and the dipolar nature of sydnone are reflected in its chemical properties. Sydnone unsubstituted at R-N-C--H R-NH2 --. R-NHCH COOH --. R-N(NO)CH2COOH --. Scheme 1. Synthetic route 2 J(±) b-o '0/ to sydnone. -44--------------------------------~~-----------R-ES-O-N--A-N-C-E-I-O--ct-o-be-r--2-0-o-6 GENERAL I ARTICLE 4-position undergoes substitution with a wide variety of electro­ Scheme 2. Substitution re­ philes, with retention of the ring, typical of aromatic substrates. action of sydnone. Tandem Reactions of Sydnone Though aromatic, sydnone ring is readily cleaved by hydrochlo­ Though aromatic, ric acid, and as dipolarophile undergoes 1,3-dipolar cycloaddi­ sydnone ring is tion reaction with unsaturated systems. This supports the readily cleaved by azomethine imine structure (ld) and suggests that the degree of hydrochloric acid, it aromaticiy of sydnone ring is about the same order as that of undergoes 1,3- furan. These two reactions are highly useful in heterocyclic dipolar synthesis as they involve one-pot ring conversions to the 5- cycloaddition membered 1,2-diazaheterocycles. Sydnone ring transformation reaction with unsaturated systems. R-N--v-H R-N::O ~/ C~' I L~ ~ \R' " + N% I R C /' H ~~' R-N-C--H oy; 'k """ / \ a ~ 1,3-DC-C0 -N 0==0 2 "0/ -co,t H"'hift ~rl1'3-DC ~ CR' R-N~H R-N--v-H / \ R' N~I~ft. G., o-R' N"""I/~ T~ R' Rl-co' ~hloranil -H2~ R-N-D--H / ~ N~/~R' Scheme 3. 1,3-dipolar addi­ f tion-elimination reactions R' ofsydnone. -RE-S-O-N-A-N-C-E--I-O-c-to-b-e-r-2-0-0-6------------~-------------------------------45- GENERAL I ARTICLE R-N-C-H R-N-C-Br R-N-C-Br I'-f=\\ - ~ I'-f=\\ - - I \ N~C-O N~C-O N, /0=0 o 0 o Scheme 4.
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