Subject Chemistry Paper No and Title 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module No and 32: ANNULENES Title Module Tag CHE_P14_M32 CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction of annulenes 3. Synthesis of annulenes 4. Reactions of annulenes 5. Summary CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES 1. Learning Outcomes After studying this module, you shall be able to understand: What are annulenes? How can you synthesize annulenes? What are the reactions possible of annulenes? 2. Introduction Annulenes are completely conjugated monocyclic hydrocarbons. They have a general formula CnHn. According to the IUPAC naming conventions, annulenes are named as [n]-annulene, where n is the number in the bracket indicates the ring size or the number of carbon atoms in their ring. For example, Cyclobutadiene is named as [4]-annulene, Benzene is named as [6]-annulene, Cyclooctatetraene is named as [8]-annulene, Cyclodecapentaene is named as [10]-annulene, Cyclododecahexaene is named as [12]-annulene, Cyclotetradecaheptaene named as [14]-annulene, Cyclohexadecaheptaene named as [16]-annulene, Cyclooctadecanonaene named as [18]-annulene and so on. The structures are given below: CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES Annulenes were first prepared by Sondheimer et al in 1962 to test the Huckel’s rule of aromaticity. According to Huckel’s rule of aromaticity ‘if the number of - electrons is equal to 4n+2 where n is equal to zero or a whole number, the system is aromatic’. The number of -electrons for Hückel's rule are 2 (n = 0), 6 (n = 1), 10 (n = 2), 14 (n = 3), 18 (n = 4) etc. Various annulenes prepared were have n= 10, 12, 14, 16, 18, 20, 24, 30. Out of these only [14], [18], [30] annulenes have (4n + 2) -electrons and are aromatic in nature. The [4]-annulene is cyclobutadiene having 4-electrons and is anti-aromatic in nature. It is highly unstable due to the ring strain. It can be isolated only under controlled conditions like in Argon matrix or using trapping agents like dienes. Studies show that it has a rectangular structure rather than a square, with C-C bond length of 1.567 Å and C=C bond length of 1.346 Å. The [8]-annulene is also known as cyclooctatetraene. It is a polyunsaturated hydrocarbon, which is a colorless to light yellow flammable liquid at room temperature. It is non-planar and adopts a tub-conformation as shown below: The [10]- and [14]-annulene are not particularly stable due to geometric factors. [10] Annulene is also known as cyclodecapentaene. Since it has conjugated 10- electrons but still it is not aromatic due to the combination of steric strain and angular strain. Many conformations are possible for [10] annulene: (1) Planar all-cis isomer: in this conformation all the bond angles are of 144° which creates large amounts of bond strain relative to the ideal 120° for sp2 hybridization, (2) Boat-like all-cis conformation: In this conformation angular strain is less in comparison to (1) but is still unstable, CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES (3) Planar trans, cis, trans, cis, cis isomer: This conformation suffers from steric repulsion between the two internal hydrogen atoms, (4) Non-planar trans, cis, cis, cis, cis isomer: This is the most stable of all the possible isomers. The [14] annulene has 14 electrons in its conjugated pi system and hence it possess some aromatic character. The [16]-annulene is non-planar, with alternating C=C and C-C like a non-aromatic polyene. The [18]-annulene is almost planar, with some aromatic stability. However, it reacts more like a polyene than benzene. It undergoes addition reaction with H2 and Br2, and a Diels-Alder with maleic anhydride. The higher annulenes (n > 18) are of less synthetic interest. This is mostly due to their significant bond alternation asymmetry, and conformational flexibility. All the higher annulenes up to n = 30 have been synthesized except n = 26 and 28. 3. Synthesis of annulenes 3.1 Synthesis of [4]-annulene Cyclobutadiene was first synthesized in 1965 by Rowland Pettit et al although they could not isolate it. In 1965, the first stable pale yellow crystalline solid cyclobutadieneiron tricarbonyl was prepared from Fe2(CO)9 and cis-dichlorocyclobutene via double dehydrohalogenation reaction. CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES Cyclobutadiene-iron tricarbonyl complex is known to decompose oxidatively as shown below: Liberated cyclobutadiene can be trapped with several unsaturated compounds as shown below: CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES If no trapping agent is used cyclobutadiene dimerizes at 35 K as shown below: 3.2 Synthesis of [8]-annulene (i) Richard Willstatter synthesis of [8]-annulene: [8]-Annulene or 1,3,5,7-Cyclooctatetraene was first synthesized by Richard Willstatter at Munich in 1905 by the following route: CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES (ii) Reppe's synthesis of [8]-Annulene: Acetylene or ethylene on reaction with a warm mixture of nickel cyanide (NiCN) and calcium carbide (CaC) under high pressure results in the formation of [8] annulene or cyclooctatetraene in ~ 90% yield. 3.3 Synthesis of [10]-annulenes Naphthalene on Birch reduction form tetrahydronaphthalene (or isotetralin). Tetrahydronaphthalene when treated with chloroform, potassium salt of tert-butanol using ether and ethanol as solvent at -30oC undergoes dichlorocarbene addition and form 11,11- dichlorotricyclo[4.4.1.0]undeca-3,8-diene. This on further Birch reduction form tricyclo[4.4.1.0]undeca-3,8-diene by the removal of the two chloride substituents. This on further oxidation with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) form 1,6- methano[10]annulene. 3.4 Synthesis of [18]-Annulene The synthesis of [18]-annulenes is achieved by following the given reaction sequence. CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES 4. Reaction of annulenes 4.1 Reactions of [4]-annulenes The cyclobutadiene reacts with ‘ene’ to give Diels-Alder products. The reaction of ethylene and cyclobutadiene form bicycle[2.2.0]hex-2-ene. CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES The reaction of cyclobutadiene and cyclopenta-1,3-diene another Diels-Alder product as shown below: In another reaction, cyclobutadiene reacts with allene to form 5- methylenebicyclo[2.2.0]hex-2-ene. The cyclobutadieneiron tricarbonyl undergo a number of different electrophilic substitution reactions, similar to ferrocene or benzene as shown by Petitt and co-workers. Although these reactions are not particularly interesting in terms of mechanism or theory, they do highlight the aromatic character of the cyclobutadieneiron tricarbonyl and serve as useful methods for derivatizing the parent structure. CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES 4.2 Reactions of [6]-annulenes Benzene is [6]-annulenes. This is a well-known aromatic compound. This is highly unsaturated but it does not undergo any of the regular reactions of alkenes such as addition or oxidation as given below: CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES Benzene undergoes electrophilic substitution reaction, a typical properties of aromatic compounds. It reacts with bromine if a Lewis acid catalyst is present however the reaction is a substitution and not an addition like alkene. Some of the important reactions of benzene are given below. CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES 4.3 Reactions of [10]-annulenes The [10]-annulene existed in two distinct but rapidly inter-converting forms I and II to which they assigned an all-cis ‘boatlike’ conformation and a mono-trans ‘twist’ conformation. Both the isomers react thermally and give dihydronaphthalenes III and IV after the cyclization respectively. They also retain their respective cis or trans character. The lack of planarity and hence aromatic stability in both the conformations are responsible for the relatively high reactivity of [10]annulene. CHEMISTRY Paper 14: Organic Chemistry –IV (Advance Organic Synthesis and Supramolecular Chemistry and carbocyclic rings) Module 32: ANNULENES The [10]-annulene on hydrogenation gives cyclodecane. 4.4 Reactions of [18]-annulenes The [18]-annulenes are another important higher annulenes. The low-temperature NMR studies have suggested the three conformations of nearly equal energy for this molecule with three internal protons above the plane and three below